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.     

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.     

Uncovering genetic and molecular interactions among floral meristem identity genes in Arabidopsis thaliana

The inflorescence meristem produces floral primordia that remain undifferentiated during the first stages of flower development. Genes controlling floral meristem identity include LEAFY (LFY), APETALA1 (AP1), CAULIFLOWER (CAL), LATE MERISTEM IDENTITY 1 (LMI1), SHORT VEGETATIVE PHASE (SVP) and AGAMOUS-LIKE24 (AGL24). The lfy mutant shows partial reversions of flowers into inflorescence shoot-like structures and this phenotype is enhanced in the lfy ap1 double mutant. Here we show that combining the lfy mutant with agl24 and svp single mutants or with the agl24 svp double mutant enhances the lfy phenotype and that the lfy agl24 svp triple mutant phenocopies the lfy ap1 double mutant. Analysis of the molecular interactions between LFY, AGL24 and SVP showed that LFY is a repressor of AGL24 and SVP, whereas LMI1 is a positive regulator of these genes. Moreover, AGL24 and SVP positively regulate AP1 and LFY by direct binding to their regulatory regions. Since all these genes are important for establishing floral meristem identity, regulatory loops are probably important to maintain the correct relative expression levels of these genes.     

<Emphasis Type="Italic">SINGLE FLOWER TRUSS</Emphasis> regulates the transition and maintenance of flowering in tomato

The characterisation of the single flower truss ( sft) mutant phenotype of tomato ( Lycopersicon esculentum Mill.), as well as its genetic interactions with other mutations affecting FALSIFLORA ( FA) and SELF PRUNING ( SP) genes, has revealed that SFT is a key gene in the control of floral transition and floral meristem identity. The single sft mutation produces a late-flowering phenotype in both long-day and short-day conditions. In combination with fa, a mutation affecting the tomato gene orthologous to LFY, sft completely blocks the transition to flowering in this species. Thus, the phenotype of the sft fa double mutants indicates that SFT and FA participate in two parallel pathways that regulate the switch from vegetative to reproductive phase in tomato, and that both genes are indispensable for flowering. On the other hand, the replacement of flowers by vegetative shoots observed in the sft inflorescence suggests that SFT regulates flower meristem identity during inflorescence development of tomato. In addition to these two main functions, SFT is involved in the development of both flowers and sympodial shoots of tomato. First, the mutation produces a partial conversion of sepals into leaves in the first floral whorl, and a reduction in the number of floral organs, particularly carpels. Secondly, the sympodial development in the mutant plants is altered, which can be related to the interaction between SFT and SP, a gene controlling the number of nodes in sympodial shoots. In fact, we have found that the sft phenotype is epistatic to that of sp, and that the level of SP mRNA in the apical buds of sft around flowering is reduced. SFT can therefore co-ordinate the regulation of two simultaneous developmental processes in the tomato apical shoot, the promotion of flowering in one sympodial segment and the vegetative development of the next segment.     

Ternary complex formation between the MADS-box proteins SQUAMOSA, DEFICIENS and GLOBOSA is involved in the control of floral architecture in Antirrhinum majus.

In Antirrhinum, floral meristems are established by meristem identity genes. Floral meristems give rise to floral organs in whorls, with their identity established by combinatorial activities of organ identity genes. Double mutants of the floral meristem identity gene SQUAMOSA and organ identity genes DEFICIENS or GLOBOSA produce flowers in which whorled patterning is partially lost. In yeast, SQUA, DEF and GLO proteins form ternary complexes via their C-termini, which in gel-shift assays show increased DNA binding to CArG motifs compared with DEF/GLO heterodimers or SQUA/SQUA homodimers. Formation of ternary complexes by plant MADS-box factors increases the complexity of their regulatory functions and might be the molecular basis for establishment of whorled phyllotaxis and combinatorial interactions of floral organ identity genes.     

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.     

A floret by any other name: control of meristem identity in maize

The life of a plant unfolds as a series of developmental stages, with each stage defined by changes in meristem identity. In maize, there are several distinct stages: the transition from vegetative growth to flowering, the elaboration of the inflorescence, and the formation of flowers. Progress in understanding meristem identity and function has been made by analyzing maize mutants with defects at each of these stages. Recently cloned genes suggest that, although the molecular mechanisms controlling floral organ identity are conserved in maize and other model species, the control of meristem identity during earlier developmental stages might be less conserved.     

The making of a flower: control of floral meristem identity in IT>Arabidopsis/IT>

During the reproductive phase of a plant, shoot meristems follow one of two developmental programs to produce either flowers or vegetative shoots. The decision as to which meristems give rise to flowers, and when they do so, determines the general morphology of an inflorescence. Molecular and genetic research in Arabidopsis and other model species has identified several genes that control the identity that a meristem will adopt. These meristem identity genes are activated in response to developmental and environmental cues, and can be assigned to three basic categories: those required either to initiate or maintain the floral program in some meristems and those required to maintain the vegetative program in others.     

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.     

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.     

