Satellite viruses are sub-viral agents that depend on a helper virus for replication. The first reported satellite virus was Tobacco necrosis satellite virus. Satellite viruses contain nucleic acids enclosed in a protein coat and lack genes for replication. Satellite genomes can be single-stranded RNA, DNA, or circular RNA. Satellite RNAs are small, linear or circular RNA strands found in certain multicomponent virus particles. They do not encode their own coat protein and depend on a helper virus for replication and encapsidation. Viroids were discovered in 1971 and are small, circular, naked RNA molecules that replicate independently using host polymerases. Well-studied viroids include potato spindle tuber viroid and av

1. Satellite viruses, Satellite
RNA and Viroid's
N. H. SHANKAR REDDY
Ph.D., Plant Pathology
Annamalai University

2. Satellite viruses
 The 1st satellite virus to be reported is Tobacco
necrosis satellite virus (TNV) associated with
Tobacco necrosis virus by Kassanis
 Satellites viruses are sub-viral agents
composed of nucleic acids enclosed by protein
coat, due to the lack of necessary genes for
replication, they depend on helper virus
(another virus) for replication, multiplication
and co-infection.
 Satellite genomes may be single-stranded RNA
or DNA or circular RNA, and are replicated by
enzymes provided by the helper virus
 Eg – Tobacco necrosis satellite virus

3. • Satellite tobacco necrosis virus (STNV) was the first satellite virus to be recognised, the
satellite virus was used to denote to 17 nm isometric particle associated with the 30 nm
Tobacco necrosis virus (TNV) isometric particle.
Properties
 Nucleic acid of small size (0.2-1.5 kbp), share no sequence homology with helper virus
for replication.
 The nucleotide sequences do not share any homology with that of helper virus.
 They are dependent on helper virus for replication.
 They code their own coat protein.
 Interferes with replication of helper virus.
 Satellite RNAs may increase or decrease the severity of viral infection.

4. Satellite RNA
 Satellite RNAs are small, linear or circular RNA strand
found in the virions of certain multicomponent viruses.
 They do not encode their own coat protein and are
dependent upon a helper virus for both replication and
encapsidation.
 These are 3 types of satellite RNAs, i.e.
1. Large satellite RNAs (B-type),
2. Small linear satellite RNAs (C-type),
3. Small circular satellite RNAs (D-type).

5. Applications of satellite RNA
• Mild strains of helper virus containing satellite RNA to field crops can protect
plants from more virulent strains
• Producing transgenic plants that expresses the sat RNA sequences
• Development of sat RNA based vector system for the expression of foreign genes
of plants
Advantages of satRNA
1. Ease of manipulation
2. High in vivo stability
3. High expression level

6. Viroid's
• They were discovered by Diener in 1971.
• Viroids are RNA molecules, about 246-401nt length;
circular having secondary structures.
• They do not encode any protein sequences.
• Replicate independently using host polymerase
• The well studied viroids are potato spindle tuber viroid,
Avocado sun blotch viroid, Hop stunt viroid, etc.
• The names of the viroids are abbreviated with a “d”
added after the disease name.

7. Symptoms
• Viroids can infect both monocots and dicots
• The symptoms are more or less similar to those caused by viruses like stunting,
mottling, leaf distortion. Sometimes they remain symptomless like Hop latent
viroid (HLVd). Inside the plant cells, plasma membrane shows corrugation,
thickness of cell wall abnormalities in chloroplast

8. Properties
• Viroids size range from 246-401nt.
• They are circular RNA molecule. While it is rod like in viroids belonging to
family Pospiviroidae, the molecules like a branched structure in viroids belonging
to Avsunviroidae. The circular molecules have lot of complementary regions
giving rise to secondary structures.
• On the basis of sequence analysis and secondary predictions following domains
are recognized in Potato spindle tuber viroid PSTVd.

9.  So far 40 viroid diseases have been reported to be caused plant diseases in the world.
 The viroid moves from cell to cell through ‘Plasmodesmata’ and ‘cell division’
 The replication mechanism of viroid is ‘rolling circle mechanism’
 Enzyme used for replication of viroids are ‘RNA polymerase II’
 Symptoms range from no symptoms to severe developmental disorders, such as leaf
chlorosis, necrosis, stunting, flowering alterations and fruit and seed deformations
• Eg – Potato spindle tuber viroid
• Coconut cadang cadang viroid

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Channel – Geeky Researcher