Overexpression of Ms DREB 6.2 results in cytokinin‐deficient developmental phenotypes and enhances drought tolerance in transgenic apple plants

Summary Dehydration‐responsive element binding factors ( DREB s) play important roles in plant growth, development, and stress signaling pathways in model plants. However, little is known about the function of DREB s in apple ( Malus × domestica ), a widely cultivated crop that is frequently threatened by drought. We isolated a DREB gene from Malus sieversii (Ledeb.) Roem., Ms DREB 6.2 , and investigated its functions using overexpression analysis and c himeric re pressor gene‐ s ilencing t echnology ( CRES ‐T). We identified possible target genes of the protein encoded by Ms DREB 6.2 using electrophoretic mobility shift assays ( EMSA s) and chromatin immunoprecipitation (Ch IP ). Overexpression of Ms DREB 6.2 increased the expression of a key cytokinin ( CK ) catabolism gene, Md CKX 4a , which led to a significant reduction in endogenous CK levels, and caused a decrease in shoot:root ratio in transgenic apple plants. Overexpression of Ms DREB 6.2 resulted in a decrease in stomatal aperture and density and an increase in root hydraulic conductance (L 0 ), and thereby enhanced drought tolerance in transgenic plants. Furthermore, manipulating the level of Ms DREB 6.2 expression altered the expression of two aquaporin ( AQP ) genes. The effect of the two AQP genes on L 0 was further characterized using the AQP inhibitor HgCl 2 . Based on these observations, we conclude that Ms DREB 6.2 enhances drought tolerance and that its function may be due, at least in part, to its influence on stomatal opening, root growth, and AQP expression. These results may have applications in apple rootstock breeding programs aimed at developing drought‐resistant apple varieties.