Melanin: A functional trait conferring tolerance to water stress in ectomycorrhizal fungi

Background/Question/Methods

Ectomycorrhizal fungi are a diverse and ubiquitous assemblage of organisms in symbiosis with trees of tropical, temperate and boreal forest ecosystems.  These fungi colonize fine roots and provide the tree with nutrients absorbed from the soil in exchange for photosynthate. While great progress has been made in identifying many of the fungi that comprise these communities, our understanding of their functional diversity is poor.  One trait that may influence the success of ectomycorrhizal fungi is the production of melanin, a group of complex polymeric, dark compounds found in their cell walls.  Typically these compounds are associated with UV stress tolerance, but there is increasing evidence suggesting that these compounds are beneficial during water stress. Cenococcum geophilum is a highly melanized ectomycorrhizal fungus that is widely distributed and often abundant in water stressed communities.  Several studies indicate that this fungus can tolerate water stress.  However, the physiological traits that confer such tolerance have yet to be identified.   To determine the importance of melanin in water stress tolerance in Cenococcum geophilum a factorial experiment was devised in which cultures of three isolates were subjected to two levels of water stress in the presence and absence of the melanin synthesis inhibitor tricyclazole (5-methyl-1,2,4-triazolo(3,4,b)-benzothiazole). 

Results/Conclusions

In terms of fungal growth, there was a significant interaction between water stress and inhibition of melanin production; water stress only reduced fungal growth significantly when melanin synthesis was inhibited.  This suggests that for Cenococcum geophilum, melanin production may be an important trait conferring water stress tolerance. Therefore, the frequency or magnitude of water stress in a community may influence the incidence of melanin production among ectomycorrhizal fungi.  Because melanin influences decomposition and mineralization, the incidence of ectomycorrhizal fungal melanization is likely to influence soil carbon sequestration and nutrient cycling.