Viroid

{{Short description|Pathogenic small single-stranded circular RNA}}

{{Virusbox

| image = PSTviroid.png

| taxon = Viroid

| subdivision_ranks = Families

Avsunviroidae

}}

Viroids are small single-stranded, circular RNAs that are infectious pathogens.{{cite journal |last1=Navarro |first1=Beatriz |last2=Flores |first2=Ricardo |last3=Di Serio |first3=Francesco |title=Advances in Viroid-Host Interactions |journal=Annual Review of Virology |date=29 September 2021 |volume=8 |issue=1 |pages=305–325|doi-access=free |doi=10.1146/annurev-virology-091919-092331 |pmid=34255541 |issn=2327-056X}}{{cite web |last1=Di Serio |first1=Francesco |last2=Owens |first2=Robert A. |last3=Li |first3=Shi-Fang |last4=Matoušek |first4=Jaroslav |last5=Pallás |first5=Vicente |last6=Randles |first6=John W. |last7=Sano |first7=Teruo |last8=Verhoeven |first8=Jacobus Th. J. |last9=Vidalakis |first9=Georgios |last10=Flores |first10=Ricardo |date=November 2020 |editor-last=Zerbini |editor-first=F. Murilo |editor2-last=Sabanadzovic |editor2-first=Sead |title=Viroids |url=https://talk.ictvonline.org/ictv-reports/ictv_online_report/subviral-agents/w/viroids |access-date=February 3, 2021 |url-status=dead |archive-date=December 2, 2020 |archive-url=https://web.archive.org/web/20201202061614/https://talk.ictvonline.org/ictv-reports/ictv_online_report/subviral-agents/w/viroids}} Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants),{{cite journal |vauthors=Hadidi A |title=Next-Generation Sequencing and CRISPR/Cas13 Editing in Viroid Research and Molecular Diagnostics |journal=Viruses |volume=11 |issue=2 |pages=120 |date=January 2019 |pmid=30699972 |pmc=6409718 |doi=10.3390/v11020120 |doi-access=free}} and most cause diseases, whose respective economic importance to humans varies widely.{{cite journal |vauthors=Adkar-Purushothama CR, Perreault JP |title=Impact of Nucleic Acid Sequencing on Viroid Biology |journal=International Journal of Molecular Sciences |volume=21 |issue=15 |date=August 2020 |page=5532 |pmid=32752288 |pmc=7432327 |doi=10.3390/ijms21155532 |url=|doi-access=free }} A recent metatranscriptomics study suggests that the host diversity of viroids and viroid-like elements is broader than previously thought and that it would not be limited to plants, encompassing even the prokaryotes.{{cite journal |vauthors=Lee BD, Neri U, Roux S, Wolf YI, Camargo AP, Krupovic M |collaboration=RNA Virus Discovery Consortium; Simmonds P, Kyrpides N, Gophna U, Dolja VV, Koonin EV |title=Mining metatranscriptomes reveals a vast world of viroid-like circular RNAs |journal=Cell |date=2 Feb 2023 |volume=186 |issue=3 |pages=646–661 |doi=10.1016/j.cell.2022.12.039 |pmid=36696902 |pmc=9911046 |biorxiv=10.1101/2022.07.19.500677 }}

The first discoveries of viroids in the 1970s triggered the historically third major extension of the biosphere—to include smaller lifelike entities—after the discoveries in 1675 by Antonie van Leeuwenhoek (of the "subvisible" microorganisms) and in 1892–1898 by Dmitri Iosifovich Ivanovsky and Martinus Beijerinck (of the "submicroscopic" viruses).

The unique properties of viroids have been recognized by the International Committee on Taxonomy of Viruses, in creating a new order of subviral agents.{{cite book | vauthors = King AM, Adams MJ, Carstens EB, Lefkovitz EJ, etal | title = Virus Taxonomy. Ninth Report of the International Committee for Virus Taxonomy. | location = Burlington, Massachusetts, US | publisher = Elsevier Academic Press | date = 2012 | pages = 1221–1259 | isbn = 978-0-12-384685-3 }}

The first recognized viroid, the pathogenic agent of the potato spindle tuber disease, was discovered, initially molecularly characterized, and named by Theodor Otto Diener, plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971.{{cite journal | vauthors = Diener TO | title = Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA | journal = Virology | volume = 45 | issue = 2 | pages = 411–28 | date = August 1971 | pmid = 5095900 | doi = 10.1016/0042-6822(71)90342-4 }}{{cite web|url=http://www.ars.usda.gov/is/timeline/viroid.htm |title=ARS Research Timeline – Tracking the Elusive Viroid |date=2006-03-02 |access-date=2007-07-18}} This viroid is now called potato spindle tuber viroid, abbreviated PSTVd. The Citrus exocortis viroid (CEVd) was discovered soon thereafter, and together understanding of PSTVd and CEVd shaped the concept of the viroid.{{cite journal |vauthors=Flores R, Hernández C, Martínez de Alba AE, Daròs JA, Di Serio F |title=Viroids and viroid-host interactions |journal=Annual Review of Phytopathology |volume=43 |issue= |pages=117–39 |date=2005 |pmid=16078879 |doi=10.1146/annurev.phyto.43.040204.140243 |url=}}

