Molecular and morphological characterization of Tuber magnatum mycorrhizas in a long-term survey

In vitro interactions between Bradyrhizobium spp. and Tuber magnatum mycelium

Tuber magnatum is the most expensive truffle, but its large-scale cultivation is still a challenge compared to other valuable Tuber species. T. magnatum mycelium has never been grown profitably until now, which has led to difficulties to studying it in vitro. This study describes beneficial interactions between T. magnatum mycelium and never before described bradyrhizobia, which allows the in vitro growth of T. magnatum mycelium. Three T. magnatum strains were co-isolated on modified Woody Plant Medium (mWPM) with aerobic bacteria and characterised through microscopic observations. The difficulties of growing alone both partners, bacteria and T. magnatum mycelium, on mWPM demonstrated the reciprocal dependency. Three bacterial isolates for each T. magnatum strain were obtained and molecularly characterised by sequencing the 16S rRNA, glnII, recA and nifH genes. Phylogenetic analyses showed that all nine bacterial strains were distributed among five subclades included in a new monophyletic lineage belonging to the Bradyrhizobium genus within the Bradyrhizobium jicamae supergroup. The nifH genes were detected in all bacterial isolates, suggesting nitrogen-fixing capacities. This is the first report of consistent T. magnatum mycelium growth in vitro conditions. It has important implications for the development of new technologies in white truffle cultivation and for further studies on T. magnatum biology and genetics.

Description, identification, and growth of Tuber borchii Vittad. mycorrhized Pinus sylvestris L. seedlings on different lime contents

Tuber borchii forms ectomycorrhiza with oaks, hazel, and pines, including Pinus sylvestris. However, its ectomycorrhiza morphotype with P. sylvestris was not comprehensively described so far, and molecular analyses are missing despite a high danger of misidentification of T. borchii ectomycorrhiza with other closely related and less valuable truffle species. We described for the first time the morphology and anatomy of T. borchii-P. sylvestris ectomycorrhiza using differential interference contrast technique and semi-thin sections in combination with molecular confirmation of identity. Color of ectomycorrhiza is reddish to dark brown, and morphotypes are unevenly but densely covered by warts-bearing pin-like cystidia. All layers of the hyphal mantle are pseudoparenchymatous with outer mantle layer formed of epidermoid cells. T. borchii ectomycorrhiza was identified by a molecular comparison with fruitbodies used for inoculation and its respective ectomycorrhizae. T. borchii has a wide ecological amplitude. To get a better insight in mycorrhization requirements, we investigated growth of P. sylvestris and its ectomycorrhiza infection rate with T. borchii in substrate with different lime content. The mycorrhization of P. sylvestris with T. borchii in the mycorrhization substrate and cultivation in greenhouse conditions was successful, with colonization of P. sylvestris varying between 36.5 and 48.1%. There was no significant correlation of mycorrhization to applied lime contents, and consequently to pH in substrate, while the increased levels of lime improved growth of the P. sylvestris seedlings.

From Nontargeted to Targeted Analysis: Feature Selection in the Differentiation of Truffle Species (Tuber spp.) Using 1H NMR Spectroscopy and Support Vector Machine

The price of different truffle types varies according to their culinary value, sometimes by more than a factor of 10. Nonprofessionals can hardly distinguish visually the species within the white or black truffles, making the possibility of food fraud very easy. Therefore, the identification of different truffle species (Tuber spp.) is an analytical task that could be solved in this study. The polar extract from a total of 80 truffle samples was analyzed by 1H NMR spectroscopy in combination with chemometric methods covering five commercially relevant species. All classification models were validated applying a repeated nested cross-validation. In direct comparison, the two very similar looking and closely related black representatives Tuber melanosporum and Tuber indicum could be classified 100% correctly. The most expensive truffle Tuber magnatum could be distinguished 100% from the other relevant white truffle Tuber borchii. In addition, signals for a potential Tuber borchii and a potential Tuber melanosporum marker for targeted approaches could be detected, and the corresponding molecules were identified as betaine and ribonate. A model covering all five truffle species Tuber aestivum, Tuber borchii, Tuber indicum, Tuber magnatum, and Tuber melanosporum was able to correctly discriminate between each of the species.

