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The response of Panax notoginseng to combined microplastics and cadmium stress and its mechanism of rhizosphere microorganisms and root metabolites
Summary
A pot experiment investigated how polyethylene microplastics interact with cadmium — a toxic heavy metal common in agricultural soils — to affect the growth and chemistry of Panax notoginseng, a valuable medicinal plant. At low microplastic concentrations combined with low cadmium, plant biomass actually increased; at higher concentrations, growth was inhibited and cadmium accumulated in the roots. Microplastics also altered the plant's production of medicinally important compounds like flavonoids, and shifted which microbes thrived in the root zone. These results show that combined microplastic-heavy metal contamination can affect both the safety and therapeutic quality of medicinal crops.
• The biomass of Panax notoginseng showed "low promotion and high inhibition" under PE-MPs and Cd coexistence in soil. • The increase of root diameter and root activity was important response characteristic to MPs and Cd coexistence. • The content of Cd in root gradually increased, and translocation factors decreased with the increase of PE-MPs treatment concentrations. • Synthesis of flavonoids and terpenoids in roots was improved and abundance of rhizosphere microorganisms increased, especially Ohtaekwangia koreensis resistance to MPs and Cd coexistence in soil. 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.