Photoperiodic control of flowering in barley

Flowering is a very important event in plant propagation. Plants other than absolute response plants can produce flowers under any conditions. Recent studies using model plants have provided important information on flowering mechanisms. Under floral inductive conditions, the long-day (LD) plant Arabidopsis and the short-day (SD) plant rice use the same gene set, CO and FT (Hd1 and Hd3a in rice), which functions as an important promoter for flowering. In rice, Hd1 represses Hd3a expression under non-inductive conditions but not under inductive conditions. The regulation of Hd3a by Hd1 is reversed under two conditions in rice. This system is not found in Arabidopsis. Barley (Hordeum vulgare) is more closely related to rice than to Arabidopsis, but barley is a LD plant like Arabidopsis. Although barley has orthologs to CO and FT, HvCO1 and HvFT1, and their expression and functions under inductive conditions have been analyzed, the pathway under non-floral-inductive conditions remains unclear. We recently reported on genes that promote flowering under SD conditions in barley. In this review, we focus on the mechanisms of photoperiodic flowering in barley, especially the promoting pathway under non-inductive conditions. Flowering mechanisms specific to barley are also presented in comparison with the mechanisms of Arabidopsis and rice.