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Diversity of gibberellins contributes to different flowering ecotypes in chrysanthemum
The gibberellins (GA) receptor GA INSENSITIVE DWARF1 (GID1) plays a central role in GA signal perception and transduction. The typical photoperiodic plant chrysanthemum (Chrysanthemum morifolium) only flowers when grown in short-day photoperiods. In addition, chrysanthemum flowering is also controlled by the aging pathway, but whether and how GAs participate in photoperiod- and age-dependent regulation of flowering remain unknown. Here, we demonstrate that photoperiod affects CmGID1B expression in response to GAs and developmental age. Moreover, we identified PHOTOLYASE/BLUE LIGHT RECEPTOR2 (PHR2), an atypical photocleavage synthase, as a CRYPTOCHROME-INTERACTING bHLH1 (CIB1) interactor with which it forms a complex in response to short days to activate CmGID1B transcription. Knocking down CmGID1B raised endogenous bioactive GA contents and GA signal perception, in turn modulating the expression of the aging-related genes MicroRNA156 (miR156) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE3 (SPL3). We propose that exposure to short days accelerates the juvenile-to-adult transition by increasing endogenous GA contents and response to GAs, leading to entry into floral transformation.
Conservative flowering behaviours, such as flowering during long days in summer or late flowering at a high leaf number, are often proposed to protect against variable winter and spring temperatures which lead to frost damage if premature flowering occurs. Yet, due the many factors in natural environments relative to the number of individuals compared, assessing which climate characteristics drive these flowering traits has been difficult. We applied a multidisciplinary approach to 10 winter‐annual Arabidopsis thaliana populations from a wide climactic gradient in Norway. We used a variable reduction strategy to assess which of 100 climate descriptors from their home sites correlated most to their flowering behaviours when tested for responsiveness to photoperiod after saturation of vernalization; then, assessed sequence variation of 19 known environmental‐response flowering genes. Photoperiod responsiveness inversely correlated with interannual variation in timing of growing season onset. Time to flowering appeared driven by growing season length, curtailed by cold fall temperatures. The distribution of FLM, TFL2 and HOS1 haplotypes, genes involved in ambient temperature response, correlated with growing‐season climate. We show that long‐day responsiveness and late flowering may be driven not by risk of spring frosts, but by growing season temperature and length, perhaps to opportunistically maximize growth.
Chrysanthemum (Chrysanthemum morifolium) is well known as a photoperiod‐sensitive flowering plant. However, it has also evolved into a temperature‐sensitive ecotype. Low temperature can promote the floral transition of the temperature‐sensitive ecotype, but little is known about the underlying molecular mechanisms. Here, we identified MADS AFFECTING FLOWERING 2 (CmMAF2), a putative MADS‐box gene, which induces floral transition in response to low temperatures independent of day length conditions in this ecotype. CmMAF2 was shown to bind to the promoter of the GA biosynthesis gene CmGA20ox1 and to directly regulate the biosynthesis of bioactive GA1 and GA4. The elevated bioactive GA levels activated LEAFY (CmLFY) expression, ultimately initiating floral transition. In addition, CmMAF2 expression in response to low temperatures was directly activated by CmC3H1, a CCCH‐type zinc‐finger protein upstream. In summary, our results reveal that the CmC3H1–CmMAF2 module regulates flowering time in response to low temperatures by regulating GA biosynthesis in the temperature‐sensitive chrysanthemum ecotype.
In apple cultivation, dwarf rootstocks are chosen for dense planting and intensive cultivation, which is beneficial to production management. Dwarf rootstocks are widely used in apple production in China. However, the dwarfing mechanisms of dwarf interstock are still unclear. Here, M9 and SH40 were selected as the dwarf interstocks for potted Fuji apples. The key transcription factor MdWRKY24 was screened via transcriptional sequencing. The open reading frame sequence of the MdWRKY24 gene was 657 bp in length, encoded 218 amino acids, and was located on the cell membrane. The MdWRKY24-overexpressing Arabidopsis line showed a dwarf phenotype and delayed flowering. The DELLA protein RGA-like (RGL) gene is a repressor of the gibberellin signaling pathway. Yeast two-hybrid analysis revealed that MdWRKY24 could interact with MdRGL1/2/3. The results indicated that MdWRKY24 might affect plant dwarfing through the synergistic effect of MdRGL1/2/3. The MdWRKY24-MdRGL may be an important pathway underlying the gibberellin-mediated regulation of apple dwarfing.
Cucumber is a model plant for studying parthenocarpy with abundant slicing- and pickling-type germplasm. This study was undertaken to understand the role of the important cytokines (CKs), auxin (AUX) and gibberellin (GA) biosynthesis and degradation genes for the induction of parthenocarpy in slicing and pickling germplasm. Two genotypes of gynoecious parthenocarpic cucumber, PPC-6 and DG-8, along with an MABC-derived gynoecious non-parthenocarpic line, IMPU-1, were evaluated in this study. The slicing and pickling cucumber genotypes PPC-6 and DG-8 were strongly parthenocarpic in nature and set fruit normally without pollination. Endogenous auxin and gibberellin were significantly higher in parthenocarpic than non-parthenocarpic genotypes, whereas the concentration of cytokinins varied among the genotypes at different developmental stages. However, the exogenous application of Zeatin and IAA + Zeatin was effective in inducing parthenocarpic fruit in IMPU-1. Expression analysis with important CK, AUX, and GA biosynthesis-related genes was conducted in IMPU-1, PPC-6, and DG-8. The expression of the CK synthase, IPT, IPT3, PaO, LOG1, LOG2, CYP735A1, and CYP735A2 was up-regulated in the parthenocarpic genotypes. Among the transcription factor response regulators (RRs), positive regulation of CSRR8/9b, CSRR8/9d, CSRR8/9e, and CSRR16/17 and negative feedback of the CK signalling genes, such as CsRR3/4a, CsRR3/4b, CsRR8/9a, and CsRR8/9c, were recorded in the parthenocarpic lines. Homeostasis between cytokinin biosynthesis and degradation genes such as CK oxidases (CKXs) and CK dehydrogenase resulted in a non-significant difference in the endogenous CK concentration in the parthenocarpic and non-parthenocarpic genotypes. In addition, up-regulation of the key auxin-inducing proteins and GA biosynthesis genes indicated their crucial role in the parthenocarpic fruit set of cucumber. This study establishes the critical role of the CKs, AUX, and GA regulatory networks and their cross-talk in determining parthenocarpy in slicing and pickling cucumber genotypes.
