Genetic Interactions That Regulate Inflorescence Development in Arabidopsis

In Arabidopsis, floral meristems arise in continuous succession directly on the flanks of the inflorescence meristem. Thus, the pathways that regulate inflorescence and floral meristem identity must operate both simultaneously and in close spatial proximity. The TERMINAL FLOWER 1 (TFL1) gene of Arabidopsis is required for normal inflorescence meristem function, and the LEAFY (LFY), APETALA 1 (AP1), and APETALA 2 (AP2) genes are required for normal floral meristem function. We present evidence that inflorescence meristem identity is promoted by TFL1 and that floral meristem identity is promoted by parallel developmental pathways, one defined by LFY and the other defined by AP1/AP2. Our analysis suggests that the acquisition of meristem identity during inflorescence development is mediated by antagonistic interactions between TFL1 and LFY and between TFL1 and AP1/AP2. Based on this study, we propose a simple model for the genetic regulation of inflorescence development in Arabidopsis. This model is discussed in relation to the proposed interactions between the inflorescence and the floral meristem identity genes and in regard to other genes that are likely to be part of the genetic hierarchy regulating the establishment and maintenance of inflorescence and floral meristems.     

LEAFY, TERMINAL FLOWER1 and AGAMOUS are functionally conserved but do not regulate terminal flowering and floral determinacy in Impatiens balsamina

In Impatiens balsamina a lack of commitment of the meristem during floral development leads to the continuous requirement for a leaf-derived floral signal. In the absence of this signal the meristem reverts to leaf production. Current models for Arabidopsis state that LEAFY (LFY) is central to the integration of floral signals and regulates flowering partly via interactions with TERMINAL FLOWER1 (TFL1) and AGAMOUS (AG). Here we describe Impatiens homologues of LFY, TFL1 and AG (IbLFY, IbTFL1 and IbAG) that are highly conserved at a sequence level and demonstrate homologous functions when expressed ectopically in transgenic Arabidopsis. We relate the expression patterns of IbTFL1 and IbAG to the control of terminal flowering and floral determinacy in Impatiens. IbTFL1 is involved in controlling the phase of the axillary meristems and is expressed in axillary shoots and axillary meristems which produce inflorescences, but not in axillary flowers. It is not involved in maintaining the terminal meristem in either an inflorescence or indeterminate state. Terminal flowering in Impatiens appears therefore to be controlled by a pathway that uses a different integration system than that regulating the development of axillary flowers and branches. The pattern of ovule production in Impatiens requires the meristem to be maintained after the production of carpels. Consistent with this morphological feature IbAG appears to specify stamen and carpel identity, but is not sufficient to specify meristem determinacy in Impatiens.     

The indeterminate floral apex1 gene regulates meristem determinacy and identity in the maize inflorescence

Meristems may be determinate or indeterminate. In maize, the indeterminate inflorescence meristem produces three types of determinate meristems: spikelet pair, spikelet and floral meristems. These meristems are defined by their position and their products. We have discovered a gene in maize, indeterminate floral apex1 (ifa1) that regulates meristem determinacy. The defect found in ifa1 mutants is specific to meristems and does not affect lateral organs. In ifa1 mutants, the determinate meristems become less determinate. The spikelet pair meristem initiates more than a pair of spikelets and the spikelet meristem initiates more than the normal two flowers. The floral meristem initiates all organs correctly, but the ovule primordium, the terminal product of the floral meristem, enlarges and proliferates, expressing both meristem and ovule marker genes. A role for ifa1 in meristem identity in addition to meristem determinacy was revealed by double mutant analysis. In zea agamous1 (zag1) ifa1 double mutants, the female floral meristem converts to a branch meristem whereas the male floral meristem converts to a spikelet meristem. In indeterminate spikelet1 (ids1) ifa1 double mutants, female spikelet meristems convert to branch meristems and male spikelet meristems convert to spikelet pair meristems. The double mutant phenotypes suggest that the specification of meristems in the maize inflorescence involves distinct steps in an integrated process.     

