Enhanced cytosolic NADP-ME2 activity in A. thaliana affects plant development, stress tolerance and specific diurnal and nocturnal cellular processes.

Arabidopsis thaliana has four NADP-dependent malic enzymes (NADP-ME 1-4) for reversible malate decarboxylation, with NADP-ME2 being the only cytosolic isoform ubiquitously expressed and responsible for most of the total activity. In this work, we further investigated its physiological function 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 mannitol and polyethylene glycol. The increased NADP-ME2 activity led to a decreased expression of other ME and malate dehydrogenase isoforms and generated a redox imbalance with opposite characteristics depending on the time point of the day analyzed. The over-expressing plants also presented a 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. Also, a lower level of several amino acids and osmoprotector compounds was observed in transgenic plants. Thus, the gain of NADP-ME2 expression has profound consequences in the modulation of primary metabolism in A. thaliana, which reflect the relevance of this enzyme and its substrates and products in plant homeostasis.