生长素在植物胚胎早期发育中的作用

有性生殖是有花植物的一个重要特征, 胚胎则是实现有性生殖和世代交替的重要载体。植物胚胎从双受精开始, 经历了合子极性建立、顶基轴形成、细胞层分化和器官形成等过程, 这些过程都受到生长素的调控。近年来的研究表明, 生长素在生物合成、极性运输和信号转导3个层面上调控胚胎的发育过程。该文以双子叶植物拟南芥(Arabidopsis thaliana)为例, 综述了生长素对胚胎早期发育过程, 包括合子极性和顶基轴建立、表皮原特化和对称模式转变、胚根原特化和根尖分生组织形成及茎尖分生组织形成等发育的调控机制。     

Auxin Depletion from the Leaf Axil Conditions Competence for Axillary Meristem Formation in Arabidopsis and Tomato

The enormous variation in architecture of flowering plants is based to a large extent on their ability to form new axes of growth throughout their life span. Secondary growth is initiated from groups of pluripotent cells, called meristems, which are established in the axils of leaves. Such meristems form lateral organs and develop into a side shoot or a flower, depending on the developmental status of the plant and environmental conditions. The phytohormone auxin is well known to play an important role in inhibiting the outgrowth of axillary buds, a phenomenon known as apical dominance. However, the role of auxin in the process of axillary meristem formation is largely unknown. In this study, we show in the model species Arabidopsis thaliana and tomato (Solanum lycopersicum) that auxin is depleted from leaf axils during vegetative development. Disruption of polar auxin transport compromises auxin depletion from the leaf axil and axillary meristem initiation. Ectopic auxin biosynthesis in leaf axils interferes with axillary meristem formation, whereas repression of auxin signaling in polar auxin transport mutants can largely rescue their branching defects. These results strongly suggest that depletion of auxin from leaf axils is a prerequisite for axillary meristem formation during vegetative development.     

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.     

Indole acetic acid distribution coincides with vascular differentiation pattern during Arabidopsis leaf ontogeny

We used an anti-indole acetic acid (IAA or auxin) monoclonal antibody-based immunocytochemical procedure to monitor IAA level in Arabidopsis tissues. Using immunocytochemistry and the IAA-driven beta-glucuronidase (GUS) activity of Aux/IAA promoter::GUS constructs to detect IAA distribution, we investigated the role of polar auxin transport in vascular differentiation during leaf development in Arabidopsis. We found that shoot apical cells contain high levels of IAA and that IAA decreases as leaf primordia expand. However, seedlings grown in the presence of IAA transport inhibitors showed very low IAA signal in the shoot apical meristem (SAM) and the youngest pair of leaf primordia. Older leaf primordia accumulate IAA in the leaf tip in the presence or absence of IAA transport inhibition. We propose that the IAA in the SAM and the youngest pair of leaf primordia is transported from outside sources, perhaps the cotyledons, which accumulate more IAA in the presence than in the absence of transport inhibition. The temporal and spatial pattern of IAA localization in the shoot apex indicates a change in IAA source during leaf ontogeny that would influence flow direction and, consequently, the direction of vascular differentiation. The IAA production and transport pattern suggested by our results could explain the venation pattern, and the vascular hypertrophy caused by IAA transport inhibition. An outside IAA source for the SAM supports the notion that IAA transport and procambium differentiation dictate phyllotaxy and organogenesis.     

