Malate decarboxylases: evolution and roles of NAD(P)-ME isoforms in species performing C4 and C3 photosynthesis

In the C-4 pathway of photosynthesis two types of malate decarboxylases release CO2 in bundle sheath cells, NADP- and NAD-dependent malic enzyme (NADP-ME and NAD-ME), located in the chloroplasts and the mitochondria of these cells, respectively. The C-4 decarboxylases involved in C-4 photosynthesis did not evolve de novo; they were recruited from existing housekeeping isoforms. NADP-ME housekeeping isoforms would function in the control of malate levels during hypoxia, pathogen defence responses, and microspore separation, while NAD-ME participates in the respiration of malate in the tricarboxylic acid cycle. Recently, the existence of three enzymatic NAD-ME entities in Arabidopsis, occurring by alternative association of two subunits, was described as a novel mechanism to regulate NAD-ME activity under changing metabolic environments. The C-4 NADP-ME is thought to have evolved from a C-3 chloroplastic ancestor, which in turn would have evolved from an ancient cytosolic enzyme. In this way, the C-4 NADP-ME would have emerged through gene duplication, acquisition of a new promoter, and neofunctionalization. In contrast, there would exist a unique NAD-ME in C-4 plants, which would have been adapted to perform a dual function through changes in the kinetic and regulatory properties of the C-3 ancestors. In addition to this, for the evolution of C-4 NAD-ME, insertion of promoters or enhancers into the single-copy genes of the C-3 ancestors would have changed the expression without gene duplication.