Structure and function of florigen and the receptor complex Ken-ichiro Taoka 1 , Izuru Ohki 1 , Hiroyuki Tsuji 1 , Chojiro Kojima 2 , and Ko Shimamoto 1 1 Graduate School of Biological Sciences, Nara Institute of Science and Technology, Nara 630-0192, Japan 2 Institute for Protein Research, Osaka University, Osaka 565-0871, Japan In the 1930s, the flowering hormone, florigen, was pro- posed to be synthesized in leaves under inductive day length and transported to the shoot apex, where it induces flowering. More recently, generated genetic and biochemical data suggest that florigen is a protein encoded by the gene, FLOWERING LOCUS T (FT). A rice (Oryza sativa) FT homolog, Hd3a, interacts with the rice FD homolog, OsFD1, via a 14-3-3 protein. Formation of this tri-protein complex is essential for flowering pro- motion by Hd3a in rice. In addition, the multifunction- ality of FT homologs, other than for flowering promotion, is an emerging concept. Here we review the structural and biochemical features of the florigen protein complex and discuss the molecular basis for the multifunctionality of FT proteins. Day length control of flowering and florigen Most plants continue to grow and produce leaves in the vegetative phase after germination. However, when envi- ronmental conditions change, plants change their growth to a reproductive phase and produce flowers and seeds for propagation. Such a developmental phase transition occurs at the shoot apical meristem (SAM), a pluripotent stem cell population on the tip of the stem [1,2]. Photope- riodic flowering is the flowering response to day length change, one of the most critical environmental stimuli for floral induction. W.W. Garner and H.A. Allard discovered that a tobacco (Nicotiana tabacum) variety, Maryland Mammoth, produced flowers only under short day condi- tions and never under long day conditions [3]. They con- cluded that day length is a critical factor for the induction of flowering. Plants can be broadly classified into three categories, based on their day length requirement for flowering. Short day plants induce flowering under short day conditions, whereas long day plants do so under long day conditions. Short and long days mean shorter or longer than the critical day length intrinsic to the respective plant species. Day neutral plants produce flowers irrespective of the environmental day length. For example, Arabidopsis thaliana is a long day plant and rice (Oryza sativa) a short day plant. In 1936, Chailakhyan, a Russian plant physiologist, discovered that changes in day length for floral induction are recognized by leaves. Considering other physiological evidence, he proposed a flowering hormone, florigen that is produced in leaves after the stimulus of inductive day length, and then transported to the SAM for floral evocation [4]. Grafting experiments clearly showed that florigen is a universal signal for flowering, because the flowering stimu- lus is graft-transmissible from induced stock to a non-in- duced scion and between different plant species, as well as for plants with different photoperiodic requirements [5,6]. Although the molecular nature of florigen was an enigma for a long time, it is now broadly accepted that the major component of florigen is a protein encoded by the FT gene or its homologs [7–9]. The FT gene was first identified as the responsible gene in a late flowering mutant of Arabidopsis (Arabidopsis thaliana) [10–12]. Phloem specific expression of FT under the inductive day length [13,14], the require- ment for intercellular trafficking of FT from companion cells [15,16], the existence of FT homologs in phloem exudate [17,18], and the long distance trafficking of FT and the rice homolog Hd3a to the SAM [19,20] strongly support the idea that the FT protein is the long-sought florigen. The virtual absence of FT RNA in graft receptor [21], phloem exudate [18] and around the SAM [20], the tissue specific knock- down of FT mRNA [16], and the synonymous substitutions of FT mRNA [22] strongly support the idea that FT RNA is not a major component of florigen responsible for the mobile flowering signal. Recently, FT RNA of Arabidopsis was reported to move over a long distance and contribute to floral induction [23,24]. It will be important to examine whether the FT RNA from other plant species is also trans- located a long distance and how the translocated FT RNA contributes to the regulation of flowering in vivo. This review summarizes the structural and biochemical aspects of the FT protein as a major protein component of florigen and the significance of the protein complex in the regulation of flowering. The expression of FT is regulated by various environmental cues: photoperiod, vernalization, ambient temperature, and gibberellin. For a more com- plete discussion of the regulation of FT, please refer to other recent reviews [25–32]. Structure of florigen (FT protein) FT is a phosphatidylethanolamine-binding protein (PEBP) family member whose crystal structure is similar to that of mammalian PEBP [33]. PEBPs have an anion-binding pocket composed of highly conserved residues. FT and its homologs, Hd3a and TFL1, have the anion-binding pocket (Figure 1a). Anions, phosphate groups, and phospholipids Review 1360-1385/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tplants.2013.02.002 Corresponding author: Shimamoto, K. (simamoto@bs.naist.jp). Trends in Plant Science, May 2013, Vol. 18, No. 5 287