The Flowering Hormone Florigen: Its Structure, Receptor Complex, and Beyond (Memorial lecture of Dr. Ko Shimamoto)

This memorial lecture will introduce our contributions to the identification of molecular nature of systemic floral signal “florigen”, intra-cellular receptors for florigen and the structure of the florigen protein complex, with the memory of working with Professor Ko Shimamoto in Nara Institute of Science and Technology, Japan, who passed away tragically from illness. The florigen research in rice is becoming exciting from these findings, and now expanding to the several new directions including the novel technologies.

 ‘Florigen’ was proposed 75 years ago to be synthesized in the leaf and transported to the shoot apex, where it induces flowering. We showed that the rice homologue of Arabidopsis FLOWERING LOCUS T, Hd3a, is a mobile floral signal in rice. It  expressed in leaves and transported to the shoot apex where it induces flowering. In shoot apical cells, Hd3a interacts with 14-3-3 proteins, yielding a complex that translocates to the nucleus and binds to OsFD1, a rice homologue of Arabidopsis transcription factor FD. The resultant ternary 'florigen activation complex' (FAC) induces transcription of OsMADS15, a homologue of Arabidopsis APETALA1, which leads to flowering. In this perspective, we propose that 14-3-3 proteins act as intra-cellular receptors for rice Hd3a florigen.

 The new directions of florigen research include the understanding of entire picture of floral transition, molecular mechanism of pleiotropic functions of florigen and application of florigen to the plant improvement. Our new findings offer clues for improving various agriculturally important traits by manipulating florigen and other interacting regulators.