Viroids: Definition, Meaning, Types, Examples, Diseases, Symptoms

Definition Of Viroids

Viroids are small, circular RNA molecules without a protein coat, different from viruses and all other pathogens. They are composed of only a single-stranded RNA genome, typically 250-400 nucleotides in length, which can initiate infectious diseases in plants. Viroids multiply autonomously on plant host cell enzymes and have been demonstrated to trigger specific diseases through the disruption of cellular processes of plants—for instance, RNA silencing mechanisms.

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This Story also Contains

1. Definition Of Viroids
2. Differentiating Viroids From Viruses And Other Pathogens
3. Structure And Composition Of Viroids
4. Replication Mechanism Of Viroids
5. Classification And Types Of Viroids
6. Transmission And Spread Of Viroids
7. Symptoms And Impact Of Viroid Infections
8. Detection And Diagnosis Of Viroids
9. Control And Management Strategies
10. Ecological And Environmental Impact Of Viroids

Differentiating Viroids From Viruses And Other Pathogens

Viroids are more primitive, with no protective coat or capsid and are much smaller than the viruses. They do not encode proteins, and point to the host cellular machinery entirely for replication, and spread. They are also small in size and cause disease in plants mainly, unlike viruses that can infect a wide diversity of living beings, plants, animals, and bacteria, among other organisms. What makes these viroids difficult to detect and manage in the agricultural setting is their uniqueness in inducing diseases where there seems to be no mechanism to encode proteins.

Structure And Composition Of Viroids

Viroids are single-stranded RNA molecules of 246 to 401 nucleotides that fold into a stable, base-paired secondary structure. Viroids do not have a coat protein and they don't code for any proteins; replication is dependent on host enzymes. This structural simplicity underlines a basic biochemical difference between these organisms and viruses or any other type of pathogen. Their variability at the level of RNA sequence and secondary structure affects their ability to cause plant diseases, underscoring the need for an understanding of their structure in coming up with effective detection strategies and management measures for agricultural applications.

Replication Mechanism Of Viroids

Viroids replicate by a mechanism called rolling circle replication, which occurs in several discrete steps. First, host RNA polymerase enzymes transcribe the viroid RNA into a complementary RNA strand. This newly synthesised strand will be used as the template for the actual rolling circle replication. In this replication, an RNA strand is synthesised continuously. It starts making a long concatemeric strand that later gets processed for new infections into host cells to create individual viroid RNA molecules.

Viroids interact intimately with host plant enzymes in the replication cycle. They command host RNA polymerase enzymes for the transcription of their RNA and make use of the host-cell apparatus for all other processes: RNA processing, transport, etc. These interactions determine the success of viroid replication and spread in plant tissues. Therefore can be targeted to design control measures for infection by viroids in agriculture, particularly in the context of yield and plant health impacts.

Classification And Types Of Viroids

Based on their genomic structure, there exist two families of viroids: Pospiviroidae and Avsunviroidae.

• Pospiviroidae: They have a rod-like secondary structure and are capable of self-replication in the nucleus.

• Avsunviroidae: A family distinguished by their circular secondary structure and replication in chloroplasts.

Examples Of Major Viroid Families

Pospiviroidae: This family of viroids contains Potato spindle tuber viroid, Citrus exocortis viroid, and Tomato chlorotic dwarf viroid.

Avsunviroidae: This family contains Avocado sunblotch viroid and Peach latent mosaic viroid.

Transmission And Spread Of Viroids

The transmission of viroids occurs through different mechanisms, of which the most prominent is direct infection from infected plant materials (seeds, pollen, vegetative propagation: cuttings, grafting). Transmission from one plant to the other may also occur because of mechanical contact during agricultural practices like pruning or harvesting.

Several factors drive the infection dynamics of viroids in plants. The ability of the viroid to transmit itself is determined by its stability and potential for survival outside host cells at various environmental temperatures and humidity conditions. Another is the susceptibility of the host plant species itself and the presence or absence of insect vectors.

Symptoms And Impact Of Viroid Infections

Viroid infections in plants often show distinctive symptoms of stunting growth, distortion in leaves, and the flowering and development of fruits. This is because of viroid replication, which is at the expense of interfering with the RNA silencing of the plant. Further, viroids may produce severe effects such as chlorosis, necrosis, and may lead to reduced vigors, hence causing significant agricultural loss.

The economic impact of viroid infection is huge on agricultural produce in terms of reduced yield and quality. Often, the presence of viroids in most cases results in a reduction in the quantity of fruit from infected plants and reduces quality by developing deformations. In most cases, it makes the plant more vulnerable to other pathogens. Therefore, control measures for their economic management in crop production systems involve strict sanitary practices, the use of viroid-free planting materials, and early detection through diagnostic techniques.

Detection And Diagnosis Of Viroids

Molecular techniques form the mainstream methods for the detection and diagnosis of viroids. This majorly focuses on Reverse Transcriptase Polymerase Chain Reaction and Northern blotting. RT-PCR is a very sensitive and specific method for identification since it can effectively amplify and detect RNA of viroids from plant samples. On the other hand, Northern blotting confirms the existence of viroid RNA by size detection and further identifies it.

Control And Management Strategies

The control and management strategies of viroids in agriculture involve strict cultural practices of sanitation. By this infected plant materials are removed and destroyed to avoid the spread of the viroid. Resistance is also conferred by the use of resistant varieties of plants that are picked from breeding programs with increased resistance to incidences of infection. Such measures ensure low viability in transmission, thus protecting crop yield and maintaining agricultural productivity in regions where incidence is recorded.

Ecological And Environmental Impact Of Viroids

Viroids play a significant role in natural ecosystems and plant communities, as is the case with other plant pathogens: they alter the health and dynamics of plant populations. In this way, they can impact biodiversity through their effects on the growth, reproduction, and survival of infected plant species. These changes in the composition of species within the ecosystem as a result of viroid-induced diseases may affect community structure and finally the functioning of the ecosystem. Therefore, an understanding of these ecological impacts is central to a complete assessment of how viroids contribute to healthy ecosystems and biodiversity conservation.

The video describing the viroids is given below: