Apoplastic retention over vacuolar sequestration: a potential more efficient strategy limiting root-to-shoot cadmium translocation in leafy vegetable crops

Cadmium (Cd) accumulation in plants is coordinated by uptake, long-distance transport, and intracellular detoxification, yet the relative contributions of these processes to Cd allocation remain unclear. Using leafy vegetables as a representative model for rapid heavy metal translocation, we conducted a global meta-analysis of 86 studies to identify the major physiological control points associated with Cd allocation. Our results showed that restricted root-to-shoot translocation represented the dominant regulatory bottleneck, with an effect size (-0.37) more than double that of root Cd reduction (-0.17). Mechanistically, this transport restriction was strongly associated with enhanced apoplastic retention within root cell walls. Increased pectin content and PME-mediated demethylation may increase Cd binding in the cell wall, thereby potentially limiting Cd availability for symplastic loading and xylem transport. In contrast, high-Cd accumulation triggers the synthesis of high-molecular-weight phytochelatins (e.g., PC4), consistent with the activation of intracellular detoxification pathways. Together, these findings suggest that cell wall immobilization may contribute more strongly than downstream vacuolar sequestration to limiting Cd translocation in leafy vegetables. This study provides a fundamental framework for understanding physiological trade-offs under Cd stress and identifies cell wall remodeling as a primary target for developing low-Cd leafy vegetable culticvars.