Pathogen-derived resistance targeted against the negative-strand RNA of tobacco mosaic virus: RNA strand-specific gene silencing?

Tobacco plants transformed with the open-reading frame (ORF) of tobacco mosaic virus strain U1 (TMV-U1) encoding a 54 kDa (54K) region of the viral replicase are resistant against TMV strain U1. These plants are not resistant against the crucifer strain of TMV or the heterologous virus, potato virus X (PVX). However, they are resistant against derivatives of PVX containing fragments of the 54K ORF inserted either in the sense or anti-sense orientation. The smallest fragment that was a target of the resistance mechanism was a 383 nucleotide region from the central part of the 54K ORF. A transient gene expression assay revealed that this central region was also the target of a post-transcriptional gene silencing mechanism. However, unlike other examples of gene silencing associated with virus resistance, the silencing was specific for the anti-sense rather than the coding strand of the target RNA. Based on these data the authors propose that the TMV resistance is due, at least in part, to a type of transgene silencing.