Molecular basis of plant adaptation to light. Example of two enzymes of the C4 photosynthesis cycle

Abstract The light-dependent mechanisms responsible for the regulation of two enzymes of the C 4 photosynthetic cycle, phosphoenolpyruvate carboxylase and NADP-dependent malate dehydrogenase in Sorghum are presented as an illustration of adaptative mechanisms of plants to light. Emphasis is put on the organization of the signaltransduction chains which upregulate both enzymes at transcriptional and post-translational levels. For both enzymes, small gene families include a photosynthesis-related gene, the expression of which is light-triggered. The light-controlled phosphorylation of C 4 PEPC implies cross-talk between photosynthetic cell types, cytosolic pH and calcium changes, and the upregulation of a specific protein-serine kinase. This post-translational modification strongly influences its regulation by photosynthesis-related metabolites. NADP-MDH activity is controlled by a chloroplastic redox cascade involving thioredoxin as the ultimate relay. The significance of these regulatory circuits is discussed in relation with the flexibility of the metabolism in the context of the C 4 photosynthesis.