花序分生组织特性基因TFL1的系统发育及其功能分析

TFL1同源基因在维持植物营养生长和花序分生组织特性方面起着非常重要的作用,其功能的丧失常导致植物提早开花,花序的正常发育受到抑制,最终茎端形成顶花。至今已经有28种植物的TFL1基因被克隆到,其中包括拟南芥、金鱼草和番茄等模式植物。TFL1 蛋白的系统发育树基本符合物种的亲缘关系。作为花序分生组织特性基因的TFL1与花分生组织特性基因LFY 和AP1相互作用,抑制花序分生组织向花分生组织的转变。TFL1和LFY等基因可用来培育早花新品种,也可用于培育无果的新品种,减少悬铃木、杨、柳等果毛的污染。     

Interactions among APETALA1, LEAFY, and TERMINAL FLOWER1 specify meristem fate.

Upon floral induction, the primary shoot meristem of an Arabidopsis plant begins to produce flower meristems rather than leaf primordia on its flanks. Assignment of floral fate to lateral meristems is primarily due to the cooperative activity of the flower meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER. We present evidence here that AP1 expression in lateral meristems is activated by at least two independent pathways, one of which is regulated by LFY. In lfy mutants, the onset of AP1 expression is delayed, indicating that LFY is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LFY, because LFY is expressed prematurely in the converted floral meristems of plants constitutively expressing AP1. Shoot meristems maintain an identity distinct from that of flower meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LFY expression from the influorescence shoot meristem. We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1. Therefore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon floral induction, is promoted by positive feedback interactions between LFY and AP1, together with negative interactions of these two genes with TFL1.     

Expression of CENTRORADIALIS (CEN) and CEN-like genes in tobacco reveals a conserved mechanism controlling phase change in diverse species.

Plant species exhibit two primary forms of flowering architecture, namely, indeterminate and determinate. Antirrhinum is an indeterminate species in which shoots grow indefinitely and only generate flowers from their periphery. Tobacco is a determinate species in which shoot meristems terminate by converting to a flower. We show that tobacco is responsive to the CENTRORADIALIS (CEN) gene, which is required for indeterminate growth of the shoot meristem in Antirrhinum. Tobacco plants overexpressing CEN have an extended vegetative phase, delaying the switch to flowering. Therefore, CEN defines a conserved system controlling shoot meristem identity and plant architecture in diverse species. To understand the underlying basis for differences between determinate and indeterminate architectures, we isolated CEN-like genes from tobacco (CET genes). In tobacco, the CET genes most similar to CEN are not expressed in the main shoot meristem; their expression is restricted to vegetative axillary meristems. As vegetative meristems develop into flowering shoots, CET genes are downregulated as floral meristem identity genes are upregulated. Our results suggest a general model for tobacco, Antirrhinum, and Arabidopsis, whereby the complementary expression patterns of CEN-like genes and floral meristem identity genes underlie different plant architectures.     

FPF1 modulates the competence to flowering in Arabidopsis

During the transition to flowing the FPF1 gene is expressed in the peripheral zone of apical meristems and in floral meristems of Arabidopsis. Constitutive expression of FPF1 causes early flowering in Arabidopsis under both long-day and short-day conditions and leads to a shortened juvenile phase as measured by the trichome distribution on the abaxial leaf surface. In the classical late flowering mutants, overexpression of FPF1 compensates partially for the late flowering phenotype, indicating that FPF1 acts downstream or in a parallel pathway to the mutated genes. The co-overexpression of 35S::AP1 with 35S::FPF1 leads to a synergistic effect on the shortening of the time to flowering under short-day conditions. The co-overexpression of 35S::FPF1 and 35S::LFY, however, shows only an additive reduction of flowering time and the conversion of nearly every shoot meristem, except the inflorescence meristem, to a floral meristem under the same light conditions. In addition, the constitutive expression of FPF1 attenuates the severe lfy-1 phenotype under short days and phenocopies to a great extent the lfy-1 mutant grown under long-day conditions. Thus, we assume that FPF1 modulates the competence to flowering of apical meristems.     

AGAMOUS-LIKE24 and SHORT VEGETATIVE PHASE determine floral meristem identity in Arabidopsis

The formation of flowers starts when floral meristems develop on the flanks of the inflorescence meristem. In Arabidopsis the identity of floral meristems is promoted and maintained by APETALA1 (AP1) and CAULIFLOWER (CAL). In the ap1 cal double mutant the meristems that develop on the flanks of the inflorescence meristem are unable to establish floral meristem identity and develop as inflorescence meristems on which new inflorescence meristems subsequently proliferate. We demonstrate in contrast to previous models that AGAMOUS-LIKE 24 (AGL24) and SHORT VEGETATIVE PHASE (SVP) are also floral meristem identity genes since the ap1-10 agl24-2 svp-41 triple mutant continuously produces inflorescence meristems in place of flowers. Furthermore, our results explain how AP1 switches from a floral meristem identity factor to a component that controls floral organ identity.     

