Viruses and Viroids
What are Viroids?
Viroids are infectious pathogens that affect only plants, therefore are also called as the plant pathogens. Structurally, viroids are smaller than viruses and possess circular strands of ribonucleic acids (RNA’s) with no protein coating. These entities hijack the cellular machinery present in plant cells to replicate new copies of itself. It primarily affects all forms of higher plants.
Structure Of Viroids
Viroids differ from the virus 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 millions of dollars in the agricultural revenue every year. Some of the plants that are affected by these pathogens are potatoes, tomatoes, cucumbers, chrysanthemums, coconut palms, avocados, etc.
Viroids were first discovered 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 cell organelles such as the 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 mechanical damage to the cell wall of the plant.
Characteristic Features Of Viroids
Some of the characteristic features of viroids are given below-
- Viroids are formed only on the RNA.
- These are known to be smaller in size and infect only the plants.
- These are among the smallest known agents causing infectious disease.
- Viroids are the species of nucleic acid with relatively low molecular weight and a unique structure.
- They reproduce within the host cell which they affect in and cause variations in them causing death.
- Viroids are mainly classified into two families namely Pospiviroidae- nuclear viroids and Avsunviroidae- chloroplastic viroids.
- Viroids are said to move in an intracellular manner, cell to cell through the plasmodesmata, and a long-distance through the phloem.
Some of the diseases that are caused by the infection of viroid in plants are citrus exocortis, cucumber pale fruit, chrysanthemum stunt. These infectious 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 include stunting of growth, stem necrosis, deformation of the leaves and fruits, and at last causing the death of the plant.
Most of the viroids are said to infect the plants, including coconut and the apple trees. The (PSTV) potato spindle tuber viroid causes 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 leaf epinasty.