The function of melanin in the ectomycorrhizal fungus Cenococcum geophilum under water stress

•The water stress tolerance of Cenococcum geophilum is poorly understood.•We studied the role of melanin production in tolerating water stress in C. geophilum.•Melanin inhibition greatly reduced the ability of C. geophilum to tolerate water stress.•Melanin production is likely a key functional trait of this important fungus.

Despite the ubiquity and importance of ectomycorrhizal fungi to ecosystem function, our understanding of their functional ecology remains poor. The highly melanized and common ectomycorrhizal fungus, Cenococcum geophilum, is drought tolerant and abundant in water-stressed habitats, yet the responsible functional traits have not been identified. The production of melanin, a class of complex dark polymers found in fungal cell walls, may be a key functional trait to water stress tolerance. To test this hypothesis, we devised a series of experiments determining the effect of the melanin biosynthesis inhibitor, tricyclazole, on response to osmotic and desiccation stresses. Melanin inhibition only had negative effects on growth when C. geophilum isolates were subjected to osmotic and desiccation stress (−1.7 MPa and desiccated) but not under control conditions (−0.01 MPa and non-desiccated). This suggests that melanin production is an important functional trait that contributes to water stress tolerance of this cosmopolitan ectomycorrhizal fungus.