The Glucose Sensor MdHXK1 Phosphorylates a Tonoplast Na⁺/H⁺ Exchanger to Improve Salt Tolerance

Glc regulates many vital processes, including plant growth, development, metabolism, and responses to biotic and abiotic stress. However, the molecular mechanism by which Glc acts as a signal to regulate salinity tolerance remains unclear. In this study, we found that the apple (Malus domestica Borkh.) Glc sensor hexokinase1 (MdHXK1) contributes to Glc-mediated salinity tolerance. A combination of split ubiquitin system, pull-down, co-immunoprecipitation, and bimolecular fluorescence complementation assays demonstrated that MdHXK1 interacts with and phosphorylates the Na⁺/H⁺ exchanger MdNHX1 at its Ser-275 residue. Phosphorylation improved the stability of MdNHX1 and enhanced its Na⁺/H⁺ transport activity in MdNHX1 overexpression transgenic apple and yeast complementation cells. Furthermore, Ser-275 of MdNHX1 was found to be crucial for MdHXK1-mediated phosphorylation. Finally, a series of transgenic analyses demonstrated that salt tolerance mediated by MdHXK1 partially depended on MdNHX1. Overall, our findings provide insights into how sugar recruits and regulates MdNHX1 in response to high salinity in plants.