Mitospore formation on pure cultures of Tuber japonicum (Tuberaceae, Pezizales) in vitro.

The members of the genus Tuber are Ascomycota that form ectomycorrhizal associations with various coniferous and broadleaf tree species. In the teleomorphic stage, the species of the genus produce fruit bodies known as true truffles. Recent studies have discovered mitosporic structures, including spore mats, of several Tuber species on forest soils, indicating the presence of a cryptic anamorphic stage or an unknown reproductive strategy. Here, we report in vitro mitospore formation on the mycelium of T. japonicum, which belongs to the Japonicum clade, collected in several regions in Japan. Twenty of the 25 strains formed mitospores on modified Melin-Norkrans agar medium, indicating that mitospore formation is likely a common trait among strains of T. japonicum. The fungus forms repeatedly branched conidiophores on aerial hyphae on colonies and generates holoblastic mitospores sympodially on the terminal and near apical parts and/or occasionally on the middle and basal parts of the conidiogenous cells. Mitospores are hyaline and elliptical, obovate, oblong, or occasionally bacilliform, with a vacuole and often distinct hilar appendices. Formation of mitospores by T. japonicum in vitro is useful in understanding the functions of mitospores in the genus Tuber under controlled environmental conditions.

Physiological characteristics of pure cultures of a white-colored truffle Tuber japonicum.

A white-colored truffle Tuber japonicum, indigenous to Japan, is an ascomycetous ectomycorrhizal fungus. To clarify the physiological characteristics of this fungus, we investigated the influence of culture medium, temperature, and sources of nitrogen (N) and carbon (C) on the growth of five strains. Tuber japonicum strains grew better on malt extract and modified Melin-Norkrans medium, and showed peak growth at 20 °C or 25 °C. This fungus utilized inorganic (NH4 + and NO3 -) and organic N sources (casamino acids, glutamine, peptone, urea, and yeast extract). Additionally, this fungus utilized various C sources, such as monosaccharide (arabinose, fructose, galactose, glucose, and mannose), disaccharide (maltose, sucrose, and trehalose), polysaccharide (dextrin and soluble starch), and sugar alcohol (mannitol). However, nutrient sources that promote growth and their effects on growth promotion widely varied among strains. This can result from the strain difference in enzyme activities involved in the assimilation and metabolism of these sources.

Component features, odor-active volatiles, and acute oral toxicity of novel white-colored truffle Tuber japonicum native to Japan.

Component analysis of a novel white-colored truffle native to Japan, Tuber japonicum, was performed to determine its characteristic features. The analysis of odor-active volatile compound showed a high contribution of 1-octen-3-ol and 3-methyl-2,4-dithiapentane to the odor of T. japonicum. Although 2,4-dithiapentane is a key odorant of well-known white truffle T. magnatum, 3-methyl-2,4-dithiapentane was detected from the ripe T. japonicum. The chemical components of T. japonicum showed no clear difference with those of edible truffles T. magnatum and T. melanosporum. It was rich in crude protein, crude fiber, and minerals (especially potassium), and low in crude fat. Glutamine and glutamic acid were detected in T. japonicum as free amino acids, while T. magnatum contained a large amount of alanine. Acute oral toxicity tests showed no abnormality, with an LD50 value of over 2000 mg/kg under the test conditions. The present study may support future market distribution of T. japonicum as a high-class foodstuff.

Inside Cover Image, Volume 8, Issue 1.

The cover image is based on the ORIGINAL RESEARCH Component features, odor-active volatiles, and acute oral toxicity of novel white-colored truffle Tuber japonicum native to Japan by Tomoko Shimokawa et al., https://doi.org/10.1002/fsn3.1325. Cover Credit: Cover image ©Tomoko Shimokawa Images.

Localization of helotialean fungi on ectomycorrhizae of Castanopsis cuspidata visualized by in situ hybridization.

Non-ectomycorrhizal fungi that associate with typical ectomycorrhizae often remain hidden, and their localization inside ectomycorrhizal (ECM) roots has remained uncharacterized. In this study, the fungal community associated with the ectomycorrhizae of Castanopsis cuspidata was investigated using a culture-dependent isolation technique. Additionally, the species composition and localization were determined using molecular techniques. The results of the isolation and identification of fungal species revealed the predominance of a few species belonging to the order Helotiales. Furthermore, the fungal community structures were significantly different depending on the taxa of the ectomycorrhiza-forming fungi. A taxon-specific probe was developed to analyze the localization of one dominant Hyaloscyphaceae (Helotiales) species in ECM tissues by in situ hybridization. Hybridization signals were detected on the surface of the fungal mantle and around the ECM fungal cells within the mantle. Hyphal penetration into ECM hyphal cells of fungal mantles was also observed. Signals were not detected in the Hartig net or plant tissues inside the mantle in healthy ectomycorrhizae. These findings suggest that the analyzed species interact not only with host plant as root endophyte but also directly with the ECM fungi.