Resistance mechanisms to plant viruses: an overview.

To obtain virus-resistant host plants, a range of operational strategies can be followed nowadays. While for decades plant breeders have been able to introduce natural resistance genes in susceptible genotypes without knowing precisely what these resistance traits were, currently a growing number of (mostly) dominant resistance genes have been cloned and analyzed. This has led not only to a better understanding of the plant's natural defence systems, but also opened the way to use these genes beyond species borders. Besides using natural resistance traits, also several novel, "engineered" forms of virus resistance have been developed over the past 15 years. The first successes were obtained embarking from the principle of pathogen-derived resistance (PDR) by transforming host plants with viral genes or sequences with the purpose to block a specific step during virus multiplication in the plant. As an unforeseen spin-off of these investments, the phenomenon of post-translational gene silencing (PTGS) was discovered, which to date is by far the most successful way to engineer resistance. It is generally believed that PTGS reflects a natural defence system of the plant, and part of the hypothesized components required for PTGS have been identified. As counteracting strategy, and confirming PTGS to be a natural phenomenon, a considerable number of viruses have acquired gene functions by which they can suppress PTGS. In addition to PDR and PTGS, further strategies for engineered virus resistance have been explored, including the use of pokeweed antiviral protein (PAP), 2',5'-oligoadenylate synthetase and "plantibodies". This paper will give a brief overview of the major strategies that have become operational during the past 10 years.