What are Viroids?

You must be aware that the virus has a protein coat or casing around them. Viroids are the smallest infectious pathogens made up of a short circular RNA without the protein coat. These pathogens affect only the plants and are therefore known as plant pathogens. Single-stranded circular RNA molecules, viroid vary in length from 246 to 463 nucleotides and are found only in plants. So far, 30 viroids have been characterised, 28 infect the dicotyledonous (double- seeded) plants, and the other 2 infect the monocotyledonous (single-seeded) plants. In this part, we will study the viroid definition and know more about the structure of viroid. 

One can call these pathogens as sub viral agents. The diseases caused by some of these viroids or pathogens are of considerable importance in agriculture. Viroids do not encode protein; however, they replicate independently when introduced into host plants. Thirty viroids have been discovered in experiments initially conducted to check for viruses. Fig.1. Potato spindle tuber viroid found in 1971 is a prototype.

Image will be uploaded soon

Types of Viroids

The 30 known viroids belong to two families- Pospiviroidae and Avsunviroidae.    

1 – Pospiviroidae:  Members of the pospiviroidae are named after the potato spindle tuber viroid. They have a rod-like structure with a small single-stranded region. They also have a central conserved region and replicate in the nucleus.

In plants that are infected with members of the Pospiviroidae, viroid RNA is imported into the nucleus and copied by plant DNA-dependent RNA polymerase II.  The viroid is copied by a rolling circle mechanism which produces complementary linear and concatemeric RNAs.

These are copied again to produce concatemeric linear molecules which are cut by the host enzyme RNA polymerase III. Their ends are joined by a host enzyme to form circles.

2 – Avsunviroidae: is named after avocado sunblotch viroid. This viroid also has rod-like branched regions. However, this viroid lacks a central conserved region and replicates in the chloroplast. RNA molecules are working ribozymes, and this activity is crucial for replication.  

In plants infected with members of the Avsunviroidae family, viroid RNA is imported into the chloroplast, and complementary concatemeric RNAs are produced by chloroplast-dependent RNA polymerase.

Cutting of cleavage of these molecules is done by a ribozyme, an enzyme encoded in the viroid RNA.

Viroids are known to affect only plants. This affliction of the disease can cause economic loss and destroy important crops, fruits, and plants. Two clear examples of economically significant viroids are coconut cadang-cadang viroid. It creates a deadly infection in coconut palm trees. The second example is apple scar skin viroid that causes a disease in apple trees that result in visually unappealing apples. Some other cases along with scientific notations are as follows,

Examples of a Viroid


Viroid sub-group





Avocado sunblotch viroid




Chrysanthemum stunt viroid


Images will be uploaded soon

Viroid Diseases 

The diseases that are caused by viroids in plants are citrus exocortis, cucumber pale fruit, chrysanthemum stunt, etc. These infections spread due to many reasons. Some infections spread due to the propagation of seeds in plants by cutting, tubers etc. The disease can even spread by mishandling of the contaminated equipment and implements.

Viroid progeny moves to adjacent cells through plasmodesmata. It can travel systematically via the phloem to infect other cells. Viroids enter the pollen and the ovule. From here they are transmitted to the seed. When seeds germinate, the new emerging plant also becomes a victim of infection. Viroids can also spread among plants by contaminated insects.

The common symptoms of viroid diseases in plants include stunting of growth, deformation of leaves and fruit, stem necrosis and finally, death of the plant.

In humans, the only disease caused by viroid is Hepatitis –D.

How to Avoid Viroids 

Viroids cause economic loss. Potato spindle tuber viroid is one such example that causes severe yield losses. The pathogen spared fast within the culture. Different chemical substances such as 1-5% sodium hypochlorite, 6% hydrogen peroxide, and 2 % formaldehyde are used as disinfectants to prevent viroid transmission in field conditions—these disinfectants clean tools such as knives, pruning, and grafting devices to eliminate viroids. The possibility of viroid inactivation by chemicals methods offers a new perspective to control viroid transmission through tools.

FAQ (Frequently Asked Questions)

1. What is the origin of Viroids?

There is a theory that the disease-causing viroids were transferred from wild plants used for breeding modern plants. Their large-scale occurrence can be traced to the use of genetically identical plants, worldwide distribution of breeding lines, and the mechanical transmission by contaminated machinery. As a result, these unusual pathogens now occupy a special place. The origin of viroids remains a mystery.  Viroids have properties that make them part of the RNA world; small genome size to avoid error in replication. They have found a place around the earth that was never available to them. Today viroids no longer self-replicate, they became parasites of plants.

2. Who discovered Viroids?

While carrying out efforts to identify the cause of potato spindle tuber disease, Dr Theodor Diener discovered viroids. The condition was supposed to be caused by a virus. Experiments yielded unexpected results. Observations revealed that most of the infectious agents present in infected samples did not sediment into pellets when subjected to centrifugal force- this is a test for the virus. The infecting agent was a different particle. It was sensitive to treatment with RNA and insensitive to DNA, phenol, chloroform, n-butanol or ethanol. In 1967, it was clear that the agent of the potato spindle was not a virus- a new term viroid was invented for this free infectious RNA.