Arbuscular mycorrhizal fungi enhance cadmium resistance and reduce translocation in maize: dependence on microplastics concentration

The coexistence of microplastics (MPs) and heavy metals (HMs) such as cadmium (Cd) in agricultural soils represents a growing threat to crop production and food security. While arbuscular mycorrhizal fungi (AMF) are recognized for their ability to enhance plant metal tolerance, their role in mediating crop responses under combined contamination with MPs and Cd, especially across different MPs concentrations, remains largely unexplored. This pot study was conducted to elucidate how AMF modulate maize growth, Cd accumulation, and soil biogeochemical processes with three polyethylene (PE)-MPs levels (0, 0.5%, and 5% w/w) and two Cd treatments (0 and 20 mg kg−1). In non-inoculated plants, the addition of 5% PE-MPs significantly aggravated Cd toxicity in maize, elevating Cd translocation to shoots by 79.6% and causing severe growth suppression relative to the treatment without PE-MPs. PE-MPs also modified key soil characteristics, increasing organic matter content and pH, which promoted the transformation of Cd into less bioavailable fractions yet failed to counteract its direct phytotoxic effects. AMF inoculation markedly alleviated these stresses. Under Cd and 5% PE-MPs co‑contamination, mycorrhizal plants showed 87.5% higher shoot biomass, 39.6% greater phosphorus uptake, and 38.5% enhanced net photosynthesis compared to non‑inoculated plants. AMF reconfigured the antioxidant network by suppressing excessive SOD activity while sustaining POD-mediated H2O2 scavenging, thereby reducing oxidative membrane damage. AMF further promoted Cd sequestration in cell walls, and decreased the biologically active Cd pool in shoots. Additionally, AMF lowered Cd bioavailability by shifting soil bacterial community composition, particularly by restoring the abundance of Pseudomonadota. Notably, AMF-mediated protection was more pronounced at 0.5% than at 5% PE-MPs, revealing a concentration-dependent efficacy. This study reveals that PE-MPs modulate Cd phytotoxicity in a concentration-dependent biphasic manner, and that AMF dynamically adjust their protective strategy accordingly. At low PE-MPs dose, AMF prioritize cellular detoxification processes, while at high PE-MPs dose they shift toward rhizosphere stabilization and passive Cd immobilization. These findings establish AMF as a resilient, context-dependent bioremediation tool and provide a mechanistic framework for assessing and mitigating risks in soils co-contaminated with MPs and HMs.