Lack of starch synthase IIIa and high expression of granule-bound starch synthase I synergistically increase the apparent amylose content in rice endosperm

Rice endosperm starch is composed of 0–30% linear amylose, which is entirely synthesized by granule-bound starch synthase I (GBSSI: encoded by Waxy, Wx). The remainder consists of branched amylopectin and is elongated by multiple starch synthases (SS) including SSI, IIa and IIIa. Typical japonica rice lacks active SSIIa and contains a low expressing Wxb causing a low amylose content (ca. 20%). WAB2-3 (SS3a/Wxa) lines generated by the introduction of a dominant indica Wxa into a japonica waxy mutant (SS3a/wx) exhibit elevated GBSSI and amylose content (ca. 25%). The japonica ss3a mutant (ss3a/Wxb) shows a high amylose content (ca. 30%), decreased long chains of amylopectin and increased GBSSI levels. To investigate the functional relationship between the ss3a and Wxa genes, the ss3a/Wxa line was generated by crossing ss3a/Wxb with SS3a/Wxa, and the starch properties of this line were examined. The results show that the apparent amylose content of the ss3a/Wxa line was increased (41.3%) compared to the parental lines. However, the GBSSI quantity did not increase compared to the SS3a/Wxa line. The amylopectin branch structures were similar to the ss3a/Wxb mutant. Therefore, Wxa and ss3a synergistically increase the apparent amylose content in rice endosperm, and the possible reasons for this increase are discussed.