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The Transcription Factor FUSCA3 Controls Developmental Timing in Arabidopsis through the Hormones Gibberellin and Abscisic Acid
Abstract
Although plants continually produce different organs throughout their life cycle, little is known about the factors that regulate the timing of a given developmental program. Here we report that the restricted expression of FUS3 to the epidermis is sufficient to control foliar organ identity in Arabidopsis by regulating the synthesis of two hormones, abscisic acid and gibberellin. These hormones in turn regulate the rates of cell cycling during organ formation to determine whether an embryonic or adult leaf will emerge. We also show that FUS3 expression is influenced by the patterning hormone, auxin, and therefore acts as a nexus of hormone action during embryogenesis. The identification of lipophillic hormones downstream of a heterochronic regulator in Arabidopsis has parallels to mechanisms of developmental timing in animals and suggests a common logic for temporal control of developmental programs between these two kingdoms.
... In A. thaliana, activation of the FUS3 protein results in a decrease in the transcriptional activity of the gibberellin biosynthesis genes AtGA3ox1 and AtGA20ox1, leading to an increase in the amount of ABA in developing seed cells [43,44]. In turn, ABI3, together with LEC1, LEC2 and FUS3, mediates ABA biosynthesis in developing seed tissues [37]. ...
... As shown in many studies, there is a positive correlation between FUS3 activity and ABA levels and a negative correlation between FUS3 activity and GA levels [44]. These hormones act in opposite ways during seed maturation, which might be due to the synergistic functionality of GA and PKL in inhibiting the activity of regulatory genes and the positive correlation between these factors and the presence of ABA [53]. ...
... In the results presented here, a very similar expression pattern to LlLEC2 was also observed for the FUS3 homolog (Figure 1c). Other factors that may modulate the activity level of LlLEC2 or LlFUS3 genes may be hormones, including ABA and GA [44,57,58]. The control of GA synthesis and degradation of gene expression by FUS3 affects hormone homeostasis, which may translate into the expression pattern of both LEC2 and FUS3. ...
Citation: Klajn, N.; Kapczyńska, K.; Pasikowski, P.; Glazińska, P.; Kugiel, H.; Kęsy, J.; Wojciechowski, W. Regulatory Effects of ABA and GA on the Expression of Conglutin Genes and LAFL Network Genes in Yellow Lupine (Lupinus luteus L.) Seeds. Int.
Abstract: The maturation of seeds is a process of particular importance both for the plant itself by assuring the survival of the species and for the human population for nutritional and economic reasons. Controlling this process requires a strict coordination of many factors at different levels of the functioning of genetic and hormonal changes as well as cellular organization. One of the most important examples is the transcriptional activity of the LAFL gene regulatory network, which includes LEAFY COTYLEDON1 (LEC1) and LEC1-LIKE (L1L) and ABSCISIC ACID INSENSITIVE3 (ABI3), FUSCA3 (FUS3), and LEC2 (LEAFY COTYLEDON2), as well as hormonal homeostasis-of abscisic acid (ABA) and gibberellins (GA) in particular. From the nutritional point of view, the key to seed development is the ability of seeds to accumulate large amounts of proteins with different structures and properties. The world's food deficit is mainly related to shortages of protein, and taking into consideration the environmental changes occurring on Earth, it is becoming necessary to search for a way to obtain large amounts of plant-derived protein while maintaining the diversity of its origin. Yellow lupin, whose storage proteins are conglutins, is one of the plant species native to Europe that accumulates large amounts of this nutrient in its seeds. In this article we have shown the key changes occurring in the developing seeds of the yellow-lupin cultivar Taper by means of modern molecular biology techniques, including RNA-seq, chromatographic techniques and quantitative PCR analysis. We identified regulatory genes fundamental to the seed-filling process, as well as genes encoding conglutins. We also investigated how exogenous application of ABA and GA 3 affects the expression of LlLEC2, LlABI3, LlFUS3, and genes encoding β-and δ-conglutins and whether it results in the amount of accumulated seed storage proteins. The research shows that for each species, even related plants, very specific changes can be identified. Thus the analysis and possibility of using such an approach to improve and stabilize yields requires even more detailed and extended research.
... and LEC2 genes during postembryonic development induces SE on the cotyledons and leaves of Arabidopsis seedlings . FUS3 and ABI3 do not induce SE when overexpressed, but they do promote embryo traits in seedlings (Gazzarrini et al., 2004;Parcy et al., 1994). The LAFL genes LEC1, LEC2, and FUS3 are needed for efficient BBM-induced SE, while ABI3 negatively regulates BBM-induced somatic embryogenesis ). ...
... Ectopic expression of LEC2 during postembryonic development induces SE on the cotyledons and leaves of arabidopsis seedlings . FUS3 and ABI3 do not induce SE when overexpressed, but they both promote embryo traits in seedlings (Gazzarrini et al., 2004;Parcy et al., 1994). The upregulation of the LAFL genes is in line with previous studies describing their early expression in microspore culture . ...
... FUS3 overexpression induces cotyledon identity in leaves. (Gazzarrini et al., 2004;Yamamoto et al., 2014) 270 PLT1 and PLT2 globular-late embryo AINTEGUMENTA-LIKE TF-Member of the AP2-domain PLETHORA (PLT) family. PLT1 and PLT2 redundantly determine embryo basal cell identity. ...
... In seed development in Arabidopsis, a peak of ABA level in the whole silique is observed in the middle of development (around nine days after flowering (DAF)), and after 12 DAF, ABA increases until late stage of development (21 DAF) [7][8][9]. ABA was detected mostly in the seeds during the middle stage and in the envelopes during the late stage of maturation [7] (Figure 1). It has been demonstrated that nine-cis epoxycarotenoid dioxygenase (NCED) is the key regulatory enzyme in the ABA biosynthetic pathway [11]. ...
... In Arabidopsis, FUS3 expression is increased by exogenously-introduced ABA [72], and FUS3 induces the increase of ABA [8]. Thus, FUS3 and ABA are positive regulators of each other [41]. ...
... Thus, FUS3 and ABA are positive regulators of each other [41]. Furthermore, the expression of FUS3 was found to be able to be positively regulated by auxin [8]. ...
Plants have evolved seeds to permit the survival and dispersion of their lineages by providing nutrition for embryo growth and resistance to unfavorable environmental conditions. Seed formation is a complicated process that can be roughly divided into embryogenesis and the maturation phase, characterized by accumulation of storage compound, acquisition of desiccation tolerance, arrest of growth, and acquisition of dormancy. Concerted regulation of several signaling pathways, including hormonal and metabolic signals and gene networks, is required to accomplish seed formation. Recent studies have identified the major network of genes and hormonal signals in seed development, mainly in maturation. Gibberellin (GA) and abscisic acids (ABA) are recognized as the main hormones that antagonistically regulate seed development and germination. Especially, knowledge of the molecular mechanism of ABA regulation of seed maturation, including regulation of dormancy, accumulation of storage compounds, and desiccation tolerance, has been accumulated. However, the function of ABA and GA during embryogenesis still remains elusive. In this review, we summarize the current understanding of the sophisticated molecular networks of genes and signaling of GA and ABA in the regulation of seed development from embryogenesis to maturation.
... At the beginning of the twenty-first century, great progress has been made in the research of FUS3. Researchers found that the FUS3 gene plays irreplaceable roles in the process of seed development, such as regulating the establishment of embryonic cotyledon identity (Keith et al. 1994), transition of developmental phase (Gazzarrini et al. 2004;Lumba et al. 2012), and accumulation of seed store biological macromolecules (Kroj et al. 2003;Zhang and Rock 2004;Kagaya et al. 2005;Wang et al. 2007Wang et al. , 2014Moreno-Risueno et al. 2008;Yamamoto et al. 2010;Chen et al. 2014Chen et al. , 2015Elahi et al. 2015;Zhang et al. 2016;Sun et al. 2017;Liu et al. 2017;Li et al. 2018aLi et al. , 2020Yang et al. 2018). In the past 10 years of research, it has been discovered that FUS3 TF not only plays key roles in seed development, but also regulates other developmental stages of plants, such as participating in the formation of plant lateral organs (Tsai and Gazzarrini 2012a;Tang et al. 2017) and responding to heat stress (Tamura et al. 2006;Toh et al. 2008;Chiu et al. 2012Chiu et al. , 2016aChan et al. 2017). ...
... The lack of the C-terminal domain is inhibited the degradation of FUS3 (Lu et al. 2010). It also mediates the regulation of ABA and GA on FUS3 stability (Gazzarrini et al. 2004). The AIP2 factor acts on the N-terminal domain of FUS3 to promote FUS3 degradation (Duong et al. 2017). ...
... The AIP2 factor acts on the N-terminal domain of FUS3 to promote FUS3 degradation (Duong et al. 2017). In addition, the phosphorylation state of the FUS3 is also related to its stability (Tsai and Gazzarrini 2012a, b) of FUS3 is strictly regulated by ABA and gibberellins (GA) (Gazzarrini et al. 2004). The C-terminal domain of AtFUS3 is a key area for sensing ABA and GA. ...
FUSCA3 (FUS3) is a member of the seed plant-specific B3 transcription factor family, a multifunctional plant regulator. In plants, the expression of the FUS3 gene is regulated mainly through transcriptional and post-transcriptional regulation, thereby giving it control over various functions. The transcription factor FUS3 can be involved in many important physiological processes in plants, such as transition of seed embryo developmental phase and accumulation of storage reserves during seed development. Recent studies have shown that FUS3 also regulates the development of plant lateral organs, activation of flowering sites, coordination of embryo and endosperm growth and helps plants resist heat stress. Therefore, it plays important roles in seed development and plant growth. Here, we provide an overview of FUS3, including the identification of the FUS3 gene, the regulation and functions of FUS3 and the functional mechanisms of FUS3. In addition, potential research directions are proposed that may shed new light on the functional mechanisms of FUS3 in plants.
... Previous studies have suggested that LEAFY genes play a role in seed development and the transition from the embryonic to the vegetative phase [16][17][18]. To examine the impact of GmLEC1 on cotyledon structure and appearance, mature green seeds of wildtype, atlec1, and GmLEC1/atlec1 were germinated on a sterilized MS medium. ...
... Previous studies have suggested that LEAFY genes play a role in seed development and the transition from the embryonic to the vegetative phase [16][17][18]. To examine the impact of GmLEC1 on cotyledon structure and appearance, mature green seeds of wild-type, atlec1, and GmLEC1/atlec1 were germinated on a sterilized MS medium. ...
Soybean is an important oilseed crop that is used as a feed for livestock and has several industrial uses. Lipid biosynthesis and accumulation primarily occur during seed development in plants. This process is regulated by several transcription factors and interconnected biochemical pathways. This study investigated the role of glycine max LEAFY COTYLEDON 1 (GmLEC1) in soybean seed development and the accumulation of storage reserves. The overexpression of GmLEC1 significantly increased the amount of triacylglycerol (TAG) in transgenic Arabidopsis seeds compared to the wild-type and an atlec1 mutant. Similarly, the high expression of GmLEC1 led to a 12% increase in TAG content in transgenic soybean hairy roots compared to the control. GmLEC1 also altered the fatty acid composition in transgenic Arabidopsis seeds and soybean hairy roots. Additionally, the overexpression of GmLEC1 resulted in a reduction in starch accumulation in seeds and vegetative tissues, as well as changes in cotyledon and seed morphology. The cotyledons of the atlec1 mutant displayed abnormal trichome development, and the seeds were smaller and less tolerant to desiccation. A complementation assay in Arabidopsis restored normal cotyledon phenotype and seed size. The main downstream targets of LEC1 are GL2 and WRI1, which were found to participate in fatty acid biosynthesis and trichome formation through the regulation of phytohormones and various transcription factors involved in seed development and maturation. The findings of this study suggest that GmLEC1 controls seed development and regulates the accumulation of seed storage compounds. Furthermore, these results demonstrate that GmLEC1 could be a reliable target for the genetic improvement of oil biosynthesis in soybean.
... AGL15 can directly target the downstream gene gibberellin 2-oxidase (GA2ox6) and further regulate GA changes during embryonic development in Arabidopsis [26]. Auxin affects the expression level of FUS3, while FUS3 negatively regulates the expression of GA biosynthetic genes GA3ox1 and GA3ox2 [27,28]. FUS3 is a link between hormones during embryogenesis [27]. ...
... Auxin affects the expression level of FUS3, while FUS3 negatively regulates the expression of GA biosynthetic genes GA3ox1 and GA3ox2 [27,28]. FUS3 is a link between hormones during embryogenesis [27]. ...
Paeonia ostii is a worldwide ornamental flower and an emerging oil crop. Zyotic embryogenesis is a critical process during seed development, and it can provide a basis for improving the efficiency of somatic embryogenesis (SE). In this study, transcriptome sequencing of embryo development was performed to investigate gene expression profiling in P. ostii and identified Differentially expressed genes (DEGs) related to transcription factors, plant hormones, and antioxidant enzymes. The results indicated that IAA (Indole-3-acetic acid), GA (Gibberellin), BR (Brassinosteroid) and ETH (Ethylene) were beneficial to early embryonic morphogenesis, while CTK (Cytokinin) and ABA (Abscisic Acid) promoted embryo morphogenesis and maturation. The antioxidant enzymes' activity was the highest in early embryos and an important participant in embryo formation. The high expression of the genes encoding fatty acid desaturase was beneficial to fast oil accumulation. Representative DEGs were selected and validated using qRT-PCR. Protein-protein interaction network (PPI) was predicted, and six central node proteins, including AUX1, PIN1, ARF6, LAX3, ABCB19, PIF3, and PIF4, were screened. Our results provided new insights into the formation of embryo development and even somatic embryo development in tree peonies.
... Many genes participating in FA synthesis are direct or indirect targets of transcription factors which are also implicated in morphogenic events during embryo development (Braybrook and Harada, 2008). Among these factors are LEAFY COTYLEDON1 (LEC1), FUSCA3 (FUS3), and WRIN-KLED1 (WRI1) (Meinke et al., 1994;Gazzarrini et al., 2004;Baud et al., 2007). As a member of the HAP3 subunit of the CCAAT binding factor family (Lotan et al., 1998), LEC1 is expressed during many stages of seed development (Parcy et al., 1994). ...
... Synthesis of oil is tightly regulated by a set of transcription factors comprising LEAFY COTYLEDON1 (LEC1), FUSCA3 (FUS3), and WRIN-KLED1 (WRI1), and supported by sugars produced through photosynthesis or released from storage products (Meinke et al., 1994;Gazzarrini et al., 2004;Baud et al., 2007). In WT seed, LEC1 was induced after 14 DAP and its expression remained stable during the following weeks (Fig. 6A). ...
To examine the function of phytoglobin 2 (Pgb2) on seed oil level in the oil-producing crop Brassica napus L., we generated transgenic plants in which BnPgb2 was over-expressed in the seeds using the cruciferin1 promoter. Over-expression of BnPgb2 elevated the amount of oil, which showed a positive relationship with the level of BnPgb2, without altering the oil nutritional value, as evidenced by the lack of major changes in composition of fatty acids (FA), and key agronomic traits. Two key transcription factors, LEAFY COTYLEDON1 (LEC1) and WRINKLED1 (WRI1), known to promote the synthesis of fatty acids (FA) and potentiate oil accumulation, were induced in BnPgb2 over-expressing seeds. The concomitant induction of several enzymes of sucrose metabolism, SUCROSE SYNTHASE1 (SUS) 1 and 3, FRUCTOSE BISPHOSPHATE ALDOLASE (FPA), and PHOSPHOGLYCERATE KINASE (PGK), and starch synthesis, ADP-GLUCOSE PHOSPHORYLASE (AGPase) suggests that BnPgb2 favors sugar mobilization for FA production. The two plastid FA biosynthetic enzymes SUBUNIT A OF ACETYL-CoA CARBOXYLASE (ACCA2), and MALONYL-CoA:ACP TRANSACYLASE (MCAT) were also up-regulated by the over-expression of BnPgb2. The requirement of BnPgb2 for oil deposition was further evidenced in natural germplasm by the higher levels of BnPgb2 in seeds of high-oil genotypes relative to their low-oil counterparts.
... The four master regulators form a highly redundant network controlling almost every aspects of seed maturation including storage protein and oil accumulation, cotyledon identity, and dormancy ). In the context of storage protein accumulation, FUS3, LEC2, and ABI3 directly induce the expression of seed storage protein genes, and together with LEC1, the four regulators are integrated into a complex network by the multiple regulatory links among themselves, hence a redundant genetic framework playing the central role in storage protein accumulation control is established (Kroj et al. 2003;Parcy et al. 1994;Kagaya et al. 2005b;Gazzarrini et al. 2004;To et al. 2006). This closely relevant network controlling seed development and storage accumulation revealed in Arabidopsis may also play key roles in soybean seeds in similar manners. ...
