Suppression of gene silencing: A threat to virus-resistant transgenic plants?

Abstract

Post-transcriptional gene silencing (PTGS) – a sequence-specific RNA degradation mechanism – is considered to be responsible for immunity to viral infection in many transformed plants that carry homologous viral transgene sequences 1. Current research on insect, nematode, fungal and plant model systems is constantly clarifying PTGS pathways 2, 3. A significant development has been the discovery of virus-encoded suppressors of PTGS. The ability of plant viruses to suppress PTGS probably evolved as a counter-defence strategy against the RNA-specific PTGS defence mechanism that is inherent in plants. When the HC-Pro protein of tobacco etch potyvirus 4, the 2b protein of cucumber mosaic virus (CMV) 5, 6 and gene products of several other RNA and DNA viruses 7 are expressed in plants, they can suppress PTGS of transgenic marker genes. This led us to investigate what might happen if a PTGS-based virus-immune transgenic plant becomes infected by another virus encoding a suppressor of PTGS. We found that such immune plants could revert to a susceptible phenotype following infection by a heterologous virus. This observation questions the stability and utility of engineered RNA-mediated virus resistance in transgenic crops under field conditions in which plants might be exposed to various viral pathogens.