Genotypic diversity of the Asiatic black truffle, Tuber himalayense, collected in spontaneous and highly productive truffle grounds

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.

Mycorrhization of Quercus spp. with Tuber huidongense and T. himalayense Collected in Korea

Fungi of the genus Tuber are ectomycorrhizal fungi that form a symbiotic relationship mainly with oak and hazel trees. Tuber spp. exhibit a highly selective host plant preference; thus, for cultivation purposes it is important to select an appropriate host plant for successful mycorrhization. In addition, as mycorrhizal characteristics differ according to Tuber spp., it is necessary to understand the differences in mycorrhizae according to the fungal species. Tuber huidongense and Tuber himalayense were recently discovered in Korea; therefore, we used spore suspensions from these two species to inoculate two species of oak trees, Quercus acutissima and Quercus dentata, to compare colonization rates and morphologies of the mycorrhizae. The colonization rates demonstrated that the different Tuber spp. favored different host plant species. In addition, unique morphological and anatomical characteristics were observed for T. huidongense and T. himalayense depending on the host species. These findings can lead to new economically important agricultural activities related to truffle cultivation in Korea.

Morphological characteristics of ectomycorrhizas formed by in vitro synthesis between conifer seedlings and Tuber mycelial strains of the Puberulum clade isolated in Japan

Seedlings of Pinus densiflora and Abies sachalinensis were inoculated with Tuber mycelial strains of the Puberulum clade in vitro to examine the morphological characteristics of their ectomycorrhizas. Axenically germinated seedlings were inoculated with the mycelia of five taxa from the Puberulum clade and grown in glass jars for 4 mo in an illuminated incubator. The seedlings were successfully colonized by the inoculated Tuber strains, as confirmed by the nuclear ribosomal internal transcribed spacer barcoding of the synthesized ectomycorrhizas. The ectomycorrhizas were characterized by a pale yellow to brown color, short needle-shaped cystidia, and net-like hyphal arrangement, and epidermoid cells on the mantle surface; notably, these features are similar to the ectomycorrhizas of various Puberulum clade members. As the ectomycorrhizas of different Tuber species are indistinguishable by morphological characters, molecular techniques are necessary to identify ectomycorrhizas formed by Tuber species within the Puberulum clade.

Life Cycle and Phylogeography of True Truffles

True truffle (Tuber spp.) is one group of ascomycetes with great economic importance. During the last 30 years, numerous fine-scale population genetics studies were conducted on different truffle species, aiming to answer several key questions regarding their life cycles; these questions are important for their cultivation. It is now evident that truffles are heterothallic, but with a prevalent haploid lifestyle. Strains forming ectomycorrhizas and germinating ascospores act as maternal and paternal partners respectively. At the same time, a number of large-scale studies were carried out, highlighting the influences of the last glaciation and river isolations on the genetic structure of truffles. A retreat to southern refugia during glaciation, and a northward expansion post glaciation, were revealed in all studied European truffles. The Mediterranean Sea, acting as a barrier, has led to the existence of several refugia in different peninsulas for a single species. Similarly, large rivers in southwestern China act as physical barriers to gene flow for truffles in this region. Further studies can pay special attention to population genetics of species with a wide distribution range, such as T. himalayense, and the correlation between truffle genetic structure and the community composition of truffle-associated bacteria.