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

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.