Post-fertilization expression of FLOWERING LOCUS T suppresses reproductive reversion

FLOWERING LOCUS T-mediated thermal signalling regulates age-dependent inflorescence development in Arabidopsis thaliana

Many plants show strong heteroblastic changes in the shape and size of organs as they transition from juvenile to reproductive age. Most attention has been focused on heteroblastic development in leaves, but we wanted to understand heteroblastic changes in reproductive organ size. We therefore studied the progression of reproductive development in the model plant Arabidopsis thaliana, and found strong reductions in the size of flowers, fruit, seed, and internodes during development. These did not arise from correlative inhibition by older fruits, or from changes in inflorescence meristem size, but seemed to stem from changes in the size of floral organ primordia themselves. We hypothesized that environmental conditions might influence this heteroblastic pattern and found that the ambient temperature during organ initiation strongly influences organ size. We show that this temperature-dependent heteroblasty is dependent on FLOWERING LOCUS T (FT)-mediated signal integration, adding to the repertoire of developmental processes regulated by this pathway. Our results demonstrate that rising global temperatures will not affect just fertility, as is widely described, but also the size and seed number of fruits produced. However, we also show that such effects are not hard-wired, and that selective breeding for FT expression during reproductive development could mitigate such effects.

Photoperiod controls plant seed size in a CONSTANS-dependent manner

Photoperiodic plants perceive changes in day length as seasonal cues to orchestrate their vegetative and reproductive growth. Although it is known that the floral transition of photoperiod-sensitive plants is tightly controlled by day length, how photoperiod affects their post-flowering development remains to be clearly defined, as do the underlying mechanisms. Here we demonstrate that photoperiod plays a prominent role in seed development. We found that long-day (LD) and short-day (SD) plants produce larger seeds under LD and SD conditions, respectively; however, seed size remains unchanged when CONSTANS (CO), the central regulatory gene of the photoperiodic response pathway, is mutated in Arabidopsis and soybean. We further found that CO directly represses the transcription of AP2 (a known regulatory gene of seed development) under LD conditions in Arabidopsis and SD conditions in soybean, thereby controlling seed size in a photoperiod-dependent manner, and that these effects are exerted through regulation of the proliferation of seed coat epidermal cells. Collectively, our findings reveal that a crucial regulatory cascade involving CO-AP2 modulates photoperiod-mediated seed development in plants and provide new insights into how plants with different photoperiod response types perceive seasonal changes that enable them to optimize their reproductive growth.

Transcriptome Analysis to Identify Genes Related to Flowering Reversion in Tomato

Flowering reversion is a common phenomenon in plant development in which differentiated floral organs switch from reproductive growth to vegetative growth and ultimately form abnormal floral organs or vegetative organs. This greatly reduces tomato yield and quality. Research on this phenomenon has recently increased, but there is a lack of research at the molecular and gene expression levels. Here, transcriptomic analyses of the inflorescence meristem were performed in two kinds of materials at different developmental stages, and a total of 3223 differentially expressed genes (DEGs) were screened according to the different developmental stages and trajectories of the two materials. The analysis of database annotations showed that these DEGs were closely related to starch and sucrose metabolism, DNA replication and modification, plant hormone synthesis and signal transduction. It was further speculated that tomato flowering reversion may be related to various biological processes, such as cell signal transduction, energy metabolism and protein post-transcriptional regulation. Combined with the results of previous studies, our work showed that the gene expression levels of CLE9, FA, PUCHI, UF, CLV3, LOB30, SFT, S-WOX9 and SVP were significantly different in the two materials. Endogenous hormone analysis and exogenous hormone treatment revealed a variety of plant hormones involved in flowering reversion in tomato. Thus, tomato flowering reversion was studied comprehensively by transcriptome analysis for the first time, providing new insights for the study of flower development regulation in tomato and other plants.

