Rice plastidial NAD ‐dependent malate dehydrogenase 1 negatively regulates salt stress response by reducing the vitamin B6 content

Summary Salinity is an important environmental factor that adversely impacts crop growth and productivity. Malate dehydrogenases ( MDH s) catalyse the reversible interconversion of malate and oxaloacetate using NAD (H)/ NADP (H) as a cofactor and regulate plant development and abiotic stress tolerance. Vitamin B6 functions as an essential cofactor in enzymatic reactions involved in numerous cellular processes. However, the role of plastidial MDH in rice ( Oryza sativa ) in salt stress response by altering vitamin B6 content remains unknown. In this study, we identified a new loss‐of‐function osmdh1 mutant displaying salt stress‐tolerant phenotype. The Os MDH 1 was expressed in different tissues of rice plants including leaf, leaf sheath, panicle, glume, bud, root and stem and was induced in the presence of NaCl. Transient expression of Os MDH 1‐ GFP in rice protoplasts showed that Os MDH 1 localizes to chloroplast. Transgenic rice plants overexpressing Os MDH 1 ( Os MDH 1 OX ) displayed a salt stress‐sensitive phenotype. Liquid chromatography–mass spectrometry ( LC ‐ MS ) metabolic profiling revealed that the amount of pyridoxine was significantly reduced in Os MDH 1 OX lines compared with the NIP plants. Moreover, the pyridoxine content was higher in the osmdh1 mutant and lower in Os MDH 1 OX plants than in the NIP plants under the salt stress, indicating that Os MDH 1 negatively regulates salt stress‐induced pyridoxine accumulation. Furthermore, genome‐wide RNA ‐sequencing ( RNA ‐seq) analysis indicated that ectopic expression of Os MDH 1 altered the expression level of genes encoding key enzymes of the vitamin B6 biosynthesis pathway, possibly reducing the level of pyridoxine. Together, our results establish a novel, negative regulatory role of Os MDH 1 in salt stress tolerance by affecting vitamin B6 content of rice tissues.