a plant pathogen

2. VIROIDS

3. INTRODUCTION
• Viroids are plant pathogens.
• They were discovered, initially characterized,
and named by Theodor Otto Diener, plant
pathologist at the Agricultural Research Service
in Maryland, in 1971.
• Contains 246-469 nucleotides
• The first viroid to be identified was Potato
spindle tuber viroid (PSTVd). Some 33 species
have been identified.

4. MULTIPLICATION
• Viriods multiply inside the nucleus of infected plant
cell by “rolling circle” mechanism, using the host
enzymes.
• Produce a long monomeric strand of RNA which are
cut into monomers ,each monomers then join to
form an individual circular viriod.
• After replication , the progeny RNAs move to
neighbouring healthy cells though the connecting
plasmodesmata or distinct parts of the plants through
phloem.

5. DISEASES
Viriods causes over 20 different plant diseases of which
the most studied are-
1. Potato spindle-tuber disease
2. Exocortis disease of citrus trees
3. Chrysanthemum stunt disease
4. The only human disease caused by viriods is
Hepatitis-D.

6. PSTVd INFECTED POTATOES
Potato spindle tuber disease.
1st viroid to be identified.
Natural hosts is potatoes and tomatoes
All potatoes and tomatoes are susceptible to PSTVd
and there is no form of natural resistance.

9. Coconut Cadang Cadang disease
• Coconut cadang-cadang
viroid (CCCVd)
• Every year one million
coconut palms are killed
by CCCVd and over 30
million coconut palms
have been killed since
Cadang-cadang has
been discovered.

10. PATHOGENICITY
• Though the pathogenicity of viriods is not well understood,it is
presumed that viriods cause disease by triggering a eukaryotic
response called RNA SILENCING,
• This silencing normally functions to protect against infection
caused by double stranded viruses, by degrading it.
• Viriods may usurp this response by hybridizing to specific host
mRNA molecules to which they have a complementary
sequence forming Hybrid viriod:host mRNA ds molecule
which will elicit RNA silencing.
• This results in destruction of the host message and thus,
silencing of the host gene. This leads to disease in the host
plant.

11. REFRENCES
• Google scholars.org.in
• National Institutes of Health. (2010).
NINDS Transmissible Spongiform
Encephalopathies Information.
• Cell and molecular biology by Gerald karp.
• Boundless. “Prions.” Boundless Microbiology.
• McGraw hill animations
• Wikipedia
• http://www.eppo.org/QUARANTINE/virus/
Coconut_cadang_cadang_viroid/CCCVD0_ds.pdf
• Haseloff, J.; Mohamed, N.A.; Symons,
R.H. (1982) Viroid RNAs of cadang-cadang
disease of coconuts. Nature 299, 316-321.