Starch Biosynthesis in Leaves and Its Regulation

Plants assimilate carbon during photosynthesis using light energy to reduce atmospheric CO2 and to produce sugars and chemical energy (ATP). Sugars are partly incorporated directly into starch granules in leaf chloroplasts for short-term storage or are exported to non-photosynthetic organs for long-term storage. Indeed, starch accumulation in photosynthetic tissues is transient since it undergoes recurrent cycles of synthesis and degradation following day/night oscillation. Transient starch is synthesized during the day while photosynthesis is active and is degraded at night to provide carbon and energy to the plant when photosynthesis is inactive. Conversely, storage starch accumulates over long periods in storage organs such as seeds or tubers where it is degraded to sustain germination before photosynthesis becomes effective. Transient and storage starch syntheses occur in plastid stroma and involve dedicated enzymatic activities typically supported by several genetically independent isoforms. Although highly similar, both processes hold specific features regarding synthesis and regulation. In this chapter, we describe the mechanism of starch synthesis in photosynthetic tissues (mostly leaves) and its regulation. Several aspects are specifically highlighted here such as: (1) the function of starch synthases for the initiation of starch synthesis and the elongation of the amylopectin- and amylose-forming glucans, (2) the implication of branching enzymes and debranching enzymes for the formation of branch points and the control of their distribution within the polysaccharides, and (3) the regulation of the pathway in leaves especially by the circadian clock , the redox state of the cell and the influence of physiological factors, and the formation of protein-protein complexes.