REQUIREMENT OF SENSE TRANSCRIPTION FOR HOMOLOGY-DEPENDENT VIRUS-RESISTANCE AND TRANS-INACTIVATION

Transgenic tobacco plants carrying post-transcriptionally silenced (L) or conditionally high-expressing (H-c) cauliflower mosaic virus 35S p romoter:beta-glucuronidase (GUS) transgenes were tested for their abil ity to support replication of a potato virus X derivative with an inse rted GUS coding sequence (PVX.GUS). PVX.GUS accumulated to high levels on non-transformed tobacco. In contrast, both L and H-c tobacco were resistant to PVX.GUS. Another line, 271, carries a 35S-driven transgen e that can downregulate transcription from any other 35S promoter in t rans. When the 271 locus was introduced into the L and H-c lines, viru s resistance was suppressed. The L tobacco line also trans-inactivated transient GUS expression from an extra-chromosomal T-DNA delivered by Agro-bacterium. This trans-inactivation was not observed when the 271 locus was present. In addition, it was found that the 271 locus suppr essed the virus resistance normally conferred by a 35S-driven transgen e derived from the PVX replicase open reading frame. These results ind icate that sense transcription is required for a transgene to conditio n homology-dependent Virus resistance and post-transcriptional trans-i nactivation.