Polycomb proteins control floral determinacy by H3K27me3-mediated repression of pluripotency genes in Arabidopsis thaliana

Polycomb group (PcG) protein-mediated histone methylation (H3K27me3) controls the correct spatiotemporal expression of numerous developmental regulators in Arabidopsis. Epigenetic silencing of the stem cell factor gene WUSCHEL (WUS) in floral meristems (FMs) depends on H3K27me3 deposition by PcG proteins. However, the role of H3K27me3 in silencing of other meristematic regulator and pluripotency genes during FM determinacy has not yet been studied. To this end, we report the genome-wide dynamics of H3K27me3 levels during FM arrest and the consequences of strongly depleted PcG activity on early flower morphogenesis including enlarged and indeterminate FMs. Strong depletion of H3K27me3 levels results in misexpression of the FM identity gene AGL24, which partially causes floral reversion leading to ap1-like flowers and indeterminate FMs ectopically expressing WUS and SHOOT MERISTEMLESS (STM). Loss of STM can rescue supernumerary floral organs and FM indeterminacy in H3K27me3-deficient flowers, indicating that the hyperactivity of the FMs is at least partially a result of ectopic STM expression. Nonetheless, WUS remained essential for the FM activity. Our results demonstrate that PcG proteins promote FM determinacy at multiple levels of the floral gene regulatory network, silencing initially floral regulators such as AGL24 that promotes FM indeterminacy and, subsequently, meristematic pluripotency genes such as WUS and STM during FM arrest.

Fine-Tuning Florigen Increases Field Yield Through Improving Photosynthesis in Soybean

Crop yield has been maintaining its attraction for researchers because of the demand of global population growth. Mutation of flowering activators, such as florigen, increases plant biomass at the expense of later flowering, which prevents crop maturity in the field. As a result, it is difficult to apply flowering activators in agriculture production. Here, we developed a strategy to utilize florigen to significantly improve soybean yield in the field. Through the screening of transgenic lines of RNAi-silenced florigen homologs in soybean (Glycine-max-Flowering Locus T Like, GmFTL), we identified a line, GmFTL-RNAi#1, with minor changes in both GmFTL expression and flowering time but with notable increase in soybean yield. As expected, GmFTL-RNAi#1 matured normally in the field and exhibited markedly high yield over multiple locations and years, indicating that it is possible to reach a trade-off between flowering time and high yield through the fine-tuning expression of flowering activators. Further studies uncovered an unknown mechanism by which GmFTL negatively regulates photosynthesis, a substantial source of crop yield, demonstrating a novel function of florigen. Thus, because of the highly conserved functions of florigen in plants and the classical RNAi approach, the findings provide a promising strategy to harness early flowering genes to improve crop yield.

Inter-species functional compatibility of the Theobroma cacao and Arabidopsis FT orthologs: 90 million years of functional conservation of meristem identity genes

BACKGROUND

In angiosperms the transition to flowering is controlled by a complex set of interacting networks integrating a range of developmental, physiological, and environmental factors optimizing transition time for maximal reproductive efficiency. The molecular mechanisms comprising these networks have been partially characterized and include both transcriptional and post-transcriptional regulatory pathways. Florigen, encoded by FLOWERING LOCUS T (FT) orthologs, is a conserved central integrator of several flowering time regulatory pathways. To characterize the molecular mechanisms involved in controlling cacao flowering time, we have characterized a cacao candidate florigen gene, TcFLOWERING LOCUS T (TcFT). Understanding how this conserved flowering time regulator affects cacao plant's transition to flowering could lead to strategies to accelerate cacao breeding.

RESULTS

BLAST searches of cacao genome reference assemblies identified seven candidate members of the CENTRORADIALIS/TERMINAL FLOWER1/SELF PRUNING gene family including a single florigen candidate. cDNA encoding the predicted cacao florigen was cloned and functionally tested by transgenic genetic complementation in the Arabidopsis ft-10 mutant. Transgenic expression of the candidate TcFT cDNA in late flowering Arabidopsis ft-10 partially rescues the mutant to wild-type flowering time. Gene expression studies reveal that TcFT is spatially and temporally expressed in a manner similar to that found in Arabidopsis, specifically, TcFT mRNA is shown to be both developmentally and diurnally regulated in leaves and is most abundant in floral tissues. Finally, to test interspecies compatibility of florigens, we transformed cacao tissues with AtFT resulting in the remarkable formation of flowers in tissue culture. The morphology of these in vitro flowers is normal, and they produce pollen that germinates in vitro with high rates.

CONCLUSION

We have identified the cacao CETS gene family, central to developmental regulation in angiosperms. The role of the cacao's single FT-like gene (TcFT) as a general regulator of determinate growth in cacao was demonstrated by functional complementation of Arabidopsis ft-10 late-flowering mutant and through gene expression analysis. In addition, overexpression of AtFT in cacao resulted in precocious flowering in cacao tissue culture demonstrating the highly conserved function of FT and the mechanisms controlling flowering in cacao.

