Dynamic changes of starch properties, sweetness, and β-amylases during the development of sweet potato storage roots

Starch and sugar properties and sweetness of storage root (SR) are vital to the economic value of sweet potato. To clarify the factors determining these properties of raw sweet potato, eleven varieties with different SR properties were selected to investigate the changes of starch and sugar-related traits, brix and amylase activities during the growth and development of SRs, and the correlation among these SR properties and the amylase activities were analyzed. Five β-amylase genes expressed in SRs were cloned, and their expression and characteristics were analyzed to reveal the involvement of amylase in these SR properties. The results showed total sugar, soluble sugar, reducing sugar content and brix varied greatly among different varieties and development periods, β-amylase activities were much higher than that of α-amylase in SRs, dynamically changed during the development of SRs, and decreased at the late stage of SR expansion. The expression of three β-amylase encoding genes were positively correlated with the total sugar content, and might play vital roles in starch and sugar metabolism in SRs. The brix of raw SRs was significantly correlated with total starch, amylose, amylopectin, total sugar content and reducing sugar content, but had no significant relationship with soluble sugar content and β-amylase activity. These results indicated that the brix of SR was determined by both starch and sugar components, and multiple β-amylases function together in starch and sugar metabolism in developing SRs. These findings provide important information for the improvement of sweetness and the quality of processed products in sweet potato.