Mycorrhizal diversity in Apostasia Orchidaceae indicates the origin and evolution of orchid mycorr.docx

Mycorrhizal diversity in Apostasia Orchidaceae indicates the origin and evolution of orchid mycorr.docx

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MycorrhizaldiversityinApostasiaOrchidaceaeindicatestheoriginandevolutionoforchidmycorr

AJBAdvanceAccess

PublishedonlineaheadofprintOctober23,2009;

doi:

10.3732/ajb.0900101

AmericanJournalofBotany

©2009BotanicalSocietyofAmerica,Inc.

MycorrhizaldiversityinApostasia（Orchidaceae）indicatestheoriginandevolutionoforchidmycorrhiza1

TomohisaYukawa2,5,6,YukiOgura-Tsujita2,5,RichardP.Shefferson3andJunYokoyama4

2TsukubaBotanicalGarden,NationalMuseumofNatureandScience,Tsukuba305-0005Japan3OdumSchoolofEcology,UniversityofGeorgia,Athens,Georgia30602USA4FacultyofScience,YamagataUniversity,Yamagata990-8560Japan

ABSTRACT

Wedemonstratedthat"orchidmycorrhiza,"aspecializedmycorrhizaltype,appearedinthecommonancestorofthelargestplantfamilyOrchidaceaeandthatthefungalpartnershiftedfromGlomeromycotatoaparticularcladeofBasidiomycotainassociationwiththischaracterevolution.Severaluniquemycorrhizalcharacteristicsmayhavecontributedtothediversificationofthefamily.However,theoriginoforchidmycorrhizaandthediversityofmycobiontsacrossorchidlineagesstillremainobscure.Inthisstudy,weinvestigatedthemycorrhizaeoffiveApostasiataxa,membersoftheearliest-divergingcladeofOrchidaceae.TheresultsofmolecularidentificationusingnrDNAITSandLSUregionsshowedthatApostasiamycorrhizalfungibelongtofamiliesBotryobasidiaceaeandCeratobasidiaceae,whichfallwithintheorderCantharellalesofBasidiomycota.MostmajorcladesinOrchidaceaealsoformmycorrhizaewithmembersofCantharellales,whilethesistergroupandothercloselyrelatedgroupstoOrchidaceae（i.e.,Asparagalesexceptfororchidsandthe"commelinid"families）ubiquitouslyformsymbioseswithGlomeromycotatoformarbuscularmycorrhizae.ThispatternofsymbiosisindicatesthatamajorshiftinfungalpartneroccurredinthecommonancestoroftheOrchidaceae.

KeyWords:

Apostasia•Asparagales•Basidiomycota•Botryobasidiaceae•Cantharellales•Ceratobasidiaceae•evolution•Orchidaceae•mycorrhiza

Orchidmycorrhiza

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OrchidmycorrhizaareasymbioticrelationshipbetweentherootsofplantsofthefamilyOrchidaceaeandavarietyoffungi.Allorchidsaremycoheterotrophicatsomepointintheirlifecycle.Orchidmycorrhizaarecriticallyimportantduringorchidgermination,asorchidseedhasvirtuallynoenergyreserveandobtainsitscarbonfromthefungalsymbiont.Manyadultorchidsretaintheirfungalsymbionts,althoughthebenefitstotheadultphotosyntheticorchidandthefungusremainlargelyunexplored.

Fungiformingorchidmycorrhiza

Thefungithatformorchidmycorrhizaaretypicallybasidiomycetes.ThesefungicomefromarangeoftaxaincludingCeratobasidium（Rhizoctonia）,Sebacina,TulasnellaandRussulaspecies.Someorchidsassociatewithsaprotrophic,orpathogenicfungi,whileotherorchidsassociatewithectomycorrhizalfungalspecies.Theselatterassociationsareoftencalledtripartateassociationsastheyinvolvetheorchid,theectomycorrhizalfungusandtheectomycorrhizalhostplant.

SymbioticCultureofTerrestrialOrchids

TerrestrialOrchidMycorrhizae

Theoverallmajorityoforchidseedsneedfungitogerminate.Allterrestrialorchidsarecharacterisedforhavingarelationshipwithfungithatliveinthesoil.Thehyphaecontacttherootswithoutdestroyingthemandthere'sanexchangeoforganicbuildingblocks-mainlycarbonandnitrogen.1）Thisisinitiatedinseedgerminationandcontinuesintoadulthood.

Thezygotethatresultedfrompollinationhastoundergoanumberofdivisionstocreatetheembryo.Thisfirstdivisionnormallytraversesthelengthoftheseed,creatingtwopoles:

oneatthetopwheregrowthhappensandoneatthebottomwithlookslikeastalk.Thisstalkiscalledthesuspensoranditsmainjobisabsorptionandthemanufactureofnutrientsfromtheendosperm-thenutritiousfoodreservethatenvelopestheembryoin“non-orchid”seeds.Thevastmajorityoforchidsdon'thaveanendosperm（exceptBletillastriata&Disauniflora）,buttheydogrowsuspensorcells.

Whenthefungusbreaksintothetestaitpenetratestheembryoviathe（large）suspensorcell,growingitshyphaeinsidetheinnerembryoniccellsaslittlecoilscalledpeletons.Theorchidsimplydigeststhesepeletonsandthusreceivesnutrition.Withthisnewlyacquiredsourcetheorchidembryoincreasesinmasstoformtheprotocorm.Meanwhilethenumberofhyphaeenteringtheseedincreasesandwhenatippingpointisreachedtheprotocormdevelopsashootwithrootsandleaves.Atthispointthefungusinvadestheouter（cortical）cellsandstartsproducingpeletonsthere.Whentherootsbecomelargertheygrowroothairstoincreasethecontactsurfacebetweenrootandsoil（andfungus）.

