Simulation of organ patterning on the floral meristem using a polar auxin transport model

An intriguing phenomenon in plant development is the timing and positioning of lateral organ initiation, which is a fundamental aspect of plant architecture. Although important progress has been made in elucidating the role of auxin transport in the vegetative shoot to explain the phyllotaxis of leaf formation in a spiral fashion, a model study of the role of auxin transport in whorled organ patterning in the expanding floral meristem is not available yet. We present an initial simulation approach to study the mechanisms that are expected to play an important role. Starting point is a confocal imaging study of Arabidopsis floral meristems at consecutive time points during flower development. These images reveal auxin accumulation patterns at the positions of the organs, which strongly suggests that the role of auxin in the floral meristem is similar to the role it plays in the shoot apical meristem. This is the basis for a simulation study of auxin transport through a growing floral meristem, which may answer the question whether auxin transport can in itself be responsible for the typical whorled floral pattern. We combined a cellular growth model for the meristem with a polar auxin transport model. The model predicts that sepals are initiated by auxin maxima arising early during meristem outgrowth. These form a pre-pattern relative to which a series of smaller auxin maxima are positioned, which partially overlap with the anlagen of petals, stamens, and carpels. We adjusted the model parameters corresponding to properties of floral mutants and found that the model predictions agree with the observed mutant patterns. The predicted timing of the primordia outgrowth and the timing and positioning of the sepal primordia show remarkable similarities with a developing flower in nature.     

The influence of cytokinin-auxin cross-regulation on cell-fate determination in Arabidopsis thaliana root development

Root growth and development in Arabidopsis thaliana are sustained by a specialised zone termed the meristem, which contains a population of dividing and differentiating cells that are functionally analogous to a stem cell niche in animals. The hormones auxin and cytokinin control meristem size antagonistically. Local accumulation of auxin promotes cell division and the initiation of a lateral root primordium. By contrast, high cytokinin concentrations disrupt the regular pattern of divisions that characterises lateral root development, and promote differentiation. The way in which the hormones interact is controlled by a genetic regulatory network. In this paper, we propose a deterministic mathematical model to describe this network and present model simulations that reproduce the experimentally observed effects of cytokinin on the expression of auxin regulated genes. We show how auxin response genes and auxin efflux transporters may be affected by the presence of cytokinin. We also analyse and compare the responses of the hormones auxin and cytokinin to changes in their supply with the responses obtained by genetic mutations of SHY2, which encodes a protein that plays a key role in balancing cytokinin and auxin regulation of meristem size. We show that although shy2 mutations can qualitatively reproduce the effect of varying auxin and cytokinin supply on their response genes, some elements of the network respond differently to changes in hormonal supply and to genetic mutations, implying a different, general response of the network. We conclude that an analysis based on the ratio between these two hormones may be misleading and that a mathematical model can serve as a useful tool for stimulate further experimental work by predicting the response of the network to changes in hormone levels and to other genetic mutations.     

The Exogenous Hormornal Control of the Development of Regenerated Flower Buds in Hyacinthus orientalis

