Molecular and biological features of umbraviruses, the unusual plant viruses lacking genetic information for a capsid protein

Abstract

The genus Umbravirus comprises plant viruses that replicate in their host plants and encode a movement protein-like product, but are unusual in that they lack genetic information for a capsid protein. For aphid transmission, umbraviruses depend on suitable luteoviruses, which act as the assistor viruses. In mixed infection the umbravirus RNA is encapsidated by the capsid protein provided from the luteovirus partner and the virion so composed is readily transmitted by an aphid vector. Umbraviruses and their assistor viruses are infectious as independent entities, but they associate in the host plants to produce specific complex diseases. Some of these diseases have highly destructive effects on crop plants. The recent progress in studies upon molecular features of plant viruses gives a chance to recognize and examine umbraviruses in more detail. This review shows the development of studies upon umbraviruses and describes the distinctive biological and molecular features of these unusual viral agents.