Structure-based discovery of flowering hormone receptor

Chojiro Kojima

Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka 565-0871, Japan

kojima@protein.osaka-u.ac.jp

Florigen, a mobile floral induction factor encoded by FLOWERING LOCUS T (FT) and its homologs, activates floral identity genes such as APETALA1 (AP1) at the shoot apex. Florigen is produced in leaves and is transmitted through the phloem to the shoot apex, where it induces flowering. A number of recent reports have provided evidence that Arabidopsis FT protein (Hd3a in rice) is a key component of florigen. In the shoot apical meristem, FT activates AP1 transcription and induces flowering by interacting with the bZIP transcription factor FD, although the detail of the interaction between FT and FD has not yet been clarified. In rice, the closest homolog of FD, OsFD1, has been identified based on its function and homology with maize DLF1. The FT-FD interaction is required for flowering, and phosphorylation of residue T282 in the C-terminal region of FD may be critical for this interaction and floral initiation. Here we found the rice florigen Hd3a directly interacts with 14-3-3 proteins, but not with a transcription factor OsFD1. These 14-3-3 proteins play a key role by mediating an indirect interaction between Hd3a and OsFD1. Therefore we have determined the 2.4-Å crystal structure of a rice floral induction complex, named the florigen activation complex (FAC), which consists of Hd3a, OsFD1, and 14-3-3 protein. This FAC structure clearly explains the activation mechanism of the floral identity genes by florigen. Finally, functional studies using rice cultured cells and transgenic rice plants proved that the 14-3-3 proteins function as intracellular receptors of florigen to activate floral identity genes [1].

1. Taoka K, Ohki I, Tsuji H, Furuita K, Hayashi K, Yanase T, Yamaguchi M, Nakashima C, Purwestri YA, Tamaki S, Ogaki Y, Shimada C, Nakagawa A, Kojima C & Shimamoto K (2011) 14-3-3 proteins act as intracellular receptors for rice Hd3a florigen. Nature 476, 332-335.