Abstract
We have analyzed molecular mechanisms of photoperiodic flowering in rice, a short-day plant, and compared them with those of Arabidopsis, a long-day plant, to elucidate molecular variations in photoperiodic flowering. Orthologous rice Hd1/Arabidopsis CO are key transcriptional factors in photoperiodic flowering. Circadian clocks express Hd1/CO mRNA mainly at night. While CO induces the FT floral inducer in LD, Hd1 represses and induces FT orthologs (Hd3a etc.) in LD and SD, respectively. We here examined transcriptional regulation of FT/Hd3a by CO/Hd1 in transient assays and demonstrated that the difference in the FT/Hd3a promoters is a key in this regulation. We further cloned a novel floral inducer termed Ehd1 in rice. Ehd1 encodes a novel B-type response regulator, whose ortholog may not exist in Arabidopsis. In addition, Ehd1 regulates Hd3a etc. in rice. Therefore, the Ehd1 novel floral pathway is integrated into the conserved floral pathways through Hd3a regulation in rice.
