Morphological and molecular analyses of ectomycorrhizal diversity in a man-made T. melanosporum plantation: description of novel truffle-like morphotypes

Successes and challenges in the sustainable cultivation of edible mycorrhizal fungi – furthering the dream

The cultivation of edible mycorrhizal fungi (EMF) has made great progress since the first cultivation of Tuber melanosporum in 1977 but remains in its infancy. Five cultivation steps are required: (1) mycorrhizal synthesis, (2) mycorrhiza development and acclimation, (3) out-planting of mycorrhizal seedlings, (4) onset of fructification, and (5) performing tree orchards. We provide examples of successes and challenges associated with each step, including fruiting of the prestigious chanterelles in Japan recently. We highlight the challenges in establishing performing tree orchards. We report on the monitoring of two orchards established between Lactarius deliciosus (saffron milk cap) and pines in New Zealand. Saffron milk caps yields reached 0.4 and 1100 kg/ha under Pinus radiata and P. sylvestris 6 and 9 y after planting, respectively. Canopy closure began under P. radiata 7 y after planting, followed by a drastic reduction of yields, while P. sylvestris yields still hovered at 690 to 780 kg/ha after 11 y, without canopy closure. The establishment of full-scale field trials to predict yields is crucial to making the cultivation of EMF a reality in tomorrow's cropping landscape. Sustainable EMF cultivation utilizing trees in non-forested land could contribute to carbon storage, while providing revenue and other ecosystem services.

Diversity of ectomycorrhizal Thelephoraceae in Tuber melanosporum-cultivated orchards of Northern Spain

Truffles are edible hypogeous ascomycetes highly appreciated worldwide, especially the black truffle (Tuber melanosporum Vittad.). In recent decades, the cultivation of the black truffle has expanded across the Mediterranean climate regions in and outside its native range. Members of the Thelephoraceae (Thelephorales, Agaricomycetes, Basidiomycota) are commonly found in truffle plantations, but their co-occurrence with Tuber species and other members of the fungal community has been scarcely reported. Thelephoraceae is one of the most represented families of the ectomycorrhizal fungal community in boreal and Mediterranean forests. To reveal the diversity of these fungi in T. melanosporum-cultivated plantations, ten orchards located in the Navarra region (Northern Spain) were surveyed for 2 years. Morphological and molecular approaches were used to detect and identify the Thelephoraceae ectomycorrhizas present in those plantations. Ten different mycorrhizal types were detected and described. Four of them were morphologically identified as Tomentella galzinii, Quercirhiza cumulosa, Q. squamosa, and T39 Thelephoraceae type. Molecular analyses revealed 4-6 operational taxonomic units (OTUs), depending on the nucleotide database used, but similarities remained under 95 % and no clear species assignments could be done. The results confirm the diversity and abundance of this fungal family in the ectomycorrhizal community of black truffle plantations, generally established in Mediterranean areas. The occurrence and relative abundance of Thelephoraceae ectomycorrhizas is discussed in relation to their possible influence on truffle production.

Temporal dynamics of ectomycorrhizal fungi and persistence of Tuber melanosporum in inoculated Quercus robur seedlings in North Europe

Tuber melanosporum is widely cultivated outside its natural habitat in various climatic conditions. This study aims to monitor the persistence of T. melanosporum in inoculated oak seedlings and document temporal changes of native ectomycorrhizal fungi in an Estonian 5-year-old plantation. Sampling of ectomycorrhizal fungi was carried out in June-July 2011-2013 to provide DNA-based identification of the inoculated truffle species and other co-occurring ectomycorrhizal fungi. The mycorrhiza of T. melanosporum proved resistant to cold winters, and the fungus persisted well in the root systems of surviving seedlings albeit slight decline in relative colonization level over 3 years. We identified the genera Hymenogaster and Hebeloma to be the dominant ectomycorrhizal competing fungi in root systems. Good vegetative growth of mycelia and the presence of two compatible mating types suggest that cultivation of T. melanosporum is possible in the Nordic climate. Evaluation of fruit-body production will be a critical next step, because fruiting efficiency allows to determine economic feasibility and ecological sustainability of the Périgord truffle cultivation in northern climate.

