Feedback from Lateral Organs Controls Shoot Apical Meristem Growth by Modulating Auxin Transport

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Cell Differentiation and Organ Initiation at the Shoot Apical Meristem

Plants continuously generate organs at the flanks of their shoot apical meristems (SAMs). The patterns in which these organs are initiated, also called patterns of phyllotaxis, are highly stereotypic and characteristic for a particular species or developmental stage. This stable, predictable behaviour of the meristem has led to the idea that organ initiation must be based on simple and robust mechanisms. This conclusion is less evident, however, if we consider the very dynamic behaviour of the individual cells. How dynamic cellular events are coordinated and how they are linked to the regular patterns of organ initiation is a major issue in plant developmental biology.     

ERECTA Family Genes Regulate Auxin Transport in the Shoot Apical Meristem and Forming Leaf Primordia

Leaves are produced postembryonically at the flanks of the shoot apical meristem. Their initiation is induced by a positive feedback loop between auxin and its transporter PIN-FORMED1 (PIN1). The expression and polarity of PIN1 in the shoot apical meristem is thought to be regulated primarily by auxin concentration and flow. The formation of an auxin maximum in the L1 layer of the meristem is the first sign of leaf initiation and is promptly followed by auxin flow into the inner tissues, formation of the midvein, and appearance of the primordium bulge. The ERECTA family genes (ERfs) encode leucine-rich repeat receptor-like kinases, and in Arabidopsis (Arabidopsis thaliana), this gene family consists of ERECTA (ER), ERECTA-LIKE1 (ERL1), and ERL2. Here, we show that ERfs regulate auxin transport during leaf initiation. The shoot apical meristem of the er erl1 erl2 triple mutant produces leaf primordia at a significantly reduced rate and with altered phyllotaxy. This phenotype is likely due to deficiencies in auxin transport in the shoot apex, as judged by altered expression of PIN1, the auxin reporter DR5rev::GFP, and the auxin-inducible genes MONOPTEROS, INDOLE-3-ACETIC ACID INDUCIBLE1 (IAA1), and IAA19. In er erl1 erl2, auxin presumably accumulates in the L1 layer of the meristem, unable to flow into the vasculature of a hypocotyl. Our data demonstrate that ERfs are essential for PIN1 expression in the forming midvein of future leaf primordia and in the vasculature of emerging leaves.Leaves are formed during postembryonic development by the shoot apical meristem (SAM), a dome-shaped organ with a stem cell reservoir at the top and with leaf initiation taking place slightly below in the peripheral zone. The initiation of leaf primordia depends on the establishment of auxin maxima at the site of initiation (Braybrook and Kuhlemeier, 2010). Auxin is polarly transported through the epidermal layer of the meristem to the incipient primordium initiation site (Heisler et al., 2005) and then moves inward, where it promotes the formation of a vascular strand (Scarpella et al., 2006; Bayer et al., 2009). The developing vascular tissue acts as an auxin sink, depleting auxin in the epidermal layer (Scarpella et al., 2006). PIN1, an auxin efflux protein, is a central player in the formation of auxin maxima and is involved in the transport of auxin in both the epidermis and the forming vascular strand during leaf initiation (Benková et al., 2003; Reinhardt et al., 2003). PIN1 is the earliest marker for midvein formation (Scarpella et al., 2006), which starts to form before a leaf primordium bulges out of the meristem. The mechanisms determining PIN1 expression and polar localization in the SAM are central to understanding leaf initiation. In the L1 layer of the SAM, PIN1 is polarly