The enhancement of tolerance to salt and cold stresses by modifying the redox state and salicylic acid content via the cytosolic malate dehydrogenase gene in transgenic apple plants

Summary In this study, we characterized the role of an apple cytosolic malate dehydrogenase gene ( Mdcy MDH ) in the tolerance to salt and cold stresses and investigated its regulation mechanism in stress tolerance. The Mdcy MDH transcript was induced by mild cold and salt treatments, and Mdcy MDH ‐ overexpressing apple plants possessed improved cold and salt tolerance compared to wild‐type ( WT ) plants. A digital gene expression tag profiling analysis revealed that Mdcy MDH overexpression largely altered some biological processes, including hormone signal transduction, photosynthesis, citrate cycle and oxidation–reduction. Further experiments verified that Mdcy MDH overexpression modified the mitochondrial and chloroplast metabolisms and elevated the level of reducing power, primarily caused by increased ascorbate and glutathione, as well as the increased ratios of ascorbate/dehydroascorbate and glutathione/glutathione disulphide, under normal and especially stress conditions. Concurrently, the transgenic plants produced a high H 2 O 2 content, but a low O 2 · −production rate was observed compared to the WT plants. On the other hand, the transgenic plants accumulated more free and total salicylic acid ( SA ) than the WT plants under normal and stress conditions. Taken together, Mdcy MDH conferred the transgenic apple plants a higher stress tolerance by producing more reductive redox states and increasing the SA level; Mdcy MDH could serve as a target gene to genetically engineer salt‐ and cold‐tolerant trees.