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Four axes of development in seed plants. The apical-basal axis (1) of the plant represents a polarity established in embryogenesis with the shoot apical meristem being at the apical end and the root meristem residing at basal tip shown here in a longitudinal section of an Arabidopsis shoot (A). The other 3 axes depicted are defined with reference to this apical-basal axis. The central (ce)-peripheral (pe) axis (2) in the stem (B) is analogous to the adaxial (ad)-abaxial (ab) axis (4) of lateral organs (D) with the central/adaxial end being adjacent to the center of the shoot and the peripheral/abaxial end being away from the center of the shoot meristem (m). Lateral organs such as leaves also exhibit a proximal-distal axis (3) that can also be defined with respect to the stem, with the distal end away from the stem and the proximal end attached to the stem, as shown in the Acer leaf in C. Positions of tissues within vascular bundles develop with respect to the central-peripheral axis with the phloem (ph) positioned peripherally, the xylem (xy) positioned centrally in the stem (B), and the xylem positioned adaxially and phloem abaxially in lateral organs (E). In addition to polar differentiation of vascular tissues, leaf asymmetry is evident with palisade mesophyll (pm) differentiating adaxially and spongy mesophyll (sp) positioned abaxially, as shown in E, a longitudinal section of an Arabidopsis leaf.
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The bodies of seed plants are comprised of two classes of organs with contrasting growth and sym- metry attributes. Stems and roots are indeterminate organs that exhibit apical growth at apical meristems and radial growth at the vascular cambium, a pattern of growth that results in primarily radially symmetric organs. Lateral organs of the shoot, f...
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Context 1
... bodies of seed plants are comprised of two classes of organs with contrasting growth and symmetry attributes. Stems and roots are indeterminate organs that exhibit apical growth at apical meristems and radial growth at the vascular cambium, a pattern of growth that results in primarily radially symmetric organs. Lateral organs of the shoot, for example cotyledons, leaves, and floral organs, are determinate and exhibit localized planer growth resulting in breaking of radial symmetry and asymmetric development. Local- ized planer growth in the leaf generates the leaf blade, the principle site of photosynthesis in most plants. The seed plant body may be described with reference to defined axes of symmetry ( Fig. 1). The primar y axis of the seed plant body is the apical-basal axis. The apical-basal axis is established during embryogenesis and runs from the center of the primary shoot apical meristem to the center of the primary root apical meristem (Fig. 1A, Axis 1). Distinct organ types are generated at specific positions along the apical-basal axis (root meristem, root, hypocotyl, cotyledon, stem-internode, and shoot apical meristem). Radially symmetric organs possess a second axis, the central- peripheral axis, which extends from the apical-basal axis at the center of the organ outward to the organ epidermis (Fig. 1, A and B, Axis 2). Asymmetric development along the central-peripheral axis is evident in the pattern of vascular tissue placement, with xylem developing in closer proximity to the apical- basal axis relative to phloem (Fig. 1B). Lateral organs of the shoot arise from the flanks of shoot meristems and consequently possess an intrinsic positional relationship with the meristem from which they are derived. Two axes of lateral organs, the proximal-distal (Fig. 1C, Axis 3) and adaxial-abaxial (Fig. 1D, Axis 4) axes, may be readily defined with respect to the position of the lateral organ in relationship to the meristem from which the organ is derived. Both these axes are exploited as references for the regulation of asymmetrical development. The proximal-distal axis runs from the base of the organ (proximal, i.e. nearest to the meristem) to