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Microplastics disrupt bacterial defense within the plant–AMF–bacteria continuum to amplify Cd bioavailability
Summary
Tiny plastic particles in soil can weaken the natural defenses that plants and helpful soil microbes use to block toxic cadmium from entering crops, and surprisingly, larger microplastic pieces caused even more contamination than smaller ones, boosting cadmium buildup in corn plants by over 50%. This matters because cadmium is a toxic heavy metal linked to kidney damage and cancer risk, and this research suggests that plastic pollution in farmland could be quietly increasing toxic metal levels in our food, even when soil tests don't show a change in contamination.
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.