Abstract: Cysteine is the precursor for the biosynthesis of glutathione, a key stress-protective metabolite, and methionine, which is imperative for cell growth and protein synthesis. The exact mechanism that governs the routing of cysteine toward glutathione or methionine during stresses remains unclear. Our study reveals that under oxidative stress, methionine and glutathione compete for cysteine and that the increased oxidized glutathione (GSSG) levels under stress hinder methionine biosynthesis. Moreover, we find that inhibition occurs as GSSG binds to and accelerates the degradation of cystathionine γ-synthase, a key enzyme in the methionine synthesis pathway. Consequently, this leads to a reduction in the flux toward methionine-derived metabolites and redirects cysteine utilization toward glutathione, thereby enhancing plant protection. Our study suggests a novel regulatory feedback loop involving glutathione, methionine, and cysteine, shedding light on the plant stress response and the adaptive rerouting of cysteine. These findings offer new insights into the intricate balance of growth and protection in plants and its impact on their nutritional value due to low methionine levels under stress.

Key words: cystathionine γ‐synthase (CGS), cysteine, glutathione, etabolic competition, methionine, oxidative stress, sulfur metabolism, tobacco