Ethylene-regulated synergy of pectin methylesterase and transport proteins in cadmium stress response: Molecular mechanisms

Cadmium (Cd) contamination poses a serious threat to pepper production and food safety, yet the molecular mechanisms underlying Cd-induced ethylene biosynthesis and cell wall remodeling in pepper roots remain unclear. In this study, pepper genotypes with contrasting Cd accumulation were subjected to different Cd stress levels (0 mg/L, 0.5 mg/L, 2.0 mg/L) and time points (0.5, 1, 3, and 7 d) to systematically analyze physiological responses, cell wall polysaccharides, ethylene metabolites, and transcriptomic profiles. Low-Cd genotypes exhibited higher antioxidant enzyme activities, with CAT activity reaching 371.82 μmol/min/g under prolonged high-Cd treatment, while H₂O₂ peaked at only 5.675 nmol/g, 71.06% of that in high-Cd genotypes. Galacturonic acid content was up to 1.2-fold higher, and root Cd sequestration reached 1.12-fold higher than in high-Cd genotypes. These results indicate that low-Cd genotypes maintain stronger antioxidant defenses and membrane stability, and enhance Cd immobilization by increasing pectin and galacturonic acid accumulation. Metabolite profiling showed significant shifts in ethylene precursors under Cd stress: SAM and methionine increased by more than 127% (P < 0.05), while ACC decreased by 65.67%, suggesting metabolic flux regulation of ethylene biosynthesis. Transcriptome analysis revealed marked genotypic differences in genes related to cell wall modification, ethylene biosynthesis and signaling, and metal transport, with low-Cd genotypes exhibiting up to 2,857 differentially expressed genes. Key candidates included PME/PMEI, Nramp, and ABCC, implicating their roles in Cd sequestration and tolerance. Collectively, these findings demonstrate that low-Cd peppers coordinate ethylene metabolism and cell wall remodeling to restrict Cd translocation to shoots, providing new molecular evidence for Cd accumulation divergence and offering theoretical and genetic resources for breeding low-Cd cultivars to ensure food safety. • Low-Cd peppers enhance root pectin and galacturonic acid for Cd sequestration. • Ethylene precursors shift (↓ACC, ↑SAM) regulate Cd-responsive metabolism. • Antioxidant enzymes coordinate ROS scavenging and membrane stability under Cd stress. • Transcriptomics identified PME/PMEI , Nramp, ABCC, ERFs as Cd tolerance genes.