Rice is a facultative short-day plant, and molecular genetic studies have identified the major genes involved in short-day flowering. However, the molecular mechanisms promoting the diversity of flowering time in cultivated rice are not known. We used a core collection of 64 rice cultivars that represent the genetic diversity of 332 accessions from around the world and studied the expression levels and polymorphisms of 6 genes in the short-day flowering pathway. The RNA levels of Heading date 3a (Hd3a), encoding a floral activator, are highly correlated with flowering time, and there is a high degree of polymorphism in the Heading date 1 (Hd1) protein, which is a major regulator of Hd3a expression. Functional and nonfunctional alleles of Hd1 are associated with early and late flowering, respectively, suggesting that Hd1 is a major determinant of variation in flowering time of cultivated rice. We also found that the type of Hd3a promoter and the level of Ehd1 expression contribute to the diversity in flowering time and Hd3a expression level. We evaluated the contributions of these 3 factors by a statistical analysis using a simple linear model, and the results supported our experimental observations.) has evolved during the last 8,000 to 10,000 years of domestication and breeding (1, 2). A major reason for the spread of rice cultivation to a wide range of geographical regions, and for the increases in yield, is the diversification of flowering time (1). In general, rice is known as a short-day plant that induces transition from the vegetative phase to the reproductive phase when it senses a decrease in day length. The molecular genetic pathway for short-day flowering in cultivated rice (Fig. 1A) is relatively well characterized (3-5). Signals from light and circadian clocks are received by OsGI, the rice orthologue of Arabidopsis GIGANTEA, and it regulates expression of Heading date 1 (Hd1) and OsMADS51 (6-8). Hd1 and its Arabidopsis orthologue CONSTANS encode zinc-finger type transcriptional activators with the CO, CO-like, and TOC1 (CCT) domains (9). Hd1 regulates Heading date 3a (Hd3a) expression (7, 9, 10). Hd3a is a rice orthologue of Arabidopsis FLOWERING LOCUS T (FT), and these genes recently were shown to encode a mobile flowering signal (11-16). RICE FLOWERING LOCUS T1 (RFT1) belongs to the rice FT-like gene family and functions as a floral activator, acting redundantly with Hd3a (17, 18). OsMADS51 encodes a type I MADSbox gene and functions upstream of Early heading date 1 (Ehd1) (8). Ehd1 encodes a B-type response regulator and acts as an activator of Hd3a independently from Hd1 (19). No clear orthologues of Ehd1 or OsMADS51 are found in the Arabidopsis genome. Although the genetic pathway for short-day flowering in rice is relatively well understood, the molecular mechanisms generating the diversity of flowering time in cultivated rice are not known. In this study, we analyzed the expression and nucleotide sequences of genes involved in short-day flowering in rice. Our study revealed that allelic vari...