Ambient temperatures are increasing due to climate change. Cereal crops development and production will be affected consequently. Flowering time is a key factor for adaptation of small grain cereals and, therefore, exploring developmental responses of barley to rising temperatures is required. In this work, we studied phasic growth, and inflorescence traits related to yield, in eight near isogenic lines of barley (Hordeum vulgare L.) differing at the VRN-H1, VRN-H2 and PPD-H1 genes, representing different growth habits. The lines were grown in contrasting vernalization treatments, under two temperature regimes (18 and 25°C), in long days. Lines with recessive ppd-H1 presented delayed development compared to lines with the sensitive PPD-H1 allele, across the two growth phases considered. High temperature delayed flowering in all unvernalized plants, and in vernalized spring barleys carrying the insensitive ppd-H1 allele, whilst it accelerated flowering in spring barleys with the sensitive PPD-H1 allele. This finding evidenced an interaction between PPD-H1, temperature and vernalization. At the high temperature, PPD-H1 lines in spring backgrounds (VRN-H1-7) yielded more, whereas lines with ppd-H1 were best in vrn-H1 background. Our study revealed new information that will support breeding high-yielding cultivars with specific combinations of major adaptation genes tailored to future climatic conditions.
Flowering is the first committed step of plant sexual reproduction. While the developing flower is a strong sink requiring large quantity of sugars from photosynthetic source tissues, this process is under-temper-spatially controlled via hormone signaling pathway and nutrient availability. Sugar transporters SUT/SUC and SWEET mediate sugars movement across membranes and play a significant role in various physiological processes, including reproductive organ development. In Petunia axillaris, a model ornamental plant, 5 SUT/SUC and 36 SWEET genes are identified in the current version of the genome. Analysis of their gene structure and chromosomal locations reveal that SWEET family is moderately expanded. Most of the transporter genes are abundantly expressed in the flower than in other organs. During the five flower developmental stages, transcript levels of PaSUT1, PaSUT3, PaSWEET13c, PaSWEET9a, PaSWEET1d, PaSWEET5a and PaSWEET14a increase with the maturation of the flower and reach their maximum in the fully open flowers. PaSWEET9c, the nectar-specific PhNEC1 orthologous, is expressed in matured and fully opened flowers. Moreover, determination of sugar concentrations and phytohormone dynamics in flowers at the five developmental stages shows that glucose is the predominant form of sugar in young flowers at the early stage but depletes at the later stage, whereas sucrose accumulates only in maturated flowers prior to the corolla opening. On the other hand, GA3 content and to a less extent IAA and zeatin decreases with the flower development; however, JA, SA and ABA display a remarkable peak at mid- or later flower developmental stage.
Plants undergo distinct phase transitions during their post-embryonic development and progresses from the juvenile to the adult and reproductive phases. These transitions are characterized by morphological and molecular changes and are differently influenced by gibberellins (GAs) and cytokinins (CKs). GAs are notably known to either induce or repress phase transition and flowering in diverse plant species. This GA-mediated modulation is ultimately related to the behavior of the DELLA transcriptional regulators. CKs influence phase transitions by promoting meristem cell divisions and flowering stimulation. Moreover, CKs and GAs can mutually repress each other or have complementary functions in processes such as branching and flowering. Therefore, the effects observed such as flower formation and vegetative growth is modulated by the co-regulation exerted by the cross-talk of both GA and CK pathways. We review the roles of GA and CK in phase transitions at the molecular level in model species such as Arabidopsis and the genes that are modulated by both GA and CK pathways. Additionally we point out perspectives of the conservation of these molecular pathways in tropical plants.
The quality of Lily cut flower was determined by the quality of bulbs. During the process of vernalization and flower bud differentiation, sugar massively accumulated in the bulb, which influenced the bulb development. However, the details of sugar genes’ regulation mechanism for these processes were not fully understood. Here, morphological physiology, transcriptomes and gene engineering technology were used to explore this physiological change. Seventy-two genes of 25 kinds of sugar metabolism-related genes were annotated after re-analyzing transcriptome data of Oriental hybrid lily ‘Sorbonne’ bulbs, which were generated on Hiseq Illumina 2000. The results showed that these genes were closely related to lily bulb vernalization and development. Combining gene expression pattern with gene co-expression network, five genes (Contig5669, Contig13319, Contig7715, Contig1420 and Contig87292) were considered to be the most potential signals, and the sucrose transporter gene (SUT) was the focus of this study. Carbohydrate transport pathway and genes’ regulation mechanism were inferred through a physiological and molecular test. SUT seemed to be the sugar sensor that could sense and regulate sugar concentration, which might have effects on other genes, such as FT, LFY and so on. LoSUT2 and LoSUT4 genes were cloned from Oriental hybrid lily ‘Sorbonne’ by RACE, which was the first time for these genes in Oriental hybrid lily ‘Sorbonne’. The physiological properties of these proteins were analyzed such as hydrophobicity and phosphorylation. In addition, secondary and tertiary structures of proteins were predicted, which indicated the two proteins were membrane proteins. Their cellular locations were verified through positioning the experiment of the fluorescent vector. They were highly expressed in cells around phloem, which illustrated the key role of these genes in sugar transport. Furthermore, transient expression assays showed that overexpressed LoSUT2 and LoSUT4 in Arabidopsis thaliana bloomed significantly earlier than the wild type and the expression of FT, SOC1 and LFY were also affected by LoSUT2 and LoSUT4, which indicated that LoSUT2 and LoSUT4 may regulate plants flowering time.
Age, as a threshold of floral competence acquisition, prevents precocious flowering when there is insufficient biomass, and ensures flowering independent of environmental conditions; however, the underlying regulatory mechanisms are largely unknown. In this study, silencing the expression of a nuclear factor gene, CmNF-YB8, from the short day plant chrysanthemum (Chrysanthemum morifolium), results in precocious transition from juvenile to adult, as well as early flowering, regardless of day length conditions. The expression of SQUAMOSA PROMOTER BINDING-LIKE (SPL) family members, SPL3, SPL5, and SPL9, is upregulated in CmNF-YB8-RNAi plants, while expression of the microRNA, cmo-MIR156, is downregulated. In addition, CmNF-YB8 is shown to bind to the promoter of the cmo-MIR156 gene. Ectopic expression of cmo-miR156, using a virus-based microRNA expression system, restores the early flowering phenotype caused by CmNF-YB8 silencing. These results show that CmNF-YB8 influences flowering time through directly regulating the expression of cmo-MIR156 in the aging pathway.
The transition to flowering marks a key adaptive developmental switch in plants which impacts on their survival and fitness. Different signaling pathways control the floral transition, conveying both endogenous and environmental cues. These cues are often relayed and/or modulated by different hormones, which might confer additional developmental flexibility to the floral process in the face of varying conditions. Among the different hormonal pathways, the phytohormone gibberellic acid (GA) plays a dominant role. GA is connected with the other floral pathways through the GA-regulated DELLA proteins, acting as versatile interacting modules for different signaling proteins. In this review, I will highlight the role of DELLAs as spatial and temporal modulators of different consolidated floral pathways. Next, building on recent data, I will provide an update on some emerging themes connecting other hormone signaling cascades to flowering time control. I will finally provide examples for some established as well as potential cross-regulatory mechanisms between hormonal pathways mediated by the DELLA proteins.