localized in the plasma membrane toward cells with higher auxin concentration (Jönsson et al., 2006; Smith et al., 2006). Formation of the vein is explained by the canalization hypothesis, in which high auxin flux reinforces PIN1 expression (Kramer, 2008). Of all plasma membrane-localized PIN family transporters, only PIN1 has been detected in the vegetative SAM and linked with the initiation of rosette leaves (Guenot et al., 2012). At the same time, rosette leaves are positioned nonrandomly in pin1 mutants, suggesting that additional PIN1-independent mechanisms also have a role in regulating leaf initiation (Guenot et al., 2012).Here, we investigate the role of ERECTA family receptor-like kinases during leaf initiation in Arabidopsis (Arabidopsis thaliana). Previously, ERECTA family genes (ERfs) have been shown to be involved in the regulation of epidermis development and of plant growth along the apical-basal/proximodistal axis in aboveground organs (Torii et al., 1996; Shpak et al., 2004, 2005). Triple erecta (er), erecta-like1 (erl1), and erl2 mutants (er erl1 erl2) form a rosette with small, round leaves that lack petiole elongation. During the reproductive stage, the main inflorescence stem exhibits striking elongation defects and reduced apical dominance. ER has been implicated in vascular development, with the er mutation causing radial expansion of xylem (Ragni et al., 2011) and premature differentiation of vascular bundles (Douglas and Riggs, 2005). Recently, the dwarfism of described mutants was attributed to the function of ERf genes in the phloem, where they perceive signals from the endodermis (Uchida et al., 2012a). In the epidermis, all three genes inhibit the initial decision of protodermal cells to become meristemoid mother cells (Shpak et al., 2005). In addition, ERL1 and to a lesser extent ERL2 are important for maintaining cell proliferative activity in stomata lineage cells and for preventing terminal differentiation of meristemoids into guard mother cells. The activity of ERf receptors in the epidermis is regulated by a different set of peptides than in the phloem. EPIDERMAL PATTERNING FACTOR1 (EPF1) and EPF2 are expressed in stomatal precursor cells. They inhibit the development of new stomata in the vicinity of a forming stoma (Hara et al., 2007, 2009; Hunt and Gray, 2009). EPF-LIKE9 (EPFL9)/stomagen is expressed in the mesophyll, and, in contrast, it promotes the development of stomata (Kondo et al., 2010; Sugano et al., 2010). EPFL4 and EPFL6/CHALLAH are expressed in the endodermis, and their perception by phloem-localized ERfs is critical for stem elongation (Uchida et al., 2012a).While ERfs are very strongly expressed in the vegetative SAM and in forming leaf primordia, only recently has it become clear that these genes are involved in the regulation of meristem size and leaf initiation (Uchida et al., 2012b, 2013). It was suggested that ERfs regulate stem cell homeostasis in the SAM via buffering its cytokinin responsiveness by an unknown mechanism (Uchida et al., 2013). Here, we further investigate the involvement of ERfs in the control of leaf initiation and phyllotaxy. Our data suggest that ERfs are essential for PIN1 expression in the vasculature of forming leaf primordia. Based on analysis of the DR5rev::GFP reporter, auxin may accumulate in the L1 layer of the SAM in the mutant but is not able to move into the vasculature, consistent with drastically reduced PIN1pro:PIN1-GFP expression there. These data suggest that the convergence of PIN1 expression in the inner tissues of the SAM during leaf initiation is a complex process involving intercellular communications enabled by ERfs. The importance of ERfs for efficient auxin transport is further supported by reduced phototropic response in the er erl1 erl2 mutant.     

