Plant biology

Florigens, Flowering, and 14-3-3 Proteins

Science Signaling  23 Aug 2011:
Vol. 4, Issue 187, pp. ec233
DOI: 10.1126/scisignal.4187ec233

Photoperiodic regulation of flowering in plants is mediated by florigens, hormones that are produced in the leaf and transported to the shoot apex. Using yeast two-hybrid screening, Taoka et al. identified a 14-3-3 protein (GF14) and a basic leucine zipper (bZIP)–containing transcription factor (OsFD1) as putative binding partners of the rice florigen Heading date 3a (Hd3a). Direct interactions between GF14 and Hd3a, and GF14 and OsFD1, but not Hd3a and OsFD1, were detected in glutathione S-transferase pull-down assays. Crystal structure analysis indicated that the ternary “florigen activation complex” is a heterohexamer containing two molecules each of GF14, Hd3a, and OsFD1, with GF14 bridging the interaction between Hd3a and OsFD1. Confocal fluorescence microscopy images of rice protoplasts expressing fluorescent protein–tagged versions of GF14, Hd3a, and OsFD1 revealed that GF14-Hd3a complexes, but not GF14-OsFD1 complexes, localized predominantly in the cytoplasm. Ternary Hd3a-GF14-OsFD1 complexes were detected in the nucleus using BiFC (bimolecular fluorescence complementation)–based FRET (fluorescence resonance energy transfer) analysis. Nuclear ternary complexes were not, however, detected in protoplasts expressing mutants of either Hd3a (Hd3aR64G/R132A) or OsFD1 (OsFD1S192A) that were unable to interact with GF14. Gel-shift assays with a DNA oligomer containing a region of the promoter of OsMADS15, a gene involved in floral induction, detected a super shift upon addition of GF14, Hd3a, and OsFD1 together, but not when GF14 was omitted. OsMADS15 expression was reduced in rice protoplasts cotransformed with either Hd3aR64G/R132A and wild-type OsFD1 or wild-type Hd3a and OsFD1S192A compared with that in protoplasts cotransformed with wild-type Hd3a and wild-type OsFD1, supporting a role for a functional Hd3a-GF14-OsFD1 complex in inducing OsMADS15 expression and flowering.

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