Recent Views on the Biosynthesis of the Plant Starch Granule

Abstract

Plant starch can be distinguished from bacterial, fungal and animal glycogen by the simultaneous presence of at least 2 distinct fractions (amylose and amylopectin) whose relative arrangement inside a huge insoluble granule remains to be determined. Amylopectin, the major branched fraction is itself a highly organized molecule displaying a succession of clusters of glucans packed in crystal arrays. The unit cluster size (9nm) is a remarkably constant feature of all plants examined which in turn suggests the existence of a highly conserved and ordered biosynthetic pathway. This review after giving a brief overview on starch structure and metabolism will focuss on the building of the granule's architecture. The genetic and biochemical evidence summarized here all point to the presence of multiple elongation and branching enzymes who are only partly redundant in function for starch biosynthesis. We suggest that each enzyme is responsible for the building of specific granule substructures.