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Model for the role of polar auxin transport in phyllotaxis. (A, B) A side view of the SAM showing a developing leaf primordium (P1) and the site of an incipient primordium (I1) in the peripheral organogenic zone (dashed lines). Arrows indicate polar auxin transport through the epidermis and sub epidermis, and arrowheads the flow of auxin from surrounding tissue into the developing leaf. Shading represents the likely distribution of auxin (darker-more auxin, lighter-less auxin). Following organ initiation, the primordium (P1) becomes an auxin sink, diverting the acropetal flow of auxin away from the apex and draining auxin from surrounding meristem tissue. As a consequence, a zone of auxin depletion forms around the expanding organ preventing organ formation. Auxin accumulates in the region of the meristem furthest from P1. Once the concentration of auxin passes a critical threshold organ formation is initiated. The newly forming leaf becomes an auxin sink causing a local depletion of auxin from surrounding tissue. This pattern of auxin depletion and accumulation is self-perpetuating and can largely account for the maintenance of phyllotaxis once it has been established. (Redrawn from Reinhardt et al., 2003a). (C). According to the above model of phyllotaxy the distribution of auxin (shaded) largely determines the pattern of organ formation. This is illustrated for several different types of phyllotaxy shown in transverse section-spiral (left), decussate (middle) and distichous (right). In spiral phyllotaxis, P1 and P2 are both auxin sinks, with P1 being a large sink than P2. As a consequence I1 forms between P1 and P2, but closer to P2. In decussate phyllotaxis the two opposing primordia are weak auxin sinks allowing auxin to accumulate in two regions of the meristem that are equidistant from P1 organs. In distichous phyllotaxis, the youngest primordium (P1) is an auxin sink whereas the older primordium (P2, not shown) is too far from the meristem to affect the distribution of auxin, allowing a newly initiating organ (I1) to form opposite P1. (Redrawn from Reinhardt and Kuhlemeier, 2002).
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A characteristic feature of plant development is the extensive role played by cell-cell signalling in regulating patterns of growth and cell fate. This is particularly apparent in the shoot apical meristem (SAM) where signalling is involved in the maintenance of a central undifferentiated stem cell population and the formation of a regular and pred...
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Context 1
... to this model, auxin is actively transported to the shoot apex from basal regions where its accumu- lation leads to organ formation. Once established, young organ primordia function as auxin sinks, generating a zone of auxin depletion in adjacent regions of the meristem (see Figure 3). Auxin arriving from basal sources will only accumulate in the region furthest from existing primordia, leading to the initiation of a new primordium and the establishment of a new auxin sink. ...
Context 2
... the link between auxin and organ forma- tion, it has been proposed that the various types of phyllotaxy arise from different patterns of auxin accumulation in the shoot apex (see Figure 3; Reinhardt and Kuhlemeier, 2002). As organ for- mation is a dynamic process, changes in auxin distribution must occur rapidly. ...
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... Plant leaves and all other lateral organs are derived from the founder cells of the peripheral zone in the shoot apical meristem (SAM) (Golz 2006). The founder cells differentiate into leaf primordia and consequently develop into flat leaves after differentiation along the adaxial-abaxial, proximodistal, and mediolateral axes (Scanlon 2000;Nelson et al. 2002). ...
The leaf is the most important organ for photosynthesis; a moderately curly leaf is beneficial for improving the light energy utilization rate. Male sterility is another key trait that can facilitate hybrid seed production. Previously we have developed a mutant line JQA showing wrinkled leaves and male sterility in sesame (Sesamum indicum L.), which is an important genetic stock for the development of super variety with ideal plant type. Before it can be fully utilized in breeding practice, a better understanding of the cytological basis for leaf and anther development in JQA is required. In this study, we investigated the developmental processes in JQA at three cytological levels. Histological observation showed that the wrinkled leaf trait was expressed during the whole life cycle. Compared with normal leaves, the JQA wrinkled leaves have much fewer chloroplasts in the differentiated palisade tissue, which affect the photosynthesis efficiency by reducing the number of chloroplasts and delaying their development, and finally leads to leaf wrinkle. In addition, transmission electron microscope observation showed that most of the microspore mother cells in JQA could not divide evenly into dyads and tetrads during meiosis and that the tapetal cells could not disintegrate in time to provide adequate nutrition for the microspores. Consequently, pollens and pollen sacs were impeded or degraded due to a severe lack of nutrition. These findings will deepen our understanding of the anatomical formation of wrinkled leaves and anther abortion in sesame.
