The influence of alterations in ADP-glucose pyrophosphorylase activities on starch structure and composition in potato tubers

In order to examine whether alterations in the supply of precursor molecule s into the starch biosynthetic pathway affected various characteristics of the starch, starch was isolated from potato (Solanum tuberosum L.) tubers c ontaining reduced amounts of the enzyme ADP-glucose pyrophosphorylase (AGPa se). It was found that although the type of crystalline polymorph in the st arch was not altered, the amylose content was severely reduced. In addition , amylopectin from the transgenic plants accumulated more relatively short chains than that from control plants and the sizes of starch granules were reduced. The starch granules from the transgenic plants contained a greater amount of granule-bound starch synthase enzyme, which led to an increase i n the maximum activity of the enzyme per unit starch tested. The K-m for AD P-glucose was, at most, only slightly altered in the transgenic lines. Pota to plants containing reduced AGPase activity were also transformed with a b acterial gene coding for AGPase to test whether this enzyme can incorporate phosphate monoesters into amylopectin. A slight increase in phosphate cont ents in the starch in comparison with the untransformed control was found, but not in comparison with starch from the line with reduced AGPase activit y into which the bacterial gene was transformed.