Background
Chrysanthemum is a leading cut flower species. Most conventional cultivars flower during the fall, but the Chrysanthemum morifolium ‘Yuuka’ flowers during the summer, thereby filling a gap in the market. To date, investigations of flowering time determination have largely focused on fall-flowering types. Little is known about molecular basis of flowering time in the summer-flowering chrysanthemum. Here, the genome-wide transcriptome of ‘Yuuka’ was acquired using RNA-Seq technology, with a view to shedding light on the molecular basis of the shift to reproductive growth as induced by variation in the photoperiod. ResultsTwo sequencing libraries were prepared from the apical meristem and leaves of plants exposed to short days, three from plants exposed to long days and one from plants sampled before any photoperiod treatment was imposed. From the ~316 million clean reads obtained, 115,300 Unigenes were assembled. In total 70,860 annotated sequences were identified by reference to various databases. A number of transcription factors and genes involved in flowering pathways were found to be differentially transcribed. Under short days, genes acting in the photoperiod and gibberellin pathways might accelerate flowering, while under long days, the trehalose-6-phosphate and sugar signaling pathways might be promoted, while the phytochrome B pathway might block flowering. The differential transcription of eight of the differentially transcribed genes was successfully validated using quantitative real time PCR. ConclusionsA transcriptome analysis of the summer-flowering cultivar ‘Yuuka’ has been described, along with a global analysis of floral transition under various daylengths. The large number of differentially transcribed genes identified confirmed the complexity of the regulatory machinery underlying floral transition.
Sotol', a protected designation of origin alcoholic beverage, is obtained from Dasylirion cedrosanum spp. plants. There is little knowledge concerning the sexual differentiation mechanisms of this species, which leads to a lack of proposals for not only its conservation and reforestation but also mechanisms to account for the dioecious nature of the plant. Phytohormones have been associated with sexual differentiation in dioecious plants because, individually or in combination, these hormones promote masculinization or feminization of their sexual structures. The objective of this study was to quantify gibberellin A4 (GA4) and trans-zeatin riboside (tZR) levels in samples of different organs of staminate and pistillate plants of D. cedrosanum, which were collected at different stages of floral development. The plant material was obtained at three locations in Coahuila at North Mexico. Gibberellin A4 (GA4) and trans-zeatin riboside (tZR) were quantified by HPLC-UV at 205 nm and 268 nm, respectively. During the later appearance of pollen and seeds, the GA4 levels in the crown and leaves were the same but exceeded those in the inflorescence. There were no differences in tZR levels between the plants of different sexes. Among organs, differences were only found during inflorescence emergence and death, stages during which the crown presented the highest levels of tZR. The results for the GA4/tZR ratio were similar to those reported for GA4. GA4 most likely plays a role in sexual determination in D. cedrosanum because its presence is associated with the appearance of staminate flowers.
The phytohormones gibberellic acid (GA) and abscisic acid (ABA) are known to play antagonistic roles in the control of seed germination and seedling establishment. Here, the regulatory roles of woodland strawberry MOTHER OF FT AND TFL1 (FvMFT) involved in GA and ABA signaling were investigated. FvMFT, which encodes a phosphatidylethanolamine-binding protein, was predominantly expressed in young strawberry fruits. Transient over-expression of FvMFT repressed the expression of several DELLA genes in strawberry fruits. Ectopic over-expression of FvMFT in Arabidopsis thaliana markedly altered the expression levels of ABA-INSENSITIVE (ABI) and DELLA genes, which is accompanied by partially restoring the impaired post-germination growth when the seeds were germinated on sugar-free medium. The same transgenic Arabidopsis plants also exhibited hypersensitivity to GA3 and ABA treatments during seed germination and post-germination growth. In addition, the fluorescence-tagged FvMFT proteins exhibited intriguing subcellular localizations in integument cells and endosperm cells of transgenic Arabidopsis. Taken together, our results support the notion that FvMFT could act as a dual regulator of seed germination and post-germination growth in response to GA and ABA signaling in plants.
Aechmea fasciata is a well-known ornamental flowering plant in the bromeliad family. It is proposed that a small burst of ethylene synthesis in the meristem triggers flowering in pineapple and other bromeliads in response to diverse environmental and endogenous signals. A. fasciata showed an age-dependent response: adult plants were induced to flower successfully under ethylene treatment but juvenile plants did not. To better understand the mechanism of different responses to ethylene in transcriptome and flowering induction by ethylene, we performed a comparative analysis of A. fasciata transcriptome. Four libraries of A. fasciata adult and juvenile plants under water and ethylene treatment were sequenced by Illumina deep sequencing, 55,238,936, 53,797,292, 53,471,812, and 53,485,862 qualified Illumina clean reads, respectively, with 90 bp mean length, respectively. Unigenes of the four libraries were assembled and then merged into an unified library with 86,609 sequences and a mean size of 987 bp. After searching against Nr, KEGG, Swiss-Prot, and COG databases, 28,350 sequences were assigned to 129 KEGG pathways, 28,289 unigenes were categorized into 64 functional groups, and 19,293 sequences were classified into 25 COG categories. Through differential expression analysis, 56 DEGs related to flowering were identified. The critical genes correlated with flowering were selected and confirmed by qRT-PCR analysis. This study provided a global survey of changes in transcriptomes of A. fasciata in response to ethylene. The analyses of transcriptome profiles imply that FT is upregulated in the adult plant and results in flowering. Moreover, the differential expression of GI, DELLA, GAD1, AP2, and so on indicated that a complicated network participated in the induction of flowering by ethylene, which will help in the future studies.
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.
The switch to reproductive development is biphasic in many plants, a feature important for optimal pollination and yield.
We show that dual opposite roles of the phytohormone gibberellin underpin this phenomenon in Arabidopsis. Although gibberellin promotes termination of vegetative development, it inhibits flower formation. To overcome this effect,
the transcription factor LEAFY induces expression of a gibberellin catabolism gene; consequently, increased LEAFY activity
causes reduced gibberellin levels. This allows accumulation of gibberellin-sensitive DELLA proteins. The DELLA proteins are
recruited by SQUAMOSA PROMOTER BINDING PROTEIN–LIKE transcription factors to regulatory regions of the floral commitment gene
APETALA1 and promote APETALA1 up-regulation and floral fate synergistically with LEAFY. The two opposing functions of gibberellin may facilitate evolutionary
and environmental modulation of plant inflorescence architecture.
