Genetical and morphological characterization of cold tolerance at fertilization stage in rice.

Cold temperature during the reproductive phase leads to seed sterility, which reduces yield and decreases the grain quality of rice. The fertilization stage, ranging from pollen maturation to the completion of fertilization, is sensitive to unsuitable temperature. Improving cold tolerance at the fertilization stage (CTF) is an important objective of rice breeding program in cold temperature areas. In this study, we characterized fertilization behavior under cold temperature to define the phenotype of CTF and identified quantitative trait loci (QTLs) for CTF. A wide variation in CTF levels has been identified among local cultivars in Hokkaido, which is one of the most northern regions for rice cultivation in the world. Clear varietal differences in pollen germination, and pollen tube elongation due to cold temperature have been observed. These differences may confer a degree of CTF among this population. We conducted QTL analysis for CTF using 120 backcrossed inbred lines derived from a cross between Eikei88223 (vigorous CTF) and Suisei (very weak CTF). Three QTLs for CTF were identified. A clear effect by QTL, qCTF7, for increasing the level of CTF was validated using advanced progeny. These results will facilitate marker-assist selection for desirable QTLs for CTF in rice breeding program.