Physicochemical properties of amylose-free and high-amylose starches from transgenic sweetpotatoes modified by RNA interference

Abstract

The transgenic sweetpotatoes having amylose-free and moderately high-amylose starches have been obtained by RNA interferences of granule-bound starch synthase I (GBSSI) and starch branching enzyme II [SBEII (class A)] genes, respectively. In this study, the physicochemical properties of such sweetpotato starches were investigated. From the amylose-free starches, structural changes in the starch molecules were found by suppressing GBSSI expression: lack of amylose, lack of long chains with more than DP 100 in amylopectin, and a slight decrease in chains with DP 6 and 7 in amylopectin. These results suggest that GBSSI participates in not only the synthesis of amylose molecules, but also the characterization of amylopectin molecules. In the case of the high-amylose starches, complicated structural changes were found by suppressing SBEII expression: increase in amylose, increase in phosphate content, alteration of crystalline structure, decrease in long chains with more than DP 100 in amylopectin, decrease in chains around DP 6-11, and increase in chains around DP 12-15 and DP 24-33 in amylopectin. As the results of the structural changes, the amylose-free sweetpotato starches showed a very slow retrogradation and high digestibility by pancreatin, whereas the high-amylose sweetpotato starches showed a higher pasting viscosity, more rapid retrogradation and lower digestibility compared to normal starch.