First production of Italian white truffle (Tuber magnatum Pico) ascocarps in an orchard outside its natural range distribution in France

Truffles are ectomycorrhizal species forming edible ascocarps. The Italian white truffle (Tuber magnatum Pico) is the most famous and expensive species harvested to date; it comes exclusively from natural habitats in European countries. The annual production of T. magnatum is generally insufficient to respond to the high demands making its cultivation a research hotspot. The first attempt to cultivate T. magnatum started in the 1970s without success; only recently have mycorrhized plants been successfully produced. The aims of this study were (1) to assess the persistence of T. magnatum in the soil of plantations realized with mycorrhized plants and (2) to characterize the first T. magnatum orchard that produced ascocarps outside the known natural geographic range of this species. In 2018, five orchards were sampled in France, and T. magnatum was investigated in the soil. We confirmed that T. magnatum survived in the soil 3 to 8 years after planting. The key finding of this study was the harvest of T. magnatum ascocarps in 2019 and 2020 from one orchard. The production of ascocarps started 4.5 years after planting, and the ascocarps were harvested under different trees and during two consecutive seasons. A detailed analysis of the productive orchards (i.e., soil features, soil water availability, cultivation techniques) is presented. These results demonstrate the feasibility of T. magnatum cultivation worldwide by planting mycorrhized plants. The cultivation of T. magnatum could therefore become a real opportunity for farmers and could respond to the high demand of this high-priced food.

Food Authentication: Truffle (Tuber spp.) Species Differentiation by FT-NIR and Chemometrics

Truffles are certainly the most expensive mushrooms; the price depends primarily on the species and secondly on the origin. Because of the price differences for the truffle species, food fraud is likely to occur, and the visual differentiation is difficult within the group of white and within the group of black truffles. Thus, the aim of this study was to develop a reliable method for the authentication of five commercially relevant truffle species via Fourier transform near-infrared (FT-NIR) spectroscopy as an easy to handle approach combined with chemometrics. NIR-data from 75 freeze-dried fruiting bodies were recorded. Various spectra pre-processing techniques and classification methods were compared and validated using nested cross-validation. For the white truffle species, the most expensive Tuber magnatum could be differentiated with an accuracy of 100% from Tuber borchii. Regarding the black truffle species, the relatively expensive Tuber melanosporum could be distinguished from Tuber aestivum and the Chinese truffles with an accuracy of 99%. Since the most expensive Italian Tuber magnatum is highly prone to fraud, the origin was investigated and Italian T. magnatum truffles could be differentiated from non-Italian T. magnatum truffles by 83%. Our results demonstrate the potential of FT-NIR spectroscopy for the authentication of truffle species.

Food Authentication: Identification and Quantitation of Different Tuber Species via Capillary Gel Electrophoresis and Real-Time PCR

Truffles are hypogeous fungi mainly found in Europe and Asia. Due to their special aroma and taste, some truffle species are sold on the international market at an extremely high price. Among the economically relevant species, the white Alba truffle (Tuber magnatum) and the black Périgord truffle (T. melanosporum) are the most appreciated species. The fruiting bodies of the Asian black truffle are morphologically very similar to T. melanosporum, and those of the Bianchetto truffle (T. albidum Pico) are similar to T. magnatum, but are of little economic value. Highly valued species are adulterated with cheaper ones, especially. Because of this problem, the aim of this study was the development of methods for detecting possible admixtures to protect consumers from fraud. This study is based on seven different truffle species (117 fruiting bodies) from different growing regions. Additionally, selected truffle products were included. Using this material, a real-time PCR (polymerase chain reaction) assay allowing the detection and quantitation of Asian black truffles in T. melanosporum up to 0.5% was developed. In addition, a capillary gel electrophoresis assay was designed, which allows the identification and quantitation of different species. The methods can be used to ensure the integrity of truffle products.

Genetic Structure and Phylogeography of Tuber magnatum Populations

The ectomycorrhizal fungus Tuber magnatum produces the white truffle appreciated worldwide for its unique aroma. With respect to other Tuber spp. of economic interest, T. magnatum presents a narrower geographical range. This species has, in fact, long been considered endemic to Italy. However, over the last few decades several reports have documented the presence of white truffles in different Mediterranean countries and in particular in various areas of south-east Europe. In this study, samples from several Pannonian and Balkan countries such as Hungary, Serbia, Romania, Bulgaria and Greece have been collected and genotyped with microsatellite markers and the data merged with those available for Italian populations. Our objectives were to test whether Italian and south-east European populations are differentiated and to evaluate the genetic diversity of T. magnatum all over its distributional range. We show the genetic structure of T. magnatum populations with the differentiation of four main groups: northern Italy, central-northern Italy, southern Italy and the Balkan/Pannonian region. The present study allowed us to refine the evolutionary history of T. magnatum and track the possible post-glacial expansion route of this species. The assessment of T. magnatum’s genetic structure is not only of scientific relevance, but it is also important for the conservation and market traceability of this prestigious fungus.

