PS 13-176 - Positive interactions between mycorrhizal fungi and bacteria are widespread and benefit plant growth

Abstract: Bacteria, ectomycorrhizal (EcM) fungi, and land plants have been coevolving for nearly 200 million years, and their interactions presumably contribute to the function of terrestrial ecosystems. The direction, stability, and strength of bacteria-EcM fungi interactions across landscapes, however, remains unclear. Moreover, the genetic mechanisms that govern them have not been addressed. To these ends, we collected soil samples from pine forests across a climate-latitude gradient in coastal California, fractionated the soil samples based on their proximity to EcM-colonized roots, characterized the microbial communities using amplicon sequencing, and generated linear regression models showing the impact that select bacterial taxa have on EcM fungal abundance. In addition, we paired greenhouse experiments with transcriptomic analyses to determine the directionality of these relationships and identify which genes EcM-synergist bacteria express during tripartite symbioses. Our data reveal that ectomycorrhizas (i.e., EcM-colonized roots) enrich for conserved bacterial taxa across climatically heterogeneous regions. We also show that dominant EcM-synergists are positively associated with plant and fungal growth and have unique gene expression profiles compared to dominant EcM-antagonist bacteria. In sum, we identify common mechanisms that facilitate widespread and diverse multipartite symbioses, which inform our understanding of how plants develop in complex environments.