the tip (distal, i.e. furthest from the meristem; Fig. 1C). Asymmetric development along the proximal-distal axis is most evident in leaves, in that the amount of lateral growth frequently correlates with location along the proximal-distal axis in a manner that is highly predictable for a given species. The adaxial- abaxial axis runs from the surface of the lateral organ closest to the meristem (ad-adjacent) to the surface of the organ furthest from the meristem (ab-away; Fig. 1D). Asymmetric development in the adaxial-abaxial axis is most evident in many species in the leaf blade, where the adaxial epidermis and underlying mesophyll differentiate to develop characteristics special- ized for efficient light capture, while the abaxial epidermis and mesophyll develop characteristics spe- cialized for efficient gas exchange (Fig. 1E). Proper asymmetric development along the adaxial-abaxial axis is therefore often critical for development of a leaf architecture optimized for photosynthesis. It should be noted that the central-peripheral axis of radially symmetric organs and the adaxial-abaxial axis of shoot lateral organs are equivalent. Both axes are defined with respect to the same reference, the apical- basal axis at the center of the shoot meristem. More- over, the pattern of vascular tissue placement in lateral organs (xylem-adaxial, phloem-abaxial; Fig. 1E) is equivalent to that in radially symmetric organs (xylem- central, phloem-peripheral; Fig. 1B). The ultimate basis of asymmetrical development in all multicellular organisms is the ability of cells to interpret their position with respect to an external reference and transduce this information into asymmetric patterns of cellular differentiation, a process termed polarity establishment. The positional relationship of lateral organs to the meristem from which they are derived indicates that the meristem could in principle serve as the reference for polarity establishment in lateral organs. That this is in fact the case is demonstrated by surgical experiments performed roughly 50 years ago (Warlaw, 1949; Sussex, 1955; Snow and Snow, 1959). When an incision is placed between the meristem and the site where a lateral organ primordia will next emerge (the P0 site), a lateral organ primordia emerges but develops as a radially symmetrical structure with apparently abaxial characteristics. Several tentative conclusions may be drawn from this simple experiment. First, some form of signal emanating from the meristem, transmission of which is interrupted by the presence of the incision, is required for interpretation of the adaxial-abaxial axis. Second, adaxial-abaxial polarity establishment is necessary for blade growth. Third, in the absence of adaxial-abaxial polarity establishment, abaxial identity may be the default state. The surgical experiments provide a conceptual framework in which the results of subsequent genetic analyses of polarity establishment may be interpreted. Beginning in 1995 with characterization of the role of PHANTASTICA ( PHAN ) in promoting adaxial cell fate in Antirrhinum, the genetic basis of adaxial-abaxial polarity establishment in lateral organs has begun to be elucidated in Antirrhinum, maize, and Arabidopsis (Waites and Hudson, 1995; Tsiantis et al., 1999; Bowman et al., 2002). Results of these studies largely reinforce the conclusions derived from the early surgical experiments. Genetic studies in Arabidopsis have identified several families of genes that play a role in promoting proper adaxial-abaxial development (Table I; for review, see Bowman et al., 2002). The class III HD- Zip genes, in particular the PHABULOSA ( PHB ), PHAVOLUTA ( PHV ), and REVOLUTA ( REV ) genes (col- lectively referred to as PHAB genes) promote adaxial identity and meristem maintenance (McConnell and Barton, 1998; McConnell et al., 2001; Emery et al., 2003). The KANADI1-3 ( KAN1-3 ) genes and members of the YABBY gene family promote abaxial identity (Eshed et al., 1999, 2001; Sawa et al., 1999; Siegfried et al., 1999; Kerstetter et al., 2001). Members of all three families are believed on the basis of ...
