Relationships among unrelated organisms are dynamic in space and time, and certain environmental conditions may be expected to favor the evolution of mutually beneficial associations. Arbuscular mycorrhizal (AM) symbioses are common in most terrestrial ecosystems, but there is a great deal of variability in the degree to which different taxa of plants and fungi benefit from each other. For example, long-lived perennial plants that evolved in nutrient- and water-limited environments generally benefit more from mycorrhizas than annual plants that evolved in highly disturbed or nutrient rich environments. Furthermore, AM fungal communities isolated from fertilized soils tend to be less beneficial to plants compared to those isolated from nutrient-limited soils. Long-term fertilization experiments in natural grasslands in North America and China have shown that nutrient enrichment changes the composition of plant and AM fungal communities and diminishes the mutually beneficial properties of AM symbioses. Mesocosm and greenhouse experiments provide further insights into the role of mycorrhizas in plant community assembly and the impact of fertilization on the assembly of AM fungal communities. Insights from these studies are the basis for the Mycorrhizal Phenotype Model, which is rooted in optimization theories, and defines mycorrhizal function as an emergent property of 1) plant genotype, 2) the AM fungal community plus its associated microbiome, and 3) environmental conditions. The goal of this talk is to describe the traits of plants and AM fungi that help predict the degree to which they benefit from each other, and the environmental conditions (both abiotic and biotic) that tend to generate mutualistic, commensal or even parasitic associations.
Results/Conclusions
Many field and greenhouse studies support the premises of the Mycorrhizal Phenotype model and show that the influence of mycorrhizas on plant performance and community structure are context dependent and vary predictably with different environmental conditions. Future research is needed to elucidate the structure and function of the microbiome associated with plants and AM fungi because these associations are likely to contribute to mycorrhizal adaptation to variability in local environmental conditions. Nutritional mutualisms are only stable when their benefits exceed their costs; consequently, symbiotic exchange of minerals for photosynthate is only beneficial to plants when the minerals supplied by AM fungi are in limiting supply in the soil. An important implication of this finding is that certain agricultural practices may be inadvertently selecting for commensal, or even parasitic AM associations.