Methyl Jasmonate Improves Cadmium Tolerance by Reducing Cadmium Accumulation, Enhancing Cell Wall Immobilization and Modulating Transcriptional Regulation in <i>Malus</i> Rootstock

The mechanisms by which methyl jasmonate (MeJA) enhances cadmium (Cd) tolerance in <i>Malus</i> remain unclear. The physicochemical and molecular responses of <i>Malus hupehensis</i> exposed to 0 or 50 μM Cd with or without exogenous 5 μM MeJA or 50 μM ibuprofen (IBU) for 3 d were explored via a hydroponic experiment. MeJA alleviated Cd-induced growth inhibition, photosynthetic suppression, and oxidative damage. Cd increased the activities of some antioxidant enzymes and levels of endogenous MeJA, which were further enhanced by MeJA application. MeJA decreased the proportion of water-soluble Cd, thereby decreasing Cd mobility and accumulation in all tissues. MeJA increased cell wall (CW) component contents and -COO/-OH groups, thereby improving Cd binding capacity and restricting cellular Cd mobility. MeJA-responsive differentially expressed genes (DEGs) regulated phenylpropanoid biosynthesis and pentose and glucuronate interconversions. MeJA downregulated Cd uptake and transport genes but upregulated Cd detoxification genes and CW metabolic genes. Weighted gene coexpression network analysis (WGCNA) revealed that two key modules strongly correlated with Cd tolerance, including hub transcription factors (TFs) that regulate hormone signaling, phenylpropanoid biosynthesis, and carbohydrate metabolism. Overall, MeJA effectively enhanced <i>Malus</i> plant tolerance by reducing Cd accumulation, enhancing cell wall immobilization and modulating transcriptional regulation.