Characterization and Utilization of Spontaneous Deficiency in Starch Branching Enzyme I of Rice (Oryza sativa L.)

Abstract

Starch, the major component of cereal grains including rice is composed of two types of α-polyglucan, amylose and amylopectin. Amylose forms a mainly linear structure, with glucose residues linked via α-1,4 bonds, whereas amylopec-tin has a highly ordered structure 1) with numerous branches linked to the middle of the linear structure by α-1,6 bonds. The ratio of amylose to amylopectin and the characteristics of the distribution of amylopectin chain-lengths affect the eating qualities and processing properties of cereal-based foods through changes in gelatinization and retrogradation of starch. Natural and induced mutants that have a defect in a gene encoding a starch-synthesizing enzyme and altered starch properties have been utilized in food production. For example, traditionally cultivated waxy rice, waxy maize, waxy barley, and the relatively recently bred waxy wheat 2) , which lack the granule-bound starch synthase I (GBSSI or Waxy) responsible for amylose synthesis have stickier or softer textures after cooking, and have been utilized for food products where such textures are desirable. In case of the rice mutants lacking starch branching enzyme IIb activity (amylose-extender or ae), the proportion of short-chains (degree of polymerisation (DP) 17 or less) of amylopectin is decreased and that of middle-length chains (DP 18 to 36) is increased, which results in higher gelatinization temperatures (GTs) and faster retrogradation of gelatinazed starch. 3) This type of maize is known as amylomaize and is utilized for the production of resistant starch 4) : i.e., starch that cannot be digested by the human small intestine and therefore acts as a dietary fiber with many health benefits. Lack of starch synthase IIa (SSIIa) function causes changes in chain-length distribution: in rice, 5) barley, 6) wheat 7) and maize, 8) there is an Abstract: Mutations in the starch synthesizing genes of cereal crops with altered starch properties has been utilized to widen the applications of grain, flour and starch. Here, rice spontaneous mutants that lacked starch branching enzyme 1 (BEI) activity were identified from among local upland rice cultivars. Two cultivars, Kurnai and Hiderishirazu-D, lacked BEI activity in the developing endosperm, and their reserved starch was rich in short chains of amylopectin and showed a low pasting temperature (PT) of rice flour. These features are similar to those of the induced starch branching enzyme 1 gene (Sbe1) mutant of rice. We detected polymorphisms in the Sbe1 of the Kurnai and Hiderishirazu-D cultivars and used them to develop PCR markers suitable for selecting breeding lines and cultivars with BEI deficiency. We assessed the possible utilization of BEI deficiency in waxy rice processing with the use of both the spontaneous and induced BEI-deficient lines. Rice cakes made from BEI-deficient lines maintained their softness for longer periods than those made from functional BEI lines. Our results suggest that use of BEI-deficient waxy rice could improve the quality and extend the shelf life of waxy rice products.