Sainsbury Laboratory, Norwich Research Park, Colney Lane, Norwich, Norfolk NR4 7UH, UK

Background and objectives
Plants employ various types of resistance mechanism against viruses. Some of these are mechanisms particular to certain types of virus, as exemplified by the resistance in Arlington cowpea against cowpea mosaic virus [1]. Other resistance mechanisms have the potential to act more generally against a broad spectrum of viruses and even non-viral pathogens. These types of resistance mechanism are often affected by dominant resistance genes. A third type of resistance mechanism, for which recent evidence is emerging, is targeted against the viral RNA. This type of RNA-mediated natural resistance mechanism may have been exploited inadvertently in transgenic plants carrying viral transgenes.

Results and conclusions
Examples will be given of resistance gene-mediated resistance and RNA-mediated resistance based on studies with potato virus X. From the studies with mechanisms controlled by the Rx and Nb resistance genes, it has become clear that virus resistance mechanisms are related to resistance mechanisms against non-viral pathogens. The Rx gene shares sequence similarity with genes conferring resistance against bacterial pathogens and, in both cases, the genetics and cell biology of the resistance mechanisms indicate mechanistic similarities of defence against viral and non-viral pathogens. The RNA-mediated natural resistance mechanism was discovered first in plants infected with cauliflower mosaic virus [2] and nepoviruses [3]. Similarities between this resistance mechanism and the mechanism of gene silencing in transgenic plants indicates that the two are the same. Recent studies have shed light on the underlying mechanism of gene silencing and are allowing us to find out whether gene silencing is exploited more generally as a means of attenuating virus accumulation in infected plants.

1. Ponz F, Glascock CB, Bruening G, 1988. Molecular Plant-Microbe Interactions 1, 25-31.
2. Covey SN, Al-Kaff NS, Langara A, Turner DS, 1997. Nature. 385, 781-782.
3. Ratcliff F, Harrison BD, Baulcombe DC, 1997. Science 276, 1558-1560.