14-3-3 proteins act as intracellular receptors for rice Hd3a florigen

How florigen says it with flowers Florigen, a plant hormone that translocates from leaves to the shoot apex to induce flowering, is encoded by the FLOWERING LOCUS T ( FT ) gene. The detailed molecular mechanism for florigen action and the identity of its receptors remained unknown. Ko Shimamoto and colleagues show that in rice, florigen binds to 14-3-3 proteins, a type of regulatory molecule present in all eukaryotic cells, and to the transcription factor OsFD1. The crystal structure of the resulting florigen activation complex (FAC) has been determined, providing a mechanistic basis for florigen function in flowering. This work offers clues for improving agriculturally important traits by manipulating florigen and its regulators. ‘Florigen’ was proposed 75 years ago 1 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 plants 2 , 3 , 4 . Nonetheless, the exact function of florigen during floral induction remains poorly understood and receptors for florigen have not been identified. Here we show that the rice FT homologue Hd3a 5 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 2.4 Å crystal structure of rice FAC, which provides a mechanistic 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.