The activation of the meristem identity gene VRN1 is a critical regulatory point in wheat flowering. In photoperiod-sensitive wheat varieties, VRN1 is expressed only under long days (LD), but mutants carrying deletions in a regulatory element in its promoter show VRN1 transcription and early spike development under short days (SD). However, complete spike development is delayed until plants are transferred to LD, indicating the existence of an additional regulatory mechanism dependent on LD. We show here that exogenous gibberellin (GA) application accelerates spike development under SD, but only in wheat lines expressing VRN1. The simultaneous presence of GA and VRN1 results in the upregulation of the floral meristem identity genes SOC1-1 and LFY, whereas inhibition of GA biosynthesis with paclobutrazol precludes the LD induction of these two genes. The inductive role of GA on wheat flowering is further supported by the upregulation of GA-biosynthetic genes in the apices of plants transferred from SD to LD, and in photoperiod insensitive and transgenic wheat plants with increased FT transcription under SD. The upregulation of GA-biosynthetic genes was not observed in the leaves of the same genetic stocks. Based on these observations we propose a model in which FT is up-regulated in the leaves under LD and is then transported to the shoot apical meristem where it simultaneously induces the expression of VRN1 and GA-biosynthetic genes, which are both required for the upregulation of the early floral meristem genes SOC1-1 and LFY and the timely development of the wheat spike.
Observations on twenty Chinese herbaceous peony (Paeonia lactiflora) cultivars under both ambient and forcing conditions revealed seven stages of plant development from dormancy release to anthesis. The flowering ability was evaluated in these cultivars under forcing conditions, and only five of them were identified as suitable for forcing. 'Da Fu Gui' was selected to study the effects of chilling and gibberellin (GA(3)) application on growth and flowering under forcing. Prior to forcing, four chilling regimes (0, 3, 4, and 5 weeks at 0-4 degrees C) in combination with 0, 200, and 500 mg/L of GA(3) were applied. The results showed that all the chilling promotes dormancy release and plant growth, while GA(3) application significantly accelerates sprouting and growth, and advances the flowering of the peony plants. However, no significant difference was found between 200 and 500 mg/L GA(3). Stem height, flower size, and flowering rate were positively affected by both chilling and GA(3), while leaf extension (maximum width of foliage) was influenced only by GA(3). A GA(3) concentration of 200 mg/L was found to be the optimal for forcing 'Da Fu Gui' peony chilled at 0-4 degrees C for 4 weeks.
Seed dormancy is an important economic trait for agricultural production. Abscisic acid (ABA) and Gibberellins (GA) are the primary factors that regulate the transition from dormancy to germination, and they regulate this process antagonistically. The detailed regulatory mechanism involving crosstalk between ABA and GA, which underlies seed dormancy, requires further elucidation. Here, we report that ABI4 positively regulates primary seed dormancy, while negatively regulating cotyledon greening, by mediating the biogenesis of ABA and GA. Seeds of the Arabidopsis abi4 mutant that were subjected to short-term storage (one or two weeks) germinated significantly more quickly than Wild-Type (WT), and abi4 cotyledons greened markedly more quickly than WT, while the rates of germination and greening were comparable when the seeds were subjected to longer-term storage (six months). The ABA content of dry abi4 seeds was remarkably lower than that of WT, but the amounts were comparable after stratification. Consistently, the GA level of abi4 seeds was increased compared to WT. Further analysis showed that abi4 was resistant to treatment with paclobutrazol (PAC), a GA biosynthesis inhibitor, during germination, while OE-ABI4 was sensitive to PAC, and exogenous GA rescued the delayed germination phenotype of OE-ABI4. Analysis by qRT-PCR showed that the expression of genes involved in ABA and GA metabolism in dry and germinating seeds corresponded to hormonal measurements. Moreover, chromatin immunoprecipitation qPCR (ChIP-qPCR) and transient expression analysis showed that ABI4 repressed CYP707A1 and CYP707A2 expression by directly binding to those promoters, and the ABI4 binding elements are essential for this repression. Accordingly, further genetic analysis showed that abi4 recovered the delayed germination phenotype of cyp707a1 and cyp707a2 and further, rescued the non-germinating phenotype of ga1-t. Taken together, this study suggests that ABI4 is a key factor that regulates primary seed dormancy by mediating the balance between ABA and GA biogenesis.
Gravitropic bending of plant organs is mediated by an asymmetric signaling of the plant hormone auxin between the upper and lower side of the respective organ. Here, we show that also another plant hormone, gibberellic acid (GA), shows asymmetric action during gravitropic responses. Immunodetection using an antibody against GA and monitoring GA signaling output by downstream degradation of DELLA proteins revealed an asymmetric GA distribution and response with the maximum at the lower side of gravistimulated roots. Genetic or pharmacological manipulation of GA levels or response affects gravity-mediated auxin redistribution and root bending response. The higher GA levels at the lower side of the root correlate with increased amounts of PIN-FORMED2 (PIN2) auxin transporter at the plasma membrane. The observed increase in PIN2 stability is caused by a specific GA effect on trafficking of PIN proteins to lytic vacuoles that presumably occurs downstream of brefeldin A-sensitive endosomes. Our results suggest that asymmetric auxin distribution instructive for gravity-induced differential growth is consolidated by the asymmetric action of GA that stabilizes the PIN-dependent auxin stream along the lower side of gravistimulated roots.
INTRODUCTION
Conventional Illumina RNA-Seq does not have the resolution to decode the complex eukaryote transcriptome due to the lack of RNA polarity information. Strand-specific RNA sequencing (ssRNA-Seq) can overcome these limitations and as such is better suited for genome annotation, de novo transcriptome assembly, and accurate digital gene expression analysis. This protocol describes a simple and robust method to generate ssRNA-Seq libraries for the Illumina sequencing platform. It has significantly increased the throughput to 96 libraries in a two-day preparation while simultaneously lowering the reagent costs to below ten dollars per library. It is compatible with both single-read and paired-end multiplex sequencing and, most importantly, its data can also be used with existing conventional RNA-Seq data. This is a significant advantage, because it enables researchers to switch to ssRNA-Seq even if a large amount of data has already been generated by the nonstrand specific methods.
Plants undergo a major physiological change as they transition from vegetative growth to reproductive development. This transition is a result of responses to various endogenous and exogenous signals that later integrate to result in flowering. Five genetically defined pathways have been identified that control flowering. The vernalization pathway refers to the acceleration of flowering on exposure to a long period of cold. The photoperiod pathway refers to regulation of flowering in response to day length and quality of light perceived. The gibberellin pathway refers to the requirement of gibberellic acid for normal flowering patterns. The autonomous pathway refers to endogenous regulators that are independent of the photoperiod and gibberellin pathways. Most recently, an endogenous pathway that adds plant age to the control of flowering time has been described. The molecular mechanisms of these pathways have been studied extensively in Arabidopsis thaliana and several other flowering plants.
