Abstract
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 (13C, 15N) 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 δ13C values of T. matsutake and other fungal groups are consistent with those of most studies, but a very high δ15N value (16.8‰ ± 2.3) is observed in T. matsutake. Such isotopic pattern of fungal δ15N suggests that matsutake has a greater proteolytic potential to digest chemically complex 15N-enriched organic matter and hydrophobic hyphae. This assumption is further supported by a significant and positive correlation between δ13Ccap–stipe and δ15Ncap–stipe exclusively in T. matsutake, which suggests common C and N sources (protein) possible for isotopically enriched cap. The 13C increase of caps relative to stipe presumably reflects greater contents of 13C-enriched protein than 13C-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.
Highlights
Samples of 3 mg or 15 mg were combusted and C and N isotope ratios were measured on a Finnigan MAT Delta plus stable isotopic ratio mass spectrometer (IRMS) equipped with an elemental analyzer (SF samples) or a Delta V Advantage equipped with an elemental analyzer (Thermo Fisher Scientific, Bremen, Germany) (NJ samples)
Δ15N in the different forest compartments from foliage, litter to deeper soil horizons, in consistent with those found previously for boreal forests elsewhere (Taylor et al 1997; Zeller et al 2007; Hobbie et al 2012) based on samples collected from Finland and Japan
We suggested the following new findings: (1) the sporocarps of T. matsutake comprised of similar range of δ13C value as in other ECM fungi; (2) the sporocarps of T. matsutake showed very high δ15N values in comparison to other ECM and SAP fungi, suggesting that matsutake may obtain N from chemically complex 15N
Summary
(Ogawa 1978; Yamada et al 1999; Vaario et al 2010). Matsutake is one of the most expensive edible mushrooms available (Hall et al 2003). While many investigators have focused on optimal growth conditions and the artificial culture of this high-value fungus (Kawai and Ogawa 1981; Lee et al 1984; Guerin-Laguette et al 2005; Yamada et al 2006), artificial cultivation remains difficult and unsatisfactory (Wang et al 2012). Earlier studies of the ecological interactions between T. matsutake and other soil microbes with the host plant as well as studies of edaphic qualities and climate (see review, Vaario et al 2017) have tried to identify the factors that regulate sporocarp formation. One aspect that has yet to be investigated thoroughly in this respect concerns the physiology of matsutake nutrition
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