SYSTEMIC ACQUIRED SILENCING - TRANSGENE-SPECIFIC POSTTRANSCRIPTIONAL SILENCING IS TRANSMITTED BY GRAFTING FROM SILENCED STOCKS TO NON-SILENCED SCIONS

Using grafting procedures, we investigated the transmission of co-supp ression of nitrate reductase and nitrite reductase host genes and tran sgenes and of posttranscriptional silencing of a uidA transgene encodi ng glucuronidase in tobacco, We demonstrate that silencing is transmit ted with 100% efficiency from silenced stocks to non-silenced scions e xpressing the corresponding transgene, Transmission is unidirectional from stock to scion, transgene specific, locus independent and require s the presence of a transcriptionally active transgene in the target s cion, The transmission of cosuppression occurs when silenced stocks an d nonsilenced target scions are physically separated by up to 30 cm of stem of a non-target wild-type plant, Taken together, these results s uggest that a non-metabolic, transgene-specific, diffusable messenger mediates the propagation of de novo post-transcriptional silencing thr ough the plant.