ABSTRACT.- The ability of plant microbial symbionts to enhance hosts ' fitness depends on the abiotic and biotic context, including the presence of co-existing symbionts. We studied how the presence of arbuscular mycorrhizal fungi (AMF) affects the performance of a host grass associated or not with fungal asexual endophytes, growing either alone or in interaction with a legume hosting nitrogen-fixing bacteria. We hypothesized that the presence of legume-rhizobia symbiosis enables endophytes and AMF to promote host grass growth and nutrition, as well as host and symbionts fitness through nitrogen acquisition-mediated effects even when their primary benefits (herbivore protection and phosphorous provision) are not required. © 2025 Springer Nature