RNA-mediated virus resistance in transgenic plants: exploitation of a cellular pathway possibly involved in RNA degradation.

Transgenic Nicotiana tabacum cv. Burley 49 plants were generated that express the 5' untranslated region of the tobacco etch potyvirus (TEV) genome ligated to a mutated version of the TEV coat protein gene sequence that rendered it untranslatable. Eight different transgenic plant lines were analyzed for transgene expression and for resistance to TEV. Three different responses were noted when the transgenic plant lines were inoculated with TEV: 1) some were highly resistant, and no virus replication occurred; 2) some were susceptible but able to recover from systemic TEV infection; and 3) some were susceptible to TEV infection. Plant tissue displaying the recovery phenotype was analyzed for virus replication and transgene expression. Recovered tissue could not be infected with TEV and had steady-state transgene RNA levels which were five- to eightfold lower than those of unchallenged transgenic plant tissue. Nuclear runoff assays suggested a post-transcriptional reduction in specific RNA levels. The highly resistant and recovery phenotypes associated with TEV challenge inoculation and the reduction of steady-state RNA levels in recovered transgenic leaf tissue may be manifestations of a common mechanism.