Viroid and virusoid

Detailed description about viroid, virusoid and prions are described in a simple and detailed manner, will be very to understand about different plant pathogens

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

2. •Viroidsaresmall,covalentlyclosed,circular,singlestandardRNA(ssRNA)
moleculewithoutproteincoat(nucleicaciddevoidofproteincoat).
•FirstreportedbyDienerandRaymerin1971
•World’sfirstreportedviroidisPotatoSpindleTuberViroid(DienerandRayer,
1971)anditisthemoststudiesviroid.

4. General properties
•Theyareusuallysmallerthanplantvirusesandareknowntoinfectonlyplants.
•Genomesizerangesformfewhundred(250-401)nucleotideslong,withhighdegreeof
specificity.
•Theydonotcodeforanyproteinsandthusdependonthehostplantorwithassociated
plantvirusfortheirreplicationandotherfunctions
•Viroidsareonlytransmittedthroughmechanicalorsaptransmission,noother(vectors)
transmissiontypehasbeenreportedsofar.
•Sofar40viroiddiseaseshavebeenreportedtobecausedplantdiseasesintheworld.
•Theviroidmovesfromcelltocellthrough‘Plasmodesmata’and‘celldivision’
•Thereplicationmechanismofviroidis‘rollingcirclemechanism’

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

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

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

8. VIRUSOID
•Virusoidsarecircularsingle-strandedRNAsdependenton
plantvirusesforreplicationandencapsidation
•DiscoveredbyRandles
•Containsseveralhundrednucleotidesanddoesnotcodefor
anyproteins.
•Theyarethussimilartoviroidsintheirmeansofreplication
(rollingcirclereplication),
•Virusoidsactasasatellitevirusandarenotnecessaryfor
viralRNA
•Themolecularweightofvirusoidsrangesfrom1.5×10
6

9. Prions
•Aprionisasmallinfectiousparticlecomposedof
abnormallyfoldedproteinandhasaabilityto
transmittheirmisfoldedshapedontonormal
variantsofthesameprotein.
•Discovered by Stanley B. Prusiner

10. Animals Diseases
Sheep,Goat Scrapie
Cattle Mad cowdisease(Bovine
spongispermencephalopathy)
DeerandMouse Chronicwastingdisease
Human
Creutzfeldt-Jacobdisease(CJD)
Kuru
Fatalfamilialinsomnia