Although viroids are composed of nucleic acid, they do not code for any protein.{{cite journal | vauthors = Tsagris EM, Martínez de Alba AE, Gozmanova M, Kalantidis K | title = Viroids | journal = Cellular Microbiology | volume = 10 | issue = 11 | pages = 2168–79 | date = November 2008 | pmid = 18764915 | doi = 10.1111/j.1462-5822.2008.01231.x | s2cid = 221581424 | doi-access = free }}{{cite journal| vauthors = Flores R, Di Serio F, Hernández C |title=Viroids: The Noncoding Genomes|journal=Seminars in Virology|date=February 1997|volume=8|issue=1|pages=65–73|doi=10.1006/smvy.1997.0107}} The viroid's replication mechanism uses 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's RNA as a template. Viroids are often ribozymes, having catalytic properties that allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.{{cite journal |vauthors=Moelling K, Broecker F |title=Viroids and the Origin of Life |journal=International Journal of Molecular Sciences |volume=22 |issue=7 |date=March 2021 |page=3476 |pmid=33800543 |pmc=8036462 |doi=10.3390/ijms22073476 |url=|doi-access=free }}

Diener initially hypothesized in 1989 that viroids may represent "living relics" from the widely assumed, ancient, and non-cellular RNA world, and others have followed this conjecture.{{cite journal |vauthors=Diener TO |date=December 1989 |title=Circular RNAs: relics of precellular evolution? |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=86 |issue=23 |pages=9370–4 |bibcode=1989PNAS...86.9370D |doi=10.1073/pnas.86.23.9370 |pmc=298497 |pmid=2480600 |doi-access=free}}{{cite journal|last1=Moelling|first1=Karin|last2=Broecker |first2=Felix|date=2021-03-28|title=Viroids and the Origin of Life|journal=International Journal of Molecular Sciences|volume=22|issue=7|pages=3476 |doi=10.3390/ijms22073476|issn=1422-0067 |pmc=8036462|pmid=33800543|doi-access=free}} Following the discovery of retrozymes, it has been proposed that viroids and other viroid-like elements may derive from this newly found class of retrotransposon.{{cite journal|last1=Cervera|first1=Amelia |last2=Urbina|first2=Denisse |last3=de la Peña|first3=Marcos|date=2016-06-23 |title=Retrozymes are a unique family of non-autonomous retrotransposons with hammerhead ribozymes that propagate in plants through circular RNAs |journal=Genome Biology|volume=17|issue=1|pages=135 |doi=10.1186/s13059-016-1002-4|issn=1474-760X|pmc=4918200|pmid=27339130 |doi-access=free }}{{cite journal |last1=de la Peña|first1=Marcos|last2=Cervera|first2=Amelia|date=2017-08-03|title=Circular RNAs with hammerhead ribozymes encoded in eukaryotic genomes: The enemy at home|journal=RNA Biology |language=en|volume=14|issue=8 |pages=985–991|doi=10.1080/15476286.2017.1321730|issn=1547-6286 |pmc=5680766|pmid=28448743}}{{cite journal|last1=Lee|first1=Benjamin D. |last2=Koonin |first2=Eugene V.|date=2022-01-12|title=Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators?|journal=Life|volume=12|issue=1|pages=103 |doi=10.3390/life12010103|pmid=35054497 |pmc=8781251 |bibcode=2022Life...12..103L |issn=2075-1729|doi-access=free}}

The human pathogen hepatitis D virus is a subviral agent similar in structure to a viroid, as it is a hybrid particle enclosed by surface proteins from the hepatitis B virus.{{cite journal | vauthors = Alves C, Branco C, Cunha C | title = Hepatitis delta virus: a peculiar virus | journal = Advances in Virology | volume = 2013 | pages = 560105 | year = 2013 | pmid = 24198831 | pmc = 3807834 | doi = 10.1155/2013/560105 | doi-access = free }}

Taxonomy

File:PSTviroid.png of the PSTVd viroid. The highlighted nucleotides are found in most other viroids.]]

