Defoliation effects on enzyme activities of the ectomycorrhizal fungus Suillus granulatus in a Pinus contorta (lodgepole pine) stand in Yellowstone National Park

Ectomycorrhizal (EM) basidiomycete fungi are obligate mutualists of pines and hardwoods that receive fixed C from the host tree. Though they often share most recent common ancestors with wood-rotting fungi, it is unclear to what extent EM fungi retain the ability to express enzymes that break down woody substrates. In this study, we tested the hypothesis that the dominant EM fungus in a pure pine system retains the ability to produce enzymes that break down woody substrates in a natural setting, and that this ability is inducible by reduction of host photosynthetic potential via partial defoliation. To achieve this, pines in replicate blocks were defoliated 50% by needle removal, and enzyme activities were measured in individual EM root tips that had been treated with antibiotics to prevent possible bacterial activity. Results indicate that the dominant EM fungal species (Suillus granulatus) expressed all enzymes tested (endocellulase D-glucosidase, laccase, manganese peroxidase, lignin peroxidase, phosphatase and protease), and that activities of these enzymes increased significantly (P < 0.001) in response to defoliation. Thus, this EM fungus (one of the more specialized mutualists of pine) has the potential to play a significant role in C, N and P cycling in this forested ecosystem. Therefore, many above-ground factors that reduce photosynthetic potential or divert fixed C from roots may have wide-reaching ecosystem effects.