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Zorb, C., Ludewig, U. and Hawkesford, M. J. 2018. Perspective on wheat
yield and quality with reduced nitrogen supply. Trends in Plant Science.
23 (11), pp. 1029-1037.
The publisher's version can be accessed at:
•
https://dx.doi.org/10.1016/j.tplants.2018.08.012
The output can be accessed at: https://repository.rothamsted.ac.uk/item/8488x.
© 21 September 2018. Licensed under the Creative Commons CC BY.
Review
Perspective
on
Wheat
Yield
and
Quality
with
Reduced
Nitrogen
Supply
Christian
Zörb,
1,* Uwe
Ludewig,
2and
Malcolm
J.
Hawkesford
3Wheatisanimportantcerealcropwithahighdemandfornitrogen(N)fertilizer to enable the grain protein accumulation that is necessary for baking and processingquality.Here,perspectivesforthedevelopmentofimprovedwheat genotypeswithhigheryieldstability,bettergrainquality,andimprovedNuse efficiencytolowerenvironmentalimpactsarediscussed.Thedevelopmentof improvedwheatgenotypes,forexample,genotypesthatlackstorageproteins thatdonotcontributetobakingquality(e.g.,bygenomeediting),in combina-tion with appropriate N fertilizer management to prevent N losses into the environmentunderpinsanovel approachtoimprovingNuseefficiency. This approach may be particularly applicable to wheats grown for animal feed, whichhavelowerqualityandfunctionalityrequirements.
WheatProductionandUse:RecentProblems
Wheatisthesecondmostwidelygrowncropintheworld,estimatedat200millionha.Wheat grainconsumptionaccountsfor19%ofthecaloriesintheglobalhumandiet[1],whileabout 40%ofwheatproducedisfedtopoultryandlivestock.Wheatgrainisrichincarbohydratesand hasahigherproteincontentthanothermajorcereals,suchasrice(Oryzasativa),maize(Zea mays),rye(Secalecereale),andmillet(Pennisetumglaucum)[2].Italsocontainssubstantial amountsofminerals(e.g.,Zn,Fe),vitamins,andphytochemicals,makingitagoodsourceof nutrition[3–6].Wheatisusedgloballyforthe productionofbread,pasta,andotherbakery productsandtoasmallextentforindustrialproducts.
ThereisanabsoluterequirementforNforwheatgrowth,andcropyieldandqualitydepend uponsubstantialNinputs.Initially,thisdrivescanopyformationrequiredforphotosynthesis that,inturn,drivesyield.Subsequently,themajorsinkisthereproductivecomponent,namely, the wheat grain. More than half of industrially fixed N is used by agriculture, worldwide amountingto inexcess of180Mt/year[7].Theproductionandtransport ofNfertilizersby theHaber–Boschprocessishighlyenergyintensiveanddependsonfossilfuels;however,the costsforthefertilizerformanyfarmerscontinuetoberelativelylow,insomecasesduetostate subsidies.Readyavailabilityandcheapsupplyencourageoveruse,andenvironmental prob-lemsinsomeagro-ecosystemsremainanissue.
N losses from the productionsystem occur as nitrate(NO3 ) leaching oras the gaseous
productsofdenitrificationinsoil:futiledi-nitrogen(N2);nitrousoxide,agreenhousegaswith
300timestheheat-trappingcapacityofcarbondioxide(CO2);andfinallyammonia.Exceptfor
N2,thesegasescontributetopollutionandclimatechange[8].Furthermore,Naccumulatesin
thesoil.N losses reduce ecosystemproductivityandbiological diversityandcontribute to eutrophication.Asaconsequence,maximalacceptableNO3 levelintheEuropeanUnion(EU)
infreshwaterresourcesislegislatedto50mg/l.Thistarget,however,isnotachievedinsome
Highlights
Wheatproductionhasbeencriticized
becauseofaJanus-facednaturefor
theuseofnitrogen(N)fertilizer:it
ben-eficiallyproduceshighqualityand/or
highyieldsbutalsohaspotential
nega-tiveenvironmentalimpacts.