... A high ABA/GA ratio promotes seed maturation, allowing the ABAinduced storage protein accumulation become dominant (Nambara and Marion-Poll 2005). To maintain such phytohormone ratio, FUS3 and LEC2 function as inhibitors to restrict GA level by repressing the expression of GA biosynthesis genes (Gazzarrini et al. 2004). Recently, Hu et al. (2021) revealed that GA signaling participates in regulating storage protein in Arabidopsis, in which the DELLA protein RGA-LIKE3 (RGL3) acts as coactivator of ABI3 to promote storage protein synthesis. ...
Unlabelled:
Soybean is an utterly important crop for high-quality meal protein and vegetative oil. Soybean seed protein content has become a key factor in nutrients for livestock feed as well as human dietary consumption. Genetic improvement of soybean seed protein is highly desired to meet the demands of rapidly growing world population. Molecular mapping and genomic analysis in soybean have identified many quantitative trait loci (QTL) underlying seed protein content control. Exploring the mechanisms of seed storage protein regulation will be helpful to achieve the improvement of protein content. However, the practice of breeding higher protein soybean is challenging because soybean seed protein is negatively correlated with seed oil content and yield. To overcome the limitation of such inverse relationship, deeper insights into the property and genetic control of seed protein are required. Recent advances of soybean genomics have strongly enhanced the understandings for molecular mechanisms of soybean with better seed quality. Here, we review the research progress in the genetic characteristics of soybean storage protein, and up-to-date advances of molecular mappings and genomics of soybean protein. The key factors underlying the mechanisms of the negative correlation between protein and oil in soybean seeds are elaborated. We also briefly discuss the future prospects of breaking the bottleneck of the negative correlation to develop high protein soybean without penalty of oil and yield.
Supplementary information:
The online version contains supplementary material available at 10.1007/s11032-023-01373-5.
... The recalcitrant Richilia dregeana seeds could maintain viability for several weeks at 16°C at the harvested water content, but they only exhibit 4% viability after 8 days of storage at the axis water content of 1.68 g·g −1 dry mass (Drew et al., 2000). The recalcitrant Araucaria angustifolia seeds show a complete loss of embryo viability after storage at 48% water content for 26 months (Gasparin et al., 2020). The dehydration sensitivity of recalcitrant seeds makes them impossible for long-term storage. ...
... Hence, ABA signaling might be amplified through the MAPK signaling cascades to maintain seed dormancy and inhibit seed development. For example, AtFUS3 influences the development of seed embryos by inhibiting GA biosynthesis and promoting ABA accumulation (Gazzarrini et al., 2004). Consistently, FUS was increased in HA-treated seeds during the after-ripening process ( Figure 6B). ...
The seeds of Panax notoginseng (Burk.) F. H. Chen are typically characterized by their recalcitrance and after-ripening process and exhibit a high water content at harvest as well as a high susceptibility to dehydration. Storage difficulty and the low germination of recalcitrant seeds of P. notoginseng are known to cause an obstacle to agricultural production. In this study, the ratio of embryo to endosperm (Em/En) in abscisic acid (ABA) treatments (1 mg·l ⁻¹ and 10 mg·l ⁻¹ , LA and HA) was 53.64% and 52.34%, respectively, which were lower than those in control check (CK) (61.98%) at 30 days of the after-ripening process (DAR). A total of 83.67% of seeds germinated in the CK, 49% of seeds germinated in the LA treatment, and 37.33% of seeds germinated in the HA treatment at 60 DAR. The ABA, gibberellin (GA), and auxin (IAA) levels were increased in the HA treatment at 0 DAR, while the jasmonic acid (JA) levels were decreased. ABA, IAA, and JA were increased, but GA was decreased with HA treatment at 30 DAR. A total of 4,742, 16,531, and 890 differentially expressed genes (DEGs) were identified between the HA-treated and CK groups, respectively, along with obvious enrichment in the ABA-regulated plant hormone pathway and the mitogen-activated protein kinase (MAPK) signaling pathway. The expression of pyracbactin resistance-like ( PYL ) and SNF1-related protein kinase subfamily 2 ( SnRK2s ) increased in the ABA-treated groups, whereas the expression of type 2C protein phosphatase ( PP2C ) decreased, both of which are related to the ABA signaling pathway. As a result of the changes in expression of these genes, increased ABA signaling and suppressed GA signaling could inhibit the growth of the embryo and the expansion of developmental space. Furthermore, our results demonstrated that MAPK signaling cascades might be involved in the amplification of hormone signaling. Meanwhile, our study uncovered that the exogenous hormone ABA could inhibit embryonic development, promote dormancy, and delay germination in recalcitrant seeds. These findings reveal the critical role of ABA in regulating the dormancy of recalcitrant seeds, and thereby provide a new insight into recalcitrant seeds in agricultural production and storage.
... In this process, the sources of seedling nutrition and energy acquisition gradually transition from consumption of seed storage substances to photoautotrophy, in conjunction with significant alteration of biosynthetic and signaling pathways (Zanten et al., 2013;Jia et al., 2014). Correspondingly, suppression of seed maturation genes, the LAFL, and activation of those involved in vegetative growth is indispensable to avoid ectopic proliferation of embryonic tissues and thus maintain the normal vegetative morphology of seedlings (Parcy and Giraudat, 1997;Lotan et al., 1998;Stone et al., 2001;Gazzarrini et al., 2004;Braybrook et al., 2006;Yang et al., 2013). ...
... In loss-of-function mutants of these genes, embryos skip late-embryonic development and enter the vegetative program prematurely (Keith et al., 1994;West et al., 1994;Nambara et al., 2000). However, when some of these genes are misexpressed in vegetative tissues, abnormally developed seedlings emerge that show induced ectopic deposition of seed storage proteins and even somatic embryo or callus formation (Parcy and Giraudat, 1997;Lotan et al., 1998;Stone et al., 2001;Gazzarrini et al., 2004;Braybrook et al., 2006;Yang et al., 2013). The factors involved in chromatin remodeling and histone modification protect normal seedling morphology mainly by repressing the transcription of LAFL genes ( Figure 1D). ...
Seeds are essential for the reproduction and dispersion of spermatophytes. The seed life cycle from seed development to seedling establishment proceeds through a series of defined stages regulated by distinctive physiological and biochemical mechanisms. The role of histone modification and chromatin remodeling in seed behavior has been intensively studied in recent years. In this review, we summarize progress in elucidating the regulatory network of these two kinds of epigenetic regulation during the seed life cycle, especially in two model plants, rice and Arabidopsis. Particular emphasis is placed on epigenetic effects on primary tissue formation (e.g., the organized development of embryo and endosperm), pivotal downstream gene expression (e.g., transcription of DOG1 in seed dormancy and repression of seed maturation genes in seed-to-seedling transition), and environmental responses (e.g., seed germination in response to different environmental cues). Future prospects for understanding of intricate interplay of epigenetic pathways and the epigenetic mechanisms in other commercial species are also proposed.
... Moreover, AGL18 induces AGL16, LEC1, PLETHORA2 (PLT2), and ABI4, while both AGL15 and AGL18 suppress GA3ox2, a gibberellin (GA) biosynthetic gene, to promote somatic embryogenesis (Paul et al., 2022). Earlier studies have shown that LEC2 and FUS3 negatively affect GA biosynthesis by repressing GA3ox2 expression, thus regulating the embryonic development (Curaba et al., 2004;Gazzarrini et al., 2004). The activation of auxin biosynthesis genes and the suppression of GA biosynthesis during embryogenesis further implies the importance of hormonal regulation during the process, however, delving into this aspect is beyond the scope of this review. ...
Transcription factors (TFs) are diverse groups of regulatory proteins. Through their specific binding domains, TFs bind to their target genes and regulate their expression, therefore TFs play important roles in various growth and developmental processes. Plant embryogenesis is a highly regulated and intricate process during which embryos arise from various sources and undergo development; it can be further divided into zygotic embryogenesis (ZE) and somatic embryogenesis (SE). TFs play a crucial role in the process of plant embryogenesis with a number of them acting as master regulators in both ZE and SE. In this review, we focus on the master TFs involved in embryogenesis such as BABY BOOM (BBM) from the APETALA2/Ethylene-Responsive Factor (AP2/ERF) family, WUSCHEL and WUSCHEL-related homeobox (WOX) from the homeobox family, LEAFY COTYLEDON 2 (LEC2) from the B3 family, AGAMOUS-Like 15 (AGL15) from the MADS family and LEAFY COTYLEDON 1 (LEC1) from the Nuclear Factor Y (NF-Y) family. We aim to present the recent progress pertaining to the diverse roles these master TFs play in both ZE and SE in Arabidopsis, as well as other plant species including crops. We also discuss future perspectives in this context.
... Mutations of LAFL are associated with misexpression of embryonic characteristics, resulting in arrested seedling development, while their action is antagonised by VIVIPAROUS1/ABI3-LIKE (VAL) proteins that repress LAFL transcription during the final stage of germination (Jia et al. 2013). Among LAFL, FUS3 represses GA biosynthesis, affecting ABA/GAs balance by promoting ABA accumulation (Gazzarrini et al. 2004). While ABI3 binds the promoter of REVERSAL OF RDO (ODR) and hinders its expression. ...
Seed germination is a crucial plant-life process whose success depends largely on the seed's ability to germinate under favourable environmental conditions. Through molecular signalling, a seed is able to perceive environmental information, assimilate it, and transmit signals that determine its destiny. Reactive Oxygen and Nitrogen Species (RONS) function as signalling molecules that influence multiple phases of plant development. In the process of seed germination, their presence generally promotes germination completion, though not to the same extent in all species and environments. As signalling molecules, they participate in the sensing of light and temperature fluctuations as favourable germination cues, but they also play a role in inhibiting germination when temperatures exceed the optimal range, preventing seedling exposure to heat. Depending on environmental conditions, RONS set up crosstalk with the major phytohormones involved in germination, ABA, GA, and even auxin, regulating their biosynthesis and signalling. Here, we show relevant studies on how RONS exert seed germination control on multiple levels, such as through protein oxidation, epigenetic control, promotion of phytohormone key-metabolism genes expression, post-translational protein modifications, and redox interactions with DOG1. This review summarises the current understanding of the role of RONS in the seed, from its maturation to the transduction of environmental conditions. Special consideration is given to the RONS-mediated germination response to favourable stimuli, such as light or temperature fluctuations, and to conditions that inhibit germination, such as high temperatures.
... AGL15 is a direct induced target of several key embryo identity TFs including LEC1, LEC2, FUS3, but not ABI3 (the so-called LAFL factors) (Braybrook et al., 2006;Pelletier et al., 2017;Wang & Perry, 2013). These LAFL TFs are also able to induce embryonic programs in tissues after completion of germination to different extents (Gazzarrini et al., 2004;Lotan et al., 1998;Stone et al., 2001). Direct and indirect targets of AGL15 were determined to investigate the mechanism of SE promotion, revealing that AGL15 can directly induce as well as repress gene expression Zheng et al., 2009). ...
Somatic embryogenesis (SE) is a process by which an embryo is derived from somatic tissue. Transcription factors (TFs) have been identified that control this process. One such TF that promotes SE is AGAMOUS-like 15 (AGL15). Prior work has shown that AGL15 can both induce and repress gene expression. One way this type of dual function TF works is via protein interactions, so a yeast 2-hybrid (Y2H) screen was undertaken. One intriguing protein with which AGL15 interacted in Y2H was LBD40. LBD40 encodes a LATERAL ORGAN BOUNDARIES (LOB)-domain TF that is unique to plants and is primarily expressed during seed development. Here, we confirm the AGL15-LBD40 interaction by quantitative assays and in planta co-immunoprecipation. We also document a role for LBD40, and the closely related protein LBD41, in supporting SE. To determine downstream genes potentially controlled by LBD40, chromatin immunoprecipitation followed by high throughput sequencing (ChIP-seq) was used. More than 400 binding regions for LBD40 were consistently found genome-wide. To determine genes responsive to LBD40/41 accumulation, RNA-seq analysis of transcriptomes of wild-type control and loss-of-function lbd40/lbd41 was performed. Combining these datasets provides insight into genes directly and indirectly controlled by these LOB domain TFs. The gene ontology (GO) enrichment analysis of these regulated genes showed an overrepresentation of biological processes that are associated with SE, further indicating the importance of LBD40 in SE. This work provides insight into SE, a poorly understood, but essential process to generate transgenic plants to meet agricultural demands or test gene function. This manuscript reports on experiments to understand the role that LDB40, a TF, plays in support of SE by investigating genes directly and indirectly controlled by LBD40 and examining physical and genetic interactions with other TFs active in SE. We uncover targets of LBD40 and an interacting TF of the MADS family and investigate targets involvement in SE.
... It has also been shown that AGL15 can be a positive regulator for the GIBBERELLIN 2-OXIDASE6 (GA2ox6) gene encoding a GA-degrading enzyme and a negative regulator for the GA3ox2 gene which generally causes a reduction in the endogenous GA concentration [86]. Furthermore, GA biosynthesis is downregulated by FUS3 through activating ABA biosynthesis and repressing GA3ox1 and GA3ox2 [87]. In contrast, our results showed that gibberellin dioxygenase genes (e.g., LOC115704748, LOC115705300, and LOC115710658) were upregulated in embryogenic calli, which is considered to be an additional sign of a failure to induce somatic embryogenesis in embryogenic callus. ...
Differential gene expression profiles of various cannabis calli including non-embryogenic and embryogenic (i.e., rooty and embryonic callus) were examined in this study to enhance our understanding of callus development in cannabis and facilitate the development of improved strategies for plant regeneration and biotechnological applications in this economically valuable crop. A total of 6118 genes displayed significant differential expression, with 1850 genes downregulated and 1873 genes upregulated in embryogenic callus compared to non-embryogenic callus. Notably, 196 phytohormone-related genes exhibited distinctly different expression patterns in the calli types, highlighting the crucial role of plant growth regulator (PGRs) signaling in callus development. Furthermore, 42 classes of transcription factors demonstrated differential expressions among the callus types, suggesting their involvement in the regulation of callus development. The evaluation of epigenetic-related genes revealed the differential expression of 247 genes in all callus types. Notably, histone deacetylases, chromatin remodeling factors, and EMBRYONIC FLOWER 2 emerged as key epigenetic-related genes, displaying upregulation in embryogenic calli compared to non-embryogenic calli. Their upregulation correlated with the repression of embryogenesis-related genes, including LEC2, AGL15, and BBM, presumably inhibiting the transition from embryogenic callus to somatic embryogenesis. These findings underscore the significance of epigenetic regulation in determining the developmental fate of cannabis callus. Generally, our results provide comprehensive insights into gene expression dynamics and molecular mechanisms underlying the development of diverse cannabis calli. The observed repression of auxin-dependent pathway-related genes may contribute to the recalcitrant nature of cannabis, shedding light on the challenges associated with efficient cannabis tissue culture and regeneration protocols.
... Overexpression of LEC2 significantly affects SE in Theobroma cacao [113,114]. LEC2 represses GA3ox2 and promotes the auxin pathway, whereas FUSCA3 (FUS3) negatively regulates gibberellin (GA) accumulation by suppressing GA3ox2 and GA3ox3 [115]. GA also has a positive role during SE, stimulating the expression of CnKNOX1 [71]. ...
The coconut palm (Cocos nucifera L.) is a perennial, cross-pollinated, oil-bearing tropical forest tree. Recently, the demand for coconut goods has surged to 5 to 10 times its former value; however, coconut production is in jeopardy. Coconut senility is one of the most apparent factors that influence productivity. Adequate replanting is urgently required to maintain the growing demand for coconut products. However, coconut palm mass replanting might not be possible with traditional approaches. To overcome this snag, micropropagation via somatic embryogenesis (SE) has enormous potential for proficient clonal propagation in the coconut palm. During SE, the stimulation of cell proliferation, acquisition of embryogenic cell competence, and induction of somatic embryos undergo a series of developmental events. This phenomenon requires regulation in gene expression patterns and the activation of specific signaling pathways. This review summarizes gene regulatory mechanisms involved in the cell cycle, dedifferentiation, totipotency, embryo initiation, and meristem development during somatic embryo formation. Plant hormonal signal transduction is also highlighted during the formation of SE in coconut.
... These results indicate that ABRE motif plays an important role in B3 protein-mediated regulation. (Curaba et al., 2004;Gazzarrini et al., 2004;Yamamoto et al., 2009). ...
B3-domain containing transcription factors (TFs) are well known to play important roles in various developmental processes, including embryogenesis, seed germination, etc. Characterizations and functional studies of the B3 TF superfamily in poplar are still limited, especially on their roles in wood formation. In this study, we conducted comprehensive bioinformatics and expression analysis of B3 TF genes in Populus alba × Populus glandulosa. A total of 160 B3 TF genes were identified in the genome of this hybrid poplar, and their chromosomal locations, syntenic relationships, gene structures, and promoter cis-acting elements were analyzed. Through domain structure and phylogenetic relationship analyses, these proteins were classified into four families LAV, RAV, ARF, and REM. Domain and conservation analyses revealed different gene numbers and different DNA-binding domains among families. Syntenic relationship analysis suggested that approximately 87% of the genes resulted from genome duplication (segmental or tandem), contributing to the expansion of the B3 family in P. alba × P. glandulosa. Phylogeny in seven species revealed the evolutionary relationship of B3 TF genes across different species. B3 domains among the eighteen proteins that were highly expressed in differentiating xylem had a high synteny, suggesting a common ancestor for these seven species. We performed co-expression analysis on the representative genes in two different ages of poplar, followed by pathways analysis. Among those genes co-expressed with four B3 genes, 14 were involved in lignin synthases and secondary cell walls biosynthesis, including PagCOMT2, PagCAD1, PagCCR2, PagCAD1, PagCCoAOMT1, PagSND2, and PagNST1. Our results provide valuable information for the B3 TF family in poplar and show the potential of B3 TF genes in engineering to improve wood properties.