{{as of|2024}}:{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Virus Taxonomy: 2023 Release (release v4) |url=https://ictv.global/taxonomy |website=ICTV Taxonomy Browser |access-date=9 December 2024 |language=en |date=30 October 2024}}

Family Pospiviroidae: relies on host Rnase III
Genus Pospiviroid; type species: Pospiviroid fusituberis (former name Potato spindle tuber viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid fusituberis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304527&taxon_name=Pospiviroid%20fusituberis}}); 356–361 nucleotides(nt){{cite book | title=Desk Encyclopedia of Plant and Fungal Virology | isbn=978-0123751485|editor=Brian W. J. Mahy, Marc H. V. Van Regenmortel | publisher= Academic Press|pages=71–81| date=2009-10-29}}
Pospiviroid chloronani (former name Tomato chlorotic dwarf viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid chloronani: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304529&taxon_name=Pospiviroid%20chloronani}}); (TCDVd); accession AF162131, genome length 360nt
Mexican papita viroid; (MPVd); accession L78454, genome length 360nt
Pospiviroid machoplantae (former name Tomato planta macho viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid machoplantae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304530&taxon_name=Pospiviroid%20machoplantae}}); (TPMVd); accession K00817, genome length 360nt
Pospiviroid exocortiscitri (former name Citrus exocortis viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid exocortiscitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304523&taxon_name=Pospiviroid%20exocortiscitri}}); 368–467 nt
Pospiviroid impedichrysanthemi (former name Chrysanthemum stunt viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid impedichrysanthemi: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304522&taxon_name=Pospiviroid%20impedichrysanthemi}}); (CSVd); accession V01107, genome length 356nt
Pospiviroid apicimpeditum (former name Tomato apical stunt viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid apicimpeditum: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304528&taxon_name=Pospiviroid%20apicimpeditum}}); (TASVd); accession K00818, genome length 360nt
Pospiviroid alphairesinis (former name Iresine 1 viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid alphairesinis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304525&taxon_name=Pospiviroid%20alphairesinis}}); (IrVd-1); accession X95734, genome length 370nt
Pospiviroid latenscolumneae (former name Columnea latent viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid latenscolumneae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304524&taxon_name=Pospiviroid%20latenscolumneae}}); (CLVd); accession X15663, genome length 370nt
Pospiviroid latensportulacae (former name Pospiviroid plvd{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid latensportulacae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313877&taxon_name=Pospiviroid%20latensportulacae}})
Pospiviroid parvicapsici (former name Pepper chat fruit viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pospiviroid parvicapsici: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304526&taxon_name=Pospiviroid%20parvicapsici}})
Genus Hostuviroid; type species: Hostuviroid impedihumuli (former name Hop stunt viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Hostuviroid impedihumuli: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304520&taxon_name=Hostuviroid%20impedihumuli}}); 294–303 nt
Hostuviroid latensdahliae (former name Dahlia latent viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Hostuviroid latensdahliae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304519&taxon_name=Hostuviroid%20latensdahliae}})
Genus Cocadviroid; type species: Cocadviroid cadangi (former name Coconut cadang-cadang viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Cocadviroid cadangi: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304511&taxon_name=Cocadviroid%20cadangi}}); 246–247 nt
Cocadviroid tinangajae (former name Coconut tinangaja viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Cocadviroid tinangajae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304512&taxon_name=Cocadviroid%20tinangajae}}); (CTiVd); accession M20731, genome length 254nt
Cocadviroid latenshumuli (former name Hop latent viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Cocadviroid latenshumuli: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304513&taxon_name=Cocadviroid%20latenshumuli}}); (HLVd); accession X07397, genome length 256nt
Cocadviroid rimocitri (former names Citrus bark cracking viroid, Citrus IV viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Cocadviroid rimocitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304510&taxon_name=Cocadviroid%20rimocitri}}); (CVd-IV); accession X14638, genome length 284nt
Genus Apscaviroid; type species: Apscaviroid cicatricimali (former name Apple scar skin viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid cicatricimali: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304500&taxon_name=Apscaviroid%20cicatricimali}}); 329–334 nt
Citrus III viroid; (CVd-III); accession AF184147, genome length 294nt
Apscaviroid fossulamali (former name Apple dimple fruit viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid fossulamali: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304499&taxon_name=Apscaviroid%20fossulamali}}); (ADFVd); accession X99487, genome length 306nt