CropproductionandNmanagement
therefore present a sustainability
dilemma.AhighNapplicationis
obli-gatory because high wheat protein
contentisimportantforbakingquality,
butsimultaneously,highNmaycause
environmentalNpollutionvialeaching
orgaseousNlosses.
Solutionsinvolveboththeoptimization
ofthemanagementofNfertilizerand
breedingstrategiestoimproveNuse
efficiencyofthecrops.
A conventional but relatively easily
achievablesolutionwouldbe
improv-ingyieldstabilityasacriticalfactorto
improve N use efficiency. A novel
approachmaybetomanipulatewheat
grainNdemandbyselectively
redu-cingsome storage proteins to help
reduceNfertilizerinputs.
1
InstituteofCropScience,Qualityof PlantProducts(340e),Universityof Hohenheim,70593Stuttgart,Schloss Westflügel,Germany
2
InstituteofCropScience,Nutritional CropPhysiology(340h),Universityof Hohenheim,70593Stuttgart,Germany
3
RothamstedResearch,Plant SciencesDepartment,HarpendenAL5 2JQ,UK
*Correspondence:
(C.Zörb).
EU areas, especially those withhigh organicmanure inputs; the disposal ofN-containing manurefromlocallyconcentratedanimalfarmingindustrymay beafurtherfuturethreatfor theseregions.TheenvironmentalproblemsassociatedwithNinputscontinuetobeanissuein manypartsandcountriesoftheworld,includingtheUSAandlargeregionsofChina.Although, substantialprogresshasrecentlybeenmade,wheatremainstheleastNuseefficient(NUE) majorcrop,whereasNUEsofmaizeandricearearound25%higher[9].Fertilizerregulationsto improve Nuse managementwiththe cleargoal to decrease N inputsand losses intothe environmentdifferineachcountry.However,majorconcernsfromfarmersarethatyieldand grainqualitywillnotbemaintainedathighlevelsifNfertilizeruseissubstantiallyreduced.This reviewarticleprovidesbackgroundinformationandsomefiguresforwhatisachievableinfuture wheat production as well as a concept for an improved strategy, by using new wheat genotypeswithhigherNUEtomaintainorevenimprovegrainyield,yieldstability,andquality.
ModernWheatFertilizer Management
Globally,agrowingpopulationandpercapitaincreasingincomewilltranslateintogreaterfood andprotein needs, usinga nearlystatic land area thatrelies onintensive managementof agriculturalinputs.Overthepast30years,therehasbeenapositivecorrelationbetweencereal productionandNfertilizeruse(bothfrommineralfertilizerandorganicrecyclingfertilizer)in developingcountries.SyntheticNfertilizeruseworldwidewas atonly 9.2MtN in1960;it increasedto80.4MtNin1995andsincethenhassteadilyincreasedto108MtNin2015[10]. GlobalanalysisidentifiedthatinthepastfivedecadestheNuseefficiencyinmanycountriesfirst decreasedandthenincreasedwitheconomicgrowth.Despitemanycounty-specific socio-economiesandNpollutionavoidancepolicies,thetotalNuseefficiency,andespeciallythatof wheatsystems,remainsrelativelylow[11].Overall,cropNUEonaworldwidescaleis47%[12]
[heredefinedas(totalgrainNremoved–Ncomingfromsoil]/fertilizerNapplied)]andforsome decadeswassubstantiallylowerforcereals(33%)[12,13].
