Abstract
Magnaporthe oryzae, the fungus that causes rice blast, is the most destructive pathogen of rice worldwide. A number of M. oryzae mycoviruses have been identified. These include Magnaporthe oryzae. viruses 1, 2, and 3 (MoV1, MoV2, and MoV3) belonging to the genus, Victorivirus, in the family, Totiviridae; Magnaporthe oryzae. partitivirus 1 (MoPV1) in the family, Partitiviridae; Magnaporthe oryzae. chrysovirus 1 strains A and B (MoCV1-A and MoCV1-B) belonging to cluster II of the family, Chrysoviridae; a mycovirus related to plant viruses of the family, Tombusviridae (Magnaporthe oryzae. virus A); and a (+)ssRNA mycovirus closely related to the ourmia-like viruses (Magnaporthe oryzae. ourmia-like virus 1). Among these, MoCV1-A and MoCV1-B were the first reported mycoviruses that cause hypovirulence traits in their host fungus, such as impaired growth, altered colony morphology, and reduced pigmentation. Recently we reported that, although MoCV1-A infection generally confers hypovirulence to fungi, it is also a driving force behind the development of physiological diversity, including pathogenic races. Another example of modulated pathogenicity caused by mycovirus infection is that of Alternaria alternata chrysovirus 1 (AaCV1), which is closely related to MoCV1-A. AaCV1 exhibits two contrasting effects: Impaired growth of the host fungus while rendering the host hypervirulent to the plant, through increased production of the host-specific AK-toxin. It is inferred that these mycoviruses might be epigenetic factors that cause changes in the pathogenicity of phytopathogenic fungi.
Highlights
Similar to animals and plants, fungi are often infected by viruses
Magnaporthe oryzae chrysovirus 1 (MoCV1)-A and MoCV1-B viral proteins were detected in the cell-free culture supernatant [17,18]. We investigated this phenomenon in the following additional mycoviruses: Magnaporthe oryzae virus 2 (MoV2) [46], Alternaria alternata victorivirus 1 (AaVV1) [47], Alternaria alternata virus 1 (AaV1) [48], alternata chrysovirus 1 (AaCV1), and L-A virus of Saccharomyces cerevisiae [49]
We discovered that infection of the Japanese pear pathotype fungus, Alternaria alternate, with Alternaria alternata chrysovirus 1 (AaCV1) simultaneously impaired growth of the host fungus and increased levels of the host-specific AK-toxin [38,57]
Summary
Similar to animals and plants, fungi are often infected by viruses. Mycoviruses that infect plant pathogenic fungi were initially discovered in the rice blast fungus, Magnaporthe oryzae [6], and have. Similar to animals and plants, many fungi have antiviral capabilities, which act through RNA interference mechanisms targeting dsRNA degradation [7]. About 8000 phytopathogenic fungi have been reported in Japan alone and they are the most common type of plant pathogen, over 10 times more frequent than viruses and bacteria [13]. We discuss MoCV1, a mycovirus that causes growth inhibition in the rice blast fungus, M. oryzae. We focus on its molecular genetic characteristics, the influence of viral proteins on host cells, and our methodology of investigating physiological activity using a yeast heterologous expression system
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