A Comprehensive Analysis of the NADP-Malic Enzyme Gene Family of Arabidopsis

The Arabidopsis (Arabidopsis thaliana) genome contains four genes encoding putative NADP-malic enzymes (MEs; AtNADP-ME1–ME4). NADP-ME4 is localized to plastids, whereas the other three isoforms do not possess any predicted organellar targeting sequence and are therefore expected to be cytosolic. The plant NADP-MEs can be classified into four groups: groups I and II comprising cytosolic and plastidic isoforms from dicots, respectively; group III containing isoforms from monocots; and group IV composed of both monocots and dicots, including AtNADP-ME1. AtNADP-MEs contained all conserved motifs common to plant NADP-MEs and the recombinant isozymes showed different kinetic and structural properties. NADP-ME2 exhibits the highest specific activity, while NADP-ME3 and NADP-ME4 present the highest catalytic efficiency for NADP and malate, respectively. NADP-ME4 exists in equilibrium of active dimers and tetramers, while the cytosolic counterparts are present as hexamers or octamers. Characterization of T-DNA insertion mutant and promoter activity studies indicates that NADP-ME2 is responsible for the major part of NADP-ME activity in mature tissues of Arabidopsis. Whereas NADP-ME2 and -ME4 are constitutively expressed, the expression of NADP-ME1 and NADP-ME3 is restricted by both developmental and cell-specific signals. These isoforms may play specific roles at particular developmental stages of the plant rather than being involved in primary metabolism

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 C-4 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. (C) 2015 Elsevier Ireland Ltd. All rights reserved