Mechanistic analysis of Paenibacillus polymyxa enhancing cadmium remediation efficiency of Medicago polymorpha based on combined analysis of root metabolome and soil microbiome

As a plant growth-promoting rhizobacterium, Paenibacillus polymyxa exhibits significant potential in enhancing the phytoremediation of heavy metal (HM)-contaminated soils. However, its specific effects on the growth and HM tolerance of Medicago polymorpha (a kind of HM-accumulating plant), as well as the mechanisms by which it promotes plant phytoextraction by altering the composition of root exudates and regulating the structure of the rhizosphere microbial community, remain unclear. In this study, a pot experiment was conducted using cadmium (Cd)-contaminated soil to investigate the mechanisms by which P. polymyxa improves the Cd-contaminated soil remediation capacity of M. polymorpha . The results showed that after inoculation with P. polymyxa , the plant height, nitrogen concentration, and Cd concentration of M. polymorpha were 15.10 cm, 46.66 g/kg·fresh weight (FW), and 2.64 mg/kg·FW, respectively. These values were 1.12 times, 1.07 times, and 1.47 times that of the uninoculated plants, respectively. Meanwhile, after inoculation, the malondialdehyde content in the plants decreased from 65.14 to 31.33 nmol/g·FW. Furthermore, P. polymyxa inoculation reshaped 40 differential root exudate metabolites, which in turn optimized the rhizospheric microbial community structure. The abundances of beneficial microbial genera, Stachybotrys and Pseudoxanthomonas , increased by 2371.11% and 155.00% after inoculation, which were recruited by 6- O -acetylglucose and dibutylphthalate respectively, while the abundance of the pathogenic genus Fusarium decreased by 38.82%. This optimization of the soil microbial community enhanced the adaptability of M. polymorpha to Cd-contaminated soil and accelerated N cycling. Furthermore, a structural equation model was employed to validate the proposed mechanistic model. This study provides theoretical support and practical references for the application of P. polymyxa in the phytoremediation of Cd-contaminated soils.