Viroids: an Ariadne's thread into the RNA labyrinth

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Abstract
Viroids are structurally, functionally and evolutionarily different from viruses. Despite their small, non‐protein‐encoding, single‐stranded circular RNA genome, viroids can infect higher plants and cause certain diseases. Members of the two viroid families, Pospiviroidae and Avsunviroidae, have evolved to usurp the transcriptional machinery of their host nuclei and chloroplasts, respectively, in which replication proceeds through a rolling‐circle mechanism involving RNA polymerization, cleavage and ligation. Remarkably, viroids subvert certain DNA‐dependent RNA polymerases to transcribe RNA templates, and, in the family Avsunviroidae, post‐transcriptional cleavage is catalysed by hammerhead ribozymes. Viroids are models for studying RNA evolution and for analysing RNA transport in plants, because they can move intracellularly, intercellularly through plasmodesmata and to distal parts of the plant through the vascular system. Viroids elicit RNA‐silencing phenomena, which might mediate some of their biological properties, including pathogenesis. As some viroids behave as catalytic RNAs, they are regarded as remnants of the RNA world.