Exogenous sodium nitroprusside alleviates cadmium toxicity in Solanum nigrum via antioxidant activation, metal...

Cadmium (Cd) contamination in the environment poses a serious threat to plant growth and physiological metabolism. Although previous studies have shown that nitric oxide (NO), as an important signaling molecule, participates in plant responses to heavy metal stress, its regulatory role and underlying molecular mechanisms in the Cd hyperaccumulator Solanum nigrum L remain poorly understood. In this study, Cd exposure significantly inhibited seedling growth (fresh weight and root length), photosynthetic pigments (chlorophyll a (Chl a), chlorophyll b (Chl b), and carotenoids (Car)), and carbon metabolites (total flavonoids (TFL), total phenolic compounds (TPC), total soluble sugars (TSS), and total starch (TS)). Treatment with 100 μmol·L⁻¹ sodium nitroprusside (SNP) was the most effective in alleviating Cd toxicity, restoring all photosynthetic pigments and carbon metabolites to control levels. Furthermore, exogenous application of 100 μmol·L⁻¹ SNP effectively mitigated Cd toxicity at multiple levels by significantly reducing stress indicators (malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and proline (Pro)) and enhancing the activities of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione reductase (GR)). In addition, SNP treatment substantially reduced total Cd accumulation in seedlings and reshaped Cd distribution among subcellular compartments and different chemical fractions, leading to a 17.96% increase in the F3 (soluble fraction) and a 4.38% decrease in the F1 (cell wall fraction). Transcriptomic analysis further revealed that SNP significantly regulated multiple gene pathways associated with metal ion transport, photosynthesis, and antioxidant defense under Cd stress. Differential expression analysis identified that transcription factors (WRKY, MYB, and bHLH) and membrane transporters (ZIP and Nramp) are major genes participating in NO-mediated regulation of Cd stress. Overall, this study demonstrates that exogenous SNP enhances Cd tolerance in S. nigrum by coordinately regulating redox homeostasis, metabolic reprogramming, and Cd distribution patterns, providing new experimental evidence for NO-mediated heavy metal tolerance mechanisms in hyperaccumulator plants.