The biology and chemistry of a mutualism between a soil bacterium and a mycorrhizal fungus.

Arbuscular mycorrhizal (AM) fungi (e.g., Rhizophagus species) recruit specific bacterial species in their hyphosphere. However, the chemical interplay and the mutual benefit of this intricate partnership have not been investigated yet, especially as it involves bacteria known as strong producers of antifungal compounds such as Bacillus velezensis. Here, we show that the soil-dwelling B. velezensis migrates along the hyphal network of the AM fungus R. irregularis , forming biofilms and inducing cytoplasmic flow in the AM fungus that contributes to host plant root colonization by the bacterium. During hyphosphere colonization, R. irregularis modulates the biosynthesis of specialized metabolites in B. velezensis to ensure stable coexistence and as a mechanism to ward off mycoparasitic fungi and bacteria. These mutual benefits are extended into a tripartite context via the provision of enhanced protection to the host plant through the induction of systemic resistance. [Display omitted] • B. velezensis colonizes the entire mycelial network more efficiently than roots • Bacillus uses fungal hyphae as highways for soil invasion and to colonize new plant • The lipopeptide surfactin plays key roles in the chemical ecology of the interaction • The microbial partnership enhances the systemic resistance of tomato against Botrytis Anckaert et al. investigate the dynamics of Rhizophagus hyphae colonization by B. velezensis. Reduced production of bacterial toxic metabolites triggered by the fungus leads to an unsuspected stable coexistence. This interaction brings benefits to both microbes and provides positive effects to the host plant, which is more resistant to disease. [ABSTRACT FROM AUTHOR]