A multi-stages biosynthetic pathway in starch granules revealed by the ultrastructure of maize mutant starches

Our previous work indicated that starches containing B-type crystallites sh ow low susceptibility to amyloids and suggested that B-type crystallites ha ve an effect on starch granule organisation. To elucidate granular ultrastr ucture, double wxae and aedu maize mutant starches containing A- (30 and 50 % respectively) and B-type (70 and 50% respectively crystallites were treat ed with porcine pancreatic alpha-amylase. the surface structure of the nati ve and degraded starches was studied by scanning electron microscopy, and t he internal ultrastructure by transmission electron microscopy after staini ng with PATAg reagents. The results confirm the influence of B-type crystal lites on granule organisation and indicate that starches containing B-type crystallites show an amylolysis attack pattern with minor exocorrosion and major endocorrosion. The granule organisation of A- and B-type starches pro posed is not consistent with an onion ring model(1) and may account for the different behavior of these starches to amylolysis. Transmission electron microscopy showed that most native wxae nad aedu starch granules are compos ed of a core with classical alternating structure and a peripheral ring. Th e peripheral ring in waxe starch was ordered and resistant to amylolysis, w hereas that of aedu was disordered and degradable. It is proposed that thes e two specific starch synthesising enzymes during the course of starch bios ynthesis. (C) 2001 Academic Press.