Persistent plant viruses: Molecular hitchhikers or epigenetic elements?

Many plants harbor persistent cytoplasmic viruses that are not transmitted horizontally and do not move from cell to cell. These viruses have extensive longevity within individual plant cultivars. Based on phylogenetic evidence they appear to undergo rare transmission events between plants and fungi. Very few functions have been attributed to persistent viruses in plants, but their longevity and protection from the plant's immune system suggest that they provide a selective advantage for their hosts, at least under some conditions. In addition, some persistent plant virus sequences have been found in plant genomes and are expressed as functional genes. Hence, rather than simply molecular hitchhikers, they may be cytoplasmic epigenetic elements that could provide genetic information to their plant hosts.