The role of alanine and aspartate aminotransferases in C4 photosynthesis

Alanine and aspartate are essential transfer metabolites for C-4 species of the NAD-malic enzyme and phosphoenolpyruvate carboxykinase subtype. To some degree both amino acids are also part of the metabolite shuttle in NADP-malic enzyme plants. In comparison with C-3 species, the majority of C-4 species are therefore characterised by enhanced expression and activity of alanine and aspartate aminotransferases (AT) in the photosynthetically active tissue. Both enzymes exist in multiple copies and have been found in different subcellular compartments. We tested whether different C-4 species show preferential recruitment of enzymes from specific lineages and subcellular compartments. Phylogenetic analysis of alanine and aspartate ATs from a variety of monocot and eudicot C-4 species and their C-3 relatives was combined with subcellular prediction tools and analysis of the subsequent transcript amounts in mature leaves. Recruitment of aspartate AT from a specific subcellular compartment was strongly connected to the biochemical subtype. Deviation from the main model was however observed in Gynandropsis gynandra. The configuration of alanine AT generally differed in monocot and eudicot species. C-4 monocots recruited an alanine AT from a specific cytosolic branch, but eudicots use alanine AT copies from a mitochondrial branch. Generally, plants display high plasticity in the setup of the C-4 pathway. Beside the common models for the different C-4 subtypes, individual solutions were found for plant groups or lineages.