Overexpression of chloroplastic Zea mays NADP-malic enzyme (ZmNADP-ME) confers tolerance to salt stress in Arabidopsis thaliana

Plants with C4 photosynthesis efficiently assimilate CO 2 under stress conditions. To probe this idea further, the cDNA of decarboxylating C4 gene, for the NADP-malic enzyme from Zea mays ( Zm NADP-ME), was overexpressed in Arabidopsis thaliana under the control of 35S promoter. The amino acids and protein contents in the transgenics were lower than in the vector control (VC). In the transgenics, the decarboxylation of malate to pyruvate resulted in reduced presence of 4-carbon acids that serve as the carbon backbone for amino acid synthesis. Consequently, amino acid and protein content were lower in the transgenics than in the VC. As a result, the photosynthetic efficiency (Fv/Fm), electron transport rate (ETR), carbon assimilation rate, overall quantum yield and starch content were reduced in the transgenics. These resulted in lower Chl content, rosette diameter, fresh weight and dry weight of the transgenics than that of the VC. Conversely, the transgenics had higher photosynthetic rate under salt stress. The overexpressers had higher Chl and protein content, Fv/Fm, ETR, and biomass than the VC grown under 150mM NaCl. NADPH generated due to the overexpression of NADP-ME in the overexpressers must have been used to synthesize proline that protected plants from reactive oxygen species, increased glutathione peroxidase activity and decreased H 2 O 2 content in the transgenics. The reduced membrane lipid peroxidation and lower malondialdehyde production resulted in better preservation of thylakoid integrity and membrane architecture in the transgenics under saline environment. Our results demonstrate the vital role of C4 gene(s) in protecting plants form abiotic stress.