Mechanism of Pectin Methylesterification Modification Mediating Pb²⁺ Accumulation in Cassava Roots

Cassava (Manihot esculenta Crantz) exhibits remarkable capacity for heavy metal accumulation, yet the molecular mechanisms governing lead (Pb²⁺) sequestration in tuberous roots remain poorly understood. This study investigates the role of pectin methylesterase inhibitor 1 (MePMEI1) in modulating pectin methylesterification status and Pb²⁺ accumulation in cassava. Through generation of MePMEI1 overexpressing lines and CRISPR/Cas9-mediated mepmei1 knockout mutants, we demonstrate that MePMEI1 overexpression significantly reduces pectin methylesterification degree, resulting in substantial accumulation of low-methylesterified pectin within cell walls. This modified pectin composition dramatically increases Pb²⁺ binding capacity in root tissues, particularly within the meristematic zone, while simultaneously enhancing plant tolerance to Pb stress. Mechanistically, MePMEI1-mediated pectin remodeling thickens cell walls, improves structural integrity, and activates antioxidant defense systems, evidenced by elevated catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) activities with concomitant reduction in malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) accumulation under Pb²⁺ exposure. Notably, mepmei1 mutants exhibited compromised Pb²⁺ sequestration and reduced stress tolerance compared to wild-type plants. Our findings establish MePMEI1 as a central regulator in the pectin-dependent Pb²⁺ immobilization pathway, providing crucial molecular insights for developing cassava cultivars with minimized Pb²⁺ accumulation in edible tuberous roots while maintaining phytoremediation potential for contaminated soils.