The transition of the shoot apical meristem (SAM) from vegetative to reproductive growth is a critical developmental switch in plants, which is regulated by both environmental and endogenous factors. Among the environmental factors, photoperiod is one of the most important regulators of flowering. Long-day (LD) or short-day (SD) photoperiod-sensitive plants can be induced to flower with the appropriate day length. A comparison of flowering pathways in Arabidopsis, an LD plant, and rice, an SD plant, has revealed that major genes involved in photoperiod flowering are highly conversed between these two species. In rice, photoperiod is the most important environmental cue for signaling flowering. Several flowering time genes have been identified however, Heading date 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1), known as florigen genes, are expressed in the vascular tissues, and their proteins from leaves move to the shoot apical meristem (SAM) through the phloem.