... In total, 8,927 AS events were detected, of which 88% were newly identified in Arabidopsis. Among them was found a new alternative 3′ splice site (A3P) event of the FUSCA3 (FUS3) gene which codes for a B3 domain transcription factor that acts as a regulator of seed development (Gazzarrini et al., 2004;Tsai and Gazzarrini, 2012). From this AS variant a truncated protein was created due to reading frame shift and formation of a premature stop codon within the B3 domain. ...
Seed germination is an essential step in a plant's life cycle. It is controlled by complex physiological, biochemical, and molecular mechanisms and external factors. Alternative splicing (AS) is a co-transcriptional mechanism that regulates gene expression and produces multiple mRNA variants from a single gene to modulate transcriptome diversity. However, little is known about the effect of AS on the function of generated protein isoforms. The latest reports indicate that alternative splicing (AS), the relevant mechanism controlling gene expression, plays a significant role in abscisic acid (ABA) signaling. In this review, we present the current state of the art about the identified AS regulators and the ABA-related changes in AS during seed germination. We show how they are connected with the ABA signaling and the seed germination process. We also discuss changes in the structure of the generated AS isoforms and their impact on the functionality of the generated proteins. Also, we point out that the advances in sequencing technology allow for a better explanation of the role of AS in gene regulation by more accurate detection of AS events and identification of full-length splicing isoforms.
... These genes show that resetting could occur at multiple time windows before microspore formation. Instead, genes related to embryo pattern formation such as FUS3, WOX11 and XND1 [46][47][48][49] , showed only a mild reduction of H3K27me3 in microspores (Fig. 4h), suggesting that their resetting occurs predominantly after microspore formation. Intriguingly, although the mRNA levels of resetting genes were generally undetectable in sperm, they were often expressed either before sperm formation or after fertilization (Fig. 4i). ...
Epigenetic reprogramming in the germline contributes to the erasure of epigenetic inheritance across generations in mammals but remains poorly characterized in plants. Here we profiled histone modifications throughout Arabidopsis male germline development. We find that the sperm cell has widespread apparent chromatin bivalency, which is established by the acquisition of H3K27me3 or H3K4me3 at pre-existing H3K4me3 or H3K27me3 regions, respectively. These bivalent domains are associated with a distinct transcriptional status. Somatic H3K27me3 is generally reduced in sperm, while dramatic loss of H3K27me3 is observed at only ~700 developmental genes. The incorporation of the histone variant H3.10 facilitates the establishment of sperm chromatin identity without a strong impact on resetting of somatic H3K27me3. Vegetative nuclei harbor thousands of specific H3K27me3 domains at repressed genes, while pollination-related genes are highly expressed and marked by gene body H3K4me3. Our work highlights putative chromatin bivalency and restricted resetting of H3K27me3 at developmental regulators as key features in plant pluripotent sperm.
... GA may also promote germination by promoting the transition from embryonic to vegetative growth, which is mediated in part by the chromatin remodeling protein PICKLE (PKL). GA-stimulated disappearance of the embryonic identity protein FUSCA 3 (FUS3), which positively regulates ABA synthesis and negatively regulates GA synthesis, and amplification of the pkl embryonic root phenotype by GA biosynthesis inhibitors both support GA promotion of this transition (Henderson et al., 2004;Gazzarrini et al., 2004). Gibberellins' role in dormancy release is debatable. ...
Seed dormancy is the result of a deficiency in germination. However, the lack of germination can be caused by various reasons. Therefore, the frequently used definition of seed dormancy can be defined as "the failure of a viable seed to germinate under environmental conditions suitable for germination". The underlying mechanisms of all types of dormancy produce a delay between seed shedding and germination. This variation is characterized according to whether germination is limited due to embryonic immaturity or physical or physiological restrictions, and if the regulating structure or chemicals are embryonic or in the seed's surrounding tissues, i.e., coat imposed. Dormancy release is controlled by a combination of endogenous and exogenous signals that have both antagonistic and synergistic effects. Regulation of dormancy is mostly regulated by plant growth regulators and other molecules. Changes in proteomes transcriptomes and plant growth regulator levels have been linked to dormancy states ranging from deep primary or secondary dormancy to varying degrees of release in molecular studies of dormancy. The amount of abscisic acid (ABA) and gibberellin (GA) levels and sensitivity is a significant, but just not single, regulator of dormancy condition. Environmental signals regulate this amount by modifying the expression of biosynthetic and catabolic enzymes. ABA stimulates dormancy initiation and maintenance, so while GA stimulates progression from release to germination. Environmental and hormonal responses mediators include both positive and negative regulators, many of which are feedback-regulated to increase or decrease the response. As a result, the reaction is slightly diverse, allowing more temporal choices for effective germination.
... GA biosynthesis genes (KO, KAO, KS, GA20ox, and DELLAs) were co-expressed with LEC1 and FUS3 and correlated with auxin and BR biosynthesis genes, YUC, IAAs, and BR6ox in K. obovata. It was reported that FUS3 represses GA biosynthesis and increases ABA in seed maturation (Gazzarrini et al., 2004). However, a study also showed that LEC1 interacts with DELLA in vivo, and the increase of GA will release the suppressing role of DELLA to LEC1; in reverse, promoting auxin accumulation facilitates embryo development in late embryogenesis (Hu et al., 2018). ...
Vivipary is a rare sexual reproduction phenomenon where embryos germinate directly on the maternal plants. However, it is a common genetic event of woody mangroves in the Rhizophoraceae family. The ecological benefits of vivipary in mangroves include the nurturing of seedlings in harsh coastal and saline environments, but the genetic and molecular mechanisms of vivipary remain unclear. Here we investigate the viviparous embryo development and germination processes in mangrove Kandelia obovata by a transcriptomic approach. Many key biological pathways and functional genes were enriched in different tissues and stages, contributing to vivipary. Reduced production of abscisic acid set a non-dormant condition for the embryo to germinate directly. Genes involved in the metabolism of and response to other phytohormones (gibberellic acid, brassinosteroids, cytokinin, and auxin) are expressed precociously in the axis of non-vivipary stages, thus promoting the embryo to grow through the seed coat. Network analysis of these genes identified the central regulatory roles of LEC1 and FUS3 , which maintain embryo identity in Arabidopsis. Moreover, photosynthesis related pathways were significantly up-regulated in viviparous embryos, and substance transporter genes were highly expressed in the seed coat, suggesting a partial self-provision and maternal nursing. We conclude that the viviparous phenomenon is a combinatorial result of precocious loss of dormancy and enhanced germination potential during viviparous seed development. These results shed light on the relationship between seed development and germination, where the continual growth of the embryo replaces a biphasic phenomenon until a mature propagule is established.
... LEC1 and LEC2 seem involved in maintaining or inducing a totipotent cell state during embryogenesis through the control of auxin biosynthetic genes (Stone et al., 2008;Wójcikowska and Gaj, 2015;Lepiniec et al., 2018). FUS3 is known to modulate the ABA/GA balance by increasing ABA levels and repressing the synthesis of GA, while ABI3 integrates ABA signalling (Curaba et al., 2004;Gazzarrini et al., 2004;Braybrook et al., 2006). In addition, LAFL activities are themselves modulated by hormone signalling feedbacks involving ABA, GA, BR or auxins Carbonero et al., 2017;Lepiniec et al., 2018). ...
Despite the importance of secondary dormancy for plant life cycle timing and survival, there is insufficient knowledge about the (epigenetic) regulation of this trait at the molecular level. Our aim was to determine the role of (epi)genetic processes in the regulation of secondary seed dormancy using natural genotypes of the widely distributed Capsella bursa-pastoris . Seeds of nine ecotypes were exposed to control conditions or histone deacetylase inhibitors [trichostatin A (TSA), valproic acid] during imbibition to study the effects of hyper-acetylation on secondary seed dormancy induction and germination. Valproic acid increased secondary dormancy and both compounds caused a delay of t50 for germination (radicle emergence) but not of t50 for testa rupture, demonstrating that they reduced speed of germination. Transcriptome analysis of one accession exposed to valproic acid versus water showed mixed regulation of ABA, negative regulation of GAs, BRs and auxins, as well as up-regulation of SNL genes, which might explain the observed delay in germination and increase in secondary dormancy. In addition, two accessions differing in secondary dormancy depth (deep vs non-deep) were studied using RNA-seq to reveal the potential regulatory processes underlying this trait. Phytohormone synthesis or signalling was generally up-regulated for ABA (e.g. NCED6 , NCED2 , ABCG40 , ABI3 ) and down-regulated for GAs ( GA20ox1 , GA20ox2 , bHLH93 ), ethylene ( ACO1 , ERF4-LIKE, ERF105 , ERF109-LIKE ), BRs ( BIA1 , CYP708A2-LIKE , probable WRKY46 , BAK1 , BEN1 , BES1 , BRI1 ) and auxin ( GH3.3 , GH3.6 , ABCB19 , TGG4 , AUX1 , PIN6 , WAT1 ). Epigenetic candidates for variation in secondary dormancy depth include SNL genes, histone deacetylases and associated genes ( HDA14 , HDA6-LIKE , HDA-LIKE , ING2 , JMJ30 ), as well as sequences linked to histone acetyltransferases ( bZIP11 , ARID1A-LIKE ), or to gene silencing through histone methylation ( SUVH7 , SUVH9 , CLF ). Together, these results show that phytohormones and epigenetic regulation play an important role in controlling differences in secondary dormancy depth between accessions.
... Second, in early lateral root formation, pericycle specific expression of the transcription factors LEC2 and FUS3 has been shown to induce the auxin biosynthesizing enzyme YUCCA4 (Tang et al. 2017). Additionally, expression of FUS3, and indirectly also LEC2 is auxin-dependent (Gazzarrini et al. 2004;Horstman et al. 2017). Combined this gives rise to another positive feedback loop ( Fig 3A) In our model we simplified this through incorporating an auxin-dependent expression of YUCCA4 specifically in the pericycle: Eq. 6 with YUCCA4 production rate, m,YUCCA4 the half-saturation constant of auxin dependent transcription, and YUCCA4 degradation rate. ...
Priming is the process through which periodic elevations in auxin signalling prepattern future sites for lateral root formation, called prebranch sites. Thusfar is has remained a matter of debate to what extent elevations in auxin concentration and/or auxin signalling are critical for priming and prebranch site formation. Recently, we discovered a reflux-and-growth mechanism for priming generating periodic elevations in auxin concentration that subsequently dissipate. Here we reverse engineer a mechanism for prebranch site formation that translates these transient elevations into a persistent increase in auxin signalling, resolving the prior debate into a two-step process of auxin concentration mediated initial signal and auxin signalling capacity mediated memorization. A critical aspect of the prebranch site formation mechanism is its activation in response to time integrated rather than instantaneous auxin signalling. The proposed mechanism is demonstrated to be consistent with prebranch site auxin signalling dynamics, lateral inhibition and symmetry breaking mechanisms and perturbations in auxin homeostasis.
... identified in ACRs with increased accessibility in hybrids (Figure 6d), including two Dof-type proteins [Dof3.6/OBP3 modulating plant development and phytochrome and cryptochrome signaling (Kang & Singh, 2000;Ward et al., 2005); and CDF2 essential for a photoperiodic flowering response (Fornara et al., 2009;Sun et al., 2015)], an ethylene response factor member ERF8 involved in abscisic acid (ABA) and immune signaling ( (Chan et al., 2017;Wang & Perry, 2013), and leaf organogenesis mediated by gibberellin and ABA signaling (Gazzarrini et al., 2004). We constructed a transcriptional regulatory network for six enriched TF binding motifs (large circles) and ACRs (small circles) that increase accessibility in hybrids, and shades of color represent accessible ACRs affected by one or more TFs of potential regulation, of which two Dof-type factors Dof3.6 and CDF2 targeted the most accessible ACRs (Figure 6e). ...
Heterosis is extensively used to improve crop productivity, yet its allelic and chromatin regulation remains unclear. Based on our resolved genomes of the maternal TGY and paternal HD, we analyzed the contribution of allele-specific expression (ASE) and chromatin accessibility of JGY and HGY, the artificial hybrids of oolong tea with the largest cultivated area in China. The ASE genes (ASEGs) of tea hybrids with maternal-biased were mainly related to the energy and terpenoid metabolism pathways, whereas the ASEGs with paternal-biased tend to be enriched in glutathione metabolism, and these parental bias of hybrids may coordinate and lead to the acquisition of heterosis in more biological pathways. ATAC-seq results showed that hybrids have significantly higher accessible chromatin regions (ACRs) compared to their parents, which may confer broader and stronger transcriptional activity of genes in hybrids. The number of ACRs with significantly increased accessibility in hybrids was much greater than decreased, and the associated alleles were also affected by differential ACRs across different parents, suggesting enhanced positive chromatin regulation and potential genetic effects in hybrids. Core ASEGs of terpene and purine alkaloid metabolism pathways with significant positive heterosis have greater chromatin accessibility in hybrids and were potentially regulated by several members of the MYB, DOF, and TRB families. The binding motif of CsMYB85 in the promoter ACR of the rate-limiting enzyme CsDXS was verified by DAP-seq. These results suggest that higher numbers and more accessible ACRs in hybrids contribute to the regulation of ASEGs, thereby affecting the formation of heterotic metabolites.
... The transcription factor ABI3 is involved in auxin signaling [116]. Expression of LEC2 activates auxin-related genes [117] and auxin activates the expression of FUS3 [118]. ...
The synthesis of seed storage reserves occurs during seed filling, and many seeds contain large and characteristic levels of polymeric reserves. Storage reserves are found in the endosperm of cereal seeds and in the endosperm and/or cotyledons of dicot seeds depending of the plant crop species. Recently progress has been made in understanding the complex network of genetic regulation associated with seed filling. These advances in storage reserve quantity and nutrient quality contribute to a comprehensive understanding of reserve composition, synthesis, and regulation. Phytohormones such as abscisic acid (ABA), cytokinin, gibberellic acid, Indole-3-acetic acid (IAA), ethylene and their interactions play critical roles in seed filling and development. At different stages of seed development, the levels of different hormones such as ABA, IAA zeatin and zeatin riboside changes gradually from the beginning of the process to maturity. In addition, the quality and yield of seed storage reserves are significantly affected by the environmental conditions before and during the synthesis of the reserves. Given the fateful importance of seed storage reserves for food and feed and their use as sustainable industrial feedstock to replace dwindling fossil reserves, understanding the metabolic and developmental control of seed filling will be an important focus of plant research.
... higher than in wild-type seeds, while those in sfl1-1;sfl4-1 dry seeds were 4-fold higher ( Figure S4a). Auxin is known to enhance the effect of ABA on seed dormancy (Gazzarrini et al., 2004;Liu et al., 2013), and in our study we found increased accumulation of the auxin, indole-3-acetic acid (IAA) in sfl1-1;sfl4-1 and sfl1-1; sfl2-1;sfl3-1 dry seeds ( Figure S4b). However, there were no significant differences in the levels of jasmonic acid (JA), JA-Ile, and salicylic acid between the mutants and wild type ( Figure S4c,d,e). ...
Seed dormancy is an adaptive trait which enables plants to survive adverse conditions and restart growth in a season and location suitable for vegetative and reproductive growth. Control of seed dormancy is also important for crop production and food quality because it can help induce uniform germination and prevent preharvest sprouting. Rice preharvest sprouting quantitative trait locus (QTL) analysis has identified Seed dormancy 4 (Sdr4) as a positive regulator of dormancy development. Here, we analyzed the loss‐of‐function mutant of the Arabidopsis ortholog, Sdr4 Like1 (SFL1), and found that the sfl1‐1 seeds showed precocious germination at mid‐ to late‐maturation stage similar to rice sdr4 mutant, but converted to become more dormant than the wild type during maturation drying. In sync with the dormancy levels, expression levels of the seed maturation and dormancy master regulator genes, ABI3, FUS3 and DOG1 in sfl1‐1 seeds were lower than in wild type at early‐ and mid‐maturation stages, but higher at the late‐maturation stage. In addition to the seed dormancy phenotype, sfl1‐1 seedlings showed a growth arrest phenotype and heterochronic expression of LAFL (LEC1, ABI3, FUS3, LEC2) and DOG1 in the seedlings. These data suggest that SFL1 is a positive regulator of initiation and termination of the seed dormancy program. We also found genetic interaction between SFL1 and the SFL2, SFL3 and SFL4 paralogs of SFL1, which impacts on the timing of the phase transition from embryo maturation to seedling growth.
