Respiratory burst oxidase G (SlRBOHG): A key regulator of H2O2-Mediated Na+ homeostasis and salt tolerance in tomato

Regulation of sodium homeostasis is vital for plant responses to salinity. In this study, we characterized two allelic tomato mutants, sodium gatherer1-2 (sga1-2), which show extreme salt sensitivity. The mutants display early severe chlorosis, swelling of aerial parts, and eventual leaf desiccation, leading to plant death. Mapping-by-sequencing identified mutations in the SlRBOHG gene, encoding a plasma membrane NADPH oxidase, as the cause of this phenotype. To the best of our knowledge, these are the first RBOH gene family knockout mutants identified in tomato to date. Physiological analyses revealed that sga1-2 mutants exhibit significantly increased Na ⁺ transport from roots to shoots, reduced K⁺ uptake, and extremely high Na⁺/K⁺ ratios, particularly in the shoots, explaining their salt hypersensitivity. CRISPR/Cas9 knockouts of SlRBOHG reproduced the sga1-2 mutant phenotype and exhibited the same ion homeostasis alterations. At the molecular level, sga1-2 mutants show reduced expression of key Na ⁺ transporter genes, including SlSOS1, SlHKT1s, and SlNHXs. Additionally, while wild-type plants (WT) show an increase in H₂O₂ concentration in the roots following salt treatment, the mutants do not exhibit this response. The inhibition of salinity-induced H₂O₂ increase in WT plants, using the NADPH oxidase inhibitor DPI, also led to the suppression of SlHKT1;2 gene expression, which was associated with Na⁺ accumulation in the leaves. However, the treatment of WT plant with DPI did not alter K⁺ homeostasis. These findings demonstrate that SlRBOHG-mediated H₂O₂ production is critical for conferring salt tolerance in tomato plants, mainly by activating mechanisms that maintain Na⁺ homeostasis in the plant.