Viruses that infect plants are responsible for reduction in both yield and quality of crops around the world, and thus are of great economic importance. This has provided the impetus for the extensive research into the molecular and cellular biology of these pathogens and into their interaction with their plant hosts and their vectors. However, interest in plant viruses extends beyond their ability to damage crops. Many plant viruses - for example, tobacco mosaic virus - have been used as model systems to provide basic understanding of how viruses express genes and replicate. Others permitted...