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.