Oomycete–plant coevolution
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Oomycete–plant coevolution
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Oomycete–plantcoevolution:recentadvancesandfutureprospects
MarcoThines1,2andSophienKamoun1
Oomycetesareadiversegroupofeukaryoticorganismsthathavecolonisedmanyecologicalniches;yetmorethan60%oftheknownspeciesareparasiticonplants.ParasitismofplantshasevolvedseveraltimesindependentlyinthreedifferentlineagesoftheOomycota.Here,weprovideanoverviewofthecurrentknowledgeofthediversity,evolutionandlifestylesofplantparasiticoomycetes.Wethenreportonrecentadvancesinmolecularstudiesonoomycete–plantinteractionswithaparticularemphasisonworkwithoomyceteeffectors.Inthefuture,genomesequencingofabroaderspectrumofoomycetespecieswillexpandourknowledgeofpathogenicitymechanismsandwilllikelyrevealnovelstructuralandfunctionalclassesofeffectors.
Addresses1
TheSainsburyLaboratory,NorwichNR47UH,UnitedKingdom2
BiodiversityandClimateResearchCentre(BiK-F),Georg-Voigt-Str.14-16,D-60325Frankfurt,Germany
Correspondingauthors:Kamoun,Sophien(sophien.kamoun@tsl.ac.uk)
andbrownalgaeinthekingdomStraminipila[4,5].Someoomycetesareparasitesthatdevelopwithinasinglehostcell,forexampleadiatom,withtheentirecytoplasmeventuallyrecruitedtoreproduction(holocarpy)[6].Ot-heroomycetescanformdensematsoflong,branchedhyphae,likeinLeptomituslacteus,thesewageoomycete[7].SeveraloomycetegroupshaveevolvedintohighlyadaptedpathogensaffectingorganismsineukaryotickingdomsasdiverseasAlveolata,Animalia,Mycota,StraminipilaandPlantae.Themajorityofoomycetespeciesareparasitesofplants,andsomespecies,suchasthepotatolateblightagentPhytophthorainfestans,causediseasesofgreatimportancetomankind.Unsurprisingly,mostoftheresearchonoomycetesfocusesonplantpathogenicspecies[8??].
Oomyceteevolutionaryhistory:currentstatus
Oomyceteshavemostlikelyevolvedinamarineenviron-ment,asthemostbasallineagesoftheOomycotaarepredominantlymarineparasitesofseaweeds,diatoms,crustaceansandnematodes(Figure1).Despitetheirgreatgeneticdivergence,thesebasallineagesaccountforonlyabout5%ofoomycetespecies,whereasmorethan60%ofthespeciesareplantparasitic.ParasitismofplantshasevolvedatleastthreetimesindependentlyindifferentlineagesoftheOomycota(Figure1).ItemergedonceintheSaprolegnialesinwhichthelegumepathogenApha-nomyceseuteichesisbecominganimportantsubjectofstudy[9–11],andatleasttwiceintheperonosporaleanlineage,whichincludeswidelyknownplantpathogens,suchasdownymildews,Phytophthora,PythiumandAlbugo.Inaddition,evolutionfromapresumablysaprophyticances-torintoobligatebiotrophicpathogenshasoccurredtwiceintheperonosporeleanlineage,leadingtowhiteblisterrusts(Albuginales)anddownymildews(partofthePer-onosporaceae)(Figure1).
TheAlbuginalesareanancientlineagewithseveraltraitsthatunderscoretheirintimateadaptationtoplantpatho-genicity.Suchtraitsincludethelackofnecrotrophyeveninseverelyinfestedplanttissue(Figure2i),andinde-pendencefromatmosphericwaterforsporulation,auniqueoomycetetraitthatisdeterminedbyhighlyregulated,enzymemediatedsporulationandpustuleopeningmechanisms[12].Apparently,thehostimmunesystemisstronglymanipulatedbywhiteblisterrustsresultinginincreasedsusceptibilitytowardsotherpatho-gens[13].Inaddition,contrarytoearlierassumptions,mostspeciesoftheAlbuginaceaewererecentlyshowntobehighlyhostspeci?c[14,15],andaspecialisedspecies,
CurrentOpinioninPlantBiology2010,13:427–433
CurrentOpinioninPlantBiology2010,13:427–433ThisreviewcomesfromathemedissueonBioticinteractions
EditedbyJaneE.ParkerandJeffreyG.EllisAvailableonline4thMay2010
1369-5266/$–seefrontmatter
#2010ElsevierLtd.Allrightsreserved.DOI10.1016/j.pbi.2010.04.001
Introductiontotheoomycetes
Oomycetesareadiversegroupofeukaryoticorganismswithaworldwidedistribution.Theycanbefoundinterrestrialmountainsaswellasinopenseaenvironments.TheyhavenotonlycolonisedthedesertsofIran[1],butalsothearcticregionsoftheworld,includingAntarctica[2,3].Nonetheless,theoomycetesremainrelativelypoorlyknowncomparedtothetruefungi,theMycota,towhichtheyareunrelatedphylogenetically.