Genetic interactions between phytohormones in the control of flowering time in Arabidopsis thaliana have not been extensively studied. Three phytohormones have been individually connected to the floral-timing program. The inductive function of gibberellins (GAs) is the most documented. Abscisic acid (ABA) has been demonstrated to delay flowering. Finally, the promotive role of brassinosteroids (BRs) has been established. It has been reported that for many physiological processes, hormone pathways interact to ensure an appropriate biological response.
We tested possible genetic interactions between GA-, ABA-, and BR-dependent pathways in the control of the transition to flowering. For this, single and double mutants deficient in the biosynthesis of GAs, ABA, and BRs were used to assess the effect of hormone deficiency on the timing of floral transition. Also, plants that over-express genes encoding rate-limiting enzymes in each biosynthetic pathway were generated and the flowering time of these lines was investigated.
Loss-of-function studies revealed a complex relationship between GAs and ABA, and between ABA and BRs, and suggested a cross-regulatory relation between GAs to BRs. Gain-of-function studies revealed that GAs were clearly limiting in their sufficiency of action, whereas increases in BRs and ABA led to a more modest phenotypic effect on floral timing. We conclude from our genetic tests that the effects of GA, ABA, and BR on timing of floral induction are only in partially coordinated action.
The mechanisms responsible for stem growth in peanut (Arachis hypogaea L.) cultivars with varying plant heights remain unclear, despite the significant impact of plant height on peanut yield. Therefore, this study aimed to investigate the underlying mechanisms of peanut stem growth using phenotypic, physiological, transcriptomic, and metabolomic analyses. The findings revealed that the tallest cultivar, HY33, exhibited the highest rate of stem growth and accumulated the most stem dry matter, followed by the intermediate cultivar, SH108, while the dwarf cultivar, Df216, displayed the lowest values. Furthermore, SH108 exhibited a higher harvest index, as well as superior pod and kernel yields compared to both HY33 and Df216. Transcriptome and metabolome analyses identified differentially expressed genes (DEGs) and differentially expressed metabolites (DEMs) associated with phenylpropanoid and flavonoid biosynthesis. Notably, downregulated DEGs in Df216/HY33 and Df216/SH108 included phenylalanine ammonia-lyase (PAL), caffeoyl-CoA O-methyltransferase (COMT), and ferulate-5-hydroxylase (F5H), while downregulated DEMs included p-coumaryl alcohol, chlorogenic acid, and L-epicatechin. Compared to HY33, the reduced activities of PAL, COMT, and F5H resulted in a decreased stem lignin content in Df216. Additionally, downregulated DEGs involved in gibberellin (GA) and brassinosteroid (BR) biosynthesis were identified in Df216/HY33, which contributed to the lowest levels of GA1, GA3, and BR contents in Df216. The results suggest that the dwarf phenotype arises from impaired GA and BR biosynthesis and signaling, resulting in a slower stem growth rate and reduced lignin accumulation.
Coumarins, the naturally occurring benzo-α-pyrone derivatives, are important group of secondary plant metabolites derived from phenylalanine metabolism. Two net house experiment was carried out concurrently to examine the effects of Gibberellic acid (GA3) (Experiment 1) and naphthalene acetic acid (NAA) (Experiment 2) on growth and physiological parameters, and active constituents of Ammi majus, which is a medicinally important plant rich in coumarins and various flavonoids, alkaloids, and terpenoids. Foliar-spray treatments, consisting of increasing concentrations of GA3 (0, 50, 100, 200, and 400 mg/L) and NAA (0, 25, 50, 100, and 200 mg/L) were applied at 45 days after sowing. Application of both of the PGRs improved all of the parameters significantly. The best results were obtained at 100 mg/L of GA3 (Experiment 1) and 50 mg/L of NAA (Experiment 2) in terms of growth characteristics (plant height, fresh weight and dry weight), physiological
parameters (photosynthetic pigments, chlorophyll fluorescence, and activities of nitrate reductase and carbonic anhydrase) and seed-quality parameters (bioactive constituents). The most effective treatments, viz. 100 mg/L of GA3 (Experiment 1) and 50 mg/L of NAA (Experiment 2), resulted in appreciable increases in seed bioactive ingredients. The primary components were Methoxsalen, 7 H-furo [3,2-g] [1] benzopyron-7-one, 4, 9 dimethoxy, and p-camphorene, according to gas liquid chromatographic analyses. In comparison to the control, foliar treatment of GA3 (100 mg/L) considerably increased the plant’s shoot length (29.54%), root length (27.58%), fresh weight (36.78%), and dry weight (41.71%). Whereas, 50 mg/L of NAA enhanced these growth attributes by 31.12%, 32.61%, 34.84%, and 30.32%, respectively, in comparison to the control. Foliar application of 100 mg/L of GA3 increased the chlorophyll fluorescence (51.8%) as well as total content of chlorophyll (39.81%), and carotenoids (47.53%), in contrast with the control. Comparing the control, 50 mg/L of NAA enhanced the chlorophyll fluorescence, and total content of chlorophyll and carotenoids by 42.3%, 30.92%, and 27.81%, respectively. The best foliar-application treatments of 100 mg/L of GA3 and 50 mg/L of NAA were most
effective for NR activity, surpassing the control by 27.7% and 21.5%, respectively. The largest increase in leaf CA activity was observed due to 100 mg/L of GA3 and 50 mg/L of NAA, which exceeded the control by 24.6% and 19.04%, respectively.
Sugar translocation between cells and between subcellular compartments in plants requires either plasmodesmata or a diverse array of sugar transporters. Interactions between plants and associated microorganisms also depend on sugar transporters. The sugars will eventually be exported transporter (SWEET) family is made up of conserved and essential transporters involved in many critical biological processes. The functional significance and small size of these proteins have motivated crystallographers to successfully capture several structures of SWEETs and their bacterial homologs in different conformations. These studies together with molecular dynamics simulations have provided unprecedented insights into sugar transport mechanisms in general and into substrate recognition of glucose and sucrose in particular. This review summarizes our current understanding of the SWEET family, from the atomic to the whole-plant level. We cover methods used for their characterization, theories about their evolutionary origins, biochemical properties, physiological functions, and regulation. We also include perspectives on the future work needed to translate basic research into higher crop yields.
Expected final online publication date for the Annual Review of Plant Biology, Volume 73 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The plant hormone concept probably derives from observations of morphogenic and developmental correlations by Sachs between 1880 and 1893. He suggested that “Morphological differences between plant organs are due to differences in their material composition” and postulated the existence of root-forming, flower forming and other substances that move in different directions through the plant (Went and Thimann, 1937).