... Several studies also illustrate DELLAs role in mediating the antagonism between ABA and GAs in light-mediated seed germination. It was noted that a transcriptional factor LEAFY COTYLEDON 2 and FUSCA 3 binds with DEL-LAs to induce ABA synthesis and GA catabolism, thus negatively regulating light-mediated seed germination and seedling development (Curaba et al., 2004;Gazzarrini et al., 2004) (Figure 1). In A. thaliana, a light-liable transcriptional factor PHYTOCHROME-INTERACTING FACTOR 3-LIKE 5 (PIL5) was also found to interact with DEL-LAs GAI and RGA (Oh et al., 2007), activating the ABA biosynthesis and inhibiting GA activity to negatively regulate phytochrome mediated seed germination (Gabriele et al., 2010) (Figure 1; Table 3). ...
Gibberellins (GAs) are a pervasive group of diterpene phytohormones. They can dominate various aspects of plant development and modulate plant phenotypic plasticity by crosstalking with multiple phytohormones. Studies on GA signaling illustrate that master negative regulator DELLA proteins regulate GAs and nearly all phytohormones growth responses to sustain a dilemma among plant growth and defense under relentless biotic and abiotic stresses. The molecular mechanism of DELLA proteins in regulating GA‐sensitive growth in an integrated manner with multiple phytohormones became apparent recently. These studies describe the DELLAs interaction with a diverse group of transcriptional regulators and factors as well as non‐transcriptional proteins in directing GA‐stimulated plant growth. In this review, we try to address the extensive recent work to delineate DELLAs and their interacting partners and summarize the latest evidence that DELLA proteins have a unique function in suppressing the GA‐related responses to sustain the tension among growth and defense. This article is protected by copyright. All rights reserved
... During the seed-to-seedling transition, however, transcription of these positive regulators must be strictly silenced, ensuring the successful transition to vegetative development (Suzuki and McCarty, 2008;Jia et al., 2014). Continued expression of LAFL genes leads to aberrant embryonic traits, and even failure to develop normal seedlings (Lotan et al., 1998;Stone et al., 2001;Gazzarrini et al., 2004;Suzuki and McCarty, 2008;Thakare et al., 2008). ...
PICKLE (PKL) is a Chromodomain Helicase DNA-binding domain 3 (CHD3) chromatin remodeler that plays essential roles in controlling the gene expression patterns that determine developmental identity in plants, but the molecular mechanisms through which PKL is recruited to its target genes remain elusive. Here, we define a cis-motif and trans-acting factors mechanism that governs the genomic occupancy profile of PKL in Arabidopsis thaliana. We show that two homologous trans-factors VIVIPAROUS1/ABI3-LIKE1 (VAL1) and VAL2, physically interact with PKL in vivo, localize extensively to PKL-occupied regions in the genome, and promote efficient PKL recruitment at thousands of target genes, including those involved in seed maturation. Transcriptome analysis and genetic interaction studies reveal a close cooperation of VAL1/VAL2 and PKL in regulating gene expression and developmental fate. We demonstrate that this recruitment operates at two master regulatory genes, ABSCISIC ACID INSENSITIVE3 (ABI3) and AGAMOUS-LIKE 15 (AGL15), to repress the seed maturation program and ensure the seed-to-seedling transition. Together, our work unveils a general rule through which the CHD3 chromatin remodeler PKL binds to its target chromatin in plants.
... In this study, we identified the TPGs FUS3, AGL15 and CYP88A3 in the MSe of B. napus. FUS3 is a master regulator of seed development in A. thaliana, and regulates embryonic development by controlling the levels of GA and ABA (Gazzarrini et al., 2004). ABA has been demonstrated to regulate seed maturation and dormancy, and to inhibit the phase transition from embryonic to germinative growth and from vegetative to reproductive growth (Finkelstein et al., 2002;Vishal, 2018). ...
Brassica napus is an important oil crop and an allotetraploid species. However, the detailed analysis of gene function and homoeologous gene expression in all tissues at different developmental stages was not explored. In this study, we performed a global transcriptome analysis of 24 vegetative and reproductive tissues at six developmental stages (totally 111 tissues). These samples were clustered into eight groups. The gene functions of silique pericarp were similar to roots, stems, and leaves. In particular, glucosinolate metabolic process was associated with root and silique pericarp. Genes involved in protein phosphorylation was often associated with stamen, anther, and the early developmental stage of seeds. Transcription factor (TF) genes were more specific than structural genes. A total of 17,100 genes that were preferentially expressed in one tissue (tissue‐preferred genes, TPGs), including 889 TFs (5.2%), were identified in the 24 tissues. Some TPGs were identified as hub genes in the co‐expression network analysis, and some TPGs in different tissues were involved in different hormone pathways. About 67.0% of the homoeologs showed balanced expression, whereas biased expression of homoeologs was associated with structural divergence. In addition, the spatiotemporal expression of homoeologs were related to the presence of transposable elements (TEs) and regulatory elements (REs); more TEs and fewer REs in the promoters resulted in divergent expression in different tissues. This study provides a valuable transcriptional map for understanding the growth and development of B. napus, for identifying important genes for future crop improvement, and for exploring gene expression patterns in the B. napus.
... BBM directly regulates expression of the LAFL genes (LEAFY COTY-LEDON1 (LEC1), ABI3, FUSCA3, and LEC2). LAFL genes are central regulators of embryo development, as they regulate the maintenance of embryo identity, embryo storage product accumulation, and desiccation tolerance (45)(46)(47) and can also induce somatic embryo formation or confer embryo identity when ectopically expressed (48)(49)(50)(51). Thus, loss of BBM (and PLT2) expression might simultaneously deregulate expression of many of these key transcription factors and their transcriptional networks, leading to embryo abortion and arrest. ...
The BABY BOOM (BBM) AINTEGUMENTA-LIKE (AIL) AP2/ERF domain transcription
factor is a major regulator of plant cell totipotency, as it induces asexual
embryo formation when ectopically expressed. Surprisingly, only limited information is available on the role of BBM during zygotic embryogenesis. Here we reexamined BBM expression and function in the model plant Arabidopsis thaliana (Arabidopsis) using reporter analysis and newly developed CRISPR mutants. BBM was expressed in the embryo from the zygote stage and also in the maternal (nucellus) and filial (endosperm) seed tissues. Analysis of CRISPR mutant alleles for BBM (bbm-cr) and the redundantly acting AIL gene PLETHORA2 (PLT2) (plt2-cr) uncovered individual roles for these genes in the timing of embryo progression. We also identified redundant roles for BBM and PLT2 in endosperm proliferation and cellularization and the maintenance of
zygotic embryo development. Finally, we show that ectopic BBM expression in the egg cell of Arabidopsis and the dicot crops Brassica napus and Solanum lycopersicon is sufficient to bypass the fertilization requirement for embryo development. Together these results highlight roles for BBM and PLT2 in seed development and demonstrate the utility of BBM genes for engineering asexual embryo development in dicot species.
... Thereby, the expression level of VvWRKY37 was downregulated with decreasing ABA levels in grape vine buds before bud break. A previous study has reported that ABA and GA had an antagonistic effect on the regulation of several biological processes (Gazzarrini et al., 2004). Thus, the interplay between the hormones determines their effectiveness. ...
Dormancy is a common survival strategy in plants to temporarily suspend visible growth under unsuitable conditions. The elaborate mechanism underlying bud break in perennial woody plants is gradually illustrated. Here, we identified a grape vine WRKY transcription factor, VvWRKY37, which was highly expressed in dormant buds. It was particularly induced by the application of exogenous abscisic acid, and depressed on exposure to gibberellin and low temperature (4°C) stress at the transcript level. The yeast one-hybrid assay confirmed that VvWRKY37 had a transcriptional activity. Ectopic over-expression of VvWRKY37 significantly delayed bud break of transgenic poplar plants. As an ABA-inducible gene, VvWRKY37 also depressed the expression of ABA catabolic gene CYP707As and enhanced the accumulation of endogenous ABA in transgenic poplar plants. The molecular pieces of evidence showed that VvWRKY37 preferentially recognized and bound W-box 5′-G/CATTGACT/C/G-3′ cis-element in vitro. Additionally, VvABI5 and VvABF2 acted as the upstream transcriptional activators of VvWRKY37 via protein-DNA interactions. Taken together, our findings provided valuable insights into a new regulatory mechanism of WRKY TF by which it modulates bud break through ABA-mediated signaling pathways.
... Embryonic cells have been shown to have a higher ratio of GA to ABA than somatic cells (Yamaguchi et al., 2001;Mitchum et al., 2006;Hu et al., 2008). The LAFL transcription factors, LEC1, LEC2, FUS3, and AGL15, downregulate GA biosynthesis genes (Curaba et al., 2004;Zheng et al., 2009), while FUS3 positively regulates the ABA pathway (Gazzarrini et al., 2004). LEC1 and LEC2 promote the expression of auxin biosynthesis genes (Braybrook et al., 2006;Junker et al., 2012), and AGL15 negatively regulates the auxin response genes, ARF6, ARF8, and TRANSPORT INHIBITOR RESPONSE1 (TIR1) (Zheng et al., 2016). ...
In vitro plant regeneration involves dedifferentiation and molecular reprogramming of cells in order to regenerate whole organs. Plant regeneration can occur via two pathways, de novo organogenesis and somatic embryogenesis. Both pathways involve intricate molecular mechanisms and crosstalk between auxin and cytokinin signaling. Molecular determinants of both pathways have been studied in detail in model species, but little is known about the molecular mechanisms controlling de novo shoot organogenesis in lettuce. This review provides a synopsis of our current knowledge on molecular determinants of de novo organogenesis and somatic embryogenesis with an emphasis on the former as well as provides insights into applying this information for enhanced in vitro regeneration in non-model species such as lettuce (Lactuca sativa L.).
... The LAFL/ AGL15 protein is necessary for the promotion of SE by BBM because the overexpression of BBM in lec1, lec2, fus3 and agl15 mutants reduces or eliminates the ability of seedlings to form somatic embryos. The abi3 mutant exhibits the same maturation defects as other LAFL mutants (Parcy 1994;Gazzarrini et al. 2004;To 2006;Jia et al. 2014). ...
Birch (Betula platyphylla Suk.) is a deciduous tree with medicinal and ornamental value. During the process of genetic transformation, somatic embryos do not easily develop into transgenic plants, which is a limitation in genetic breeding. The Arabidopsis thaliana WUSCHEL (AtWUS) gene, which is a transcription factor, plays an important role in maintaining and regulating stem cell characteristics, which determines whether the stem cell population is differentiated. To explore methods for inducing somatic embryogenesis (SE) in birch, we overexpressed the AtWUS gene and transferred it to birch. The expression of AtWUS increased the SE rate from 101.4 to 717.1%. The expression of the AtWUS gene led to the downregulation of BpWUS gene expression in both calli and globular embryos as well as bud meristems. The expression of a few genes, i.e., BpLEC1 (LEAFY COTYLEDON 1), BpLEC2 (LEAFY COTYLEDON 2) and BpFUS3 (FUSCA 3), was upregulated during both embryogenesis and bud meristem development. However, BpABI3 (ABSCISIC ACID INSENSITIVE 3) gene expression was upregulated only in calli embryos, while BpSTM (SHOOT MERISTEMLESS) and BpCUC2 (CUP-SHAPED COTYLEDON 2) gene expression was upregulated only in bud meristems. This result indicated that overexpression of the AtWUS gene promoted SE by increasing the expression of SE-related genes. In conclusion, this study focused on the role of the AtWUS gene in birch SE and the molecular mechanism by which SE was promoted.
... Interestingly, it has been reported that FUS3 can bind to the MIR156A/C promoter 31 . However, being expressed at the triangular stage of embryo development 32 , it is unlikely that FUS3 contributes to the de novo activation of MIR156A/C after fertilization. Since it is documented that LEC2 exerts an important function in the resetting of the vernalization requirement in Arabidopsis 30 , we speculate that LEC2 may also be involved in MIR156A/C re-activation during embryogenesis. ...
Multicellular organisms undergo several developmental transitions during their life cycles. In contrast to animals, the plant germline is derived from adult somatic cells. As such, the juvenility of a plant must be reset in each generation. Previous stud�ies have emonstrated that the decline in the levels of miR156/7 with age drives plant maturation. Here we show that the resetting of plant juvenility during each generation is mediated by de novo activation of MIR156/7 in Arabidopsis. Blocking this process leads to a shortened juvenile phase and premature flowering in the offspring. In particular, an Arabidopsis plant devoid
of miR156/7 flowers even without formation of rosette leaves in long days. Mechanistically, we find that different MIR156/7 genes are reset at different developmental stages through distinct reprogramming routes. Among these genes, MIR156A, B and C are activated de novo during sexual reproduction and embryogenesis, while MIR157A and C are reset upon seed germination. This redundancy generates a robust reset mechanism that ensures accurate restoration of the juvenile phase in each plant generation.
... Interestingly, it has been reported that FUS3 can bind to the MIR156A/C promoter 31 . However, being expressed at the triangular stage of embryo development 32 , it is unlikely that FUS3 contributes to the de novo activation of MIR156A/C after fertilization. Since it is documented that LEC2 exerts an important function in the resetting of the vernalization requirement in Arabidopsis 30 , we speculate that LEC2 may also be involved in MIR156A/C re-activation during embryogenesis. ...
Multicellular organisms undergo several developmental transitions during their life cycles. In contrast to animals, the plant germline is derived from adult somatic cells. As such, the juvenility of a plant must be reset in each generation. Previous stud�ies have emonstrated that the decline in the levels of miR156/7 with age drives plant maturation. Here we show that the resetting of plant juvenility during each generation is mediated by de novo ctivation of MIR156/7 in Arabidopsis. Blocking this process leads to a shortened juvenile phase and premature flowering in the offspring. In particular, an Arabidopsis plant devoid
of miR156/7 flowers even without formation of rosette leaves in long days. Mechanistically, we find that different MIR156/7 genes are reset at different evelopmental stages through distinct reprogramming routes. Among these genes, MIR156A, B and C are activated de novo during sexual reproduction and embryogenesis, while MIR157A and C are reset upon seed germination. This redundancy generates a robust reset mechanism that ensures accurate restoration of the juvenile phase in each plant generation.
... ABI3 expression is activated in embryogenesis and further upregulated along embryo maturation, similar to embryonic FLC reactivation To explore the role of the three master B3 TFs (LEC2, FUS3, and ABI3) in FLC re-activation along seed development, we examined the expression of these genes from zygote (1 day after pollination [DAP]) through mature seed (16 DAP) from a vernalized winter-annual reference line FRI-Col, in which a functional FRI was introgressed into the rapid-cycling Col (Lee et al., 1994), and found that LEC2 expression peaked at heartstage embryo (6 DAP) and subsequently was repressed in embryo maturation (note that LEC2 is specifically expressed in embryos; Tao et al., 2019), whereas FUS3 expression peaked in mid-stage of embryo maturation (10 DAP) and repressed in late maturation ( Figure 1A), largely in line with previous findings (Gazzarrini et al., 2004;To et al., 2006). Interestingly, ABI3 expression was turned on in late embryogenesis (4 DAP), and further upregulated along the course of embryo maturation ( Figure 1, A and B and Supplemental Figure S1A). ...
Many over-wintering plants grown in temperate climate acquire competence to flower upon prolonged cold exposure in winter, through vernalization. In Arabidopsis thaliana, prolonged cold exposure induces the silencing of the potent floral repressor FLOWERING LOCUS C (FLC) through repressive chromatin modifications by Polycomb proteins. This repression is maintained to enable flowering after return to warmth, but is reset during seed development. Here, we show that embryonic FLC reactivation occurs in two phases: resetting of cold-induced FLC silencing during embryogenesis and further FLC activation during embryo maturation. We found that the B3 transcription factor ABSCISIC ACID-INSENSITIVE 3 (ABI3) mediates both FLC resetting in embryogenesis and further activation of FLC expression in embryo maturation. ABI3 binds to the cis-acting Cold Memory Element (CME) at FLC and recruits a scaffold protein with active chromatin modifiers to reset FLC chromatin into an active state in late embryogenesis. Moreover, in response to abscisic acid (ABA) accumulation during embryo maturation, ABI3, together with the bZIP transcription factor ABI5, binds to an ABA-responsive cis-element to further activate FLC expression to high level. Therefore, we have uncovered the molecular circuitries underlying embryonic FLC reactivation following parental vernalization, which ensures that each generation must experience winter cold prior to flowering.
... The differentiation of seed tissues at different stages also involves the ordered elimination of cells by the programmed cell death (PCD) process (Domínguez & Cejudo, 2014). Furthermore, cell fate is regulated by LEAFY COTYLEDON1 (LEC1), LEC2, and FUSCA3 (FUS3), which are principally expressed in the embryo and endosperm with a determinant function for both embryo development and for initiation and maintenance of the maturation phase (Lotan et al., 1998;Gazzarrini et al., 2004). ...
