# Avsunviroidae
**Avsunviroidae** is a family of viroids, small infectious plant pathogens consisting of a single-stranded, circular RNA of 246 to 434 nucleotides that encodes no proteins and replicates autonomously in the plastids (mostly chloroplasts) of host cells through a symmetric rolling-circle mechanism.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) It is one of the two families of viroids recognized by the [International Committee on Taxonomy of Viruses](https://www.edgepedia.org/international-committee-on-taxonomy-of-viruses) (ICTV), the other being [Pospiviroidae](https://www.edgepedia.org/pospiviroidae), and its members are distinguished by hammerhead ribozymes embedded in strands of both polarities, which cleave their replicative intermediates.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) • [2](https://www.annualreviews.org/content/journals/10.1146/annurev.phyto.43.040204.140243) The type species is avocado sunblotch viroid (ASBVd), from which the family name derives.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
| Key fact | Detail |
|---|---|
| Genome | Single-stranded, circular RNA of 246–434 nt; G+C content above 50% in most members, except ASBVd at 38%[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) |
| Replication site | Plastids, mostly chloroplasts, via a symmetric rolling-circle mechanism[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) |
| Cleavage | Hammerhead ribozymes in strands of either polarity self-cleave the oligomeric replicative intermediates[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) |
| Taxonomy | Three genera, Avsunviroid, Pelamoviroid, and Elaviroid, collectively including five species per the current ICTV report chapter[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) |
| Host range | Naturally infects only angiosperms, usually a single plant species or a few related species[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) |
| Enzymes used | A nuclear-encoded plastid RNA polymerase transcribes RNA templates; a tRNA ligase circularizes the monomeric strands[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) |
## Distinguishing features and comparison with Pospiviroidae
Viroids are classified into two families on multiple criteria: subcellular localization, mode of rolling-circle replication, sequence of the central conserved region, presence or absence of conserved motifs, overall sequence identity, and biological features such as host specificity.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC8403530/) The two families differ on four basic points. Members of Pospiviroidae replicate in the nucleus by an asymmetric rolling-circle pathway, using nuclear [RNA polymerase II](https://www.edgepedia.org/rna-polymerase-ii) for strand synthesis, and their concatemers are cleaved by host enzymes. Members of Avsunviroidae replicate in chloroplasts by a symmetric rolling-circle pathway, use chloroplast [RNA polymerase](https://www.edgepedia.org/rna-polymerase)(s), and cleave their concatemers with hammerhead ribozymes.[4](https://www.sciencedirect.com/topics/neuroscience/avsunviroidae) Avsunviroidae members also lack the central conserved region typical of Pospiviroidae.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
The symmetric mechanism is supported by the presence of circular monomeric RNAs of both (+) and (−) polarity in infected tissue, a feature of Avsunviroidae but not of Pospiviroidae.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC11210808/) The (+) polarity is arbitrarily assigned to the most abundant strand in vivo.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
## Genera
Genus demarcation within the family uses the type of hammerhead structure, the G+C content, solubility in 2 M LiCl, and clustering in phylogenetic trees derived from whole genome sequences.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
**Avsunviroid.** The single species, avocado sunblotch viroid (ASBVd, formally Avsunviroid albamaculaperseae), has a genome of 246 to 251 nt with an unusually low G+C content of 38%, against 53 to 60% for the other family members, and adopts a rod-like conformation.[6](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae/avsunviroid) It is soluble in 2 M LiCl. Its hammerhead structures are thermodynamically unstable with a short helix III, so double-hammerhead structures have been proposed to operate in self-cleavage.[6](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae/avsunviroid) Avocado is the only known natural host.[6](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae/avsunviroid)
**Pelamoviroid.** Its two species, peach latent mosaic viroid (PLMVd) and chrysanthemum chlorotic mottle viroid (CChMVd), have branched genomes stabilized by a kissing-loop interaction in the (+) strand, are insoluble in 2 M LiCl, and form stable single-hammerhead structures.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC12662183/) Host range is restricted to the original hosts and some closely related species.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC12662183/)
**Elaviroid.** The single species, eggplant latent viroid (ELVd), has a quasi rod-like genome soluble in 2 M LiCl, and both strands form stable single-hammerhead structures. Its host range is restricted to several eggplant cultivars.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC12662183/)
The current ICTV report chapter counts five species across the three genera, including apple hammerhead viroid (AHVd) alongside ASBVd, PLMVd, CChMVd, and ELVd; an earlier ICTV taxonomy profile described four species.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC12662183/) A 2024 review likewise placed 5 of 44 formal viroid species in Avsunviroidae, with 39 in Pospiviroidae.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC11210808/)
## Replication
Replication proceeds in three steps through an RNA-based rolling-circle mechanism: synthesis of longer-than-unit strands by host RNA polymerases, processing to unit length, and circularization.[2](https://www.annualreviews.org/content/journals/10.1146/annurev.phyto.43.040204.140243) In Avsunviroidae, a nuclear-encoded plastid RNA polymerase (NEP), normally an enzyme that transcribes DNA, is conscripted to transcribe RNA templates, generating complementary oligomeric RNAs of both polarities.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) These oligomers are self-cleaved co-transcriptionally by the embedded hammerhead ribozymes, which operate as single or double structures, producing linear monomeric RNAs that are ligated into circular forms by a tRNA ligase, a nuclear-encoded enzyme translocated into plastids.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) Experimental work identified the chloroplastic isoform of the plant tRNA ligase as the host enzyme mediating circularization of both (+) and (−) monomeric linear intermediates, though the hammerhead ribozyme itself can also catalyze ligation when its tertiary stabilizing motifs are preserved.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3421689/)
