Viroids are the smallest known infectious agents and consist of circular single-stranded RNA molecules that infect plants. They lack a protein coat and do not encode any proteins, yet they are capable of autonomous replication within susceptible hosts.
Explanation
Viroids were first described by Theodor Diener in 1971 while studying potato spindle tuber disease. Unlike viruses, which encapsidate genetic material and often encode structural and replicative proteins, viroids are naked RNA molecules of 246‑401 nucleotides that adopt highly base‑paired, rod‑like secondary structures. They replicate through a rolling‑circle mechanism using host DNA‑dependent RNA polymerase II, and replication intermediates are processed by host ribonucleases and ligases. Viroids are classified into two families: Pospiviroidae, which replicate in the nucleus and rely on host enzymes for processing, and Avsunviroidae, which replicate in chloroplasts and possess ribozyme activity that self‑cleaves multimeric intermediates. Because they do not encode proteins, viroids depend entirely on host factors for movement and replication. Disease symptoms result from interactions between viroid RNA and host pathways; some viroids interfere with RNA silencing and alter host gene expression, leading to stunting, leaf distortion or bark scaling. Viroid infections are restricted to plants and have significant agricultural impact on crops such as potato, citrus, hop and avocado. Research on viroids has contributed to understanding RNA structure–function relationships and RNA‑based regulation in plants.
Plant diseases caused by viroids
Potato spindle tuber viroid causes elongated, spindle‑shaped tubers and stunted growth in potato plants, resulting in yield losses. Citrus exocortis viroid leads to bark scaling, stunting and leaf epinasty in citrus trees. Hop stunt viroid reduces cone yield and quality in hops and also infects grapevines, causing diminished vine growth and fruit development. Avocado sunblotch viroid induces yellow streaks and cracking on fruit and can severely reduce avocado production. Chrysanthemum stunt viroid produces stunted growth and chlorotic leaves in ornamental chrysanthemums. Detection and control rely on sensitive molecular techniques such as reverse transcription PCR, sanitation of propagation material and removal of infected plants, as there are no curative treatments. Viroids illustrate how a small RNA molecule can parasitize cellular machinery and cause disease without encoding proteins. Their study has expanded knowledge of plant pathology, RNA replication and host–pathogen interactions. Related Terms: Prion, Satellite RNA, Rolling‑circle replication, RNA silencing, Plant pathology