The plastidic C4 Zea mays NADP-malate dehydrogenase (ZmNADP-MDH), responsible for catalysis of oxaloacetate to malate, was overexpressed in Arabidopsis thaliana to assess its impact on photosynthesis and tolerance to salinity stress. Different transgenic lines were produced having ~3-6-fold higher MDH protein abundance and NADP-MDH enzyme activity than vector control. The overexpressors had similar chlorophyll, carotenoid, and protein content as that of vector control. Their photosynthetic electron transport rates, carbon assimilation rate, and consequently fresh weight and dry weight were almost similar. However, these overexpressors were tolerant to salt stress (150 mM NaCl). In saline environment, the Fv/Fm ratio, yield of photosystem II, chlorophyll, and protein content were higher in ZmNADP-MDH overexpressor than vector control. Under identical conditions, the generation of reactive oxygen species (H2O2) and production of malondialdehyde, a membrane lipid peroxidation product, were lower in overexpressors. In stress environment, the structural distortion of granal organization and swelling of thylakoids were less pronounced in ZmNADP-MDH overexpressing plants as compared to the vector control. Chloroplastic NADP-MDH in consort with cytosolic and mitochondrial NAD-MDH plays an important role in exporting reducing power (NADPH) and exchange of metabolites between different cellular compartments that maintain the redox homeostasis of the cell via malate valve present in chloroplast envelope membrane. The tolerance of NADP-MDH overexpressors to salt stress could be due to operation of an efficient malate valve that plays a major role in maintaining the cellular redox environment.
Keywords: Abiotic stress; Arabidopsis thaliana; C3 and C4 photosynthesis; Malate dehydrogenase; Malate valve; Reactive oxygen species; Salt stress.