Multi-Omics Analysis Provides Insights Into the Molecular Mechanism of Melatonin-Mediated Cadmium Tolerance in Populus tomentosa.

Cadmium (Cd) contamination, a major constraint on plant growth, causes a serious threat to ecological safety. Populus tomentosa is a promising woody species for Cd phytoremediation, but the mechanism of Cd phytoremediation by melatonin (MT) in this species remains unclear. To elucidate how MT confers Cd tolerance in P. tomentosa, four treatments (control, MT: 100 μmol·L-1 melatonin, Cd: 100 μmol·L-1 CdCl2, and MT + Cd, CM: 100 μmol·L-1 CdCl2 + 100 μmol·L-1 melatonin) on seedlings were conducted, and integrated analyses of physiological traits, noninvasive micro-test (NMT) data, transcriptomes, and metabolomes were further performed. Cd stress significantly inhibited seedling growth, reduced chlorophyll content, promoted excessive Cd accumulation, and triggered a burst of reactive oxygen species (ROS). Exogenous MT application reversed Cd-induced growth suppression by promoting root Cd2+ efflux while reducing Cd influx, decreasing Cd deposition in leaves, and activating superoxide dismutase and peroxidase to scavenge ROS under Cd treatment. Transcriptomic and metabolomic profiling suggested that metabolic pathways, including hormone signaling, glutathione metabolism, and phenylpropanoid biosynthesis, might be participated in the MT-mediated Cd response. Furthermore, three transcription factors, ERF098, bHLH041, and MYB308, were also identified as candidate regulators of MT-mediated Cd detoxification. Heterologous expression in yeast revealed that MYB308 not only enhanced Cd tolerance but also reduced Cd accumulation under Cd stress. These findings elucidate the multilayered regulatory network underlying MT-enhanced Cd tolerance in P. tomentosa, and provide genetic resources for breeding Cd-resistant tree varieties and theoretical guidance for remediating Cd-contaminated soils.