Diversity and persistence of ectomycorrhizal fungi and their effect on nursery-inoculated Pinus pinaster in a post-fire plantation in Northern Portugal

Ectomycorrhizal fungi (ECMF) play an important role in forest ecosystems, often mitigating stress factors and increasing seedling performance. The aim of this study was to investigate the effects of a nursery inoculation on Pinus pinaster growth and on the fungal communities established when reforesting burned areas. Inoculated P. pinaster saplings showed 1.5-fold higher stem height than the non-inoculated controls after a 5 year growth period, suggesting that fungal inoculation could potentiate tree growth in the field. Ordination analysis revealed the presence of different ECMF communities on both plots. Among the nursery-inoculated fungi, Laccaria sp., Rhizopogon sp., Suillus bovinus and Pisolithus sp. were detected on inoculated Pinus saplings on both sampling periods, indicating that they persisted after field establishment. Other fungi were also detected in the inoculated plants. Phialocephala sp. was found on the first assessment, while Terfezia sp. was detected on both sampling periods. Laccaria sp. and Rhizopogon sp. were identified in the control saplings, belonging however to different species than those found in the inoculated plot. Inocybe sp., Thelephora sp. and Paxillus involutus were present on both sampling periods in the non-inoculated plots. The results suggest that ECMF inoculation at nursery stage can benefit plant growth after transplantation to a post-fire site and that the inoculated fungi can persist in the field. This approach has great potential as a biotechnological tool to aid in the reforestation of burned areas.

Effects of soil tillage on Tuber magnatum development in natural truffières

Tuber magnatum Pico, the Italian white truffle, commands the highest prices of any truffle. Despite its commercial value, it is the only edible European truffle which has not yet been successfully cultivated. Because of this, it is essential to safeguard natural truffières and to identify cultural practices to maximize their productivity. Soil tillage is successfully and extensively used in black truffle cultivation to enhance productivity, but its effects are not known on the development of T. magnatum. A recently developed real-time PCR assay was applied to evaluate the effects of tillage (10-15 cm depth) on T. magnatum mycelium in two different natural truffle grounds located in Tuscany and Emilia-Romagna. Tillage effects on bulk density, ectomycorrhizal fungal communities, and ascoma production were also assessed. Tilling significantly increased the quantity of T. magnatum mycelium which seemed to be related to an increase in soil porosity by up to 34%, and the diversity of ectomycorrhizal fungal communities. On the contrary, no significant effects were found on ascoma production. The results highlight that real-time PCR is the most reliable method for evaluating the effects of cultural practices on the development of T. magnatum in soil avoiding long-term studies on fruiting body production.

Ectomycorrhizal fungus diversity and community structure with natural and cultivated truffle hosts: applying lessons learned to future truffle culture

Since the first truffle plantations were established in France, Italy and other parts in the world, many studies have been carried out to improve their productivity and sustainability. Success of plantations is clearly related to the mycorrhizal status of the host trees over the years, from inoculated seedlings to truffle-producing trees. The experience gained in monitoring the ectomycorrhizal fungus status in cultivated truffle grounds has allowed us to develop an extensive catalogue of the ectomycorrhizal fungi present in truffle plantations. Herein, we summarize fungal community data from 85 references that represent different truffle studies in natural habitats and plantations. Approximately 25% of the ectomycorrhizae reported in the 85 references are common to most of the studies. In general, more fungal species are detected in productive plantations than in the non-productive ones. Truffle plantations display a diverse ectomycorrhizal fungal community, in which species of the genus Tuber are well represented. Tuber rufum and some members of Boletales are typically restricted to productive truffle plots. On the other hand, Hebeloma, Laccaria and Russula species are mostly associated with unproductive plots. Ectomycorrhizae belonging to Thelephoraceae are frequently found in mature truffle orchards but do not seem to affect sporocarp production. Several biotic and abiotic factors affect the ectomycorrhizal fungus communities associated with truffle orchards. Among them are plantation age, host species and its growth, the surrounding environment (particularly the presence of other ectomycorrhizal hosts), and plantation management. Understanding the ectomycorrhizal fungal communities inhabiting different plantations may give us clues about the dynamics of the targeted truffles and the possibility of identifying mycorrhizal fungal species that are good indicators of successful truffle plantations.

