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viroids, viroid diseases

Viroids are the smallest infectious pathogens known They are composed solely of a short strand of circular, single-stranded RNA without protein coat All known viroids are inhabitants of higher plants, in which most cause diseases, some of which are of slight to catastrophic economic importance

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 US Department of Agriculture's Research Center in Beltsville, Maryland, in 1971 This viroid is now called Potato spindle tuber viroid, abbreviated PSTVd

Discovery of the viroid triggered the third major extension of the biosphere in history to include smaller lifelike entities—after the discovery of the "subvisible" microorganisms by Antonie van Leeuwenhoek in 1675 and the "submicroscopic" viruses by Dmitri Iosifovich Ivanovsky in 1892

The unique properties of viroids have been recognized by the International Committee for Virus Taxonomy with the creation of a new order of subviral agents

In a year 2000 compilation of the most important Millennial Milestones in Plant Pathology, the American Phytopathological Society has ranked the 1971 discovery of the viroid as one of the Millennium's ten most important pathogen discoveries

As cogently expressed by Flores et al: Viruses and viroids share the most characteristic property of living beings: In an appropriate environment, they are able to generate copies of themselves, in other words, they are endowed with autonomous replication and evolution It is in this framework where viroids represent the frontier of life 246 to 467nt, an aspect that should attract the attention of anybody interested in biology

Although viroids are composed of nucleic acid, they do not code for any protein 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 Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates

With Diener's 1989 hypothesis that viroids may represent "living relics" from the widely assumed, ancient, and non-cellular RNA world—extant before the evolution of DNA or proteins—viroids have assumed significance beyond plant pathology to evolutionary science, by representing the most plausible RNAs capable of performing crucial steps in abiogenesis, the evolution of life from inanimate matter

The human pathogen hepatitis D virus is a "defective" RNA virus similar to a viroid


  • 1 Taxonomy
  • 2 Transmission
  • 3 Replication
  • 4 RNA silencing
  • 5 RNA world hypothesis
  • 6 History
  • 7 See also
  • 8 References
  • 9 External links


  • Family Pospiviroidae
    • Genus Pospiviroid; type species: Potato spindle tuber viroid; 356–361 nucleotidesnt
    • Genus Pospiviroid; type species: Citrus exocortis; 368–467 nt
    • Genus Hostuviroid; type species: Hop stunt viroid; 294–303 nt
    • Genus Cocadviroid; type species: Coconut cadang-cadang viroid; 246–247 nt
    • Genus Apscaviroid; type species: Apple scar skin viroid; 329–334 nt
    • Genus Coleviroid; type species: Coleus blumei viroid 1; 248–251 nt
Putative secondary structure of the PSTVd viroid
  • 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


The reproduction mechanism of a typical viroid Leaf contact transmits the viroid The viroid enters the cell via its plasmodesmata RNA polymerase II catalyzes rolling-circle synthesis of new viroids

Viroid infections can be transmitted 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


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

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 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 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

RNA world hypothesis

Diener's 1989 hypothesis 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, in which the authors summarized Diener's evidence supporting his hypothesis as:

  1. Viroids' small size, imposed by error-prone replication
  2. Their high guanine and cytosine content, which increases stability and replication fidelity
  3. Their circular structure, which assures complete replication without genomic tags
  4. Existence of structural periodicity, which permits modular assembly into enlarged genomes
  5. Their lack of protein-coding ability, consistent with a ribosome-free habitat
  6. 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


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

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 However, a fungus or bacterium could not be found consistently associated with symptom-bearing plants, 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 nucleobases of single-stranded RNA and, unlike viruses, did not have a protein coat Compared with other infectious plant pathogens, viroids are extremely small in size, ranging from 246 to 467 nucleobases; 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 nucleobases long

In 1976, Sänger et al 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 together Sänger et al also provided evidence for the true circularity of viroids by finding that the RNA could not be phosphorylated at the 5' terminus Then, 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

The single-strandedness and circularity of viroids was confirmed by electron microscopy, and Gross et al determined the complete nucleotide sequence of potato spindle tuber viroid in 1978 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

In 2014, New York Times science writer Carl Zimmer published a popularized piece that mistakenly credited Flores et al with the hypothesis' original conception

See also

  • Viruses portal
  • Circular RNA
  • Microparasite
  • Non-cellular life
  • Plant pathology
  • Plasmid
  • RNA world hypothesis
  • Satellite biology
  • Virus
  • Virus classification
  • Virusoid