The critical role of exogenous hormone on inducing the initiation of different floral organs in the regenerated flower bud and controlling their numbers was further evidenced. The initiation of the flower buds was first induced from the perianth explants of Hyacinthus orientalis L. cv. White pearl by a combination of 2 mg/L 6-BA and 0.1 mg/L 2,4-D, and then a continuous initiation of over 100 tepals (a flower bud of H. orientalis in situ has only 6 tepals) was successfully controlled by maintenance of such a hormone concentration. However, a change of hormonal concentration (2 mg/L 6-BA and 0-0.000 1 mg/L 2,4-D) caused cessation of continuous initiation of the tepals but gave rise to induction of stamen initiation. Keeping the changed hormone concentrations could successfully control the continuous initiation of over 20 stamens (a flower bud of H. orientalis in situ has only 6 stamens). The experiment showed that the number of identical floral organs in the regenerated flower buds can be controlled by certain defined concentrations of the exogenous hormones, and the amount of the induced identical floral organs has no effect on the differentiation sequence of the different floral organs in the regenerated flower bud. Based on a systematic research on controlling the differentiation of the floral organs from both the perianth explants and the regenerated flower buds by the exogenous hormones in H. orientalis over the past decade, the authors put forward here a new idea on the role of phytohormone in controlling the automatic and sequential differentiation of the different floral organs in flower development. The main points are as follows: 1. the development of flower bud in plant is a process in which all of the floral organs are automatically and sequentially differentiated from the flower meristem. 2. Experiments in vitro showed that the effect of exogenous hormones in controlling the initiation of different floral organs is strictly concentration dependent, i.e., one kind of the floral organ can continuously and repeatedly initiate from the flower meristem as long as it is maintained in that specific concentration of the exogenous hormone which is suitable for the initiation of that particular kind of floral organ. 3. It shows that the flower buds in situ must be automatically able to adjust the endogenous hormonal concentrations just after the completion of the differentiation of one whorl of floral organ to suit the differentiation of the next whorl. Thus, the phytohormone in different concentrations takes after many change-over switches of the organ differentiation and plays a connective and regulatory role between the differentiation of every two whorls of the floral organ. In other words, these change-over switches play the roles of inhibiting the expression of the genes which control the initiation of the floral organs in the first whorl, meanwhile, activating the expression of the genes which control the initiation of the floral organs in the second whorl during the successive initiation of the different floral organs from the flower bud. It results in the automatic and sequential initiation of the various floral organs from the floral meristem.      

Auxin regulates the initiation and radial position of plant lateral organs

Leaves originate from the shoot apical meristem, a small mound of undifferentiated tissue at the tip of the stem. Leaf formation begins with the selection of a group of founder cells in the so-called peripheral zone at the flank of the meristem, followed by the initiation of local growth and finally morphogenesis of the resulting bulge into a differentiated leaf. Whereas the mechanisms controlling the switch between meristem propagation and leaf initiation are being identified by genetic and molecular analyses, the radial positioning of leaves, known as phyllotaxis, remains poorly understood. Hormones, especially auxin and gibberellin, are known to influence phyllotaxis, but their specific role in the determination of organ position is not clear. We show that inhibition of polar auxin transport blocks leaf formation at the vegetative tomato meristem, resulting in pinlike naked stems with an intact meristem at the tip. Microapplication of the natural auxin indole-3-acetic acid (IAA) to the apex of such pins restores leaf formation. Similarly, exogenous IAA induces flower formation on Arabidopsis pin-formed1-1 inflorescence apices, which are blocked in flower formation because of a mutation in a putative auxin transport protein. Our results show that auxin is required for and sufficient to induce organogenesis both in the vegetative tomato meristem and in the Arabidopsis inflorescence meristem. In this study, organogenesis always strictly coincided with the site of IAA application in the radial dimension, whereas in the apical-basal dimension, organ formation always occurred at a fixed distance from the summit of the meristem. We propose that auxin determines the radial position and the size of lateral organs but not the apical-basal position or the identity of the induced structures.     

A crosstalk of auxin and GA during tuber development

Several hormones have been studied for their effect on tuber initiation and development. Until recently, the hormone with the most prominent role in tuber initiation was attributed to GA. Genes involved in GA degradation do exhibit an upregulated profile during early stages of tuber development, leading to a rapid decrease of active GA content, thereby facilitating stolon-tip swelling. While GA is known to be involved in shoot and stolon elongation, the development of the new tuberorgan requires changes in meristem identity and the reorientation ofthe plane of cell division. In other developmental processes, such as embryo patterning, flower development and lateral root initiation auxin plays a key role. Recent evidence on the involvement of auxin in tuber formation was providedby the measurement of auxin content in swelling stolons. Auxin content in the stolon tips increased several fold prior to tuber swelling. In vitro tuberisation experiments with auxin applications support the role of auxin during tuber initiation. Taken together, it is becoming clear that the initiation and induction of tubers in potato is a developmental process that appears to be regulated by a crosstalk between GA and auxin.     