Rapid MALDI-TOF MS identification of commercial truffles

Truffles are edible mushrooms with similar morphological characteristics, that make it difficult to distinguish between highly prized truffles (such as the Périgord black T. melanosporum) and inexpensive truffles (such as the Asian Black T. indicum). These biological and economic features have led to several misidentifications and/or fraudulent profit in the truffle markets. In this paper, we investigate Matrix-assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF MS) biotyping to identify 34 commercial fresh truffles from Europe and Asia. The MALDI-TOF MS clustering rapidly distinguished seven Tuber species identified by ITS phylogenetic analysis. The tasty T. melanosporum was clearly differentiated from the Chinese and less expensive truffles. These cheaper mushrooms were marketed as T. indicum but corresponded to a mix of three species. In total, the method confirmed misidentifications in 26% of commercial specimens. Several unknown blind-coded truffles were rapidly identified, with scores >= 2, using the Bruker Biotyper algorithm against MS databases. This study demonstrates that MALDI-TOF MS is a reliable, rapid and cheaper new tool compared with molecular methods for the identification of truffle species and could be used to control frauds in the truffle markets. It could also be useful for the certification of truffle-inoculated seedlings and/or diversity in forest ecosystems.

Forty years of inoculating seedlings with truffle fungi: past and future perspectives

The first commercialization of seedlings inoculated with truffle fungi occurred in 1973. Over the last 40 years, considerable progress has been made relative to quality control for inoculated seedlings. A recently published paper by Andrés-Alpuente and colleagues (Mycorrhiza 24:29-37, 2014) reviewed and tested the different methods of mycorrhization assessment currently used in Europe. The aim of this paper is to augment their findings by adding information to the discussion about the most important steps in seedling quality control. Additionally, the history of seedlings inoculated with truffles, procedures currently used in France for seedling control quality, and a reflection on future research aimed at increasing truffle production will be presented.

Assessment of ectomycorrhizal fungal communities in the natural habitats of Tuber magnatum (Ascomycota, Pezizales)

The ectomycorrhizal (ECM) fungal communities of four natural Tuber magnatum truffle grounds, located in different Italian regions (Abruzzo, Emilia-Romagna, Molise, and Tuscany), were studied. The main objective of this study was to characterize and compare the ECM fungal communities in the different regions and in productive (where T. magnatum ascomata were found) and nonproductive points. More than 8,000 (8,100) colonized root tips were counted in 73 soil cores, and 129 operational taxonomic units were identified using morphological and molecular methods. Although the composition of the ECM fungal communities studied varied, we were able to highlight some common characteristics. The most plentiful ECM fungal taxa belong to the Thelephoraceae and Sebacinaceae families followed by Inocybaceae and Russulaceae. Although several ectomycorrhizas belonging to Tuber genus were identified, no T. magnatum ectomycorrhizas were found. The putative ecological significance of some species is discussed.

Development and validation of a real-time PCR assay for detection and quantification of Tuber magnatum in soil

BACKGROUND

Tuber magnatum, the Italian white truffle, is the most sought-after edible ectomycorrhizal mushroom. Previous studies report the difficulties of detecting its mycorrhizas and the widespread presence of its mycelium in natural production areas, suggesting that the soil mycelium could be a good indicator to evaluate its presence in the soil. In this study a specific real-time PCR assay using TaqMan chemistry was developed to detect and quantify T. magnatum in soil. This technique was then applied to four natural T. magnatum truffières located in different regions of Italy to validate the method under different environmental conditions.

RESULTS

The primer/probe sets for the detection and quantification of T. magnatum were selected from the ITS rDNA regions. Their specificity was tested in silico and using qualitative PCR on DNA extracted from 25 different fungal species. The T. magnatum DNA concentration was different in the four experimental truffières and higher in the productive plots. T. magnatum mycelium was however also detected in most of the non-productive plots. Ascoma production during the three years of the study was correlated with the concentration of T. magnatum DNA.

CONCLUSIONS

Taken together, these results suggest that the specific real-time PCR assay perfected in this study could be an useful tool to evaluate the presence and dynamics of this precious truffle in natural and cultivated truffières.

Self-anastomosing ability and vegetative incompatibility of Tuber borchii isolates

In this work, different mycelial isolates of the ectomycorrhizal fungus Tuber borchii were analysed for their ability to form self-anastomoses, which were monitored by time-lapse live-cell imaging, providing a description of the anastomosis process. Self-fusions were evidenced in three out of five isolates, with frequencies ranging between 50 and 88% of hyphal contacts. Time-lapse video microscopy evidenced that during pre-contact events, hyphae were capable of growth re-orientation functional to hyphal contact: the time elapsed between hyphal growth re-direction and complete fusion ranged from 115 to 200 min. After anastomosis, protoplasmic flow occurred between fused hyphae and nuclei could be detected in fusion bridges. Vegetative incompatibility was also assessed by analysing macroscopic and microscopic hyphal interactions between paired T. borchii mycelia. Both plate-pairing tests and microscopic analyses showed vegetative compatibility only within the same isolate, whereas different degrees of incompatible responses were observed in inter-isolate pairings. The diversity of T. borchii strains revealed by cytomorphological approaches is consistent with their genetic diversity obtained by molecular methods.

