Satellite viruses, satellite rna and viroid's

detail descrption about different plant pathogens i.e. Satellite viruses, satellite rna and viroid's

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

2. Satellite viruses
The1
st
satellitevirustobereportedisTobacco
necrosissatellitevirus(TNV)associatedwith
TobacconecrosisvirusbyKassanis
Satellitesvirusesaresub-viralagents
composedofnucleicacidsenclosedbyprotein
coat,duetothelackofnecessarygenesfor
replication,theydependonhelpervirus
(anothervirus)forreplication,multiplication
andco-infection.
Satellitegenomesmaybesingle-strandedRNA
orDNAorcircularRNA,andarereplicatedby
enzymesprovidedbythehelpervirus
Eg–Tobacconecrosissatellitevirus

3. •Satellitetobacconecrosisvirus(STNV)wasthefirstsatellitevirustoberecognised,the
satelliteviruswasusedtodenoteto17nmisometricparticleassociatedwiththe30nm
Tobacconecrosisvirus(TNV)isometricparticle.
Properties
Nucleicacidofsmallsize(0.2-1.5kbp),sharenosequencehomologywithhelpervirus
forreplication.
Thenucleotidesequencesdonotshareanyhomologywiththatofhelpervirus.
Theyaredependentonhelpervirusforreplication.
Theycodetheirowncoatprotein.
Interfereswithreplicationofhelpervirus.
SatelliteRNAsmayincreaseordecreasetheseverityofviralinfection.

4. Satellite RNA
SatelliteRNAsaresmall,linearorcircularRNAstrand
foundinthevirionsofcertainmulticomponentviruses.
Theydonotencodetheirowncoatproteinandare
dependentuponahelpervirusforbothreplicationand
encapsidation.
Theseare3typesofsatelliteRNAs,i.e.
1.LargesatelliteRNAs(B-type),
2.SmalllinearsatelliteRNAs(C-type),
3.SmallcircularsatelliteRNAs(D-type).

5. Applications of satellite RNA
•MildstrainsofhelperviruscontainingsatelliteRNAtofieldcropscanprotect
plantsfrommorevirulentstrains
•ProducingtransgenicplantsthatexpressesthesatRNAsequences
•DevelopmentofsatRNAbasedvectorsystemfortheexpressionofforeigngenes
ofplants
AdvantagesofsatRNA
1.Easeofmanipulation
2.Highinvivostability
3.Highexpressionlevel

6. Viroid's
•TheywerediscoveredbyDienerin1971.
•ViroidsareRNAmolecules,about246-401ntlength;
circularhavingsecondarystructures.
•Theydonotencodeanyproteinsequences.
•Replicateindependentlyusinghostpolymerase
•Thewellstudiedviroidsarepotatospindletuberviroid,
Avocadosunblotchviroid,Hopstuntviroid,etc.
•Thenamesoftheviroidsareabbreviatedwitha“d”
addedafterthediseasename.

7. Symptoms
•Viroidscaninfectbothmonocotsanddicots
•Thesymptomsaremoreorlesssimilartothosecausedbyviruseslikestunting,
mottling,leafdistortion.SometimestheyremainsymptomlesslikeHoplatent
viroid(HLVd).Insidetheplantcells,plasmamembraneshowscorrugation,
thicknessofcellwallabnormalitiesinchloroplast

8. Properties
•Viroidssizerangefrom246-401nt.
•TheyarecircularRNAmolecule.Whileitisrodlikeinviroidsbelongingto
familyPospiviroidae,themoleculeslikeabranchedstructureinviroidsbelonging
toAvsunviroidae.Thecircularmoleculeshavelotofcomplementaryregions
givingrisetosecondarystructures.
•Onthebasisofsequenceanalysisandsecondarypredictionsfollowingdomains
arerecognizedinPotatospindletuberviroidPSTVd.

9. Sofar40viroiddiseaseshavebeenreportedtobecausedplantdiseasesintheworld.
Theviroidmovesfromcelltocellthrough‘Plasmodesmata’and‘celldivision’
Thereplicationmechanismofviroidis‘rollingcirclemechanism’
Enzymeusedforreplicationofviroidsare‘RNApolymeraseII’
Symptomsrangefromnosymptomstoseveredevelopmentaldisorders,suchasleaf
chlorosis,necrosis,stunting,floweringalterationsandfruitandseeddeformations
•Eg–Potatospindletuberviroid
• Coconutcadangcadangviroid

12. Video available at you tube
Channel –Geeky Researcher