Cross protection

Explained in detail about cross protection

1. Cross protection
N. H. SHANKAR REDDY
Ph.D., Plant Pathology
Annamalai University

2. •Infectionofasusceptibleplantwithamildorattenuatedstrainofvirushelpstoprevent
againstthelatersystemicinfectionofmoreseverestrainofthesamevirusiscalled“cross
protection”.
(or)
•Cross-protectionisanaturalphenomenonwherebytoleranceorresistanceofaplanttoone
virusstrainisinducedbysystemicinfectionwithasecond.
•ThephenomenonwasfirstdemonstratedbyMcKinney(1929),whoobservedthatin
tobaccoplantssystemicallyinfectedwitha“lightgreenstrain”ofTobaccomosaicvirus
(TMV:GenusTobamovirus)
•LaterSalaman(1933)demonstratedthatanavirulentstrainofPotatovirusX(PVX:
GenusPotexvirus)providedprotectionagainstsuperinfectionwithavirulentstrainofPVX
inpotato
•Plantsmaybepurposelyinfectedwithamildstrainasaprotectivemeasure
againstseveredisease.

3. •Mildstrainsoftomatomosaicvirusareparticularlyhelpfulincontrollinginfection
byseverestrainsintomatoplants
•ProtectionbyCTVwithmildstrainisalsohelpfulinprotectingcitruscrops
againstseverestrainsofCitrustristezavirus.
•ThenaturallyoccurringsatelliteinCMVstrains(CM-S)hasbeenusedasa
biologicalcontrolagenttoprotecttomatoplantsagainstdiseaseinducedbysevere
strainsofCMV.

4. •Webbetal.(1952)showedthatcrossprotectionagainstthephloem-limitedvirus,
Potatoleafrollvirus(PLRV:GenusPolerovirus)couldbeachievedbyinfection
withtheaphidvector.
•Thefirstdemonstrationsofvirus-diseasecontrolbymildstrainsweredonewith
Citrustristezavirus(CTV:GenusClosterovirus)(GrantandCosta,1951),and
Cacaoswollenshootdisease(PosnetteandTodd,1955).

5. •Varioustermswereusedtodescribethisphenomenon,including“acquired
immunity”,“antagonism”,“crossimmunization”,“inducedimmunity”,
“interference”,“pre-immunity”“protectioninoculation”.
•Theterm“cross-protection”(Matthews,1949)seemedmostappropriate,sinceit
describedanaspectofthephenomenonconcernedwithdiseasecontrolincrops
(Fulton,1986;Lecoq,1998)

6. Mechanism of cross protection
•Coat-protein (CP)-mediated resistance and
•RNA-mediated resistance.

7. i. Coat-protein (CP)-mediated resistance
•CP-mediatedresistancehasbeenshowntobeinvolvedincrossprotection
betweenstrainsofTMVandCucumbermosaicvirus(CMV:Genus
Cucumovirus)
•CP-mediatedresistanceintransgenicplantsdependsontheexpressionlevelofthe
transgeneCP,andahigherleveloftransgeneexpressionelicitsbetterprotection.
Ingeneral,CP-mediatedresistanceisbrokenbyahighlevelofchallengevirus
•ThemostcommonlyproposedmodelforCP-mediatedresistanceisbasedon
preventionoftheuncoatingofthechallengevirusasitenterstheplantcell,which
interfereswiththetranslationandreplicationprocesses

8. ii. RNA-mediated resistance
•Protectorvirusproducesexcessprogenypositive-senseRNA,whichhybridizesto
thefirstminus-strandRNAofthechallengevirus,therebyblockingfurther
replicationandtranslationoftheincomingvirus

9. Current uses of cross-protection
•Protectionoforchardcrops.Thereareonlytwoexamplesofcrossprotectionof
majoreconomicimportanceinorchardsthatareinusetoday:againstCTV,PRSV,
CMVandZYMV
•CTVInseveralregionsoftheworldCTVwasthemostdisastrousepidemic
diseaseincitrusorchards,andprotectionbygraftinoculation(budding)withmild
naturallyoccurringstrainsprovidedgoodprotection
•PRSVThisvirus,identifiedabout30yearsago,isthemostdestructivepapaya
diseaseworldwide,andhasbecomealimitingfactorinproduction.Cross-
protectionagainstPRSVhasbeenachievedsuccessfullyandiswidelyusedin
Taiwan,Thailand,Mexico,FloridaandHawaii,withseveraldifferentmutated
mildviruses

10. Successful application of cross protection
•Citrus tristeza virus
•Tomato mosaic
OTHER DISEASES AS CANDIDATES FOR CONTROL BY CROSS PROTECTION
•Coconutswollenshotvirus-Mildstrainswerelessreadilytransmittedbyinsect
vectorsthanseverestrains
•PapayaRingspot
•PassionFruitWoodiness-mildisolatesofthisvirusbygraftingfromstemtipsin
summer,protectedfromseverestrainsofthewoodinessvirus
•AvocadoSunBlotch
•ConcaveGumandPsoriasisofCitrus-Mildstrainsoftheseviruseshavebeen
reportedtoprotectagainstbuddinginoculationswithmoreseverestrains
•Stone and Pome Fruit Virus Diseases

11. Limitation in cross protection
•Lossofyieldsasaresultofthemildstraininfectionincertainphysiological
instances.
•Incompleteprotectionandbreakdownofprotection
•Astrainthatismildinonecropmightbesevereinanother
•Difficultyinrestrictingtheprotectorvirustothetreatedfield,becauseofnatural
vectors.
•Unavailabilityofmildstrainsofpracticalvalue.
•Synergismandruinousinteractionswithotherviruses
•Geneticinstabilityoftheprotectorvirusbecauseofmutationorrecombination

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