The chloroplastic, NADP-dependent form is essential for the photosynthesis C4 cycle, which allows plants to circumvent the problem of photorespiration. In C4 plants, NADP-MDH activity acts to convert oxaloacetate to malate in chloroplasts of mesophyll cells for transport to the bundle sheath cells. (S)-malate + NADP(+) = H(+) + NADPH + oxaloacetate Chloroplast NADP-MDH is activated upon illumination. In order to be enzymatically active, disulfide bridges on the protein must be reduced by thioredoxin which receives electrons from ferredoxin and the electron transport system of photosynthesis. Homodimer. Expression decreases transiently in the leaves after salt stress and then increases to levels greater than two-fold higher than in unstressed plants. Belongs to the LDH/MDH superfamily. MDH type 2 family.