Viroids

AbstractViroids are infectious, nonprotein‐coding, highly structured small circularribonucleic acids(RNAs) able to replicate autonomously and induce diseases in higher plants. Viroids and viruses differ in structure, function and evolutionary origin (with the former regarded as relics of a primitive RNA world). Viroids are classified into the familiesPospiviroidaeandAvsunviroidae, the members of which replicate in the nucleus and chloroplast, respectively, through an RNA‐based rolling circle mechanism with three steps catalysed by: (i) hostdeoxyribonucleic acid (DNA)‐dependent RNA polymerasesredirected to accept RNA templates, (ii) processing enzymes or, in the familyAvsunviroidae, hammerhead ribozymes and (iii)RNA ligases. When infecting a cell, the viroid RNA must travel to its replication organelle, with the resulting progeny moving cell‐to‐cell through plasmodesmata and reaching distal parts through the phloem. Pathogenesis could be triggered by the replicating viroid itself, or via RNA silencing. Viroids are detected by nucleic acid‐based approaches, some can be eliminated from infected plants by meristem tip culturing, and biotechnological approaches are being developed for their control.Key Concepts:Viroids are small nonprotein‐coding RNAs that infect, replicate and induce disease in higher plants.Viroids differ from viruses in fundamental aspects that include structure, function and evolutionary origin.Viroids are classified into the familiesPospiviroidae(nuclear viroids) andAvsunviroidae(chloroplastic viroids).Viroid RNAs adopt compact secondary structures additionally stabilised by elements of tertiary structure.Viroids replicate through a rolling circle mechanism with only RNA intermediates and enzyme or ribozyme processing.Different viroids have distinct host ranges and closely related viroids display cross‐protection effects when co‐infecting a common host.Viroids move intracellularly, cell‐to‐cell through plasmodesmata, and long distance through the phloem.Viroids may infect their host plants latently or induce different pathogenic alterations including death.Viroids propagate in their hosts as a population of closely related variants (quasispecies).Viroids are considered molecular fossils of the RNA world postulated to have preceded our present world dominated by DNA and proteins.