TRANSGENES ARE DISPENSABLE FOR THE RNA DEGRADATION STEP OF COSUPPRESSION

Cosuppression results in the degradation of RNA from host genes and ho mologous transgenes after transcription in the nucleus. By using graft ing experiments, we have shown previously that a systemic signal media tes the propagation of cosuppression of Nia host genes and 35S-Nia2 tr ansgenes from silenced 35S-Nia2 transgenic stocks to non-silenced 35S- Nia2 transgenic scions but not to wild-type scions. Here, we examined the requirements for triggering and maintenance of cosuppression in va rious 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-f ree plants over-accumulating host Nia mRNA caused by metabolic derepre ssion. When grafting-induced silenced scions were removed from the sil enced stocks and regrafted onto wild-type plants, silencing was not ma intained in the 35S-Nia2 transgene-free plants and in the 35S-Nia2 tra nsgenic lines that are not able to trigger cosuppression spontaneously . Conversely, silencing was maintained in the 35S-Nia2 transgenic line s that are able to trigger cosuppression spontaneously. Our results in dicate that the presence of a 35S-Nia2 transgene is dispensable for th e RNA degradation step of posttranscriptional silencing when host Nia mRNA over-accumulate above the level of wild-type plants. They also su ggest that grafting-induced RNA degradation does not result in the pro duction of the systemic silencing signal required for spontaneous trig gering and maintenance.