Abstract
Background
Cadmium (Cd) is a prevalent and toxic heavy metal widely present in soils. Cotton (Gossypium hirsutum L.), which readily accumulates Cd, exhibits inhibited seed germination under Cd stress. Melatonin (MT), a potent indoleamine antioxidant, has been demonstrated to facilitate seed germination under adverse conditions, but its regulatory mechanisms in cotton under Cd stress remain unclear. In this study, cotton seeds of the cultivar 'Nongda 601' were used to investigate the effects of exogenous MT on germination characteristics, antioxidant enzyme activities, and hormone levels under Cd stress. Experimental treatments included a control (CK), MT treatment, Cd treatment, and a combined Cd + MT treatment (CM). Transcriptome sequencing was further conducted to elucidate the underlying mechanisms.Results
Cd stress significantly inhibited cotton seed germination, reduced amylase activity, and increased starch accumulation. It also decreased antioxidant enzyme activities while increasing reactive oxygen species (ROS) and malondialdehyde (MDA) contents. In addition, Cd stress increased IAA, ABA, and endogenous MT contents, while decreasing GA3 and ZR levels. Exogenous MT significantly enhanced amylase and antioxidant enzyme activities, decreased ROS, MDA, and starch contents, and modulated hormone levels by increasing IAA, GA3, ZR, and endogenous MT, while decreasing ABA content. Transcriptome analysis indicated that MT upregulated the expression of antioxidant-related genes under Cd stress, including SODA, FSD, CAT2, and pod. Additionally, exogenous MT alleviated Cd toxicity by regulating hormone signal transduction and the phenylpropanoid biosynthesis pathway.Conclusions
Exogenous MT alleviates Cd-induced inhibition of cotton seed germination by enhancing antioxidant capacity, promoting starch mobilization, and rebalancing hormone signaling, while modulating phenylpropanoid metabolism. These findings provide a theoretical foundation for improving cotton cultivation in Cd-contaminated soils.