Down-regulation of MdHDA6 enhanced drought tolerance in apple. A) Identification of MdHDA6-RNAi transgenic lines at DNA level with 2 specific primers corresponding to the 2 RNAi fragments (Forward and Reverse fragments). M: marker; WT: wild-type apple plants (GL-3); #6, #10, #12: 3 MdHDA6-RNAi transgenic lines; H₂O: Negative control, we used H₂O as a template for the PCR reaction to be a negative control. B) Identification of MdHDA6 RNAi lines at RNA level. MdMDH was used as the internal reference gene. Error bars indicate SD (n = 3). C) Phenotypes of GL-3 (Wild-type) and MdHDA6-RNAi transgenic plants. Bars = 5 cm. Three-month-old plants were treated under drought for 14 d, and then rewatered for 3 d. D) Survival rates of GL-3 and 3 independent MdHDA6-RNAi lines in (C). Data are mean ± SD (n = 3). At least 18 seedlings were used per line in each biological replicate. E) Water loss of detached leaves from GL-3 and MdHDA6-RNAi plants at 25 °C. Data are mean ± SD (n = 6). F) Leaf relative water content (RWC) of GL-3 and MdHDA6-RNAi plants under drought. Data are mean ± SD (n = 6). G) Photosynthesis rate analysis of GL-3 and MdHDA6-RNAi plants under drought. Data are mean ± SD (n = 9). H) Leaf electrolyte leakage assays of GL-3 and MdHDA6-RNAi plants under drought treatment. Data are mean ± SD (n = 7). Detection of MDA (I) and H₂O₂ (J) contents in GL-3 and MdHDA6-RNAi plants under drought treatment. Data are mean ± SD (n = 3). K) DAB staining was performed to detect the H₂O₂ contents. MDA, malondialdehyde; H₂O₂, hydrogen peroxide. DAB, 3,30-DAB tetrahydrochloride. One-way ANOVA (Tukey's test) was performed and statistically significant differences were indicated by *P < 0.05; **P < 0.01; ***P < 0.001.