Cross-protection between viruses -- these have RNA genomes for the most part -- is a function of their sequence similarity.
1. Baulcombe’s lab showed that cross-protection between viruses that normally don’t cross protect could be transferred by introducing a sequence from one into the other in such a way that it does not affect expression of the viral proteins.
2. This implies that the viral RNA itself can trigger a process in which closely related sequences are targetted for destruction (or, perhaps, aborted replication).
3. When a plant is infected with a virus into whose genome a copy of either an endogenous plant gene or a resident transgene has been inserted, the resident genes are silenced, much as is observed with co-suppression.
All of these observations suggest a close relationship between the induction of viral immunity and PTGS.