One Shroom to Rule Them All: Identifying the mechanisms employed in ectomycorrhizal mutualisms for the generalist fungus Thelephora terrestris and seven genetically diverse host tree species

This summer, through the Stanford EARTH Young Investigators Internship, I worked in the Peay fungal ecology lab to set up an experiment to identify what fungal mechanisms are at work in ectomycorrhizal mutualisms between seven phylogenetically distinct tree species and the generalist fungus Thelephora terrestris. Ectomycorrhizal fungi occupy an important niche in terrestrial ecology through their symbiotic mutualisms with plant hosts that allow for the exchange of carbon and nitrogen. However, very little is known about what determines partner choice for ectomycorrhizal fungal mutualists. Among pathogenic fungi, specialization on particular hosts is common, likely because the pathogen must work in specialized ways with the host's immune system. Ectomycorrhizal mutualists, however, tend to be generalists, even though their associations with plants are physically intimate and chemically complex. In order to understand how ectomycorrhizal fungi maintain a broad host range, I grew and planted seedlings and cuttings of Pinus muricata (bishop pine), Pseudotsuga menziesii (Douglas fir), Salix lasiolepis (arroyo willow), Populus trichocarpa (black cottonwood), Quercus agrifolia (coastal live oak), Eucalyptus globulus (blue gum), and Arbutus menziesii (Pacific madrone). Within each pot, the seven seedlings was planted around a previously planted donor bishop pine in potting mixture inoculated with Thelephora terrestris so that the fungus could spread from the donor pine to the others. I also helped analyze the extent of Thelephora terrestris growth on the plant roots from a preliminary round of the experiment in order to refine the data collection protocol for the coming experiment. Several months from now, my research mentor will label the carbon and nitrogen moving between the fungus and the plant to find out how well the symbiosis is working for each partner, and will sequence the RNA from the fungus to see if it uses different genes to communicate and associate with each host. The work I did growing and planting seedlings under sterile conditions ultimately emphasizes the importance of combining ecological and physiological experimental design with molecular data collection in order to better understand how molecular function influences the ecology of ectomycorrhizal fungi.