1 December 2023

The viroid concept is related to an unusual entity in the field of viruses. Unlike conventional viruses, viroids are very small circular RNA molecules, devoid of the protein coat that characterizes viruses. These infectious molecules can cause disease in plants. For this reason, microbiology and phytopathology study these microorganisms to avoid the threat it poses to agriculture and food security

viroids are a subject of great interest to biologists and farmers, since its study and control can help prevent plant diseases and improve its production. In this introduction, we will explore the most important aspects of viroids, their structure, their mode of replication, and their role in plant disease.

Definition of a viroid in biology

In the field of biology, a viroid is a circular RNA molecule devoid of a protein envelope, which can cause diseases in plants. Unlike viruses, viroids do not contain prions and their genome is composed of a single, short RNA strand, generally between 246 and 399 nucleotides.

Viroids are capable of interfering with the DNA replication of plant cells and their normal process of growth and development, which can have a negative impact on the quality and quantity of the harvest. Due to its unique structure and its ability to cause infection In plants, viroids turn out to be a relevant topic in the investigation of plant biology and agronomy.

The structure and composition of the virion varies according to the type of virus. Some viruses have a lipid envelope that protects them and allows them to interact with host cells, while other viruses are unenveloped and comprised solely of a protein capsid. The capsid is a protein structure that contains the genetic material of the virus that protects and maintains nucleic acid integrity during virus transmission.

What is the function of a viroid?

In biological terms, the viroids do not fulfill a specific function in the infected plant. Unlike viruses, viroids do not code for viral proteins, enzymes, or structures, and have not been shown to provide any evolutionary advantage to infected plants. Instead, viroids appear to be cell parasiteswhich take advantage of the replication process of the host cell for their own reproduction and propagation.

By doing so, they can interfere with normal host cell processes and cause damage to plants. In terms of their function, viroids are pathogenic infectious agents that cause diseases in plants, which can lead to significant losses in agricultural production.

How are viroids formed?

Viroids are formed from a single-stranded RNA molecule that replicates autonomously in the host cell. Viroids are thought to have evolved from introns, which are non-coding segments of RNA found within lThe genes of plants and other organisms. During evolution, some of these introns developed the ability to replicate independently and became viroids.

Viroid replication is mediated by the enzyme RNA polymerase II, which normally synthesizes RNA in plant cells. However, viroids are capable of hijacking this enzyme and using it for their own replication. During replication, the viroid RNA folds into a highly stable double helix structure, which helps protect it from degradation by cellular enzymes. This double helix structure also appears to be essential for viroid infectivity and its ability to cause disease in plants.

What do viroids infect?

viroids infect plants and are responsible for diseases known as viroidosis. Viroids have been identified in a wide variety of cultivated plants, including crops of economic importance such as potatoes, tomatoes, cucumbers, strawberries, avocados, and coconuts, among others.

viroids are specific to plants and do not infect animals or other organisms. Although viroids can infect different parts of plants, such as leaves, stems, and fruits, they have been found more frequently in reproductive organ tissues, such as seeds and cuttings.

The symptoms of viroid infection in plants can vary, some examples are:

  • Deformation of leaves and stems.
  • Stunted growth.
  • Reduction in the production of fruits or seeds.
  • Necrosis.
  • Eventual death of the plant.

What are the characteristics of viroids?