The life cycle of flowering plants is a dynamic process that involves successful passing through several developmental phases and tremendous progress has been made to reveal cellular and molecular regulatory mechanisms underlying these phases, morphogenesis, and growth. Although several key regulators of plant growth or developmental phase transitions have been identified in Arabidopsis, little is known about factors that become active during embryogenesis, seed development and also during further postembryonic growth. Much less is known about accession-specific factors that determine plant architecture and organ size. Bur-0 has been reported as a natural Arabidopsis thaliana accession with exceptionally big seeds and a large rosette; its phenotype makes it an interesting candidate to study growth and developmental aspects in plants, however, the molecular basis underlying this big phenotype remains to be elucidated. Thus, the general aim of this PhD project was to investigate and unravel the molecular mechanisms underlying the big phenotype in Bur-0. Several natural Arabidopsis accessions and late flowering mutant lines were analysed in this study, including Bur-0. Phenotypes were characterized by determining rosette size, seed size, flowering time, SAM size and growth in different photoperiods, during embryonic and postembryonic development. Our results demonstrate that Bur-0 stands out as an interesting accession with simultaneously larger rosettes, larger SAM, later flowering phenotype and larger seeds, but also larger embryos. Interestingly, inter-accession crosses (F1) resulted in bigger seeds than the parental self-crossed accessions, particularly when Bur-0 was used as the female parental genotype, suggesting parental effects on seed size that might be maternally controlled. Furthermore, developmental stage-based comparisons revealed that the large embryo size of Bur-0 is achieved during late embryogenesis and the large rosette size is achieved during late postembryonic growth. Interestingly, developmental phase progression analyses revealed that from germination onwards, the length of developmental phases during postembryonic growth is delayed in Bur-0, suggesting that in general, the mechanisms that regulate developmental phase progression are shared across developmental phases. On the other hand, a detailed physiological characterization in different tissues at different developmental stages revealed accession-specific physiological and metabolic traits that underlie accession-specific phenotypes and in particular, more carbon resources during embryonic and postembryonic development were found in Bur-0, suggesting an important role of carbohydrates in determination of the bigger Bur-0 phenotype. Additionally, differences in the cellular organization, nuclei DNA content, as well as ploidy level were analyzed in different tissues/cell types and we found that the large organ size in Bur-0 can be mainly attributed to its larger cells and also to higher cell proliferation in the SAM, but not to a different ploidy level. Furthermore, RNA-seq analysis of embryos at torpedo and mature stage, as well as SAMs at vegetative and floral transition stage from Bur-0 and Col-0 was conducted to identify accession-specific genetic determinants of plant phenotypes, shared across tissues and developmental stages during embryonic and postembryonic growth. Potential candidate genes were identified and further validation of transcriptome data by expression analyses of candidate genes as well as known key regulators of organ size and growth during embryonic and postembryonic development confirmed that the high confidence transcriptome datasets generated in this study are reliable for elucidation of molecular mechanisms regulating plant growth and accession-specific phenotypes in Arabidopsis. Taken together, this PhD project contributes to the plant development research field providing a detailed analysis of mechanisms underlying plant growth and development at different levels of biological organization, focusing on Arabidopsis accessions with remarkable phenotypical differences. For this, the natural accession Bur-0 was an ideal outlier candidate and different mechanisms at organ and tissue level, cell level, metabolism, transcript and gene expression level were identified, providing a better understanding of different factors involved in plant growth regulation and mechanisms underlying different growth patterns in nature.
... Among these genes, over expression of BBM1 [33], WUX [34], LEC2 [35], FUS3 [36], AIL5, AIL7 and PLT2 genes [37] could enhance SEis in A. thaliana. Although, it is well known that plant hormones, especially auxin, cytokinins and GAs, participate in the process of cell rejuvenation [32]. ...
Somatic embryogenesis is a preferred method for large-scale production of forest trees due to its high propagation efficiency. In this study, hybrid sweetgum leaves with phase changes from mature to embryogenic state were selected as experimental material to study somatic embryo initiation. Embryogenicity ranged from high to low, i.e. from 45%, 25%, and 12.5% to 0, with the samples of embryogenic callus (EC), whiten leaf edge (WLI), whiten leaf (WLII), and green leaf (GL) respectively. High correlations existed between embryogenicity and endogenous brassinosteroids (BRs) (r = 0.95, p < 0.05). Similarly, concentrations of endogenous BRs of the sample set correlated positively (r = 0.93, 0.99, 0.87, 0.99, 0.96 respectively, P < 0.05) to expression of somatic embryo (SE)-related genes, i.e. BBM, LEC2, ABI3, PLT2, and WOX2. Hierarchical cluster and weighted gene coexpression network analysis identified modules of coexpressed genes and network in 4820 differentially expressed genes (DEGs) from All-BR-Regulated Genes (ABRG). Moreover, exogenously-supplemented epiBR, together with 2,4-D and 6-BA, increased embryogenicity of GL-sourced callus, and expression of SE- and auxin-related genes, while brassinazole (BRZ), a BR biosynthesis inhibitor, reduced embryogenicity. Evidences obtained in this study revealed that BRs involved in phase change of leaf explants and may function in regulating gene expression and enhancing auxin effects. This study successfully established protocols for inducing somatic embryogenesis from leaf explants in hybrid sweetgum, which could facilitate the propagation process greatly, and provide theoretical basis for manipulating SE competence of explants in ornamental woody plants.
... Consistent with this, loss-of-function SnRK1 garden pea plants have decreased ABA levels and show ABI3 repression 71,72 . SnRK1 phosphorylates and positively regulates FUS3, which promotes ABA biosynthesis and is in turn regulated by ABA in Arabidopsis [73][74][75] (Figure 4). SnRK1 overexpression delays Arabidopsis seed germination and genetic analysis indicates that FUS3 acts downstream of SnRK1 73 . ...
Amongst the myriad of metabolites produced by plants, primary metabolites and hormones play crucial housekeeping roles in the cell and are essential for proper plant growth and development. While the biosynthetic pathways of primary metabolism are well characterized, those of hormones are yet to be completely defined. Central metabolism provides precursors for hormone biosynthesis and the regulation and function of primary metabolites and hormones are tightly entwined. The combination of reverse genetics and technological advances in our ability to evaluate the levels of the molecular entities of the cell (transcripts, proteins and metabolites) has led to considerable improvements in our understanding of both the regulatory interaction between primary metabolites and hormones and its coordination in response to different conditions. Here, we provide an overview of the interaction of primary and hormone metabolism at the metabolic and signaling levels, as well as a perspective regarding the tools that can be used to tackle our current knowledge gaps at the signaling level.
FUSCA 3 (FUS3), a seed master regulator, plays critical roles in seed dormancy and oil accumulation. However, its downstream regulation mechanisms remain poorly understood. Here, we explored the roles of AINTEGUMENTA-like 6 (AIL6), a seed transcription factor, in these processes. The activation of AIL6 by FUS3 was demonstrated by dual-LUC assay. Seeds of ail6 mutants showed alterations of fatty acid compositions, and both AtAIL6 (AIL6 from Arabidopsis thaliana) and BnaAIL6 (AIL6 from Brassica napus) rescued the phenotype. Over-expression (OE) of AIL6s reversed changes in seed fatty acid composition. Notably, OE lines showed low seed germination rates down to 12% compared to 100% of wild-type Col-0. Transcriptome analysis of the mutant and an OE line indicated widespread expression changes of genes involved in lipid metabolism and phytohormone pathways. In OE mature seeds, GA4 content decreased more than 15-fold, while ABA and IAA contents clearly increased. Exogenous GA3 treatments did not effectively rescue the low germination rate. Nicking seed coats increased germination rates from 25% to nearly 80% while the wild-type rdr6-11 is 100% and 98% respectively, and elongation of storage time also improved seed germination. Furthermore, dormancy imposed by AIL6 was fully released in the della quintuple mutant. Together, our results indicate AIL6 acts as a manager downstream of FUS3 in seed dormancy and lipid metabolism.
In rice (Oryza sativa) tissue culture, callus can be induced from the scutellum in embryo or from the vasculature of non-embryonic organs such as leaves, nodes, or roots. Here we show that the auxin signaling pathway triggers cell division in the epidermis of the scutellum to form an embryo-like structure, which leads to callus formation. Our transcriptome data show that embryo-, stem cell-, and auxin-related genes are upregulated during scutellum-derived callus initiation. Among those genes, the embryo-specific gene OsLEC1 is activated by auxin and involved in scutellum-derived callus initiation. However, OsLEC1 is not required for vasculature-derived callus initiation from roots. In addition, OsIAA11 and OsCRL1, which are involved in root development, are required for vasculature-derived callus formation but not for scutellum-derived callus formation. Overall, our data indicate that scutellum-derived callus initiation is regulated by an embryo-like development program, and this is different from vasculature-derived callus initiation which borrows a root development program.
Plant leaves feature epidermal stomata that are organized in stereotyped patterns. How does the pattern originate? We provide transcriptomic, imaging, and genetic evidence that Arabidopsis embryos engage known stomatal fate and patterning factors to create regularly spaced stomatal precursor cells. Analysis of embryos from 36 plant species indicates that this trait is widespread among angiosperms. Embryonic stomatal patterning in Arabidopsis is established in three stages: first, broad SPEECHLESS (SPCH) expression; second, coalescence of SPCH and its targets into discrete domains; and third, one round of asymmetric division to create stomatal precursors. Lineage progression is then halted until after germination. We show that the embryonic stomatal pattern enables fast stomatal differentiation and photosynthetic activity upon germination, but it also guides the formation of additional stomata as the leaf expands. In addition, key stomatal regulators are prevented from driving the fate transitions they can induce after germination, identifying stage-specific layers of regulation that control lineage progression during embryogenesis.
Lipid accumulation is regulated during seed development. However, the molecular link between the development of seed components and lipid accumulation remains largely unknown, especially in tung tree seeds. Tung tree (Vernicia fordii Hemsl.) is an oil-bearing woody plant that produces seeds rich in α-eleostearic acid. In this study, it was found that the tissue outside the cotyledon in the kernel of tung tree can be further divided into the inner and outer parts. The inner part was gradually enlarged to form the mature endosperm with a large lipid volume. The outer part turned into a dry, thin layer with little oil accumulation. Transcriptome data revealed that the inner part enlargement could be due to the rapid cell proliferation and the activation of the lipid biosynthesis pathway during the oil accumulation period. The outer part should function as the delivery of maternal nutrition to support the inner part expansion during seed development. Furthermore, a set of candidate transcription factors were identified that regulated different fates of the inner and outer parts during the oil accumulation period. The transcript repressor, VfVAL3 was highly expressed in the outer part but low level in the inner part. Conversely, VfVAL3 potential target genes in the LEC1/B3 network had higher expressions in the inner part. The Sph/RY motif recognized by the VAL protein was highly enriched in the promoters or introns of the LEC1/B3 network genes. However, the yeast one-hybrid assay showed that VAL3 didn’t bind to the promoter or intron sequences of the LEC1/B3 network genes. Overexpression of VfVAL3 reduced oil content in transgenic Arabidopsis. Therefore, VfVAL3 may block the development and lipid accumulation of the outer part by suppressing the LEC1/B3 network. This study provides essential information on understanding the molecular connection of different seed parts and lipid accumulation during seed development in tung tree.
Bud dormancy is an important trait in geophytes that largely affects their flowering process and vegetative growth after dormancy release. Compared with seed dormancy, the regulation of bud dormancy is still largely unclear. Abscisic acid (ABA) acts as the predominant hormone that regulates the whole dormancy process. In Gladiolus (Gladiolus hybridus), cold storage promotes corm dormancy release (CDR) by repressing ABA biosynthesis and signaling. However, the mechanisms governing ABA-related processes during CDR via epigenetics are poorly understood. Here, we show that class I BASIC PENTACYSTEINE2, (GhBPC2) directly binds to 9-CIS-EPOXYCAROTENOID DIOXYGENASE (GhNCED) and ABA INSENSITIVE5 (GhABI5) loci and down-regulates their expression to accelerate CDR. During CDR, histone modifications change dramatically at the GhBPC2-binding loci of GhABI5 with an increase in H3K27me3 and a decrease in H3K4me3. GhBPC2 is involved in both H3K27me3 and H3K4me3 and fine-tunes GhABI5 expression by recruiting Polycomb Repressive Complex 2 (PRC2) and the chromatin remodeling factor EARLY BOLTING IN SHORT DAYS (GhEBS). These results show GhBPC2 epigenetically regulates corm dormancy release in Gladiolus by mediating GhABI5 expression with PRC2 and GhEBS.
Introduction:
Melatonin is a multipotent molecule that exists widely in animals and plants and plays an active regulatory role in abiotic stresses. The B3 superfamily is a ubiquitous transcription factor with a B3 functional domain in plants, which can respond temporally to abiotic stresses by activating defense compounds and plant hormones. Despite the fact that the B3 genes have been studied in a variety of plants, their role in soybean is still unknown.
Methods:
The regulation of melatonin on cold resistance of soybean and the response of B3 genes to cold stress were investigated by measuring biochemical indexes of soybean. Meanwhile, the genome-wide identification of B3 gene family was conducted in soybean, and B3 genes were analyzed based on phylogeny, motifs, gene structure, collinearity, and cis-regulatory elements analysis.
Results:
We found that cold stress-induced oxidative stress in soybean by producing excessive reactive oxygen species. However, exogenous melatonin treatment could increase the content of endogenous melatonin and other hormones, including IAA and ABA, and enhance the antioxidative system, such as POD activity, CAT activity, and GSH/GSSG, to scavenge ROS. Furthermore, the present study first revealed that melatonin could alleviate the response of soybean to cold stress by inducing the expression of B3 genes. In addition, we first identified 145 B3 genes in soybean that were unevenly distributed on 20 chromosomes. The B3 gene family was divided into 4 subgroups based on the phylogeny tree constructed with protein sequence and a variety of plant hormones and stress response cis-elements were discovered in the promoter region of the B3 genes, indicating that the B3 genes were involved in several aspects of the soybean stress response. Transcriptome analysis and results of qRT-PCR revealed that most GmB3 genes could be induced by cold, the expression of which was also regulated by melatonin. We also found that B3 genes responded to cold stress in plants by interacting with other transcription factors.
Discussion:
We found that melatonin regulates the response of soybean to cold stress by regulating the expression of the transcription factor B3 gene, and we identified 145 B3 genes in soybean. These findings further elucidate the potential role of the B3 gene family in soybean to resist low-temperature stress and provide valuable information for soybean functional genomics study.
Seeds are the major source of nutrient compounds for human and animal livestock worldwide. With the improved living standard, high nutritional quality has become one of the main breeding targets. Storage protein content in seeds acts as a pivotal criterion of nutritional quality, which is highly variable depending on species considered. In the last few decades, our understanding has been greatly advanced regarding the molecular genetics and regulatory mechanism of storage protein synthesis. Here, we systematically and comprehensively summarize the breakthroughs for the conservation and divergence of storage protein synthesis in dicots and monocots. We discuss storage protein accumulation about evolutionary origins, developmental process, characters of main storage protein fractions, regulatory network and genetic modification. In addition, we explore the potential breeding strategies to improve storage protein and some unanswered key problems to be addressed.
SNF1-related protein kinases (SnRKs) get their name from their fungal counterpart SFN1, which is a regulator of carbon metabolism. SnRK performs critical roles in biotic and abiotic stress responses by activating protein phosphorylation pathways. 1 SnRK has been identified as the critical switch in plant sugar signaling. SnRKs in plants may be classified into three subfamilies based on sequence similarity and gene structure: SnRK1, SnRK2, and SnRK3. Because of its important involvement at the interface of metabolic and stress signaling, SnRKs are promising candidates for modification to improve crop performance in adverse environments. The importance of the SnRK in abiotic stress tolerance was highlighted in this chapter, and its applications are likely to benefit crop breeding.
The B3 superfamily comprises a class of transcription factors containing B3 functional domains that play crucial roles in plant growth and development. However, the identification and analysis of the B3 superfamily have not been reported in pecans. In this study, 75 B3 superfamily members were identified and divided into four families, namely ARF (31), RAV (10), LAV (8), and REM (26). The gene structure, conserved motifs and domain analysis indicated that each family was highly similar. The Ka/Ks results indicated that the CiB3s underwent purifying selection. Several growth- and development-related cis-elements, such as Box 4, GATA-motif, and G-Box, were found in the promoter of the B3 transcription factor. The results obtained from the transcriptomic analysis of different kernel development stages and different tissues of pecan and qRT-PCR, indicated that several transcription factors, particularly CiABI3, CiFUS3, and CiLEC2, were involved in regulating kernel development. In particular, the expression of CiFUS3 changed significantly between S3 and S5. Additionally, CiFUS3 is localized in the nucleus and has no self-transcriptional activity. A yeast one-hybrid experiment showed that CiFUS3 could bind to the RY-motif related to seed development. Overall, our study provides a reference for the role of CiFUS3 in oil synthesis and the functional study of B3 genes in pecan.
