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

The genus Phellodon (Thelephorales, Basidiomycota) in Europe: Four new species, one new combination, four new typifications and a first European record

Phellodon is a globally distributed genus of stipitate hydnoid fungi recognised by a white spore print and an ectomycorrhizal life strategy. In Europe, many Phellodon species have declining populations and are used as indicators of forests with high conservation values. However, their use in this context and the correct assessment of their extinction risk, according to IUCN guidelines, are currently hampered by inconsistent name usage and unclear species delimitations, primarily within the species complexes of P. melaleucus, P. niger and P. tomentosus. We analysed 286 ITS sequences of Phellodon, of which 51 were also analysed in conjunction with the corresponding LSU regions. This work included 102 newly sequenced collections, primarily sourced from Sweden and the UK. Our phylogenetic results show that four species are new to science and hence we formally describe P. castaneoleucus within the P. melaleucus morphological complex, P. aquiloniniger and P. frondosoniger within the P. niger complex and P. dititomentosus as a sister species to P. tomentosus. The combination P. melilotinus is made to accommodate an additional, previously described, species that belongs in the P. niger complex. We designate lectotypes and epitypes for P. melaleucus, P. niger and P. tomentosus as well as an epitype for P. melilotinus. Phellodon ellisianus is recorded as new to Europe and the ecological amplitude of P. secretus and P. violaceus is revealed to be wider than previously believed. We provide a key to all 13 species currently known in Europe and assign them to three habitat-based assemblages based on data from the Nordic countries and the UK. We hope this will facilitate the assessment of such habitats for legal protection and other conservation-related actions. Citation: Svantesson S, Larsson E, Larsson K-H, Parfitt D, Suz LM, Ainsworth AM (2025). The genus Phellodon (Thelephorales, Basidiomycota) in Europe: Four new species, one new combination, four new typifications and a first European record. Fungal Systematics and Evolution 15: 1-45. doi: 10.3114/fuse.2025.15.01.

Saprotrophic-mycorrhizal divide in stable isotope composition throughout the whole fungus: from mycelium to hymenophore

Mycorrhizal and saprotrophic macromycetes contribute strongly to the carbon and nitrogen cycles of forest ecosystems, often studied by tracing stable isotope composition of carbon and nitrogen. The phenomenon of the saprotrophic-mycorrhizal divide highlights the difference in the stable isotope composition of fruiting bodies of mycorrhizal and saprotrophic fungi. Much less is known about the isotopic composition of the mycelium, which plays an important role in the formation of the soil organic matter and fuels the fungal trophic channel in soil food webs. In this study, we assessed whether the saprotrophic-mycorrhizal divide in the natural δ13С and δ15N values can be traced throughout entire fungal organisms. This hypothesis was tested using 16 species of ectomycorrhizal and six species of saprotrophic basidiomycetous fungi. We showed that not only fruiting bodies, but also the mycelium of ectomycorrhizal and saprotrophic fungi differs in the δ13C and δ15N values. In both ectomycorrhizal and saprotrophic fungi, the δ13C and δ15N values increased from mycelium to hymenophores and correlated positively with the total N content in the corresponding tissues. The differences between ectomycorrhizal and saprotrophic mycelium can be used to reconstruct the fungal-driven belowground carbon and nitrogen allocation, and the contribution of saprotrophic and mycorrhizal fungi to soil food webs.

Functional genomics gives new insights into the ectomycorrhizal degradation of chitin

Ectomycorrhizal (EcM) fungi play a crucial role in the mineral nitrogen (N) nutrition of their host trees. While it has been proposed that several EcM species also mobilize organic N, studies reporting the EcM ability to degrade N-containing polymers, such as chitin, remain scarce. Here, we assessed the capacity of a representative collection of 16 EcM species to acquire ¹⁵ N from ¹⁵ N-chitin. In addition, we combined genomics and transcriptomics to identify pathways involved in exogenous chitin degradation between these fungal strains. Boletus edulis, Imleria badia, Suillus luteus, and Hebeloma cylindrosporum efficiently mobilized N from exogenous chitin. EcM genomes primarily contained genes encoding for the direct hydrolysis of chitin. Further, we found a significant relationship between the capacity of EcM fungi to assimilate organic N from chitin and their genomic and transcriptomic potentials for chitin degradation. These findings demonstrate that certain EcM fungal species depolymerize chitin using hydrolytic mechanisms and that endochitinases, but not exochitinases, represent the enzymatic bottleneck of chitin degradation. Finally, this study shows that the degradation of exogenous chitin by EcM fungi might be a key functional trait of nutrient cycling in forests dominated by EcM fungi.

