Amylopectin has a highly regulated branched structure called cluster structure, which causes the glucan to be hydrophobic and to form a semicrystalline architecture of starch granules. It is known that lesions of the isoamylase1 (ISA1) gene result in accumulation of a water-soluble glucan (WSG) called phytoglycogen instead of amylopectin in various plant species. This type of cereal mutant is referred to as sugary-1 and accumulates a large amount of phytoglycogen in the endosperm. In this study, another WSG-synthesizing mutant has been isolated from japonica rice. This mutant accumulated a significant amount of WSG in the center of endosperm. No significant changes were found in activities of ISA and pullulanase and in the compositions of ISA1 homomer and ISA1-ISA2 heteromer. In addition, activities of starch branching enzyme and starch synthase isoforms were not altered. The accumulated WSG had a specific fine structure, that differed from that of phytoglycogen. Thus, we designated this new mutant as the sugary-2 mutant. Our study results strongly suggest that the Sugary-2 gene product plays an important role in amylopectin synthesis of rice endosperm and identifies a new factor that controls the normal amylopectin structure, especially at the early developmental stage of the endosperm.
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