植物茎尖分生组织分化调控机制研究进展

茎尖分生组织是位于植物顶端具有持续分化能力的组织,通过细胞分裂、分化产生茎、叶和花等器官,形成植株地上部分。茎尖分生组织在分化过程中受外界环境因素、内源激素水平和分子调控等影响,表现出明显变化。该文综合国内外近年来有关茎尖分生组织分化调控的研究进展,从茎尖分生组织的形态结构和环境影响因素,以及激素调控和分子调控等方面,对茎尖分生组织分化活动的研究进行综述,并对目前研究现状存在问题及未来研究方向进行了分析和展望。     

被子植物的茎端分生组织及其细胞的命运

茎端分生组织的活动是植物地上部分所有器官的来源,在植物个体发育过程中具有重要作用.介绍了茎端分生组织的基本形态、起始与维持的分子基础及其衍生细胞的命运.     

Rates of Cell Division in the Shoot Apical Meristem of Pisum

The relative rates of cell division in different regions ofthe pea shoot apical meristem were obtained by measuring theincrease in the numbers of metaphases following applicationof colchicine to the plants. Absolute values for the rates ofcell division could be calculated since the average rate ofcell division for the whole apex was known. Measurements ofthe rates of cell division were obtained at defined intervalsduring the course of a single plastochron. Within each regionof the apex the rate of cell division did not change more thanabout two-fold throughout the plastochron. There was very littleor no increase in the rate of cell division associated withleaf initiation. The formation of a leaf primordium and thesubsequent growth of the apical dome apparently result fromchanges in the direction of growth rather than changes in therates of growth. Three main regions were discernible withinthe apical meristem: a region with a slow rate of cell divisionin the apical dome, a region of a faster rate of cell divisionat the base of the apical dome and at the site of initiationof procambial strands, and a region of an intermediate rateof cell division in the newly initiated leaf primordium andthe adjacent part of the shoot axis.     

Elevation in the Sucrose Content of the Shoot Apical Meristem of Sinapis alba at Floral Evocation

Nanogram tissue samples from apical meristems of Sinapis alba were assayed for sucrose, total soluble hexosyl equivalents ( glucose and fructose plus hexoses from sucrose hydrolysis), and total soluble glucosyl equivalents ( glucose plus glucose from sucrose hydrolysis). On dry weight basis, sucrose concentration increased by more than 50% within 10 hours after the start of either a long photoperiod or a short photoperiod displaced by 10 hours in the 24-hour cycle (`displaced short day'). (These treatments induce flower initiation) Glucose and fructose concentrations were close to zero in vegetative meristems and remained low compared to sucrose in meristems of induced plants. Within a single meristem, the peripheral and the central zones had similar concentrations of sucrose. Our results indicate that an early physiological event in floral transition is the accumulation of sucrose in the meristem.     

植物茎端分生组织中的茎干细胞调控机制

介绍了高等植物茎端分生组织茎干细胞维持自我更新和产生分化细胞之问平衡的分子机制研究进展.     

Studies of aberrant phyllotaxy1 Mutants of Maize Indicate Complex Interactions between Auxin and Cytokinin Signaling in the Shoot Apical Meristem

One of the most fascinating aspects of plant morphology is the regular geometric arrangement of leaves and flowers, called phyllotaxy. The shoot apical meristem (SAM) determines these patterns, which vary depending on species and developmental stage. Auxin acts as an instructive signal in leaf initiation, and its transport has been implicated in phyllotaxy regulation in Arabidopsis (Arabidopsis thaliana). Altered phyllotactic patterns are observed in a maize (Zea mays) mutant, aberrant phyllotaxy1 (abph1, also known as abphyl1), and ABPH1 encodes a cytokinin-inducible type A response regulator, suggesting that cytokinin signals are also involved in the mechanism by which phyllotactic patterns are established. Therefore, we investigated the interaction between auxin and cytokinin signaling in phyllotaxy. Treatment of maize shoots with a polar auxin transport inhibitor, 1-naphthylphthalamic acid, strongly reduced ABPH1 expression, suggesting that auxin or its polar transport is required for ABPH1 expression. Immunolocalization of the PINFORMED1 (PIN1) polar auxin transporter revealed that PIN1 expression marks leaf primordia in maize, similarly to Arabidopsis. Interestingly, maize PIN1 expression at the incipient leaf primordium was greatly reduced in abph1 mutants. Consistently, auxin levels were reduced in abph1, and the maize PIN1 homolog was induced not only by auxin but also by cytokinin treatments. Our results indicate distinct roles for ABPH1 as a negative regulator of SAM size and a positive regulator of PIN1 expression. These studies highlight a complex interaction between auxin and cytokinin signaling in the specification of phyllotactic patterns and suggest an alternative model for the generation of altered phyllotactic patterns in abph1 mutants. We propose that reduced auxin levels and PIN1 expression in abph1 mutant SAMs delay leaf initiation, contributing to the enlarged SAM and altered phyllotaxy of these mutants.     

Formation, Maintenance and Function of the Shoot Apical Meristem in Rice

In higher plants, the process of embryogenesis establishes the plant body plan (body axes). On the basis of positional information specified by the body axes, the shoot apical meristem (SAM) and root apical meristem (RAM) differentiate at fixed positions early in embryogenesis. After germination, SAM and RAM are responsible for the development of the above-ground and below-ground parts, respectively, of the plant. Because of the importance of SAM function in plant development, the mechanisms of SAM formation during embryogenesis and of SAM maintenance and function in post-embryonic development are priority questions in plant developmental biology. Recent advances in molecular and genetic analysis of morphogenetic mutations in Arabidopsis have revealed several components required for SAM formation, maintenance and function. Although these processes are fundamental to the life cycle of every plant, conservation of the components does not explain the diversity of plant morphologies. Rice is used as a model plant of the grass family and of monocots because of the progress in research infrastructure, especially the collection of unique mutations and genome information. In comparison with the dicot Arabidopsis, rice has many unique organs or processes of development. This review summarizes what is known of the processes of SAM formation, maintenance and function in rice.     

