Dissection of RNA-mediated ryegrass mosaic virus resistance in fertile transgenic perennial ryegrass (Lolium perenne L.).

Ryegrass mosaic virus (RgMV) frequently reduces yield and persistence of perennial ryegrass. We introduced an untranslatable RgMV coat protein (RgMV-CP) gene into perennial ryegrass using particle bombardment to explore the potential of RNA-mediated virus resistance. Nine months after inoculation with the RgMV-Bulgaria strain, the most resistant transgenic line showed no immuno-detectable RgMV-CP in all replications of the primary transformant and a sexual progeny. A significant influence of the transgenic line, virus strain and the period after inoculation on the RgMV level was observed. Typical examples of recovery resistance from initially susceptible plants were not identified. Molecular analysis revealed RgMV resistance operates by targeted RNA degradation resulting in post-transcriptional transgene silencing (PTGS) along with inhibition of virus RNA replication and was not associated with a threshold of transgene expression. Suppression of PTGS by RgMV differed significantly in two progenies with identical RgMV-CP transgene integration and a similar RgMV-CP transgene methylation pattern, suggesting the presence of quantitative components controlling the stability of PTGS in perennial ryegrass. This study extends RNA-mediated virus resistance, post transcriptional gene silencing and crop improvement by genetic engineering to an agronomically important perennial monocot.