PIN-FORMED 1 regulates cell fate at the periphery of the shoot apical meristem

The process of organ positioning has been addressed, using the pin-formed 1 (pin1) mutant as a tool. PIN1 is a transmembrane protein involved in auxin transport in Arabidopsis. Loss of function severely affects organ initiation, and pin1 mutants are characterised by an inflorescence meristem that does not initiate any flowers, resulting in the formation of a naked inflorescence stem. This phenotype, combined with the proposed role of PIN1 in hormone transport, makes the mutant an ideal tool to study organ formation and phyllotaxis, and here we present a detailed analysis of the molecular modifications at the shoot apex caused by the mutation. We show that meristem structure and function are not severely affected in the mutant. Major alterations, however, are observed at the periphery of the pin1 meristem, where organ initiation should occur. Although two very early markers of organ initiation, LEAFY and AINTEGUMENTA, are expressed at the periphery of the mutant meristem, the cells are not recruited into distinct primordia. Instead a ring-like domain expressing those primordium specific genes is observed around the meristem. This ring-like domain also expresses a boundary marker, CUP-SHAPED COTYLEDON 2, involved in organ separation, showing that the zone at the meristem periphery has a hybrid identity. This implies that PIN1 is not only involved in organ outgrowth, but that it is also necessary for organ separation and positioning. A model is presented in which PIN1 and the local distribution of auxin control phyllotaxis.     

The Relationship between auxin transport and maize branching

Maize (Zea mays) plants make different types of vegetative or reproductive branches during development. Branches develop from axillary meristems produced on the flanks of the vegetative or inflorescence shoot apical meristem. Among these branches are the spikelets, short grass-specific structures, produced by determinate axillary spikelet-pair and spikelet meristems. We investigated the mechanism of branching in maize by making transgenic plants expressing a native expressed endogenous auxin efflux transporter (ZmPIN1a) fused to yellow fluorescent protein and a synthetic auxin-responsive promoter (DR5rev) driving red fluorescent protein. By imaging these plants, we found that all maize branching events during vegetative and reproductive development appear to be regulated by the creation of auxin response maxima through the activity of polar auxin transporters. We also found that the auxin transporter ZmPIN1a is functional, as it can rescue the polar auxin transport defects of the Arabidopsis (Arabidopsis thaliana) pin1-3 mutant. Based on this and on the groundbreaking analysis in Arabidopsis and other species, we conclude that branching mechanisms are conserved and can, in addition, explain the formation of axillary meristems (spikelet-pair and spikelet meristems) that are unique to grasses. We also found that BARREN STALK1 is required for the creation of auxin response maxima at the flanks of the inflorescence meristem, suggesting a role in the initiation of polar auxin transport for axillary meristem formation. Based on our results, we propose a general model for branching during maize inflorescence development.     

ABERRANT PANICLE ORGANIZATION 2/RFL, the rice ortholog of Arabidopsis LEAFY, suppresses the transition from inflorescence meristem to floral meristem through interaction with APO1

The temporal and spatial control of meristem identity is a key element in plant development. To better understand the molecular mechanisms that regulate inflorescence and flower architecture, we characterized the rice aberrant panicle organization 2 (apo2) mutant which exhibits small panicles with reduced number of primary branches due to the precocious formation of spikelet meristems. The apo2 mutants also display a shortened plastochron in the vegetative phase, late flowering, aberrant floral organ identities and loss of floral meristem determinacy. Map-based cloning revealed that APO2 is identical to previously reported RFL gene, the rice ortholog of the Arabidopsis LEAFY (LFY) gene. Further analysis indicated that APO2/RFL and APO1, the rice ortholog of Arabidopsis UNUSUAL FLORAL ORGANS, act cooperatively to control inflorescence and flower development. The present study revealed functional differences between APO2/RFL and LFY. In particular, APO2/RFL and LFY act oppositely on inflorescence development. Therefore, the genetic mechanisms for controlling inflorescence architecture have evolutionarily diverged between rice (monocots) and Arabidopsis (eudicots).     