After entry into a host cell, the viroid RNA must be translocated across the double plastid membrane into chloroplasts, a pathway that remains unknown; cell-to-cell and systemic spread are assumed to occur through plasmodesmata and the phloem, though conclusive data are lacking.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
## Biology and transmission
Members of the family naturally infect only angiosperms, with host ranges restricted to a single plant species or, occasionally, a few related species.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) This narrow range has limited experimental study of these RNAs.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC1866107/)
Transmission modes vary by species. Seed transmission has been documented for ASBVd and ELVd but not for PLMVd; PLMVd is instead transmitted through infected pollen, while the situation for ASBVd pollen transmission is unclear. PLMVd has been experimentally transmitted by the aphid *Myzus persicae*, but this mode appears minor under natural conditions. Horizontal transmission by grafting infected material and by sap-contaminated cutting implements occurs frequently in the field, especially in woody hosts.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) Superinfection exclusion between mild and severe strains of PLMVd and CChMVd was reported even before their viroid nature was discovered and was used to develop early detection methods.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
## Avocado sunblotch disease
The family's type member causes one of the economically important viroid diseases. Sunblotch of avocado was first described in 1928, with symptoms initially attributed to sunburn; graft-transmissibility was shown in 1932, suggesting a viral cause, and a viroid was purified from infected leaves and characterized as a covalently closed circular RNA in 1979.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC6631365/) ASBVd was confirmed as the causal agent in 1981, when healthy seedlings developed symptoms 2 to 5 months after inoculation with bark from infected trees, or 4 to 8 months after inoculation with purified viroid.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC6631365/) The disease produces discolored, depressed stem streaks, fruit lesions and discoloration, and reduced yield, and symptomless carrier trees are a major source of spread through grafting and seed.[11](https://www.avocadosource.com/Journals/SAAGA/SAAGA_2014/SAAGA_2014_37_PG_69.pdf)
Survey data illustrate the difficulty of control. In South Africa from 2008 to 2013, 15.6% of 24,685 nursery mother trees from 14 commercial nurseries tested positive for ASBVd by RT-PCR, compared with 2% in a 1987 survey that used dot-blot hybridization; the increase was attributed partly to more sensitive detection methods.[11](https://www.avocadosource.com/Journals/SAAGA/SAAGA_2014/SAAGA_2014_37_PG_69.pdf) Because viroid infections are incurable, infected plants are destroyed and new plantings established with certified viroid-free material.[11](https://www.avocadosource.com/Journals/SAAGA/SAAGA_2014/SAAGA_2014_37_PG_69.pdf)
## Evolutionary relationships
Phylogenetic analysis has related Avsunviroidae to Pospiviroidae and to ribozyme-containing viroid-like satellite RNAs, although this relationship has been questioned with alternative phylogenetic approaches; the apparent monophyletic origin of these biologically different circular RNAs supports a possible origin in the [RNA world](https://www.edgepedia.org/rna-world).[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) Certain small circular satellite RNAs of viruses in the genera Sobemovirus and Polerovirus also contain hammerhead ribozymes in strands of both polarities, but unlike viroids they depend on a co-infecting helper virus for replication.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae) A viroid-like RNA with hammerhead ribozymes in both strands, found in grapevine, resembles members of the genus Pelamoviroid, but whether it is actually a viroid awaits data on its independent replication and movement.[1](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
## References
1. [Family: Avsunviroidae, ICTV Report](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae)
2. [Flores R, Hernández C, Martínez de Alba AE, Daròs JA, Di Serio F (2005). Viroids and Viroid-Host Interactions. Annual Review of Phytopathology 43:117–139](https://www.annualreviews.org/content/journals/10.1146/annurev.phyto.43.040204.140243)
3. [Progress in 50 years of viroid research—Molecular structure, pathogenicity, and host adaptation, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8403530/)
4. [Avsunviroidae - an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/neuroscience/avsunviroidae)
5. [Understanding viroids, endogenous circular RNAs, and viroid-like RNAs in the context of biogenesis, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11210808/)
6. [Genus: Avsunviroid, ICTV Report](https://ictv.global/report/chapter/avsunviroidae/avsunviroidae/avsunviroid)
7. [ICTV Virus Taxonomy Profile: Avsunviroidae, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12662183/)
8. [Involvement of the Chloroplastic Isoform of tRNA Ligase in the Replication of Viroids Belonging to the Family Avsunviroidae, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3421689/)
9. [Processing of RNAs of the Family Avsunviroidae in Chlamydomonas reinhardtii Chloroplasts, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC1866107/)
10. [The Avocado Sunblotch Viroid: An Invisible Foe of Avocado, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6631365/)
11. [An overview of avocado sunblotch viroid disease in South Africa from 2008 to 2013, South African Avocado Growers' Association Yearbook 37](https://www.avocadosource.com/Journals/SAAGA/SAAGA_2014/SAAGA_2014_37_PG_69.pdf)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Viruses and acellular agents › Viroids, satellites, and prions › Viroids › Avsunviroidae*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 11, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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