Umbravirus

The genomes of umbraviruses consist of one linear segment of positive-sense single-stranded RNA (ssRNA). Umbraviruses differ from other plant viruses in that they do not encode a coat protein (CP), and no viral particles are formed in infected plants. To compensate for the lack of virus particles, umbraviruses depend for survival on an assistor virus, which is always a member of the family Luteoviridae. For transmission between plants, the CP of the luteovirus forms aphid-transmissible hybrid virus particles encapsidating umbraviral RNA. Satellite RNAs are also associated with some umbraviruses. Umbravirus genomes encode an RNA-dependent RNA polymerase and two other proteins. One of these is a cell-to-cell movement protein that can mediate transport of viral RNA through plasmodesmata. The other, the ORF3 protein, binds to RNA to form filamentous ribonucleoprotein (RNP) particles. This serves to stabilize RNA and facilitate its long-distance movement through the phloem. The ORF3 protein enters the cell nucleus targeting the nucleolus. There is a correlation between the ORF3 protein nucleolar localization and its ability to form the RNP particles and transport viral RNA long distances. Information on the biological properties of umbraviruses, such as host range and mode of aphid transmission as well as on the control measures, is presented