Context 2
... bodies of seed plants are comprised of two classes of organs with contrasting growth and symmetry attributes. Stems and roots are indeterminate organs that exhibit apical growth at apical meristems and radial growth at the vascular cambium, a pattern of growth that results in primarily radially symmetric organs. Lateral organs of the shoot, for example cotyledons, leaves, and floral organs, are determinate and exhibit localized planer growth resulting in breaking of radial symmetry and asymmetric development. Local- ized planer growth in the leaf generates the leaf blade, the principle site of photosynthesis in most plants. The seed plant body may be described with reference to defined axes of symmetry ( Fig. 1). The primar y axis of the seed plant body is the apical-basal axis. The apical-basal axis is established during embryogenesis and runs from the center of the primary shoot apical meristem to the center of the primary root apical meristem (Fig. 1A, Axis 1). Distinct organ types are generated at specific positions along the apical-basal axis (root meristem, root, hypocotyl, cotyledon, stem-internode, and shoot apical meristem). Radially symmetric organs possess a second axis, the central- peripheral axis, which extends from the apical-basal axis at the center of the organ outward to the organ epidermis (Fig. 1, A and B, Axis 2). Asymmetric development along the central-peripheral axis is evident in the pattern of vascular tissue placement, with xylem developing in closer proximity to the apical- basal axis relative to phloem (Fig. 1B). Lateral organs of the shoot arise from the flanks of shoot meristems and consequently possess an intrinsic positional relationship with the meristem from which they are derived. Two axes of lateral organs, the proximal-distal (Fig. 1C, Axis 3) and adaxial-abaxial (Fig. 1D, Axis 4) axes, may be readily defined with respect to the position of the lateral organ in relationship to the meristem from which the organ is derived. Both these axes are exploited as references for the regulation of asymmetrical development. The proximal-distal axis runs from the base of the organ (proximal, i.e. nearest to the meristem) to the tip (distal, i.e. furthest from the meristem; Fig. 1C). Asymmetric development along the proximal-distal axis is most evident in leaves, in that the amount of lateral growth frequently correlates with location along the proximal-distal axis in a manner that is highly predictable for a given species. The adaxial- abaxial axis runs from the surface of the lateral organ closest to the meristem (ad-adjacent) to the surface of the organ furthest from the meristem (ab-away; Fig. 1D). Asymmetric development in the adaxial-abaxial axis is most evident in many species in the leaf blade, where the adaxial epidermis and underlying mesophyll differentiate to develop characteristics special- ized for efficient light capture, while the abaxial epidermis and mesophyll develop characteristics spe- cialized for efficient gas exchange (Fig. 1E). Proper asymmetric development along the adaxial-abaxial axis is therefore often critical for development of a leaf architecture optimized for photosynthesis. It should be noted that the central-peripheral axis of radially symmetric organs and the adaxial-abaxial axis of shoot lateral organs are equivalent. Both axes are defined with respect to the same reference, the apical- basal axis at the center of the shoot meristem. More- over, the pattern of vascular tissue placement in lateral organs (xylem-adaxial, phloem-abaxial; Fig. 1E) is equivalent to that in radially symmetric organs (xylem- central, phloem-peripheral; Fig. 1B). The ultimate basis of asymmetrical development in all multicellular organisms is the ability of cells to interpret their position with respect to an external reference and transduce this information into asymmetric patterns of cellular differentiation, a process termed polarity establishment. The positional relationship of lateral organs to the meristem from which they are derived indicates that the meristem could in principle serve as the reference for polarity establishment in lateral organs. That this is in fact the case is demonstrated by surgical experiments performed roughly 50 years ago (Warlaw, 1949; Sussex, 1955; Snow and Snow, 1959). When an incision is placed between the meristem and the site where a lateral organ primordia will next emerge (the P0 site), a lateral organ primordia emerges but develops as a radially symmetrical structure with apparently abaxial characteristics. Several tentative conclusions may be drawn from this simple experiment. First, some form of signal emanating from the meristem, transmission of which is interrupted by the presence of the incision, is required for interpretation of the adaxial-abaxial axis. Second, adaxial-abaxial polarity establishment is necessary for blade growth. Third, in the absence of adaxial-abaxial polarity establishment, abaxial identity may be the default state. The surgical experiments provide a conceptual framework in which the results of subsequent genetic analyses of polarity establishment may be interpreted. Beginning in 1995 with characterization of the role of PHANTASTICA ( PHAN ) in promoting adaxial cell fate in Antirrhinum, the genetic basis of adaxial-abaxial polarity establishment in lateral organs has begun to be elucidated in Antirrhinum, maize, and Arabidopsis (Waites and Hudson, 1995; Tsiantis et al., 1999; Bowman et al., 2002). Results of these studies largely reinforce the conclusions derived from the early surgical experiments. Genetic studies in Arabidopsis have identified several families of genes that play a role in ...
