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12FairTradeintheUnderworld:theEctomycorrhizalSymbiosisF.Martin1CONTENTSI.Introduction........................291II.BiologyoftheEctomycorrhizalSymbiosis.292A.TradingintheUnderground.........292III.SymbiosisDevelopment...............293A.BuildingtheSymbiosis.............293B.Cross-TalkintheEctomycorhizosphere294IV.GenomicsofEctomycorrhizalSymbioses..296A.GenomeSequenceoftheEctomycorrhizalBasidiomyceteLaccariabicolor...................296B.TheTranscriptomeofEctomycorrhizalFungi...........2981.TranscriptsExpressedinFree-LivingMycelium..........2982.TranscriptsExpressedinSymbioticTissues.............2993.Early-TranscriptionallyResponsiveGenestoSymbiosisFormation.....2994.Middle-andLate-TranscriptionallyResponsiveGenestoSymbiosisFormation...........3005.FunctionalSpecializationoftheHyphalWebCompartments..3026.PlantResponsesuponSymbiosisDevelopment......302C.EctomycorrhizalMutants............302V.FutureResearch.....................303References..........................304I.IntroductionBiologicaldiversityisoftenexclusivelyconsideredatthelevelofplantsandanimals,whereasthebulkofglobalbiodiversityisinfactatthemicrobiallevel.Althoughitisclearthattheecologyofourplanetisdrivenbymicrobialecosystems,weareseverelyhamperedbyourlimitedunderstandingofthedi-versityandfunctionofsuchmicrobialecosystems.Livinginthevicinityofweatheredrockandgrow-ingonmineralparticlesanddecayingorganicmat-terareacastofhundredsoffungalspecies.Withintherhizospherewhichhostslargeanddiversecom-1UMRINRA/UHP1136‘InteractionsArbres/Micro-Organismes’,IFR110,CentreINRAdeNancy,54280Champenoux,Francemunitiesofprokaryoticandeukaryoticmicroor-ganismsthatcompeteandinteractwitheachotherandwithplantroots,mycorrhizalfungiarealmostubiquitous.Thesespeciesarenomarginaloddity,havingbeenshowntoaccountforhalfofthefun-galspeciesinmosttemperate,montaneandborealforests(Fiereretal.2005).Thesymbioticrelation-shipbetweenrootsandthesefungiisoneofthemostprevalentassociationsinallterrestrialecosys-tems.Knowingwhichprocessesthesesoilfungiareresponsiblefor,andhow,isthusincreasinglyim-portantforunderstandingtheinputsandoutputsinforestecosystems.Thehyphaeofmycorrhizalfungiandplantshortrootsformanovelcompositeorgan,theso-calledmycorrhiza,whichisthesiteofnutrientandcarbontransferbetweenthetwosymbioticpartners.Thisinteractionallowsterrestrialplantstogrowefficientlyinsuboptimalenvironments(Brundrett2002;ReadandPerez-Moreno2003).Thefirstmycorrhizalassociationsmusthavebeenderivedfromearliertypesofplant–fungusinteraction,suchasendophyticfungiinthebryophyte-likeprecursorsofvascularplants(Re-deckeretal.2000).StructuressimilartoarbuscularmycorrhizahavebeenobservedinplantfossilsfromtheEarlyDevonian(SelosseandLeTacon1998),whereasfossilectomycorrhizahavebeenfoundinthemiddleEocene(LePageetal.1997).Mycorrhizalassociationsarefoundonmostan-nualandperennialplants(probably>90%).Abouttwo-thirdsoftheseplants,mostlyherbaceousspecies,aresymbioticwitharbuscularmycor-rhizal(AM)glomaleanfungi(Glomeromycota).Ericoidmycorrhizasareecologicallyimportantasefficientorganicmatter(OM)degraders,butaremainlyrestrictedtoheathlands(ReadandPerez-Moreno2003).Whilearelativelysmallnumberofplants,ca.8000,formectomycorrhizas(ECM),theirecologicalimportanceisamplifiedbytheirwideoccupancyofbiomes.TreesofBe-tulaceae,Cistaceae,Dipterocarpaceae,Fagaceae,BiologyoftheFungalCell,2ndEditionTheMyc...