FT Florigen proteins in photoperiodic signaling: conservation and diversity in their regulation, structure and function

Seasonal changes in day length regulate plant growth and development. FLOWERING LOCUS T (FT) proteins are widely-conserved effectors of photoperiod-induced flowering, and also promote tuberization in potato and bud growth in trees. We integrate data from several model and crop species to illustrate the major features of FT function and regulation. The day lengths that induce developmental responses differ among species, and diverse examples are selected to show how this is conferred by photoperiod-dependent FT transcription in leaf vasculature. FT protein movement into the phloem sieve elements and to the shoot apical meristem is then described. The functionally important domains of FT and how they contribute to a transcriptional complex with bZIP transcription factors and 14-3-3 proteins are outlined. Functional FT is contrasted with diverged FT paralogues and related TERMINAL FLOWER 1 proteins that act as negative regulators of FT activity to modulate developmental responses. A relay mechanism in which FT genes or closely related paralogues are transcriptionally induced at the shoot apex after the arrival of FT protein is described in cereals, tomato and Arabidopsis and in the stolon of potato, and we argue that it plays a role in sustaining photoperiod-induced developmental transitions. Finally, we discuss unresolved questions in FT signaling and how these might be addressed.