A counterdefensive strategy of plant viruses: suppression of posttranscriptional gene silencing.

Posttranscriptional gene silencing (PTGS) in plants inactivates some aberrant or highly expressed RNAs in a sequence-specific manner in the cytoplasm. A silencing mechanism similar to PTGS appears to function as an adaptive antiviral response. We demonstrate that the P1/HC-Pro polyprotein encoded by tobacco etch virus functions as a suppressor of PTGS. A locus comprised of a highly expressed beta-glucuronidase (GUS) transgene was shown to exhibit PTGS. Genetic crosses and segregation analyses revealed that a P1/ HC-Pro transgene suppressed PTGS of the GUS sequence. Nuclear transcription assays indicated that the silencing suppression activity of P1/HC-Pro was at the posttranscriptional level. These data reveal that plant viruses can condition enhanced susceptibility within a host through interdiction of a potent defense response.