Epistasis among the three major flowering time genes in rice: coordinate changes of photoperiod sensitivity,basic vegetative growth and optimum photoperiod

Flowering time is affected not only by photoperiod sensitivity (PS) but also by basic vegetative growth (BVG) and optimum photoperiod (OP), although their developmental and genetic relationships are not well understood. The present study was carried out in rice to examine to what extent these three developmental components are modified by the three flowering time genes, Se1 (= Hd1), Ef1 and e1 (= m-Ef1), which are known to contribute to flowering time in temperate and tropical regions of rice cultivation. Photoperiodic response curves were estimated under controlled conditions of different growth regimes, using eight near-isogenic lines possessing different combinations of the alleles at the three loci. The results showed that each of the components is greatly affected by the main effect of the genes, temperature and their epistasis, indicating that none of the three genes controls flowering time by altering any single component in PS, BVG or OP. Epistasis was detected more frequently among the three genes than reported before, suggesting that epistasis contributes to flowering time by changing PS, BVG and OP differently. The comparison of the nucleotide sequences suggested that Ef1 is the same as Early heading date 1 (Ehd1). Since the two genes Se1 (= Hd1) and Ef1 (= Ehd1) are known to up-regulate the rice homolog of Arabidopsis FT, it is suggested that the detected epistasis may respond to diverse environments by modulating the CO/FT system conserved in flowering plants.