Integrated roles of nitric oxide and melatonin in enhancing chromium resilience in cotton plants: modulation of thiol metabolism and antioxidant responses.

Chromium (Cr) is a hazardous metal found in various oxidation states, posing significant environmental and health risks. The study focuses on understanding how melatonin (MT), known for its diverse functions, including stress alleviation and antioxidant properties, interacts with nitric oxide (NO) to regulate sulfur metabolism and enhance cotton resilience under Cr toxicity. Cr toxicity negatively affected plant growth and photosynthesis and induced oxidative stress. MT treatment ameliorated these effects by enhancing photosynthetic pigments and gas exchange traits and upregulating the activities of antioxidant enzymes and the expression of FeSOD, CuZnSOD, and APX1. Moreover, MT reduced Cr accumulation in leaves, protecting photosynthetic organs from direct toxicity. Additionally, MT promotes the level of sulfur-based defense substances like glutathione (GSH) and phytochelatins (PC), which are crucial for detoxifying heavy metals. This is achieved by upregulating genes involved in cysteine metabolism (CYC1, CYC2, CAS1, CAS2, DES1, DES2, and SSCS), increasing cysteine availability for GSH and PC synthesis, and enhancing Cr sequestration in vacuoles. However, when the inhibitor of MT biosynthesis (p-CPA) and NO scavenger (cPTIO) were used along with MT in Cr-stressed plants, they hindered the stimulatory effects of MT. The study highlights the importance of the NO-MT interaction in mediating these protective effects, indicating a potential signaling role for NO in plant defense mechanisms against Cr toxicity. Overall, the findings reveal that MT fertilizing may serve as an effective strategy to enhance Cr resistance in cotton plants, with NO potentially playing a signaling role in this response pathway.