WheathasthreemainphasesofgrowthwithconsiderableNdemands(Figure1).Aftersowing, the approximate 6mg of total protein reserves of the kernel is sufficient to maintain the
Leaf development
Wheat developmental phases
Canopy closure
Nitrogen ferƟlizaƟon
Senescence Ripening
Flowering Ear
development Shoot
growth Tillering
Leaf development
Increase of kernel weight grain protein concentraion
Figure1. Wheat Developmental Phases.Nitrogen (N) fertilizerapplicationmanagement ofwheat duringthe
vegetationperiodandinfluencesongrainyieldandqualityparameters.Fertilization,markedwitharrows.
germinationandgrowthoftheseedlinguntilthefirstleafemerges.FurtherNhastobeacquired bytherootsystem,butatthisstagetherootisverysmall.Thus,additionalfertilizermightbe bestapplieddirectlyasasmallleaching-resistantammoniumplacementbelowtheseedrow, butonlyifthereisinsufficientNfrommineralizationavailable.DespiteglobalwheatNUEbeing apparently poor, root N uptake of individual wheat plants, mostly in the form of NO3 ,
ammonium,orevenurea[14],isgenerallyveryefficient.Untilnow,thereisnotasingleexample thatagronomicNUEofthebest-performingcropvarieties(i.e.,thosethataregrownbyfarmers) was further improved by genetic targeting of uptake systems or primary N assimilation. Breeding progress over several decades inthe UK,Argentina, and Italy did not increase theNuptakeperunitrootlengthandreducedrootlengthdensity[15].However,fivedecades ofselectionforyieldreducedrootlengthdensityandincreasedNuptakeperunitrootlengthin Australianwheat varieties.Forwheat to obtain high yield andquality ina humid Northern Hemisphere,afirstrateofNfertilizerapplication(upto60kgNha 1)isconsiderednecessary
attheendofwinter,aroundgrowthstage31(earat1cm;[16]),beforethesecondleafemerges (Figure1).Asecondapplication(upto60kgNha 1)isattillering.Withinthat,theN
concen-tration in the tissue is responsible for the formation of the numbers of tillers per plant. N-accumulatinggenotypesthatlatertranslocateN(asareserveforlatergrain filling)tothe grainappearhighlyefficientinlowNconditions,buttheNUEinwellfertilizedconditionsremains poor[17].ThethirdapplicationofNfertilizerapplicationisbeforegrowthstage37(flagleafjust visible),andtheaimistopromoteproteinbuildupintheears[18,19].Currently,inmodern wheatvarieties,thegrainproteinconcentrationisrequiredtobeabove12%drymatter,which means that amino acids must be synthesized in high amountsin vegetative tissues and transportedtothedevelopinggrain,wherestorageproteinsareformed(Figure1).Thisprocess is only moderately influenced by late application of high amounts of N fertilizer (up to 150kgNha 1),asrootactivitydeclinesduringmaturationandmaythusberesponsiblefor the environmentalproblem of N losses. Excessive use of fertilizerN withtotal average N applicationratesupto>500kgNha 1forwinterwheatisgettingrare,eveninChina[20].Such highNratesgreatlywillinevitablyleadtolargelosses ofN [21,22].However,ifN ratesare massivelyreduced,thewheatplantcannotexploitthegeneticpotentialtobuildupasmuchas possible protein during kernel development [23]. However, recent farm trials in southern GermanyshowedthatsingleNapplicationsofvariousfertilizerformsaftertilleringgavethe samehighyieldandgrainproteinconcentrations,withoutincreasedriskofNO3 leaching,as
ModernWheatGenotypes
Wheatisapolyploid,combiningthreegenomesfromgrasses:anAgilopsspeciesthatbrought theBBgenome,TriticumuratuthatbroughttheAAgenome,andTriticumtauschiithatbrought theDDgenomeandhasbeencultivatedfor10000years.TheAABBDDhexaploidgenomemix ofourmodernwheatvarietiesincludesbreadwheat,aswellasspelt[5,27].Anotherwidely usedwheatform isTriticumdurumwithan AABBgenome,whichisusefulforhardwheat productssuchaspastaandotherproductsaroundtheMediterraneanbasin.Modernwheat varietiesmaybeclassifiedaswinterwheat.Winterwheatissowninautumnandisrelatively frostresistant.Springwheat,whichissowninspring,usuallyhasloweryieldsandlowerprotein concentrationsthanwinterwheat.Winterwheatcanbeusedforbreadmaking.Therearesome hundredsofmodernwheatvarietiesthatmightbeusedinacertaincountry,besuitableina specialclimate,orbesuitableforadifferentsoiltypeorforspecificproducts.Apartfromyield, highresistanceagainstfungaldiseasesandhighgrainproteinconcentrationaremajortargets ofmodernwheatbreeding.Whilewheathybrids(comparedtolines)donotappeartohavethe largeyieldadvantagefromheterosisthatisfoundinmaize,yieldstabilityandpotentiallyquality traitsbenefitfromcrossinglinesofdifferentheteroticgroups[28,29].Varietieswithhigheryield stabilitymayprovideanoverallbenefitwithrespecttoNlosses,becausefarmersprojectand applytheseasonalfertilizerrequirementbeforeknowingwhetherweatherconditionsallowthe genotypetofullyretrieveitsyieldpotential.Thus,stabilehigheryieldingvarietiespreventexcess lossesinunfavorableyears.
