Viroids – Structure, Characteristics, Features, and Diseases

Viroid is an infectious particle that causes plant diseases. Viroids differ from viruses in having a single-stranded RNA as genetic material and lacking a protein coat. Viroid was discovered by Theodor Otto Diener.

Viroids are transmitted mechanically from one cell to another through cellular debris. The first viroid was identified in potato tuber called potato spindle tuber viroid (PSTVd). Most viroid causes plant diseases and about 33 viroids have been identified.

The human disease caused by a viroid is hepatitis D. After reading this article you will be able to write a short note on Viroids and understand the viroids structure, characteristics, and types of viroids.

What is Viroid?

Viroid Definition: Viroids can be defined as the smallest infectious agents having single-stranded, highly complementary, circular RNAs without any protein coat. They are even smaller than viruses but can replicate autonomously in susceptible host cells.

The term ‘viroid’ means virus (‘vir’) like particles (‘oid’). Viroid discovered by plant pathologist Theodor Otto Diener are small, circular single-stranded RNA molecules lacking any protein capsule, that infect several crop plants and cause diseases of economic importance. They contain 246 to 467 nucleotide long genomes. Replication of viroids occur autonomously in host cells but they do not code for proteins.

Diseases casued by viroids primarily infect plants and are known to cause diseases such as potato spindle tuber disease, citrus exocortis, and avocado sunblotch disease. Viroids examples are – the potato spindle tuber viroid (PSTVd), citrus exocortis viroid (CEVd), and avocado sunblotch viroid (ASBVd).

What are Virusoids?

Virusoids are small, circular, non-self-replicating single-stranded pathogenic RNA molecules that can infect commercially important crops. Unlike viroid, virusoid depends on virus for replication that’s why they need the cells to be infected with a specific ‘helper’ virus. The helper viruses belong to the family of Sobemoviruses. When the helper virus enters the host cell, the virusoids are released from the helper virus within the plant cell and show ribozyme activity.

The genome size of the virusoid is small, only 220 to 388 nucleotides long which does not code for any proteins. Virusoids are also called satellite RNAs. The virusoid that infects humans has been identified by some scientists as hepatitis delta virusoid (HDV).

Structure of Viroids

The structure of the viroid can be seen by an electron microscope. The structures is as follows:

• The viroids are 240 to 380 nucleotide-long molecules having dumbbell structures.
• They contain a few hundred base pairs of highly complementary circular single-stranded RNA lacking protein capsules with molecular weights between 1,07,000 and 1,27,000.
• Most of the nucleotides are base-paired to form double-stranded RNA molecules.
• The extensive intra-strand base pairings with unpaired loops at the intervals are found in the single-stranded circular DNA. This structure protects the viroid from degradation by ribonuclease.
• Based on structure, viroids can be classified into two groups: self-cleaving and non-self-cleaving.
• Non-self-cleaving viroids form “dog bone” or rod-like structures after replication in the nucleus.
• Non-self-cleaving viroids consist of five domains which are as follows.
  • Conserved central domain (C)
  • Pathogenic domain (P)
  • Variable domain (V)
  • Terminal left domain (TL)
  • Terminal right domain (TR)

Viroid Diagram

The diagram of viroids is given below:

Characteristics of Viroid

The characteristics of viroid are cited below:

• Viroids are intracellular obligatory parasites.
• They are very small, even smaller than viruses.
• They do not have any protein coat (capsid).
• They are covalently closed single-stranded Circular RNA molecules.
• Their size varies from 264 to 400 nucleotides.
• Viroid RNA is unable to code for any protein due to the absence of an initiation codon.
• They replicate with the help of the host’s polymerases.

Types of Viroid

Viroids belong to two families – Pospiviroidae and Avsunviroidae.

• Pospiviroidae: The potato spindle tuber viroid belongs to the Pospiviroidae family. They are single-stranded rod-like structures. They replicate by symmetric rolling circular mechanism. The activity of the RNA pol II enzyme produces the multimetric negative strand from the positive circular RNA strand. These viroids replicate within the nucleus and have a core conserved area.
• Avsunviroidae: The avocado sunblotch virus belongs to Avsunviroidae family. These viroids have branching portions which have also rod-like shapes. These viroids replicate within the chloroplast and do not have a core conserved area.

Examples of Viroid

Some of the examples of viroids are as follows.

• Genus Pospiviroids: PSTV (potato spindle tuber viroid), TBTVd (Tomato bunchy top viroid), CSVd (Chrysanthemum stunt viroid)
• Genus Coleviroids: CbVd 1 (coleus blumei 1)
• Genus Hostuviroids: HSV (hop stunt viroid), CPFVd (Cucumber pale fruit viroid)
• Genus Avsunviroids: ASBV (avocado sunblotch viroid)
• Genus Cocadviroids CCCV (coconut cadang-cadang viroid)
• Genus Pelamoviroids: PLMVD (peach latent mosaic viroid)
• Genus Apscaviroids: ASSVd (apple scar skin viroid), CEVd (Citrus excortis viroid).

Functions of Viroid

The functions of viroid are stated below:

• Mature viroid autonomously replicate and infect the host plants.
• They utilize proteins of the host for replication, processing, transport, and pathogenesis.
• They provide appropriate signals to the host machinery based on their sequence or their structure.

How do Viroids Spread?

