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Genetic and molecular dissection of flowering time in rice

Flowering time (heading date) is a major determinant of regional and seasonal adaptation of cultivated rice. A large amount of variation is observed in heading date and photoperiodic response among rice cultivars and strains, including wild relatives. Quantitative trait locus (QTL) analyses of progeny derived from several cross combinations of rice cultivars suggest that more than 15 loci are involved in heading date. Map-based cloning has been performed on several QTLs for photoperiodic response. We have demonstrated that Heading date 1 (Hd1) is an ortholog of CONSTANS (CO) in Arabidopsis and is involved in the promotion of heading under short-day (SD) conditions and inhibition under long-day (LD) conditions. Hd6 is involved in inhibition under LD conditions and encodes the alpha-subunit of protein kinase CK2. Hd3a shows a high level of similarity to T Early heading date 1 (Ehd1) is involved in promotion under SD conditions and encodes a B-type response regulator. Hd5 is involved in inhibition under LD conditions and encodes a putative subunit of a CCAAT-box-binding protein. Late heading date 4 (Lhd4) is involved in inhibition under LD conditions and encodes a protein with a CCT motif. The combining of information from genetic and sequencing analyses reveals that the combination of natural alleles with loss or gain of function at particular QTLs, such as Hd1, Hd5, Hd6, Ehd1, and Lhd4, seems to generate a wide range of continuous variation in photoperiodic flowering in rice. These genetic and molecular analyses have allowed us to propose a pathway for the genetic control of photoperiodic flowering in rice, and analysis of the mRNA levels of genes in near-isogenic lines has clearly revealed their hierarchical relationship in the genetic control pathway. Identification and expression analyses of genes suggest the conservation and divergence of various features in the photoperiodic control of flowering in rice, an SD plant, and Arabidopsis, an LD plant.