Shaping up: the genetic control of leaf shape

Leaf initiation at the shoot apical meristem involves a balance between cell proliferation and commitment to make primordia. Several genes, such as CLAVATA1, CLAVATA3, WUSCHEL, KNOTTED1, and PHANTASTICA, play key roles in these processes. When expressed in the leaf primordium, however, these 'meristem' genes can profoundly affect leaf shape and size, possibly by regulating hormone gradients and transport. The KNOTTED1-like genes are involved in regulating changes in hormonal levels. Recent studies have elaborated on the role that hormones, such as auxin, play in releasing biophysical constraints on leaf initiation and growth. Final leaf form is elaborated by a coordination of these hormonally regulated processes, cell division and cellular differentiation.     

New Clothes for the Jasmonic Acid Receptor COI1: Delayed Abscission,Meristem Arrest and Apical Dominance

In a screen for delayed floral organ abscission in Arabidopsis, we have identified a novel mutant of CORONATINE INSENSITIVE 1 (COI1), the F-box protein that has been shown to be the jasmonic acid (JA) co-receptor. While JA has been shown to have an important role in senescence, root development, pollen dehiscence and defense responses, there has been little focus on its critical role in floral organ abscission. Abscission, or the detachment of organs from the main body of a plant, is an essential process during plant development and a unique type of cell separation regulated by endogenous and exogenous signals. Previous studies have indicated that auxin and ethylene are major plant hormones regulating abscission; and here we show that regulation of floral organ abscission is also controlled by jasmonic acid in Arabidopsis thaliana. Our characterization of coi1-1 and a novel allele (coi1-37) has also revealed an essential role in apical dominance and floral meristem arrest. In this study we provide genetic evidence indicating that delayed abscission 4 (dab4-1) is allelic to coi1-1 and that meristem arrest and apical dominance appear to be evolutionarily divergent functions for COI1 that are governed in an ecotype-dependent manner. Further characterizations of ethylene and JA responses of dab4-1/coi1-37 also provide new information suggesting separate pathways for ethylene and JA that control both floral organ abscission and hypocotyl growth in young seedlings. Our study opens the door revealing new roles for JA and its interaction with other hormones during plant development.     

The ULTRAPETALA gene controls shoot and floral meristem size in Arabidopsis

The regulation of proper shoot and floral meristem size during plant development is mediated by a complex interaction of stem cell promoting and restricting factors. The phenotypic effects of mutations in the ULTRAPETALA gene, which is required to control shoot and floral meristem cell accumulation in Arabidopsis thaliana, are described. ultrapetala flowers contain more floral organs and whorls than wild-type plants, phenotypes that correlate with an increase in floral meristem size preceding organ initiation. ultrapetala plants also produce more floral meristems than wild-type plants, correlating with an increase in inflorescence meristem size without visible fasciation. Expression analysis indicates that ULTRAPETALA controls meristem cell accumulation partly by limiting the domain of CLAVATA1 expression. Genetic studies show that ULTRAPETALA acts independently of ERA1, but has overlapping functions with PERIANTHIA and the CLAVATA signal transduction pathway in controlling shoot and floral meristem size and meristem determinacy. Thus ULTRAPETALA defines a novel locus that restricts meristem cell accumulation in Arabidopsis shoot and floral meristems.     

Arabidopsis thaliana (L.) Heynh. as a model object for studying genetic control of morphogenesis

A vast amount of information on the genetic control of plant development has been obtained in Arabidopsis thaliana with classical genetic and molecular biological methods. The genes involved in multistep regulation of floral morphogenesis have been identified. The formation of floral meristem is controlled by the LEAFY (LFY), UNUSUAL FLORAL ORGANS (UFO), APETALA1 (AP1), and APETALA2 (AP2) genes. Studies of the abruptus and bractea recessive monogenic mutants from the collection of the Department of Genetics and Selection, Moscow State University, showed that the ABRUPTUS (ABR) and BRACTEA (BRA) genes also play an important role in inflorescence differentiation. The ABR gene controls the early formation of organ primordia on the inflorescence and the formation of floral organ primordia after floral initiation. Further differentiation of inflorescence organ primordia in vegetative or generative organs depends on the activity of the LFY gene, and floral organ identity is determined by the homeotic genes. Presumably, the major function of the ABR gene is to determine the auxin polar transport. The BRA gene suppresses the development of bracts on the inflorescence and constrains cell division at the base of primordia of rosette and cauline leaves.     

Auxin: A major regulator of organogenesis

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.     

