A critical review on structural properties and formation mechanism of heterogeneous starch granules in cereal endosperm lacking starch branching enzyme

Abstract

In cereal endosperm, the deficiency of starch branching enzyme (SBE) can change starch morphology and structure, forming heterogeneous starch granules. The contents and types of heterogeneous granules in endosperm influence the quality and utilizations of cereal seeds. This review summarizes the morphologies, physicochemical properties, and formation mechanisms of heterogeneous granules in cereal endosperm lacking SBE. Some future researches on heterogeneous granules are also suggested. The heterogeneous granules can be divided into heteromorphous and biphasic granules. Heteromorphous granules including aggregate, elongated, hollow and sickle granules have different morphologies, and are detected in rice, maize, wheat and barley endosperms lacking SBE. The different heteromorphous granules show different starch components, molecular structures, thermal properties, and hydrolysis resistances. Biphasic granules have different organizational and molecular structures between their inner and outer regions, and are mainly reported in rice and maize endosperms lacking SBEIIb. The heterogeneous granules have regional distribution in endosperm. The amylose molecules from adjacent starch subgranules in one amyloplast form anti-parallel double helices, leading to the fusion of subgranules and the formation of aggregate and elongated granules. The changed amylopectin with long branch-chains and low branching degree is mainly responsible for the formation of biphasic granules. The differently changed extents of SBE in different regions of endosperm and at different development stages of starch result in different heterogeneous granules and their regional distribution in endosperm.