VIROIDS
Dr. Sanjeev Kumar
Assistant Professor/Scientist Plant Pathology JNKVV-Jabalpur
[email protected]
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VIROIDS
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1. Viroids are small, low molecular weight ribonucleic acids that can infect plant cells, replicate themselves, and cause disease . 2. Viroids differ from viruses in at least two main characteristics: (1) the size of RNA in viroids, which consists of 250 to 370 bases, is much smaller compared to that in viruses, which is 4 to 20 kilobases. (2) Virus RNA is enclosed in a protein coat whereas viroids lack protein coat and apparently exist as free (naked) RNA. 3. Their small size (250–370 nucleotides), viroids lack sufficient information to code for even one protein, even for a replicase enzyme required to replicate the viroid. 4. The existence of viroids as free RNAs rather than as nucleoproteins necessitates the use of phenol in the sap to inactivate the plant ribonucleases and makes their visualization with an electron microscope
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Viroids are circular, single-stranded RNA molecules with extensive base pairing in parts of the RNA strand. The base pairing results in some sort of hairpin structure with single-stranded and double-stranded regions of the same viroid and contributes to the stability of the RNA, given that it lacks a protein coat. It appears that, in its double-stranded form, each viroid consists of five structural regions: a left and a right terminal region, a pathogenicity region, a conserved central region, and a variable region. The terminal and pathogenicity regions determine the pathogenicity of a viroid, i.e., its ability to infect and multiply, and also the severity of the symptoms that will develop on the host plants. The severity of the symptoms, however, can be altered by changes in one or two bases in these regions. The other two regions of viroids, the conserved central region and the variable region, have not been implicated in any function of viroids.
Taxonomy (Grouping) of Viroids
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1. The taxonomy of viroids is based on the absence in some of them (the avocado sunblotch viroid group, group A, or Avsunviroids) of a conserved central region or the presence in them (the potato spindle tuber viroid group, group B, or Pospiviroids) of a central conserved region. 2. The avocado sunblotch viroid (ASBVd) group has only four members, whereas the potato spindle tuber viroid (PSTVd) group has all the rest of the 40 viroids. 3. All Avsunviroids have a ribozyme activity that enables them to self-cleave their RNA multimers during viroid replication. 4. Avsunviroids replicate in chloroplasts, whereas Pospiviroids replicate in the nucleus and nucleolus. 5. Both groups are subdivided into subgroups depending on sequence similarities in the conserved central region.
Classification
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Viroids ASBVd group or Avsunviroids Avsunviroideae Avsunviroid Avocado sunblotch viroid Pelamoviroid Chrysanthemum chlorotic mottle viroid Peach latent mosaic viroid PSTVd group or Pospiviroids Pospiviroideae Pospiviroid subgroup Potato spindle tuber viroid Chrysanthemum stunt viroid Citrus exocortis viroid Columnea latent viroid Iresine viroid 1 Mexican papita viroid Tomato apical stunt viroid Tomato planta macho viroid Apscaviroid subgroup
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Grapevine yellow speckle viroid 1 Grapevine yellow speckle viroid 2 Pear blister canker viroid Cocadviroid subgroup Coconut cadang-cadang viroid Citrus viroid 4 Coconut tinangaja viroid Hop latent viroid Coleviroid subgroup Coleus blumei viroid C. blumei viroid 1 C. blumei viroid 2 C. blumei viroid 3 Hostuviroid subgroup Hop stunt viroid Unassigned viroids Apple fruit crinkle viroid Cherry small circular viroid-like RNA Citrus viroid Ia Citrus viroid II Citrus viroid OS
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Disease caused by some viroids
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The control of diseases caused by viroids is based on the use of viroid-free propagating stock, removal anddestruction of viroid-infected plants, and washing of hands or sterilizing of tools after handling viroidinfected plants before moving on to healthy plants.
READ THINK LEARN Dr. Sanjeev Kumar
1. Viroids seem to be associated with cell nuclei, particularly the chromatin, and possibly with the endomembrane system of the cell. 2. How viroids replicate themselves is still not known. 3. Their small size is sufficient to code for a very small protein, but such a protein would be considerably smaller than known RNA polymerase (replicase) subunits and would therefore be unable to carry out replication of the viroid. 4. In addition, viroids have been shown to be inactive as a messenger RNA in all in vitro protein-synthesizing systems tested. 5. Also, no new proteins could be detected in viroidinfected plants.
Viroid Replication
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1. Viroids replicate by direct RNA copying in which all components required for viroid replication, including the RNA polymerase, are provided by the host. 2. During viroid replication, the circular (+) strand of the viroid is replicated while it acts as a rolling drum producing multimeric linear strands of (-) RNA . 3. The linear (-) strand then serves as a template for replication of multimeric strands of (+) RNA. 4. The (+) RNA is subsequently processed (cleaved) by enzymes that release linear, unit-length viroid (+) RNAs, which circularize and produce many copies of the original viroid RNA .
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How viroids cause disease
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1. How viroids cause disease is also not known. 2. Viroid diseases show a variety of symptoms that resemble those caused by virus infections. 3. The amount of viroids formed in cells seems to be extremely small, and it is therefore unlikely that they cause a shortage of RNA nucleotides in cells. 4. Even one or two base changes at specific sites of the viroid are sufficient to change the disease from mild to severe and vice versa. 5. Thus, viroids apparently interfere with the host metabolism in ways resembling those of viruses, but which ways are also unclear. 6. It has been shown that both virus-specific RNAs synthesized during infection and viroid RNA in vitro activate a protein kinase enzyme, which in turn activates other cellular enzymes while it impedes the initiation of protein synthesis. 7. As viroid strains that cause mild to severe plant symptoms activate the protein kinase more than 10 times as much as mild strains it is possible that activation of the protein kinase represents the triggering event in viroid pathogenesis and in disease development by the plant.
Transmission of Viroids
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1. Viroids are spread from diseased to healthy plants primarily by mechanical means, i.e., through sap carried on hands or tools during propagation or cultural practices and, of course, by vegetative propagation. 2. Some, such as potato spindle tuber, chrysanthemum stunt, and chrysanthemum chlorotic mottle viroids, are transmitted through sap quite readily, whereas others, such as citrus excortis viroid, are transmitted through sap with some difficulty. 3. Several viroids, e.g., those causing potato spindle tuber, cadang-cadang, tomato bunchy top, and apple scar skin, appear to be transmitted through the pollen and seed, but the rates of such transmission are usually very small. 4. No specific insect or other vectors of viroids are known, although viroids seem to be transmitted on the
Survival of Viroids
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1. Viroids apparently survive in nature outside the host or in dead plant matter for periods of time varying from a few minutes to a few months. 2. Generally, they seem to overwinter and oversummer in perennial hosts, which include the main hosts of almost all known viroids. 3. Viroids are usually quite resistant to high temperatures 4. Cannot be inactivated in infected plants by heat treatment.
Management
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1.Use of viroid-free propagating stock,. 2.Removal and destruction of viroidinfected plants. 3.Washing of hands or sterilizing of tools after handling viroid infected plants before moving on to healthy plants.
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