During long day (LD), rice flowering is delayed about 30 days and Hd3a expression under LD conditions is quite low to promote flowering. The central mechanism for Hd3a suppression comes from modification of the conserved OsGI-Hd1-Hd3a pathway where Hd1 activates Hd3a during short day (SD), but its function is converted into a repressor to attenuate Hd3a expression during LD In rice. Flowering is induced by SD and suppressed by LD. By the analysis of transgenic rice plants overexpressing OsGI or suppressing OsGI by RNAi, it was shown that OsGI acts as an activator of Hd1 in both SD and LD (Hayama et al., 2003). However, OsGI, hence Hd1 as well, functions as negative regulators of flowering in LD by suppressing Hd3a (Hayama et al., 2003). In rice, a major genetic pathway for the photoperiodic regulation of flowering is OsGI?Hd1?Hd3a. Hd1 has a dual role in regulating Hd3a, depending on the photoperiod. Rice is a facultative short-day plant and can finally flower during non-inductive LD conditions. Although Hd3a expression is quite low during LD, there is another factor RFT1, the closest paralog of Hd3a, which induces flowering in LD. RFT1 expression increased during LD in leaf phloem tissue, and RFT1 protein was shown to move from the leaves to the shoot apex. When both Hd3a and RFT1 activities are suppressed, flowering is completely abolished, suggesting that flowering is fully dependent on florigen activity in rice.
