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The community dynamics of soil micro-environments are influenced by root, fungal, and bacterial exudates that can preferentially select for functional classes of microbes. Arbuscular mycorrhizal fungi (AM) fungi and plant growth promoting rhizobacteria (PGPR) are members of this community form symbiotic relationships with plants and play an important role in triggering induced systemic resistance (ISR), resulting in defensive “priming.’ Consequently, ‘primed’ plants can activate stronger and faster defense responses to future attacks by pathogens and insects. The biological system in this study involved four genotypes of barrel medic plants (Medicago truncatula), pea aphids (Acyrthosiphon pisum), and microbial communities present in three field-collected soils and one commercial topsoil. In the first experiment, wild-type (WT) A17 M. truncatula grown with an inoculant harvested from a M. sativa field demonstrated lower aphid colony weight than the commercial Pioneer topsoil (p = .0205), indicating that the resident plant community of this soil was effective in eliciting defensive priming. In the second experiment, WT A17 M. truncatula and mutants Mtdmi1, Mtdmi3, and Mtram1 were used to investigate the role of microbial symbionts in plant-aphid interactions. Although there were no observed differences in aphid colony weight between genotypes, treatments with active microbial communities did have significantly lower aphid colony weight than autoclaved treatments (p < .0001). This indicated that functional microbes were more important than richness of microbes and were a greater driver of aphid resistance in M. truncatula. Together, these experiments demonstrated a significant effect of the soil microbial community on plant-aphid interactions.