Imaging florigen distribution in vivo

27 INTRODUCTION Florigen was postulated as a plant hormone that is produced in the leaves, transported to the shoot apical meristem (SAM), and induces floral transition (Figure 1) (Chailakhyan, 1936, Zeevaart 2006). The concept of florigen came from the finding of photoperiodic flowering (Garner and Allard 1920), where the seasonal cue for flowering response in plants depends on the measurement of day-length. Plants usually complete processes of flower formation with the advent of seasons suitable for reproduction or before coming seasons, which are not suitable for reproduction. Garner and Allard (1920) discovered that the environmental factor used by plants to predict the seasons is day length, from experiments that shift the time of transplanting of several crops and experiments using tobacco mutants. Actually, chrysanthemum and rice measure the changes in day length of 30 minutes and can reflect it to initiation of flowering (Itoh et al. 2010, Oda et al. 2012). The discovery of photoperiodism, which is a physiological reaction of organisms to measure the day-length as information to predict seasonal change, later spreads from the study of plant reproduction to that of seasonal behavior of animals. The plant organ that measures the changes in day-length is leaves, and when the suitable day-length is recognized this information is transported to the SAM where the floral organs are developed (Zeevaart 2006). This model was supported by the experiments where plants treated by inducible and non-inducible day length are grafted, and the stimulus that travels from leaves to the SAM was called as florigen. Recent advances in the molecular biology of flowering time control in plants revealed molecular identity of florigen, its receptors, and protein complexes that are essential for its function (Andres and Coupland 2012, Tsuji and Taoka 2014). Imaging techniques contributed to achieve these findings, through revealing dynamic changes of florigen protein complex inside a cell and florigen distribution around the SAM. MOLECULAR IDENTITY OF FLORIGEN Molecular identity of florigen was revealed from the efforts to identify genes that regulate the timing of flowering, through the genetic analysis of mutants for flowering time regulation in Arabidopsis thaliana, and quantitative trait loci (QTL) analysis of heading date in rice (Andres and Coupland 2012, Tsuji et al. 2013). FT in A. thaliana and Heading date 3a (Hd3a) in rice are genes identified through these studies, and these genes are orthologs that encode a protein similar to the phosphatidylehtanolamine binding protein (PEBP) in animals (Kardailsky et al. 1999, Kobayashi et al. 1999, Kojima et al. 2002). FT in A. thaliana plays an essential role to promote flowering in a long day. Mutation in FT attenuates promotion of flowering under long days, and the expression of FT is induced under long-day conditions in leaf phloem (Kardailsky et al. 1999, Kobayashi et al. 1999). Rice Hd3a gene was identified Plant Morphology vol. 29 pp. 27-31 INVITED REVIEW