Circadian rhythms: FLOWERING LOCUS T extends opening hours

Plants are more sensitive to light in the day than at night due to the circadian clock. The protein that acts downstream from the clock to modulate blue light signalling in stomata comes as a surprise; it is FT, which is thought to be the long-distance regulator of flowering.     

Repression of FLOWERING LOCUS C and FLOWERING LOCUS T by the Arabidopsis Polycomb repressive complex 2 components

Polycomb group (PcG) proteins are evolutionarily conserved in animals and plants, and play critical roles in the regulation of developmental gene expression. Here we show that the Arabidopsis Polycomb repressive complex 2 (PRC2) subunits CURLY LEAF (CLF), EMBRYONIC FLOWER 2 (EMF2) and FERTILIZATION INDEPENDENT ENDOSPERM (FIE) repress the expression of FLOWERING LOCUS C (FLC), a central repressor of the floral transition in Arabidopsis and FLC relatives. In addition, CLF directly interacts with and mediates the deposition of repressive histone H3 lysine 27 trimethylation (H3K27me3) into FLC and FLC relatives, which suppresses active histone H3 lysine 4 trimethylation (H3K4me3) in these loci. Furthermore, we show that during vegetative development CLF and FIE strongly repress the expression of FLOWERING LOCUS T (FT), a key flowering-time integrator, and that CLF also directly interacts with and mediates the deposition of H3K27me3 into FT chromatin. Our results suggest that PRC2-like complexes containing CLF, EMF2 and FIE, directly interact with and deposit into FT, FLC and FLC relatives repressive trimethyl H3K27 leading to the suppression of active H3K4me3 in these loci, and thus repress the expression of these flowering genes. Given the central roles of FLC and FT in flowering-time regulation in Arabidopsis, these findings suggest that the CLF-containing PRC2-like complexes play a significant role in control of flowering in Arabidopsis.     

The multifaceted roles of FLOWERING LOCUS T in plant development

One of the key developmental processes in flowering plants is the differentiation of the shoot apical meristem into a floral meristem. This transition is regulated through the integration of environmental and endogenous stimuli, involving a complex, hierarchical signalling network. In arabidopsis, the FLOWERING LOCUS T (FT) protein, a mobile signal recognized as a major component of florigen, has a central position in mediating the onset of flowering. FT-like genes seem to be involved in regulating the floral transition in all angiosperms examined to date. Evidence from molecular evolution studies suggests that the emergence of FT-like genes coincided with the evolution of the flowering plants. Hence, the role of FT in floral promotion is conserved, but appears to be restricted to the angiosperms. Besides flowering, FT-like proteins have also been identified as major regulatory factors in a wide range of developmental processes including fruit set, vegetative growth, stomatal control and tuberization. These multifaceted roles of FT-like proteins have resulted from extensive gene duplication events, which occurred independently in nearly all modern angiosperm lineages, followed by sub- or neo-functionalization. This review assesses the plethora of roles that FT-like genes have acquired during evolution and their implications in plant diversity, adaptation and domestication.     

HISTONE DEACETYLASE6 interacts with FLOWERING LOCUS D and regulates flowering in Arabidopsis

Histone acetylation and deacetylation play an important role in epigenetic controls of gene expression. HISTONE DEACETYLASE6 (HDA6) is a REDUCED POTASSIUM DEPENDENCY3-type histone deacetylase, and the Arabidopsis (Arabidopsis thaliana) hda6 mutant axe1-5 displayed a late-flowering phenotype. axe1-5/flc-3 double mutants flowered earlier than axe1-5 plants, indicating that the late-flowering phenotype of axe1-5 was FLOWERING LOCUS C (FLC) dependent. Bimolecular fluorescence complementation, in vitro pull-down, and coimmunoprecipitation assays revealed the protein-protein interaction between HDA6 and the histone demethylase FLD. It was found that the SWIRM domain in the amino-terminal region of FLD and the carboxyl-terminal region of HDA6 are responsible for the interaction between these two proteins. Increased levels of histone H3 acetylation and H3K4 trimethylation at FLC, MAF4, and MAF5 were found in both axe1-5 and fld-6 plants, suggesting functional interplay between histone deacetylase and demethylase in flowering control. These results support a scenario in which histone deacetylation and demethylation cross talk are mediated by physical association between HDA6 and FLD. Chromatin immunoprecipitation analysis indicated that HDA6 bound to the chromatin of several potential target genes, including FLC and MAF4. Genome-wide gene expression analysis revealed that, in addition to genes related to flowering, genes involved in gene silencing and stress response were also affected in hda6 mutants, revealing multiple functions of HDA6. Furthermore, a subset of transposons was up-regulated and displayed increased histone hyperacetylation, suggesting that HDA6 can also regulate transposons through deacetylating histone.     

