Genetic Control of Flowering Time in Rice, a Short-Day Plant

Endogenous genetic factors and environmental signals control the time of flowering in plants. One of the environmental signals is photoperiod. Genetic control mechanisms for the photoperiodic response of flowering of long-day plants (LDPs) have been extensively analyzed through the use of Arabidopsis as a model plant (for review, see Coupland, 1998; Levy and Dean, 1998;Samach and Coupland, 2000). In contrast, mechanisms in short-day plants (SDPs) remain unclear, although many physiological studies have been performed on SDPs, such as Pharbitis nil (for review, seeLumsden, 1998). Recent progress in genome analysis has provided a new strategy for analyzing the genetic control of flowering in rice (Oryza sativa; SDP). Several studies have demonstrated that the structure of genes involved in the photoperiodic response of flowering in rice show remarkable similarity to those in Arabidopsis. In rice, genetic analyses of flowering time (often called heading date) have been performed on mutants and natural variants. Several genes involved in the photoperiodic response (photoperiod sensitivity) have been identified (Yokoo et al., 1980; Yamagata et al., 1986; Yokoo and Okuno, 1993; Okumoto and Tanisaka, 1997). A series of nearly isogenic lines (NILs) for several photoperiod sensitivity genes have been developed to facilitate genetic analysis of flowering time in rice (Yamagata et al., 1986). However, the nature of the quantitative inheritance of flowering time has prevented us from performing more detailed analyses, including analysis of epistatic interactions and determination of chromosomal locations of genes. In the last decade, the progress in development of DNA markers made quantitative trait locus (QTL) analysis possible to clarify the number and nature of the genes controlling flowering time in rice (Yano and Sasaki, 1997).