TERMINAL FLOWER 1-FD complex target genes and competition with FLOWERING LOCUS T

Plants monitor seasonal cues to optimize reproductive success by tuning onset of reproduction and inflorescence architecture. TERMINAL FLOWER 1 (TFL1) and FLOWERING LOCUS T (FT) and their orthologs antagonistically regulate these life history traits, yet their mechanism of action, antagonism and targets remain poorly understood. Here, we show that TFL1 is recruited to thousands of loci by the bZIP transcription factor FD. We identify the master regulator of floral fate, LEAFY (LFY) as a target under dual opposite regulation by TFL1 and FT and uncover a pivotal role of FT in promoting flower fate via LFY upregulation. We provide evidence that the antagonism between FT and TFL1 relies on competition for chromatin-bound FD at shared target loci. Direct TFL1-FD regulated target genes identify this complex as a hub for repressing both master regulators of reproductive development and endogenous signalling pathways. Our data provide mechanistic insight into how TFL1-FD sculpt inflorescence architecture, a trait important for reproductive success, plant architecture and yield.

Evolution and functional diversification of FLOWERING LOCUS T/TERMINAL FLOWER 1 family genes in plants

Plant growth and development, particularly the induction of flowering, are tightly controlled by key regulators in response to endogenous and environmental cues. The FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family of phosphatidylethanolamine-binding protein (PEBP) genes is central to plant development, especially the regulation of flowering time and plant architecture. FT, the long-sought florigen, promotes flowering and TFL1 represses flowering. The balance between FT and TFL1 modulates plant architecture by switching the meristem from indeterminate to determinate growth, or vice versa. Recent studies in a broad range of plant species demonstrated that, in addition to their roles in flowering time and plant architecture, FT/TFL1 family genes participate in diverse aspects of plant development, such as bamboo seed germination and potato tuber formation. In this review, we briefly summarize the evolution of the FT/TFL1 family and highlight recent findings on their conserved and divergent functions in different species.

Plant Inflorescence Architecture: The Formation, Activity, and Fate of Axillary Meristems

The above-ground plant body in different plant species can have very distinct forms or architectures that arise by recurrent redeployment of a finite set of building blocks-leaves with axillary meristems, stems or branches, and flowers. The unique architectures of plant inflorescences in different plant families and species, on which this review focuses, determine the reproductive success and yield of wild and cultivated plants. Major contributors to the inflorescence architecture are the activity and developmental trajectories adopted by axillary meristems, which determine the degree of branching and the number of flowers formed. Recent advances in genetic and molecular analyses in diverse flowering plants have uncovered both common regulatory principles and unique players and/or regulatory interactions that underlie inflorescence architecture. Modulating activity of these regulators has already led to yield increases in the field. Additional insight into the underlying regulatory interactions and principles will not only uncover how their rewiring resulted in altered plant form, but will also enhance efforts at optimizing plant architecture in desirable ways in crop species.

Functional divergence between soybean FLOWERING LOCUS T orthologues FT2a and FT5a in post-flowering stem growth

Genes in the FLOWERING LOCUS T (FT) family integrate external and internal signals to control various aspects of plant development. In soybean (Glycine max), FT2a and FT5a play a major role in floral induction, but their roles in post-flowering reproductive development remain undetermined. Ectopic overexpression analyses revealed that FT2a and FT5a similarly induced flowering, but FT5a was markedly more effective than FT2a for the post-flowering termination of stem growth. The down-regulation of Dt1, a soybean orthologue of Arabidopsis TERMINAL FLOWER1, in shoot apices in early growing stages of FT5a-overexpressing plants was concomitant with highly up-regulated expression of APETALA1 orthologues. The Dt2 gene, a repressor of Dt1, was up-regulated similarly by the overexpression of FT2a and FT5a, suggesting that it was not involved in the control of stem termination by FT5a. In addition to the previously reported interaction with FDL19, a homologue of the Arabidopsis bZIP protein FD, both FT2a and FT5a interacted with FDL12, but only FT5a interacted with FDL06. Our results suggest that FT2a and FT5a have different functions in the control of post-flowering stem growth. A specific interaction of FT5a with FDL06 may play a key role in determining post-flowering stem growth in soybean.