The microRNA156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE3 module regulates ambient temperature-responsive flowering via FLOWERING LOCUS T in Arabidopsis

The flowering time of plants is affected by modest changes in ambient temperature. However, little is known about the regulation of ambient temperature-responsive flowering by small RNAs. In this study, we show that the microRNA156 (miR156)-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE3 (SPL3) module directly regulates FLOWERING LOCUS T (FT) expression in the leaf to control ambient temperature-responsive flowering. Overexpression of miR156 led to more delayed flowering at a lower ambient temperature (16°C), which was associated with down-regulation of FT and FRUITFULL expression. Among miR156 target genes, SPL3 mRNA levels were mainly reduced, probably because miR156-mediated cleavage of SPL3 mRNA was higher at 16°C. Overexpression of miR156-resistant SPL3 [SPL3(-)] caused early flowering, regardless of the ambient temperature, which was associated with up-regulation of FT and FRUITFULL expression. Reduction of miR156 activity by target mimicry led to a phenotype similar to that of SUC2::rSPL3 plants. FT up-regulation was observed after dexamethasone treatment in GVG-rSPL3 plants. Misexpression and artificial microRNA-mediated suppression of FT in the leaf dramatically altered the ambient temperature-responsive flowering of plants overexpressing miR156 and SPL3(-). Chromatin immunoprecipitation assay showed that the SPL3 protein directly binds to GTAC motifs within the FT promoter. Lesions in TERMINAL FLOWER1, SHORT VEGETATIVE PHASE, and EARLY FLOWERING3 did not alter the expression of miR156 and SPL3. Taken together, our data suggest that the interaction between the miR156-SPL3 module and FT is part of the regulatory mechanism controlling flowering time in response to ambient temperature.     

The FLOWERING LOCUS T/TERMINAL FLOWER 1 family in Lombardy poplar

Genes in the FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1)family have been shown to be important in the control of theswitch between vegetative and reproductive growth in severalplant species. We isolated nine members of the FT/TFL1 familyfrom Lombardy poplar (Populus nigra var. italica Koehne). Sequenceanalysis of the members of the FT/TFL1 family revealed considerablehomology within their coding regions both among family membersand to the members of the same family in Arabidopsis, tomatoand grapevine. Moreover, members of this family in all fourspecies examined display a common exon–intron organization.Phylogenetic analysis revealed that the genes fall into fourdifferent clades: two into the TFL1 clade; five into the FTclade; and one each into the MOTHER OF FT AND TFL1 and BROTHEROF FT AND TFL1 clades. One gene in the TFL1 clade, PnTFL1, isexpressed in vegetative meristems, and transgenic Arabidopsisthat ectopically expressed PnTFL1 had a late-flowering phenotype.The expression patterns of two genes in the FT clade, PnFT1and PnFT2, suggested a role for them in the promotion of flowering,and transgenic Arabidopsis that ectopically expressed eitherPnFT1 or PnFT2 had an early-flowering phenotype.     

Post-transcriptional Regulation of FLOWERING LOCUS T Modulates Heat-Dependent Source-Sink Development in Potato

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Cloning of a FLOWERING LOCUS T ortholog in Wasabia japonica (Matsum)

A FLOWERING LOCUS T ortholog (WjFT) was identified in Wasabia japonica. Heterologous expression of WjFT remarkably promoted the flowering of Arabidopsis. The expression of WjFT was examined in field-grown wasabi in October and November of 2009, and February of 2010 because the differentiation of flower buds occurs in autumn in field-grown wasabi. No expression of WjFT was detected in October, it was slightly increased in November, and highly increased in February. WjFT might be useful for examining the flowering response of wasabi.     

Cloning of a FLOWERING LOCUS T Ortholog in Wasabia japonica (Matsum)

A FLOWERING LOCUS T ortholog (WjFT) was identified in Wasabia japonica. Heterologous expression of WjFT remarkably promoted the flowering of Arabidopsis. The expression of WjFT was examined in field-grown wasabi in October and November of 2009, and February of 2010 because the differentiation of flower buds occurs in autumn in field-grown wasabi. No expression of WjFT was detected in October, it was slightly increased in November, and highly increased in February. WjFT might be useful for examining the flowering response of wasabi.