Hydrogen Gas Enhances Salinity Tolerance in Tomato Seedlings by Regulating the <i>S</i> ‐Nitrosylation of MEK1

Hydrogen gas (H₂) effectively alleviates abiotic stress in horticultural plants. Protein S-nitrosylation, a key post-translational modification, serves as a critical mechanism for nitric oxide (NO) to exert its biological function under adverse conditions. However, the relationship among H₂, NO and S-nitrosylation in response to salt stress remains poorly understood. In this study, we demonstrate that NO participated in H₂-enhanced salt tolerance in tomato seedlings (Solanum lycopersicum cv. Micro-Tom). H₂ triggered an increase in NO levels and S-nitrosothiol (SNO) content under salt stress, enhancing the enrichment of S-nitrosylated proteins. S-nitrosoproteomic analysis revealed that MEK1, a conserved MAPK component, was notably induced and S-nitrosylated by NO and H₂ under salt stress. Furthermore, SlMEK1-overexpressing tomato lines exhibited enhanced salt stress tolerance, while knockout lines showed reduced tolerance, indicating the positive regulatory role of SlMEK1 in salt tolerance. Additionally, SlMEK1 may contribute to H₂-enhanced salt tolerance. Meanwhile, MEK1 was shown to undergo S-nitrosylation at Cys 172, and this modification was involved in H₂-enhanced salinity tolerance. Moreover, site-specific mutation analysis at Cys 172 confirmed that MEK1 S-nitrosylation positively contributed to both intrinsic and H₂-facilitated salt tolerance. Finally, S-nitrosylation of MEK1 increased its interaction with GSNOR, and H₂ further promoted this interaction under salt stress. Collectively, our findings indicate that H₂ may enhance the salinity tolerance in tomato seedlings by orchestrating the interplay between S-nitrosylated MEK1 and GSNOR.