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 H2O2 content, but a low [ABSTRACT FROM AUTHOR]