Methyl jasmonate enhances cadmium retention in root cell walls of Cosmos bipinnatus: Involvement of hemicellulose remodeling

The high mobility and toxicity of cadmium (Cd) pose a serious threat to plant health, yet the mechanism by which methyl jasmonate (MeJA) regulates cadmium tolerance in Cosmos bipinnatus remains unclear. This study integrated physiological, ultrastructural, transcriptomic, and metabolomic approaches to systematically investigate the response mechanism of C. bipinnatus under exogenous MeJA (1.0 μmol/) treatment. The results showed that, compared with Cd-alone stress (30 μmol/), MeJA combined treatment increased cell wall thickness by 28.2% and primarily retained Cd within the cell walls, while shoot and root lengths increased by 9.6% and 12.0%, respectively. Meanwhile, the proportions of the FNaCl and FW fractions decreased by 41.2% and 28.4%, respectively. X-ray photoelectron spectroscopy confirmed increased abundances of COO⁻, C O, and C-O groups. In addition, the contents of hemicellulosic monosaccharides (except fructose) all increased, whereas xylan and xyloglucan decreased by 21.0% and 22.6%, respectively. At the molecular level, XTH and IRX genes were significantly upregulated, and their corresponding enzyme proteins—XTH26, XTH30, IRX9, and IRX7—increased by 17.12%, 20.62%, 7.6%, and 11.64%, respectively. Metabolomic analysis revealed elevated concentrations of osmoprotective sugars and sugar alcohols, along with enhanced activities of glycolysis and the oxidative pentose phosphate pathway, suggesting that the antioxidant defense mechanism relies on higher NADPH levels. Based on these findings, we reveal a dual strategy combining cell wall reinforcement and metabolic reprogramming, which not only restricts cytoplasmic Cd accumulation but also promotes biomass production to enhance stress tolerance, highlighting the potential of MeJA for optimizing industrial plant cultivation under heavy metal stress.