CONICET | Buscador de Institutos y Recursos Humanos

Arabidopsis thaliana contains four malic enzymes (NADP-ME 1-4) to catalyze the reversible oxidative decarboxylation of malate in the presence of NADP, being NADP-ME2 the only cytosolic isoform ubiquitously expressed and providing most of the total NADP-ME activity. Analysis of NADP-ME2 loss-of-function mutants indicated its role in the generation of reactive oxygen species during plant basal defense to pathogen infection. In this work, we further investigated the physiological function of NADP-ME2 by characterizing Arabidopsis plants over-expressing NADP-ME2 constitutively. In comparison to wild type, these plants exhibited reduced rosette and root sizes, delayed flowering time and increased sensitivity to the osmotic agents mannitol and polyethylene glycol. The increased NADP-ME2 activity led to decreased expression of other ME and malate dehydrogenase isoforms and generated a redox imbalance with opposite characteristics depending on the time of the day analyzed. The over-expressing plants also presented higher content of C4 organic acids and sugars under normal growth conditions. However, the accumulation of these metabolites in the over-expressing plants was substantially less pronounced after osmotic stress exposure compared to wild type. The transgenic plants also have lower level of several amino acids and osmoprotectant compounds. Thus, altering malate pathways by enhancing NADP-ME2 expression in A. thaliana has profound consequences for the response of the entire primary metabolism under both normal and osmotic stress conditions. Overall, the over-expression of NADP-ME2 produces changes in gene expression, enzymatic activities, redox status and metabolite levels, which reflect the relevance of this enzyme and its substrates and products in plant homeostasis maintenance