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'Florigen' was proposed 75 years ago1 to be synthesized in the leaf and transported to the shoot apex, where it induces flowering. Only recently have genetic and biochemical studies established that florigen is encoded by FLOWERING LOCUS T (FT), a gene that is universally conserved in higher plants2-4.Nonetheless, the exact function of florigen during floral induction remains poorly understood and receptors for florigen have not been identified. Herewe showthat the rice FT homologueHd3a5 interacts with 14-3-3 proteins in the apical cells of shoots, yielding a complex that translocates to the nucleus and binds to the Oryza sativa (Os)FD1 transcription factor, a rice homologue of Arabidopsis thaliana FD. The resultant ternary 'florigen activation complex' (FAC) induces transcription of OsMADS15, a homologue of A. thaliana APETALA1 (AP1), which leads to flowering. We have determined the 2.4Å crystal structure of rice FAC, which provides amechanistic basis for florigen function in flowering. Our results indicate that 14-3-3 proteins act as intracellular receptors for florigen in shoot apical cells, and offer new approaches to manipulate flowering in various crops and trees.
In the current model, florigen is transported fromleaves to the shoot apex, where it interacts with the basic leucine zipper (bZIP) transcription factor FD to activate expression of floral-meristem identity genes such as AP1 (refs 6, 7). We previously demonstrated that the rice FT homologue Hd3a is a strong candidate for florigen, based on an analysis of transgenic rice expressing GFP-tagged Hd3a5. Furthermore, we obtained evidence that another homologue of Hd3a, RICE FLOWERING LOCUS T1 (RFT1), also acts as a rice florigen under long-day conditions8, and that Hd3a and RFT1 activate the expression of OsMADS15 (refs 8-10), indicating that the mechanism of florigen action is conserved in rice and Arabidopsis.
To understandmore precisely howtheHd3a florigen induces flowering, we performed yeast two-hybrid screening11 and identified GF14c (a rice 14-3-3 protein), OsKANADI1 (a rice homologue of Arabidopsis KANADI) and OsBIP116b (a rice homologue of Arabidopsis WAVEDAMPENED2) as three Hd3a-interacting partners (Fig. 1a). It had previously been reported that 14-3-3 proteins interact with FT proteins12,13. We also identified OsFD1, a rice FD homologue (Supplementary Fig. 1). Three Hd3a interactors, but not GF14c, share a common SAP motif (Ser-Ala-Pro) (Fig. 1a), which resembles the FTinteracting motif in FD6. Alanine substitutions in the...