Cadmium hyperaccumulation in Sedum alfredii: from rhizosphere mobilization to vacuolar sequestration and field application

Abstract

Cadmium (Cd) contamination of agricultural soils threatens human health through food-chain exposure and prevents the safe reuse of degraded land. Phytoextraction is attractive because it removes metals in situ with limited soil disturbance, yet field performance is constrained by the scarcity of plant systems that simultaneously sustain biomass production, acquire Cd efficiently, tolerate high internal Cd, and allocate Cd to harvestable shoots. The non-Brassicaceae hyperaccumulator Sedum alfredii Hance provides a useful model for connecting rhizosphere processes with membrane transport, cellular detoxification, and remediation performance. Here, we critically synthesize evidence along a directional pathway from rhizosphere mobilization and root uptake to radial transport, xylem loading, shoot sequestration, repeated harvest, and management of Cd-enriched biomass. Species-specific evidence shows that tartaric-acid exudation by the hyperaccumulating ecotype promotes Cd mineral dissolution and plant uptake, whereas exudate-responsive rhizosphere microorganisms and selected endophytes can improve plant growth, Cd bioavailability, or Cd removal under defined soil and inoculation conditions. Functional evidence implicates SaZIP4 and SaNRAMP1 in Cd/Zn uptake or redistribution, Plant Cadmium Resistance 2 (SaPCR2) in root Cd efflux, SaHMA3 in tonoplast sequestration, and a Cd-responsive SaABCC transporter in Cd tolerance and accumulation. Cd-Zn interactions shape both Cd acquisition and essential-metal homeostasis, while glutathione-centered buffering, organic-acid complexation, antioxidant control, cell-wall binding, and vacuolar sequestration reduce Cd toxicity. Direct radiotracer evidence also supports a Ca-sensitive component of Cd uptake and translocation, although neither SaZIP4 nor SaNRAMP1 has been shown to be Ca-permeable and the responsible Ca pathway remains unresolved. We distinguish well-supported mechanisms from candidates that still rely on expression patterns, heterologous systems, or short-term assays. Translation to field phytoextraction requires experiments that couple molecular mechanisms to soil Cd bioavailability, biomass production, annual Cd removal, repeated-harvest performance, cost, and traceable biomass management. This integrated framework identifies the causal validation steps needed to move S. alfredii from a hyperaccumulation model toward a more predictable field-remediation platform.