Graft transmission of post-transcriptional gene silencing: target specificity for RNA degradation is transmissible between silenced and non-silenced plants, but not between silenced plants.

Using the grafting procedure, we examined the transmission of post-transcriptional gene silencing (PTGS) in Nicotiana benthamiana which had been transformed with the coat protein gene, including the 3' non-translated region of the sweet potato feathery mottle potyvirus. Transmission of PTGS from silenced lines to non-silenced ones was bidirectional, but occurred efficiently from root stocks to scions. The level of transgene methylation in non-silenced scions grafted onto silenced root stocks was not increased. When grafted scions which had become silenced were removed from silenced root stocks and regrafted onto non-silenced or vector-transformed root stocks, PTGS was maintained. However, their progeny did not show PTGS. Previously we reported that our transgenic lines had different target specificities of PTGS for RNA degradation: one line recognized only the 3' part of the transgene mRNA while others involved the whole transgene mRNA (Sonoda et al. 1999, Phytopathology, 89, 385-391). Using these lines, we showed that target specificity of PTGS induced in non-silenced scions after grafting was determined by that in silenced root stocks. However, unexpectedly, target specificity of PTGS induced in silenced scions was not changed [corrected] by grafting onto silenced root stocks showing different target specificity, indicating that the second PTGS from silenced root stocks was not superimposed to silenced scions.