Isolation and characterization of some mycelia inhabiting Tuber ascomata

Tuber spp. are ectomycorrhizal ascomycetes that produce subterranean ascomata known as truffles. Truffles can be regarded as complex microhabitats hosting bacteria and yeasts. In this paper we show that guest filamentous fungi are also associated to truffle ascomata, regardless of the Tuber spp., and report the morpho-molecular characterization of seven truffle-hosted mycelia isolated from healthy and intact Tuber ascomata. Some of these isolates were shown to be related to the fungal endophytes of plants. Interestingly, the truffle-hosted mycelia grew stuck to the hyphal wall of their partner when co-cultivated with the Tuber borchii mycelium, but not when co-cultivated with the test species Agaricus macrosporus. The present data suggest that guest filamentous fungi can be added to the list of truffle-interacting microorganisms.

A revision of the descriptions of ectomycorrhizas published since 1961

All available publications providing descriptions of ectomycorrhizas (ECM) were reviewed in order to build a database containing details on fungus forming the ECM, host tree, country where the material for description was collected, and habitat of the ECM. Other secondary data were also recorded. In all 1244 descriptions of ECM published since 1961 in 479 papers were reviewed. The number of different ECM morphotypes described was 814. Most ECM described were collected in Europe and North America. Gymnosperms were the most common tree associates, and boreal and temperate forests the most studied ecosystems. Fungal symbionts were mostly Basidiomycota, epigeous, and with mushroom-like morphology. The paper also addresses the gaps in ECM knowledge that mycorrhizologists should address in future studies.

Morphological and molecular typing of the below-ground fungal community in a natural Tuber magnatum truffle-ground

The aims of the work were to elucidate the distribution of the ectomycorrhizal fungus Tuber magnatum Pico during its symbiotic stage, and to identify the root-associated fungi in a natural truffle-ground located in North Italy. Ectomycorrhizal root tips were harvested in the truffle ground, sorted in morphotypes and analyzed by ITS. Morphological and molecular analyses revealed that (i) T. magnatum mycorrhizae were rare and independent on the fruitbody productions and (ii) the dominant fungal species belonged to Thelephoraceae, followed by Tuberaceae and Sebacinaceae.

Tuber magnatum Pico, a species of limited geographical distribution: its genetic diversity inside and outside a truffle ground

The aim of this work was to clarify the genetic structure of the ectomycorrhizal fungus, Tuber magnatum Pico, in a natural truffle ground located in north Italy. Ascomata of this population of T. magnatum were collected over a period of up to 5 years. For comparative analysis, T. magnatum fruit bodies of different geographical origin were also considered. We used single locus markers, such as the variable region of ribosomal genes (ITS), the beta-tubulin gene and sequence-characterized amplified regions (SCAR), as tools to identify single-nucleotide polymorphisms (SNPs). On the basis of the molecular results, which were indirectly supported by a karyological analysis, a self-fertilization mechanism is suggested. A SCAR region was polymorphic within the samples of the truffle ground, leading to the identification of two genotypes. In addition, both the SCAR and the ITS proved to be polymorphic among samples coming from different geographical regions, revealing a genetic differentiation in T. magnatum.

PCR amplification and polymorphism analysis of the intergenic spacer region of ribosomal DNA in Tuber borchii

PCR amplification of the complete intergenic spacer region (IGS) of the Tuber borchii nuclear ribosomal repeat was obtained using universal primers CNL 12 and NS1rev. In order to improve amplification yield a specific primer, T1, was selected from a partial sequence of the IGS product. IGS diversity was characterized both at the intraindividual and intraspecific level. The results obtained at the intraindividual level showed 10% varying repeats on ten screened colonies, while at the intraspecific level the IGS polymorphism was evident as difference in length amplification between mycelial strains and fruit bodies: 3.5 kb and 2 kb respectively.

Morphological and molecular characterization of mycelia of some Tuber species in pure culture

•  Pure cultures of Tuber maculatum , Tuber melanosporum , Tuber aestivum , Tuber macrosporum , Tuber rufum and Tuber brumale were isolated and characterized by morphological and molecular methods. •  The Tuber mycelia were isolated from fruit bodies and molecular identification was performed using specific primers, restriction fragment length polymorphism and/or sequence analyses of the ITS region. •  The species grew between 1.1 mm wk ^-1 and 14 mm wk ^-1 on the selected medium. The mycelium of different Tuber species showed several common morphological features such as hyphal anastomoses, vesicle formation and hyphal aggregation. Differences were found in the frequency of these morphological features and in the hyphal pattern. The isolated mycelia also showed differences in the hyphal branch angle, septal distance, hyphal diameter and rate of growth of the hyphae. •  This result opens the possibility of using pure mycelial cultures of Tuber spp. for experimental purposes and for the commercial production of infected truffle plants.