Synergistic cysteine-glutathione foliar application enhances wheat tolerance to cadmium-lead co-stress in calcareous soils: Mechanisms and phytoremediation implications

• Synergistic Cys-GSH resolves the metabolic trade-off for glutathione. • Upregulates TaPCS1 (9-fold) and restores GSH/GSSG redox homeostasis. • Enhances root sequestration, maintaining metal translocation factors <0.5. • Secures grain safety below Codex limits in contaminated calcareous soil. Co-contamination of agricultural soils with cadmium (Cd) and lead (Pb) presents a significant threat to wheat productivity and food safety, particularly in calcareous regions. Although thiol compounds are recognized as stress alleviators, the effectiveness of their synergistic application in addressing the metabolic competition for glutathione (GSH) between antioxidant defense and phytochelatin (PC) synthesis has not been thoroughly investigated. In this study, response surface methodology (RSM) was employed to optimize the foliar co-application of cysteine (Cys; 0.41 mM) and GSH (2 mM) to enhance the phytostabilization of Cd and Pb in wheat cultivated in contaminated alkaline soil. The synergistic Cys-GSH treatment significantly mitigated phytotoxicity, increasing shoot biomass by 78% and reducing malondialdehyde (MDA) by 39% compared to the stress control. Mechanistically, this tolerance is facilitated by a coordinated dual-defense response. Firstly, the antioxidant capacity was enhanced, as evidenced by a 133% increase in glutathione reductase activity and the complete restoration of the GSH/GSSG redox ratio. Second, a robust metal sequestration pathway was activated, characterized by a 9.0-fold upregulation of the phytochelatin synthase gene ( TaPCS1 ) and a 117% increase in PC accumulation. This enhanced root-level chelation reduced shoot Cd and Pb concentrations by 47% and 44%, respectively, maintaining translocation factors below 0.5. These findings demonstrate that optimized Cys-GSH synergy effectively circumvents metabolic bottlenecks to promote phytostabilization, offering a sustainable strategy to secure wheat production in metal-polluted calcareous soils.