Acommonfeatureofalloomycetesistheirabilitytodirectlyabsorbnutrients(osmotrophiclifestyle),whichisthemainreasontheyhavelongbeenconfusedwiththefungi,withwhichtheyshareseveralconvergenttraitssuchashyphalgrowthanddispersalbymitoticallyformedspores.However,thetinsel?agellumwithtripartitehairsonthebi?agellatemotilestagesoftheoomycetesrevealedtheirtruephylogeneticaf?nitywithdiatoms
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428Bioticinteractions
Figure
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http://wendang.chazidian.comsininvertedcommasdenotetaxainneedoftaxonomicrevision.Taxathatarenotyetformallyintroducedaremarkedwithanasterisk.Speciesnumbersareindicatedforthetwolargestoomyceteorders.
Albugolaibachii,wasrecentlyfoundtooccuronthemodelplantArabidopsisthaliana[16????].Albugocandidaistheonlywhiteblisterrustspeciesknownsofarwithabroadhostrangeincludingthreerelatedfamilies,theBrassicaceae,Capparaceae,andCleomaceae[17].Thebasisofthisbroadhostrangeisenigmatic,butispossiblymediatedbyyettobeidenti?edeffectorsthatinterferewithimmunepathwaysthatareconservedamongtheserelatedhostfamilies.
InthePeronosporales,biotrophyhasevolvedgradually,fromopportunisticpathogensamongPythium-likeancestors,tohemibiotrophyinPhytophthoraspecies,which,unliketheobligatebiotrophs,arealsoabletogrowondeadhosttissue(necrotrophy).SomePeronos-poraceaeevolvedahighdegreeofhostspecialisationandaprolongedbiotrophicphaseofinfection,ulti-matelyleadingtoobligatebiotrophywithdecreasingimpactonhost?tness(Figure2a–f).Evolutiontowardsobligatebiotrophywasmostlikelyaccompaniedbytheremarkableexpansionanddiversi?cationineffectorproteins,suchasthehost-translocatedRXLReffectors,whichhavenotbeenidenti?edoutsidethePeronos-porales[18–20].Geneticisolationoftheobligatebiotrophsdrivenbyhostspecialisationispronounced.Thishasledtoextensiveradiationandspeciationinseveraldownymildewgenera,especiallyinHyaloper-onospora,PeronosporaandPlasmopara,resultinginatotalofabout800downymildewspeciesdescribedtodate.Recently,thebasalgraminicolousdowny
CurrentOpinioninPlantBiology2010,13:427–433
mildewlineageswereshowntohaveintermediatecharacteristicsbetweendownymildewsandPhy-tophthorasuggestingstep-by-stepevolutionfromnecro-trophytoobligatebiotrophy[21].