A facultative ectomycorrhizal association is triggered by organic nitrogen

Increasing nitrogen (N) deposition often tends to negatively impact the functions of belowground ectomycorrhizal networks, although the exact molecular mechanisms underlying this trait are still unclear. Here, we assess how the root-associated fungus Clitopilus hobsonii establishes an ectomycorrhiza-like association with its host tree Populus tomentosa and how this interaction is favored by organic N over mineral N. The establishment of a functional symbiosis in the presence of organic N promotes plant growth and the transfer of ¹⁵N from the fungus to above ground plant tissues. Genomic traits and in planta transcriptional signatures suggest that C. hobsonii may have a dual lifestyle with saprotrophic and mutualistic traits. For example, several genes involved in the digestion of cellulose and hemicellulose are highly expressed during the interaction, whereas the expression of multiple copies of pectin-digesting genes is tightly controlled. Conversely, the nutritional mutualism is dampened in the presence of ammonium (NH₄⁺) or nitrate (NO₃^-). Increasing levels of NH₄⁺ led to a higher expression of pectin-digesting genes and a continuous increase in hydrogen peroxide production in roots, whereas the presence of NO₃^- resulted in toxin production. In summary, our results suggest that C. hobsonii is a facultative ectomycorrhizal fungus. Access to various forms of N acts as an on/off switch for mutualism caused by large-scale fungal physiological remodeling. Furthermore, the abundance of pectin-degrading enzymes with distinct expression patterns during functional divergence after exposure to NH₄⁺ or organic N is likely to be central to the transition from parasitism to mutualism.

Cultivation studies of edible ectomycorrhizal mushrooms: successful establishment of ectomycorrhizal associations in vitro and efficient production of fruiting bodies

Most edible ectomycorrhizal mushrooms are harvested in forests or controlled tree plantations; examples include truffles, chanterelles, porcinis, saffron milk caps, and matsutake. This study explored recent advances in in vitro ectomycorrhizal cultivation of chanterelles and matsutakes for successful ectomycorrhizal seedling establishment and the subsequent manipulation of these seedlings for efficient fruiting body production. Chanterelle cultivation studies have been limited due to the difficulty of establishing pure cultures. However, once pure cultures were established in the Japanese yellow chanterelle (Cantharellus anzutake), its ectomycorrhizal manipulation produced fruiting bodies under controlled laboratory conditions. As C. anzutake strains have fruited repeatedly under ectomycorrhizal symbiosis with pine and oak seedlings, mating tests for the cross breeding are ongoing issues. As one of the established strains C-23 has full-genome sequence, its application for various type of ectomycorrhizal studies is also expected. By contrast, Tricholoma matsutake fruiting bodies have not yet been produced under controlled conditions, despite successful establishment of ectomycorrhizal seedlings. At present, the shiro structure of ≈1L in volume can be provided in two y incubation with pine hosts under controlled environmental conditions. Therefore, further studies that provides larger shiro on the host root system are desired for the outplantation trial and fruiting.

Comparative Mitogenomic Analysis Reveals Dynamics of Intron Within and Between Tricholoma Species and Phylogeny of Basidiomycota

The genus of Tricholoma is a group of important ectomycorrhizal fungi. The overlapping of morphological characteristics often leads to the confusion of Tricholoma species classification. In this study, the mitogenomes of five Tricholoma species were sequenced based on the next-generation sequencing technology, including T. matsutake SCYJ1, T. bakamatsutake, T. terreum, T. flavovirens, and T. saponaceum. These five mitogenomes were all composed of circular DNA molecules, with sizes ranging from 49,480 to 103,090 bp. Intergenic sequences were considered to be the main factor contributing to size variations of Tricholoma mitogenomes. Comparative mitogenomic analysis showed that the introns of the Agaricales mitogenome experienced frequent loss/gain events. In addition, potential gene transfer was detected between the mitochondrial and nuclear genomes of the five species of Tricholoma. Evolutionary analysis showed that the rps3 gene of the Tricholoma species was under positive selection or relaxed selection in the evolutionary process. In addition, large-scale gene rearrangements were detected between some Tricholoma species. Phylogenetic analysis using the Bayesian inference and maximum likelihood methods based on a combined mitochondrial gene set yielded identical and well-supported tree topologies. This study promoted the understanding of the genetics, evolution, and phylogeny of the Tricholoma genus and related species.

Evidence for mycorrhizal cheating in Apostasia nipponica, an early-diverging member of the Orchidaceae

Most land plants, from liverworts to angiosperms, form mutualistic mycorrhizal symbioses with fungal partners. However, several plants known as mycoheterotrophs exploit fungal partners by reversing the polarity of carbon movement, which usually moves from plant to fungus. We investigated the physiological ecology of a photosynthetic orchid, Apostasia nipponica, which belongs to the first branching group within the Orchidaceae, to improve our understanding of mycoheterotrophic evolution in orchids. The fungal symbionts and nutrition modes of A. nipponica were investigated using molecular barcoding and carbon-13 (¹³ C) and nitrogen-15 (¹⁵ N) measurements, respectively. Community profiling based on a metabarcoding technique revealed that A. nipponica associates with specific Ceratobasidium spp. within ectomycorrhizas-forming clades, whereas isotope analysis revealed that A. nipponica was similar to fully mycoheterotrophic orchids in its ¹³ C signature and was even more enriched in ¹⁵ N than most of the fully mycoheterotrophic orchids that exploit ectomycorrhizal fungi. Our molecular and mass-spectrometric approaches demonstrated, for the first time, that a member of the Apostasioideae, the earliest-diverging lineage of the Orchidaceae, gains carbon through both photosynthesis and fungal cheating (i.e. partial mycoheterotrophy) during the adult stage.