Mycorrhiza analyses in New Zealand truffières reveal frequent but variable persistence of Tuber melanosporum in co-existence with other truffle species

This study compiles the results from an examination of mycorrhizae on root samples from Tuber melanosporum truffières in New Zealand. Samples were taken over 5 years from 328 trees in 43 truffières established with nursery-inoculated trees. Mycorrhizae were analysed using a combination of morphological and molecular techniques, focusing on the identification of Tuber species. Results show that 49% of the trees, and nearly 90% of the truffières, retained T. melanosporum mycorrhizae up to 21 years after planting. Tuber mycorrhizae with spiky cystidia were found on 26.9% of the tested trees: Tuber brumale (5.5%), Tuber maculatum (10.7%), and unidentified Tuber species (10.7%), and were detected in 67% of the truffières tested. T. brumale was found in 28% and T. maculatum in 35% of the truffières. In 56% of the truffières, T. melanosporum was found to occur with spiky Tuber species. The existence of T. brumale and T. maculatum in the same truffière was recorded only once. Forty-four percent of trees examined had Scleroderma-like (SCL) mycorrhizae and 50% of trees hosted other ectomycorrhizal species (OE). For all categories of mycorrhizal species examined, the variation between truffières was greater than variation within each truffière. Overall results indicate that Corylus avellana tends to be more receptive to mycorrhizae of Tuber species than Quercus robur but is not necessarily more productive. In productive truffières, Q. robur appears to host SCL mycorrhizae more often than C. avellana. This is the first study of its scale to analyse the mycorrhizal species associated with T. melanosporum truffières in the Southern Hemisphere.

Genetic diversity and population differentiation of Guignardia mangiferae from "Tahiti" acid lime

Among the citrus plants, "Tahiti" acid lime is known as a host of G. mangiferae fungi. This species is considered endophytic for citrus plants and is easily isolated from asymptomatic fruits and leaves. G. mangiferae is genetically related and sometimes confused with G. citricarpa which causes Citrus Black Spot (CBS). "Tahiti" acid lime is one of the few species that means to be resistant to this disease because it does not present symptoms. Despite the fact that it is commonly found in citric plants, little is known about the populations of G. mangiferae associated with these plants. Hence, the objective of this work was to gain insights about the genetic diversity of the G. mangiferae populations that colonize "Tahiti" acid limes by sequencing cistron ITS1-5.8S-ITS2. It was verified that "Tahiti" acid lime plants are hosts of G. mangiferae and also of G. citricarpa, without presenting symptoms of CBS. Populations of G. mangiferae present low-to-moderate genetic diversity and show little-to-moderate levels of population differentiation. As gene flow was detected among the studied populations and they share haplotypes, it is possible that all populations, from citrus plants and also from the other known hosts of this fungus, belong to one great panmictic population.

Ectomycorrhizal fungal diversity in orchards of cultivated pecan (Carya illinoinensis; Juglandaceae)

Carya illinoinensis (pecan) belongs to the Juglandaceae (walnut family) and is a major economic nut crop in the southern USA. Although evidence suggests that some species in the Juglandaceae are ectomycorrhizal, investigations on their ectomycorrhizal fungal symbionts are quite limited. Here we assessed the ectomycorrhizal fungal diversity in cultivated orchards of C. illinoinensis. Five pecan orchards in southern Georgia, USA, were studied, three of which were known to fruit the native edible truffle species Tuber lyonii. We sequenced rDNA from single ectomycorrhizal root tips sampled from a total of 50 individual trees. Mycorrhizae were identified by ITS and LSU rDNA sequence-based methods. Forty-four distinct ectomycorrhizal taxa were detected. Sequestrate taxa including Tuber and Scleroderma were particularly abundant. The two most abundant sequence types belonged to T. lyonii (17%) and an undescribed Tuber species (~20%). Because of our interest in the ecology of T. lyonii, we also conducted greenhouse studies to determine whether this species would colonize and form ectomycorrhizae on roots of pecan, oak, or pine species endemic to the region. T. lyonii ectomycorrhizae were formed on pecan and oak seedlings, but not pine, when these were inoculated with spores. That oak and pecan seedling roots were receptive to truffle spores indicates that spore slurry inoculation could be a suitable method for commercial use and that, ecologically, T. lyonii may function as a pioneer ectomycorrhizal species for these hosts.

