Increased Amylose Content and Altered Molecular Structures of Wheat Starch by SGP-1 Null Mutation

Abstract

Amylose content and molecular structures of starch from a mutant of wheat (Triticum aestivum L.) that lacks a starch granule bound protein, SGP-1, have been compared with those from normal wheat (cv. Chinese Spring). Actual amylose contents, calculated by taking iodine affinity of amylopectin into consideration, for the mutant (SGP-1 null) and normal wheat starches were 34 and 26%, respectively. The amylopectins from SGP-1 null and normal starches had number-average degree of polymerization (DPn) of 4200 and 4700, respectively. Three molecular species of amylopectin with different size were observed by fluorescent labeling/GPC. Number-average chain length (CLn) of the amylopectins was close (∼21) but their CL distributions were significantly different. SGP-1 null amylopectin contained 9.5% by weight of long unit-chains having CL of >100 while such long chains were absent in amylopectin from the normal starch. Molar ratio of A chains increased slightly and that of B₁ chains conversely decreased in the mutant, and the molar ratio of (A+B₁)/(B₂+B₃) was 13.8 and 13.1 for the mutant and normal amylopectins, respectively. DPn of amylose from SGP-1 null (∼800) was lower than that for the normal (∼1000). DP distribution of the amyloses on molar and weight bases showed the ratio of smaller amyloses was higher for the mutant than the normal starch. Branched molecules of amylose consisted of 24 and 23% by mole and had average number of chains of branched molecules (NCB) 13.1 and 7.5, respectively. The above structural data on a molar basis also revealed that the mutation caused the increase in number of amylose molecules but had no effect on the number of amylopectin molecules.