Apscaviroid alphaflavivitis (former name Grapevine yellow speckle viroid 1{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid alphaflavivitis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304506&taxon_name=Apscaviroid%20alphaflavivitis}}); (GVYSd-1); accession X06904, genome length 367nt
Apscaviroid betaflavivitis (former name Grapevine yellow speckle viroid 2{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid betaflavivitis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304507&taxon_name=Apscaviroid%20betaflavivitis}}); (GVYSd-2); accession J04348, genome length 363nt
Apscaviroid curvifoliumcitri (former name Citrus bent leaf viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid curvifoliumcitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304502&taxon_name=Apscaviroid%20curvifoliumcitri}}); (CBLVd); accession M74065, genome length 318nt
Apscaviroid pustulapyri (former name Pear blister canker viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid pustulapyri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304508&taxon_name=Apscaviroid%20pustulapyri}}); (PBCVd); accession D12823, genome length 315nt
Apscaviroid austravitis (former name Australian grapevine viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid austravitis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304501&taxon_name=Apscaviroid%20austravitis}}); (AGVd); accession X17101, genome length 369nt
Apscaviroid maculamali (former name Apscaviroid aclsvd{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid maculamali: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313867&taxon_name=Apscaviroid%20maculamali}})
Apscaviroid etacitri (former name Apscaviroid cvd-VII{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid etacitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313868&taxon_name=Apscaviroid%20etacitri}})
Apscaviroid dendrobii (former name Apscaviroid dvd{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid dendrobii: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313869&taxon_name=Apscaviroid%20dendrobii}})
Apscaviroid latensvitis (former name Apscaviroid glvd{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid latensvitis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313870&taxon_name=Apscaviroid%20latensvitis}})
Apscaviroid litchis (former name Apscaviroid lvd{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid litchis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313871&taxon_name=Apscaviroid%20litchis}})
Apscaviroid latenspruni (former name Apscaviroid plvd-I{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid latenspruni: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313874&taxon_name=Apscaviroid%20latenspruni}})
Apscaviroid diospyri (former name Apscaviroid pvd{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid diospyri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313872&taxon_name=Apscaviroid%20diospyri}})
Apscaviroid betadiospyri (former name Apscaviroid pvd-2{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid betadiospyri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313873&taxon_name=Apscaviroid%20betadiospyri}})
Apscaviroid nanocitri (former name Citrus dwarfing viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid nanocitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304503&taxon_name=Apscaviroid%20nanocitri}})
Apscaviroid epsiloncitri (former name Citrus viroid V{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid epsiloncitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304504&taxon_name=Apscaviroid%20epsiloncitri}})
Apscaviroid zetacitri (former name Citrus viroid VI{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid zetacitri: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304505&taxon_name=Apscaviroid%20zetacitri}})
Apscaviroid japanvitis{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Apscaviroid japanvitis: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202319071&taxon_name=Apscaviroid%20japanvitis}}
Genus Coleviroid; type species: Coleviroid alphacolei (former name Coleus blumei viroid 1{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Coleviroid alphacolei: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304515&taxon_name=Coleviroid%20alphacolei}}); (CbVd-1); 248–251 nt
Coleviroid betacolei (former name Coleus blumei viroid 2{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Coleviroid betacolei: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304516&taxon_name=Coleviroid%20betacolei}}); (CbVd-2); accession X95365, genome length 301nt
Coleviroid gammacolei (former name Coleus blumei viroid 3{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Coleviroid gammacolei: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202304517&taxon_name=Coleviroid%20gammacolei}}); (CbVd-3); accession X95364, genome length 361nt
Coleviroid epsiloncolei (former name Coleviroid cbvd-5{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Coleviroid epsiloncolei: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313875&taxon_name=Coleviroid%20epsiloncolei}})
Coleviroid zetacolei (former name Coleviroid cbvd-6{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Coleviroid zetacolei: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202313876&taxon_name=Coleviroid%20zetacolei}})
Family Avsunviroidae: autocatalytic clevage
Genus Avsunviroid; type species: Avsunviroid albamaculaperseae (former name Avocado sunblotch viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Avsunviroid albamaculaperseae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202302648&taxon_name=Avsunviroid%20albamaculaperseae}}); 246–251 nt
Genus Pelamoviroid; type species: ''[[Peach latent mosaic viroid|Pelamoviroid latenspruni