The plant-specific transcription factor B3 superfamily members mediate plant growth, development, and stress response. Although the B3 gene has been studied in various plants, its role in the common bean (Phaseolus vulgaris L) is unknown. In this study, 110 PvB3 genes were identified from the common bean genome and analyzed based on the phylogeny, gene structure, motifs, cis-regulatory elements, collinearity, and Ka/Ks. PvB3s were divided into four subgroups based on motifs and gene structure. The cis-element composition of the most PvB3 were involved in hormone production, abiotic stress response, and germination. Most PvB3 were highly expressed in the vigorously growth tissue of common bean, such as nodules, young pods, and young trifoliate. Moreover, RNA-seq results showed that PvB3-001/010/027/032/047/048/059/090 were enriched in tryptophan metabolism pathway (involves in AUX/IAA, GH3, and SAUR) of common bean under salt stress. Notably, exogenous IAA treatment could increase endogenous IAA content and the number of lateral roots, while altering the expression level of PvB3s in salt-sensitive common bean variety under salt stress. Taken together, the result indicated that PvB3 transcription factors may be involved in common bean salt stress response by participating in the synthesis and metabolic regulation of IAA. This study provides a theoretical basis for further study on the role of PvB3s in common bean abiotic stresses response.
Flax (Linum usitatissimum L.) as a self-pollinated annual diploid crop is grown worldwide primarily for its seed storage reserves of oil and storage proteins. The B3 domain transcription factor AtFUSCA3 (AtFUS3) has been identified as a master regulator of seed storage reserve accumulation in Arabidopsis thaliana. However, the function of LuFUS3 from L. usitatissimum has not yet been assessed. Here, we found that there were two LuFUS3 homologs, LuFUS3-1 and LuFUS3-2, in the L. usitatissimum genome. The subcellular localization and yeast transcriptional activation assays indicated that LuFUS3-1 functions as a transcription factor. Heterogeneous expression of LuFUS3-1 in the A. thaliana wild type background changed the plant architecture, including moderately dwarf stature, wrinkled leaves, increased branch number, altered floral morphology, and shorter and wider siliques, which was at least partially caused by the significantly decreased level of endogenous gibberellins. On the other hand, we demonstrated that LuFUS3-1 enhances the accumulation of seed storage reserves inclusive of oil and storage proteins in A. thaliana seeds. Consistently, LuFUS3-1 promotes the expression of many genes involved in the biosynthesis of seed oil and storage proteins during seed development. These findings provide new insights into the FUS3 function in plant architecture and seed storage reserve accumulation, and also represent a promising target for genetic modification of L. usitatissimum.
Grass inflorescence development is diverse and complex and involves sophisticated but poorly understood interactions of genes regulating branch determinacy and leaf growth. Here, we use a combination of transcript profiling and genetic and phylogenetic analyses to investigate tasselsheath1 (tsh1) and tsh4, two maize genes that simultaneously suppress inflorescence leaf growth and promote branching. We identify a regulatory network of inflorescence leaf suppression that involves the phase change gene tsh4 upstream of tsh1 and the ligule identity gene liguleless2 (lg2). We also find that a series of duplications in the tsh1 gene lineage facilitated its shift from boundary domain in nongrasses to suppressed inflorescence leaves of grasses. Collectively, these results suggest that the boundary domain genes tsh1 and lg2 were recruited to inflorescence leaves where they suppress growth and regulate a nonautonomous signaling center that promotes inflorescence branching, an important component of yield in cereal grasses.
The gibberellin class of plant hormones has been implicated in the control of flowering in several species. In Arabidop-sis, severe reduction of endogenous gibberellins delays flowering in long days and prevents flowering in short days. We have investigated how the differential effects of gibberellins on flowering correlate with expression of LEAFY , a floral meristem identity gene. We have found that the failure of gibberellin-deficient ga1-3 mutants to flower in short days was paralleled by the absence of LEAFY promoter induction. A causal connection between these two events was confirmed by the ability of a constitutively expressed LEAFY transgene to restore flowering to ga1-3 mutants in short days. In contrast to short days, impairment of gibberellin biosynthesis caused merely a reduction of LEAFY expression when plants were grown in long days or with sucrose in the dark. As a first step toward identifying other small molecules that might regulate flowering, we have developed a rapid in vitro assay for LEAFY promoter activity.
Postembryonic shoot development in maize (Zea mays L.) is divided into a juvenile vegetative phase, an adult vegetative phase, and a reproductive phase that differ in the expression of many morphological traits. A reduction in the endogenous levels of bioactive gibberellins (GAs) conditioned by any one of the dwarf1, dwarf3, dwarf5, or another ear1 mutations in maize delays the transition from juvenile vegetative to adult vegetative development and from adult vegetative to reproductive development. Mutant plants cease producing juvenile traits (e.g. epicuticular wax) and begin producing adult traits (e.g. epidermal hairs) later than wild-type plants. They also cease producing leaves and begin producing reproductive structures later than wild-type plants. These mutations greatly enhance most aspects of the phenotype of Teopod1 and Teopod2, suggesting that GAs suppress part but not all of the Teopod phenotype. Application of GA3 to Teopod2 mutants and Teopod1, dwarf3 double mutants confirms this result. We conclude that GAs act in conjunction with several other factors to promote both vegetative and reproductive maturation but affect different developmental phases unequally. Furthermore, the GAs that regulate vegetative and reproductive maturation, like those responsible for stem elongation, are downstream of GA20 in the GA biosynthetic pathway.
Because plant cells do not move and are surrounded by a rigid cell wall, cell division rates and patterns are believed to be directly responsible for generating new structures throughout development. To study the relationship between cell division and morphogenesis, transgenic tobacco and Arabidopsis plants were constructed expressing dominant mutations in a key regulator of the Arabidopsis cell cycle, the Cdc2a kinase. Plants constitutively overproducing the wild-type Cdc2a or the mutant form predicted to accelerate the cell cycle did not exhibit a significantly altered development. In contrast, a mutation expected to arrest the cell cycle abolished cell division when expressed in Arabidopsis, whereas some tobacco plants constitutively producing this mutant protein were recovered. These plants had a reduced histone H1 kinase activity and contained considerably fewer cells. These cells were, however, much larger and underwent normal differentiation. Morphogenesis, histogenesis and developmental timing were unaffected. The results indicate that, in plants, the developmental controls defining shape can act independently from cell division rates.
The accumulation kinetics of 18 mRNAs were characterized during Arabidopsis silique development. These marker mRNAs could be grouped in distinct classes according to their coordinate temporal expression in the wild type and provided a basis for further characterization of the corresponding regulatory pathways. The abscisic acid (ABA)-insensitive abi3-4 mutation modified the expression pattern of several but not all members of each of these wild-type temporal mRNA classes. This indicates that the ABI3 protein directly participates in the regulation of several developmental programs and that multiple regulatory pathways can lead to the simultaneous expression of distinct mRNA markers. The ABI3 gene is specifically expressed in seed, but ectopic expression of ABI3 conferred the ability to accumulate several seed-specific mRNA markers in response to ABA in transgenic plantlets. This suggested that expression of these marker mRNAs might be controlled by an ABI3-dependent and ABA-dependent pathway(s) in seed. However, characterization of the ABA-biosynthetic aba mutant revealed that the accumulation of these mRNAs is not correlated to the ABA content of seed. A possible means of regulating gene expression by developmental variations in ABA sensitivity is apparently not attributable to variations in ABI3 cellular abundance. The total content of ABI3 protein per seed markedly increased at certain developmental stages, but this augmentation appears to result primarily from the simultaneous multiplication of embryonic cells. Our current findings are discussed in relation to their general implications for the mechanisms controlling gene expression programs in seed.
Homeobox genes are master regulatory genes that specify the body plan and control development of many eukaryotic organisms, including plants. We isolated and characterized a cDNA designated ATML1 (for Arabidopsis thaliana meristem L1 layer) that encodes a novel homeodomain protein. The ATML1 protein shares high sequence homology inside and outside of the homeodomain with both the Phalaenopsis O39 and the Arabidopsis GLABRA2 (GL2) homeodomain proteins, which together define a new class of plant homeodomain-containing proteins, designated HD-GL2. The ATML1 gene was first expressed in the apical cell after the first asymmetric division of the zygote and continued to be expressed in all proembryo cells until the eight-cell stage. In the 16-cell proembryo, the ATML1 gene showed a distinct pattern of expression, with its mRNA becoming restricted to the protoderm. In the torpedo stage of embryo development, ATML1 mRNA disappeared altogether but reappeared later only in the L1 layer of the shoot apical meristem in the mature embryo. After germination, this L1 layer-specific pattern of expression was maintained in the vegetative shoot apical meristem, inflorescence, and floral meristems, as well as in the young floral organ primordia. Finally, ATML1 mRNA accumulated in the protoderm of the ovule primordia and integuments and gradually became restricted in its expression to the endothelium surrounding the embryo sac. We propose that ATML1 may be involved in setting up morphogenetic boundaries of positional information necessary for controlling cell specification and pattern formation. In addition, ATML1 provides an early molecular marker for the establishment of both apical-basal and radial patterns during plant embryogenesis.
Seed dormancy and germination in higher plants are partially controlled by the plant hormones abscisic acid (ABA) and gibberellic acid (GA). ABA establishes dormancy during embryo maturation, whereas GA breaks dormancy and induces germination. Previous attempts to identify GA response genes were confounded because GA mutants are not expected to germinate and, unlike GA auxotrophs, should fail to be rescued by exogenous GA. Here, we describe a screen for suppressors of the ABA-insensitive mutant ABI1-1 that enriches for GA auxotrophs and GA-insensitive mutants. The vast majority (76%) of the suppressors of ABI1-1 strongly resemble GA auxotrophs in that they are severely dwarfed and have dark green foliage and flowers with underdeveloped petals and stamen. Three isolates were alleles of the GA auxotroph ga1. The remaining severe dwarves were not rescued by GA and belong to a single complementation group that we designate sly1 (Sleepy 1). The alleles of sly1 identified are the first recessive GA-insensitive mutations to reflect the full spectrum of GA-associated phenotypes, including the failure to germinate in the absence of the ABI1-1 lesion. Thus, we postulate that SLY1 is a key factor in GA reception.
The Arabidopsis LEAFY COTYLEDON1 (LEC1) gene is required for the specification of cotyledon identity and the completion of embryo maturation. We isolated the LEC1 gene and showed that it functions at an early developmental stage to maintain embryonic cell fate. The LEC1 gene encodes a transcription factor homolog, the CCAAT box-binding factor HAP3 subunit. LEC1 RNA accumulates only during seed development in embryo cell types and in endosperm tissue. Ectopic postembryonic expression of the LEC1 gene in vegetative cells induces the expression of embryo-specific genes and initiates formation of embryo-like structures. Our results suggest that LEC1 is an important regulator of embryo development that activates the transcription of genes required for both embryo morphogenesis and cellular differentiation.
Despite extensive studies on the roles of phytochrome in photostimulated seed germination, the mechanisms downstream of the photoreceptor that promote germination are largely unknown. Previous studies have indicated that light-induced germination of Arabidopsis seeds is mediated by the hormone gibberellin (GA). Using RNA gel blot analyses, we studied the regulation of two Arabidopsis genes, GA4 and GA4H (for GA4 homolog), both of which encode GA 3beta-hydroxylases that catalyze the final biosynthetic step to produce bioactive GAs. The newly isolated GA4H gene was expressed predominantly during seed germination. We show that expression of both GA4 and GA4H genes in imbibed seeds was induced within 1 hr after a brief red (R) light treatment. In the phytochrome B-deficient phyB-1 mutant, GA4H expression was not induced by R light, but GA4 expression still was, indicating that R light-induced GA4 and GA4H expression is mediated by different phytochromes. In contrast to the GA4 gene, the GA4H gene was not regulated by the feedback inhibition mechanism in germinating seeds. Our data demonstrate that expression of GA 3beta-hydroxylase genes is elevated by R light, which may result in an increase in biosynthesis of active GAs to promote seed germination. Furthermore, our results suggest that each GA 3beta-hydroxylase gene plays a unique physiological role during light-induced seed germination.
The continuous growth of the plant embryo is interrupted during the seed maturation processes which results in a dormant seed. The embryo continues development after germination when it grows into a seedling. The embryo growth phase starts after morphogenesis and ends when the embryo fills the seed sac. Very little is known about the processes regulating this phase. We describe mutants that affect embryo growth in two sequential developmental stages. Firstly, embryo growth arrest is regulated by the FUS3/LEC type genes, as mutations in these genes cause a continuation of growth in immature embryos. Secondly, a later stage of embryo dormancy is regulated by ABI3 and abscisic acid; abi3 and aba1 mutants exhibit premature germination only after embryos mature. Mutations affecting both developmental stages result in an additive phenotype and double mutants are highly viviparous. Embryo growth arrest is regulated by cell division activities in both the embryo and the endosperm, which are gradually switched off at the mature embryo stage. In the fus3/lec mutants, however, cell division in both the embryo and endosperm is not arrested, but rather is prolonged throughout seed maturation. Furthermore ectopic cell division occurs in seedlings. Our results indicate that seed dormancy is secured via at least two sequential developmental processes: embryo growth arrest, which is regulated by cell division and embryo dormancy.
The Arabidopsis LEAFY COTYLEDON2 (LEC2) gene is a central embryonic regulator that serves critical roles both early and late during embryo development. LEC2 is required for the maintenance of suspensor morphology, specification of cotyledon identity, progression through the maturation phase, and suppression of premature germination. We cloned the LEC2 gene on the basis of its chromosomal position and showed that the predicted polypeptide contains a B3 domain, a DNA-binding motif unique to plants that is characteristic of several transcription factors. We showed that LEC2 RNA accumulates primarily during seed development, consistent with our finding that LEC2 shares greatest similarity with the B3 domain transcription factors that act primarily in developing seeds, VIVIPAROUS1/ABA INSENSITIVE3 and FUSCA3. Ectopic, postembryonic expression of LEC2 in transgenic plants induces the formation of somatic embryos and other organ-like structures and often confers embryonic characteristics to seedlings. Together, these results suggest that LEC2 is a transcriptional regulator that establishes a cellular environment sufficient to initiate embryo development.
In Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type producing extracellular pectinaceous mucilage and a volcano-shaped secondary cell wall. Differentiation involves a regulated series of cytological events including growth, cytoplasmic rearrangement, mucilage synthesis, and secondary cell wall production. We have tested the potential of Arabidopsis seed coat epidermal cells as a model system for the genetic analysis of these processes. A screen for mutants defective in seed mucilage identified five novel genes (MUCILAGE-MODIFIED [MUM]1-5). The seed coat development of these mutants, and that of three previously identified ones (TRANSPARENT TESTA GLABRA1, GLABRA2, and APETALA2) were characterized. Our results show that the genes identified define several events in seed coat differentiation. Although APETALA2 is needed for differentiation of both outer layers of the seed coat, TRANSPARENT TESTA GLABRA1, GLABRA2, and MUM4 are required for complete mucilage synthesis and cytoplasmic rearrangement. MUM3 and MUM5 may be involved in the regulation of mucilage composition, whereas MUM1 and MUM2 appear to play novel roles in post-synthesis cell wall modifications necessary for mucilage extrusion.
Auxin-regulated gene expression is mediated by two families of transcription factors. The ARF proteins bind to a conserved DNA sequence called the AuxRE and activate transcription. The Aux/IAA proteins repress ARF function, presumably by forming dimers with ARF proteins. Recent genetic studies in Arabidopsis indicate that auxin regulates this system by promoting the ubiquitin-mediated degradation of the Aux/IAA proteins, thus permitting ARF function. Mutations in components of SCF(TIR1), a ubiquitin protein ligase (E3) result in stabilization of Aux/IAA proteins and decreased auxin response. Further, recent biochemical experiments indicate that the Aux/IAA proteins bind SCF(TIR1) in an auxin-dependent manner.
Molecular studies of late embryogenesis and seed development have emphasized differential gene expression as a means of identifying discrete stages of embryogenesis. Little has been done to identify factors that regulate the length of a given developmental stage or the degree of overlap between adjacent developmental programs. We designed a genetic screen to identify mutations that disrupt late embryo development in Arabidopsis without loss of hormonal responses. One such mutation, fusca3 (fus3), alters late embryo functions, such as the establishment of dormancy and desiccation tolerance, and reduces storage protein levels. fus3 cotyledons bear trichomes, and their ultrastructure is similar to that of leaf primordia. Immature fus3 embryos enter germinative development, and the shoot apical meristems develop leaf primordia before seed desiccation begins. The cotyledons resemble leaf primordia, yet retain some cotyledon characteristics; thus, cotyledon- and leaf-specific functions are expressed simultaneously. Together, these observations are consistent with a heterochronic interpretation of the fus3 mutation.