Citations
... On the other hand, radial tissue organization in Arabidopsis roots is modulated by the interplay between miR165/miR166 regulatory hubs and auxin and abscisic acid (ABA) (Fig. 2; Carlsbecker et al., 2010;Ursache et al., 2014;Ramachandran et al., 2018;Bloch et al., 2019). The miR165/miR166 family regulates the class III homeodomain zipper (HD-ZIPIII) transcription factors, known to control the expression of several auxin biosynthesis, transport, and response genes (reviewed in Turchi et al., 2015), and are involved in the adaxial identity of lateral organs as well as meristem and vascular development (Engstrom et al., 2004;Williams et al., 2005;Nogueira et al., 2009). In the root, HD-ZIPIII genes are largely restricted to the vascular cylinder due to production of miR165/miR166 in the endodermis. ...
The root system commonly lies underground, where it provides anchorage to the aerial organs, as well as nutrients and water. Both endogenous and environmental cues contribute to the establishment of the root system. Among the endogenous cues, microRNAs (miRNAs), transcription factors and phytohormones modulate root architecture. miRNAs belong to a subset of endogenous hairpin-derived small RNAs that post-transcriptionally control target gene expression, mostly transcription factors, comprising the miRNA regulatory hubs. Phytohormones are signaling molecules involved in most developmental processes. Some miRNAs and targets participate in more than one hormonal pathway, thereby providing new bridges in plant hormonal crosstalk. Unraveling the intricate network of molecular mechanisms underlying the establishment of root systems is a central aspect in the development of novel strategies for plant breeding to increase yield and optimize agricultural land use. In this review, we summarize recent findings describing the molecular mechanisms associated with the interplay between miRNAs regulatory hubs and phytohormones to ensure the establishment of a proper root system. We focus on postembryonic growth and development of primary, lateral, and adventitious roots. In addition, we discuss novel insights for future research on the interaction between miRNAs and phytohormones in root architecture.
... Meristems are radially symmetrical with a central/peripheral axis. During initiation of determinate lateral organs of the shoot, including leaves and floral organs, radial symmetry is broken with the onset of asymmetric patterning (2). The localized planar growth of lateral organs is derived from their positional relationship to the meristem (3)(4)(5) in which the adaxial surface faces the meristem while the abaxial surface faces away from the meristem (2). ...
... During initiation of determinate lateral organs of the shoot, including leaves and floral organs, radial symmetry is broken with the onset of asymmetric patterning (2). The localized planar growth of lateral organs is derived from their positional relationship to the meristem (3)(4)(5) in which the adaxial surface faces the meristem while the abaxial surface faces away from the meristem (2). Thus, the central/peripheral axis continues into the adaxial/abaxial axis of shoot lateral organs (2). ...
... The localized planar growth of lateral organs is derived from their positional relationship to the meristem (3)(4)(5) in which the adaxial surface faces the meristem while the abaxial surface faces away from the meristem (2). Thus, the central/peripheral axis continues into the adaxial/abaxial axis of shoot lateral organs (2). ...
Significance
The maize ear is unbranched and terminates in a single point. The ear and tassel inflorescences of Fascicled ear mutants fail to grow as a single point and instead are branched. This phenotype results from the misexpression of duplicated transcription factors, ZMM8 and DRL2. We hypothesize that these gene rearrangements create regulatory sequences that cause misexpression in early inflorescence meristems, thus activating a laminar program, ablating the meristem, and producing branches. This work demonstrates that zmm8 and drl2 must be restricted from the inflorescence meristem to maintain its terminal point, and conversely, a mechanism by which branching may be imposed. Manipulation of these genes can be used to alter plant architecture, potentially to improve agronomic traits.