]] (former name Peach latent mosaic viroid''{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pelamoviroid latenspruni: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202302653&taxon_name=Pelamoviroid%20latenspruni}}); 335–351 nt

Pelamoviroid maculachrysanthemi (former name Chrysanthemum chlorotic mottle viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pelamoviroid maculachrysanthemi: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202302652&taxon_name=Pelamoviroid%20maculachrysanthemi}})
Pelamoviroid malleusmali (former name Apple hammerhead viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Pelamoviroid malleusmali: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202307651&taxon_name=Pelamoviroid%20malleusmali}})
Genus Elaviroid; type species: Elaviroid latensmelongenae (former name Eggplant latent viroid{{cite web |author1=ICTV |author1-link=International Committee on Taxonomy of Viruses |title=Elaviroid latensmelongenae: Taxon Details |website=ICTV Taxonomy Browser |access-date=9 December 2024 |url=https://ictv.global/taxonomy/taxondetails?taxnode_id=202302650&taxon_name=Elaviroid%20latensmelongenae}}); 332–335 nt

Transmission and replication

File:Viroids- how it do.gif. RNA polymerase II catalyzes rolling-circle synthesis of new viroids.]]

Viroids are only known to 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.{{cite journal | vauthors = De Bokx JA, Piron PG | year = 1981 | title = Transmission of potato spindle tuber viroid by aphids. | journal = Netherlands Journal of Plant Pathology | volume = 87 | issue = 2 | pages = 31–34 | doi=10.1007/bf01976653| bibcode = 1981EJPP...87...31D | s2cid = 44660564 }} 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.{{cite journal | vauthors = Flores R, Serra P, Minoia S, Di Serio F, Navarro B | title = Viroids: from genotype to phenotype just relying on RNA sequence and structural motifs | journal = Frontiers in Microbiology | volume = 3 | pages = 217 | year = 2012 | pmid = 22719735 | pmc = 3376415 | doi = 10.3389/fmicb.2012.00217 | doi-access = free }}

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.{{cite journal |vauthors=Flores R, Navarro B, Kovalskaya N, Hammond RW, Di Serio F |title=Engineering resistance against viroids |journal=Current Opinion in Virology |volume=26 |issue= |pages=1–7 |date=October 2017 |pmid=28738223 |doi=10.1016/j.coviro.2017.07.003}} Evidence suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence.{{cite journal |vauthors=Hammond RW |title=Analysis of the virulence modulating region of potato spindle tuber viroid (PSTVd) by site-directed mutagenesis |journal=Virology |volume=187 |issue=2 |pages=654–662 |date=April 1992 |pmid=1546460 |doi=10.1016/0042-6822(92)90468-5 |url=https://zenodo.org/record/1258280}} This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding 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.{{cite journal |vauthors=Wang MB, Bian XY, Wu LM, Liu LX, Smith NA, Isenegger D, Wu RM, Masuta C, Vance VB, Watson JM, Rezaian A, Dennis ES, Waterhouse PM |display-authors=6 |title=On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=101 |issue=9 |pages=3275–3280 |date=March 2004 |pmid=14978267 |pmc=365780 |doi=10.1073/pnas.0400104101 |bibcode=2004PNAS..101.3275W |doi-access=free}} This 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 the RNA-induced silencing complex. The viroid siRNAs contain sequences capable of complementary base pairing with the plant's own messenger RNAs, and induction of degradation or inhibition of translation causes the classic viroid symptoms.{{cite book |vauthors=Pallas V, Martinez G, Gomez G |chapter=The Interaction Between Plant Viroid-Induced Symptoms and RNA Silencing |title=Antiviral Resistance in Plants |volume=894 |pages=323–343 |year=2012 |pmid=22678590 |doi=10.1007/978-1-61779-882-5_22 |isbn=978-1-61779-881-8 |series=Methods in Molecular Biology |hdl=10261/74632 |hdl-access=free}}

Viroid-like elements

"Viroid-like elements" refer to pieces of covalently closed circular (ccc) RNA molecules that do not share the viroid's lifecycle. The category encompasses satellite RNAs (including small plant satRNAs "virusoids", fungal "ambivirus", and the much larger HDV-like Ribozyviria) and "retroviroids". Most of them also carry some type of a ribozyme.

=Viroid-like satellite RNAs=

Viroid-like satellite RNAs are infectious circular RNA molecules that depend on a carrier virus to reproduce, being carried in their capsids. Like Avsunviroidae, however, they are capable of self-clevage.{{cite journal |last1=Balázs |first1=E |last2=Hegedűs |first2=K |last3=Divéki |first3=Z |title=The unique carnation stunt-associated pararetroviroid. |journal=Virus Research |date=15 April 2022 |volume=312 |pages=198709 |doi=10.1016/j.virusres.2022.198709 |pmid=35183574|s2cid=246987005 }}

