Emergent benefits of arbuscular mycorrhizal fungi in multisymbiotic grass-legume mixtures

Background and aims

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.

Methods

In pots with sterile, nitrogen-limited soil either inoculated or not with AMF, we grew Lolium multiflorum grass plants associated or not with a vertically-transmitted endophyte (Epichloë occultans), either in monocultures or in mixtures with rhizobia-inoculated Trifolium repens.

Results

In monocultures, grass C, N and P acquisition were reduced by AMF. Conversely, in mixtures with legumes, AMF increased grass growth, soil N uptake, and transfer of biologically fixed N from the legume to the grass. Endophyte and AMF both decreased grass fitness, but endophyte presence increased AMF spore density.

Conclusions

AMF can increase nitrogen transfer and increase grass growth, a benefit that relies on the presence of rhizobia-associated neighboring legumes. Notably, the fitness of plants and symbionts does not always align with the benefits provided to each other. The success of each host or symbiont may depend on their ability to capitalize on the benefits.