Abstract: Excessive application of nitrogen (N) fertilizers, particularly nitrate (NO₃⁻), in protected agricultural systems has led to secondary soil salinization, posing a global threat to crop yield and quality. The regulatory network underlying tolerance to excess nitrate in tomato (Solanum lycopersicum) remains largely unexplored. Here, we report that S-nitrosoglutathione reductase (GSNOR) positively regulates nitrate tolerance by reducing nitric oxide (NO) and S‑nitrosothiol (SNO) accumulation. We then identified the transcription factor SlMYB86 as a direct upstream activator of SlGSNOR, and further found that nitrate‑induced NO triggers S‑nitrosylation of SlMYB86 at Cys68 and Cys263. This post‑translational modification enhances SlMYB86 protein stability by blocking 26S proteasome‑mediated degradation, while also increasing its DNA‑binding affinity and the transcriptional activation of SlGSNOR, thereby enhancing nitrate tolerance. Moreover, Growth-regulating factor 4 (SlGRF4) functions as an upstream regulator of this pathway, activating SlMYB86 expression under nitrate stress and thereby enhancing tomato tolerance to excessive nitrate stress. Collectively, these findings establish a SlGRF4–SlMYB86–SlGSNOR module in which SlMYB86 S‑nitrosylation establishes a negative feedback loop that couples NO signaling to NO clearance, maintaining redox homeostasis and promoting nitrate tolerance in tomato.

Key words: excessive nitrate stress, nitric oxide, S-nitrosylation, soil secondary salinization, tomato