Systemic acquired silencing: Transgene-specific post-transcriptional silencing is transmitted by grafting from silenced stocks to non-silenced scions

Using grafting procedures, we investigated the transmission of co-suppression of nitrate reductase and nitrite reductase host genes and transgenes and of posttranscriptional silencing of a uidA transgene encoding glucuronidase in tobacco, We demonstrate that silencing is transmitted with 100% efficiency from silenced stocks to non-silenced scions expressing the corresponding transgene, Transmission is unidirectional from stock to scion, transgene specific, locus independent and requires the presence of a transcriptionally active transgene in the target scion, The transmission of cosuppression occurs when silenced stocks and nonsilenced target scions are physically separated by up to 30 cm of stem of a non-target wild-type plant, Taken together, these results suggest that a non-metabolic, transgene-specific, diffusable messenger mediates the propagation of de novo post-transcriptional silencing through the plant.