Diversityofextendedphenotypescausedbyoomycetesonhostplants
Severalplantpathogensalterthemorphologyoftheirhostsinwaysthatimprovecolonisation,survivalanddispersal.Suchextendedphenotypessuggestthathostmanipulationbypathogeneffectorsismorecomplexthanthealterationofhostimmunity[22].Amongthemostdramaticextendedphenotypesofoomycetepathogensarethewitchesbroomphenotypescausedbyseveralgraminicolousdownymildews(Figure2g)[23,24].Inthesecases,systemicinfectionbytheoomyceteleadstothereplacementof?owersbyshoots,whichareoftenheavilyinfectedandwillproducemassiveamountsofbothasexuallyformedsporangiaandsexuallyformedoospores.Witchesbroomsprovidethepathogentheadvantageofbeingexposedtowindandrain,ensuringmoreef?cientdistributionofspores.Thisstrategyhasbeenfurtheroptimisedinthe?oricolousdownymildews,whicharemembersofamonophyleticcladewithinthegenusPeronospora[25].The?oricolousdownymildewsparasitiseAsteranaesensuChase&Reveal[26],andexclusivelysporulateon?oralpartsoftheplantstherebyexploitingpollinatinginsectsfordispersal.Manydownymildewsandwhiteblisterrustscaninfectthegenerativeorgansoftheirhosts(Figure2h–k)anddispersewith
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Oomycete–plantcoevolutionThinesandKamoun429
Figure
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Diversityofsymptomscausedbyoomycetesontheirhostplants.(a)NecrotrophicgrowthofPhytophthorainfestansontomato.(b)NecrosisofAnemonenemorosaaftersporulationofPlasmovernapygmaea.(c)ChloroticlesioncausedbythesystemicgrowthofPseudoperonosporahumuliinhop.(d)SystemiccolonisationandsporulationofPeronosporasp.onViciatetrasperma,whichhardlyaltersthehostappearanceexceptforslightchlorosis.(e)ProfusesporulationofHyaloperonosporathlaspeos-perfoliationitshost(Microthlaspiperfoliatum)withoutadditionalvisiblesymptoms.(f)DistortionofCardaminehirsutacausedbyHyaloperonosporasp.,withonlylimitedsporulationandnoadverseeffectoninflorescencedevelopment.(g)WitchesbroomofPennisetumglaucumcausedbySclerosporagraminicola(imagecourtesyofSabineTelle).(h)Limitedsporulationof
Hyaloperonosporathlaspeos-arvensisonseedpodsofThlaspiarvense.(i)IntenseinfectionofCardaminepratensisbyAlbugosp.,withsporulation,butwithoutnecrosis.(j)SystemicinfectionandsporulationofHyaloperonosporacrispulaonaseedpodofResedalutea.(k)OpenedseedcapsuleofViciatetraspermasystemicallyinfectedbyPeronosporasp.,withnearlynormalseeddevelopment.
seeds[27–30].Somestudieshavedemonstratedthepre-
senceofoomyceteswithintheseedsoftheirhosts,which
mostlikelyenablespathogensurvivalanddispersal[31–
33](HellerandThines,unpublishedresults).
http://wendang.chazidian.comRecentadvancesinthestudyofoomycete–plantinteractionsLikeotherplantpathogens,oomycetesmanipulatetheirhostsbysecretinganarsenalofproteins,collectivelyCurrentOpinioninPlantBiology2010,13:427–433
430Bioticinteractions
knownaseffectors,whichtargetplantmoleculesandalterplantprocesses[22,34,35].Here,wehighlightsomerecentadvancesinthestudyofoomycete–plantinter-actionswithanemphasisoneffectorbiology
Oomycetegenomics:effectorspopulatedynamicandexpandedgenomeregions
thatenablethesegenome-levelstructuralpolymorphismstoimpactphenotypicvariationremaintobedetermined.
Effectortraf?ckingintoplantcells:How?
ThegenomesofseveraloomyceteshavebeensequencedbutsofaronlythoseofP.sojae,P.ramorumandP.infestanshavebeendescribedinpublications[18,20].Thegen-omeofP.infestansisremarkablebyitslargesize.At240MbpitisthreetofourfoldlargerthanthegenomesofP.sojaeandP.ramorum[18].ThisexpansionwasdrivenbyadramaticincreaseinmobileelementsandotherrepetitiveDNAwhichaccountfor 74%oftheP.infes-tansgenome[18].Mostgenefamiliesarenotexpanded.However,theRXLRandCRNfamiliesofeffectorsareexpandedtwofoldormoreinP.infestansrelativetoP.sojaeandP.ramorum[18].Theseeffectorgenesoccupyregionsofthegenomethatarerichinrepeatsandpooringenes[18].Haasetal.[18]proposedthatthesegenomeregionsaredynamicandaccelerateeffectorevolution,possiblybyenablingnonallelichomologousrecombina-tionandtandemgeneduplication.Genomesequencesofspeciesthatcoverthebroaderphylogeneticspectrumofoomycetepathogenspromiseadditionalinsights.Already,analysesofthetranscriptomesofSaprolegniaparasitica[36],Aphanomyceseuteiches[9]andPythiumulti-mum[37]revealeddistinctrepertoiresofeffectorsandpathogenicitydeterminants,highlightingthevalueofcomparativegenomics.