=Ambiviruses=

"Ambiviruses" are mobile genetic elements that were recently (2020s) discovered in fungi. Their RNA genomes are circular, circa 5 kb in length. One of at least two open reading frames encodes a viral RNA-directed RNA polymerase, that firmly places "ambiviruses" into ribovirian kingdom Orthornavirae; a separate phylum Ambiviricota has been established since the 2023 ICTV Virus Taxonomy Release because of the unique features of encoding RNA-directed RNA polymerases but also having divergent ribozymes in various combinations in both sense and antisense orientation – the detection of circular forms in both sense orientations suggest that "ambiviruses" use rolling circle replication for propagation.{{cite journal |last1=Sutela |first1=Suvi |last2=Forgia |first2=Marco |last3=Vainio |first3=Eeva J |last4=Chiapello |first4=Marco |last5=Daghino |first5=Stefania |last6=Vallino |first6=Marta |last7=Martino |first7=Elena |last8=Girlanda |first8=Mariangela |last9=Perotto |first9=Silvia |last10=Turina |first10=Massimo |title=The virome from a collection of endomycorrhizal fungi reveals new viral taxa with unprecedented genome organization |journal=Virus Evolution |date=1 July 2020 |volume=6 |issue=2 |pages=veaa076 |doi=10.1093/ve/veaa076 |pmid=33324490 |pmc=7724248}}{{cite journal |last1=Chong |first1=Li Chuin |last2=Lauber |first2=Chris |title=Viroid-like RNA-dependent RNA polymerase-encoding ambiviruses are abundant in complex fungi |journal=Frontiers in Microbiology |date=12 May 2023 |volume=14 |doi=10.3389/fmicb.2023.1144003 |doi-access=free |pmid=37275138 |pmc=10237039}}{{cite web |title=ICTV Virus Taxonomy: 2023 Release |url=https://ictv.global/taxonomy |publisher=ICTV |access-date=2 May 2024}}

=Retroviroids=

"Retroviroids", more formally "retroviroid-like elements", are viroid-like circular RNA sequences that are also found with homologous copies in the DNA genome of the host.{{cite journal| vauthors=Daròs JA, Flores R| title=Identification of a retroviroid-like element from plants. | journal=Proceedings of the National Academy of Sciences of the United States of America | year= 1995 | volume= 92 | issue= 15 | pages= 6856–6860 | pmid= 7542779| doi= 10.1073/pnas.92.15.6856| pmc= 41428| bibcode=1995PNAS...92.6856D | doi-access=free }} The only types found are closely related to the original "carnation small viroid-like RNA" (CarSV).{{cite journal |vauthors=Hegedűs K, Palkovics L, Tóth EK, Dallmann G, Balázs E |title=The DNA form of a retroviroid-like element characterized in cultivated carnation species |journal=The Journal of General Virology |volume=82 |issue=Pt 3 |pages=687–691 |date=March 2001 |pmid=11172112 |doi=10.1099/0022-1317-82-3-687 |url=|doi-access=free }}{{cite journal| vauthors= Hegedűs K, Dallmann G, Balázs E| title=The DNA form of a retroviroid-like element is involved in recombination events with itself and with the plant genome. | journal=Virology| year= 2004 | volume= 325 | issue= 2 | pages= 277–286 | pmid= 15246267| doi= 10.1016/j.virol.2004.04.035| pmc= | doi-access= free}} These elements may act as a homologous substrate upon which recombination may occur and are linked to double-stranded break repair.{{cite journal | vauthors = Truong LN, Li Y, Shi LZ, Hwang PY, He J, Wang H, Razavian N, Berns MW, Wu X | display-authors = 6 | title = Microhomology-mediated End Joining and Homologous Recombination share the initial end resection step to repair DNA double-strand breaks in mammalian cells | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 19 | pages = 7720–25 | date = May 2013 | pmid = 23610439 | pmc = 3651503 | doi = 10.1073/pnas.1213431110 | bibcode = 2013PNAS..110.7720T | doi-access = free }}

These elements are dubbed retroviroids as the homologous DNA is generated by reverse transcriptase that is encoded by retroviruses.{{cite book | vauthors = Hull R | chapter = Chapter 5: Agents Resembling or Altering Virus Diseases | chapter-url = https://books.google.com/books?id=PYrZAAAAQBAJ&pg=PA199 |title=Plant virology |date= October 2013 |location=London, UK | publisher = Academic Press |isbn=978-0-12-384872-7 |edition=Fifth}} They are neither true viroids nor viroid-like satellite RNAs: there is no extracellular form of these elements; instead, they are spread only through pollen or egg-cells. They appear to co-occur with a pararetrovirus.{{cite journal |last1=Breit |first1=TM |last2=de Leeuw |first2=WC |last3=van Olst |first3=M |last4=Ensink |first4=WA |last5=van Leeuwen |first5=S |last6=Dekker |first6=RJ |title=Genome Sequence of a New Carnation Small Viroid-Like RNA, CarSV-1. |journal=Microbiology Resource Announcements |date=16 March 2023 |volume=12 |issue=3 |pages=e0121922 |doi=10.1128/mra.01219-22 |pmid=36840552 |pmc=10019309 |doi-access=free}}

