[Current situation and prospect of studies on genus Umbravirus]

Umbraviruses are a group of imperfectly characterized plant viruses, which are distinguished from most other viruses by their genomes lack of a gene for coat protein (CP) , and as a result umbraviruses do not form conventional virus particles. Umbraviruses are mechanically transmissible, and can be aphid transmitted in the persistent manner by an unrelated assistor virus, which is always a member of the family Luteoviridae . In nature, each umbravirus depends for survival on one particular luteovirus. The genus Umbravirus comprises seven distinct virus specieses and three tentative members. Only Tobacco bushy top virus (TBTV) has been reported in China as an umbravirus. Tobacco bushy top disease, caused by TBTV and its helper, Tobacco vein distorting virus(TVDV), which resulted in severe tobacco losses in western of Yunan. Umbraviruses had a restricted host range in nature, and their infectivity and longevity in vitro are not so stable. Plants infected umbraviruses contain abundant double-stranded RNA (dsRNA), and some umbraviruses possess one or more additional dsRNA species associated with the presence of a satellite RNA. The genomes of the umbraviruses consist of one linear segment of positive sense single-stranded RNA(ssRNA), and the nucleotide sequences possess ORFs for four potential non-structural protein products. The umbravirus-encoded ORF3 proteins play essential roles in stabilization of viral RNA and mediation of its long-distance movement. The current research progresses have been reviewed detailly, and the future research tendency and research fields about umbraviruses and umbravirus-caused diseases are put forward.