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Organic acids are involved in numerous metabolic pathways in all plants. The finding that some plants, known as C 4  plants, have four-carbon dicarboxylic acids as the first product of carbon fixation showed these organic acids play essential roles as photosynthetic intermediates. Oxaloacetate (OAA), malate, and aspartate (Asp) are substrates for the C 4  acid cycle that underpins the CO 2  concentrating mechanism of C 4  photosynthesis. In this cycle, OAA is the immediate, short-lived, product of the initial CO 2  fixation step in C 4  leaf mesophyll cells. The malate and Asp, resulting from the rapid conversion of OAA, are the organic acids delivered to the sites of carbon reduction in the bundle-sheath cells of the leaf, where they are decarboxylated, with the released CO 2  used to make carbohydrates. The three-carbon organic acids resulting from the decarboxylation reactions are returned to the mesophyll cells where they are used to regenerate the CO 2  acceptor pool. NADP-malic enzyme-type, NAD-malic enzyme-type, and phospho enol pyruvate carboxykinase-type C 4  plants were identified, based on the most abundant decarboxylating enzyme in the leaf tissue. The genes encoding these C 4  pathway-associated decarboxylases were co-opted from ancestral C 3  plant genes during the evolution of C 4  photosynthesis. Malate was recognized as the major organic acid transferred in NADP-malic enzyme-type C 4  species, while Asp fills this role in NAD-malic enzyme-type and phospho enol pyruvate carboxykinase-type plants. However, accumulating evidence indicates that many C 4  plants use a combination of organic acids and decarboxylases during CO 2  fixation, and the C 4 -type categories are not rigid. The ability to transfer multiple organic acid species and utilize different decarboxylases has been suggested to give C 4  plants advantages in changing and stressful environments, as well as during development, by facilitating the balance of energy between the two cell types involved in the C 4  pathway of CO 2  assimilation. The results of recent empirical and modeling studies support this suggestion and indicate that a combination of transferred organic acids and decarboxylases is beneficial to C 4  plants in different light environments.

The Roles of Organic Acids in C4 Photosynthesis