True Truffle Host Diversity

Physiological characteristics of pure cultures of a white-colored truffle <i>Tuber japonicum</i>

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.

Synthesis and ultrastructural observation of arbutoid mycorrhizae of black truffles (Tuber melanosporum and T. aestivum)

Arbutus unedo (the strawberry tree) is a Mediterranean shrub which forms arbutoid mycorrhizae with a variety of Asco- and Basidiomycetes. After the discovery of the mycorrhizal symbiosis between A. unedo and Tuber borchii, in this study, arbutoid mycorrhizae were synthetized in greenhouse with Tuber aestivum and Tuber melanosporum. Six months after inoculation, both species colonized the roots of all inoculated A. unedo seedlings, but mature mycorrhizae were only observed after 12 months. Ultrastructure analysis of Tuber arbutoid mycorrhizae was described for the first time, showing, as observed in typical endosymbiosis, a rearrangement of host cells and the creation of an interface compartment with both truffle species. Immunolabelling experiments suggested that pectins are not present in the interface matrix surrounding the intracellular hyphae. Thus, the ability to establish symbiosis with A. unedo seems to be a common feature in the genus Tuber, opening up the possibility to use this plant for mycorrhization with valuable truffles. This could represent an important economic opportunity in Mediterranean areas by combining the production of truffles, edible fruits and valued honey.

Persistence of Tuber melanosporum in truffle orchards in North Carolina, USA

A survey was conducted to determine the persistence of mycorrhization by Tuber melanosporum in truffle orchards established with European and American species of oak and common hazel trees in North Carolina. The trees had reportedly been inoculated and colonized by T. melanosporum prior to planting. Root samples were collected from 95 trees among seven orchards in 2015 and roots were analyzed by morphology and quantitative PCR. Samples that tested negative for T. melanosporum or where ectomycorrhizal morphology was not observed were analyzed by sequencing to identify the mycorrhizal fungal symbiont present. The presence of T. melanosporum was detected in all seven orchards. In six orchards, T. melanosporum was detected on all trees, but in only two of fifteen trees in one orchard. Other species of Tuber including T. brennemanii, T. canaliculatum, and T. lyonii, species of Scleroderma, and members of the Pezizales were also detected by sequence analysis. Sporocarps of T. aestivum and T. brumale were found in 2017 and 2018 in separate orchards in North Carolina after the survey was conducted. Overall, results indicate that T. melanosporum has persisted in truffle orchards sampled in North Carolina. Indigenous and contaminating fungal species, including Tuber species, were also detected and present a challenge to the truffle industry in North Carolina.

Tuberomics: a molecular profiling for the adaption of edible fungi (Tuber magnatum Pico) to different natural environments

BACKGROUND

Truffles are symbiotic fungi that develop underground in association with plant roots, forming ectomycorrhizae. They are primarily known for the organoleptic qualities of their hypogeous fruiting bodies. Primarily, Tuber magnatum Pico is a greatly appreciated truffle species mainly distributed in Italy and Balkans. Its price and features are mostly depending on its geographical origin. However, the genetic variation within T. magnatum has been only partially investigated as well as its adaptation to several environments.

RESULTS

Here, we applied an integrated omic strategy to T. magnatum fruiting bodies collected during several seasons from three different areas located in the North, Center and South of Italy, with the aim to distinguish them according to molecular and biochemical traits and to verify the impact of several environments on these properties. With the proteomic approach based on two-dimensional electrophoresis (2-DE) followed by mass spectrometry, we were able to identify proteins specifically linked to the sample origin. We further associated the proteomic results to an RNA-seq profiling, which confirmed the possibility to differentiate samples according to their source and provided a basis for the detailed analysis of genes involved in sulfur metabolism. Finally, geographical specificities were associated with the set of volatile compounds produced by the fruiting bodies, as quantitatively and qualitatively determined through proton transfer reaction-mass spectrometry (PTR-MS) and gas-chromatography-mass spectrometry (GC-MS). In particular, a partial least squares-discriminant analysis (PLS-DA) model built from the latter data was able to return high confidence predictions of sample source.

CONCLUSIONS

Results provide a characterization of white fruiting bodies by a wide range of different molecules, suggesting the role for specific compounds in the responses and adaptation to distinct environments.

Truffle biogeography-A case study revealing ecological niche separation of different Tuber species

Ecology of hypogeic mycorrhizal fungi, such as truffles, remains largely unknown, both in terms of their geographical distribution and their environmental niches. Occurrence of true truffles (Tuber spp.) was therefore screened using specific polymerase chain reaction (PCR) assays and subsequent PCR amplicon sequencing in tree roots collected at 322 field sites across the Czech Republic. These sites spanned a wide range of climatic and soil conditions. The sampling was a priori restricted to areas thought to be suitable for Tuber spp. inasmuch as they were characterized by weakly acidic to alkaline soils, warmer climate, and with tree species previously known to host true truffles. Eight operational taxonomic units (OTUs) corresponding to Tuber aestivum, T. borchii, T. foetidum, T. rufum, T. indicum, T. huidongense, T. dryophilum, and T. oligospermum were detected. Among these, T. borchii was the OTU encountered most frequently. It was detected at nearly 19% of the sites. Soil pH was the most important predictor of Tuber spp. distribution. Tuber borchii preferred weakly acidic soils, T. foetidum and T. rufum were most abundant in neutral soils, and T. huidongense was restricted to alkaline soils. Distribution of T. aestivum was mainly dictated by climate, with its range restricted to the warmest sites. Host preferences of the individual Tuber spp. were weak compared to soil and climatic predictors, with the notable exception that T. foetidum appeared to avoid oak trees. Our results open the way to better understanding truffle ecology and, through this new knowledge, also to better-informed trufficulture.