stunt viroid; (CSVd); accession V01107, genome length 356nt Tomato apical stunt viroid; (TASVd); accession K00818, genome length 360nt Iresine 1 viroid; (IrVd-1); accession X95734, genome length 370nt Columnea latent viroid; (CLVd); accession X15663, genome length 370nt Genus Hostuviroid; type species: Hop stunt viroid; 294–303 nt Genus Cocadviroid; type species: Coconut cadang-cadang viroid; 246–247 nt Coconut tinangaja viroid; (CTiVd); accession M20731, genome length 254nt Hop latent viroid; (HLVd); accession X07397, genome length 256nt Citrus IV viroid; (CVd-IV); accession X14638, genome length 284nt Genus Apscaviroid; type species: Apple scar skin viroid; 329–334 nt Citrus III viroid; (CVd-III); accession AF184147, genome length 294nt Apple dimple fruit viroid; (ADFVd); accession X99487, genome length 306nt Grapevine yellow speckle 1 viroid; (GVYSd-1); accession X06904, genome length 367nt Grapevine yellow speckle 2 viroid; (GVYSd-2); accession J04348, genome length 363nt Citrus bent leaf viroid; (CBLVd); accession M74065, genome length 318nt Pear blister canker viroid; (PBCVd); accession D12823, genome length 315nt Australian grapevine viroid; (AGVd); accession X17101, genome length 369nt Genus Coleviroid; type species: Coleus blumei viroid 1; 248–251 nt Coleus blumei 2 viroid; (CbVd-2); accession X95365, genome length 301nt Coleus blumei 3 viroid; (CbVd-3); accession X95364, genome length 361nt Family Avsunviroidae Genus Avsunviroid; type species: Avocado sunblotch viroid; 246–251 nt Genus Pelamoviroid; type species: Peach latent mosaic viroid; 335–351 nt Genus Elaviroid; type species: Eggplant latent viroid; 332–335 nt Transmission and replication Viroids only infect plants, and infectious viroids can be transmitted to new plant hosts by aphids, by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact. Upon infection, viroids replicate in the nucleus (Pospiviroidae) or chloroplasts (Avsunviroidae) of plant cells in three steps through an RNA-based mechanism. They require RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid as template. Unlike plant viruses which produce movement proteins, viroids are entirely passive, relying entirely on the host. This is useful in the study of RNA kinetics in plants. RNA silencing There has long been uncertainty over how viroids induce symptoms in plants without encoding any protein products within their sequences. Evidence suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base