| Rice false smut caused by Ustilaginoidea virens (teleomorph:Villosiclava virens) is a fungal disease infecting rice panicle. In recent years, because of high yielding hybrid rice varieties and improvement of fertilizer levels, the occurrence of rice false smut increased year by year. Mycoviruses are commonly found in all major taxa belonging to the kingdom Fungi, but always be ignored. There is little information regarding mycoviruses in U. virens, and previous studies have concentrated on single isolates of U. virens. There have been no reports describing the populations of mycovriuses infecting U. virens.1. In this study,198 U. virens isolates collected from different geographical regions (Guangxi, Guizhou, Hunan and Fujian Provinces in mountainous areas; Hubei, Hebei and Zhejiang Provinces in lowland regions) in China are examined for dsRNA elements. Result reveals a high occurrence:94.9% of 198 isolates appeare to harbor dsRNAs with viral characteristics, and the dsRNA elements show various electrophoretic profiles. Furthermore, mixed mycoviral infections seem frequent because only 7 of the 188 virus-infected isolates have only a single dsRNA band, whereas others containe multiple dsRNA elements in this survey. The number of dsRNA bands per fungal isolate varies from 1 to 11 and ranges in size from 0.5- to 6.0 kb.77.8% of the tested isolates contain a 5.5 kb dsRNA band.29.3% of the tested isolates contain a 3.1 kb dsRNA element. In addition, several dsRNA elements (1.8-, 1.7-,1.5-, and 1.2 kb) are often present together or in combinations in isolates of different regions. Several approaches are used to eliminate mycoviruses from the isolate GX-1, including single conidial isolation, protoplast isolation and regeneration, thermotherapy and chemotherapy (incorporation of ribavirin in the culture medium). Regardless of the treatments applied, the dsRNA elements are retained in GX-1. These results suggest that these mycoviruses are highly stable in U. virens, and the mycoviruses infecting U. virens are widespread and highly diverse.2. The GX-1 isolate is chosen for further characterization because this isolate contain dsRNA elements of varying size, including major elements characterized as large (ca.5.5 kb), medium (ca.3.1 kb), and small (ca.1.8-,1.7-,1.6-,1.5- and 1.2 kb). The GX-1 isolate containes four dsRNA mycoviruses. Ustilaginoidea virens RNA virus 1 (UvRV1) corresponding to 5.5 kb dsRNA element, which belongs to Totiviridae. Ustilaginoidea virens RNA virus 4 (UvRV4) corresponding to 3.1 kb dsRNA element, which belongs to an unclassified family includes the Heterobasidion RNA virus 6 (HetRV6). However, a low level of sequence similarity is also detected with sequences in the Partitiviridae and Potyviridae families. And the last two corresponding to 1.8-,1.7-,1.6-,1.5- and 1.2 kb dsRNA elements, which probably belong to Partitiviridae.3. Seventeen isolates that containe a 5.5 kb dsRNA and 17 isolates that containe a 3.1 kb dsRNA element element are randomly selected.11 of the 17 isolates of U. virens harboring the 5.5 kb dsRNA element hybridize with the UvRV1 strain GX-1 probe. Moreover,13 of the 17 isolates harboring the 3.1 kb dsRNA element hybridize with the UvRV4 probe.4. After horizontally transmitted, we obtain a derivative strain containing UvRV1 strain GX-1 and UvRV4, another derivative strain containing UvRV4. Biological comparisons of virus-free and infected fungal isolates reveal that UvRVl strain GX-1 and UvRV4 are likely cryptic, since the infect strains do not show apparent symptoms or debilitation.5. The bisegmented genome of a putative double-stranded (ds) RNA virus from Ustilaginoidea virens isolate HP-30 is sequenced and analyzed. The larger genomic segment encodes a putative RNA-dependent RNA polymerase (RdRp), and the smaller one encodes a putative coat protein (CP). Phylogenetic analysis indicates that this novel partitivirus, named Ustilaginoidea virens partitivirus 2 (UvPV2), can be assigned to the family Partitiviridae. |
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