Hydrogen sulfide as a key mediator in melatonin-induced enhancement of cold tolerance in tomato (Solanum lycopersicum L.) seedlings

Melatonin (MT) is a key bioactive molecule that enhances plant tolerance to cold stress; however, the underlying mechanisms remain unclear. This study demonstrates that foliar application of 100 μM MT improves cold tolerance in tomato seedlings by activating antioxidant enzyme systems and cold-responsive pathways. Notably, endogenous hydrogen sulfide (H₂S) levels are also elevated, suggesting a potential role in MT-mediated responses. Moreover, treatment with sodium hydrosulfide (NaHS, an H₂S donor) further enhances MT-induced cold tolerance by increasing antioxidant capacity, promoting the accumulation of photosynthetic pigments and secondary metabolites, and reducing cold-induced oxidative damage. In contrast, inhibition of H₂S signaling using HT (an H₂S scavenger) or PAG (an inhibitor of H₂S biosynthesis) significantly attenuates the protective effects of MT. Additionally, tomato lines overexpressing the H₂S biosynthetic gene SlDCD1 exhibit a more pronounced response to MT-induced cold tolerance than wild-type plants. Conversely, CRISPR/Cas9-mediated knockout of SlDCD1 significantly compromises the protective effects of MT under cold stress. Collectively, the results suggest that H₂S signaling plays a central role in MT-mediated cold stress tolerance in tomato seedlings. This work provides new insights into how MT contributes to plant adaptation under cold stress.