Activity of starch biosynthetic enzymes and their association with endosperm modification in quality protein maize

The formation of vitreous endosperm in quality protein maize (QPM) is associated with changes in the composition and structure of starch granules, but little is known about the role of alterations in the structure and activity of the main starch biosynthetic enzymes. Developing endosperms from K0326Y-QPM, W64Ao2, and derived recombinant inbred lines were used to analyze the activity of ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), starch-branching enzyme (SBE) and pullulanase (PULL). GBSS activity correlated positively with kernel vitreousness and the opposite was observed for SBE. SBEIIb enzymes from K0326Y-QPM and W64Ao2 differed in five amino acids. Three amino acid changes appear to affect the SBEIIb catalytic site, which may be responsible for the lower activity and amylopectin content in vitreous lines. However, recombinant SBEIIb enzymes produced in Escherichia coli showed similar activities. The results suggest that endosperm modification in QPM is associated with changes in the activity of starch biosynthetic enzymes, which affect the starch composition, physicochemical and structural properties during endosperm development. The higher activity of GBSS and the lower activity of SBE produce starch granules with larger proportions of amylose and amorphous regions, favoring a greater compaction between these structures, contributing to the vitreous phenotype in QPM.