Exogenous melatonin decreased Cd2+ accumulation inside cells through the phenylpropanoid biosynthesis and reduced cell damage in rice (Oryza sativa) seedlings under CdCl2 stress

Heavy metal-induced stress is an abiotic form of stress that significantly restricts crop yield and quality. This stress affects plants at all stages, but they are particularly vulnerable as seedlings, when it can directly influence later growth and development. Cd²⁺ is an important heavy metal stressor and negatively influences plant growth. However, the regulation mechanism underlying Cd²⁺ stress resistance has not been adequately elucidated, especially in major cultivars, which restricts the application of Cd²⁺ resistance. Here, exogenously applied melatonin (N-acetyl-5-methoxytryptamine) was tested on rice seedlings as a practical solution to enhance the plants' stress tolerance. The modern variety Longjing 203 was used for the experiments due to its extensive cultivation in Heilongjiang Province, China. Seedlings were treated with 50μMol/L CdCl₂ and 100μMol/L exogenous melatonin to investigate the molecular mechanism underlying exogenous melatonin's ability to enhance Cd²⁺ tolerance. The results revealed that Cd²⁺-induced stress limited growth, while melatonin alleviated the stress-induced damage on seedlings. Specifically, differentially expressed gene (DEGs) analysis showed that the phenylpropanoid biosynthesis pathway was enriched in plants treated with melatonin, which was also verified by qRT-PCR, enriched enzyme activity assays, and molecular docking. Also, the results of lignin content and Cd²⁺ distribution in subcellular compartments indicated that melatonin promoted lignin accumulation and intercepted Cd²⁺ into the cell wall, limiting influx into organelles and the cytoplasm. Then, the group of applied melatonin had shown to enhance stress tolerance by reducing DNA damage, as evidenced by the DNA cross-linking, 8-hydroxy-20-deoxyguanine levels, relative density of apurinic sites, and random amplified polymorphic DNA (RAPD) analysis. These findings also revealed that exogenous melatonin relieved cellular damage caused by Cd²⁺ by reinforcing the cell wall lignin barrier to regulate cellular homeostasis.