When most folks think of fungi, they think of rotten fruit, decaying trees, or mushrooms. These are all good examples, but there is much more to the vast world of fungi than meets the eye. (Of course that makes sense since most fungi are microscopic.) One particular group of fungi, the ectomycorrhizal (ECM) fungi, form mutualistic symbiotic relationships with tree roots where the fungi integrate intimately with the roots of the tree and help it absorb water and minerals from the soil. This relationship is critical to the growth, health, and stress tolerance of the tree. The most commonly found species of ECM fungi is Cenococcum geophilum, which is the only member of the Dothideomycetes class capable of forming these relationships. Most other Dothideomycetes are saprophytes or plant pathogens. What about Cenococcum geophilum gene expression causes it to be a mutualist, while many other in its class are pathogens? A team led by researchers at the French National Institute for Agricultural Research (INRA) and the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, and including researchers at the U.S. Department of Energy Joint Genome Institute (DOE JGI) set out to answer this question. Their method combined the power of next-generation sequencing with a clever protocol. The gene expression of C. geophilum was compared to that of Lepidopterella palustris and Glonium stellatum, two closely related species, neither of which are ECM fungi. The researchers discovered that C. geophilum has a marked decrease in plant cell wall degrading enyzmes, and an increase in the expression of proteins that are involved in the symbiosis. C. geophilum is also very good at holding onto water under dry conditions, increasing the drought tolerance of its host tree. Illuminating of the relationship of C. geophilum and its host tree will help us to understand drought tolerance and the lessons learned may be applicable to important agricultural crops and trees. You can read the research article in Nature Commnications.