Plant diversity improves protection against soil-borne pathogens by fostering antagonistic bacterial communities

1. Rhizosphere bacteria antagonistic to fungal pathogens improve plant performance by preventing infection. In temperate grasslands, primary productivity often increases with plant diversity, and we hypothesized that this effect may in part rely on the interactions between plants and antagonistic bacteria. 2. We investigated the impact of plant diversity and functional group composition on soil bacteria producing the antifungal compounds 2,4-diacetylphloroglucinol (DAPG) and pyrrolnitrin (PRN) in a long-term grassland biodiversity experiment, as well as their impact on soil suppressiveness. Soil suppressiveness was investigated in a model infection assay with Beta vulgaris and the pathogen Rhizoctonia solani. 3. The abundance of DAPG and PRN producers increased with plant diversity and that of PRN also increased in the presence of grasses. Moreover, legume species richness and coverage decreased the abundance of DAPG and PRN producers, respectively, contrary to beneficial effects of legumes on soil microorganisms reported previously. In turn, soil suppressiveness was at maximum when DAPG and PRN producer abundance was high. 4. Synthesis. Our results suggest that plant diversity contributes to plant community resistance against pathogens by fostering beneficial bacterial communities. This indirect soil feedback mechanism may contribute to the positive relationship between plant diversity and productivity and could also help the development of more sustainable and environmentally friendly agricultural management strategies.