The MdVQ22-MdWRKY33d complex regulates MdPER55 and MdDREB1D expression to modulate apple salt tolerance by reactive...

Apple is a globally significant fruit tree with considerable economic value. However, it is severely affected by soil salinization, which leads to a reduction in both yield and quality. Since salt tolerance in grafted apple trees largely depends on the rootstock, this study compared transcriptomic responses between salt-tolerant (R5-5) and salt-sensitive (M9T337) rootstocks under salt stress. A key salt-responsive gene, MdWRKY33d, was identified. Overexpressing MdWRKY33d in apple improved salt tolerance, while silencing it had the opposite effect. Additionally, under salt stress, compared with the control, chlorophyll fluorescence values, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) levels were elevated in MdWRKY33d transgenic apple and poplar materials. Functional analyses demonstrated that MdWRKY33d directly activates the expression of Peroxidase55 (MdPER55) and Dehydration-Responsive Element-Binding protein1D (MdDREB1D), thereby enhancing reactive oxygen species (ROS) scavenging and salt tolerance in transgenic plants. Yeast two-hybrid screening identified MdVQ22 as an interacting partner of MdWRKY33d, which was confirmed by pull-down, bimolecular fluorescence complementation (BiFC), and luciferase complementation imaging (LCI) assays. MdVQ22 negatively regulated apple salt tolerance by suppressing MdWRKY33d-mediated transcriptional activation of MdPER55 and MdDREB1D. Collectively, this study explores a putative regulatory module MdVQ22-MdWRKY33d-MdPER55/MdDREB1D to coordinate the responses to apple salt stress and provides potential targets for engineering salt-tolerant varieties.