Viroids- Definition & Structure

Viroids are the infectious pathogens, highly structured and small circular ribonucleic acids (RNA’ s) with nonprotein‐coding that are able to replicate independently in the host plants and which induces diseases in the higher plants.

Viroids differ from the viruses in structure and form. These consists of solely short strands of circular, and single-stranded RNA without the protein coats.

The plants that are infected by viroids are responsible for the crop failures and also causes loss of million of dollars in the agricultural revenue every year. Some of the plants that are affected by this pathogens are potatoes, tomatoes, cucumbers, chrysanthemums, coconut palms, avocados, etc.

Viroids was first found by T.O. Diener in the year 1971. It was first examined in the potato spindle tuber viroid that caused a huge loss to the potato industry.

Viroids are the plant parasites like transcriptional machinery of the organelles such as nucleus or the chloroplast, since they are known to be non-coding. These replicate by the process of RNA–RNA transcription. They mainly infect the epidermis of the hosts after causing a mechanical damage to the cell wall of the plant.  

Viroids- Characteristics & functions

Some of the characteristic features and functions of viroids are given below-

  • Viroids are the species of nucleic acid with relatively low molecular weight and a unique structure
  • These are among the smallest known agents causing infectious disease.
  • They reproduce within the host cell which they affect in and cause variations in them causing death.
  • Viroids are formed only on the RNA
  • These are known to be smaller in size and infect only the plants.
  • Viroids are said to move in intracellular manner, cell to cell through the plasmodesmata, and a long distance through phloem.
  • Viroids are mainly classified into two families namely Pospiviroidae- nuclear viroids and Avsunviroidae- chloroplastic viroids.

Viroid- Diseases

Some of the diseases that are caused by the infection of viroid in plants are citrus exocortis, cucumber pale fruit, chrysanthemum stunt. These diseases are spread by the propagation of seeds in plants by cutting, tubers etc and also by mishandling the contaminated implements. The only disease that is caused by viroids in humans is Hepatitis- D.

The symptoms that are caused by the infection of viroid in plants includes stunting of growth, stem necrosis, deformation of the leaves and fruits, and at last causing the death of plant.

Most of the viroids are said to infect the plants, including coconut and the apple trees. The (PSTV) potato spindle tuber viroid causes a significant crop damage to the potato yields causing the tubers to elongate and then crack. The other common type of viroid infection symptoms includes stunting and the leaf epinasty.

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