Regulation of photosynthetic carbon assimilation at the cellular level: a review.

In green leaves and a number of algae, photosynthetically derived carbon is ultimately converted into two carbohydrate end-products, sucrose and starch. Drainage of carbon from the Calvin cycle proceeds via triose phosphate, fructose 6-phosphate and glycollate. Gluconeogenesis in photosynthetic cells is controlled by light, inorganic phosphate and phosphorylated sugars. Light stimulates the production of dihydroxyacetone phosphate, the initial substrate for sucrose and starch synthesis, and inhibits the degradative pathways in the chloroplast. Phosphate inactivates reactions of synthesis and activates reactions of degradation. Among the phosphorylated sugars a special role is allocated to fructose 2,6-bisphosphate, which is present in the cytoplasm at very low concentrations and inhibits sucrose synthesis directly by inactivating pyrophosphatedependent phosphofructokinase. The synthesis of sucrose plays a central role in the partitioning of photosynthetic carbon. The cytoplasmic enzymes, fructose bisphosphate phosphatase and sucrose phosphate synthase are likely key points of regulation. The regulation is carried out by several effector metabolites. Fructose 2,6-bisphosphate is likely to be the main coordinator of the rate of sucrose synthesis, hence of photosynthetic carbon partitioning between sucrose and starch.