... These leaf primordia are separated from the main shoot through the establishment of a boundary between the organ-forming cells and the meristem cells that provides a permissive environment for differentiation. During the final stages of leaf development, cell and tissue specification occurs through coordinated processes of cell division, expansion, and differentiation along the proximal-distal, adaxial-abaxial and central-lateral axes 29 (Waites & Hudson, 1995;Engstrom et al., 2004). Dependent on the species-specific genetic program and environmental conditions, leaf primordia develop either into simple leaves or into compound (complex) leaves. ...
The Medicago truncatula NODULE-ROOT, the Arabidopsis thaliana BLADE-ON-PETIOLE, and the Pisum sativum COCHLEATA genes are members of a highly conserved NOOT-BOP-COCH-LIKE1 (NBCL1) specific clade that belongs to the NON-EXPRESSOR OF PATHOGENESIS RELATED PROTEIN1 LIKE gene family. In legumes, the members of this NBCL1 clade are known as key regulators of the symbiotic organ identity. The members of the NBCL2 clade (MtNOOT2) also play a key role in the establishment and maintenance of the symbiotic nodule identity, redundantly with NBCL1 while without significant phenotype alone. These NBCL plant genes were also shown to be involved in abscission. In addition, NBCL genes are also conserved in monocotyledon plants in which they also control different aspects of development. The present thesis work aims to better understand the roles of the NBCL1 and NBCL2 genes in development in both legume and Brachypodium plants and to discover new molecular actors involved in the NBCL1-dependent regulation of the nodule identity using novel TILLING and Tnt1 insertional mutants in two legume species, Medicago, and Pisum. In addition we used CRISPR knock-out mutations in Brachypodium to better understand their roles in monocotyledon plants. This thesis work unraveled new functions of the NBCL1 genes in plant shoot development and plant architecture. We also revealed that the members of the legume-specific NBCL2 redundantly function with NBCL1 sub-clade and play important roles in leaf, stipule, inflorescence and flower development. In addition we showed a role in nodule development, identity establishment and maintenance, and consequently in the success and efficiency of the symbiotic association. In this thesis, we also explored the roles of the highly conserved NBCL genes, BdUNICULME4 and BdLAXATUM-A, in the development of B. distachyon using double mutants. We confirmed previous results and reveal a new function for these two genes in plant architecture, ligule and inflorescence formation, and also lignin content. This thesis work has finally allowed the identification and the characterization of new mutants for M. truncatula ALOG (Arabidopsis LSH1 and Oryza G1) genes. ALOG proteins are potential interacting partners for NBCL. We showed that some ALOG members play important roles in nodule and aerial organ development. Altogether, this thesis work suggests that during evolution, the nodule developmental program was recruited from pre-existing regulatory programs for nodule development and identity.
... Moreover, the expression of MIR165/166 was repressed by HD-ZIPIIIs acting together with HD-ZIPIIs (Weigel et al., 1992). MIR165 and MIR166 have different potential targets, but it is not yet clear whether these interactions are specific because there is only a single nucleotide difference between the two microRNAs (Engstrom et al., 2004). Dominant HD-ZIPIII alleles are from mutations abrogating the MIR165/166 target site, leading to ectopic expression of mutant transcripts in the abaxial domain and the formation of adaxialized leaves (McConnell et al., 2001;Emery et al., 2003;Juarez et al., 2004;Zhong and Ye, 2004;Prigge et al., 2005;Ochando et al., 2006). ...
Leaves are derived from shoot apical meristem with three distinct dorsoventral, proximodistal and mediolateral axes. Different regulators are involved in the establishment of leaf polarity. Members of the class III homeodomain‐leucine zipper (HD‐ZIPIII) gene family are critical players in the determination of leaf adaxial identity mediated by microRNA165/166. However, their roles in compound leaf development are still unclear. By screening of a retrotransposon‐tagged mutant population of the model legume plant Medicago truncatula, a mutant line with altered leaflet numbers was isolated and characterized. Mutant leaves partially lost their adaxial identity. Leaflet numbers in the mutant were increased along the proximodistal axis, showing pinnate pentafoliata leaves in most cases, in contrast to the trifoliate leaves of wild type. Detailed characterization revealed that a lesion in a HD‐ZIPIII gene, REVOLUTA (MtREV1), resulted in the defects of the mutant. Overexpression of MtMIR166‐insensitive MtREV1 led to adaxialized leaves and ectopic leaflets along the dorsoventral axis. Accompanying the abnormal leaf patterning, the content of free auxin was affected. Our results demonstrated that MtREV1 plays a key role in determination of leaf adaxial‐abaxial polarity and compound leaf patterning, which is associated with proper auxin homeostasis. This article is protected by copyright. All rights reserved.