... In plants, cell groups remain in the shoot and root apexes, which consist of undifferentiated (stem) cells, and which correspond to a special type of cells emerged from embryonic stem cells. The regularity of cell events in these shoots and root meristems is much higher than in other cells, thus confirming the statement about deterministic behavior of stem (stem-like) cells (Theise and Harris, 2005;Golz, 2006;Mohn and Schubeler, 2009;Shapiro et al., 2015;Greb and Lohmann, 2016;Factor, 2017). ...
Along with a strict determinism of early embryogenesis in most living organisms, some of them exhibit variability of cell fates and developmental pathways. Here we discuss the phenomena of determinism and variability of developmental pathways, defining its dependence upon cell potency, cell sensitivity to the external signals and cell signaling. We propose a set of conjectures on the phenomenon of variability of developmental pathways, and denote a difference between a normal (local) variability, leading to an invariant final structure (e.g., embryo shape), and fundamental one, which is a switching between different developmental pathways, leading to different possible structures.
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... Mature miRNAs are then exported to the cytoplasm and incorporated into an RNA-induced silencing complex (RISC), which controls how complementary mRNAs undergo cleavage or translational repression (Li and Mao 2007). To date, the regulatory role of plant miRNAs has been exemplified by the critical regulatory behavior of miRNAs at key positions in a variety of pathways, such as root (Wang et al. 2005), shoot (Goiz 2006), leaf (Kidner and Martienssen 2004), and flower development (Mallory et al. 2004) and cell fate (Carraro et al. 2006). Heterochromatin has long been considered inert, but it is now believed to give rise to sRNAs, which, via RNA interference, direct the modification of proteins, DNA in heterochromatic repeats and transposable elements (Lippman and Martienssen 2004). ...
Supernumerary B chromosomes (Bs) are nonessential chromosomes that are considered genetically inert. However, the maize B carries control elements that direct its behavior, such as that of nondisjunction, during the second pollen mitosis, and affects normal A chromosomes during cell division. Recently, the maize B has been found to contain transcriptionally active sequences and to affect the transcription of genes on A chromosomes. To better understand the regulatory mechanisms underlying the maize B, we constructed two small RNA libraries from maize B73 inbred lines with and without Bs. The sequencing results revealed that 18 known microRNAs (miRNAs) were significantly differentially expressed in response to the presence of the B, and most target mRNAs were characterized as transcription factors. Moreover, three novel B-derived miRNAs were identified via stem-loop reverse transcriptase-polymerase chain reaction (RT-PCR)-based analysis, and all showed consistent B-specific expression in almost all analyzed inbred lines and in all tissue types, including leaves, roots, and pollen grains. By the use of B-10L translocations, the three B-derived miRNAs were mapped to specific B regions. The results from this study suggest that the maize B can express miRNAs and affect the expression of A-derived miRNAs, which could regulate the expression of A-located genes.
... miRNAs are reported to be involved in a broad range of metabolic and physiological processes in plants, such as growth (Jones-Rhoades et al., 2006), development (Rubio-Somoza and Weigel, 2011) and responses to various stresses (Khraiwesh et al., 2012). There gulatory role of miRNAs is exemplified by their critical regulatory behavior at key steps in a variety of pathways, such as root (Wang et al., 2005), shoot (Golz, 2006), leaf (Kidner and Martienssen, 2004), and flower (Teotia and Tang, 2015) development and cell fate (Carraro et al., 2006), and it is likely that their gene regulation function is as critical in maturing seeds as in other tissues (Martin et al., 2006). To date, researchers have been able to identify conserved and novel miRNAs in Arabidopsis (659) (Mallory et al., 2005;Reyes and Chua, 2007),rice (1500) (Xue et al., 2009;Zhang et al., 2012;Yi et al., 2013;Peng et al., 2014), maize (158) (Kang et al., 2012;Li et al., 2013), barley (101) (Curaba et al., 2012), wheat (1920) (Meng et al., 2013;Han et al., 2014), soybean(399) (Song et al., 2011) and rapeseed (90) (Xie et al., 2007;Korbes et al., 2012;Zhao et al., 2012) seeds. ...
Seed development has a critical role during the spermatophyte life cycle. In Brassica napus, a major oil crop, fatty acids are synthesized and stored in specific tissues during embryogenesis, and understanding the molecular mechanism underlying fatty acid biosynthesis during seed development is an important research goal. In this study, we constructed three small RNA libraries from early seeds at 14, 21, and 28 days after flowering (DAF) and used high-throughput sequencing to examine microRNA (miRNA) expression. A total of 85 known miRNAs from 30 families and 1160 novel miRNAs were identified, of which 24, including 5 known and 19 novel miRNAs, were found to be involved in fatty acid biosynthesis.bna-miR156b, bna-miR156c, bna-miR156g, novel_mir_1706, novel_mir_1407, novel_mir_173, and novel_mir_104 were significantly down-regulated at 21 DAF and 28 DAF, whereas bna-miR159, novel_mir_1081, novel_mir_19 and novel_mir_555 were significantly up-regulated. In addition, we found that some miRNAs regulate functional genes that are directly involved in fatty acid biosynthesis and that other miRNAs regulate the process of fatty acid biosynthesis by acting on a large number of transcription factors. The miRNAs and their corresponding predicted targets were partially validated by quantitative RT-PCR. Our data suggest that diverse and complex miRNAs are involved in the seed development process and that miRNAs play important roles in fatty acid biosynthesis during seed development.