Repression of floral meristem fate is crucial in shaping tomato inflorescence

Tomato is an important crop and hence there is a great interest in understanding the genetic basis of its flowering. Several genes have been identified by mutations and we constructed a set of novel double mutants to understand how these genes interact to shape the inflorescence. It was previously suggested that the branching of the tomato inflorescence depends on the gradual transition from inflorescence meristem (IM) to flower meristem (FM): the extension of this time window allows IM to branch, as seen in the compound inflorescence (s) and falsiflora (fa) mutants that are impaired in FM maturation. We report here that Jointless (J), which encodes a MADS-box protein of the same clade than Short Vegetative Phase (SVP) and Agamous Like 24 (AGL24) in Arabidopsis, interferes with this timing and delays FM maturation, therefore promoting IM fate. This was inferred from the fact that j mutation suppresses the high branching inflorescence phenotype of s and fa mutants and was further supported by the expression pattern of J, which is expressed more strongly in IM than in FM. Most interestingly, FA--the orthologue of the Arabidopsis LEAFY (LFY) gene--shows the complementary expression pattern and is more active in FM than in IM. Loss of J function causes premature termination of flower formation in the inflorescence and its reversion to a vegetative program. This phenotype is enhanced in the absence of systemic florigenic protein, encoded by the Single Flower Truss (SFT) gene, the tomato orthologue of Flowering Locus T (FT). These results suggest that the formation of an inflorescence in tomato requires the interaction of J and a target of SFT in the meristem, for repressing FA activity and FM fate in the IM.     

Characterization of tomato (Solanum lycopersicum L.) mutants affected in their flowering time and in the morphogenesis of their reproductive structure

The impact of the season on flowering time and the organization and morphogenesis of the reproductive structures are described in three tomato mutants: compound inflorescence (s), single flower truss (sft), and jointless (j), respectively, compared with their wild-type cultivars Ailsa Craig (AC), Platense (Pl), and Heinz (Hz). In all environmental conditions, the sft mutant flowered significantly later than its corresponding Pl cultivar while flowering time in j was only marginally, but consistently, delayed compared with Hz. The SFT gene and, to a lesser extent, the J gene thus appear to be constitutive flowering promoters. Flowering in s was delayed in winter but not in summer compared with the AC cultivar, suggesting the existence of an environmentally regulated pathway for the control of floral transition. The reproductive structure of tomato is a raceme-like inflorescence and genes regulating its morphogenesis may thus be divided into inflorescence and floral meristem identity genes as in Arabidopsis. The s mutant developed highly branched inflorescences bearing up to 200 flowers due to the conversion of floral meristems into inflorescence meristems. The S gene appears to be a floral meristem identity gene. Both sft and j mutants formed reproductive structures containing flowers and leaves and reverting to a vegetative sympodial growth. The SFT gene appears to regulate the identity of the inflorescence meristem of tomato and is also involved, along with the J gene, in the maintenance of this identity, preventing reversion to a vegetative identity. These results are discussed in relation to knowledge accumulated in Arabidopsis and to domestication processes.     

barren inflorescence2 Encodes a co-ortholog of the PINOID serine/threonine kinase and is required for organogenesis during inflorescence and vegetative development in maize

Organogenesis in plants is controlled by meristems. Axillary meristems, which give rise to branches and flowers, play a critical role in plant architecture and reproduction. Maize (Zea mays) and rice (Oryza sativa) have additional types of axillary meristems in the inflorescence compared to Arabidopsis (Arabidopsis thaliana) and thus provide an excellent model system to study axillary meristem initiation. Previously, we characterized the barren inflorescence2 (bif2) mutant in maize and showed that bif2 plays a key role in axillary meristem and lateral primordia initiation in the inflorescence. In this article, we cloned bif2 by transposon tagging. Isolation of bif2-like genes from seven other grasses, along with phylogenetic analysis, showed that bif2 is a co-ortholog of PINOID (PID), which regulates auxin transport in Arabidopsis. Expression analysis showed that bif2 is expressed in all axillary meristems and lateral primordia during inflorescence and vegetative development in maize and rice. Further phenotypic analysis of bif2 mutants in maize illustrates additional roles of bif2 during vegetative development. We propose that bif2/PID sequence and expression are conserved between grasses and Arabidopsis, attesting to the important role they play in development. We provide further support that bif2, and by analogy PID, is required for initiation of both axillary meristems and lateral primordia.     

