Role of Plant Growth-Promoting Bacteria in Reshaping Rhizosphere Bacterial and Fungal Microbiomes Under Multi-Metal–Microplastic Composite Pollution in Spinach

Microplastics (MPs) often co-occur with heavy metals (HMs), posing combined stress that inhibits plant growth. While plant growth-promoting bacteria (PGPB) are known to alleviate heavy metal toxicity, their role under MP–HM co-contamination and the differential responses of rhizosphere microbial communities remain unclear. This study evaluated the effects of cadmium (Cd) and lead (Pb), polylactic acid (PLA) MPs, and their combined contamination on spinach growth using pot experiments, and assessed the mitigation potential of two PGPB strains. PGPB inoculation significantly increased plant height and dry weight. High-throughput sequencing revealed that pollution treatments and PGPB altered rhizosphere bacterial and fungal community composition and diversity. Microbial shifts were closely associated with soil chemical properties and plant growth. Notably, bacteria and fungi exhibited distinct response patterns to combined stress and remediation. Functional prediction (PICRUSt2) indicated that microbial communities enhanced metabolic processes and nutrient (N and P) cycling to cope with stress. PGPB inoculation reduced heavy metal toxicity, improved soil nutrient status (P and K), increased microbial diversity, and regulated microbial functions, thereby supporting soil ecological stability. These findings provide insights into rhizosphere microbial mechanisms and support the application of PGPB for remediation of MP–HM co-contaminated soils.