=Obelisks=

After applying metatranscriptomics – the computer-aided search for RNA sequences and their analysis – biologists reported in January 2024 the discovery of "obelisks", a new class of viroid-like elements, and "oblins", their related group of proteins, in the human microbiome. Given that the RNA sequences recovered do not have homologies in any other known life form, the researchers suggest that the obelisks are distinct from viruses, viroids and viroid-like entities, and thus form an entirely new class of organisms.{{cite journal |last=Koumoundouros |first=Tessa |title='Obelisks': Entirely New Class of Life Has Been Found in The Human Digestive System |url=https://www.sciencealert.com/obelisks-entirely-new-class-of-life-has-been-found-in-the-human-digestive-system |date=29 January 2024 |journal=ScienceAlert |url-status=live |archiveurl=https://archive.today/20240129133614/https://www.sciencealert.com/obelisks-entirely-new-class-of-life-has-been-found-in-the-human-digestive-system |archivedate=29 January 2024 |accessdate=29 January 2024 }}{{Cite Q|Q124389714 |last1=Zheludev |first1=Ivan N. |last2=Edgar |first2=Robert C. |last3=Lopez-Galiano |first3=Maria Jose |last4=de la Peña |first4=Marcos |last5=Artem |first5=Babaian |last6=Bhatt |first6=Ami S. |last7=Fire |first7=Andrew Z. |author-link6=Ami Bhatt |author-link7=Andrew Fire |doi-access=free |url-status=live |accessdate=29 January 2024}}

RNA world hypothesis

Diener's 1989 hypothesisDiener, T O. "Circular RNAs: relics of precellular evolution?."Proc.Natl.Acad.Sci.USA, 1989;86(23):9370-9374 had proposed that the unique properties of viroids make them more plausible macromolecules than introns, or other RNAs considered in the past as possible "living relics" of a hypothetical, pre-cellular RNA world. If so, viroids have assumed significance beyond plant virology for evolutionary theory, because their properties make them more plausible candidates than other RNAs to perform crucial steps in the evolution of life from inanimate matter (abiogenesis). Diener's hypothesis was mostly forgotten until 2014, when it was resurrected in a review article by Flores et al.,{{cite journal | vauthors = Flores R, Gago-Zachert S, Serra P, Sanjuán R, Elena SF | title = Viroids: survivors from the RNA world? | journal = Annual Review of Microbiology | volume = 68 | pages = 395–414 | date = June 18, 2014 | pmid = 25002087 | doi = 10.1146/annurev-micro-091313-103416 | url = https://digital.csic.es/bitstream/10261/107724/1/Annu.%20Rev.%20Microbiol.%20Flores%20et%20al%202014.pdf | hdl = 10261/107724 | hdl-access = free }} in which the authors summarized Diener's evidence supporting his hypothesis as:

Viroids' small size, imposed by error-prone replication.
Their high guanine and cytosine content, which increases stability and replication fidelity.
Their circular structure, which assures complete replication without genomic tags.
Existence of structural periodicity, which permits modular assembly into enlarged genomes.
Their lack of protein-coding ability, consistent with a ribosome-free habitat.
Replication mediated in some by ribozymes—the fingerprint of the RNA world.

The presence, in extant cells, of RNAs with molecular properties predicted for RNAs of the RNA world constitutes another powerful argument supporting the RNA world hypothesis. However, the origins of viroids themselves from this RNA world has been cast into doubt by several factors, including the discovery of retrozymes (a family of retrotransposon likely representing their ancestors) and their complete absence from organisms outside of the plants (especially their complete absence from prokaryotes including bacteria and archaea). However, recent studies suggest that the diversity of viroids and others viroid-like elements is broader than previously thought and that it would not be limited to plants, encompassing even the prokaryotes. Matches between viroid cccRNAs and CRISPR spacers suggest that some of them might replicate in prokaryotes.

Control

The development of tests based on ELISA, PCR, and nucleic acid hybridization has allowed for rapid and inexpensive detection of known viroids in biosecurity inspections, phytosanitary inspections, and quarantine.

History

In the 1920s, symptoms of a previously unknown potato disease were noticed in New York and New Jersey fields. Because tubers on affected plants become elongated and misshapen, they named it the potato spindle tuber disease.{{cite journal | vauthors = Owens RA, Verhoeven JT | title = Potato Spindle Tuber | journal = Plant Health Instructor | year = 2009 | doi = 10.1094/PHI-I-2009-0804-01 }}

The symptoms appeared on plants onto which pieces from affected plants had been budded—indicating that the disease was caused by a transmissible pathogenic agent. A fungus or bacterium could not be found consistently associated with symptom-bearing plants, however, and therefore, it was assumed the disease was caused by a virus. Despite numerous attempts over the years to isolate and purify the assumed virus, using increasingly sophisticated methods, these were unsuccessful when applied to extracts from potato spindle tuber disease-afflicted plants.

