The role of plant viroids in diseases - new developments

Viroids are tiny infectious agents of plants, consisting exclusively of a small (246-401 nucleotides) circular RNA lacking protein-coding ability. Likely mimicking structural features of cellular RNAs, they can redirect the pre-existing host transcription and RNA trafficking machineries for their own replication and spread, and by doing so they eventually elicit severe diseases. Therefore, in addition to causing phytopathological problems, viroids also offer opportunities for basic research on cellular RNA-based networks. After briefly summarizing the biological, molecular and epidemiological properties of viroids, this review focuses on relevant novelties of these infectious agents, with major emphasis on the molecular pathways presumably mediating viroid pathogenesis and on current phytosanitary emergences in Europe associated with the dissemination of viroids potentially causing severe diseases in vegetable crops.