Structures of Endosperm Starch from a Rice wx Cultivar Expressing Wx.ALPHA. Transgene

Structural changes caused by the introduction of the Wxα transgene, which encodes granule-bound starch synthase I (GBSSI), into the non-transgenic wx cultivar Musashimochi were examined and compared with the previous results (Plant Cell Physiol., 49, 925-933 (2008), using a transgenic line (WAB3-5) established independently from those used previously. Increases of amylose content of starch and extra-long chain (ELC) content of amylopectin were observed in the line WAB3-5 (actual amylose, 15.2%; ELC, 11.6% by weight). An HPSEC profile (size distribution) of WAB3-5 amylose showed characteristics in-between of those of Wxα and Wxb cultivars, and WAB3-5 amylose had a comparably high degree of branching, which was indicated by the molar fraction of branched molecules (MFB, 0.38) and number of chains of branched molecules (NCB, 10.5). Amylose from cv. Yumetoiro (Wxα) had a comparable degree of branching (MFB, 0.35; NCB, 13.7) with the WAB3-5 amylose. Chain-length distribution of WAB3-5 amylopectin showed a slightly higher amount of B2+B3 chains than cv. Musashimochi. These results are consistent with those of the previous study, providing additional evidence for the proposed role of GBSSI in both amylose and ELC synthesis in rice endosperm. ELCs of amylopectins from line WAB3-5 and cvs. IR36 and Yumetoiro showed size distributions that were basically similar to each other but distinct from those of their amylose counterpart. ELCs were hydrolyzed by β-amylolysis of amylopectins with different extents of trimming, 66.3, 92.0 and 77.0% for line WAB3-5, cv. IR36 and cv. Yumetoiro, respectively, suggesting the branched structure of the ELCs is different among the three amylopectins.