In 1971, Theodor O. Diener showed that the agent was not a virus, but a totally unexpected novel type of pathogen, 1/80th the size of typical viruses, for which he proposed the term "viroid". Parallel to agriculture-directed studies, more basic scientific research elucidated many of viroids' physical, chemical, and macromolecular properties. Viroids were shown to consist of short stretches (a few hundred nucleotides) of single-stranded RNA and, unlike viruses, did not have a protein coat. Viroids are extremely small, from 246 to 467 nucleotides, smaller than other infectious plant pathogens; they thus consist of fewer than 10,000 atoms. In comparison, the genomes of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleotides long.

In 1976, Sanger et al.{{cite journal | vauthors = Sanger HL, Klotz G, Riesner D, Gross HJ, Kleinschmidt AK | title = Viroids are single-stranded covalently closed circular RNA molecules existing as highly base-paired rod-like structures | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 73 | issue = 11 | pages = 3852–6 | date = November 1976 | pmid = 1069269 | pmc = 431239 | doi = 10.1073/pnas.73.11.3852 | bibcode = 1976PNAS...73.3852S | doi-access = free }} presented evidence that potato spindle tuber viroid is a "single-stranded, covalently closed, circular RNA molecule, existing as a highly base-paired rod-like structure"—believed to be the first such molecule described. Circular RNA, unlike linear RNA, forms a covalently closed continuous loop, in which the 3' and 5' ends present in linear RNA molecules have been joined. Sanger et al. also provided evidence for the true circularity of viroids by finding that the RNA could not be phosphorylated at the 5' terminus. In other tests, they failed to find even one free 3' end, which ruled out the possibility of the molecule having two 3' ends. Viroids thus are true circular RNAs.{{cite journal |vauthors=Wang Y |title=Current view and perspectives in viroid replication |journal=Current Opinion in Virology |volume=47 |issue= |pages=32–37 |date=April 2021 |pmid=33460914 |doi=10.1016/j.coviro.2020.12.004 |pmc=8068583 |url=}}

The single-strandedness and circularity of viroids was confirmed by electron microscopy,{{cite journal | vauthors = Sogo JM, Koller T, Diener TO | title = Potato spindle tuber viroid. X. Visualization and size determination by electron microscopy | journal = Virology | volume = 55 | issue = 1 | pages = 70–80 | date = September 1973 | pmid = 4728831 | doi = 10.1016/s0042-6822(73)81009-8 }} The complete nucleotide sequence of potato spindle tuber viroid was determined in 1978.{{cite journal | vauthors = Gross HJ, Domdey H, Lossow C, Jank P, Raba M, Alberty H, Sänger HL | title = Nucleotide sequence and secondary structure of potato spindle tuber viroid | journal = Nature | volume = 273 | issue = 5659 | pages = 203–8 | date = May 1978 | pmid = 643081 | doi = 10.1038/273203a0 | s2cid = 19398777 | bibcode = 1978Natur.273..203G }} PSTVd was the first pathogen of a eukaryotic organism for which the complete molecular structure has been established. Over thirty plant diseases have since been identified as viroid-, not virus-caused, as had been assumed.{{cite book |last=Pommerville|first=Jeffrey C |date=2014 |title= Fundamentals of Microbiology|location=Burlington, MA |publisher= Jones and Bartlett Learning |page= 482|isbn=978-1-284-03968-9}}{{cite journal | vauthors = Hammond RW, Owens RA | title = Viroids: New and Continuing Risks for Horticultural and Agricultural Crops. | journal = APSnet Feature Articles | date = 2006 | doi = 10.1094/APSnetFeature-2006-1106 }}

Four additional viroids or viroid-like RNA particles were discovered between 2009 and 2015.{{cite journal |vauthors=Wu Q, Ding SW, Zhang Y, Zhu S |title=Identification of viruses and viroids by next-generation sequencing and homology-dependent and homology-independent algorithms |journal=Annual Review of Phytopathology |volume=53 |issue= |pages=425–44 |date=2015 |pmid=26047558 |doi=10.1146/annurev-phyto-080614-120030 |url=|doi-access=free }}

In 2014, New York Times science writer Carl Zimmer published a popularized piece that mistakenly credited Flores et al. with the virioid - RNA world hypothesis' original conception.{{cite news |last=Zimmer |first=C |author-link=Carl Zimmer |title=A Tiny Emissary From the Ancient Past |url=https://www.nytimes.com/2014/09/25/science/a-tiny-emissary-from-the-ancient-past.html |date=September 25, 2014 |work=New York Times |access-date=November 22, 2014 }}

In January 2024, biologists reported the discovery of "obelisks", a new class of viroid-like elements, and "oblins", their related group of proteins, in the human microbiome.

References

External links

[http://www.atsu.edu/faculty/chamberlain/Website/Lects/Prions.htm Viroids/ATSU]
[https://viroids.org/ ViroidDB], a database of viroids and viroid-like circular RNAs

Category:Subviral agents