Microplastics disrupt bacterial defense within the plant-AMF-bacteria continuum to amplify Cd bioavailability.
The plant-arbuscular mycorrhizal fungi (AMF)-bacteria continuum provides a critical barrier against heavy-metal toxicity, but how microplastics (MPs) disrupt rhizosphere functions and exacerbate phytotoxicity remains unresolved. Using a maize-AMF-bacteria system in cadmium (Cd)-contaminated soil, we investigated two MP fractions differing in size and morphology added at increasing Cd contents. Although AMF colonization remained resilient, MPs induced fraction-dependent bacterial functional decoupling. Small MPs shifted the microbiome from extracellular Cd-immobilizing taxa (Sphingomonadaceae and Rhizobiaceae) toward intracellular stress-tolerant lineages. Large MPs restricted bacterial contacts, suppressing density-dependent cooperation. Metagenomic profiling and analysis of metagenome-assembled genomes (MAGs) revealed reduced potential for quorum sensing, ABC transporters, and alpha-linolenic acid metabolism under large MP exposure, compromising biofilm formation and extracellular Cd sequestration. Partial least squares path modeling indicated that bulk-soil chemistry did not define Cd uptake by plants. Instead, depletion of available Cd in soil reflected a biological sink associated with enhanced plant uptake. Enhanced Cd accumulation was associated with loss of rhizosphere defense mechanisms: potential root-barrier disruption by large MPs and weakened microbial buffering. Consequently, large MPs increased the Cd bioconcentration factor by 57.5%, compared with 32.1% for small MPs. These findings show that MPs amplify legacy Cd risks without increasing bulk-soil Cd availability, through disruption of root-interface integrity and microbial protection.