color = violet
unranked_ordo = Viroid
subdivision_ranks = Families
subdivision = "
plant pathogensthat consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNAwithout the protein coat that is typical for viruses. The smallest so far is a 220 nucleobasescRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV). [cite journal |author=Collins RF, Gellatly DL, Sehgal OP, Abouhaidar MG |title=Self-cleaving circular RNA associated with rice yellow mottle virus is the smallest viroid-like RNA |journal=Virology |volume=241 |issue=2 |pages=269–75 |year=1998 |pmid=9499801 |doi=10.1006/viro.1997.8962] In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2 kilobases in size. The human pathogen hepatitis Dis similar to viroids.cite journal |author=Rocheleau L, Pelchat M |title=The Subviral RNA Database: a toolbox for viroids, the hepatitis delta virus and satellite RNAs research |journal=BMC Microbiol. |volume=6 |issue= |pages=24 |year=2006 |pmid=16519798 |doi=10.1186/1471-2180-6-24]
Viroids were discovered and given this name by
Theodor Otto Diener, a plant pathologist at the Agricultural Research Service in Maryland, in 1971.cite journal |author=Diener TO |title=Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA |journal=Virology |volume=45 |issue=2 |pages=411–28 |year=1971 |month=August |pmid=5095900 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/0042-6822(71)90342-4] [ |url=http://www.ars.usda.gov/is/timeline/viroid.htm |title=ARS Research Timeline - Tracking the Elusive Viroid |date= 2006-03-02|accessdate=2007-07-18] [ [http://www2.oakland.edu/biology/chaudhry/pics/Viroids%5B1%5D.pdf Discovery of Viroids] ]
Viroid RNA does not code for any
protein. [cite journal |author=Tsagris EM, de Alba AE, Gozmanova M, Kalantidis K |title=Viroids |journal=Cell. Microbiol. |year=2008 |month=September |pmid=18764915 |doi=10.1111/j.1462-5822.2008.01231.x] The replication mechanism involves RNA polymerase II, an enzyme normally associated with synthesis of messenger RNAfrom DNA, which instead catalyzes " rolling circle" synthesis of new RNA using the viroid's RNA as template. Some viroids are ribozymes, having catalyticproperties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates. [cite journal |author=Daròs JA, Elena SF, Flores R |title=Viroids: an Ariadne's thread into the RNA labyrinth |journal=EMBO Rep. |volume=7 |issue=6 |pages=593–8 |year=2006 |pmid=16741503 |doi=10.1038/sj.embor.7400706]
The first viroid to be identified was the "
potato spindle tuber viroid" (PSTVd). Some 33 species have been identified.
Primary [cite journal |author=Schrader O, Baumstark T, Riesner D |title=A mini-RNA containing the tetraloop, wobble-pair and loop E motifs of the central conserved region of potato spindle tuber viroid is processed into a minicircle |journal=Nucleic Acids Res. |volume=31 |issue=3 |pages=988–98 |year=2003 |pmid=12560495 | doi = 10.1093/nar/gkg193 ] and secondary structure of the PSTVd viroid:
Pospiviroid"; type species: " Potato spindle tuber viroid"
Hostuviroid"; type species: " Hop stunt viroid"
Cocadviroid"; type species: " Coconut cadang-cadang viroid"
Apscaviroid"; type species: " Apple scar skin viroid"
Coleviroid"; type species: " Coleus blumei viroid 1"
Avsunviroid"; type species: " Avocado sunblotch viroid"
Pelamoviroid"; type species: " Peach latent mosaic viroid"
Viroids and RNA silencing
There has long been confusion over how viroids are able to induce
symptoms in plants without encoding any proteinproducts within their sequences. Evidence now suggests that RNA silencingis involved in the process. First, changes to the viroid genomecan dramatically alter its virulence. [cite journal |author=Elizabeth Dickson, Hugh D. Robertson, C. L. Niblett, R. K. Horst & Milton Zaitlin |title=Minor differences between nucleotide sequences of mild and severe strains of potato spindle tuber viroid |year=1979 |journal=Nature |volume=277 |pages=60–62 |doi=10.1038/277060a0] This reflects the fact that any siRNAs produced would have less complementary base pairingwith target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants. [cite journal |author=Papaefthimiou I, Hamilton A, Denti M, Baulcombe D, Tsagris M, Tabler M |title=Replicating potato spindle tuber viroid RNA is accompanied by short RNA fragments that are characteristic of post-transcriptional gene silencing |journal=Nucleic Acids Res. |volume=29 |issue=11 |pages=2395–400 |year=2001 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=55696 |pmid=11376158 |doi=10.1093/nar/29.11.2395] Finally, transgenicexpression of the noninfectious hpRNA of potato spindle tuber viroiddevelops all the corresponding viroid like symptoms. [cite journal |author=Wang MB, Bian XY, Wu LM, "et al" |title=On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=101 |issue=9 |pages=3275–80 |year=2004 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=365780 |pmid=14978267 |doi=10.1073/pnas.0400104101]
This evidence indicates that when viroids replicate via a double stranded intermediate
RNA, they are targeted by a dicerenzyme and cleaved into siRNAs that are then loaded onto the RNA-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.
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