Viroid spread in the following ways:

• Vegetative Transmission: Viroid are transmitted through vegetative propagation from one plant to the next plant like grafting, budding, cuttings, bulbs, tubers, and other methods.
• Mechanical Transmission: Mechanical transmission of viroids includes transmission through viroid-infected sap or nucleic acids by use of viroid-contaminated farming tools and instruments in agricultural and horticultural practices.
• Seed and Pollen Transmission: Many viroids spread through seeds and/or pollen. These modes of transmission are responsible for epidemic outbreaks among the plants during the start of the growing season.
• Vector Transmission: Vector transmission includes the spread of viroid through insects, parasitic plants, goats, and fungi.

Viroid Diseases

Viroid diseases in plants are potato spindle tuber disease, citrus exocortis, avocado sunblotch disease, and many others, which can lead to reduced crop yields and economic losses in agricultural settings.

Different viroid species cause different diseases. Some viroid diseases are discussed below:

• The most common viroid diseases are caused by potato spindle tuber viroid (PSTVd). Potatoes and other Solanaceous plants are infected by PSTVd which causes serious damage to potato crops by causing elongation and cracking of the tubers.
• Potatoes infected by the potato spindle tuber viroid (PSTV).
• Apple proliferation viroid (APVd) infects apples and other Malus species.
• Tomato yellow leaf curl viroid (TYLCVd) causes damage to tomatoes and other Solanaceous plants.
• Almond leaf scorch viroid (ALSVd) affects almonds and other Prunus species.
• Grapevine fanleaf virus (GFLV) infects grapes and other Vitis species.
• Avocado sunblotch viroid (ASBV) is another important viroid that affects avocado trees resulting in lower yields and poorer quality fruit.
• Other diseases of plants caused by viroids include citrus exocortis, cucumber pale fruit, and chrysanthemum stunt.
• The only human disease caused by viroid is Hepatitis –D.

Symptoms of Viroid Diseases in Plants

The symptoms of plant diseases caused by viroids are as follows:

• Plant growth is stunted and retarded resulting in smaller plants.
• Necrosis and chlorosis of stems and leaves are observed which may lead to the death of the plant.
• Leaves and fruits are often deformed and discolouration and distortion of leaves may happen.
• Other symptoms include dwarfing, epinasty, distortion, mottling, development of chlorotic or necrotic spots, cankers, scaling and cracking of bark, and malformation of tubers, flowers, and fruits.

Prevention of Viroid

Different measures should be taken to prevent viroid diseases which are as follows:

• As the viroids are spread mechanically, planting materials should be made disease-free by cleaning cutting knives, and all planting and field equipment thoroughly.
• The instruments which were used on infected crops should not be reused before sterilization.
• To prevent the spread of viroid, different disinfectants such as 1-5% sodium hypochlorite, 6% hydrogen peroxide, 2% formaldehyde, etc. should be used to clean instruments.
• To prevent the spread of viroid disease, humans should stay up-to-date on vaccinations.

Conclusion on Viroid

Viroids are single-stranded circular RNA without any capsid which is mainly categorized into two families – Pospiviroidae and Avsunviroidae. They are highly infectious mainly to plants. Viroids diseases in plants leads to stunted growth and development of plants, stem necrosis, deformation of leaves and fruits, and ultimately leads to death. The damages caused by viroid to economically important plants can be overcome by using some preventive measures.

FAQs on Viroid – Structure, Characteristics, Features

What is Viroids in Biology?

Viroids are small, infectious RNA molecules that lack a protein coat and cause diseases in plants by disrupting normal cellular functions. They are simpler in structure compared to viruses and are responsible for various plant diseases.

What is the Shape of a Viroid?

The viroid is a single-stranded piece of circular RNA that has an unusual structure due to complementary base pairing. Some form a simple rod-like structure, whereas other viroids have a complex branched structure.

Which Feature of Viroid has Genetic Material?

Viroids contain genetic material in the form of RNA. RNA is a nucleic acid molecule that is made up of nucleotides.

What is the Structure and Characteristics of Viroids?

Viroids are single-stranded, covalently closed circular RNA molecules with extensive regions of intramolecular complementarity. They exist in their native state as highly base-paired rods.

What is the Structure of the Viroid Genome?

Viroids are small, non-encapsidated, single-stranded, circular RNAs. The viroid genome consists of 246–434 nucleotides. It autonomously replicates when it invades a host cell.

What is Difference Between Virus and Viroids class 11th?

Viruses are infectious agents composed of nucleic acid surrounded by a protein coat, while viroids are smaller infectious agents consisting only of a circular RNA molecule without a protein coat.

What is the Difference Between a Viroid and Prion?

Prions are infectious proteins that cause neurodegenerative diseases in animals, while viroids are small infectious agents consisting of circular RNA molecules that primarily infect plants, leading to various diseases.

Are Viroids Living or Non Living?

Viroids are non-living infectious agents consisting only of a circular RNA molecule without a protein coat, lacking the ability to replicate on their own and relying on host cells for their reproduction.

What is an Example of a Viroid Disease?

An example of a viroid disease is the Potato spindle tuber disease caused by the Potato spindle tuber viroid (PSTVd), which affects potato plants, leading to reduced yields and quality.

What is Rhinovirus?

Rhinovirus, belonging to the Picornaviridae family, is a common respiratory virus responsible for causing the common cold in humans, characterized by symptoms like runny nose, sore throat, coughing, and sneezing.