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Transgenes are dispensable for the RNA degradation step of cosuppression

The National Academy of Sciences
Publication Date
  • Biological Sciences


Cosuppression results in the degradation of RNA from host genes and homologous transgenes after transcription in the nucleus. By using grafting experiments, we have shown previously that a systemic signal mediates the propagation of cosuppression of Nia host genes and 35S-Nia2 transgenes from silenced 35S-Nia2 transgenic stocks to nonsilenced 35S-Nia2 transgenic scions but not to wild-type scions. Here, we examined the requirements for triggering and maintenance of cosuppression in various types of scions. Grafting-induced silencing occurred in 35S-Nia2 transgenic lines over-accumulating Nia mRNA whether they are able to spontaneously trigger cosuppression or not and in 35S-Nia2 transgene-free plants over-accumulating host Nia mRNA caused by metabolic derepression. When grafting-induced silenced scions were removed from the silenced stocks and regrafted onto wild-type plants, silencing was not maintained in the 35S-Nia2 transgene-free plants and in the 35S-Nia2 transgenic lines that are not able to trigger cosuppression spontaneously. Conversely, silencing was maintained in the 35S-Nia2 transgenic lines that are able to trigger cosuppression spontaneously. Our results indicate that the presence of a 35S-Nia2 transgene is dispensable for the RNA degradation step of posttranscriptional silencing when host Nia mRNA over-accumulate above the level of wild-type plants. They also suggest that grafting-induced RNA degradation does not result in the production of the systemic silencing signal required for spontaneous triggering and maintenance.

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