Experimental and genomic evidence clarifies the mycorrhizal helper role of a widespread bacterium in Bishop pine forests.
Whether a widespread bacterial strain of Paraburkholderia can enhance the physiological responses of ectomycorrhizal fungi (EcMF) and host Bishop pine seedling growth remains unclear. We developed a 'top-down meets bottom-up' approach that harmonized data from molecular field surveys, experimental forest soil manipulations, statistical interaction models, metabolomics studies, bacterial isolations, controlled growth chamber experiments, and comparative genomics analyses to test the direction and strength of Paraburkholderia-EcMF interactions on host seedling physiology and identify potential mechanisms that support these tripartite interactions. Paraburkholderia sp. D1E increased host root colonization of Suillus pungens - a keystone EcMF taxon for seedling establishment. Paraburkholderia-Suillus co-inoculations also often drove additive seedling growth responses (e.g. biomass and foliar chemistry) and generated nonadditive, positive effects on seedling shoot height. Genomic comparisons identified low chitin and high arabinitol utilization potential as distinguishing features of Paraburkholderia-EcMF symbioses. Our analyses provide experimental evidence, genomic resources, and cross-data validation that highlight potential mechanisms involved in a widespread bacteria-EcMF-tree interaction. Given the diversity of bacteria and fungi in the rhizosphere, however, this approach should continue to be applied to other species combinations to generalize interaction mechanisms among bacterial, fungal, and plant partners.