Taxonomic and transcriptional associations between arbuscular mycorrhizal fungi and the soil microbiome are maintained under biotic perturbation

Arbuscular mycorrhizal fungi (AMF) structure soil microbiomes and support plant growth. However, the stability of established AMF-microbiome interactions to biotic perturbation by introduced microbial inocula is unclear. We investigated how AMF shape soil microbial communities and transcriptional activity when challenged by the introduction of foreign microbial inocula. Using established maize-AMF mesocosms, we introduced microbial communities from forest and agricultural soils and assessed taxonomic composition and metatranscriptomic profiles. Despite clear differences between introduced inocula, neither microbiome treatment altered resident microbial community composition or transcriptional profiles. Instead, AMF exerted a strong and consistent influence on microbial gene expression and bacterial taxa abundances, with effects scaling quantitatively with the degree of mycorrhizal colonisation. Co-expression analyses revealed coordinated transcription among plant, AMF, and microbial genes, suggesting multi-kingdom interactions. Overall, we find AMF drive the structure of soil microbial communities, with biotic perturbation exerting limited influence under the conditions tested.