SPring-8, the large synchrotron radiation facility

Japanese scientists have discovered, ahead of other countries, receptors of florigen, a hormone that is responsible for flowering in plants and is synthesized in the leaf, at the shoot apex. They include Ko Shimamoto (Professor), Ken-ichiro Taoka (Assistant Professor), Izuru Ohki (Assistant Professor), and Hiroyuki Tsuji (Assistant Professor) of the Graduate School of Biological Sciences, Nara Institute of Science and Technology (President, Akira Isogai) and Chojiro Kojima (Associate Professor) of the Institute for Protein Research, Osaka University. They clarified that 14-3-3 proteins act as receptors of florigen and determined the crystal structure of the complex composed of florigen and its receptors at the atomic level. They also succeeded in manipulating the flowering period by variously changing the strength of binding between florigen and its receptors.

The flowering hormone florigen is synthesized in the leaf when plants are stimulated by changes in the environment, such as number of daytime hours and temperature. After its synthesis, florigen is transported to the shoot apex, where flower buds are produced, and induces flowering. Although florigen has long been called a phantom hormone with an unclear nature, in 2007 Professor Shimamoto found it to be a protein named Hd3a or flowering locus T (FT). However, the exact function of florigen within cells during flowering induction and receptors that bind to florigen within the cells have not been identified thus far.

Florigen and its receptors were found to be substances that are universally conserved in all plants, which may lead to the development of techniques of complete manipulation of the flowering period of plants reminiscent of Hanasaka Jiisan, an old man in a Japanese fairy tale who induced the blossoming of withered trees. In the future, it is hoped that the findings of this study will be applied to the development of techniques of producing grains and fruit even in poor environments, as well as biofuel crops. The achievements of this research were published online in the British scientific journal Nature on 1 August 2011.

Professor Shimamoto, Associate Professor Kojima, and their colleagues considered that the rice florigen Hd3a binds to several proteins to induce flowering and they examined the proteins to which Hd3a binds. They discovered that Hd3a binds to 14-3-3 proteins conserved in a wide range of eukaryotes and DNA-binding Oryza sativa (Os)FD1 proteins to form a ternary complex, Hd3a-14-3-3-OsFD1 (florigen activation complex). The crystal structure of this complex was determined mainly by nuclear magnetic resonance (NMR) analysis and X-ray crystallography using SPring-8 beamline BL41XU. Detailed analysis with modeling techniques revealed that the complex is a hexamer and forms a symmetric W-shaped structure on DNA. Moreover, when the mechanism of formation of the complex within cells was analyzed by bioimaging techniques, it was clarified that florigen binds to 14-3-3 proteins in the cytoplasm and that the resulting Hd3a-14-3-3 complex is transported into the nucleus, where it binds to OsFD1 to form a higher complex, the florigen activation complex; thus, genes triggering flower bud formation are activated. This finding indicates that the receptors of florigen Hd3a within cells are 14-3-3 proteins and that the Hd3a-14-3-3-OsFD1 complex is responsible for the flowering induction.

Publication:
"14-3-3 proteins act as intracellular receptors for rice Hd3a florigen"
Ken-ichiro Taoka, Izuru Ohki, Hiroyuki Tsuji, Kyoko Furuita, Kokoro Hayashi, Tomoko Yanase, Midori Yamaguchi, Chika Nakashima, Yekti Asih Purwestri, Shojiro Tamaki, Yuka Ogaki, Chihiro Shimada, Atsushi Nakagawa, Chojiro Kojima and Ko Shimamoto
Nature 476, 332–335 (2011), published online 31 July 2011