Tissue histogenesis during plant development depends on regulation of cell division plane, timing and frequency to produce cell units of correct size and shape for mature function. Differences among the dermal, ground and vascular tissue systems arise during development, largely through regulation of these aspects of cell cycling in relation to overall tissue expansion. Using a cyclin1At::GUS reporter construct, we demonstrate quantitative differences in cell cycling frequency among tissue systems and among primary, secondary, and tertiary veins; these differences are superimposed upon the more general longitudinal gradient of cell division frequency in developing leaves of Arabidopsis thaliana (L.) Heynh. Patterns of cell cycling frequency coincide almost exactly with those of the earliest known molecular marker of procambial identity, the HD-ZIP III homeobox gene ATHB-8, suggesting that ATHB-8 may play a role in regulating the early events of procambial development, including procambium-specific patterns of cell cycling. Cellular localization of cyc1At::GUS and ATHB-8::GUS within developing vascular strands indicates, however, that ATHB-8 has additional functions related to dorsiventral patterning within veins and cell differentiation events.
Axis formation occurs in plants, as in animals, during early embryogenesis. However, the underlying mechanism is not known. Here we show that the first manifestation of the apical-basal axis in plants, the asymmetric division of the zygote, produces a basal cell that transports and an apical cell that responds to the signalling molecule auxin. This apical-basal auxin activity gradient triggers the specification of apical embryo structures and is actively maintained by a novel component of auxin efflux, PIN7, which is located apically in the basal cell. Later, the developmentally regulated reversal of PIN7 and onset of PIN1 polar localization reorganize the auxin gradient for specification of the basal root pole. An analysis of pin quadruple mutants identifies PIN-dependent transport as an essential part of the mechanism for embryo axis formation. Our results indicate how the establishment of cell polarity, polar auxin efflux and local auxin response result in apical-basal axis formation of the embryo, and thus determine the axiality of the adult plant.
Despite extensive studies on the roles of phytochrome in photostimulated seed germination, the mechanisms downstream of the photoreceptor that promote germination are largely unknown. Previous studies have indicated that light-induced germination of Arabidopsis seeds is mediated by the hormone gibberellin (GA). Using RNA gel blot analyses, we studied the regulation of two Arabidopsis genes, GA4 and GA4H (for GA4 homolog), both of which encode GA 3beta-hydroxylases that catalyze the final biosynthetic step to produce bioactive GAs. The newly isolated GA4H gene was expressed predominantly during seed germination. We show that expression of both GA4 and GA4H genes in imbibed seeds was induced within 1 hr after a brief red (R) light treatment. In the phytochrome B-deficient phyB-1 mutant, GA4H expression was not induced by R light, but GA4 expression still was, indicating that R light-induced GA4 and GA4H expression is mediated by different phytochromes. In contrast to the GA4 gene, the GA4H gene was not regulated by the feedback inhibition mechanism in germinating seeds. Our data demonstrate that expression of GA 3beta-hydroxylase genes is elevated by R light, which may result in an increase in biosynthesis of active GAs to promote seed germination. Furthermore, our results suggest that each GA 3beta-hydroxylase gene plays a unique physiological role during light-induced seed germination.
The hormone-mediated control of plant growth and development involves both synthesis and response. Previous studies have shown that gibberellin (GA) plays an essential role in Arabidopsis seed germination. To learn how GA stimulates seed germination, we performed comprehensive analyses of GA biosynthesis and response using gas chromatography-mass spectrometry and oligonucleotide-based DNA microarray analysis. In addition, spatial correlations between GA biosynthesis and response were assessed by in situ hybridization. We identified a number of transcripts, the abundance of which is modulated upon exposure to exogenous GA. A subset of these GA-regulated genes was expressed in accordance with an increase in endogenous active GA levels, which occurs just before radicle emergence. The GA-responsive genes identified include those responsible for synthesis, transport, and signaling of other hormones, suggesting the presence of uncharacterized crosstalk between GA and other hormones. In situ hybridization analysis demonstrated that the expression of GA-responsive genes is not restricted to the predicted site of GA biosynthesis, suggesting that GA itself, or GA signals, is transmitted across different cell types during Arabidopsis seed germination.
A position-dependent pattern of epidermal cell types is produced during root development inArabidopsis thaliana. This pattern is reflected in the expression pattern of GLABRA2 (GL2), a homeobox gene that regulates cell differentiation in the root epidermis. GL2 promoter::GUSfusions were used to show that the TTG gene, a regulator of root epidermis development, is necessary for maximalGL2 activity but is not required for the pattern ofGL2 expression. Furthermore, GL2-promoter activity is influenced by expression of the myc-like maize R gene (35S::R) in Arabidopsis but is not affected by gl2 mutations. A position-dependent pattern of cell differentiation andGL2-promoter activity was also discovered in the hypocotyl epidermis that was analogous to the pattern in the root. Non-GL2-expressing cell files in the hypocotyl epidermis located outside anticlinal cortical cell walls exhibit reduced cell length and form stomata. Like the root, the hypocotylGL2 activity was shown to be influenced byttg and 35S::R but not bygl2. The parallel pattern of cell differentiation in the root and hypocotyl indicates that TTG andGL2 participate in a common position-dependent mechanism to control cell-type patterning throughout the apical-basal axis of the Arabidopsis seedling.
Glc has hormone-like functions and controls many vital processes through mostly unknown mechanisms in plants. We report here on the molecular cloning of GLUCOSE INSENSITIVE1 (GIN1) and ABSCISIC ACID DEFICIENT2 (ABA2) which encodes a unique Arabidopsis short-chain dehydrogenase/reductase (SDR1) that functions as a molecular link between nutrient signaling and plant hormone biosynthesis. SDR1 is related to SDR superfamily members involved in ret-inoid and steroid hormone biosynthesis in mammals and sex determination in maize. Glc antagonizes ethylene signal-ing by activating ABA2/GIN1 and other abscisic acid (ABA) biosynthesis and signaling genes, which requires Glc and ABA synergistically. Analyses of aba2/gin1 null mutants define dual functions of endogenous ABA in inhibiting the post-germination developmental switch modulated by distinct Glc and osmotic signals and in promoting organ and body size and fertility in the absence of severe stress. SDR1 is sufficient for the multistep conversion of plastid-and carote-noid-derived xanthoxin to abscisic aldehyde in the cytosol. The surprisingly restricted spatial and temporal expression of SDR1 suggests the dynamic mobilization of ABA precursors and/or ABA.
In Arabidopsis thaliana 37 independent irradiation or EMS induced mutants were isolated which have an absolute or almost absolute gibberellin (GA) requirement for germination and successive elongation growth. These are called ‘non-germinating GA-dwarfs’, since without further addition of GA they develop into typical GA-dwarfs, being dark green, stunted and sterile. However, with repeated GA-treatment they develop into fertile plants with a completely wild type phenotype, or nearly so. In addition, 19 independently induced ‘germinating GA-dwarfs’ were obtained, i.e. mutants which do germinate without GA but develop into typical GA-dwarfs. With repeated GA-treatment these too grow to become completely wild type phenotypes, or nearly so. ‘Germinating dwarfs’ have been found by previous authors in a number of other plant species. The ‘non-germinating dwarfs’ form a new class of mutants. The system of non-germinating mutants offers a resolving power unique in higher plants, so that self-detecting rare events like induced revertants or intragenic recombinants can be efficiently screened for.
The 56 GA-sensitive mutants represent mutations at 5 loci, located on three of five Arabidopsis chromosomes. At three of the five loci both mutant classes were represented in similar frequency ratio's, whilst at the other two loci only germinating dwarfs were found.
[1,2-13C2]ABA, which is designed as a stable and pure internal standard for GC/MS analysis, is synthesized through the Witting reaction of 1-hydroxy-4-keto-α-ionone with carbomethoxymethylenetriphenylphosphorane prepared from [1,2-13C2]bromoacetic acid followed by saponification of the product methyl [1,2-13C2]ABA.
The fus3 mutation of Arabidopsis thaliana affects several aspects of seed development. Mutant seeds are desiccation intolerant, viviparous and accumulate anthocyanins. Two major classes of storage proteins, the 12S cruciferins and the 2S albumins, are nearly absent, storage lipids are reduced and their composition is changed. The transcription of heterologous storage protein gene promoters in a fus3 genetic background is similarly affected. Our data suggest that the FUS3 gene is together with other genes like ABI3 and LEC1 central to the regulation of developmental processes during late embryogenesis.
Conditionally lethal mutant alleles of theFUSCA3(FUS3) gene ofArabidopsis thalianaare specifically defective in the gene expression program responsible for seed maturation.FUS3was isolated by map-based cloning and expression of theFUS3cDNA resulted in complementation of the Fus3– phenotype. In the predicted FUS3 gene product, a continuous stretch of more than 100 amino acids shows significant sequence similarity to the B3 domains of the polypeptides encoded byABI3(Arabidopsis) andVP1(maize).FUS3transcription was detected mainly in siliques and was found to be developmentally regulated during embryogenesis. Transcripts of abnormal sizes were observed infus3mutants due to aberrant splicing caused by point mutations at intron termini. Sequence analysis of mutant and wild-typeFUS3alleles, as well as sequencing offus3cDNAs, revealed small in-frame deletions at two different sites of the coding region. While a deletion between B3 and the C-terminus of the predicted polypeptide was found in conjunction with normal FUS3 function, another deletion located within the conserved B3 domain (as well as truncations therein) were associated with the Fus3– phenotype. It is apparent, therefore, that an intact B3 domain is essential for the regulation of seed maturation by FUS3.
SummaryA novel chemical induction system for transcription in plants has been developed, taking advantage of the regulatory mechanism of vertebrate steroid hormone receptors. A chimeric transcription factor, designated GVG was constructed, consisting of the DNA-binding domain of the yeast transcription factor GAL4, the transactivating domain of the herpes viral protein VP16, and the receptor domain of the rat glucocorticoid receptor (GR). The GVG gene was introduced into transgenic tobacco and Arabidopsis together with a luciferase (Luc) gene which was transcribed from a promoter containing six tandem copies of the GAL4 upstream activating sequence. Induction of luciferase activity was observed when the transgenic tobacco plants were grown on an agar medium containing dexamethasone (DEX), a strong synthetic glucocorticoid. Induction levels of the luciferase activity were well correlated with DEX concentrations in the range from 0.1 to 10 µM and the maximum expression level was over 100 times that of the basal level. Analysis of the induction kinetics by Northern blot analysis showed that the Luc mRNA was first detected 1 h after DEX treatment and increased to the maximum level in 4 h. The stationary induction level and the duration of the induction varied with the glucocorticoid derivative used. The GVG gene activity can also be regulated by DEX in transgenic Arabidopsis plants. The results indicate that a stringent chemical control of transcription can be achieved in plants with the GVG system. Advantages and potential uses of this system are also discussed.
The control of cell fate was investigated in the root epidermis of Arabidopsis thaliana . Two distinct types of differentiated epidermal cells are normally present: root-hair-bearing Cells and hairless cells. In wild-type Arabidopsis roots, epidermal cell fate was found to be correlated with cell position, with root-hair cells located over radial walls between cortical cells, and with hairless cells located directly over cortical cells. This normal positional relationship was absent in ttg (transparent testa glabrous) mutants (lacking trichomes, anthocyanins, and seed coat mucilage); epidermal cells in all positions differentiate into root-hair cells. The opposite condition was generated in roots of transgenic Arabidopsis expressing the maize R (R-Lc) gene product (a putative TTG homologue) under the control of a strong promoter (CaMV35S), which produced hairless epidermal cells in all positions. In both the ttg and R -expressing roots, epidermal cell differentiation was affected at an early stage, prior to the onset of cell elongation or root-hair formation. The ttg mutations were also associated with abnormalities in the morphology and organization of cells within and surrounding the root apical meristem. The results indicate that alterations in TTG activity cause developing epidermal cells to misinterpret their position and differentiate into inappropriate cell types. This suggests that, in wild-type roots, TTG provides, or responds to, positional signals to cause differentiating epidermal cells that lie over cortical cells to adopt a hairless cell fate. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/31169/1/0000068.pdf
Higher plants pass through several phases of shoot growth during which they may produce morphologically distinct vegetative structures. In Arabidopsis thaliana this phenomenon is apparent in the distribution of trichomes on the leaf surface. Leaves produced early in rosette development lack trichomes on their abaxial (lower) surface, leaves produced later have trichomes on both surfaces, and leaves in the inflorescence (bracts) may have few or no trichomes on their adaxial (upper) surface. Here we describe some of the factors that regulate this distribution pattern. We found that the timing of abaxial trichome production and the extent to which bracts lack adaxial trichomes varies in different ecotypes. The production of abaxial trichomes appears to be regulated by the age, rather than the size of the plant. This conclusion is based on the observation that mutations that affect either the rate (altered meristem programming1) or onset (paused) of leaf initiation respectively increase or decrease the number of leaves that lack abaxial trichomes, but have only a minor effect on the time at which the first leaf with abaxial trichomes is produced. The production of abaxial trichomes is coordinated with the reproductive development of the shoot as this trait is delayed by photoperiodic conditions and some mutations that delay flowering. The loss of adaxial trichomes is likely to be a consequence of floral induction, and is accelerated by terminal flower1-10, a mutation that accelerates inflorescence development. We demonstrate that gibberellins promote trichome production in Arabidopsis and present evidence indicating that abaxial trichome production is regulated by both the level of a trichome inducer and the competence of the abaxial epidermis to respond to this inducer.
We describe mutations of three genes in Arabidopsis thaliana-extra cotyledon1 (xtc1), extra cotyledon2 (xtc2), and altered meristem programming1 (amp1)-that transform leaves into cotyledons. In all three of these mutations, this transformation is associated with a change in the timing of events in embryogenesis. xtc1 and xtc2 delay the morphogenesis of the embryo proper at the globular-to-heart transition but permit the shoot apex to develop to an unusually advanced stage late in embryogenesis. Both mutations have little or no effect on seed maturation and do not affect the viability of the shoot or the rate of leaf initiation after germination. amp1 perturbs the pattern of cell division at an early globular stage, dramatically increases the size of the shoot apex and, like xtc1 and xtc2, produces enlarged leaf primordia during seed development. These unusual phenotypes suggest that these genes play important regulatory roles in embryogenesis and demonstrate that the development of the shoot apical meristem and the development of the embryo proper are regulated by independent processes that must be temporally coordinated to ensure normal organ identity.
Heterochrony describes the phylogenetic variation in the relative timing of major developmental events. Such heterochronic variation has been noted across phylogeny, including closely related species, suggesting that particular genetic loci control global aspects of developmental timing, and that variation at those loci may play important roles in evolutionary change. Genetic analyses of heterochronic mutations in the nematode Caenorhabditis elegans reveal that control of temporal patterning is analogous to the dedicated genetic pathways that control the patterning of the spatial axes in Drosophila and other metazoans. These pathways generate graded or binary levels of regulatory factors that pattern particular axes of the developing animal. C. elegans heterochronic genes constitute a regulatory cascade that both generates a temporal decrease in the level of the LIN-14 and LIN-28 proteins and responds to the changes in these gene activities to coordinate the temporal sequence of many cell fates as the animal develops. The temporal regulation of lin-14 and lin-28 gene activities is posttranscriptional and mediated by the antisense RNA product of the lin-4 gene. Hormonal control of developmental timing is a common theme throughout phylogeny. Heterochronic genes that involve hormonal signaling have been identified in vertebrates as well as C. elegans.
Conditionally lethal mutant alleles of the FUSCA3 (FUS3) gene of Arabidopsis thaliana are specifically defective in the gene expression program responsible for seed maturation. FUS3 was isolated by map-based cloning and expression of the FUS3 cDNA resulted in complementation of the Fus3- phenotype. In the predicted FUS3 gene product, a continuous stretch of more than 100 amino acids shows significant sequence similarity to the B3 domains of the polypeptides encoded by ABI3 (Arabidopsis) and VP1 (maize). FUS3 transcription was detected mainly in siliques and was found to be developmentally regulated during embryogenesis. Transcripts of abnormal sizes were observed in fus3 mutants due to aberrant splicing caused by point mutations at intron termini. Sequence analysis of mutant and wild-type FUS3 alleles, as well as sequencing of fus3 cDNAs, revealed small inframe deletions at two different sites of the coding region. While a deletion between B3 and the C-terminus of the predicted polypeptide was found in conjunction with normal FUS3 function, another deletion located within the conserved B3 domain (as well as truncations therein) were associated with the Fus3- phenotype. It is apparent, therefore, that an intact B3 domain is essential for the regulation of seed maturation by FUS3.
The Agrobacterium vacuum infiltration method has made it possible to transform Arabidopsis thaliana without plant tissue culture or regeneration. In the present study, this method was evaluated and a substantially modified transformation method was developed. The labor-intensive vacuum infiltration process was eliminated in favor of simple dipping of developing floral tissues into a solution containing Agrobacterium tumefaciens, 5% sucrose and 500 microliters per litre of surfactant Silwet L-77. Sucrose and surfactant were critical to the success of the floral dip method. Plants inoculated when numerous immature floral buds and few siliques were present produced transformed progeny at the highest rate. Plant tissue culture media, the hormone benzylamino purine and pH adjustment were unnecessary, and Agrobacterium could be applied to plants at a range of cell densities. Repeated application of Agrobacterium improved transformation rates and overall yield of transformants approximately twofold. Covering plants for 1 day to retain humidity after inoculation also raised transformation rates twofold. Multiple ecotypes were transformable by this method. The modified method should facilitate high-throughput transformation of Arabidopsis for efforts such as T-DNA gene tagging, positional cloning, or attempts at targeted gene replacement.