Experimental and genetic analysis of root development inArabidopsis thaliana

The cellular organisation of theArabidopsis thaliana root is remarkably regular. A fate map of the primary root and root meristem that predicts the developmental destinies of cells within the embryonic root primordium has been constructed. Nevertheless, laser ablation experiments demonstrate that root meristem cells develop according to position and not according to lineage. Mutational analysis has identified genes required for cell specification in the radial as well as in the apical-basal dimension. The corresponding gene functions appear to be necessary during embryogenesis for the formation of a correctly patterned primary root. H Lambers Section editor     

The auxin influx carrier is essential for correct leaf positioning

Auxin is of vital importance in virtually every aspect of plant growth and development, yet, even after almost a century of intense study, major gaps in our knowledge of its synthesis, distribution, perception, and signal transduction remain. One unique property of auxin is its polar transport, which in many well-documented cases is a critical part of its mode of action. Auxin is actively transported through the action of both influx and efflux carriers. Inhibition of polar transport by the efflux inhibitor N-1-naphthylphthalamic acid (NPA) causes a complete cessation of leaf initiation, a defect that can be reversed by local application of the auxin, indole-3-acetic acid (IAA), to the responsive zone of the shoot apical meristem. In this study, we address the role of the auxin influx carrier in the positioning and outgrowth of leaf primordia at the shoot apical meristem of tomato. By using a combination of transport inhibitors and synthetic auxins, we demonstrate that interference with auxin influx has little effect on organ formation as such, but prevents proper localization of leaf primordia. These results suggest the existence of functional auxin concentration gradients in the shoot apical meristem that are actively set up and maintained by the action of efflux and influx carriers. We propose a model in which efflux carriers control auxin delivery to the shoot apical meristem, whereas influx and efflux carriers regulate auxin distribution within the meristem.     

Regulatory networks that function to specify flower meristems require the function of homeobox genes PENNYWISE and POUND-FOOLISH in Arabidopsis

Flowering is a major developmental phase change that transforms the fate of the shoot apical meristem (SAM) from a leaf-bearing vegetative meristem to that of a flower-producing inflorescence meristem. In Arabidopsis, floral meristems are specified on the periphery of the inflorescence meristem by the combined activities of the FLOWERING LOCUS T (FT)–FD complex and the flower meristem identity gene, LEAFY ( LFY ). Two redundant functioning homeobox genes, PENNYWISE ( PNY ) and POUND-FOOLISH ( PNF ), which are expressed in the vegetative and inflorescence SAM, regulate patterning events during reproductive development, including floral specification. To determine the role of PNY and PNF in the floral specification network, we characterized the genetic relationship of these homeobox genes with LFY and FT . Results from this study demonstrate that LFY functions downstream of PNY and PNF. Ectopic expression of LFY promotes flower formation in pny pnf plants, while the flower specification activity of ectopic FT is severely attenuated. Genetic analysis shows that when mutations in pny and pnf genes are combined with lfy , a synergistic phenotype is displayed that significantly reduces floral specification and alters inflorescence patterning events. In conclusion, results from this study support a model in which PNY and PNF promote LFY expression during reproductive development. At the same time, the flower formation activity of FT is dependent upon the function of PNY and PNF.     

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.     

PIN1-Independent Leaf Initiation in Arabidopsis

Phyllotaxis, the regular arrangement of leaves and flowers around the stem, is a key feature of plant architecture. Current models propose that the spatiotemporal regulation of organ initiation is controlled by a positive feedback loop between the plant hormone auxin and its efflux carrier PIN-FORMED1 (PIN1). Consequently, pin1 mutants give rise to naked inflorescence stalks with few or no flowers, indicating that PIN1 plays a crucial role in organ initiation. However, pin1 mutants do produce leaves. In order to understand the regulatory mechanisms controlling leaf initiation in Arabidopsis (Arabidopsis thaliana) rosettes, we have characterized the vegetative pin1 phenotype in detail. We show that although the timing of leaf initiation in vegetative pin1 mutants is variable and divergence angles clearly deviate from the canonical 137° value, leaves are not positioned at random during early developmental stages. Our data further indicate that other PIN proteins are unlikely to explain the persistence of leaf initiation and positioning during pin1 vegetative development. Thus, phyllotaxis appears to be more complex than suggested by current mechanistic models.     

