10 - Starch Biosynthesis and Degradation

Abstract

This chapter presents the information known about starch biosynthesis and degradation, and the regulatory phenomena associated with it in leaf and in storage tissue. It also presents at least three possible enzymatic routes from glucose-1-P to the biosynthesis of α1 - 4 glucosidic linkages in plant systems: the phosphorylase route, via UDP-glucose, or via ADP-glucose. Furthermore, many plant cell extracts contain potent amylase activities that can interfere with the starch synthase assay. The products of starch degradation comprise a major portion of the metabolites of germinating seeds or other reserve tissue that it is relatively straightforward to determine the major products of starch degradation in these tissues. However, in photosynthetic tissues, where starch degradation accounts for only part of the sugar, sugar phosphate and organic acid transformations occurring in the dark, it is more difficult to assign the appearance of intermediates to starch breakdown. Additionally, starch degradation in leaves may be regulated by phosphate concentration.