Mitochondria in photosynthetic cells: Coordinating redox control and energy balance.

In photosynthetic tissues in the light, the function of energy production is associated primarily with chloroplasts, while mitochondrial metabolism adjusts to balance ATP supply, regulate the reduction level of pyridine nucleotides, and optimize major metabolic fluxes. The tricarboxylic acid cycle in the light transforms into a non-cyclic open structure (hemicycle) maintained primarily by influx of malate and export of citrate to the cytosol. The exchange of malate and citrate forms the basis of feeding redox energy from the chloroplast into the cytosolic pathways. This supports the level of NADPH in different compartments, contributes to the biosynthesis of amino acids, and drives secondary metabolism via the supply of substrates for 2-oxoglutarate dioxygenase and for cytochrome P450-catalyzed monooxygenase reactions. This results in the maintenance of redox and energy balance in photosynthetic plant cells and in the formation of numerous bioactive compounds specific for any particular plant species. The non-coupled mitochondrial respiration operates in coordination with the malate and citrate valves and supports intensive fluxes of respiration and photorespiration. The metabolic system of plants has features associated with remarkable metabolic plasticity of mitochondria that permit the use of energy accumulated in photosynthesis in a way that all anabolic and catabolic pathways become optimized and coordinated.