... The shape and architecture of leaf need the orchestration of auxin, KNOX genes and miRNA regulation. KNOX genes could be down-regulated by CUC transcriptional regulators, which are important for organ boundaries building (Takada and Tasaka, 2002;Chen, 2009), floral patterning, and leaf morphogenesis (Micol and Hake, 2003;Engstrom et al., 2004). NAC (NAM, CUC1/2-like) is one branch of CUC gene family regulated by miR164. ...
Tea tree [Camellia sinensis (L.) O. Kuntze] is an important leaf (sometimes tender stem)-using commercial plant with many medicinal uses. The development of newly sprouts would directly affect the yield and quality of tea product, especially significant for Pingyang Tezaocha (PYTZ) which takes up a large percent in the early spring tea market. MicroRNA (miRNA), particularly the conserved miRNAs, often position in the center of subtle and complex gene regulatory systems, precisely control the biological processes together with other factors in a spatio-temporal pattern. Here, quality-determined metabolites catechins, theanine and caffeine in PYTZ sprouts including buds (sBud), different development stages of leaves (sL1, sL2) and stems (sS1, sS2) were quantified. A total of 15 miRNA libraries of the same tissue with three repetitions for each were constructed to explore vital miRNAs during the biological processes of development and quality formation. We analyzed the whole miRNA profiles during the sprout development and defined conserved miRNA families in the tea plant. The differentially expressed miRNAs related to the expression profiles buds, leaves, and stems development stages were described. Twenty one miRNAs and eight miRNA-TF pairs that most likely to participate in regulating development, and at least two miRNA-TF-metabolite triplets that participate in both development and quality formation had been filtered. Our results indicated that conserved miRNA act boldly during important biological processes, they are (i) more likely to be linked with morphological function in primary metabolism during sprout development, and (ii) hold an important position in secondary metabolism during quality formation in tea plant, also (iii) coordinate with transcription factors in forming networks of complex multicellular organism regulation.
... Hong et al., 2003), 'promoter-bashing' reporter analysis (e.g. Engstrom et al., 2004), and/or through genome-wide techniques that facilitate the analysis of the GRNs (e.g. Mateos et al., 2017). ...
During seed development, carbon is reallocated from maternal tissues to support germination and subsequent growth. As this pool of resources is depleted post-germination, the plant begins autotrophic growth through leaf photosynthesis. Photoassimilates derived from the leaf are used to sustain the plant and form new organs, including other vegetative leaves, stems, bracts, flowers, fruits, and seeds. In contrast to the view that reproductive tissues act only as resource sinks, many studies demonstrate that flowers, fruits, and seeds are photosynthetically active. The photosynthetic contribution to development is variable between these reproductive organs and between species. In addition, our understanding of the developmental control of photosynthetic activity in reproductive organs is vastly incomplete. A further complication is that reproductive organ photosynthesis (ROP) appears to be particularly important under suboptimal growth conditions. Therefore, the topic of ROP presents the community with a challenge to integrate the fields of photosynthesis, development, and stress responses. Here, we attempt to summarize our understanding of the contribution of ROP to development and the molecular mechanisms underlying its control.
