The relationship between the fine structure of amylopectin and the type of crystalline allomorph of starch granules in rice endosperm

Abstract

AbstractBackground and ObjectivesIt is known that any wild‐type varieties of cereals so far examined have A‐type starch crystals in their endosperm whereas their starch branching enzyme 2b (be2b) mutants that are usually referred to as amylose‐extender (ae) mutants produce B‐type starch crystals. The present study aimed to examine the structural features of amylopectin which are responsible for the A‐type or B‐type crystalline allomorphs of starch granules by using starches in wild‐type japonica and indica rice varieties and their be2b (ae) mutants.FindingsThe average length of chains of A‐type amylopectin was markedly shorter than that of B‐type amylopectin. It was also thought that A‐type amylopectin had two types of branches, namely the first branches were formed mainly by BEI in the basal region of the cluster and the second branches were synthesized specifically by BEIIb inside the cluster. These differences caused the average length of double helices formed by cluster constituent chains to be shorter in wild‐types than that in their be2b mutants, and these differences changed the crystalline allomorph of starch granules to B‐type in the mutants from A‐type in their wild‐types, as examined by wide angle X‐ray diffraction and sum frequency generation spectroscopy.ConclusionsCombined with these observations and calculations of chain‐length of hypothetical A chains in four types of amylopectin molecules, it was concluded that both the average length of external segments of cluster chains and the formation of the second branches inside the cluster greatly affect the crystal types of starch granules.Significance and NoveltyThe novel findings in the present study can provide new insights into the structural features of amylopectin which determine the A‐type or B‐type crystalline allomorph of starch granules of cereals.