Plant viruses Historical aspects

The  early  studies on  infectious diseases in the  late  19th  century quickly  demonstrated that microscopic organisms were  the  cause of many illnesses, and  a number of bacteria were  isolated by filtering the  lysate  of diseased cells and  examining them by cultivation  and  microscopy. However, several diseases failed to reveal an agent that could be grown or seen with the microscopes of the day, and yet diseased cell lysates could faithfully transmit the symptoms to new hosts. Many  of these first steps into  the discovery of viruses (the word ‘virus’ comes from the Latin for ‘poison’) were done using plant viruses and  our  modern understanding of virus  structure, replication, assembly, and  even  the nature of virus  genetics was  derived from  studies of the  tobacco mosaic virus  ( TMV ). The virus, which causes mosaic patterning of the leaves of the tobacco plant, was first described in 1892. TMV was the first virus to be observed under the electron microscope, the first to demonstrate the intrinsic infectivity of a naked viral genome, and the first virus to be assembled in vitro from purified preparations of viral RNA and coat protein molecules. Studies with turnip yellow mosaic virus and tomato bushy stunt virus revealed the morphological details of icosahedral viruses. Ironically, in more recent times the molecular study of plant viruses has trailed behind that of animal viruses, as plant cells are more difficult to culture than animal cells.

Plant viruses

Plant viruses have traditionally been named with a combination of the host plant and the type of disease produced (e.g. tobacco mosaic, turnip yellow mosaic, tomato bushy stunt, cauliflower mosaic, and tomato spotted wilt). Plant viruses are diverse in their morphology, nucleic acid composition, and replication. It is beyond the scope of this book to detail all viruses but Figures highlights the various morphologies and groupings of plant viruses. They may be dsDNA (Badnaviridae, e.g. rice tungrobacilliform virus), ssDNA (Geminivir­ idae, e.g. maize streak virus), dsRNA (Reoviridae, e.g. clover wound tumor virus) or ssRNA viruses of positive (Comoviridae, e.g. tobacco ringspot virus) or negative (Rhabdoviridae, e.g. potato yellow dwarf virus) sense. Like animal viruses, plant virus genomes can be segmented or nonsegmented, and enclosed in helical or icosahedral capsids, although several have amorphous capsids with no defined shape. Envelopes are found only within the Bunyaviridae and Rhabdoviridae families and there are no Class VI plant viruses.