This document discusses viroids, virusoids, and prions. It defines viroids as small, circular, single-stranded RNA molecules without a protein coat that can infect plants. Viroids were first reported in 1971 and the most studied is the Potato Spindle Tuber Viroid. Virusoids are also circular single-stranded RNAs that depend on plant viruses for replication and encapsidation. Prions are small infectious particles composed of abnormally folded protein that can transmit their misfolded shape to normal variants of the same protein.

1. VIROID, VIRUSOID AND
PRIONS
N.H. SHANKAR REDDY
2ND PH.D. PLANT PATHOLOGY
ANNAMALAI UNIVERSITY

2. • Viroids are small, covalently closed, circular, single standard RNA (ssRNA)
molecule without protein coat (nucleic acid devoid of protein coat).
• First reported by Diener and Raymer in 1971
• World’s first reported viroid is Potato Spindle Tuber Viroid (Diener and Rayer,
1971) and it is the most studies viroid.

4. General properties
• They are usually smaller than plant viruses and are known to infect only plants.
• Genome size ranges form few hundred (250-401) nucleotides long, with high degree of
specificity.
• They do not code for any proteins and thus depend on the host plant or with associated
plant virus for their replication and other functions
• Viroids are only transmitted through mechanical or sap transmission, no other (vectors)
transmission type has been reported so far.
• 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’

5. Viroid diseases
• Potato spindle tuber viroid (PSTVd)

6. Citrus exocortis viroid (CEVd)
• Shelling of bark
• Yield loss
• Transmitted through stock and graft

7. Coconut cadang cadang viroid
(CCCVd)
• Reduced the nut size and husk
production
• Reduced tree length and
stunting
• Withering of leaves and look
like telephone pole appearance

8. VIRUSOID
• Virusoids are circular single-stranded RNAs dependent on
plant viruses for replication and encapsidation
• Discovered by Randles
• Contains several hundred nucleotides and does not code for
any proteins.
• They are thus similar to viroids in their means of replication
(rolling circle replication),
• Virusoids act as a satellite virus and are not necessary for
viral RNA
• The molecular weight of virusoids ranges from 1.5 × 106

9. Prions
• A prion is a small infectious particle composed of
abnormally folded protein and has a ability to
transmit their misfolded shaped onto normal
variants of the same protein.
• Discovered by Stanley B. Prusiner

10. Animals Diseases
Sheep, Goat Scrapie
Cattle Mad cow disease (Bovine
spongisperm encephalopathy)
Deer and Mouse Chronic wasting disease
Human
Creutzfeldt -Jacob disease (CJD)
Kuru
Fatal familial insomnia