A novel tobamo-like mycovirus with filamentous particles replicates in plant cells

Parasitic and symbiotic plant-fungus relationships have existed for millions of years, and phylogenetic analyses of certain virus families indicate transmission between plants and fungi. A group of tobamo-like viruses from various fungi has recently been identified. Tobamo-like viruses are homologous to plant viruses of the Tobamovirus genus in the Virgaviridae family, but it was unknown whether they form typical virus particles and can replicate and exhibit cell-to-cell movement in plants. Here, a novel tobamo-like virus, Nigrospora aurantiaca tobamo-like virus 1 (NaTLV1), obtained from the phytopathogenic fungus Nigrospora aurantiaca, was characterized. Its genome (10,301 nucleotides) comprises four open reading frames (ORFs) and a poly(A) tail. ORF1 encodes a methyltransferase and helicase, and ORF2 encodes RNA-dependent RNA polymerase, which are closely related to proteins of Virgaviridae viruses. ORF3 encodes a putative movement protein, and ORF4 encodes a putative coat protein, which are closely related to proteins of Gammaflexiviridae and Betaflexiviridae viruses. NaTLV1 formed filamentous virus particles and could replicate in Nicotiana benthamiana. However, the cell-to-cell movement of NaTLV1 was not observed in N. benthamiana but could not be ruled out. NaTLV1 underwent horizontal and vertical transmissions via hyphal anastomosis and conidia, respectively. There were no significant differences in phenotype or virulence between NaTLV1-infected (A4) and NaTLV1-free (V1) N. aurantiaca strains. In general, our findings may be useful in elucidating the origination and evolution of tobamo-like viruses, plant virus families (Virgaviridae and Betaflexiviridae), and a fungal virus family (Gammaflexiviridae).IMPORTANCECross-kingdom infections involving plant- and fungal-associated viruses have been directly observed in nature, and some of these viruses share a high degree of genetic similarity. A group of novel tobamo-like viruses with nonsegmented single-stranded RNA genomes was recently isolated from diverse fungal groups. Here, we identified a novel virus, Nigrospora aurantiaca tobamo-like virus 1 (NaTLV1), in the phytopathogenic fungus Nigrospora aurantiaca. NaTLV1 was phylogenetically related to other tobamo-like viruses, plant-associated viruses in the Virgaviridae and Betaflexiviridae families, and fungus-associated viruses in the Gammaflexiviridae family. NaTLV1 formed filamentous virus particles and could replicate in Nicotiana benthamiana, but the cell-to-cell movement of NaTLV1 was not observed. This study provides potential insights into the origins and evolution of mycoviruses originating from plant viruses and indicates that tobamo-like viruses may have adapted to an intracellular lifestyle during evolution. The identification of novel tobamo-like viruses is crucial for understanding fungal and plant virus evolution.