The main characteristics of viroids are:

  • Structure: Viroids are single-stranded circular RNA molecules, which can range in length from 246 to 399 nucleotides. The single-stranded RNA is highly folded and forms a highly stable double-helical secondary structure that is essential for its infectivity. The secondary structure of viroid RNA is very complex and can be composed of a variety of structural elements, such as loops, hairpins, stems, and mating regions.
  • Replication: Viroids are capable of reproducing themselves within the nucleus of host plant cells, using the cell’s replication machinery. During this process, the RNA strand of the viroid folds into a highly stable double helix structure, which is essential for its infectiveness. Viroids take advantage of the host cell’s RNA polymerase II to produce new copies of viroid RNA and can also use the host cell’s proteins to complete their replication cycle.
  • specificity: Viroids are specific to plants and do not infect animals or other organisms. Each plant species may be infected by a specific set of viroids, and within a plant species, different cultivars may have different susceptibilities to viroid infection.
  • Transmission: Viroids are transmitted from one plant to another of the same species through vegetative propagation, such as using infected cuttings or planting infected seeds. Viroids can also be transmitted by insect vectors, such as aphids and thrips, but this is less common.
  • pathogenicity: Viroids can cause diseases in plants, which can vary in severity and symptoms, such as deformation of leaves and stems, growth retardation, reduced fruit or seed production, necrosis, or subsequent death of the plant. The pathogenicity of a viroid depends on factors such as the host plant species and variety, the specific strain of the viroid, and environmental conditions.

What is the size of a viroid?

viroids are single-stranded circular RNA molecules they are very small in size compared to viruses or other infectious organisms. Their size varies between 246 and 399 nucleotides, which allows them to be transported through nuclear pores and replicate in the nucleus of plant cells.

In terms of physical size, viroids are about 10 to 100 times smaller than a typical virus. Due to their small size, viroids cannot be observed with conventional light microscopes and require high-resolution microscopy techniques, such as transmission electron microscopy, for visualization.

What is the structure of a viroid?

Viroids are single-stranded circular RNA molecules that fold into a highly stable double helix structure which is essential for its infectious capacity. The structure of viroid RNA is very complex and can be composed of a variety of structural elements, such as loops, hairpins, stems, and mating regions.

  • Stem: It is a structure that is formed when two complementary regions of RNA are joined by base pairs and paired forming a double helix structure.
  • Fork: It is a structure that forms when a region of RNA folds on itself and pairs with itself, forming a structure similar to that of a hairpin.
  • Loop: It is a structure that forms when a region of RNA cannot pair with another complementary region and folds into a loop-shaped structure.
  • Region of mating: It is a region of RNA that pairs with another complementary region to form a double helix structure.

What is the importance of viroids in the cell?

Despite its small size, viroids are of great importance in the cell, since its presence can have significant effects on the physiology of the infected plant. Some of the main implications of the presence of viroids in cells are:

  • Interference in gene expression: Viroids can interact with cellular RNA and alter the regulation of gene expression, which may have effects on protein synthesis and cellular metabolism.
  • Induction of symptoms: Viroids can cause symptoms in infected plants, such as leaf curl, reduced growth and fruit set, and necrosis.
  • Interference in defense against pathogens: Viroids can affect the host plant’s response to other pathogens and reduce its ability to defend itself against other infections.
  • Alteration in cell structure: The presence of viroids in cells can have an impact on the structure of infected cells, which can alter their function and ultimately their survival.

Examples of viroids in biology

Viroids are very small pathogens that infect plants. Here are some examples of viroids in biology:

  • Potato viroid (PSTVd): It is the best known viroid and was the first to be discovered. It infects potatoes and other solanaceous crops, such as tomatoes and peppers. It produces symptoms such as the rolling of the leaves, the reduction in the size of the tubers and the decrease in the quality of the crop.

  • Coconut cadang-cadang viroid (CCCVd): This viroid infects the coconut palm and causes coconut ringspot, which produces a significant reduction in crop yield.

  • Citrus exocortis viroid (CEVd): This viroid infects citrus and produces a variety of symptoms, including reduced growth and fruit set, as well as deformation of leaves and fruits.

  • Chrysanthemum stunt viroid (CSVd): This viroid infects chrysanthemums and causes growth retardation, leaf deformation, and reduced flower bud size.

  • Tomato apical stunt viroid (TASVd): This viroid infects tomato and produces symptoms such as reduced growth and deformation of the leaves.

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