The timing of flowering is determined by endogenous genetic components as well as various environmental factors, such as day length, temperature, and stress. The genetic elements and molecular mechanisms that rule this process have been examined in the long-day-flowering plant Arabidopsis thaliana and short-day-flowering rice (Oryza sativa). However, reviews of research on the role of those factors are limited. Here, we focused on how flowering time is influenced by nutrients, ambient temperature, drought, salinity, exogenously applied hormones and chemicals, and pathogenic microbes. In response to such stresses or stimuli, plants either begin flowering to produce seeds for the next generation or else delay flowering by slowing their metabolism. These responses vary depending upon the dose of the stimulus, the plant developmental stage, or even the cultivar that is used. Our review provides insight into how crops might be managed to increase productivity under various environmental challenges. This article is protected by copyright. All rights reserved.
Gibberellin (GA) and photoperiod pathways have recently been demonstrated to collaboratively modulate flowering under long-days (LDs). However, the molecular mechanisms underlying this collaboration remain largely unclear. In this study, we found that GA-induced expression of FLOWERING LOCUS T (FT) under LDs was dependent on CONSTANS (CO), a critical transcription factor positively involved in photoperiod signaling. Mechanistic investigation revealed that DELLA proteins, a group of crucial repressors in GA signaling, physically interacted with CO. The DELLA-CO interactions repressed the transcriptional function of CO protein. Genetic analysis demonstrated that CO acts downstream of DELLA proteins to regulate flowering. Disruption of CO rescued the earlier-flowering phenotype of the gai-t6 rga-t2 rgl1-1 rgl2-1 mutant (dellap), while a gain-of-function mutation in GA INSENSITIVE (GAI, a member of the DELLA gene) repressed the earlier-flowering phenotype of CO-overexpressing plants. In addition, the accumulation of DELLA proteins and mRNAs was rhythmic, and REPRESSOR OF GA1-3 (RGA) protein was noticeably decreased in the long-day afternoon, a time when CO protein is abundant. Collectively, these results demonstrate that the DELLA-CO cascade inhibits CO/FT-mediated flowering under LDs, which thus provide evidence to directly integrate GA and photoperiod signaling to synergistically modulate flowering under LDs.
The effects of the plant-growth regulator, prohexadione-calcium, on the levels of the endogenous gibberellins in rice shoots were measured by GC-SIM using 2H-labeled gibberellins as internal standards. The compound was applied at the 4-leaf stage and shoots were harvested 5 and 12 days after treatment. Plant length was reduced to 78% and 66%, respectively, relative to control by application of the compound at 1 and 30 mg m-2, when plant length was measured 12 days after treatment. The level of GA, was reduced to 36% and 18%, respectively, relative to control in the treated plants. The levels of GA19and GA20 increased but that of GA44 was reduced in the treated plants. The level of GA53 was unchanged. These results suggest that primary mode of action of the compound in vivo is the inhibition of the 3β-hydroxylation of GA20 to GA1 and further support the hypothesis that GA1 not GA19 nor GA20 is active in promoting shoot elongation in rice.
Proper timing of flowering is essential for reproduction of plants. Although it is well known that both light and gibberellin (GA) signaling play critical roles in promoting flowering in Arabidopsis thaliana, whether and how they are integrated to regulate flowering remain largely unknown. Here we show through biochemical studies that DELLA proteins physically interact with CONSTANS (CO). Furthermore, the interaction of CO with NF-YB2 is inhibited by the DELLA protein, RGA. Our findings suggest that regulation of flowering by GA signaling in leaves under long days is mediated, at least in part, through repression of DELLA proteins on CO, providing a molecular link between DELLA proteins, key components in GA signaling pathway, and CO, a critical flowering activator in photoperiod signaling pathway. This article is protected by copyright. All rights reserved.
The B-box (BBX) family is a subgroup of zinc finger transcription factors that regulate flowering time, light-regulated morphogenesis, and abiotic stress in Arabidopsis. Overexpression of CmBBX24, a zinc finger transcription factor gene in chrysanthemum, results in abiotic stress tolerance. We have investigated and characterized the promoter of CmBBX24, isolating a 2.7-kb CmBBX24 promoter sequence and annotating a number of abiotic stress-related cis-regulatory elements, such as DRE, MYB, MYC, as well as cis-elements which respond to plant hormones, such as GARE, ABRE, and CARE. We also observed a number of cis-elements related to light, such as TBOX and GBOX, and some tissue-specific cis-elements, such as those for guard cells (TAAAG). Expression of the CmBBX24 promoter produced a clear response in leaves and a lower response in roots, based on β-glucuronidase histochemical staining and fluorometric analysis. The CmBBX24 promoter was induced by abiotic stresses (mannitol, cold temperature), hormones (gibberellic acid, abscisic acid), and different light treatments (white, blue, red); activation was measured by fluorometric analysis in the leaves and roots. The deletion of fragments from the 5'-end of the promoter led to different responses under various stress conditions. Some CmBBX24 promoter segments were found to be more important than others for regulating all stresses, while other segments were relatively more specific to stress type. D0-, D1-, D2-, D3-, and D4-proCmBBX24::CmBBX24 transgenic Arabidopsis lines developed for further study were found to be more tolerant to the low temperature and drought stresses than the controls. We therefore speculate that CmBBX24 is of prime importance in the regulation of abiotic stress in Arabidopsis and that the CmBBX24 promoter is inductive in abiotic stress conditions. Consequently, we suggest that CmBBX24 is a potential candidate for the use in breeding programs of important ornamental plants.
Plant growth and development are highly regulated processes that are coordinated by hormones including the brassinosteroids (BRs), a group of steroids with structural similarity to steroid hormones of mammals. Although it is well understood how BRs are produced and how their signals are transduced, BR targets, which directly confer the hormone's growth-promoting effects, have remained largely elusive. Here, we show that BRs regulate the biosynthesis of gibberellins (GAs), another class of growth-promoting hormones, in Arabidopsis thaliana. We reveal that Arabidopsis mutants deficient in BR signaling are severely impaired in the production of bioactive GA, which is correlated with defective GA biosynthetic gene expression. Expression of the key GA biosynthesis gene GA20ox1 in the BR signaling mutant bri1-301 rescues many of its developmental defects. We provide evidence that supports a model in which the BR-regulated transcription factor BES1 binds to a regulatory element in promoters of GA biosynthesis genes in a BR-induced manner to control their expression. In summary, our study underscores a role of BRs as master regulators of GA biosynthesis and shows that this function is of major relevance for the growth and development of vascular plants.
© 2015 American Society of Plant Biologists. All rights reserved.
Arabidopsis thaliana (L.) Heynh. is a quantitative long-day (LD) rosette plant in which stem growth is mediated by gibberellins (GAs). Application of GAs to plants in short-day (SD) conditions resulted in rapid stem elongation and flower formation, with GA 4 and GA 9 being equally effective, and GA 1 showing lower activity. The effects of photoperiod on the levels of endogenous GAs were measured by combined gas chromatography-mass spectrometry with selected ion monitoring. When plants were transferred from SD to LD conditions there was a slight decrease in the level of GA 53 and an increase in the levels of C 19 -GAs, GA 9 , GA 20 , GA 1 , and GA 8 , indicating that GA 20-oxidase activity is stimulated in LD conditions. Expression of GA5, which encodes GA 20-oxidase, was highest in elongating stems and was correlated with the rate of stem elongation. By contrast, GA4, which encodes 3β-hydroxylase, showed low expression in stems and its expression was not correlated with the rate of stem elongation. We conclude that stem elongation in LD conditions is at least in part due to increased expression of GA5, whereas expression of GA4 is not under photoperiodic control.
