Symbiotic nitrogen fixation in the reproductive structures of a basidiomycete fungus

Nitrogen (N) fixation is a driving force for the formation of symbiotic associations between N-2-fixing bacteria and eukaryotes.(1) Limited examples of these associations are known in fungi, and none with sexual structures of non-lichenized species.(2-6) The basidiomycete Guyanagaster necrorhizus is a sequestrate fungus endemic to the Guiana Shield.(7) Like the root rot-causing species in its sister genera Armillaria and Desarmillaria, G. necrorhizus sporocarps fruit from roots of decaying trees (Figures 1A-1C),(8) and genome sequencing is consistent with observations that G. necrorhizus is a white-rotting decomposer. This species also represents the first documentation of an arthropod-dispersed sequestrate fungus. Numerous species of distantly related wood-feeding termites, which scavenge for N-rich food, feed on the mature spore-bearing tissue, or gleba, of G. necrorhizus. During feeding, mature spores adhere to termites for subsequent dispersal.(9) Using chemical assays, isotope analysis, and high-throughput sequencing, we show that the sporocarps harbor actively N-2-fixing Enterobacteriaceae species and that the N content within fungal tissue increases with maturation. Untargeted proteomic profiling suggests that ATP generation in the gleba is accomplished via fermentation. The use of fermentation-an anaerobic process-indicates that the sporocarp environment is anoxic, likely an adaptation to protect the oxygen-sensitive nitrogenase enzyme. Sporocarps also have a thick outer covering, possibly to limit oxygen diffusion. The enriched N content within mature sporocarps may offer a dietary inducement for termites in exchange for spore dispersal. These results show that the flexible metabolic capacity of fungi may facilitate N-2-fixing associations, as well as higher-level organismal associations.

Automated summary

Research has discovered that the sequestrate fungus Guyanagaster necrorhizus can form mutually beneficial symbiotic relationships with termites to disperse spores. This fungus can digest oligosaccharide-rich food, produce ATP, and be rich in nitrogen. The nitrogen-enriched fungal sporocarps may induce termite parasitism through maintaining anaerobic conditions and specific protein molecules.