Giving rice the time of day. Molecular identification of a major photoperiod sensitivity quantitative trait locus.

Daylength controls flowering in many plant species. Early in the twentieth century, Garner and Allard (1920) first described the phenomenon of plants that flower only when daylength is longer or shorter than a particular threshold, named long-day and shortday plants, respectively. Rice is a shortday plant; flowering (termed “heading” in rice and other cereals) is promoted by short daylength. However, there is a large degree of genetic variability in this trait among rice cultivars, from photoperiod insensitivity to strong photoperiod sensitivity. The manipulation of photoperiod sensitivity is an important breeding objective for rice grown in many tropical and temperate regions to optimize heading date and maturity for local environments (Poonyarit et al., 1989; Li et al., 1995). Like many other important traits in plant breeding, heading date is a complex trait that shows continuous phenotypic variation among progeny and is controlled by multiple genes known as quantitative trait loci (QTLs). QTLs typically are difficult to identify because of the lack of discrete phenotypic segregation and because the phenotypic effects of each gene associated with a complex trait are relatively small (Yano and Sasaki, 1997). QTL analysis involves selecting and hybridizing parental lines that differ in one or more quantitative traits and analyzing the segregating progeny to link the QTL to known DNA markers. Chromosomal QTL regions often are quite large and can include many open reading frames. This situation can exacerbate “linkage drag” in the application of QTL analysis to plant breeding or introgression into elite germplasm of undesirable characters that are linked to a desirable QTL (Tanksley Giving Rice the Time of Day: Molecular Identification of a Major Photoperiod Sensitivity Quantitative Trait Locus