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... Leaf initiation is taking place through the continuous proliferation of pluripotent cells in the SAM and the synchronous transition in the fate of a group of these pluripotent cells to determinate cells (Byrne 2012). The change in fate is associated with changes in gene expression and new patterns of cell division and expansion (Golz 2006). As these cells proliferate, new axes of growth are established lateral to the SAM resulting in an outgrowth (leaf primordium) from Chapter 1 the flanks of the SAM (Golz 2006). ...
... The change in fate is associated with changes in gene expression and new patterns of cell division and expansion (Golz 2006). As these cells proliferate, new axes of growth are established lateral to the SAM resulting in an outgrowth (leaf primordium) from Chapter 1 the flanks of the SAM (Golz 2006). The direction of this outgrowth and thus the positioning of the new leaf primordium on the SAM in relation to the earlier initiated primordia (i.e. ...
... Briefly, auxin once in the SAM, is absorbed by the existing developing primordia which are acting as auxin sinks depleting auxin from the surrounded tissue (Reinhardt et al. 2003). Therefore, auxin accumulates in the region of the SAM furthest from the previously formed primordia and, as a consequence, when auxin passes a critical threshold in this region a new primordium is initiated (Golz 2006). Consequently, leaf initiation and its spatial arrangement are determined by a complex signaling network between the SAM and the earlier initiated leaf primordia (Ha et al. 2010). ...
The initiation of new leaves, which takes place at the shoot apical meristem, is essential for plant growth and development. Leaf initiation rate (LIR) is very sensitive to meristem temperature. However, in practice meristem temperature is hardly ever monitored and air temperature is often used instead. It can be questioned whether relating LIR solely to air temperature is valid. This thesis aims at linking LIR to the aerial environment in two main horticultural crops: tomato and cucumber. It was shown that meristem temperature often differs from air temperature, depending on other environmental factors (e.g. radiation, humidity and wind speed) and species-specific traits. LIR was solely influenced by meristem temperature even when it largely deviated from air temperature. In addition, LIR was reduced at low light levels. Consequently, air temperature is not the whole story when relating leaf initiation to the environment.
... Although the mechanisms of RNAi initiated by miR- NAs are about the same as siRNAs, however, miRNAs are said to play an essential role in plant growth, development and stress response [17,18]. MicroRNA play a critical regulatory behaviour in root, leaf, flower and shoot development1819202122. Recent findings have demonstrated that majority of the plant miRNAs are conserved. ...
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In line with this, we have generated two small RNAs libraries from samples with contrasting lignin content using Illumina GAII sequencer. About 10 million sequence reads were obtained in secondary xylem of Am48 with high lignin content (41%) and a corresponding 14 million sequence reads were obtained in secondary xylem of Am54 with low lignin content (21%). Our results suggested that A. mangium small RNAs are composed of a set of 12 highly conserved miRNAs families found in plant miRNAs database, 82 novel miRNAs and a large proportion of non-conserved small RNAs with low expression levels. The predicted target genes of those differentially expressed conserved and non-conserved miRNAs include transcription factors associated with regulation of the lignin biosynthetic pathway genes. Some of these small RNAs play an important role in epigenetic silencing. Differential expression of the small RNAs between secondary xylem tissues with contrasting lignin content suggests that a cascade of miRNAs play an interconnected role in regulating the lignin biosynthetic pathway in Acacia species.
Our study critically demonstrated the roles of small RNAs during secondary wall formation. Comparison of the expression pattern of small RNAs between secondary xylem tissues with contrasting lignin content strongly indicated that small RNAs play a key regulatory role during lignin biosynthesis. Our analyses suggest an evolutionary mechanism for miRNA targets on the basis of the length of their 5' and 3' UTRs and their cellular roles. The results obtained can be used to better understand the roles of small RNAs during lignin biosynthesis and for the development of gene constructs for silencing of specific genes involved in monolignol biosynthesis with minimal effect on plant fitness and viability. For the first time, small RNAs were proven to play an important regulatory role during lignin biosynthesis in A. mangium.
