Tricholoma matsutake may take more nitrogen in the organic form than other ectomycorrhizal fungi for its sporocarp development: the isotopic evidence

Tricholoma matsutake is an ectomycorrhizal (ECM) fungus capable of in vitro saprotrophic growth, but the sources of C and N used to generate sporocarps in vivo are not well understood. We examined natural abundance isotope data to investigate this phenomenon. For this purpose, C, N and their stable isotopes (C-13, N-15) content of fungal sporocarps and their potential nutrient sources (i.e., foliage, litter, fine roots, wood, and soil) were investigated from two well-studied sites in Finland and Japan. Our results show that C-13 values of T. matsutake and other fungal groups are consistent with those of most studies, but a very high N-15 value (16.8 parts per thousand +/- 2.3) is observed in T. matsutake. Such isotopic pattern of fungal N-15 suggests that matsutake has a greater proteolytic potential to digest chemically complex N-15-enriched organic matter and hydrophobic hyphae. This assumption is further supported by a significant and positive correlation between C-13(cap-stipe) and N-15(cap-stipe) exclusively in T. matsutake, which suggests common C and N sources (protein) possible for isotopically enriched cap. The C-13 increase of caps relative to stipe presumably reflects greater contents of C-13-enriched protein than C-13-depleted chitin. We conclude that T. matsutake is a typical ECM fungus which obtains for its sporocarp development for both C and N from a common protein source (vs. photosynthetic carbon) present in soil organic matter.