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

Abstract

SummaryUsing 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 ef®ciently 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 speci®cities 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 speci®city of PTGS induced in non-silenced scions after grafting was determined by that in silenced root stocks. However, unexpectedly, target speci®city of PTGS induced in silenced scions after grafting was not changed by grafting onto silenced root stocks showing different target speci®city, indicating that the second PTGS from silenced root stocks was not superimposed to silenced scions.

Virus-encoded suppressor of posttranscriptional gene silencing targets a maintenance step in the silencing pathway

“…In plants, PTGS of transgenes is typically associated with methylation of nuclear DNA corresponding to the transcribed region of the target RNA, although transcription levels of the transgene are generally unaffected (1). In addition, systemic signaling to trigger PTGS at a distance can occur in plants, presumably through transport of a nucleic acid signal (7)(8)(9). Several genes encoding factors required for PTGS or RNA interference have been isolated, and these include proteins with similarities to an RNA-dependent RNA polymerase (5,10,11), a RecQ-like DNA helicase (10), an RNaseD-like protein (12), and a protein encoded by the piwi͞sting͞argonaute͞zwille͞eIF2C gene family (13).…”

A Cucumber Mosaic Virus (CMV) RNA 1 Transgene Mediates Suppression of the Homologous Viral RNA 1 Constitutively and Prevents CMV Entry into the Phloem

“…2 is not unusual (20,26). By contrast, PTGS of viral vectors usually shows a reduction of accumulation of 1to 2 orders of magnitude (20,34,41). Thus, these results indicate that the presence of CMV RNA 1 sequences alone is not sufficient to suppress the accumulation of an RNA containing such sequences.…”

Spreading of RNA silencing and DNA methylation in transgenic hybrid plants with the coat protein gene of Sweet potato feathery mottle virus

“…In previous study we described that RNA silencing as well as transitive silencing was induced in non-silenced scions grafted onto silenced rootstocks using transgenic N. benthamiana carrying the entire or 5′ one third of the coat protein gene (CP) of Sweet potato feathery mottle virus (SPFMV) (Haque et al 2007a, Sonoda andNishiguchi 2000). Similar transitive silencing was observed by crossing between these plant lines (Haque et al 2007a).…”

“…Moreover, spreading of DNA methylation in association with the transitive RNA silencing outside the initial target has also been reported in transgenic plants (Jones et al 1999, Vaistij et al 2002, Van Houdt et al 2003. However, RNA silencing was induced but transgene methylation level was not changed in non-silenced scions when grafted onto silenced rootstocks (Mallony et al 2002, Sonoda andNishiguchi 2000). From this point of view, RNA silencing and DNA methylation seem to be independent from each other, and dependent upon the system used (grafting versus agroinfiltration, crossing and virus induced gene silencing).…”