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 rice slender mutant (slr1-1) is caused by a single recessive mutation and results in a constitutive gibberellin (GA) response phenotype. The mutant elongates as if saturated with GAs. In this mutant, (1) elongation was unaffected by an inhibitor of GA biosynthesis, (2) GA-inducible α-amylase was produced by the aleurone layers without gibberellic acid application, and (3) endogenous GA content was lower than in the wild-type plant. These results indicate that the product of the SLR1 gene is an intermediate of the GA signal transduction pathway. SLR1 maps to OsGAI in rice and has significant homology with height-regulating genes, such as RHT-1Da in wheat, D8 in maize, and GAI and RGA in Arabidopsis. The GAI gene family is likely to encode transcriptional factors belonging to the GRAS gene superfamily. DNA sequence analysis revealed that the slr1-1 mutation is a single basepair deletion of the nuclear localization signal domain, resulting in a frameshift mutation that abolishes protein production. Furthermore, introduction of a 6-kb genomic DNA fragment containing the wild-type SLR1 gene into the slr1-1 mutant restored GA sensitivity to normal. These results indicate that the slr1-1 mutant is caused by a loss-of-function mutation of the SLR1 gene, which is an ortholog of GAI, RGA, RHT, and D8. We also succeeded in producing GA-insensitive dwarf rice by transforming wild-type rice with a modified SLR1 gene construct that has a 17–amino acid deletion affecting the DELLA region. Thus, we demonstrate opposite GA response phenotypes depending on the type of mutations in SLR1.
The tetracyclic diterpenoid carboxylic acids, gibberellins (GAs), orchestrate a broad spectrum of biological programs. In nature, GAs or GA-like substance is produced in bacteria, fungi, and plants. The function of GAs in microorganisms remains largely unknown. Phytohormones GAs mediate diverse growth and developmental processes through the life cycle of plants. The GA biosynthetic and metabolic pathways in bacteria, fungi, and plants are remarkably divergent. In vascular plants, phytohormone GA, receptor GID1, and repressor DELLA shape the GA-GID1-DELLA module in GA signaling cascade. Sequence reshuffling, functional divergence, and adaptive selection are main driving forces during the evolution of GA pathway components. The GA-GID1-DELLA complex interacts with second messengers and other plant hormones to integrate environmental and endogenous cues, which is beneficial to phytohormones homeostasis and other biological events. In this review, we first briefly describe GA metabolism pathway, signaling perception, and its second messengers. Then, we examine the evolution of GA pathway genes. Finally, we focus on reviewing the crosstalk between GA-GID1-DELLA module and phytohormones. Deciphering mechanisms underlying plant hormonal interactions are not only beneficial to addressing basic biological questions, but also have practical implications for developing crops with ideotypes to meet the future demand.
‘Alaska’ and ‘Redwing’ azaleas having dormant flower buds were sprayed with gibberellins (GA3 or GA4 + 7) alone and in combination with thiourea, N6 benzyl adenine (BA) or kinetin weekly for 3 or 4 weeks to test the efficacy of these materials in breaking bud dormancy. Additional plants received 6 weeks of cold storage at 4.5°C or glasshouse day temperatures of 21°C and above. The 2000 and 3000 mg l−1 GA3 and Ga4 + 7 sprays were better than 1000 mg l−1 in promoting flowering, with ‘Redwing’ responding better than ‘Alaska’. GA-treated plants flowered in fewer days than those receiving cold storage. Flower diameter and pedicel length increased with higher levels of GA, and flower uniformity was comparable to cold-stored plants on most GA-treated ‘Redwing’-plants. Thiourea, BA and kinetin applied alone had no effect and considerable cytokinin activity was highest in GA-treated buds 14–21 days after treatment application. No increase in activity occurred on plants not receiving GA.
Hibiscus rosa-sinensis ‘Jane Cowl’ in 1.5–1 pots were given a soil drench of 0.2 mg uniconazole, pruned 2 weeks later, and treated with a foliar
application of GA4+7 at 0, 25 (once or four times every 2 weeks), 50 (once or twice every 4 weeks), or 100 mg L-1. One application of GA4+7 at 100 mg L-1, two applications at 50 mg L-1, and four applications at 25 mg L-1 were more active in partially restoring stem elongation and caused nearly normal leaf production than other GA treatments,
but promoted the abscission of the lower leaves. The size of the individual leaves, but not stem diameter, increased following
GA4+7 application. Multiple applications of GA4+7 stimulated flowering of the retarded plants. Uniconazole resulted in short pedicels bearing short cells with increased diameter,
as well as larger pith, vascular, and cortical tissues than the untreated control. Four applications of 25 mg L-1 GA4+7 to uniconazole-treated plants resulted in long pedicels, having long cells similar to the control. Results of the histological
study suggest that uniconazole either slowed cell division or caused cell division to cease early.
Plants monitor changes in photoperiod and temperature to synchronize their flowering with seasonal changes to maximize fitness. In the Arabidopsis photoperiodic flowering pathway, the circadian clock-regulated components, such as FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 and CONSTANS, both of which have light-controlled functions, are crucial to induce the day-length specific expression of the FLOWERING LOCUS T (FT) gene in leaves. Recent advances indicate that FT transcriptional regulation is central for integrating the information derived from other important internal and external factors, such as developmental age, amount of gibberellic acid, and the ambient temperature. In this review, we describe how these factors interactively regulate the expression of FT, the main component of florigen, in leaves.
In the period 1982–1988, the effects of gibberellin A3 (GA3) applied at concentrations of 0.002, 0.004, 0.006, 0.008, 0.01, 0.02 and 0.03% a.i. and chloromequat (CCC) applied at concentrations of 0.3, 0.6, 1.2, 2.4 and 4.8% a.i., as well as of a combination of GA3 (0.008%) and CCC (1.2%) and of a treatment with 0.008% GA3 directly after removal of the fruit-bearing parts of the mother plants, were studied in strawberry propagation fields with elite mother plants.With 0.008% GA3, the number of usable runner plants increased by 22%, whereas 1.2% CCC increased the number of usable runner plants by about 10%. No after-effects of GA3 on strawberry fruit production were found, whereas CCC increased fruit yield.
