Viroids: Explanation, Structure, Types and Symptoms

Viroids are an independent class of plant pathogens. T.O. Diener made the discovery of two rounds of rolling viroids in 1971. It was discovered when the potato business suffered a significant loss after closely analyzing the potato spindle tuber. 

• They are contagious diseases made out of short, single-stranded circular RNAs.
•  They lack a protein covering, in contrast to viruses. 
• Only plants are known to have the single-stranded circular RNA molecules known as viroid, which range in length from 246 to 463 nucleotides. 
• As of now, 30 viroids have been identified; 28 of them affect monocotyledonous (single-seeded) plants, and the other 2 affect dicotyledonous (double-seeded) plants.

Read more: Virology

KeyTerms: Viroid, Crops, Plant cells, RNA strands, Protein cover, Virusoids

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Definition of a Viroid

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Viruses are infectious diseases that solely harm plants; as a result, they are often referred to as plant pathogens. Viroids have a lower structural size than viruses and circular RNA strands without a protein covering. To make more copies of themselves, these creatures use the cellular machinery found in plant cells. It mostly has an impact on higher plant species.

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Structure of a Viroid

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Viroids are different from viruses in form and structure. These are made up entirely of single-stranded, circular, short RNA without any protein coverings.

Viroid particles

Crop failures and the loss of tens of millions of dollars in agricultural revenue each year are caused by viroids-infected plants. These diseases can damage a variety of plants, including avocados, chrysanthemums, coconut palms, potatoes, tomatoes, and cucumbers.

Since they are known to be non-coding, viroids resemble the transcriptional machinery of cell organelles like the nucleus or chloroplast, making them plant parasites. These carry out their replication via RNA-RNA transcription. They mostly infect their hosts' epidermis after mechanically breaking down the plant's cell wall.

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Characteristic Features of Viroids

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The following are some viroids' distinguishing characteristics:

• Only RNA is present in viroids.
• These are known for being smaller in size and exclusively infecting plants.
• These are some of the tiniest infectious disease-causing agents ever discovered.
• Viroids are a kind of nucleic acid with a distinct structure and a low molecular weight.
• They replicate inside the host cell, which they influence and change, ultimately leading to death.
• It is said that viroids travel intracellularly, large distances through the phloem, and from cell to cell by plasmodesmata.
• Pospiviroidae, which includes nuclear viroids, and Avsunviroidae, which includes chloroplastic viroids, are the two primary families into which viroids fall.

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Types of Viroids

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Pospiviroidae and Avsunviroidae are two families to which the 30 known viroids belong.

• Pospiviroidae: The potato spindle tuber viroid serves as the surname for members of the pospiviroidae family. They feature a single-stranded area and a rod-like shape. They replicate in the nucleus and feature a core conserved area.
• Avsunviroidae: It takes its name from the avocado sunblotch virus. The branching portions of this viroid similarly resembles rods. However, this viroid replicates in the chloroplast and lacks a core conserved area. Working ribozymes are found in RNA molecules, and replication depends on their activity.

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Viroid Disease

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Citrus exocortis, cucumber pale fruit, chrysanthemum stunt, etc. are a few of the plant diseases brought on by viroids. Any sort of interaction with a plant that has already been infected is the most frequent cause of the dissemination of viroids. 

There are several factors that contribute to the spread of these illnesses.

•  Some illnesses are transferred as a result of plants' ability to reproduce their seeds by cuttings, tubers, etc. 
• Even improper handling of infected tools and objects might transmit the illness.
• Plasmodesmata allow the transmission of viroid progeny to neighboring cells. 
• The phloem can be used to spread the infection to additional cells in a systematic manner. 
• Pollen and ovules are both entry points for viroids. 
• They go to the seed from this point. 
• As soon as seeds are planted, an infection spreads to the newly emerged plant. 
• Furthermore, tainted insects can transmit viruses across plants. 
• There have been reports of grasshoppers, plant bugs, flea beetles, and other insects spreading the Potato Spindle Tuber Viroid. 
• Although the spread by insects is significant, it infects fewer plants than the contamination of hands and instruments.
• Viroid infections commonly manifest in plants as stunted development, malformed leaves and fruit, stem necrosis, and ultimately plant death.
• Hepatitis-D is the only virus-based illness that affects humans.

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Symptoms of Viroid Infection in Crops

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Viroid infections are invisible until the crop is infected, the symptoms are not very noticeable. Retardation of Plant Growth and Stunting, often known as no growth or development in the crop and leading to dwarfism in crops, is the most typical sign of the viroid infection. The potato is smaller than a healthy potato as a result of the potato spindle tuber virus.

Potato spindle tuber virus

Citrus trees with the citrus exocortis virus infection are stunted (Dwarfism). Scaling of the bark below the graft union is one sign of the disease in citrus trees.

The tomato bunchy top viroid infection in tomatoes causes stunting and when chrysanthemum and hop plants are both infected with the chrysanthemum chlorotic mottle viroid and the hop stunt viroid, the affected plants exhibit the stunting symptom.

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How to Avoid Viroids?

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Viroids result in financial loss. Some of the measures to avoid viroids are as follows:

•  It is necessary to use clean instruments and hands since viroids are transmitted via touch. 
• Any leaf touch or the use of instruments that have previously been used on infected crops should be prevented. 
• To prevent the spread of viroids in field settings, several chemical compounds including sodium hypochlorite, hydrogen peroxide, and formaldehyde are employed as disinfectants to clean instruments like knives etc. 
• The potential of controlling viroid transmission through tools is made possible by chemical viroid inactivation.

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What are Virusoids?

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In addition to the various features of viroids, certain viruses also include a satellite RNA that resembles viroids. Very little sequence homology exists between these virusoids and viroids. Significant similarities between the linear satellite RNA and the Tobacco ringspot virus have been found. The viroids are circular, autonomous molecules that lack polypeptide coding.

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Things to Remember

• Viroids are made up entirely of single-stranded, circular, short RNA without any protein coverings.
• Pospiviroidae and Avsunviroidae are two families to which the viroids belong.
• Viroids travel intracellularly, large distances through the phloem, and from cell to cell by plasmodesmata.
• Viroid infections commonly manifest in plants as stunted development, malformed leaves and fruit, stem necrosis, and ultimately plant death.
• It is necessary to use clean instruments and hands since viroids are transmitted via touch.
• To prevent the spread of viroids in field settings, several chemical compounds are employed as disinfectants to clean instruments to get rid of viroids.