The Medicago FLOWERING LOCUS T Homolog, MtFTa1, Is a Key Regulator of Flowering Time 1[C][W][OA] Rebecca E. Laurie, Payal Diwadkar, Mauren Jaudal, Lulu Zhang, Vale ´rie Hecht, Jiangqi Wen, Million Tadege 2 , Kirankumar S. Mysore, Joanna Putterill, James L. Weller, and Richard C. Macknight* Department of Biochemistry, University of Otago, Dunedin 9054, New Zealand (R.E.L., P.D., M.J., R.C.M.); School of Biological Science, University of Auckland, Auckland 1010, New Zealand (L.Z., J.P.); School of Plant Science, University of Tasmania, Hobart, Tasmania 7001, Australia (V.H., J.L.W.); and Plant Biology, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73401 (J.W., M.T., K.S.M.) FLOWERING LOCUS T (FT) genes encode proteins that function as the mobile floral signal, florigen. In this study, we characterized five FT-like genes from the model legume, Medicago (Medicago truncatula). The different FT genes showed distinct patterns of expression and responses to environmental cues. Three of the FT genes (MtFTa1, MtFTb1, and MtFTc) were able to complement the Arabidopsis (Arabidopsis thaliana) ft-1 mutant, suggesting that they are capable of functioning as florigen. MtFTa1 is the only one of the FT genes that is up-regulated by both long days (LDs) and vernalization, conditions that promote Medicago flowering, and transgenic Medicago plants overexpressing the MtFTa1 gene flowered very rapidly. The key role MtFTa1 plays in regulating flowering was demonstrated by the identification of fta1 mutants that flowered significantly later in all conditions examined. fta1 mutants do not respond to vernalization but are still responsive to LDs, indicating that the induction of flowering by prolonged cold acts solely through MtFTa1, whereas photoperiodic induction of flowering involves other genes, possibly MtFTb1, which is only expressed in leaves under LD conditions and therefore might contribute to the photoperiodic regulation of flowering. The role of the MtFTc gene is unclear, as the ftc mutants did not have any obvious flowering-time or other phenotypes. Overall, this work reveals the diversity of the regulation and function of the Medicago FT family. To precisely control the timing of flowering, plants have evolved mechanisms to integrate seasonally pre- dictable environmental cues (such as changes in pho- toperiod and prolonged periods of cold temperatures) and developmental cues (such as maturity; Amasino, 2010). To allow this diversity of floral cues to influ- ence when flowering occurs in Arabidopsis (Arabidop- sis thaliana), multiple pathways converge on a small number of genes, the floral integrator genes, including the floral promoters FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF; Amasino, 2010). FT and TSF are members of a family of proteins that contain a phosphatidylethanolamine-binding protein (PEBP) domain (Kardailsky et al., 1999; Kobayashi et al., 1999). In addition to the FT-like proteins, the plant PEBP family consists of two other phylogenetically distinct groups of proteins: the TERMINAL FLOWER1 (TFL1)- like proteins and the MOTHER OF FT AND TFL (MFT)-like proteins (Bradley et al., 1997; Mimida et al., 2001; Yoo et al., 2004, 2010; Yamaguchi et al., 2005). FT and TSF act redundantly to promote flower- ing under long-day (LD) photoperiods (Michaels et al., 2005; Yamaguchi et al., 2005; Jang et al., 2009). The B-box zinc finger transcription factor CONSTANS (CO) protein induces the expression of TF and TSF in the vascular tissues under LD-inductive conditions (Kardailsky et al., 1999; Kobayashi et al., 1999; Samach et al., 2000; An et al., 2004). The LD-specific production of CO protein is achieved through the coincidence of the circadian expression of CO mRNA and the stabi- lization of the CO protein in the light (Sua ´rez-Lo ´pez et al., 2001; Valverde et al., 2004). FT and TSF proteins, produced in the phloem (Takada and Goto, 2003; Yamaguchi et al., 2005; Jang et al., 2009), are trans- ported to the apex, where they are able to dimerize with the bZIP transcription factor, FD, to activate the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1; Michaels et al., 2005; Yoo et al., 2005) and the floral meristem identity genes APETALA1 (AP1) and LEAFY (Abe et al., 2005; Wigge et al., 2005). Thus, FT/TSF constitute the long-sought mobile floral signal molecule, florigen (Corbesier et al., 1 This work was supported by the New Zealand Foundation for Research Science and Technology (grant no. C10X0704) and by an AGMARDT Postdoctoral Fellowship (to R.E.L.). 2 Present address: Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078–6028. * Corresponding author; e-mail richard.macknight@otago.ac.nz. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Richard C. Macknight (richard.macknight@otago.ac.nz). [C] Some figures in this article are displayed in color online but in black and white in the print edition. [W] The online version of this article contains Web-only data. [OA] Open Access articles can be viewed online without a sub- scription. www.plantphysiol.org/cgi/doi/10.1104/pp.111.180182 Plant Physiology Ò , August 2011, Vol. 156, pp. 2207–2224, www.plantphysiol.org Ó 2011 American Society of Plant Biologists 2207 Downloaded from https://academic.oup.com/plphys/article/156/4/2207/6109097 by guest on 20 February 2023