Copynumbervariationineffectorgenesimpactspathogen?tness
OomyceteRXLReffectorscarryanN-terminaldomainthatenablestranslocationinsidehostcellsandissimilarinsequence,positionandfunctiontothePEXELhosttranslocationsignalofmalariaparasites[41–45].HowtheRXLRdomainenablestranslocationinsidehostcellsisanimportantandchallengingquestion.Douetal.[46]reportedthatRXLReffectorsofP.sojaeenterplantcellsintheabsenceofthepathogen.TheyshowedthattheRXLRmotifofP.sojaeAvr1beffectorisrequiredforcelldeathinductionwhenafull-lengthconstructwiththesignalpeptideisexpressedinsoybeancells,presumablytoenablere-entryoftheproteinfollowingsecretion[46].Inaddition,Douetal.[46]demonstratedthatrecombi-nantfusionproteinsoftheRXLRdomainandthegreen?uorescentproteindirectlypenetratesoybeanrootcells.TheseexperimentssuggestthatRXLReffectorsdonotrequireapathogen-derivedmachinerytotraf?cinsidehostcellsandexclusivelyexploithost-derivedmolecules.Surprisingly,theseresultsmarkedlydifferfromthepre-vailingmodelofhosttranslocationofPEXELeffectorsreportedinPlasmodium,whichinvolvesprocessingofthePEXELmotifbyanendoplasmicreticulum-speci?cpro-teasefollowedbytherecruitmentofthecleavedeffectorsintoaparasite-derivedtranslocon[47–50].
Effectoractivities:desperatelyseekinghosttargets
Segmentalduplicationofnear-identicalcopiesandcopynumbervariation(CNV)arefrequentinRXLReffectorgenesofP.sojaeandP.ramorum[38????].Asigni?cantnumberofpredictedRXLReffectorgenes,rangingfrom17%inP.sojaeto65%inP.ramorum,wereestimatedtobepresentintwoormorecopies[38????].ThreeP.sojaeeffectorgenes,Avr1a,Avr3aandAvr3c,whichconferavirulencetotheircognatesoybeanRpsresistancegenes,occurintandemrepeatsanddisplayCNV[38????,http://wendang.chazidian.comVimpactstranscriptaccumulationofAvr1aandAvr3aandvirulenceonresistantsoybeanplantshighlightinganunusualmechanismofgeneticadaptationtohostresist-ance[38????].AnotherintriguingcaseofCNVinvolvestheP.infestansAvr3b-Avr10-Avr11locus,whichdeterminesavirulencetothreepotatoresistancegenesR3b,R10andR11[40].Acandidateavirulencegene,pi3.4,wasident-i?edinthislocusandfoundtoencodealargeproteinwiththefeaturesofatranscriptionalregulator[40].Inavirulentstrains,theAvr3b-Avr10-Avr11locusdisplayscopynum-bervariationresultinginampli?cationofupto25trun-catedcopiesofpi3.4[40].ThiscasefurtheremphasizestheoccurrenceofCNVinPhytophthoragenesassociatedwithvirulence/avirulence.However,themechanisms
CurrentOpinioninPlantBiology2010,13:427–433
Onecriticalquestioninthestudyofoomyceteeffectorsistheidenti?cationoftheirbiochemicalactivitiesandmodeofactiononhostcells.The?rststeptowardsthisobjec-tiveistoidentifythehosttargetmoleculesoftheeffec-tors.Sofar,thehosttargetsofRXLReffectorshavenotbeendescribedintheliteraturealthoughitisobviousfromconferenceproceedingsthatsomeprogresshasbeenmade[51].Incontrast,thetargetproteinsofseveraloomyceteapoplasticeffectors,manyofwhichfunctionashydrolaseinhibitors,havebeendetermined[35,52–54].Interestingly,theP.infestanscysteineproteaseinhibitorsEPIC1andEPIC2BbindandinhibitthetomatodefenseproteaseRcr3[55].TheysharethisactivitywithAvr2,anapoplasticeffectorsecretedbythefungusCladosporiumfulvum[56].The?ndingthatunrelatedpathogensevolvedeffectorstoinhibitthesamehosttarget,suggeststhattheinhibitedprotease,Rcr3,isanimportantcom-ponentofplantdefense[55].Inthefuture,additionalstudiesonoomyceteeffectorsandtheirhosttargetswillundoubtedlyhelptodiscoveranddissectnovelplantimmunepathways.