Transient activity of the florigen complex during the floral transition in Arabidopsis thaliana

FLOWERING LOCUS T (FT) is an essential component of florigen in Arabidopsis thaliana Transcription of FT is induced in leaves, and the resulting FT protein is transported to the shoot apex, in which it initiates floral development. Previous analyses suggest that, together with the b-ZIP transcription factor FD, FT regulates the transcription of downstream targets such as APETALA1 (AP1) in floral anlagen. However, conclusive in vivo evidence that FT is transported to the shoot apex to form an FT-FD complex is lacking. Here, using an innovative in vivo imaging technique, we show that the FT-FD complex and AP1 colocalise in floral anlagen. In addition, the FT-FD complex disappears soon after the floral transition owing to a reduction in FD transcripts in the shoot apex. We further show that misinduction of FD activity after the transition leads to defective reproductive development. Taken together, our results indicate that the FT-FD complex functions as a transient stimulus and imply that a regulatory mechanism exists during the floral transition that reduces FT-FD complex levels via modulation of FD expression.

Natural variation in Brassica FT homeologs influences multiple agronomic traits including flowering time, silique shape, oil profile, stomatal morphology and plant height in B. juncea

Natural structural variants of regulatory proteins causing quantitative phenotypic consequences have not been reported in plants. Herein, we show that 28 natural structural variants of FT homeologs, isolated from 6 species of Brassica, differ with respect to amino-acid substitutions in regions critical for interactions with FD and represent two evolutionarily distinct categories. Analysis of structural models of selected candidates from Brassica juncea (BjuFT_AAMF1) and Brassica napus (BnaFT_CCLF) predicted stronger binding between BjuFT and Arabidopsis thaliana FD. Over-expression of BjuFT and BnaFT in wild type and ft-10 mutant backgrounds of Arabidopsis validated higher potency of BjuFT in triggering floral transition. Analysis of gain-of-function and artificial miRNA mediated silenced lines of B. juncea implicated Brassica FT in multiple agronomic traits beyond flowering, consistent with a pleiotropic effect. Several dependent and independent traits such as lateral branching, silique shape, seed size, oil-profile, stomatal morphology and plant height were found altered in mutant lines. Enhanced FT levels caused early flowering, which in turn was positively correlated to a higher proportion of desirable fatty acids (PUFA). However, higher FT levels also resulted in altered silique shape and reduced seed size, suggesting trait trade-offs. Modulation of FT levels for achieving optimal balance of trait values and parsing pair-wise interactions among a reportoire of regulatory protein homeologs in polyploid genomes are indeed future areas of crop research.

Extended Vernalization Regulates Inflorescence Fate in Arabis alpina by Stably Silencing PERPETUAL FLOWERING1

The alpine perennial Arabis alpina initiates flower buds during prolonged exposure to cold. In the accession Pajares, we demonstrate that the length of vernalization influences flowering time and inflorescence fate but does not affect the axillary branches that maintain vegetative growth. The expression of floral organ identity genes gradually increases in the main shoot apex during vernalization, correlating with an increase in floral commitment. In northern Arabidopsis (Arabidopsis thaliana) accessions, the length of vernalization modulates the stable silencing of the floral repressor FLOWERING LOCUS C (FLC). We demonstrate that expression of PERPETUAL FLOWERING1 (PEP1), the ortholog of FLC in A. alpina, is similarly influenced by the duration of the exposure to cold. Extended vernalization results in stable silencing of PEP1 in the inflorescence. In contrast, insufficient vernalization leads to PEP1 reactivation after cold treatment, which correlates with delayed flowering and the appearance of floral reversion phenotypes such as bracts and vegetative inflorescence branches. Floral reversion phenotypes are reduced in the pep1-1 mutant, suggesting that PEP1 regulates the fate of the inflorescence after vernalization. The effect of vernalization duration on stable silencing of PEP1 is specific to meristems that initiate flowering during cold treatment. Extended vernalization fails to silence PEP1 in young seedlings and axillary branches that arise from buds initiated during cold treatment, which remain vegetative. We conclude that the duration of vernalization in A. alpina differentially regulates PEP1 in the inflorescence and axillary branches. PEP1 has a dual role regulating meristem fate; it prevents meristems from flowering and antagonizes inflorescence development after vernalization.

