The MdERF1B-MdWRKY75-MdSOS3 Module Confers Salt Tolerance by Regulating Sodium-Potassium Homoeostasis in Apple

Apple is an economically important crop worldwide, but its production is constrained by soil salinization. Excess salt impairs growth through ion toxicity, osmotic stress, and oxidative damage, ultimately compromising fruit quality and yield. Breeding salt-tolerant varieties is therefore a key strategy for sustaining production in saline environments. This study showed that a salt response transcription factor, MdWRKY75, enhances apple salt tolerance by regulating the sodium-potassium ion balance. Under salt stress, the Na⁺ content and Na⁺/K⁺ ratio in the roots and leaves of the MdWRKY75-overexpressing lines were significantly reduced compared with the wild type (WT). Moreover, the expression of genes responsible for salt ion transport was significantly upregulated. The RNA interference (RNAi) lines displayed the opposite phenotype. Further analysis revealed that MdWRKY75 binds to the MdSOS3 promoter and positively regulates its expression. The roots of plants overexpressing MdSOS3 lines showed salt tolerance and could rescue the salt-sensitive phenotype of MdWRKY75-Ri3. Yeast one-hybrid revealed a gene in the ethylene signalling pathway, MdERF1B, as the upstream transcription factor of MdWRKY75. Electrophoretic mobility shift assay (EMSA), dual-luciferase, and β-glucuronidase (GUS) assays confirmed that MdERF1B activates the expression of MdWRKY75. Overexpressing MdERF1B in both WT and MdWRKY75-Ri3 backgrounds reduced the Na⁺ and Na⁺/K⁺ ratios in plants and improved salt tolerance. Additionally, exogenous application of 1-aminocyclopropane-1-carboxylic acid (ACC) enhanced apple salt tolerance by activating MdERF1B. Therefore, this study demonstrated that an ethylene-mediated MdERF1B-MdWRKY75-MdSOS3 module enhances apple salt tolerance by regulating the sodium-potassium ion balance.