Boron Mitigates Cadmium Toxicity by Reducing Cadmium Accumulation, Enhancing Cell Wall Immobilization and Regulating gene expression in Malus Rootstock

To investigate the mitigating role and underlying mechanisms of exogenous boron (B) in cadmium (Cd)-stressed woody fruit trees, a hydroponic study was conducted using Malus hupehensis Rehd. seedlings treated with different B concentrations (0, 12.5, 50, and 150 µM H₃BO₃). Cd stress significantly inhibited plant growth, reduced photosynthetic parameters, pigment content, biomass, and root activity, but induced reactive oxygen species (ROS) accumulation and impaired the antioxidant defense system. In contrast, the 50 µM B treatment (B2) effectively alleviated Cd toxicity. This treatment significantly decreased Cd accumulation, bioconcentration factor, and translocation factor across tissues. The B2 treatment enhanced Cd immobilization in root cell walls by increasing pectin content and pectin methylesterase activity. Additionally, it shifted Cd chemical forms toward lower-toxicity forms—increasing pectin- and protein-bound, phosphate-bound, and oxalate-bound Cd, while reducing inorganic and water-soluble Cd fractions. The B2 treatment further activated the antioxidant system, elevating the activities of superoxide dismutase and peroxidase, and increasing non-enzymatic antioxidant levels (free proline and ascorbic acid), thereby reducing ROS and malondialdehyde accumulation. The B2 treatment also downregulated key genes including ZIP6 and IRT1 involved in Cd uptake. In conclusion, an optimal B concentration of 50 µM alleviates Cd stress in Malus hupehensis Rehd. by regulating Cd uptake and translocation, enhancing cell wall fixation, altering Cd chemical forms, activating antioxidant defenses, and regulating stress-related gene expression.