Structures and properties of arnylopectin and phytoglycogen in the endosperm of sugary-1 mutants of rice

Starch debranching enzyme (DBE)-deficient, sugary-1 mutations of rice caused the biosynthesis of more highly branched alpha-glucan in the form of phytoglycogen and sugary-amylopectin rather than wild-type amylopectin. Phytoglycogen and sugary-amylopectin are located in the inner and outer regions of the rice endosperm, respectively. Detailed analyses of these glucans from a variety of allelic mutants with different severity of sugary phenotype revealed that sugary-amylopectins consisted of more short chains of DP < 12 but fewer chains of DP13-24 than did wild-type amylopectin, and the alterations reflected on lower molecular weights, smaller gyration radii, and denser molecules. These changes resulted in lower onset gelatinization temperature, lower peak viscosity, lesser degree of crystallinity, and smaller and cracked granules with rough surface of sugary starch granules as compared with that of the wild-type. These results indicate that changes in the fine structure of amylopectin induced by lesion of DBE greatly influence the morphological and physicochemical properties of starch. Phytoglycogens from sugary-1 mutants were composed of multiple components of smaller molecular weights and gyration radii than sugary-amylopectin and amylopectin. The highly branched phytoglycogens were more tightly packed molecules exhibiting greater dispersed densities than sugary-amylopectin and amylopectin. These results suggest that the levels of DBE in rice endosperm control the biosynthetic pathway of branched alpha-glucans and produce amylopectin, sugary-amylopectin and phytoglycogen. (C) 2002 Elsevier Science Ltd. All rights reserved.