Biosynthesis of reserve starch

Abstract

Plants have developed two distinct starch biosynthetic systems composed of over 30 kinds of enzymatic reaction network in photosynthetic and nonphotosynthetic cells. Higher plants have also evolved a process in which cells can accumulate huge amounts of starch as granules inside the amyloplast of the reserve organs. Primarily the coordinated expression of several sets of distinct isozymes with specific enzymatic properties enables plant cells to synthesize starch with distinct fine structure and form the starch granules with specific semicrystalline structure, granular morphology, and physicochemical properties in plastids. This chapter overviews the current status of our understanding of metabolic regulation of starch biosynthesis in reserve organs by focusing on functions of individual isozymes examined by numerous in vivo and in vitro studies that have been performed to reveal how individual isozymes contribute to the synthesis of the reserve starch. The results raised the high possibility that at least some isozymes have multiple functions in starch biosynthesis under different conditions depending on the presence of various glucans and interaction/association with other enzymes. The features of starch biosynthetic process in plant tissues are also discussed with emphasis on the biochemical mechanism(s) underlying the coordinate actions among various enzymes.