The timing of the induction of flowering determines to a large extent the reproductive success of plants. Plants integrate
diverse environmental and endogenous signals to ensure the timely transition from vegetative growth to flowering. Carbohydrates
are thought to play a crucial role in the regulation of flowering, and trehalose-6-phosphate (T6P) has been suggested to function
as a proxy for carbohydrate status in plants. The loss of TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1) causes Arabidopsis thaliana to flower extremely late, even under otherwise inductive environmental conditions. This suggests that TPS1 is required for the timely initiation of flowering. We show that the T6P pathway affects flowering both in the leaves and
at the shoot meristem, and integrate TPS1 into the existing genetic framework of flowering-time control.
Gibberellin-growth retardant interactions on the vegetative growth and flowering of the vine Clerodendrum thomsoniae Balf. were studied using both exogenous treatments and biologically testing the acid fraction attained from the plant extract. The growth retardant, ancymidol, greatly retarded stem elongation and markedly increased flowering under inductive environments. Gibberellin A3 (GA3) application to the shoot tip stimulated vine growth, prevented flowering under inductive environments, and completely overcame ancymidol-induced effects. In contrast to GA3, treatment with GA7 had little effect on vegetative growth but increased flowering under inductive environments. The elevated activity of gibberellin-like compounds, as determined by bioassay, were similar except for a marked increase in levels in ancymidol-treated plants grown under inductive environmental conditions. Microscopic examination of the stem tip indicated that the action of the growth regulators involved the induction of floral buds. Thus, in Clerodendrum, ancymidol appears to stimulate an unknown gibberellin(s) and simultaneously acts antagonistically with GA3.
The plant hormones gibberellin and abscisic acid regulate gene expression, secretion and cell death in aleurone. The emerging picture is of gibberellin perception at the plasma membrane whereas abscisic acid acts at both the plasma membrane and in the cytoplasm – although gibberellin and abscisic acid receptors have yet to be identified. A range of downstream-signalling components and events has been implicated in gibberellin and abscisic acid signalling in aleurone. These include the Gα subunit of a heterotrimeric G protein, a transient elevation in cGMP, Ca2+-dependent and Ca2+-independent events in the cytoplasm, reversible protein phosphory-lation, and several promoter cis-elements and transcription factors, including GAMYB. In parallel, molecular genetic studies on mutants of Arabidopsis that show defects in responses to these hormones have identified components of gibberellin and abscisic acid signalling. These two approaches are yielding results that raise the possibility that specific gibberellin and abscisic acid signalling components perform similar functions in aleurone and other tissues.
Plant development progresses through distinct phases: vegetative growth, followed by a reproductive phase and eventually seed set and senescence. The transitions between these phases are controlled by distinct genetic circuits that integrate endogenous and environmental cues. In recent years, however, it has become evident that the genetic networks that underlie these phase transitions share some common factors. Here, we review recent advances in the field of plant phase transitions, highlighting the role of two microRNAs - miR156 and miR172 - and their respective targets during these transitions. In addition, we discuss the evolutionary conservation of the functions of these miRNAs in regulating the control of plant developmental phase transitions.
Stamen development is governed by a conserved genetic pathway, within which the role of hormones has been the subject of considerable recent research. Our understanding of the involvement of gibberellin (GA) signalling in this developmental process is further advanced than for the other phytohormones, and here we review recent experimental results in rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana) that have provided insight into the timing and mechanisms of GA regulation of stamen development, identifying the tapetum and developing pollen as major targets. GA signalling governs both tapetum secretory functions and entry into programmed cell death via the GAMYB class of transcription factor, the targets of which integrate with the established genetic framework for the regulation of tapetum function at multiple hierarchical levels.
The initiation of flowering is a critical life-history trait; plants have presumably evolved to flower at a time of year that ensures maximal reproductive success in a given region. Decades of physiological studies have revealed that flowering is initiated in response to both environmental cues and endogenous pathways. Commonly studied environmental cues include changes in temperature and daylength. Endogenous pathways function independently of environmental signals and are related to the developmental state of the plant; such pathways are sometimes referred to as “autonomous” to indicate the lack of environmental influence. The relative contributions of autonomous and environmental inputs to the flowering “decision” vary among, and even within, species. For example, flowering is considered entirely due to autonomous pathways in a variety of tobacco (Nicotiana tabacum) that forms a fixed number of nodes before flowering regardless of the environment in which it is grown (McDaniel and Hsu, 1976). Yet, a single-gene change can cause tobacco to require short days to flower (Allard, 1919), which indicates that the underlying biochemical differences between environment-sensing and endogenous pathways can be minimal. Also, endogenous and environmental pathways can interact. For example, some plants pass through a juvenile phase in which they are not responsive to environmental cues that promote flowering (Poethig, 1990); that is, the transition from the juvenile to adult phase is a type of endogenous pathway that is necessary to provide competence for environmental pathways to promote flowering. The recent addition of molecular genetics to the range of approaches used to study the initiation of flowering has provided some molecular insights into these endogenous and environment-sensing pathways and has revealed how inputs from multiple pathways are integrated into the flowering decision.
(Due to the sustained efforts of a multitude of scientists working in many species, we have learned much about the timing of flowering that is worth celebrating. Unfortunately, only a small part of this extensive body of work can be covered in this article because of length and reference limits. Accordingly, we frequently refer readers to recent review articles for more in-depth discussions, and we apologize to our colleagues whose work was not cited due to these constraints.)
We examined the gibberellin (GA) and ethylene regulation of submergence-induced elongation in seedlings of the submergence-tolerant lowland rice (Oryza sativa L.) cvs Senia and Bomba. Elongation was enhanced after germination to facilitate water escape and reach air. We found that submergence-induced elongation depends on GA because it was counteracted by paclobutrazol (an inhibitor of GA biosynthesis), an effect that was negated by GA(3). Moreover, in the cv Senia, submergence increased the content of active GA(1) and its immediate precursors (GA(53), GA(19) and GA(20)) by enhancing expression of several GA biosynthesis genes (OsGA20ox1 and -2, and OsGA3ox2), but not by decreasing expression of several OsGA2ox (GA inactivating genes). Senia seedlings, in contrast to Bomba seedlings, did not elongate in response to ethylene or 1-aminocyclopropane-1-carboxylic-acid (ACC; an ethylene precursor) application, and submergence-induced elongation was not reduced in the presence of 1-methylcyclopropene (1-MCP; an ethylene perception inhibitor). Ethylene emanation was similar in Senia seedlings grown in air and in submerged-grown seedlings following de-submergence, while it increased in Bomba. The expression of ethylene biosynthesis genes (OsACS1, -2 and -3, and OsACO1) was not affected in Senia, but expression of OsACS5 was rapidly enhanced in Bomba upon submergence. Our results support the conclusion that submergence elongation enhancement of lowland rice is due to alteration of GA metabolism leading to an increase in active GA (GA(1)) content. Interestingly, in the cv Senia, in contrast to cv Bomba, this was triggered through an ethylene-independent mechanism.