Laboratoire de Chimie Biologique, UMR 8576 du CNRS, Universite des Sciences et Technologies de Lille, Cite Scientifique, 59655 Villeneuve d’Ascq, France
Transient or long-term storage of photosynthate in starch granules is the last step of eukaryote photosynthesis. The storage of glucose into structures larger than an individual bacterial cell is a complex mechanism that distinguishes the chloroplast from its ancestor prochloron or cyanobacterial-like cell. 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 a powerful model system to select for mutants defective in various aspects of granule biogenesis, degradation or overproduction. A description of the 11 loci involved is presented and the involvement of the 3-PGA/Pi ratio in controlling the rates of polysaccharide synthesis is demonstrated genetically. 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. In addition, two enzymes have unexpected important roles in starch biosynthesis in Chlamydomonas reinhardtii. These biochemical steps define novel targets for improving the structure of starch in cereals.