Specification of reproductive meristems requires the combined function of SHOOT MERISTEMLESS and floral integrators FLOWERING LOCUS T and FD during Arabidopsis inflorescence development

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. The floral specification activity of FT is dependent upon two related BELL1-like homeobox (BLH) genes PENNYWISE (PNY) and POUND-FOOLISH (PNF) which are required for floral evocation. PNY and PNF interact with a subset of KNOTTED1-LIKE homeobox proteins including SHOOT MERISTEMLESS (STM). Genetic analyses show that these BLH proteins function with STM to specify flowers and internodes during inflorescence development. In this study, experimental evidence demonstrates that the specification of flower and coflorescence meristems requires the combined activities of FT-FD and STM. FT and FD also regulate meristem maintenance during inflorescence development. In plants with reduced STM function, ectopic FT and FD promote the formation of axillary meristems during inflorescence development. Lastly, gene expression studies indicate that STM functions with FT-FD and AGAMOUS-LIKE 24 (AGL24)-SUPPRESSOR OF OVEREXPRESSION OF CONTANS1 (SOC1) complexes to up-regulate flower meristem identity genes during inflorescence development.     

Determination of Arabidopsis floral meristem identity by AGAMOUS.

Determinate growth of floral meristems in Arabidopsis requires the function of the floral regulatory gene AGAMOUS (AG). Expression of AG mRNA in the central region of floral meristems relies on the partially overlapping functions of the LEAFY (LFY) and APETALA1 (AP1) genes, which promote initial floral meristem identity. Here, we provide evidence that AG function is required for the final definition of floral meristem identity and that constitutive AG function can promote, independent of LFY and AP1 functions, the determinate floral state in the center of reproductive meristems. Loss-of-function analysis showed that the indeterminate central region of the ag mutant floral meristem undergoes conversion to an inflorescence meristem when long-day-dependent flowering stimulus is removed. Furthermore, gain-of-function analysis demonstrated that ectopic AG function results in precocious flowering and the formation of terminal flowers at apices of both the primary inflorescence and axillary branches of transgenic Arabidopsis plants in which AG expression is under the control of the 35S promoter from cauliflower mosaic virus. Similar phenotypes were also observed in lfy ap1 double mutants carrying a 35S-AG transgene. Together, these results indicate that AG is a principal developmental switch that controls the transition of meristem activity from indeterminate to determinate.     

Homeosis, morphogenetic gradient and the determination of floral identity in the inflorescences ofPhilodendron solimoesense (Araceae)

A comparative developmental study of the inflorescence ofPhilodendron solimoesense was conducted using scanning electron microscopy. The spadix ofP. solimoesense is characterized by unisexual flowers. Staminate flowers are initiated on the upper portion of the spadix while pistillate flowers develop on the lower portion of the spadix. An intermediate zone located between the upper male and lower female portion of the inflorescence consists of sterile male flowers. Within this intermediate zone a row of flowers exhibit polarity with respect to the identity of sexual organs. Stamens are initiated on the flank of the floral meristem facing the upper male zone and carpels are initiated on the portion of the floral meristem facing the lower female zone. The resulting flowers therefore assume a bisexual identity. At the level of the inflorescence, all floral buds are initiated along a series of contact parastichies and the continuity of these parastichies is not disrupted at any level in the male, intermediate, and female zones on the spadix. Results from this study support the presence of a morphogenetic gradient acting at the level of the inflorescence and appears to be independent of the boundaries of floral primordia.     

A short history of MADS-box genes in plants

Evolutionary developmental genetics (evodevotics) is a novel scientific endeavor which assumes that changes in developmental control genes are a major aspect of evolutionary changes in morphology. Understanding the phylogeny of developmental control genes may thus help us to understand the evolution of plant and animal form. The principles of evodevotics are exemplified by outlining the role of MADS-box genes in the evolution of plant reproductive structures. In extant eudicotyledonous flowering plants, MADS-box genes act as homeotic selector genes determining floral organ identity and as floral meristem identity genes. By reviewing current knowledge about MADS-box genes in ferns, gymnosperms and different types of angiosperms, we demonstrate that the phylogeny of MADS-box genes was strongly correlated with the origin and evolution of plant reproductive structures such as ovules and flowers. It seems likely, therefore, that changes in MADS-box gene structure, expression and function have been a major cause for innovations in reproductive development during land plant evolution, such as seed, flower and fruit formation.     

The blooming of grass flower development

The past half decade has provided a wealth of information concerning the molecular and genetic control of floral organ and meristem identity in dicotyledonous plants. Comparatively little is understood about these processes in grass species in spite of the importance that these species play in human agriculture. The isolation of grass genes that are homologous to dicot floral homeotic genes in combination with recent advances in reverse genetic technology and improvements in cereal transformation opens the door for understanding molecular mechanisms of grass flower development. Such information will also focus attention on the evolutionary relationships between grass and dicot flowers and the degree to which the developmental pathways leading to reproductive organ development in divergent angiosperms have utilized conserved mechanisms.