3Dstructuresofsecretedtoxin-likeproteins
Thestructuresoftwosecretedoomyceteproteinswithtoxinactivitieshavebeenreportedrecently[57,58].Sev-eralspeciesofbacteria,fungiandoomycetessecreteNep1-likeproteins(NLPs),conservedproteinsthathave
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Oomycete–plantcoevolutionThinesandKamoun431
evolvedorhavebeenacquiredbylargelyunrelatedorganismsandelicitnecrosisandimmuneresponsesintheleavesofdicotyledonousplants[59,60].InP.infestansandP.sojae,someNLPgenesareupregulatedduringtransitionfrombiotrophictonecrotrophicgrowth[18,61,62].ThecrystalstructureofanNLPfromtheoomycetePythiumaphanidermatumrevealedstructuralsimilaritytoactinoporins,cytolytictoxinsproducedbymarineorganisms[57].MutantanalysesrevealedthatthesamestructuralfeaturesarerequiredforNLPcytolysisandmembranepermeabilizationofplantcells,aswellascomplementationtofullvirulenceofanNLPmutantofthephytopathogenicbacteriumPectobacteriumcarotovorum[57].TheseresultsstrengthentheviewthatNLPsarevirulencefactorsthatfacilitatePythiumandPhytophthoranutrientacquisitionandtherebycolonisationofhostplantsduringthenecrotrophicphaseofinfection[61,62].PcFisasecreted52-amino-acidpeptideofPhytophthoracactorumthatcontainssix6cysteinesanda4-hydroxypro-lineatresidue49[63,64].PcFtriggersnecrosisintomatoandstrawberry,aneconomicallyimportanthostforP.cactorum[64].PcFisthoughttofunctionasatoxinandhasmultiplehomologsinseveralPhytophthoraspecies[18,20,65,66].Nuclearmagneticresonance(NMR)solutionstructureofPcFrevealedahelix–loop–helixmotiffoldwithsimilaritytoOlee6,amajorallergenfromolivetreepollensuggestingthatPcFmaymimicplantproteins[58].However,theexactmodeofactionofPcFfamilyproteinsremainsunclear.
structuralandfunctionalclassesofeffectors.Inparticular,itwillbeinterestingtodeterminetheextenttowhichtheknowledgegeneratedonPhytophthoraeffectorswillapplytoother,morespecialisedandmorebiotrophicoomycetepathogens.
Acknowledgements
MTissupportedbyafellowshipfromtheMaxPlanckSociety,theElite
¨rttemberg,andtheProgramforPostdocsoftheLandesstiftungBaden-Wu
GermanSciencefoundation(DFG);SKisfundedbytheGatsbyCharitableFoundation,theBiotechnologyandBiologicalSciencesResearchCouncil(BBSRC),andBASFPlantSciences.
Referencesandrecommendedreading
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Thestudyofoomycetesandtheireffectorsisahighlydynamicandexpandingareaofresearch.However,mostoftheworkfocusesonafewspecies,particularlyP.infestans,P.sojaeandH.arabidopsidis.Littleisknownabouttheeffectorsofotherplantparasiticoomycetes,someofwhichhaveevolvedremarkableadaptationstotheirparasiticlifestyle(Figure2).Indeed,theinteractionsbetweenoomyceteparasitesandplantscanbemorecomplexthancolonisationofleaforroottissueandsomespeciestriggerstrikingsymptoms,suchaswitchesbrooms(Figure2g),orspecialiseonparticularplantorgansasinthe?oricolousdownymildews.Theseobservationspromptanumberofintriguingquestionsgiventhatnaturalselectionwouldfavoureffectorsthatmediateanytypeofphenotypicexpressionaslongastheyimprovepathogen?tness.Arethereoomyceteeffectorsthatactbeyondalteringhostimmunity?Arethereeffectorsthatalterplantdevelopmenttofacilitatepathogensurvivalanddispersal?Arethereeffectorsthatareexpressedonlyinparticularplantorgans?Forinstance,downymildewsandwhiteblisterruststhatareseedtransmittedmayhaveevolvedeffectorsthataffectseedsurvivalandgermina-tion.Undoubtedly,genomesequencingofabroaderspectrumofoomycetespecieswillexpandourknowledgeofpathogenicitymechanismsandlikelyrevealnovel
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