Unravelling proximate cues of mass flowering in the tropical forests of South-East Asia from gene expression analyses

Elucidating the physiological mechanisms of the irregular yet concerted flowering rhythm of mass flowering tree species in the tropics requires long-term monitoring of flowering phenology, exogenous and endogenous environmental factors, as well as identifying interactions and dependencies among these factors. To investigate the proximate factors for floral initiation of mast seeding trees in the tropics, we monitored the expression dynamics of two key flowering genes, meteorological conditions and endogenous resources over two flowering events of Shorea curtisii and Shorea leprosula in the Malay Peninsula. Comparisons of expression dynamics of genes studied indicated functional conservation of FLOWERING LOCUS T (FT) and LEAFY (LFY) in Shorea. The genes were highly expressed at least 1 month before anthesis for both species. A mathematical model considering the synergistic effect of cool temperature and drought on activation of the flowering gene was successful in predicting the observed gene expression patterns. Requirement of both cool temperature and drought for floral transition suggested by the model implies that flowering phenologies of these species are sensitive to climate change. Our molecular phenology approach in the tropics sheds light on the conserved role of flowering genes in plants inhabiting different climate zones and can be widely applied to dissect the flowering processes in other plant species.

Flowering time in banana (Musa spp.), a day neutral plant, is controlled by at least three FLOWERING LOCUS T homologues

Banana is an important day neutral food crop with a long flowering/fruiting cycle that is affected by hot summers or cold winters in different places. Manipulating its life cycle requires an understanding of its flowering time machinery to bypass these stresses. Twelve FLOWERING LOCUS T (FT) and two TWIN SISTER OF FT (TSF) members were isolated from banana and their organization and expression pattern studied during development in two varieties that differ in flowering time namely Grand Nain (AAA genotype) and Hill banana (AAB genotype). The expression of at least 3 genes namely MaFT1, MaFT2 and MaFT5 (and to some extent MaFT7) increases just prior to initiation of flowering. These four genes and five others (MaFT3, MaFT4, MaFT8, MaFT12 and MaTSF1 could suppress the delayed flowering defect in the Arabidopsis ft-10 mutant and induce early flowering upon over-expression in the Col-0 ecotype. Most genes are diurnally regulated and differentially expressed during development and in various vegetative and reproductive tissues suggesting roles besides flowering. Subtle amino acid changes in these FT/TSF-like proteins provide interesting insights into the structure/function relationships of banana FTs vis-à-vis Arabidopsis. The studies provide a means for manipulation of flowering in banana for better management of resources and to reduce losses through abiotic stresses.

Key developmental transitions during flower morphogenesis and their regulation

The arrangement of flowers on flowering stems called inflorescences contributes to the beauty of the natural world and enhances seed yield, impacting species survival and human sustenance. During the reproductive phase, annual/monocarpic plants like Arabidopsis and most crops form two types of lateral structures: indeterminate lateral inflorescences and determinate flowers. Their stereotypical arrangement on the primary inflorescence stem determines the species-specific inflorescence architecture. This architecture can be modulated in response to environmental cues to enhance reproductive success. Early botanists already appreciated that flowers and lateral inflorescences are analogous structures that are interconvertible. Here I will discuss the molecular underpinnings of these observations and explore the regulatory logic of the developmental fate transitions that lead to the formation of a flower.

Florigen distribution determined by a source-sink balance explains the diversity of inflorescence structures in Arabidopsis

The ability to continue flowering after loss of inductive environmental cues that trigger flowering is termed floral commitment. Reversible transition involving a switch from floral development back to vegetative development has been found in Arabidopsis mutants and many plant species. Although the molecular basis for floral commitment remains unclear, recent studies suggest that the persistent activity of FLOWERING LOCUS T (FT) at inflorescences is required for floral commitment in Arabidopsis thaliana. Because FT encodes a mobile signal, florigen, which is generally transported from leaves to meristems through the phloem, understanding the transportation dynamics of FT is required to explore the role of FT on floral commitment. Here we developed a transportation model of leaf- and inflorescence-derived florigen and sucrose based on pressure-flow hypothesis. Depending on the demanded level of florigen supply for floral commitment of each floral meristem, the model predicted the change in inflorescence pattern from stable commitment to flower, transient flowering, and complete reversion. FT activity in inflorescence partly suppressed floral reversion, but complete suppression was achieved only when inflorescence became a source of sucrose. This finding highlights the importance of monitoring the spatio-temporal sucrose distribution and floral stimulus to understand inflorescence development mechanism.

