BROAD RESISTANCE TO TOBAMOVIRUSES IS MEDIATED BY A MODIFIED TOBACCO MOSAIC-VIRUS REPLICASE TRANSGENE

Tobacco plants made transgenic to express the wild type tobacco mosaic virus (TMV) 183-kDa replicase gene were not resistant to TMV. However , transgenic plants containing essentially the same sequences, but wit h an additional insertion that would terminate translation in the midd le of the 183-kDa gene, were highly resistant to systemic infection by TMV and other tobamoviruses. The 1.4-kbp insertion in the replicase o pen reading frame (ORF) of the resistant plants was shown by DNA seque ncing to be an IS10-like transposable element, which apparently insert ed itself into the TMV sequence at nucleotide 2875 sometime during the propagation of this replicase ORF plasmid (pREP21). Because of four s top codons, in frame with the TMV replicase ORF on the immediate 5' bo rder of the IS insertion, REP21 effectively represents domain 1 (putat ive methylase domain) and a portion of domain 2 (putative helicase dom ain) of the TMV replicase ORF. REP21 Xanthi tobacco plants had a level of resistance to TMV similar to other reported transgenic replicase p lants. No TMV was detected in upper leaves of these plants at 1-mo pos tinoculation. In addition, REP21 plants were resistant to an unusually broad range of tobamoviruses including tomato mosaic virus, tobacco m ild green mosaic virus, TMV-U5, green tomato atypical mosaic virus, an d ribgrass mosaic virus. These plants were not resistant to cucumber m osaic cucumovirus. The lack of systemic infection by TMV was due to re duced multiplication in inoculated leaves rather than complete prevent ion of replication. Inoculation of progeny of the cross of a resistant REP21 line with the TMV local lesion host, Xanthi nc tobacco, resulte d in production of local lesions that were delayed, smaller, and less numerous.