Control of Flowering

Whilst many environmental and endogenous factors regulate when plants flowers, one of the most pivotal of these is day length. Plants can generally be divided into those which flower in response to short (SD) or long days (LD). Arabidopsis thaliana has long been used as a model for the analysis of flowering of LD plants. In this species the G1 gene product activates CO, which then activates a downstream floral activator (FT) resulting in flowering under LD conditions. In the April 17 Nature, Hiyama and colleagues from The Nara Institute of Science and Technology, Japan investigated the role of the orthologues of GI, CO and FT - OsGI, Hd1 and Hd3a respectively - in rice, a SD plant (Nature 2003, 422:719-722). They find that the rice orthologues act differently to control flowering.

Hiyama et al. generated transgenic rice that overexpress OsGI under both SD and LD conditions. These mutants flower later than wild-type plants, suggesting that in rice OsGI is a suppressor of flowering, in contrast to the situation in A. thaliana. Comparison of mRNA levels of the downstream targets of OsGI - Hd1 and Hd3a - showed no differences in Hd1 mRNA levels relative to wild-type, regardless of growth conditions. In wild-type plants Hd3a expression is inhibited under LD conditions, and has diurnal levels under SD conditions. No Hd3a mRNA could be detected in the OsGI transgenics, however, consistent with the late-flowering phenotype. The authors suggest that under LD conditions Hd1 expression suppresses expression of Hd3a in rice.

Thus, in A. thaliana, under LD conditions GI activates FT via CO, but in rice OsGI activates Hd1 (the CO ortholog), which under LD conditions suppresses Hd3a (the FT ortholog) expression - resulting in suppression of flowering.

The authors conclude that "an important gene network for the photoperiod control of flowering is conserved between Arabidopsis and rice, but that the regulation of the downstream gene by an upstream regulatory gene is reversed in the two species. These findings suggest the existence of common mechanisms for the photoperiodic control of various processes in diverse plant species."