Many insects show polyphenisms, or alternative morphologies, which are based on differential gene expression rather than genetic polymorphism. Queens and workers are alternative forms of the adult female honey bee and represent one of the best known examples of insect polyphenism. Hormonal regulation of caste determination in honey bees has been studied in detail, but little is known about the proximate molecular mechanisms underlying this process, or any other such polyphenism. We report the success of a molecular-genetic approach for studying queen- and worker-specific gene expression in the development of the honey bee (Apis mellifera). Numerous genes appear to be differentially expressed between the two castes. Seven differentially expressed loci described here belong to at least five distinctly different evolutionary and functional groups. Two are particularly promising as potential regulators of caste differentiation. One is homologous to a widespread class of proteins that bind lipids and other hydrophobic ligands, including retinoic acid. The second locus shows sequence similarity to a DNA-binding domain in the Ets family of transcription factors. The remaining loci appear to be involved with downstream changes inherent to queen- or worker-specific developmental pathways. Caste determination in honey bees is typically thought of as primarily queen determination; our results make it clear that the process involves specific activation of genes in workers as well as in queens.
Cell cycling plays an important role in plant development, including: (1) organ morphogenesis, (2) cell proliferation within tissues, and (3) cell differentiation. In this study we use a cyclin::beta-glucuronidase reporter construct to characterize spatial and temporal patterns of cell cycling at each of these levels during wild-type development in the model genetic organism Arabidopsis thaliana (Columbia). We show that a key morphogenetic event in leaf development, blade formation, is highly correlated with localized cell cycling at the primordium margin. However, tissue layers are established by a more diffuse distribution of cycling cells that does not directly involve the marginal zone. During leaf expansion, tissue proliferation shows a strong longitudinal gradient, with basiplastic polarity. Tissue layers differ in pattern of proliferative cell divisions: cell cycling of palisade mesophyll precursors is prolonged in comparison to that of pavement cells of the adjacent epidermal layers, and cells exit the cycle at different characteristic sizes. Cell divisions directly related to formation of stomates and of vascular tissue from their respective precursors occur throughout the period of leaf extension, so that differing tissue patterns reflect superposition of cycling related to cell differentiation on more general tissue proliferation. Our results indicate that cell cycling related to leaf morphogenesis, tissue-specific patterns of cell proliferation, and cell differentiation occurs concurrently during leaf development and suggest that unique regulatory pathways may operate at each level.
The life cycle of angiosperms is punctuated by a dormant phase that separates embryonic and postembryonic development of the sporophyte. In the pickle (pkl) mutant of Arabidopsis, embryonic traits are expressed after germination. The penetrance of the pkl phenotype is strongly enhanced by inhibitors of gibberellin biosynthesis. Map-based cloning of the PKL locus revealed that it encodes a CHD3 protein. CHD3 proteins have been implicated as chromatin-remodeling factors involved in repression of transcription. PKL is necessary for repression of LEC1, a gene implicated as a critical activator of embryo development. We propose that PKL is a component of a gibberellin-modulated developmental switch that functions during germination to prevent reexpression of the embryonic developmental state.
The Arabidopsis thaliana MERISTEM LAYER 1 (ATML1) gene is expressed in the epidermis of developing embryos and shoot meristems. To identify regulatory sequences necessary for epidermis-specific expression, three fusions of overlapping ATML1 genomic sequences to the GUS reporter gene were introduced into Arabidopsis plants. All fusion genes conferred epidermis-specific expression of both GUS mRNA and protein activity but varied in both the timing and relative levels of expression, suggesting partial redundancy of ATML1 regulatory elements. This study defines L1-specific regulatory sequences that are sufficient to direct foreign gene expression in a layer-specific manner.
Arabidopsis abi3 and fus3 mutants are defective in late embryo development and their embryos show precocious growth. To understand the function and role of ABI3 and FUS3, we analyzed expression patterns of genes which were normally activated during late embryo development and germination in these mutants. Using the differential display method, both upregulated and downregulated genes were observed in immature siliques of the abi3 fus3 double mutant. Four clones having more abundant expression in the abi3 fus3 double mutant than in wild type were isolated. These genes were activated during wild-type germination, suggesting that some genes that are activated during wild-type germination are precociously activated in the abi3 fus3 mutant during late embryo development. Also, genes that were activated during wild-type germination were isolated and their expression patterns during late embryo development in the wild type and in abi3, fus3, and abi3 fus3 mutants were analyzed. Sixteen such clones were found, and 11 of these showed derepression or precocious activation of gene expression in the mutants. These results indicate that ABI3 and FUS3 negatively regulate a particular set of genes during late embryo development. We also showed that immature fus3 siliques accumulated one-third of the wild-type level of abscisic acid (ABA), but mature fus3 siliques accumulated ABA at a level comparable to that in the wild type. The possible mechanisms of controlling developmental timing in late embryo development as well as collaborative and distinct roles of ABI3 and FUS3 are discussed.
Characterization of the heterochronic genes has provided a strong foundation for understanding the molecular mechanisms of developmental timing in C. elegans. In apparent contrast, studies of developmental timing in Drosophila have demonstrated a central role for gene cascades triggered by the steroid hormone ecdysone. In this review, I survey the molecular mechanisms of developmental timing in C. elegans and Drosophila and outline how common regulatory pathways are beginning to emerge.
RGA and GAI are homologous genes that encode putative transcriptional regulators that repress gibberellin (GA) signaling in Arabidopsis. Previously we showed that the green fluorescent protein (GFP)-RGA fusion protein is localized to the nucleus in transgenic Arabidopsis, and expression of this fusion protein rescues the rga null mutation. The GA signal seems to derepress the GA response pathway by degrading the repressor protein RGA. The GA-insensitive, semidominant, semidwarf gai-1 mutant encodes a mutant protein with a 17-amino acid deletion within the DELLA domain of GAI. It was hypothesized that this mutation turns the gai protein into a constitutive repressor of GA signaling. Because the sequences missing in gai-1 are identical between GAI and RGA, we tested whether an identical mutation (rga-Delta 17) in the RGA gene would confer a phenotype similar to gai-1. We demonstrated that expression of rga-Delta 17 or GFP-(rga-Delta 17) under the control of the RGA promoter caused a GA-unresponsive severe dwarf phenotype in transgenic Arabidopsis. Analysis of the mRNA levels of a GA biosynthetic gene, GA4, showed that the feedback control of GA biosynthesis in these transgenic plants was less responsive to GA than that in wild type. Immunoblot and confocal microscopy analyses indicated that rga-Delta17 and GFP-(rga-Delta 17) proteins were resistant to degradation after GA application. Our results illustrate that the DELLA domain in RGA plays a regulatory role in GA-induced degradation of RGA. Deletion of this region stabilizes the rga-Delta 17 mutant protein, and regardless of the endogenous GA status rga-Delta 17 becomes a constitutively active repressor of GA signaling.
If the last common ancestor of plants and animals was unicellular, comparison of the developmental mechanisms of plants and animals would show that development was independently invented in each lineage. And if this is the case, comparison of plant and animal developmental processes would give us a truly comparative study of development, which comparisons merely among animals, or merely among plants, do not-because in each of these lineages, the fundamental mechanisms are similar by descent. Evidence from studies of developmental mechanisms in both kingdoms, and data from genome-sequencing projects, indicate that development evolved independently in the lineages leading to plants and to animals.
We have previously described a homeotic leafy cotyledon (lec) mutant of Arabidopsis that exhibits striking defects in embryonic maturation and produces viviparous embryos with cotyledons that are partially transformed into leaves. In this study, we present further details on the developmental anatomy of mutant embryos, characterize their response to abscisic acid (ABA) in culture, describe other mutants with related phenotypes, and summarize studies with double mutants. Our results indicate that immature embryos precociously enter a germination pathway after the torpedo stage of development and then acquire characteristics normally restricted to vegetative parts of the plant. In contrast to other viviparous mutants of maize (vp1) and Arabidopsis (abi3) that produce ABA-insensitive embryos, immature lec embryos are sensitive to ABA in culture. ABA is therefore necessary but not sufficient for embryonic maturation in Arabidopsis. Three other mutants that produce trichomes on cotyledons following precocious germination in culture are described. One mutant is allelic to lec1, another is a fusca mutant (fus3), and the third defines a new locus (lec2). Mutant embryos differ in morphology, desiccation tolerance, pattern of anthocyanin accumulation, presence of storage materials, size and frequency of trichomes on cotyledons, and timing of precocious germination in culture. The leafy cotyledon phenotype has therefore allowed the identification of an important network of regulatory genes with overlapping functions during embryonic maturation in Arabidopsis.
LEAFY COTYLEDON1 (LEC1) is an embryo defective mutation that affects cotyledon identity in Arabidopsis. Mutant cotyledons possess trichomes that are normally a leaf trait in Arabidopsis, and the cellular organization of these organs is intermediate between that of cotyledons and leaves from wild-type plants. We present several lines of evidence that indicate that the control of late embryogenesis is compromised by the mutation. First, mutant embryos are desiccation intolerant, yet embryos can be rescued before they dry to yield homozygous recessive plants that produce defective embryos exclusively. Second, although many genes normally expressed during embryonic development are active in the mutant, at least one maturation phase-specific gene is not activated. Third, the shoot apical meristem is activated precociously in mutant embryos. Fourth, in mutant embryos, several genes characteristic of postgerminative development are expressed at levels typical of wild-type seedlings rather than embryos. We conclude that postgerminative development is initiated prematurely and that embryonic and postgerminative programs operate simultaneously in mutant embryos. The pleiotropic effects of the mutation indicate that the LEC1 gene plays a fundamental role in regulating late embryogenesis. The role of LEC1 and its relationship to other genes involved in controlling late embryonic development are discussed.
The shoot apical meristem (SAM) is an indeterminate structure that gives rise to the aerial parts of higher plants. Leaves arise from the differentiation of cells at the flanks of the SAM. Current evidence suggests that the precise regulation of KNOTTED1-like homeobox (KNOX) transcription factors is central to the acquisition of leaf versus meristem identity in a wide spectrum of plant species. Factors required to repress KNOX gene expression in leaves have recently been identified. Additional factors such as the CHD3 chromatin remodeling factor PICKLE (PKL) act to restrict meristematic activity in Arabidopsis leaves without repressing KNOX gene expression. Less is known regarding downstream targets of KNOX function. Recent evidence, however, has suggested that growth regulators may mediate KNOX activity in a variety of plant species.
Here we show that reduced activity of the gibberellin (GA) growth regulator pathway promotes meristematic activity, both in the natural context of KNOX function in the SAM and upon ectopic KNOX expression in Arabidopsis leaves. We show that constitutive signaling through the GA pathway is detrimental to meristem maintenance. Furthermore, we provide evidence that one of the functions of the KNOX protein SHOOTMERISTEMLESS (STM) is to exclude transcription of the GA-biosynthesis gene AtGA20ox1 from the SAM. We also demonstrate that AtGA20ox1 transcript is reduced in the pkl mutant in a KNOX-independent manner. Moreover, we show a similar interaction between KNOX proteins and GA-biosynthesis gene expression in the tomato leaf and implicate this interaction in regulation of the dissected leaf form.
We suggest that repression of GA activity by KNOX transcription factors is a key component of meristem function. Transfer of the KNOX/GA regulatory module from the meristem to the leaf may have contributed to the generation of the diverse leaf morphologies observed in higher plants.
Jasmonates (JAs) regulate Arabidopsis thaliana wound and defence responses, pollen development, and stress-related growth inhibition. Significantly, each of these responses requires COI1, an F-box protein. Other F-box proteins interact with SKP1 and cullin proteins to form SCF complexes that selectively recruit regulatory proteins targeted for ubiquitination. To determine whether COI1 also functions in an SCF complex, we have characterized Arabidopsis proteins that bind to COI1. An Arabidopsis cDNA expression library was screened in yeast for clones that produce proteins which can bind to COI1. We recovered two SKP1 homologues and a histone deacetylase. The Arabidopsis F-box protein TIR1 interacted with SKP1 proteins, but not with the histone deacetylase. Mutant COI1 proteins revealed that the F-box is required for interaction with SKP1s, but that sequences in leucine-rich repeat domains are required for interaction with the histone deacetylase. Epitope-tagged COI1 was introduced into Arabidopsis plants and cell cultures. Co-immunoprecipitation experiments confirmed the interaction in planta of COI1 with SKP1-like proteins and histone deacetylase, and also indicated that COI1 interacted with cullin. These results suggest that COI1 forms an SCFCOI1 complex in vivo. COI1 is therefore expected to form a functional E3-type ubiquitin ligase in plants and to regulate expression of jasmonate responsive genes, possibly by targeted ubiquitination of a histone deacetylase.
In angiosperms, germination represents an important developmental transition during which embryonic identity is repressed and vegetative identity emerges. PICKLE (PKL) encodes a CHD3-chromatin-remodeling factor necessary for the repression of expression of LEAFY COTYLEDON1 (LEC1), a central regulator of embryogenesis. A candidate gene approach and microarray analysis identified nine additional genes that exhibit PKL-dependent repression of expression during germination. Transcripts for all three LEAFY COTYLEDON genes, LEC1, LEC2, and FUS3, exhibit PKL-dependent repression, and all three transcripts are elevated more than 100-fold in pkl primary roots that inappropriately express embryonic traits (pickle roots). Three other genes that exhibit PKL-dependent regulation have expression patterns correlated with zygotic or somatic embryogenesis, and one gene encodes a putative Lin-11, Isl-1, MEC-3 (LIM) domain transcriptional regulator that is preferentially expressed in siliques. Genes that exhibit PKL-dependent repression during germination are not necessarily regulated by PKL at other points in development. Our data suggest that PKL selectively regulates a suite of genes during germination to repress embryonic identity. In particular, we propose that PKL acts as a master regulator of the LEAFY COTYLEDON genes, and that joint derepression of these genes is likely to contribute substantially to expression of embryonic identity in pkl seedlings.
Plants produce different types of organs at different times in shoot development. Along with the major changes in organ morphology
that take place during developmental transitions, more gradual patterns of variation occur. The identity of organs produced
at a particular position on the shoot is determined by interactions between several independently regulated, temporally coordinated
processes. Two of these processes are organ production and the specification of organ identity. Coordination of these processes
is accomplished in part by a thermal clock and by signal transduction pathways that mediate the response of plants to light.
The juvenile hormones of insects regulate an unusually large diversity of processes during postembryonic development and adult reproduction. It is a long-standing puzzle in insect developmental biology and physiology how one hormone can have such diverse effects. The search for molecular mechanisms of juvenile hormone action has been guided by classical models for hormone-receptor interaction. Yet, despite substantial effort, the search for a juvenile hormone receptor has been frustrating and has yielded limited results. We note here that a number of lipid-soluble signaling molecules in vertebrates, invertebrates and plants show curious similarities to the properties of juvenile hormones of insects. Until now, these signaling molecules have been thought of as uniquely evolved mechanisms that perform specialized regulatory functions in the taxon where they were discovered. We show that this array of lipid signaling molecules share interesting properties and suggest that they constitute a large set of signal control and transduction mechanisms that include, but range far beyond, the classical steroid hormone signaling mechanism. Juvenile hormone is the insect representative of this widespread and diverse system of lipid signaling molecules that regulate protein activity in a variety of ways. We propose a synthetic perspective for understanding juvenile hormone action in light of other lipid signaling systems and suggest that lipid activation of proteins has evolved to modulate existing signal activation and transduction mechanisms in animals and plants. Since small lipids can be inserted into many different pathways, lipid-activated proteins have evolved to play a great diversity of roles in physiology and development.
The expression of seed storage proteins is under tight developmental regulation and represents a powerful model system to study the regulation of gene expression during plant development. In this study, we show that three homologous B3 type transcription factors regulate the model storage protein gene, At2S3, via two distinct mechanisms: FUSCA3 (FUS3) and LEAFY COTYLEDON2 (LEC2) activate the At2S3 promoter in yeast suggesting that they regulate At2S3 by directly binding its promoter; ABSCISIC ACID INSENSITIVE3 (ABI3), however, appears to act more indirectly on At2S3, possibly as a cofactor in an activation complex. In accordance with this, FUS3 and LEC2 were found to act in a partially redundant manner and differently from ABI3 in planta: At2S3 expression is reduced to variable and sometimes only moderate extent in fus3 and lec2 single mutants but is completely abolished in the lec2 fus3 double mutant. In addition, we found that FUS3 and LEC2 expression patterns, together with an unsuspected regulation of FUS3 by LEC2, enable us to explain the intriguing expression pattern of At2S3 in lec2 or fus3 single mutants. Based on these results, we present a model of At2S3 regulation and discuss its implications for other aspects of seed maturation.