Auxin polar transport and flower formation in Arabidopsis thaliana transformed with indoleacetamide hydrolase (iaaH) gene

Involvement of auxin polar transport in flower formation of Arabidopsis thaliana was studied using a pinformed (pin) mutant (Rpin) transformed with the indoleacetamide hydrolase (iaaH) gene and the phenocopy of the pin mutant, which was induced by 9-hydroxyfluorene-9-carboxylic acid (HFCA). The application of indoleacetamide (IAM) did not change aberrant structure of the aerial part of Rpin (pin/pin), but extremely inhibited its root growth. Treatment with IAM increased the endogenous concentrations of free and conjugated IAA in Rpin normal (pin/+ or +/+) due to the expression of the iaaH gene, to 140% and 428% of those in non-treated plants, respectively, and those in Rpin to 378% and 120%, respectively. The activity of IAA polar transport in the inflorescence axis of Rpin remained low even in the presence of IAM, the activity being almost similar, to that in the pin mutant. The activity of IAA polar transport in the HFCA-induced phenocopy of the pin mutant was also extremely low, and it was not restored by the simultaneous application of IAA. Arabidopsis thaliana responded to HFCA applied from 7 to 11 d and from 25 to 29 d after germination in the wild-type plant (Enkheim ecotype) and the late flowering mutant (fb mutant), respectively. These results suggest that the construction of the system of auxin polar transport and its normal activities are essential for the differentiation and the formation of floral meristem in the early growth stage of Arabidopsis thaliana.     

The CUP-SHAPED COTYLEDON2 and 3 genes have a post-meristematic effect on Arabidopsis thaliana phyllotaxis

Background and Aims The arrangement of flowers in inflorescence shoots of Arabidopsis thaliana represents a regular spiral Fibonacci phyllotaxis. However, in the cuc2 cuc3 double mutant, flower pedicels are fused to the inflorescence stem, and phyllotaxis is aberrant in the mature shoot regions. This study examined the causes of this altered development, and in particular whether the mutant phenotype is a consequence of defects at the shoot apex, or whether post-meristematic events are involved.Methods The distribution of flower pedicels and vascular traces was examined in cross-sections of mature shoots; sequential replicas were used to investigate the phyllotaxis and geometry of shoot apices, and growth of the young stem surface. The expression pattern of CUC3 was analysed by examining its promoter activity.Key Results Phyllotaxis irregularity in the cuc2 cuc3 double mutant arises during the post-meristematic phase of shoot development. In particular, growth and cell divisions in nodes of the elongating stem are not restricted in the mutant, resulting in pedicel–stem fusion. On the other hand, phyllotaxis in the mutant shoot apex is nearly as regular as that of the wild type. Vascular phyllotaxis, generated almost simultaneously with the phyllotaxis at the apex, is also much more regular than pedicel phyllotaxis. The most apparent phenotype of the mutant apices is a higher number of contact parastichies. This phenotype is associated with increased meristem size, decreased angular width of primordia and a shorter plastochron. In addition, the appearance of a sharp and deep crease, a characteristic shape of the adaxial primordium boundary, is slightly delayed and reduced in the mutant shoot apices.Conclusions The cuc2 cuc3 double mutant displays irregular phyllotaxis in the mature shoot but not in the shoot apex, thus showing a post-meristematic effect of the mutations on phyllotaxis. The main cause of this effect is the formation of pedicel–stem fusions, leading to an alteration of the axial positioning of flowers. Phyllotaxis based on the position of vascular flower traces suggests an additional mechanism of post-meristematic phyllotaxis alteration. Higher density of flower primordia may be involved in the post-meristematic effect on phyllotaxis, whereas delayed crease formation may be involved in the fusion phenotype. Promoter activity of CUC3 is consistent with its post-meristematic role in phyllotaxis.