Ehd1 , a B-type response regulator in rice, confers short-day promotion of flowering and controls FT-like gene expression independently of Hd1

Abstract

Two evolutionarily distant plant species, rice ( Oryza sativa L.), a short-day (SD) plant, and Arabidopsis thaliana , a long-day plant, share a conserved genetic network controlling photoperiodic flowering. The orthologous floral regulators—rice Heading date 1 ( Hd1 ) and Arabidopsis CONSTANS ( CO )—integrate circadian clock and external light signals into mRNA expression of the FLOWERING LOCUS T ( FT ) group floral inducer. Here, we report that the rice Early heading date 1 ( Ehd1 ) gene, which confers SD promotion of flowering in the absence of a functional allele of Hd1 , encodes a B-type response regulator that might not have an ortholog in the Arabidopsis genome. Ehd1 mRNA was induced by 1-wk SD treatment, and Ehd1 may promote flowering by inducing FT-like gene expression only under SD conditions. Microarray analysis further revealed a few MADS box genes downstream of Ehd1 . Our results indicate that a novel two-component signaling cascade is integrated into the conserved pathway in the photoperiodic control of flowering in rice.