The age of multiplexity: recruitment and interactions of Polycomb complexes in plants

Polycomb group (PcG) proteins form distinct complexes that modify chromatin by histone H3 methylation and H2A mono-ubiquitination leading to chromatin compaction and epigenetic repression of target genes. A network of PcG protein complexes, associated partners and antagonistically acting chromatin modifiers is essential to regulate developmental transitions and cell fate in all multicellular eukaryotes. In this review, we discuss insights on the subfunctionalization of PcG complexes and their modes of recruitment to target sites based on data from the model organism Arabidopsis thaliana.

Extensive Analysis of GmFTL and GmCOL Expression in Northern Soybean Cultivars in Field Conditions

The FLOWERING LOCUS T (FT) gene is a highly conserved florigen gene among flowering plants. Soybean genome encodes six homologs of FT, which display flowering activity in Arabidopsis thaliana. However, their contributions to flowering time in different soybean cultivars, especially in field conditions, are unclear. We employed six soybean cultivars with different maturities to extensively investigate expression patterns of GmFTLs (Glycine max FT-like) and GmCOLs (Glycine max CO-like) in the field conditions. The results show that GmFTL3 is an FT homolog with the highest transcript abundance in soybean, but other GmFTLs may also contribute to flower induction with different extents, because they have more or less similar expression patterns in developmental-, leaf-, and circadian-specific modes. And four GmCOL genes (GmCOL1/2/5/13) may confer to the expression of GmFTL genes. Artificial manipulation of GmFTL expression by transgenic strategy (overexpression and RNAi) results in a distinct change in soybean flowering time, indicating that GmFTLs not only impact on the control of flowering time, but have potential applications in the manipulation of photoperiodic adaptation in soybean. Additionally, transgenic plants show that GmFTLs play a role in formation of the first flowers and in vegetative growth.

Molecular memories in the regulation of seasonal flowering: from competence to cessation

Plants commit to flowering based on endogenous and exogenous information that they can remember across mitotic cell divisions. Here, we review how signal perception and epigenetic memory converge at key integrator genes, and we show how variation in their regulatory circuits supports the diversity of plant lifestyles.

Global Transcriptome Profiling of Developing Leaf and Shoot Apices Reveals Distinct Genetic and Environmental Control of Floral Transition and Inflorescence Development in Barley

Timing of the floral transition and inflorescence development strongly affect yield in barley (Hordeum vulgare). Therefore, we examined the effects of daylength and the photoperiod response gene PHOTOPERIOD1 (Ppd-H1) on barley development and analyzed gene expression changes in the developing leaves and main shoot apices (MSAs) of barley by RNA sequencing. The daylength sensitivity of MSA development had two phases, floret primordia initiated under long and short days, whereas successful inflorescence development occurred only under long days. The transcripts associated with floral transition were largely regulated independently of photoperiod and allelic variation at Ppd-H1. The photoperiod- and Ppd-H1-dependent differences in inflorescence development and flower fertility were associated with the induction of barley FLOWERING LOCUS T orthologs: FT1 in leaves and FT2 in MSAs. FT1 expression was coregulated with transcripts involved in nutrient transport, carbohydrate metabolism, and cell cycle regulation, suggesting that FT1 might alter source-sink relationships. Successful inflorescence development correlated with upregulation of FT2 and transcripts related to floral organ development, phytohormones, and cell cycle regulation. Identification of photoperiod and stage-specific transcripts gives insights into the regulation of reproductive development in barley and provides a resource for investigation of the complexities of development and yield in temperate grasses.

Keeping the gate closed: functions of the polycomb repressive complex PRC2 in development

Plant ontogeny relies on the correct timing and sequence of transitions between individual developmental phases. These are specified by gene expression patterns that are established by the balanced action of activators and repressors. Polycomb repressive complexes (PRCs) represent an evolutionarily conserved system of epigenetic gene repression that governs the establishment and maintenance of cell, tissue and organ identity, contributing to the correct execution of the developmental programs. PRC2 is a four-subunit histone methyltransferase complex that catalyzes trimethylation of lysine 27 on histone H3 (H3K27me3), which contributes to the change of chromatin structure and long-lasting gene repression. Here, we review the composition and molecular function of the different known PRC2 complexes in plants, and focus on the role of PRC2 in mediating the establishment of different developmental phases and transitions between them.

