Complete nucleotide sequence of TaV1, a novel totivirus isolated from a black truffle ascocarp (Tuber aestivum Vittad.)

Excerpt

New viruses associated with fungi are discovered frequently and have been reported since the early 1960s in members of numerous fungal taxa [8, 9, 12‐14, 16]. Here, we report the complete genome sequence of the first virus infecting an ectomycorrhizal fungus. Mycoviruses, most of which have double-stranded (ds) RNA genomes, are currently not known to have any natural vectors. They are transmitted intracellularly by anastomosis of the hyphae or by heterokaryosis (lateral transmission), or they are spread via spores (serial transmission) [6, 7]. Cytoplasmatic exchange in different periods of the fungal life-cycle allows highly efficient virus transmission via different types of fungal propagules, such as sexually and asexually produced spores. Virus-infected fungi may show symptoms such as abnormal colony morphology or sporocarps [7] but in most cases remain symptomless. In contrast, mycoviruses can be responsible for attenuation and enhancement of fungal virulence itself (hypo- and hypervirulence), especially in the well-studied group of phytopathogenic fungi [2, 7, 15]. The commercial importance of several saprophytic and phytopathogenic fungi, such as Cryphonectria parasitica, which causes chestnut blight; Ustilago maydis, which causes corn smut, Botrytis cineria; which causes spoilage of strawberries and grapes; and the commonly important white mushroom, Agaricus bisporus, has consequently focused research on commercially relevant fungi to develop biocontrol agents to prevent massive loss of crop yields [10, 15, 19, 21]. In contrast, the mycorrhizas, which have a profound impact on the colonization of terrestrial life, have so far been screened only rudimentarily for the presence of viruses. Considering the extraordinary number of mycorrhizal fungi, it seems odd that ectomycorrhizal macrofungi, which are present in almost any global forest ecosystem, have received much less attention as a source of viruses than other organisms. Here, we report the entire genome sequence of a putatively new totivirus. The virus reported here was isolated from the ascocarp of the hypogeous ectomycorrhizal fungus Tuber aestivum, which is commonly known as black noble truffle, and the name Tuber aestivum virus 1 (TaV1) is proposed. Accordingly, these findings provide the first evidence for the presence of mycoviruses in ectomycorrhizal fungi belonging to the largest fungal phylum (Ascomycota). However, Bai et al. [1] were the first to isolate a smaller dsRNA of 2.2 kbp from the ectomycorrhizal basidomycete Hebeloma circinans (Cortinariaceae). The genomes of members of the family Totiviridae typically contain two open reading frames (ORFs) that can be translated as a single fusion-protein by a −1 frameshift. The predicted proteins encoded by ORF1 and ORF2 show sequence motifs characteristic of viral coat proteins (CP) and viral RNA-dependent RNA polymerases (RdRp), respectively. …