... Among the genes showing altered expression, HST (AK101049), NF-YB (AK241920), ATS3 (AK067237), FIE2 (AK24220), LTP (AK242537), PPROL17 (AK242325), CRA1 (AK107343), OsGRP1 (AK288031), GASA2 (AK110640), RPM1 (AK100303) and the NBS-LRR family (Os.92013) were involved in polarity specification of the adaxial/abaxial axis, as well as embryonic and seed development [38][39][40][41][42][43][44][45][46][47]. ...
... Consistent with the results from molecular and morphological analyses, some changes of gene expression appeared to be associated with the developmental defects visualized in fst mutant and FST-RNAi repression lines. For instance, remarkably down-regulated expression of genes observed in auxin efflux, GA signaling, or other cellular processes, such as PCD, polarity and cell fate determination [38][39][40][47][48][49][50], may influence chalaza positioning and integument formation, and subsequently result in defective zygotic embryo and endosperm development. ...
Many homeotic MADS-box genes have been identified as controllers of the floral transition and floral development. However, information regarding Bsister (Bs)-function genes in monocots is still limited. Here, we describe the functional characterization of a Bs-group MADS-box gene FEMALE-STERILE (FST), whose frame-shift mutation (fst) results in abnormal ovules and the complete abortion of zygotic embryos and endosperms in rice. Anatomical analysis showed that the defective development in the fst mutant exclusively occurred in sporophytic tissues including integuments, fertilized proembryos and endosperms. Analyses of the spatio-temporal expression pattern revealed that the prominent FST gene products accumulated in the inner integument, nucellar cell of the micropylar side, apical and base of the proembryos and free endosperm nuclei. Microarray and gene ontology analysis unraveled substantial changes in the expression level of many genes in the fst mutant ovules and seeds, with a subset of genes involved in several developmental and hormonal pathways appearing to be down-regulated. Using both forward and reverse genetics approaches, we demonstrated that rice FST plays indispensable roles and multiple functions during ovule and early seed development. These findings support a novel function for the Bs-group MADS-box genes in plants.
... Mutation of multiple PLTs results in rootless seedlings and embryo lethality; importantly, PLT overexpression induces formation of ectopic root meristems, indicating that PLTs have a master regulatory role in embryonic root meristem formation (Aida et al., 2004;Galinha et al., 2007). HD-ZIPIII proteins participate in various shoot developmental processes such as specification of the central domain of the shoot apical meristem and lateral organ polarity (reviewed in Engstrom et al., 2004). Similar to the PLT genes, only mutation of multiple HD-ZIPIII genes results in defective embryogenesis (Prigge et al., 2005). ...
Development of multicellular organisms requires specification of diverse cell types. In plants, development is continuous and because plant cells are surrounded by rigid cell walls, cell division and specification of daughter cell fate must be carefully orchestrated. During embryonic and postembryonic plant development, the specification of cell types is determined both by positional cues and cell lineage. The establishment of distinct transcriptional domains is a fundamental mechanism for determining different cell fates. In this review, we focus on four examples from recent literature of switches operating in cell fate decisions that are regulated by transcriptional mechanisms. First, we highlight a transcriptional mechanism involving a mobile transcription factor in formation of the two ground tissue cell types in roots. Specification of vascular cell types is then discussed, including new details about xylem cell-type specification via a mobile microRNA. Next, transcriptional regulation of two key embryonic developmental events is considered: establishment of apical-basal polarity in the single-celled zygote and specification of distinct root and shoot stem cell populations in the plant embryo. Finally, a dynamic transcriptional mechanism for lateral organ positioning that integrates spatial and temporal information into a repeating pattern is summarized.
... Members of three classes of transcription factors contribute to the establishment of adaxial and abaxial cell fates in lateral organs of Arabidopsis (Arabidopsis thaliana; for review, see Engstrom et al. 2004). A number of studies have recently identified several genes that play important roles in establishing leaf ab-/adaxial polarity. ...
