Map-based cloning of the rice cold tolerance gene Ctb1

Low temperatures during the booting stage reduce rice yields by causing cold-induced male sterility. We previously mapped a quantitative trait locus for cold tolerance, Ctb1, to a 56-kb region containing 7 putative genes. In this study we further mapped Ctb1 to a 17-kb region containing two genes that encode an F-box protein and a ser/thr protein kinase. The F-box protein gene was preferentially expressed in young panicles, while the ser/thr protein kinase gene was expressed in leaves and young panicles. Both genes were cloned from a cold-tolerant variety, Norin-PL8, and introduced into a cold-sensitive variety, Hokkai241, and a cold-sensitive line, BT4-74-8. The cold tolerance of T2 and T3 progenies was assessed by measuring the degree of spikelet fertility in plants treated with cool water irrigation (19 °C, 25 cm) or cool air (12 °C, 4 days). The results indicated that the F-box protein gene confers cold tolerance. Cold tolerance is associated with greater anther length, and the transgenic plants had longer anthers than non-transformed controls. The F-box protein interacts with a subunit of the E3 ubiquitin ligase, Skp1, suggesting that an ubiquitin–proteasome pathway is involved in cold tolerance at the booting stage.