Abstract
SSI and SSIIIa are the first and second major SS isozymes, respectively, in amylopectin biosynthesis of developing rice endosperm. We previously clarified the functions of these SS isozymes using the respective mutant lines (Fujita et al., (2006), Plant Physiol. 140: 1070-1084; Fujita etal., (2007), Plant Physiol. 144: 2009-2023). To isolate the double mutant of these major SS isozymes, the SSI mutant was crossed with the SSIIIa mutant. Two types of opaque seeds having ss1ss1/SS3ass3a and SS1ss1/ss3ass3a genotypes appeared in F2 generation and SS activity of these lines was very low. On the other hand, we failed to detect developing endosperm that was deficient in both SSI and SSIIIa activity bands, indicating that the double recessive mutant was sterile. Two types of opaque seeds had unique amylopectin and high amylose content in the endosperm. The amount of granule-bound starch synthase I (GBSSI) protein that responsible for the biosynthesis of amylose and the activity of ADP-glucose pyrophosphorylase (AGPase) were higher than those of the wild type and their parent mutant lines.