SwollenseedofOrchismasculainthesoilwithroothairsandhyphae,

aprerequisiteforgermination.

Alltuberousorchidsstartasasmallsphere.

ThefungalpartofthispartnershipistypicallyaspeciesofRhizoctonia（basidiomycota）thatplayalargeroleindecomposingcelluloseandorganicforestsoil2）.It'sdifficulttodeterminewhatthefungusactuallybenefitsfromitsrelationshipwiththeorchid,forthisreasonitisoftensaidthattheorchidisaparasiteonthefungus.Theorchiddigeststhepeletonsasacarbonsourceuntilthechlorophyllintheleaveshavedeveloped,thiscantakefrommonthstoaneternityassomeorchidsneverbuildchlorophyll.Dependencyonthefunguscandecimateoncetheorchidreachesmaturity.Especiallyepiphyticorchidsareknownforlosingtheneedforpeletons,terrestrialorchidstypicallykeeptherelationshipgoinguntildeathdoesthempart.3）4）

Aswithjustaboutanythinginnature,thereisagradient.Gastrodiasesamoides（thePotatoOrchid）,Dipodiumpunctatum（theSpottedHyacinthOrchid）andthetwospeciesofthegenusRhizanthella（RhizanthellagardneriandRhizanthellaslateri）alllackchlorophyll.Thismakesitimpossibleforthemtocapturetheenergyofsunlightdirectlyviaphotosynthesis,theycan'ttaptheenergysourceotherplantsrelyonforusingCO2asasourceofcarbon.Thesespeciesachievethisbyrelyingonafungusforthis,makingthem100%dependentontheirpartnerfortheirentirelife.TheOrchidaceaecontainabout200achlorophyllousspecies,whichobtaintheirCandmineralnutritioncompletelyfromsymbioticfungi,andarethereforecalledmycoparasitic,mycotrophicormyco-heterotrophic5）.Initially,nearlyallorchidsaremyco-heterotrophic,butsomeneverdevelopleavesandlosethephotosyntheticfunction.

Orchidgeneraandspecieswhichdependheavilyonthesymbiont

Aphyllorchis-SEAsia,Indomalaysia:

A.caudata,A.prainii,A.unguiculata

Corallorhiza-northtemperateregions;Europe（1sp.）,E.NorthAmericatoGuatemala:

C.maculata,C.mertensiana,C.striata,C.striatavar.vreelandii,C.trifida,C.wisteriana

Cymbidium:

C.macrorhizonofJapan

Cyrtosia-Indomalaysia（relatedtoGaleola）:

C.javanica

Cystorchis-ChinaandAsia.OnlyC.aphyllamycotrophic

Didymoplexis-OldWorldtropicsexceptW.AfricaandIndia:

D.pallensinJapan

Cephalanthera:

C.austinae

Epipogium-temperateEurasia:

E.aphyllum,E.roseum

Eulophia-tropical:

E.zolingeri

Galeola-Madagascar,IndomalaysiatoAustralia:

G.nudifolia,G.septentrionalis

Gastrodia-EAsia,IndomalaysiatoNewZealand,Australia:

G.confusa,G.elata,G.lacista,G.nipponica,G.procera,G.pubilabiata,G.sesamoides,G.siamensis

Hexalectris-USandMexico:

H.nitida,H.spicata,H.spicata,H.warnockii

Lecanorchis-IndomalaysiatoJapan:

L.nigricans,L.trachycaula,L.trachycaula,L.sp.1.（Japan）

Limodorum-MediterraneanandEuropetoIran:

L.abortivum

Neottia-temperateEurasia:

N.nidus-avis

Pterostylis-Australia,MalaysiatoNewCaledonia.Onlysomemycotrophic.

Rhizanthella-SWandEAustralia:

R.gardneri,R.slateri

Stereosandra-ofSEAsiaandWMalaysia:

S.javanica

Stigmatodactylus-EAsiatoMalaysia:

S.sikokianus

Wullschlaegelia-tropicalAmerica:

W.calcarata

Yoania-HimalayasandJapan:

Y.amagiensis,Y.japonica

AfewspeciesofDisafromSouthAfricaontheotherhandcanbegerminatedonpuresphagnummosswithoutafungus.Inotherwords,thereareterrorchidsthatcouldn'tbebotheredlessaboutfungi,therearethosethatcouldn'tlivewithout,butmostofthemfallsomewherein-between.Thisbizarrelationshipisconsideredtobeoneofthereasonswhyorchidsproduceahumongousamountoftinydust-likeseeds:

sincethefungusprovidestheseedlingwithnourishment,theplantsaren'trequiredtopacktheseedswithstarchesandsugarslikeinotherplantstogettheiroffspringthroughthefirstdaysofgermination.Otherplantsneedtoprovidethisreservesothattheseedlingcandevelopitsfirstleavesandroots,fromthatpointontheseedlingreliesonitsrootsandleavestogetorganicresourcesandenergy.Orchidseedsaretinyandlackthebuilt-innutritionofbiggerseeds;orchidsthenpassthroughanongreen（“achlorophyllous”）developmentalstagewhentheycannotusefats,breakdownstarch,obtainphosphatesorphotosynthesise,andthereforerelyonanexternalsource.Thisisprovidedeitherbymanintheformofsimplecarbon-containingfoodsinsterileseedgermination,orbyafunguswhichbreaksdowncomplexcompoundsintosimpleronesinsymbioticgermination.Thefungalhyphaepenetratethetestaoftheseedandenterseitherthroughepidermalhairsorthesuspensoroftheundifferentiatedembryoviathebaseendoftheseed.Throughinvaginationoftheplasmamenbranethehyphaeenterthecellsandcoilin