Efficiency of the traditional practice of traps to stimulate black truffle production, and its ecological mechanisms

Cultivation and mating of the truffle Tuber japonicum in plantations of ectomycorrhizal Quercus serrata seedlings

Tuber japonicum, a white-colored truffle that is endemic to Japan, is promising for culinary purposes due to its unique aroma. We were able to cultivate T. japonicum in plantations of inoculated Quercus serrata seedlings for the first time. Ascocarps were found after 43 months at one site and after 61 months at another. We developed simple sequence repeat markers for multilocus genotyping of glebal tissue and ascospores and confirmed that the harvested ascocarps were derived from inocula. All paternal individuals matched the multilocus genotypes of neighboring maternal individuals, indicating frequent hermaphroditism and the absence of externally introduced individuals. Our findings provide important clues to understanding the reproductive biology of T. japonicum during the early period after the truffle plantation establishment.

IMPORTANCE

Truffles are highly prized as a delicacy, but only a select few species have been successfully cultivated. In our study, we succeeded for the first time in cultivating Tuber japonicum. Two out of four plantations produced ascocarps shortly after planting, with one of them yielding a comparable weight to other cultivated truffle species. This promising productivity suggests that the fungus has potential when cultivated. Our analysis of the ascocarps' maternal and paternal genotypes, using simple sequence repeat markers, revealed hermaphroditic behavior in the fungus at our planting site. Our findings provide crucial insights into the truffle mating events.

Advances in molecular genetics have increased knowledge of Tuber species' life cycle and population genetic structure, indicating ways to improve yield

Truffles are possibly the only high-value cultivated organisms for which some aspects of the habit and life cycle have only recently been elucidated or remain unknown. Molecular techniques have helped explain the biological basis for some traditional empirical management techniques, such as inoculating soil with ascospores to improve yield, and have enhanced the detection of competitive or pathogenic soil microorganisms. Improved precision of assessment of the quality of inoculated seedlings is now possible. New knowledge of the genetic structure of populations has indicated that as trees age, the genotypes of mycorrhizae on inoculated trees change, and that there are large differences in the number of female and male genotypes participating in ascocarp formation. The plasticity of Tuber species has also been revealed, with maternal genotypes growing as an ectomycorrhiza in host tree roots and as surface mycelium or an endophyte in roots of adjacent non-mycorrhizal species. Refinement of management techniques has resulted from applying the new information, and the tools are now available to resolve the many outstanding gaps in our knowledge of Tuber biology.

Volatilome changes during black truffle (Tuber melanosporum) ontogeny

The aroma is critical in the reproductive biology of truffles and in their commercial quality. However, previous research has almost exclusively focused on characterizing ripe ascocarps. We characterized the volatilome of the highly-prized black truffle (Tuber melanosporum) ascocarps from July, in an early development stage, to March, in the late harvesting season, and investigated the relationships among aroma, ascocarp growth and morphogenetic development. The aroma profile was analyzed using a head space gas chromatography technique coupled with mass spectrometer. Seventy-one volatile compounds were identified and three development stages were clearly distinguished according to the volatile profile. In unripe ascocarps of July-September, the profile was dominated by methanethiol (19 %), 4-penten-2-ol (11 %) and acetone (11 %), the monthly mean weight of ascocarps ranged 2-20 g, and morphogenetic stages 4-6a were prevalent. In unripe ascocarps of October-December, the most abundant volatiles were 4-penten-2-ol (21 %), methanethiol (20 %) and ethanol (13 %), the monthly mean ascocarp weight ranged 28-43 g, and morphogenetic stages 6a, 6b-c were prevalent. In ripe ascocarps (December-March), the most abundant volatiles were 4-penten-2-ol (17 %), dimethyl sulfide (16 %) and ethanol (10 %), ascocarp weight did not increase significantly, and 6b-c was practically the sole morphogenetic stage. Thirty volatiles were associated to one of these three development stages. Amongst those with higher occurrence, 4-penten-2-ol, dimethyl sulfide, ethyl acetate, 2-pentanol and 2-butanone were associated to ripe truffles, whereas methanethiol, isobutyl isobutyrate, butanedione and 3-methylanisole were associated to unripe truffles.

Habitat is more important than climate for structuring soil fungal communities associated in truffle sites

The ectomycorrhizal fungi Tuber melanosporum Vittad. and Tuber aestivum Vittad. produce highly valuable truffles, but little is known about the soil fungal communities associated with these truffle species in places where they co-occur. Here, we compared soil fungal communities present in wild and planted truffle sites, in which T. melanosporum and T. aestivum coexist, in Mediterranean and temperate regions over three sampling seasons spanning from 2018 to 2019. We showed that soil fungal community composition and ectomycorrhizal species composition are driven by habitat type rather than climate regions. Also, we observed the influence of soil pH, organic matter content and C:N ratio structuring total and ectomycorrhizal fungal assemblages. Soil fungal communities in wild sites revealed more compositional variability than those of plantations. Greater soil fungal diversity was found in temperate compared to Mediterranean sites when considering all fungal guilds. Ectomycorrhizal diversity was significantly higher in wild sites compared to plantations. Greater mould abundance at wild sites than those on plantation was observed while tree species and seasonal effects were not significant predictors in fungal community structure. Our results suggested a strong influence of both ecosystem age and management on the fungal taxa composition in truffle habitats.

Effect of Bacteria Inoculation on Colonization of Roots by Tuber melanosporum and Growth of Quercus ilex Seedlings

Tuber melanosporum is an ascomycete that forms ectomycorrhizal (ECM) symbioses with a wide range of host plants, producing edible fruiting bodies with high economic value. The quality of seedlings in the early symbiotic stage is important for successful truffle cultivation. Numerous bacterial species have been reported to take part in the truffle biological cycle and influence the establishment of roots symbiosis in plant hosts and the development of the carpophore. In this work, three different bacteria formulations were co-inoculated in Quercus ilex L. seedlings two months after T. melanosporum inoculation. At four months of bacterial application, the T. melanosporum ECM root tip rate of colonization and bacterial presence were assessed using both morphological and molecular techniques. A 2.5-fold increase in ECM colonization rate was found in the presence of Pseudomonas sp. compared to the seedlings inoculated only with T. melanosporum. The same treatment caused reduced plant growth either for the aerial and root part. Meanwhile, the ECM colonization combined with Bradyrhizobium sp. and Pseudomonas sp. + Bradyrhizobium sp. reduced the relative density of fibrous roots (nutrient absorption). Our work suggests that the role of bacteria in the early symbiotic stages of ECM colonization involves both the mycorrhizal symbiosis rate and plant root development processes, both essential for improve the quality of truffle-inoculated seedlings produced in commercial nurseries.