The response of Panax notoginseng to combined microplastics and cadmium stress and its mechanism of rhizosphere microorganisms and root metabolites

Microplastics coexist with cadmium in agricultural soils, and their complex effects on crop growth and cadmium accumulation have garnered increasing attention. Pot experiments were conducted to elucidate the response mechanisms of rhizosphere microorganisms and root metabolites of Panax notoginseng (Burk.) F. H. Chen to polyethylene microplastics (PE-MPs) at concentrations of 0.1 %, 1 %, and 2 %, as well as cadmium (Cd) at levels of 0.6 mg kg⁻¹ and 6.0 mg kg⁻¹. The results indicated that: (1) The biomass of shoots and roots under the treatment of 0.1 % PE-MPs combined with 0.6 mg kg⁻¹ Cd increased by 18.1 % and 137.5 %, respectively, compared to the treatment with only 0.6 mg kg⁻¹ Cd; conversely, biomass decreased under the treatment of 0.1 % PE-MPs combined with 6 mg kg⁻¹ Cd. (2) The root diameter increased with increase of PE-MPs treatment concentrations. (3) Under 2 % PE-MPs + 0.6 mg kg^-1 Cd treatment, the Cd content in fibrous roots was 1.89 times of that under 0.6 mg kg^-1 Cd treatment. The migration was inhibited from fibrous roots to main root or from main root to shoot under 2 % PE + Cd treatment. (4) The presence of PE-MPs influenced secondary metabolite pathways including pantothenate biosynthesis, phenylpropanoid biosynthesis, flavonoid biosynthesis along with sulfur relay systems, while up-regulating ABC transporters based on KEGG analysis. (5) Under 2 % PE-MPs + Cd treatment, the relative abundances of rhizosphere microorganism Candidatur koribacter, Bradyrhizobium, Ktedonosporobacter, Pedosphaera, Novosplingobium and Chthoniobacter at the genus level and Ktedonosporobacter rubrisoli, Chthoniobacter flavus, Silvibacterium bohemicum, Novosplingobium flavum and Fimbriilglobus ruber at the species level increased compared to 1 % PE-MPs + Cd treatment. There was negative correlated between the relative abundance of Ohtaekwangia koreensis and Cd accumulation in fibrous roots. In general, the biomass and accumulation of Cd in roots of P. notoginseng under the combined microplastics and Cd stress depended on the concentrations of PE-MPs, and the roots developed resistance by synthesizing secondary metabolites and recruiting Ohtaekwangia koreensis in the rhizosphere.