Gene silencing without DNA: RNA-mediated cross-protection between viruses

Previously, it was shown that the upper leaves of plants infected with nepo viruses and caulimoviruses are symptom free and contain reduced levels of v irus. These leaves are said to be recovered. Recovery is associated with RN A-mediated cross-protection against secondary virus infection. Here, by ana lyzing plants infected with viruses that are quite distinct from the nepovi rus or caulimovirus groups, we demonstrate that this RNA-mediated defense i s a general response to virus infection. Upon infection with a tobravirus, plants exhibited RNA-mediated cross-protection and recovery, as occurs in n epovirus-infected plants. However, upon infection with a potexvirus, plants exhibited RNA-mediated cross-protection without recovery. In both instance s, a transient gene expression assay showed that RNA-mediated cross-protect ion was functionally equivalent to post-transcriptional gene silencing. Com bined, these data provide direct evidence that post-transcriptional gene si lencing of nuclear genes is a manifestation of a natural defense mechanism that is induced by a wide range of viruses.