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In this article we will discuss about viroids which are smallest known agents of infectious diseases and are not related to viruses.
Subject-Matter of Viroids:
Diener (1971), during his studies on Potato spindle Tuber disease assumed to be caused by a virus, discovered that the causal organism was made up of naked free RNA devoid of protein coat.
Except for this difference, the symptoms appeared to be similar to viral diseases. In order to differentiate this causal organism from viruses, Diener (1971, 1972, 1974) termed it as ‘Viroid’. Since then, some other diseases like, citrus exocortis, chrysanthemum stunt and chrysanthemum chlorotic mottle were observed to be caused by Viroids.
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According to Diener (1973) Viroids are smallest known agents of infectious diseases and are not related to viruses.
The following evidences suggest that these causative agents are Viroids and not viruses:
(i) Particles of viroids have not been observed in infected tissues.
(ii) The viroids can be concentrated by ethanol precipitation.
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(iii) Sedimentation rates of viroids are much below the range at which nucleo-proteins normally sediment.
(iv) The viroids are made up of free RNA without protein coat.
(v) Viruses are usually insensitive to RNase because of the protein coat around RNA while viroids are sensitive to RNase but not to DNase.
(vi) Viroids are insensitive to phenol,
(vii) No virus specific antisera is produced by viroid-infected tissues, and
(viii) Isolation of subcellular components of viroid-infected tissues indicate that infectivity resides only in the nuclear fraction and in the tissue debris.
Structure of Viroids:
Potato spindle Tuber Viroid (PSTV) disease has been studied in detail. Electron microscopic studies of PSTV revealed that it is a single stranded RNA molecule containing 250-350 nucleotides.
The RNA molecule is about 50 nm long, confined within the nucleus of infected cells, associated with chromatin and found in exceedingly small amounts. Major fraction has a molecular weight of 5 x 104 daltons while minor fractions have molecular weights ranging between 7.5 x 104 and 9.4 x 104 daltons.
The adenine: Uracil ratio is 21.7: 20.9 while the ratio of guanine: cytosine is 28.9: 28.3 i.e. the ratios are close to unity.
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The viroids do not possess protein coat around RNA molecule.
Replication of Viroids:
The precise mechanism of viroid replication has been a matter of controversy.
Two hypotheses have been put forward to explain viroid replication:
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(a) DNA-directed Replication:
It is assumed that replication of viroids takes place at the direction of DNA which produces DNA templates. These DNA templates are either present in repressed form in uninfected tissues or are synthesised as a result of viroid infection.
(b) RNA-directed Replication:
This hypothesis suggests that a replicase enzyme exists in uninfected tissues of the host. This enzyme accepts a wide variety of RNA species as templates. RNA-directed RNA polymerase has been observed to be present in healthy Chinese cabbage and tobacco plants.
Symptoms and Transmission of Viroids:
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Symptoms of Viroids:
Symptoms of diseases caused by viroids are similar to those caused by viruses. Symptoms associated with viroid diseases include chlorosis, stunting, veinal discolouration, epinasty, vein clearing, localised necrotic spots, mottling of leaves and death of the host.
Generally, the viroid-caused diseases are persistent infections, meaning thereby that the diseased plants do not recover and the viroids can be isolated from the diseased plants as long as it lives.
Transmission of Viroids:
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The viroids known so far, are generally transmitted through mechanical means. In PSTV, the viroid is also transmitted through seed and pollen of infected plants. Insect transmission has not been reported.
Origin of Viroids:
There are four possibilities of the origin of viroids:
(i) Viroids are derived from viruses in which protein coat is not required or is lost. Therefore, the viroids are sub-viral agents.
(ii) In many cases such as leaves infected with TMV, leaves infected with broad bean mottle virus and in Escherichia coli infected with phase Qβ low molecular weight RNAs have been discovered. Ultimately, these might have given rise to viroids.
(iii) Normal cells and particularly nuclei and nucleoli are known to contain RNA of low molecular weight. It has been assumed that some of these may have some regulatory function during RNA transcription from genome and may be essential for normal development of the organism.
It is possible that viroids may have originated from these regulatory RNAs which have become abnormal by mutation.
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(vi) Self-replicating RNAs normally occur in healthy animal cells as well as in tomato leaves. The viroids might have originated when one of these self-replicating RNAs becomes pathogenic and induces diseases. Potato spindle tuber viroid could be one such self-replicating RNA and PSTV, one such disease.