ASYMMETRIC LEAVES2-LIKE5/LATERAL ORGAN BOUNDARIES DOMAIN 12 (ASL5/LBD12), isolated from Arabidopsis, is a member of the LATERAL ORGAN BOUNDARIES (LOB) domain gene family. Previously, it has been reported that the dominant peacock1-D (pck1-D) mutant, wherein an ASL5 gene activated by a T-DNA lost apical dominance, had epinastic or abaxial leaves and was sterile; this suggested that ASL5 gene might be involved in ab-/adaxial determination of leaves or in the development of shoot apical meristems (SAMs), but no evidence for this has been provided hitherto. In this study, 35S:AtASL5-GFP transgenic cockscomb plants were obtained. In leaf epidermal cells, the AtASL5-GFP fusion protein displayed nuclear localization suggesting that AtASL5 might be a potential transcription factor. The 35S:AtASL5 transgenic cockscombs have dramatically altered phenotype. Histological analysis of petioles, leaf blades and lateral roots shows that these lateral organs have vascular-pattern modifications, suggesting the leaves might have ab-/adaxial defect, and the lateral roots have a central-peripheral defect. Moreover, ectopic protuberances were observed in transgenic plants, indicated that AtASL5 might have function in development of shoot apical meristem.
... The adaxial–abaxial, proximal–distal and medial–lateral axes are established at early stages of leaf organogenesis (Tsukaya 2006; Kidner and Timmermans 2007). Some genes that participate in adaxial– abaxial patterning encode members of the YABBY (YAB), KANADI (KAN) and class III homeodomain/leucine zipper (HD-ZIPIII) families of transcription factors (Siegfried et al. 1999; Emery et al. 2003; Engstrom et al. 2004; Eshed et al. 2004; Byrne 2005 ). Mutations in these genes disrupt leaf adaxial–abaxial polarity: leaf abaxialization is seen in double and triple kan mutant combinations (Eshed et al. 2001; Kerstetter et al. 2001; Emery et al. 2003; Engstrom et al. 2004; Byrne 2005), and adaxialization occurs in gain-of-function alleles of HD-ZIPIII genes (Sessa et al. 1993; Talbert et al. 1995; McConnell and Barton 1998; Zhong et al. 1999; McConnell et al. 2001; Otsuga et al. 2001; Ochando et al. 2006). ...
... Some genes that participate in adaxial– abaxial patterning encode members of the YABBY (YAB), KANADI (KAN) and class III homeodomain/leucine zipper (HD-ZIPIII) families of transcription factors (Siegfried et al. 1999; Emery et al. 2003; Engstrom et al. 2004; Eshed et al. 2004; Byrne 2005 ). Mutations in these genes disrupt leaf adaxial–abaxial polarity: leaf abaxialization is seen in double and triple kan mutant combinations (Eshed et al. 2001; Kerstetter et al. 2001; Emery et al. 2003; Engstrom et al. 2004; Byrne 2005), and adaxialization occurs in gain-of-function alleles of HD-ZIPIII genes (Sessa et al. 1993; Talbert et al. 1995; McConnell and Barton 1998; Zhong et al. 1999; McConnell et al. 2001; Otsuga et al. 2001; Ochando et al. 2006). In contrast, disruption of proximal–distal polarity in the leaves of Arabidopsis is a less common phenotype, and has been documented in the literature only a few times. ...
We isolated Arabidopsis thaliana mutants with incurved vegetative leaves. Positional cloning of incurvata8 (icu8), icu9 and icu15 has identified them as new loss-of-function alleles of the HYPONASTIC LEAVES1 (HYL1), ARGONAUTE1 (AGO1) and HUA ENHANCER1 (HEN1) genes, respectively, which encode known components of the microRNA pathway. The morphological and histological characterization of these mutants and of dicer-like1-9 indicates that small RNAs participate in the proximal-distal and adaxial-abaxial patterning of leaves, as well as in stomatal number establishment. The abnormal vasculature of ago1 and hyl1 leaves also suggests a role for AGO1 and HYL1 in venation patterning. Our mutants expand the allelic series of AGO1, HYL1 and HEN1, and might help to understand the developmental and cellular significance of miRNA-mediated posttranscriptional regulation.
