Editing Viroid (section)

==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.<ref>{{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 }}</ref>

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 [[Bacteria|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.<ref name = "ARS_timeline"/>

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".<ref name="pmid5095900"/> 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.<ref name=Pommerville/>

In 1976, Sanger et al.<ref name="pmid1069269">{{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 }}</ref> 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.<ref name="pmid33460914">{{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=}}</ref>

The single-strandedness and circularity of viroids was confirmed by electron microscopy,<ref name="pmid4728831">{{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 }}</ref> The complete nucleotide sequence of potato spindle tuber viroid was determined in 1978.<ref>{{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 }}</ref> 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.<ref name=Pommerville>{{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}}</ref><ref name="Hammond_2006">{{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 }}</ref>

Four additional viroids or viroid-like RNA particles were discovered between 2009 and 2015.<ref name="pmid26047558">{{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 }}</ref>

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.<ref name= Zimmer>{{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 }}</ref>

In January 2024, biologists reported the discovery of "[[Obelisk (life form)|obelisks]]", a new class of viroid-like elements, and "oblins", their related group of proteins, in the [[human microbiome]].<ref name="SA-20240129" /><ref name="BRX-20240121" />