Viroids: molecular implements for dissecting RNA trafficking in plants.

Viroids are infectious agents isolated so far only from plants. They are small, non-protein-coding RNAs that must interact directly with and usurp components of their host machinery for replication, systemic spread and, eventually, expression of pathogenic effects. Viroids, therefore, are invading RNAs that may be instrumental for getting deeper insights into the structural-funtional relationships of host RNAs. Specifically, here we discuss past and potential future contributions of viroids to RNA trafficking in plants, including the possible involvement of RNA silencing.