Abstract
Homology-dependent gene silencing contributes to variation between transgenic plants with respect to transgene and/or endogenous gene expression levels. Recent studies have linked post-transcriptional gene silencing and virus resistance in plants expressing virus-derived transgenes. Using a potato virus X vector, we present three examples in which silencing of nonviral transgenes prevented virus accumulation. This effect was dependent on sequence homology between the virus and the silenced transgene. Analysis of potato virus X derivatives carrying bacterial [beta]-glucuronidase (GUS) sequences showed that the 3[prime] region of the GUS coding sequence was a target of the silencing mechanism in two independent tobacco lines. Methylation of the silenced GUS transgenes in these lines was also concentrated in the 3[prime] region of the GUS coding sequence. Based on this concurrence, we propose a link between the DNA-based transgene methylation and the RNA-based gene silencing process.