  1. ^ a b Diener TO August 1971 "Potato spindle tuber "virus" IV A replicating, low molecular weight RNA" Virology 45 2: 411–28 doi:101016/0042-68227190342-4 PMID 5095900 
  2. ^ a b "ARS Research Timeline – Tracking the Elusive Viroid" 2006-03-02 Retrieved 2007-07-18 
  3. ^ King AMQ, Adams MJ, Carstens EB, Lefkovitz EJ, et al Virus Taxonomy Ninth Report of the International Committee for Virus Taxonomy Burlington, MA, USA: Elsevier Academic Press; 2012 pp 1221–1259, TN: 949565
  4. ^ From Y1K to 2K: Millennial Milestones in Plant Pathology, American Phytopathological Society Publications, APSnet Features, prepared by Thor Kommedahl,University of Minnesota, 2000, 7pp Online doi: 101094/APSFeature-2000-0100
  5. ^ Flores R, Ruiz-Ruiz S, Serra P Viroids and Hepatitis Delta Virus BSemin Liver Dis 2012, 323: 201–210
  6. ^ Tsagris EM, Martínez de Alba AE, Gozmanova M, Kalantidis K September 2008 "Viroids" Cell Microbiol 10 11: 2168–79 doi:101111/j1462-5822200801231x PMID 18764915 
  7. ^ Flores, Ricardo; DiSerio, Francesco; Hernández, Carmen February 1997 "Viroids: The Noncoding Genomes" Seminars in Virology 8 1: 65–73 doi:101006/smvy19970107SA 
  8. ^ name="Daròs JA, Elena SF, Flores R 2006 593–8">Daròs JA, Elena SF, Flores R 2006 "Viroids: an Ariadne's thread into the RNA labyrinth" EMBO Rep 7 6: 593–8 doi:101038/sjembor7400706 PMC 1479586  PMID 16741503 
  9. ^ name="ProcNatlAcadSciUSA,1989-TOD">Diener TO 1989 "Circular RNAs: relics of precellular evolution" Proc Natl Acad Sci USA 86 23: 9370–4 Bibcode:1989PNAS869370D doi:101073/pnas86239370 PMC 298497  PMID 2480600 
  10. ^ Alves C, Branco C, Cunha C 2013 "Hepatitis delta virus: a peculiar virus" Adv Virol 2013: 560105 doi:101155/2013/560105 PMC 3807834  PMID 24198831 
  11. ^ a b c d e f g h i j Brian W J Mahy, Marc H V Van Regenmortel ed Desk Encyclopedia of Plant and Fungal Virology Academic Press pp 71–81 ISBN 978-0123751485 
  12. ^ De Bokx JA, Piron PG 1981 "Transmission of potato spindle tuber viroid by aphids" Netherlands Journal of Plant Pathology 87 2: 31–34 doi:101007/bf01976653 
  13. ^ Flores R, Serra P, Minoia S, Di Serio F, Navarro B 2012 "Viroids: from genotype to phenotype just relying on RNA sequence and structural motifs" Front Microbiol 3: 217 doi:103389/fmicb201200217 PMC 3376415  PMID 22719735 
  14. ^ authors = Daròs JA, Elena SF, Flores R | title = "Viroids: an Ariadne's thread into the RNA labyrinth" | journal = Embo Rep | year = 2006 | volume = 7 | issue = 6 | pages = 593–8"
  15. ^ Hammond RW 1992 "Analysis of the virulence modulating region of potato spindle tuber viroid PSTVd by site-directed mutagenesis" Virology 187 2: 654–62 doi:101016/0042-68229290468-5 PMID 1546460 
  16. ^ 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 2004 "On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites" Proc Natl Acad Sci USA 101 9: 3275–80 Bibcode:2004PNAS1013275W doi:101073/pnas0400104101 PMC 365780  PMID 14978267 
  17. ^ Pallas V, Martinez G, Gomez G 2012 "The interaction between plant viroid-induced symptoms and RNA silencing" Methods Mol Biol Methods in Molecular Biology 894: 323–43 doi:101007/978-1-61779-882-5_22 ISBN 978-1-61779-881-8 PMID 22678590 
  18. ^ Diener, T O "Circular RNAs: relics of precellular evolution"ProcNatlAcadSciUSA, 1989;8623:9370-9374
  19. ^ Flores R, Gago-Zachert S, Serra P, Sanjuán R, Elena SF June 18, 2014 "Viroids: survivors from the RNA world" Annu Rev Microbiol 68: 395–414 doi:101146/annurev-micro-091313-103416 PMID 25002087 
  20. ^ Owens RA, Verhoeven JT 2009 "Potato Spindle Tuber" Plant Health Instructor doi:101094/PHI-I-2009-0804-01 
  21. ^ a b Pommerville, Jeffrey C 2014 Fundamentals of Microbiology Burlington, MA: Jones and Bartlett Learning p 482 ISBN 978-1-284-03968-9 
  22. ^ Sänger HL, Klotz G, Riesner D, Gross HJ, Kleinschmidt AK 1976 "Viroids are single-stranded covalently closed circular RNA molecules existing as highly base-paired rod-like structures" Proc Natl Acad Sci USA 73 11: 3852–6 doi:101073/pnas73113852 PMC 431239  PMID 1069269 
  23. ^ Sogo JM, Koller T, Diener TO 1973 "Potato spindle tuber viroid X Visualization and size determination by electron microscopy" Virology 55 1: 70–80 doi:101016/s0042-68227381009-8 PMID 4728831 
  24. ^ Gross HJ, Domdey H, Lossow C, Jank P, Raba M, Alberty H, Sänger HL 1978 "Nucleotide sequence and secondary structure of potato spindle tuber viroid" Nature 273 5659: 203–8 doi:101038/273203a0 PMID 643081 
  25. ^ Hammond RW, Owens RA 2006 "Viroids: New and Continuing Risks for Horticultural and Agricultural Crops" APSnet Feature Articles doi:101094/APSnetFeature-2006-1106 
  26. ^ Zimmer, C September 25, 2014 "A Tiny Emissary From the Ancient Past" New York Times Retrieved November 22, 2014 

External links

  • Viroids/ATSU

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