What are Viroids?
You must be aware that the virus has a protein coat or casing around them. Viroids are the smallest infectious pathogens (agents with the ability to cause disease) 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 characterized, 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 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.
Structure of Viroids
Viroids are an independent class of plant pathogens. Two rounds of rolling Viroids were discovered in the year 1971 by T.O Diener. It was found after the huge loss in the potato industry after examining the potato spindle tuber Viroid.
The structure and form of Viroids are different from that of viruses. Viroids contain short strands of circular and single-stranded RNA without the protein coats. The plants that are usually affected by Viroids are potatoes, tomatoes, coconut palms, avocados, etc.
The crops that are infected by Viroids are the reason for crop failure causing huge losses to the agriculture industry. The agriculture industry also loses a lot of revenue every year due to this infection in crops.
In comparison with other infectious plant pathogens, Viroids are very small in size and hence contain only Ten Thousand atoms. Viroids cause mechanical damage to the cell wall of the crops by infecting the epidermis.
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 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 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.
Symptoms of Viroid Infection in Crops
The infection in Viroids are not visible to the naked eye and hence the symptoms are not very prominent unless the crop is infected. The most common symptom of the Viroid infection is no growth or development in the crop, also known as Retardation of Plant Growth and Stunting, which causes dwarfism in crops.
The potato spindle tuber viroid causes the potato to be smaller in size in comparison with a healthy potato. The tuber symptoms are quite prominent as the tubers are spindled in shape.
The infected citrus trees with the citrus exocortis viroid are stunted (Dwarfism). The symptom in infected citrus trees includes scaling of the bark below the graft union. The fruit of a stunted citrus tree is normal.
Stunting is an important symptom in tomatoes infected by the tomato bunchy top viroid. The hop plants and chrysanthemum plants also show the symptom of stunting if infected by the hop stunt viroid in the case of hop plants and chrysanthemum stunt viroid and the chrysanthemum chlorotic mottle viroid.
How do Viroids Spread?
Viroids are contagious pathogens and are easily transmitted from one crop to another crop. The most common reason for the spread of Viroids is any kind of contact with the already infected plant.
For example Leaf contact or using contaminated pieces of equipment (Planting and Cultivating). The Viroids spread widely in case there is an operation issue where contaminated tools and hands are used. There are also reports of Viroids being spread through insects. The Potato spindle tuber viroid was reported to be spread by grasshoppers, plant bugs, flea beetles, etc. Even though the spread through insects is worrisome it does not infect as many plants as it does by contamination of tools and hands.
How to Avoid Viroids?
Viroids cause economic loss. Potato spindle tuber viroid is one such example that causes severe yield losses. As viroids spread through transmission, uninfected tools and hands have to be used. The operation issue has to be managed well to avoid any leaf contact or usage of the same tools used in infected crops. The pathogen was 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 chemical methods offers a new perspective to control viroid transmission through tools.
What are Virusoids?
As we know Viroids contain a linear, single-stranded molecule of genomic RNA. There are some Viruses that contain a Viroid like satellite RNA in addition to the other characteristics of Viroids. These Virusoids show very little sequence homology with viroids. The Tobacco ringspot virus shows significant homology with the linear satellite RNA. The Viroids are independent circular molecules and they do not code for polypeptides (a single linear chain of many amino acids).
FAQs on Viroids
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 replication error. 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.
3. Do Viroids contain DNA?
Previously, scientists did believe that Viroids have DNA and RNA as they are infectious particles. Stanley Prusiner discovered diseases were caused by prions (abnormal, pathogenic agents that are transmissible) in the year 1982. As Proteins are acellular and do not contain DNA and RNA the findings by Stanley Prusiner were not accepted. Later, in 1997 this research was validated and Stanley Prusiner received a Nobel Prize in Physiology. Thus, Viroids have an RNA genome (the complete set of genes or genetic material present in a cell or organism) and have a DNA homologue (One member of a chromosome pair).
4. Which disease is caused by Viroids?
Viroids can only reproduce within a host cell. They do not have an outer envelope or Capsid (Protein shell of a Virus) present. After the discovery of PSTV (Potato Spindle Tuber Viroids) other Viroids were discovered as well that caused diseases in plants. For example, the (TPMV) Tomato Planta Macho Viroid infects Tomato plants. The only human disease that can be infected with humans is Hepatitis D. Hepatitis D was initially classified as a defective Virus called the Delta Agent.
5. How do Viroids enter plant cells?
Though Viroids are different from viruses in all aspects (Structure, Functions & Evolution). Viroids are small and do not have a protein-coding but they still can infect higher plants and cause infection in them. Viroids enter the nucleus or the chloroplast for replication (intracellular movement) to infect a plant cell. The infection then exits to the cytoplasm passing through the plasmodesmata to neighboring cells (cell to cell movement) and reaching the vasculature to invade different parts of the plant through the vascular system.