... However, their broad expression patterns, extending in the meristem, indicate that the non-cell autonomous signaling present in Petunia may not be necessarily conserved in Arabidopsis. A second signal from organs to shoot further links organ polarity with meristem maintenance: ectopic expression of abaxial genes or repression of adaxial genes causes meristem arrest; conversely, overexpression of adaxial genes produces bigger meristem (reviewed in [55]). Recently, Goldshmidt et al. [53] demonstrated that the abaxially expressed YABBY gene non-autonomously repress Arabidopsis inflorescence meristem. ...
In multicellular organisms, the coordination of cell behaviors largely relies on biochemical and biophysical signals. Understanding how such signals control development is often challenging, because their distribution relies on the activity of individual cells and, in a feedback loop, on tissue behavior and geometry. This review focuses on one of the best-studied structures in biology, the shoot apical meristem (SAM). This tissue is responsible for the production of all the aerial parts of a plant. In the SAM, a population of stem cells continuously produces new cells that are incorporated in lateral organs, such as leaves, branches, and flowers. Organogenesis from stem cells involves a tight regulation of cell identity and patterning as well as large-scale morphogenetic events. The gene regulatory network controlling these processes is highly coordinated in space by various signals, such as plant hormones, peptides, intracellular mobile factors, and mechanical stresses. Many crosstalks and feedback loops interconnecting these pathways have emerged in the past 10 years. The plant hormone auxin and mechanical forces have received more attention recently and their role is more particularly detailed here. An integrated view of these signaling networks is also presented in order to help understanding how robust shape and patterning can emerge from these networks.
... In plants the target site shows near perfect complementarity to the miRNA sequence, and as a consequence most target mRNAs are cleaved by RISC, although there are examples where the translation of the mRNA is suppressed without a cleavage [8]. MicroRNAs' regulatory role has been exemplified by the critical regulatory behavior of miRNAs at key positions in a variety of pathways, such as root [9], shoot [10], leaf [11], flower development [12] and cell fate [13] . Additionally , they also include responses to phytohormones [14], nutrient [15] and other environmental stresses161718. ...
MicroRNAs (miRNAs) are a new class of endogenous small RNAs that play essential regulatory roles in plant growth, development and stress response. Extensive studies of miRNAs have been performed in model plants such as rice, Arabidopsis thaliana and other plants. However, the number of miRNAs discovered in maize is relatively low and little is known about miRNAs involved in the very early stage during seed germination.
In this study, a small RNA library from maize seed 24 hours after imbibition was sequenced by the Solexa technology. A total of 11,338,273 reads were obtained. 1,047,447 total reads representing 431 unique sRNAs matched to known maize miRNAs. Further analysis confirmed the authenticity of 115 known miRNAs belonging to 24 miRNA families and the discovery of 167 novel miRNAs in maize. Both the known and the novel miRNAs were confirmed by sequencing of a second small RNA library constructed the same way as the one used in the first sequencing. We also found 10 miRNAs that had not been reported in maize, but had been reported in other plant species. All novel sequences had not been earlier described in other plant species. In addition, seven miRNA* sequences were also obtained. Putative targets for 106 novel miRNAs were successfully predicted. Our results indicated that miRNA-mediated gene expression regulation is present in maize imbibed seed.
This study led to the confirmation of the authenticity of 115 known miRNAs and the discovery of 167 novel miRNAs in maize. Identification of novel miRNAs resulted in significant enrichment of the repertoire of maize miRNAs and provided insights into miRNA regulation of genes expressed in imbibed seed.
... What is the mechanism behind this regular spacing between primordia? Historical surgical experiments, consisting in removing an existing primordium, demonstrated that existing primordia determine future sites of organ initiation in adjacent regions of the SAM (for a review, see [20]). These results and others support a lateral inhibition model where developing organs and the centre of the meristem produce an inhibitory signal that acts locally to prevent organ formation. ...
Plant development is characterized by the continuous initiation of tissues and organs. The meristems, which are small stem cell populations, are involved in this process. The shoot apical meristem produces lateral organs at its flanks and generates the growing stem. These lateral organs are arranged in a stereotyped pattern called phyllotaxis. Organ initiation in the peripheral zone of the meristem involves accumulation of the plant hormone auxin. Auxin is transported in a polar way by influx and efflux carriers located at cell membranes. Polar localization of the PIN1 efflux carrier in meristematic cells generates auxin concentration gradients and PIN1 localization depends, in turn, on auxin gradients: this feedback loop generates a dynamic auxin distribution which controls phyllotaxis. Furthermore, PIN-dependent local auxin gradients represent a common module for organ initiation, in the shoot and in the root.
