Fine mapping of a minor-effect QTL, DTH12, controlling heading date in rice by up-regulation of florigen genes under long-day conditions

Heading date is a key determinant of regional and seasonal adaptation in rice (Oryza sativa L.). A minor-effect quantitative trait locus (QTL), QTL for D ays t o h eading 12a (qDTH-12a), with unknown genetic action was previously coarsely detected in a recombinant inbred line population. The study reported here was designed to better define the qDTH-12a locus (designated as DTH12) in advanced segregating populations. DTH12 was initially verified in chromosome segment substitution line CSSL84. A CSSL84/Asominori//Asominori BC4F2 population was then developed, and a near-isogenic line (NIL), NIL(DTH12), was subsequently selected from this population using marker-assisted tracking that headed 8 days later than Asominori under long-day (LD) conditions but which was not significantly different in heading date in short-day environments. Using 358 Asominori/NIL(DTH12) F2:3 families grown under LD conditions, we were able to initially map DTH12 to a 26-cM interval between markers InDel12-1 and RM6296. F3 individuals heterozygous for the DTH12 regions were then chosen, and 2,388 F4:5 families were used for fine mapping. DTH12 was finally dissected as a single gene and delimited to a 153-kb genomic region with 32 open reading frames. Compared with Asominori, NIL(DTH12) showed reduced transcription of the florigen genes Heading date 3a and RICE FLOWERING LOCUS T 1, suggesting that DTH12 functions as an up-regulator of florigen genes during floral induction under LD conditions. DTH12 was also found to have an important role in rice adaptation and breeding for precise control of seed maturity. These findings provide a firm basis for cloning this minor-effect QTL involved in rice flowering.