A Phylogenetic Meta-analysis of Biotic and Abiotic Factors Affecting Plant Response to Mycorrhizal Fungi

Background/Question/Methods Symbiotic associations between microbes and terrestrial plants have a long evolutionary history, are ubiquitous, and can have dramatic consequences for plant growth, local biodiversity, and ecosystem function. Research on the symbiosis between plants and mycorrhizal fungi has grown dramatically in recent decades, generating a complex body of empirical results. These studies demonstrate wide variation in the outcome of the interaction for both the host plant and mycorrhizal fungi, exhibiting a continuum from mutualism to parasitism. In addition to the biotic and abiotic context of the symbiosis, evolutionary histories of plants and mycorrhizal fungi may influence the outcome of the symbiosis. These hypotheses are usually tested either in isolation using single experiments, or in research syntheses using simple single-factor statistical models. To build on these previous approaches, we used a phylogenetic meta-analysis with a multi-factor mixed-model design to analyze a database of more than 2100 mycorrhizal inoculation experiments. Specifically, we assessed the relative importance of factors in four categories: host plant phylogeny and characteristics, fungal phylogeny, interaction between plant and fungal phylogeny, biotic complexity in the soil, and abiotic experimental conditions. Results/Conclusions Overall, host plant phylogeny was consistently important for modeling heterogeneity among experiments in plant response to both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. In addition, variation in plant response to AM fungi but not EM fungi was influenced by an interaction between host plant and fungal phylogeny. These results suggest that variation in plant response to mycorrhizal fungi may be strongly driven by diversification in plants, and also by phylogenetic correlations between plant and AM fungal lineages. The absence of an overall effect of EM fungal phylogeny is consistent with efficiency of growth promotion evolving rapidly within EM fungi. In addition, plant response to AM fungi was substantially more positive when soils were fertilized by N and were not fertilized with P, supporting the trade balance model of mycorrhizal function. Finally, biotic context had different effects for plant response to AM versus EM fungi--addition of non-mycorrhizal microbes increased and decreased plant response to AM and EM fungi, respectively. Taken together, these results emphasize the joint influence of abiotic context, biotic context, and evolutionary history for ecological outcomes in mycorrhizal symbioses