植物茎分枝的分子调控

植物茎分枝结构决定了不同植物的不同形态结构.本文从腋生分生组织的发生、腋芽的生长两个方面综述了近年来植物分枝发生发育相关的分子机理研究及其进展.发现在不同植物中腋分生组织形成的基本机制是相似的,LS（lateral suppressor）及其同源基因在不同植物中都参与腋生分生组织的形成,而BL(blind)及其同源基因也参与调控腋生分生组织的形成.腋生分生组织的形成可能也是受激素调控的.目前,对腋芽生长的分子调控机制的认识主要集中于生长素通过二级信使的作用调控腋芽的生长.而生长素调控腋芽生长的机制已经较为清楚的有两条途径：一是生长素通过抑制细胞分裂素合成来调控腋芽的生长;另一途径是一种类胡萝卜素衍生的信号物质参与生长素的运输调控(MAX途径)来调控腋芽的生长.最新研究表明,TB1的拟南芥同源基因在MAX途径的下游负调控腋芽的生长.此外,增强表达OsNAC2也促进腋芽的生长.     

Hormone signaling in plant development

Hormone signaling plays diverse and critical roles during plant development. In particular, hormone interactions regulate meristem function and therefore control formation of all organs in the plant. Recent advances have dissected commonalities and differences in the interaction of auxin and cytokinin in the regulation of shoot and root apical meristem function. In addition, brassinosteroid hormones have recently been discovered to regulate root apical meristem size. Further insights have also been made into our understanding of the mechanism of crosstalk among auxin, cytokinin, and strigolactone in axillary meristems.     

生长素对拟南芥叶片发育调控的研究进展

叶片(包括子叶)是茎端分生组织产生的第一类侧生器官, 在植物发育中具有重要地位。早期叶片发育包括三个主要过程: 叶原基的起始, 叶片腹背性的建立和叶片的延展。大量证据表明叶片发育受到体内遗传机制和体外环境因子的双重调节。植物激素, 尤其是生长素在协调体内外调节机制中起着不可或缺的作用。生长素的稳态调控、极性运输和信号转导影响叶片发育的全过程。本文着重介绍生长素在叶片生长发育和形态建成中的调控作用, 试图了解复杂叶片发育调控网络。     

生长素对拟南芥叶片发育调控的研究进展

叶片（包括子叶）是茎端分生组织产生的第一类侧生器官,在植物发育中具有重要地位。早期叶片发育包括三个主要过程：叶原基的起始,叶片腹背性的建立和叶片的延展。大量证据表明叶片发育受到体内遗传机制和体外环境因子的双重调节。植物激素,尤其是生长素在协调体内外调节机制中起着不可或缺的作用。生长素的稳态调控、极性运输和信号转导影响叶片发育的全过程。本文着重介绍生长素在叶片生长发育和形态建成中的调控作用,试图了解复杂叶片发育调控网络。     

SUPERMAN regulates floral whorl boundaries through control of auxin biosynthesis

Proper floral patterning, including the number and position of floral organs in most plant species, is tightly controlled by the precise regulation of the persistence and size of floral meristems (FMs). In Arabidopsis, two known feedback pathways, one composed of WUSCHEL (WUS) and CLAVATA3 (CLV3) and the other composed of AGAMOUS (AG) and WUS, spatially and temporally control floral stem cells, respectively. However, mounting evidence suggests that other factors, including phytohormones, are also involved in floral meristem regulation. Here, we show that the boundary gene SUPERMAN (SUP) bridges floral organogenesis and floral meristem determinacy in another pathway that involves auxin signaling. SUP interacts with components of polycomb repressive complex 2 (PRC2) and fine‐tunes local auxin signaling by negatively regulating the expression of the auxin biosynthesis genes YUCCA1/4 (YUC1/4). In sup mutants, derepressed local YUC1/4 activity elevates auxin levels at the boundary between whorls 3 and 4, which leads to an increase in the number and the prolonged maintenance of floral stem cells, and consequently an increase in the number of reproductive organs. Our work presents a new floral meristem regulatory mechanism, in which SUP, a boundary gene, coordinates floral organogenesis and floral meristem size through fine‐tuning auxin biosynthesis.