Differential toxicity mechanisms of Al³ ⁺ and Fe²⁺ in ginseng roots and ligand-specific defense induction in acidic soil

Soil acidification mobilizes aluminum (Al) and iron (Fe) into phytotoxic forms, posing a severe threat to perennial medicinal crops like ginseng, often manifesting as destructive red-skin syndrome. However, the tissue-specific differential phytotoxicity mechanisms and precision mitigation strategies have not been fully elucidated. This study combined field surveys with controlled experiments to dissect the tissue-specific injury patterns and adaptive responses of ginseng to Al³ ⁺ and Fe²⁺ toxicity. Field surveys confirmed that soil acidification elevates metal bioavailability and exacerbates red-skin syndrome. Metabolomics further demonstrated that metal stress redirects plant carbon flux toward defensive metabolites. Controlled histochemical assays uncovered two divergent metal toxicity and resistance routes. For Al³ ⁺: the ion enters fibrous roots, is transported upward through xylem, and spreads radially across root tissues. This process damages lateral and fibrous roots, then triggers two protective structures—pectin-rich vacuoles within suberized layers and proliferated secretory ducts—to sequester Al. Unlike Al³ ⁺, Fe²⁺ penetrates taproots from the epidermis inward. It first triggers cortical oxidative stress, and subsequently drives the biosynthesis of a unique suberized defensive ring in the mid-cortex. We further established a ligand-specific alleviation strategy: boric acid reinforced the pectin-based barrier against Al³ ⁺, while salicylic acid primed the suberized ring formation against Fe²⁺, both surpassing mere antioxidant effects. These defense mechanisms were also observed in other medicinal plants (Platycodon grandiflorum and Polygonatum sibiricum). Our findings elucidate a metal-specific plant adaptation principle and propose a precision phytoprotection framework, leveraging plant innate architectures for sustainable agriculture, particularly for high-value medicinal crops in acidified, metal-contaminated soils.