Sulfur-Induced Thiol Metabolism Confers Arsenic Stress Tolerance Despite Increased Arsenic Accumulation in Rice Seedlings

Arsenic (As) contamination poses a serious threat to rice production and food safety. Although sulfur (S) nutrition is known to alleviate As toxicity through thiol-based detoxification, the mechanisms underlying the relationship between S-induced As accumulation and tolerance remain poorly understood. We investigated the effects of contrasting S levels (0, 1, and 4 mM) on As accumulation, translocation, oxidative stress responses, thiol metabolite pools, and mineral nutrient profiles in hydroponically grown rice seedlings exposed to 15 µM As for 15 days. S supplementation alleviated As-induced growth inhibition in a dose-dependent manner despite increasing total As accumulation in both roots and shoots, demonstrating that S-mediated tolerance is independent of reduced As accumulation. Elevated S supply reduced oxidative stress markers (O2•–, H2O2, and MDA), while markedly enlarged the pools of γ-glutamylcysteine (γ-EC), glutathione, and phytochelatins (PCs), particularly in roots. S supplementation also improved the uptake of major essential nutrients, including nitrogen and potassium in shoots, while selectively modulating micronutrient distributions under As stress. Correlation analysis showed that root As content was strongly and positively correlated with γ-EC and PC2–PC4, and principal component analysis separated the treatments along the S gradient in both roots and shoots. Collectively, these results suggest that S enhances As tolerance in rice seedlings through thiol-dependent detoxification, thereby decoupling As tolerance from As accumulation rather than by restricting As accumulation.