Regulation of Starch Biosynthesis

Abstract

SummaryTransient or long-term storage of photosynthate in starch granules can be considered as the last step of eukaryotic photosynthesis. Storage of glucose into structures larger than the size of an individual bacterial cell is slowly uncovering as an exceedingly complex mechanism, which distinguishes the chloroplast from its ancestor prochloron or cyanobacterial-like cell. There is no question that starch biosynthesis has evolved from a pre-existing simpler bacterial glycogen synthesis pathway. However the number of enzymes involved in plant starch synthesis appears considerably higher. Chlamydomonas reinhardtii is now emerging as the most powerful model system to select for mutants defective in various aspects of granule biogenesis, degradation or overproduction. A full description of the eight loci reported to be involved is presented. A genetic demonstration is made of the involvement of the 3-PGA/Pi ratio in controlling the rates of polysaccharide synthesis in algae. The evidence for the respective functions of the starch synthases in the building of specific sub-structures of the granule is detailed. The selection of starchless C. reinhardtii mutants, in which macrogranular starch is replaced with disorganized glycogen-like structures, has paved the way for a deeper understanding of plant amylopectin synthesis. A model is thus presented proposing the existence of pre-amylopectin, a branched precursor that is subsequently trimmed into an ordered structure. The trimming is proposed to relieve the physical constraints on the upper size limit imposed on glycogen granule biogenesis. An account of the compartmentation of glycolysis and of both the pentose-phosphate and the starch biosynthesis pathways is given. The relevance of this compartmentation with respect to starch synthesis regulation is discussed.