.The majority of plant-infecting viruses have RNA genomes
that contain replication, movement and coat-protein genes.
Initially it was thought that over-expressing one or more of
these proteins in a normal or a dysfunctional state in transgenic
plants would confer protection against the virus from which the
transgene was derived. Although there have been some examples
where this appears to be true, there are several others in which the
transgene appears to have conferred resistance through its mRNA
rather than by its encoded protein. This was shown first in 1992
when virus-resistant plants expressing untranslatable coat-protein
mRNA were produced1. Since then there have been many examples
of RNA-mediated resistance (RMVR) and they appear to
share several features2: