Analysis of molecular mechanism of action of flowering-time genes in Arabidopsis thaliana

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In the long-day plant, Arabidopsis, environmental and endogenous signals transduced by several pathways are integrated at the transcriptional regulation of so-called "floral pathway integrators". FLOWERING LOCUS T(FI) is one of the floral pathway integrators, and elucidation of molecular mechanism of action of FT gene product is an important step toward understanding of regulation of flowering. With this in mind, we investigated molecular mechanism of the FT action through analysis of two genes. One is a flowering-time gene FD, whose function is required for FT to promote flowering. The other is TWIN SISTER OF FT (TSF) which is a closest homelog of FT in Arabidopsis.We identified the FD gene by a map-based approach. The FD gene encodes a bZIP transcription factor preferentially expressed in the shoot apex. We showed that a floral meristem identity gene APETALA1 (AP1) which acts at the earliest step in floral fate specification is a regulatory target of FD and that FD requires the FT fu … More nction to activate AP1 transcription. Mutual dependence between FT and FD is mediated by protein-protein interaction. which we demonstrated by yeast two-hybrid assay, in vitro pull-down assay, and bimolecular fluorescent complementation assay in plant cells. Expression of FT in either vasculature, which is the site of expression, or shoot apex could rescue ft mutants. By contrast, FD expressed in the shoot apex, but not in vasculature was effective to rescue fd mutants. These results and mutual dependence of FT and FD via protein interaction strongly suggest that the site of action of FT is shoot apex where it may modulate the activity of FD. This led us to propose that FT may represent a long-sought long distance signal in flowering, called florigen (Abe et al., Science, in press).We investigated the role of TSF in regulation flowering. Previous work has suggested a potential role of TSF in promotion of flowering. We showed that TSF and FT share similar modes of regulation : direct positive regulation by CONSTANS, diurnal oscillation with an evening peak, response to photoperiod, negative regulation by FLOWERING LOCUS C, and vernalization response. TSF and FT also share regulation by EARLY BLTING IN SHORT DAYS and phyB. tsf enhanced ft phenotype in both long-day and short-day conditions. These strongly suggest that TSF is a floral pathway integrator acting redundantly with FT. TSF and FT share preferential expression in phloem tissues, which led us to propose that the phloem represents on of the sites of pathway integration (Yamaguchi et al., Plant & Cell Physiology, in press).A current hypothesis from our work is : FT and TSF integrate signals from various pathways in-phloem tissues and their gene products (possibly proteins) are somehow transported to the shoot apex where they modulate the activity of a bZIP transcription factor FD which in turn activate AP! transcription and initiate floral morphogenesis. To test this is an important step forward in understanding flowering. Less