Rhizobacterium Bacillus cereus 2K alleviates aluminum toxicity in Panax notoginseng through metal immobilization and auxin-mediated growth promotion

Panax notoginseng, a commercially valuable traditional Chinese medicinal herb, faces significant cultivation challenges due to aluminum (Al) toxicity in acidic red soils. Developing biological approaches to mitigate Al accumulation is essential for alleviating toxicity and ensuring medicinal safety. In this study, we isolated Bacillus cereus strain 2 K from P. notoginseng rhizosphere, demonstrating remarkable tolerance to low pH and high Al concentrations. This strain exhibited potassium (K)-solubilizing capacity (6.48 mg·L⁻¹) and produced indole-3-acetic acid (IAA, 1.4 mg·mL⁻¹) and lactic acid (120 mg·mL⁻¹). Mechanistic studies revealed that B. cereus 2 K immobilized Al³⁺ through extracellular adsorption (15 %–25 %) and intracellular accumulation (40 %–56 %), involving -CH₃/-OH functional groups on bacterial cell surfaces and bioprecipitation into Al₂(CO₃)₃ and Al₂O₃ particles. Under axenic conditions, B. cereus 2 K inoculation significantly enhanced P. notoginseng growth under Al stress by improving photosynthetic efficiency, antioxidant capacity, and triterpenoid saponin biosynthesis while reducing Al accumulation. These beneficial effects were associated with upregulated expression of genes involved in IAA signaling and triterpenoid saponin biosynthesis pathways. Non-sterile pot experiments further confirmed that B. cereus 2 K promoted plant growth through enhanced IAA production and reduced Al uptake. Our findings elucidate the dual biological mechanisms whereby B. cereus 2 K promotes P. notoginseng growth via Al³⁺ immobilization and IAA pathway activation, offering an effective microbial strategy for mitigating Al toxicity in acidic agricultural soils.