Plants are routinely colonized by both beneficial and detrimental microorganisms. These two microbial
guilds may indirectly interact with each other via their host, either by modifying its vigor, or by altering
its hormonal/defense status. Here, we studied indirect interactions between arbuscular mycorrhizal (AM)
fungi and three plant pathogens. We show that AM fungal sporulation was only triggered by the least
aggressive fungal pathogen, which is known to induce a jasmonate-based hormonal response by the host
without affecting its vegetative growth and vigor. Conversely, the most aggressive fungal pathogen
considerably reduced host vigor but did not alter AM fungal growth and sporulation. Our results thus
suggest that the plant hormonal system is an important component of indirect interactions between AM
fungi and plant pathogens.