Root cortical aerenchyma improves the drought tolerance of m

Rootcorticalaerenchyma(RCA)reducesrootrespirationinmaizebyconvertinglivingcorticaltissuetoairvolume.WehypothesizedthatRCAincreasesdroughttolerancebyreducingrootmetaboliccosts,permittinggreaterrootgrowthandwateracquisitionfromdryingsoil.Totestthishypothesis,recombinantinbredlineswithhighandlowRCAwereobservedunderwaterstressinthe eldandinsoilmesocosmsinagreenhouse.Inthe eld,lineswithhighRCAhad30%moreshootbiomassat oweringcomparedwithlineswithlowRCAunderwaterstress.Rootlengthdensityindeepsoilwassigni cantlygreaterinthehighRCAlinescomparedwiththelowRCAlines.Mid-dayleafrelativewatercontentinthehighRCAlineswas10%greaterthaninthelowRCAlinesunderwaterstress.ThehighRCAlinesaveragedeighttimestheyieldofthelowRCAlinesunderwaterstress.Inmesocosms,highRCAlineshadlessseminalrootrespiration,deeperrooting,andgreatershootbiomasscomparedwithlowRCAlinesunderwaterstress.TheseresultssupportthehypothesisthatRCAisbene cialfordroughttoleranceinmaizebyreducingthemetaboliccostofsoilexploration.

Key-words:aerenchyma;drought;rootgrowth;rootrespira-tion;soilwatercontent;yield;Zeamays.

Abbreviations:DAP,daysafterplanting;RCA,rootcorticalaerenchyma;RILs,recombinantinbredlines;RWC,relativewatercontent;TDR,timedomainre ectometry.

INTRODUCTION

Droughtstressisaprimarylimitationtocropproduction(Boyer1982;Tuberosaetal.2007),andimportantagroeco-sytemsmayfaceincreasingdroughtriskastheresultofglobalclimatechange(Easterlingetal.2000).Itisestimatedthatby2025,over60%ofthehumanpopulationwillinhabitcountrieswithwatershortage(Arnel1999).Theidenti ca-tionandunderstandingoftraitsimprovingcropdroughttoleranceareessentialforthedevelopmentofmoredrought-tolerantcropsandcroppingsystems.Plantshave

Correspondence:J.P.Lynch.Fax:8148636139;e-mail:JPL4@psu.edu740

evolvedavarietyofmechanismstoadapttodroughtcon-ditions(Blum1996;Cattivellietal.2008).Inrecentdecades,researchershaveidenti edanumberoftraitsthatmighthelpplantsuseacquiredwateref cientlyandtoleratedes-iccation,aswellasasmallernumberoftraitsthatmayassistsoilwateracquisition(Sinclair,Bennett&Muchow1990;Bruce,Edmeades&Barker2002;Richards2006;Nelsonetal.2007).Forinstance,rootgrowthoftencontinuesunderseveredroughtstressthatcompletelyinhibitsshootelonga-tioninmaize(Westgate&Boyer1985;Sharp&Davies1989;Zhuetal.2007),whichisconsideredtobeamecha-nismofplantadaptationtowater-limitedconditions(Sharp&Davies1989;Sharpetal.2004).

Aerenchymaisthegeneraltermfortissuewithlargeintercellularspaces(Esau1977).Theformationofcorticalaerenchymainmaizeroots(‘RCA’)variesamonggeno-typesandisassociatedwithtoleranceofhypoxiacausedby ooding(Jackson&Armstrong1999;Manoetal.2006).MaizerootaerenchymacanalsobeinducedbysuboptimalnutrientavailabilitysuchasN,PandSde ciencyinwell-aeratedsolution(Konings&Verschuren1980;Drew,He&Morgan1989;He,Morgan&Drew1992;Fanetal.2003;Bouranisetal.2006).Ethylenecantriggeraerenchymafor-mationinmaizerootsatlowphosphorusavailability(Heetal.1992;Fanetal.2003)andinhypoxia(Drew,Jackson&Giffard1979;Evans2004).Justin&Armstrong(1991)reportedthatalowconcentrationofNAApromotesrootaerenchymaformationinmaizeplants,possiblybecauseofauxin-inducedethylenerelease.TheinductionofRCAbynutrientstressinoxygenatedrootspromptedthehypoth-esisthatRCAenhancesadaptationtonutrientstressbyconvertinglivingcorticaltissuetoairspace,therebyreduc-ingtherespiratoryandnutrientcostofsoilexploration(Lynch&Brown1998).Evidenceinfavourofthishypoth-esiswasreportedformaizeandbeanunderlowPstress(Fanetal.2003).

Traitsthataffectthemetabolicef ciencyofrootgrowthandsoilexplorationshouldbeimportantcomponentsofdroughttoleranceinmaize(Palta&Gregory1997;Liedgens&Richner2001;Bruceetal.2002).Studieshaveshownthatthemetaboliccostsofsoilexplorationbyrootsystemsarequitesubstantial,andcanexceed50%ofdailyphotosynthesis(Lambers,Atkin&Millenaar2002).RCAcouldbeconsideredtocontributetodroughttoleranceif

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