Pea VEGETATIVE2 Is an FD Homolog That Is Essential for Flowering and Compound Inflorescence Development

As knowledge of the gene networks regulating inflorescence development in Arabidopsis thaliana improves, the current challenge is to characterize this system in different groups of crop species with different inflorescence architecture. Pea (Pisum sativum) has served as a model for development of the compound raceme, characteristic of many legume species, and in this study, we characterize the pea VEGETATIVE2 (VEG2) locus, showing that it is critical for regulation of flowering and inflorescence development and identifying it as a homolog of the bZIP transcription factor FD. Through detailed phenotypic characterizations of veg2 mutants, expression analyses, and the use of protein-protein interaction assays, we find that VEG2 has important roles during each stage of development of the pea compound inflorescence. Our results suggest that VEG2 acts in conjunction with multiple FLOWERING LOCUS T (FT) proteins to regulate expression of downstream target genes, including TERMINAL FLOWER1, LEAFY, and MADS box homologs, and to facilitate cross-regulation within the FT gene family. These findings further extend our understanding of the mechanisms underlying compound inflorescence development in pea and may have wider implications for future manipulation of inflorescence architecture in related legume crop species.

Maternal temperature history activates Flowering Locus T in fruits to control progeny dormancy according to time of year

Seasonal behavior is important for fitness in temperate environments but it is unclear how progeny gain their initial seasonal entrainment. Plants use temperature signals to measure time of year, and changes to life histories are therefore an important consequence of climate change. Here we show that in Arabidopsis the current and prior temperature experience of the mother plant is used to control germination of progeny seeds, via the activation of the florigen Flowering Locus T (FT) in fruit tissues. We demonstrate that maternal past and current temperature experience are transduced to the FT locus in silique phloem. In turn, FT controls seed dormancy through inhibition of proanthocyanidin synthesis in fruits, resulting in altered seed coat tannin content. Our data reveal that maternal temperature history is integrated through FT in the fruit to generate a metabolic signal that entrains the behavior of progeny seeds according to time of year.

Induced and natural variation of promoter length modulates the photoperiodic response of FLOWERING LOCUS T

FLOWERING LOCUS T (FT) regulates the floral transition in many plant species by integrating environmental seasonal signals and internal cues. Here we show that two interdependent regulatory regions are necessary and sufficient to convey photoperiod responsiveness to FT. While a minimal distance between the regulatory regions is required to fully suppress FT expression under short days, increased distance reduces promoter response to long days. Natural variation at FT creating promoter length differences is widespread, correlates with longitudinal and latitudinal clines and affects a promoter region physically interacting with both photoperiod control regions. Three major FT promoter variants correlate with differences in FT allele usage in F1 hybrids. We propose that FT variation in cis could be adaptive by conferring differences in FT transcriptional control ultimately translating to increased fitness.

Gene expression at FLOWERING LOCUS T controls floral transition in many plants and is regulated by both environmental signals and internal cues. Liu et al. show that the distance between two regulatory sequences in the FT promoter varies with geographical location and determines responsiveness to photoperiod.

Polycomb-Group Proteins and FLOWERING LOCUS T Maintain Commitment to Flowering in Arabidopsis thaliana

The switch from vegetative to reproductive growth is extremely stable even if plants are only transiently exposed to environmental stimuli that trigger flowering. In the photoperiodic pathway, a mobile signal, florigen, encoded by FLOWERING LOCUS T (FT) in Arabidopsis thaliana, induces flowering. Because FT activity in leaves is not maintained after transient photoperiodic induction, the molecular basis for stable floral commitment is unclear. Here, we show that Polycomb-group (Pc-G) proteins, which mediate epigenetic gene regulation, maintain the identity of inflorescence and floral meristems after floral induction. Thus, plants with reduced Pc-G activity show a remarkable increase of cauline leaves under noninductive conditions and floral reversion when shifted from inductive to noninductive conditions. These phenotypes are almost completely suppressed by loss of FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE, which both delay flowering and promote vegetative shoot identity. Upregulation of FLC in Pc-G mutants leads to a strong decrease of FT expression in inflorescences. We find that this activity of FT is needed to prevent floral reversion. Collectively, our results reveal that floral meristem identity is at least partially maintained by a daylength-independent role of FT whose expression is indirectly sustained by Pc-G activity.