Euphytica 126: 227–234, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 227 Interactive effects of two heading-time loci, Se1 and Ef1, on pre-flowering developmental phases in rice (Oryza sativa L.) Katsuyuki Ichitani 1 , Hiromo Inoue 2 , Hidetaka Nishida 2 , Yutaka Okumoto 2 & Takatoshi Tanisaka 2,∗ 1 Faculty of Agriculture, Kagoshima University, Kagoshima 890-0065, Japan; 2 Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan; ( ∗ author for correspondence) Received 25 August 2000; accepted 25 October 2001 Key words: basic vegetative growth, gene analysis, gene interaction, heading time, Oryza sativa, photoperiod sensitivity Summary The interaction between the Se1 and the Ef1 loci, which chiefly control the photoperiod sensitivity (PS) and the basic vegetative growth (BVG) period of rice (Oryza sativa L.) respectively, was investigated using four tester lines different in genotype for the two heading time loci from each other. The four tester lines were grown under 10, 13, 14, 15, and 16h daylengths to estimate their BVG period and PS. The Taiwanese cultivar Taichung 65 (T65), one of the tester lines, has an extremely long BVG period that has been considered to be conferred by a late heading- time allele ef1 at the Ef1 locus. Experimental results, however, showed that the extremely long BVG of T65 was conferred not by a single effect of ef1 but by a complementary effect of ef1 and Se1-e, a photoperiod insensitivity allele, at the Se1 locus. It was also found that a complementary effect of a PS allele Se1-n at the Se1 locus and ef1 stimulates the PS of rice. Gene analysis for heading time under an optimum daylength (10 h) as well as under natural daylength confirmed the presence of the complementary effect of the two nonallelic genes on BVG, which was found only with homozygosity of both the genes. Based on these results and earlier reports on the Se1 locus, the roles of the Se1 and Ef1 loci on the durations of pre-flowering developmental phases in rice were discussed. Introduction The period from germination to flower initiation in rice (Oryza sativa L.) comprises the photoperiodically insensitive phase called the basic vegetative growth (BVG) period and the subsequent photoperiodically sensitive phase (Chang et al., 1969). The BVG period and the degree of photoperiod sensitivity (PS) respons- ible for the second phase both display a considerable intervarietal variation (Wada, 1954; Chang et al., 1969; Hosoi, 1981; Vergara & Chang, 1985). To date, as many as 12 heading-time (flowering-time) loci con- trolling PS have been identified in rice (e.g. Syakudo & Kawase, 1953; Yokoo & Fujimaki, 1971; Poonyarit et al., 1989; Okumoto & Tanisaka, 1997; Ichitani et al., 1998b). Among them, the Se1 (Photosensitivity-1) locus on chromosome 6 has been considered espe- cially important for the regional differentiation of rice cultivars because of its intensive effect on PS (Yokoo & Fujimaki, 1971; Yokoo & Kikuchi, 1977, 1982). This locus has at least three alleles, Se1-u, Se1-n and Se1-e: the first two are incomplete dominant alleles stimulating PS (Se1-u has a slightly stronger effect than Se1-n), while the second is a recessive allele not having such a function (Yokoo & Kikuchi, 1977). This locus is also known to interact with many heading- time genes; Chang et al. (1969) indicated that i-Se1 (recessive inhibitor for photosensitivity) inhibits the expression of a PS allele at this locus, though its crit- ical chromosome has not been identified as yet; Sano (1990, 1992) identified a suppressor Su-Se1 (Sup- pressor for photosensitivity) on chromosome 6 and an enhancer En-Se1 (Strong sensitivity to photoperiod / Enhancer to Se1) on chromosome 7, both of which modify the expression of a PS allele; Ohshima et al.