Engineered sulfur and iron nanoparticles enhance cotton tolerance to lead stress via distinct molecular and physiological strategies

Lead (Pb) toxicity significantly hinders plant development, physiology, and biochemistry. This study examined the efficacy of sulfur (S) and iron (Fe) nanoparticles (NPs) in mitigating Pb-induced toxicity in cotton. Seventeen-day-old seedlings were subjected to Pb (50 µM), either independently or in conjunction with SNPs (50 mg/L) or FeNPs (50 mg/L). Pb exposure markedly diminished plant height and biomass, compromised photosynthetic pigment levels, and increased oxidative stress indicators, including hydrogen peroxide (H₂O₂), superoxide anion, methylglyoxal (MG), and malondialdehyde (MDA). Despite the upregulation of antioxidant enzyme activity (CAT, SOD, APX, GR) and the glyoxalase system (Gly I, Gly II) under Pb stress, the introduction of NPs further augmented these defensive responses, revealing significant distinctions between the two NP types. The addition of SNPs significantly elevated the concentrations of sulfur-containing substances (cysteine, glutathione) and phytochelatins, facilitating lead retention primarily in the roots, hence restricting its transfer to the shoots. Conversely, FeNPs treatment more efficiently reinstated Fe homeostasis, enhanced chlorophyll production, and restored photosynthetic function. Gene expression study indicated that Pb stress inhibited the expression of Fe uptake-related genes (IRT1, IRT2, YSL2, YSL13, FRDL1) while enhancing the expression of Pb transporter genes (HMA2, HMA3, HMA4) and thiol metabolism genes (GSH1, PCS1, ABCC1). Both NPs partially mitigated these gene expression patterns; however, FeNPs primarily augmented the expression of Fe transport genes, whereas SNPs more significantly stimulated thiol-related genes. The work demonstrates that SNPs and FeNPs mitigate Pb toxicity through independent but complementary defensive mechanisms: SNPs primarily facilitate Pb detoxification by augmenting thiol metabolism and limiting Pb mobility, while FeNPs restore nutritional equilibrium and enhance photosynthetic efficiency. These findings offer novel insights into the customized application of nanoparticles for alleviating heavy metal stress in plants.