Reproductive meristem fates in Gerbera

Flowering plants go through several phases between regular stem growth and the actual production of flower parts. The stepwise conversion of vegetative into inflorescence and floral meristems is usually unidirectional, but under certain environmental or genetic conditions, meristems can revert to an earlier developmental identity. Vegetative meristems are typically indeterminate, producing organs continuously, whereas flower meristems are determinate, shutting down their growth after reproductive organs are initiated. Inflorescence meristems can show either pattern. Flower and inflorescence development have been investigated in Gerbera hybrida, an ornamental plant in the sunflower family, Asteraceae. Unlike the common model species used to study flower development, Gerbera inflorescences bear a fixed number of flowers, and the architecture of the flowers differ in that Gerbera ovaries are inferior (borne below the perianth). This architectural difference has been exploited to show that floral meristem determinacy and identity are spatially and genetically distinct in Gerbera, and we have shown that a single SEPALLATA-like MADS domain factor controls both flower and inflorescence meristem fate in the plant. Although these phenomena have not been directly observed in Arabidopsis, the integrative role of the SEPALLATA function in reproductive meristem development may be general for all flowering plants.     

Specification of Arabidopsis floral meristem identity by repression of flowering time genes

Flowering plants produce floral meristems in response to intrinsic and extrinsic flowering inductive signals. In Arabidopsis, the floral meristem identity genes LEAFY (LFY) and APETALA1 (AP1) are activated to play a pivotal role in specifying floral meristems during floral transition. We show here that the emerging floral meristems require AP1 to partly specify their floral identities by directly repressing a group of flowering time genes, including SHORT VEGETATIVE PHASE (SVP), AGAMOUS-LIKE 24 (AGL24) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1). In wild-type plants, these flowering time genes are normally downregulated in emerging floral meristems. In the absence of AP1, these genes are ectopically expressed, transforming floral meristems into shoot meristems. By post-translational activation of an AP1-GR fusion protein and chromatin immunoprecipitation assays, we further demonstrate the repression of these flowering time genes by induced AP1 activity and in vivo AP1 binding to the cis-regulatory regions of these genes. These findings indicate that once AP1 is activated during the floral transition, it acts partly as a master repressor in floral meristems by directly suppressing the expression of flowering time genes, thus preventing the continuation of the shoot developmental program.     

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.     

拟南芥LEAFY基因在花发育中的网络调控及其生物学功能

重点综述了拟南芥花分生组织特征基因——LEAFY（LFY）基因及其同源基因在花发育中的网络调控及其生物学功能。LFY基因广泛表达于高等植物的营养性和生殖性组织。LFY基因需要与其他基因相互作用,並且表达量达到一定水平时才能促进成花。LFY基因处于成花调控网络的关键位置,不仅调控开花时间和花转变,而且在花序和花的发育中也起重要作用。碳源、植物激素等因子直接或间接地影响LFY基因的表达和作用。提示通过掌握LFY基因的表达调控规律进一步探讨成花机理的可行性。 Abstract:Recent research progress on regulation network and biological roles of LFY gene in Arabidopsis thaliana and its homologue genes in floral development are reviewed emphatically in the present paper.LFY gene expresses widely in both vegetative and reproductive tissues in different higher plants,therefore investigation on role of LFY gene on flowering is of general significance.LFY gene plays an important role to promote flower formation by interaction and coordination with other genes,such as TFL,EMF,AP1,AP2,CAL,FWA,FT,AP3,PI,AG,UFO,CO,LD,GA1 etc,and a critical level of LFY expression is essential.LFY gene not only controls flowering-time and floral transition,but also plays an important role in inflorescence and floral organ development.It was situated at the central site in gene network of flowering regulation,positively or negatively regulates the level or activities of flowering-related genes.Some physiological factors,such as carbon sources,phytohormones,affect directly or indirectly the expression and actions of LFY gene.This indicates that level of LFY expression can also be regulated with physiological methods.It is probable that we can explain the principal mechanism of flowering by regulation network of LFY gene.