* Family Pospiviroidae
* Genus Pospiviroid; type species: Potato spindle tuber viroid * Genus Hostuviroid; type species: Hop stunt viroid
* Genus Cocadviroid; type species: Coconut cadang-cadang viroid * Genus Apscaviroid; type species: Apple scar skin viroid * Genus Coleviroid; type species: Coleus blumei viroid 1 * Family Avsunviroidae
* Genus Avsunviroid; type species: Avocado sunblotch viroid * Genus Pelamoviroid; type species: Peach latent mosaic viroid * Genus Elaviroid; type species: Eggplant latent viroid -------------------------------------------------
Viroids and RNA silencing
There has long been confusion over how viroids are able to induce symptoms in plants without encoding any protein products within their sequences. Evidence now suggests thatRNA 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 pairing with target messenger RNA. Secondly, siRNAscorresponding to sequences from viroid genomes have been isolated from infected plants. Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develops all the corresponding viroid like symptoms. This evidence indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto theRNA-induced silencing complex. The viroid siRNAs actually contain sequences capable of complementary base pairing with the plant's own messenger RNAs and induction of degradation or inhibition of translation is what causes the classic viroid symptoms.
Virus classification is the process of naming viruses and placing them into a taxonomicsystem. Similar to the classification systems used for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is mainly due to the pseudo-livingnature of viruses, which are not yet definitively classified as living or non-living. As such, they do not fit neatly into the established biological classification system in place for cellular organisms. Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. Currently there are two main schemes used for the classification of viruses: the ICTV system and Baltimore classification system, which places viruses into one of seven groups. Accompanying this broad method of classification are...