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 <i>Tobamovirus</i> genus in the <i>Virgaviridae</i> 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 <i>Nigrospora aurantiaca</i>, 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 <i>Virgaviridae</i> viruses. ORF3 encodes a putative movement protein, and ORF4 encodes a putative coat protein, which are closely related to proteins of <i>Gammaflexiviridae</i> and <i>Betaflexiviridae</i> viruses. NaTLV1 formed filamentous virus particles and could replicate in <i>Nicotiana benthamiana</i>. However, the cell-to-cell movement of NaTLV1 was not observed in <i>N. benthamiana</i> 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) <i>N. aurantiaca</i> strains. In general, our findings may be useful in elucidating the origination and evolution of tobamo-like viruses, plant virus families (<i>Virgaviridae</i> and <i>Betaflexiviridae</i>), and a fungal virus family (<i>Gammaflexiviridae</i>).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 <i>Nigrospora aurantiaca</i>. NaTLV1 was phylogenetically related to other tobamo-like viruses, plant-associated viruses in the <i>Virgaviridae</i> and <i>Betaflexiviridae</i> families, and fungus-associated viruses in the <i>Gammaflexiviridae</i> family. NaTLV1 formed filamentous virus particles and could replicate in <i>Nicotiana benthamiana,</i> 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.