| Abiotic factors such as soil nutrient levels can dramatically influence interactions between plants and microbial symbionts. Though less studied, variation in the biotic environment, especially the presence of additional interactions, can also influence these interactions. I used three important microbial symbionts of plants—arbuscular mycorrhizal fungi (AMF), fungal endophytes of grasses, and rhizobia bacteria—to examine how the co-occurrence microbial partners influences the dynamics of these interactions from the perspectives of all partners. Using an analysis of published studies, I found that the negative effect of antagonistic fungal endophytes was ameliorated by the presence of AMF, resulting in increased plant response. In an experimental study, I showed that the effects of AMF and fungal endophytes on a host grass are additive when these symbionts coinfect the same host individual. Further, endophyte infection decreased the colonization rate of plant roots by one AMF species, while increasing colonization in the second AMF species. This result reveals a species-specific mechanism by which these symbionts influence one another on a common host plant. These results raised the question of how important biotic environmental variation is relative to the consequences of variation in the abiotic environment in shaping plant-microbe interactions. To examine this question, I conducted an experiment manipulating the presence of two AMF inocula and two rhizobia strains on a native tallgrass prairie legume under varying soil nutrient levels. Coinfection by AMF and rhizobia synergistically improved plant growth. Interestingly, the effects of specific combinations of AMF and rhizobia on the plant depended on phosphorus levels. Taken together these results demonstrate that both the abiotic and biotic contexts of species interactions are important to understand their dynamics in nature. |