Tuber melanosporum: mating type distribution in a natural plantation and dynamics of strains of different mating types on the roots of nursery-inoculated host plants

• In light of the recent finding that Tuber melanosporum, the ectomycorrhizal ascomycete that produces the most highly prized black truffles, is a heterothallic species, we monitored the spatial distribution of strains with opposite mating types (MAT) in a natural truffle ground and followed strain dynamics in artificially inoculated host plants grown under controlled conditions. • In a natural truffle ground, ectomycorrhizas (ECMs), soil samples and fruit bodies were sampled and genotyped to determine mating types. Simple sequence repeat (SSR) markers were also used to fingerprint ECMs and fruit bodies. The ECMs from nursery-inoculated host plants were analysed for mating type at 6 months and 19 months post-inoculation. • In open-field conditions, all ECMs from the same sampling site showed an identical mating type and an identical haploid genotype, based on SSR analysis. Interestingly, the gleba of fruit bodies always demonstrated the same genotype as the surrounding ECMs. Although root tips from nursery-grown plants initially developed ECMs of both mating types, a dominance of ECMs of the same MAT were found after several months. • The present study deepens our understanding of the vegetative and sexual propagation modes of T. melanosporum. These results are highly relevant for truffle cultivation.

The AD-type ectomycorrhizas, one of the most common morphotypes present in truffle fields, result from fungi belonging to the Trichophaea woolhopeia species complex

Belowground ectomycorrhizal communities are often species rich. Characterization of the ectomycorrhizas (ECMs) underneath native truffle areas and/or cultivation sites is particularly relevant to identifying fungal species that might interfere with or promote truffle propagation and fruiting. Fungal identification at the genus/species level can now be achieved by combining detailed morphological and anatomical descriptions with molecular approaches. In a survey of the mycorrhizal biodiversity of Tuber melanosporum orchards and inoculated host plants in nurseries, we repeatedly sampled ECMs with morphological features resembling those of the ECMs widely known as the AD type. Despite the fact that the AD type is regarded as one of the most competitive fungal species towards Tuber spp., its taxonomical rank has yet to be resolved. By analyzing the 28S and internal transcribed spacer (ITS) rDNA regions, here, we show that AD-type ECMs result from host plant colonization by the pyronemataceous species Trichophaea woolhopeia. Further to this, the 28S and ITS phylogenetic trees built from the AD-type ECMs analyzed sustain the hypothesis that T. woolhopeia is a species complex.

Can NPK fertilizers enhance seedling growth and mycorrhizal status of Tuber melanosporum-inoculated Quercus ilex seedlings?

Although successful cultivation of the black truffle (Tuber melanosporum) has inspired the establishment of widespread truffle orchards in agricultural lands throughout the world, there are many unknowns involved in proper management of orchards during the 6-10 years prior to truffle production, and there are conflicting results reported for fertilizer treatments. Here, we systematically evaluate the combined effects of nitrogen, phosphorous, and potassium with different doses of each element, applied to either foliage or roots, on plant growth parameters and the mycorrhizal status of outplanted 3-year-old seedlings in five experimental Quercus ilex-T. melanosporum orchards. Fertilization did not significantly improve seedling aboveground growth, but the plants treated with the fertilizer 12-7-7 applied to the roots (HNr) displayed longer field-developed roots. Only the fertilizer with the highest dose of K (10-6-28) applied to the foliage (HKf) increased the probability of fine root tip colonization by T. melanosporum in field-developed roots. However, the plants treated with the same fertilizer applied to the soil (HKr) presented the highest probability for colonization by other competing mycorrhizal soil fungi. Potassium seems to have an important role in mycorrhizal development in these soils. Apart from T. melanosporum, we found 14 ectomycorrhizal morphotypes, from which seven were identified to species level, three to genus, two to family, and two remained unidentified by their morphological characteristics and DNA analyses.