Unit of Cell Biology, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK

Background and objectives
Viruses, as obligate genetic parasites, are dependent on productive interactions with their hosts for every step of the infection process. Following a successful challenge a virus will subvert the host's biosynthetic machinery to direct the accumulation and spread of progeny virus. We have used molecular biological and cell biological techniques to study the pathogenic process in potato virus X (PVX)-infected plants and to investigate the key interactions between host and pathogen required for successful infection.

Results and conclusions
Within PVX-infected cells, viral activity is spatially restricted to inclusion bodies. These virus-induced structures represent the site of viral replication and assembly and are rich in both host and viral constituents. The organization of inclusion bodies was investigated in cells infected with PVX mutants, allowing the contribution of different viral proteins to the development of the inclusion bodies to be determined. For cell-to-cell movement to occur, virus must leave the inclusion body and move intracellularly to plasmodesmata. The cell-to-cell and long-distance movement process of PVX was studied using a modified viral genome expressing the green fluorescent protein as a fusion to the viral coat protein. This virus assembles into particles coated with the green fluorescent protein, allowing the infection to be monitored at the cellular and subcellular levels [1]. Of the five proteins encoded by PVX, one is required for replication and the remaining four viral proteins are essential for cell-to-cell movement. PVX mutants, lacking one or more of these 'movement' proteins, were used to demonstrate that the coat protein, although essential for cell-to-cell movement, is unable either to modify plasmodesmata or traffic between cells in the absence of other virus-coded proteins. However, in the course of infection the coat protein is transported between cells, most probably as an assembled virion [2].

1. Santa Cruz S, Chapman S, Roberts AG et al., 1996. Proceedings of the National Academy of Sciences, USA 93, 6286-6290.
2. Santa Cruz S, Roberts AG, Prior DAM et al., 1998. Plant Cell, in press.