Melatonin-Induced Chromium Tolerance Requires Hydrogen Sulfide Signaling in Maize

Both melatonin and hydrogen sulfide (H₂S) mitigate chromium (Cr) toxicity in plants, but the specific interaction between melatonin and H₂S in Cr detoxification remains unclear. In this study, the interaction between melatonin and H₂S in Cr detoxification was elucidated by measuring cell wall polysaccharide metabolism and antioxidant enzyme activity in maize. The findings revealed that exposure to Cr stress (100 μM K₂Cr₂O₇) resulted in the upregulation of L-/D-cysteine desulfhydrase (LCD/DCD) gene expression, leading to a 77.8% and 27.3% increase in endogenous H₂S levels in maize leaves and roots, respectively. Similarly, the endogenous melatonin system is activated in response to Cr stress. We found that melatonin had a significant impact on the relative expression of LCD/DCD, leading to a 103.3% and 116.7% increase in endogenous H₂S levels in maize leaves and roots, respectively. In contrast, NaHS had minimal effects on the relative mRNA expression of serotonin-Nacetyltransferase (SNAT) and endogenous melatonin levels. The production of H₂S induced by melatonin is accompanied by an increase in Cr tolerance, as evidenced by elevated gene expression, elevated cell wall polysaccharide content, increased pectin methylesterase activity, and improved antioxidant enzyme activity. The scavenging of H₂S decreases the melatonin-induced Cr tolerance, while the inhibitor of melatonin synthesis, p-chlorophenylalanine (p-CPA), has minimal impact on H₂S-induced Cr tolerance. In conclusion, our findings suggest that H₂S serves as a downstream signaling molecule involved in melatonin-induced Cr tolerance in maize.