Viroid

Key takeaways

Viroid belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Viroid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Viroid from memory before moving on to harder problems.

Reference excerpt

Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. A 2023 metatranscriptomics study suggests that viroids and viroid-like elements can be found in all domains of life. The first discoveries of viroids in the 1970s triggered the historically third major extension of the biosphere—to include smaller entities—after the discoveries in 1675 by Antonie van Leeuwenhoek (of the "subvisible" protozoa) 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. The first recognized viroid, the pathogenic agent of the potato spindle tuber disease, was discovered, initially molecularly characterized, and named by Theodor Otto Diener, a plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971. 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. 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. Viroids are often ribozymes, having catalytic properties that allow self-cleavage and ligation of unit-size genomes from larger replication intermediates. 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. 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.

Taxonomy

As of 2024:

Family Pospiviroidae: relies on host Rnase III Genus Pospiviroid; type species: Pospiviroid fusituberis (former name Potato spindle tuber viroid); 356–361 nucleotides(nt) Pospiviroid chloronani (former name Tomato chlorotic dwarf viroid); (TCDVd); accession AF162131, genome length 360nt Mexican papita viroid; (MPVd); accession L78454, genome length 360nt Pospiviroid machoplantae (former name Tomato planta macho viroid); (TPMVd); accession K00817, genome length 360nt Pospiviroid exocortiscitri (former name Citrus exocortis viroid); 368–467 nt Pospiviroid impedichrysanthemi (former name Chrysanthemum stunt viroid); (CSVd); accession V01107, genome length 356nt Pospiviroid apicimpeditum (former name Tomato apical stunt viroid); (TASVd); accession K00818, genome length 360nt Pospiviroid alphairesinis (former name Iresine 1 viroid); (IrVd-1); accession X95734, genome length 370nt Pospiviroid latenscolumneae (former name Columnea latent viroid); (CLVd); accession X15663, genome length 370nt Pospiviroid latensportulacae (former name Pospiviroid plvd) Pospiviroid parvicapsici (former name Pepper chat fruit viroid) Genus Hostuviroid; type species: Hostuviroid impedihumuli (former name Hop stunt viroid); 294–303 nt Hostuviroid latensdahliae (former name Dahlia latent viroid) Genus Cocadviroid; type species: Cocadviroid cadangi (former name Coconut cadang-cadang viroid); 246–247 nt Cocadviroid tinangajae (former name Coconut tinangaja viroid); (CTiVd); accession M20731, genome length 254nt Cocadviroid latenshumuli (former name Hop latent viroid); (HLVd); accession X07397, genome length 256nt Cocadviroid rimocitri (former names Citrus bark cracking viroid, Citrus IV viroid); (CVd-IV); accession X14638, genome length 284nt Genus Apscaviroid; type species: Apscaviroid cicatricimali (former name Apple scar skin viroid); 329–334 nt Citrus III viroid; (CVd-III); accession AF184147, genome length 294nt Apscaviroid fossulamali (former name Apple dimple fruit viroid); (ADFVd); accession X99487, genome length 306nt Apscaviroid alphaflavivitis (former name Grapevine yellow speckle viroid 1); (GVYSd-1); accession X06904, genome length 367nt Apscaviroid betaflavivitis (former name Grapevine yellow speckle viroid 2); (GVYSd-2); accession J04348, genome length 363nt Apscaviroid curvifoliumcitri (former name Citrus bent leaf viroid); (CBLVd); accession M74065, genome length 318nt Apscaviroid pustulapyri (former name Pear blister canker viroid); (PBCVd); accession D12823, genome length 315nt Apscaviroid austravitis (former name Australian grapevine viroid); (AGVd); accession X17101, genome length 369nt Apscaviroid maculamali (former name Apscaviroid aclsvd) Apscaviroid etacitri (former name Apscaviroid cvd-VII) Apscaviroid dendrobii (former name Apscaviroid dvd) Apscaviroid latensvitis (former name Apscaviroid glvd) Apscaviroid litchis (former name Apscaviroid lvd) Apscaviroid latenspruni (former name Apscaviroid plvd-I) Apscaviroid diospyri (former name Apscaviroid pvd) Apscaviroid betadiospyri (former name Apscaviroid pvd-2) Apscaviroid nanocitri (former name Citrus dwarfing viroid) Apscaviroid epsiloncitri (former name Citrus viroid V) Apscaviroid zetacitri (former name Citrus viroid VI) Apscaviroid japanvitis Genus Coleviroid; type species: Coleviroid alphacolei (former name Coleus blumei viroid 1); (CbVd-1); 248–251 nt Coleviroid betacolei (former name Coleus blumei viroid 2); (CbVd-2); accession X95365, genome length 301nt Coleviroid gammacolei (former name Coleus blumei viroid 3); (CbVd-3); accession X95364, genome length 361nt Coleviroid epsiloncolei (former name Coleviroid cbvd-5) Coleviroid zetacolei (former name Coleviroid cbvd-6) Family Avsunviroidae: autocatalytic clevage Genus Avsunviroid; type species: Avsunviroid albamaculaperseae (former name Avocado sunblotch viroid); 246–251 nt Genus Pelamoviroid; type species: Pelamoviroid latenspruni (former name Peach latent mosaic viroid); 335–351 nt Pelamoviroid maculachrysanthemi (former name Chrysanthemum chlorotic mottle viroid) Pelamoviroid malleusmali (former name Apple hammerhead viroid) Genus Elaviroid; type species: Elaviroid latensmelongenae (former name Eggplant latent viroid); 332–335 nt

Transmission and replication

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Viroid

Start with the simplest possible case. Write down what Viroid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Viroid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Viroid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Viroid

In research: Viroid appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Viroid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Viroid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Subviral agents, Viroids, so understanding it makes those chapters shorter.
In everyday life: Look for Viroid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Viroid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Viroid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Viroid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Viroid in simple terms?

Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating.

Why does Viroid matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Viroid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Viroid.