New Evidence for the Role of Mechanical Forces in the Shoot Apical Meristem

Abstract The mechanism for initiation of lateral organs in the shoot apical meristem is still unknown. In this article we investigate one critical component of a buckling mechanism of organ initiation (that is, the presence and distribution of compressive stresses in the meristem). Direct evidence for compression in the sunflower capitulum was obtained from the gaping pattern of shallow cuts and the propagation of fractures. Cuts gaped widely in the central region of the capitulum but remained closed, or nearly so, in the generative and differentiation regions, suggesting the presence of circumferential compression at these locations. Fractures were initiated in the generative region and propagated circumferentially over most of their length. They did not cross the generative region perpendicularly, suggesting again the presence of compressive stresses in the circumferential direction. This conclusion was confirmed by the stress distribution computed from the geometry of the capitulum at three stages of development. One interpretation of these results is that the generative region corresponds to a zone of compression that could control the initiation of new primordia by means of buckling of the tunica layer. Received 12 January 2000; accepted 2 February 2000     

拟南芥茎顶端分生组织相关突变体的表型与结构分析

以拟南芥野生型(C24)和T-DNA插入诱发的突变体(155系)为材料,通过表型分析、组织切片、GUS基因表达的组织化学定位等研究方法对155系的形态结构和生长发育进行了较为细致的观察分析,结果发现:(1)T-DNA插入诱发的155系突变体植株矮化,叶片等器官体积减小,营养生长阶段延长,发育较C24缓慢;(2)同一时期155系的茎顶端分生组织面积较C24减小,顶端平坦,细胞层数减少,两侧叶原基基部之间的距离缩短,呈现出发育迟缓、从茎顶端分生组织向花分生组织转变延迟等特征;(3)GUS基因特异性地在155系茎顶端分生组织和维管组织中表达.结果表明,T-DNA诱捕基因可能在茎顶端分生组织中发挥作用,由于T-DNA的插入使该基因的功能受到了影响,进而影响了155系中茎顶端分生组织的发育模式,产生了155系的一系列表型改变.     

茎顶端分生组织在植物发育过程中的保持、转变和逆转

顶端分生组织(shoot apical meristems,SAM)为产生新的器官和组织而不断提供新的细胞,它的活性依赖于平衡分生组织细胞的增殖和器官发生之间关系的调控基因.来自不具备光合能力的顶端分生组织的细胞可形成具有光合能力的营养器官.在从营养生长到生殖发育的转变过程中,茎顶端分生组织,转变为花序分生组织,最终形成花分生组织.在进入开花决定状态以前,SAM的状态很大程度上受到环境信号和转录调控因子的影响.以模式植物拟南芥为主,对在顶端分生组织的保持和转变中复杂同时又有差异的基因调控网络进行讨论.在花和花序分生组织逆转过程中,SAM中的细胞也受到相关基因的调控,且表达方式存在明显的时空差异.因此,具有决定性的和未决定性双重特性的分生组织之间的转变和相互协调,对于器官发生和形态建成起到至关重要的作用.     

Apical Shoot Meristem Culture in Red Pepper (Capsicum annuum L.)

A protocol has been developed to obtain whole plants from apical shoot meristems of red pepper (Capsicum annuum L. cv. Bhivapuri), susceptible to viral infections. The meristems (～ 0.8 mm long), from aseptically grown seedlings (one-month-old), cultured on filter paper bridge in liquid Murashige and Skoog medium supplemented with 2 mg/l benzylaminopurine produced multiple shoots (5–7 per explant). The differentiated shoots developed further upon transfer to agar-solidified medium. Complete plantlets were obtained after rooting of shoots on MS medium fortified with 1 mg/l naphthaleneacetic acid.