InnorthernEuropeancountriessuchasGermanyandtheUK,wheatgrainsareratedaccording toproteinconcentration,andgrowersarepaidaccordingtothegrainproteinconcentration. Therearemillsandbakeriesthatrequireaminimumof12.8%orevenhigherprotein concen-trationinwheatflour.Thisproteinconcentrationrequireshighfertilizerratesafteranthesis,for highbuildupofstorageprotein.Despitelargeeffortsbybreeders,thenegativerelationshipof yieldwithgrainproteinconcentrationisdifficulttobreak(Figure2A)[30],althoughgrainquality ofmodernvarietieshasincreasedbyenhancingstorageproteinconcentration(measuredby nearinfraredspectroscopy-Ncontent)inthegrain(Figure2B).BecauseofincreasedNfertilizer andgenotypicimprovement,theproteinconcentrationingrains,onaverage,inGermanwheat
2.5
2.0
1.5
6 7 8 9 10
720 710 700 690 680 670 660 650
12.2 12.4 Mirage
Magnus Pamier
Potenzial
Cubus Turkis
Schamane Impression
Sokrates
BaƟs
Tommi Kranich Skagen Akteur
Enorm
Format
12.6 12.8 13.0 13.2 13.4 13.6 13.8 14.0 Grain protein concentraƟon (%)
Yield [t/ha at 85% DM]
Gr
ain %N [% DM]
Baking v
olume [ml/100 g
fl
our]
(A) (B)
Figure2.RelationshipofYieldwithGrainProteinConcentration.(A)Negativecorrelationofsingle-yeargrainnitrogen(N)concentrationsofsixBritishwheat
varieties(differentcolors),withyieldatthreeNlevels:100kg/ha(squares),200kg/ha(triangles),and250kg/ha(opensquares)[30].DM,drymatter.(B)Scatterplotof
grainproteinconcentrationandbakingvolumeofloafafterastandardbakingtest(rapidmixtest)for1kgofflour(breadwheat).NorthernEuropewinterwheatvarieties
[33,34].
genotypesrosefromabout7–8%crudeproteininthe1960sto12–16%inmoderngenotypes (Figure2B)[31].ThereisalimitationforincreasingNingrainfurtherbecauseoftheinverse relationofyieldandproteinconcentration,whichleadstohigh-yieldinggenotypesontheone hand and high-quality genotypes, as defined by high protein content, on the other hand (Figure2).Although,someauthorshavesuggestedthatgrainproteinconcentrationmaybe energylimited[32],thereisnoclearindicationthatthisistrueforwheat.Therefore,farmershave todecidewhichmarkettotarget:thebread-makingmarketwithhighqualityortheanimalfeed marketwithhighyield.Ifafarmerdecidestoproducebreadwheat,heorsheneedstouse varietieswithhighbakingpotential,whichinthepastdecadesequatedwithhighrawprotein content.
However,somenewvarietieswithahighrawproteincontent(13–16%)performedequally (bakingvolume)comparedtoothervarietieswitha1–2%lesserproteincontent(Figure2B)
[33,34].Itmaybeconcludedthat(i)thestrategyofincreasingrawproteincontenttoachieve higher baking quality is exhausted and (ii) farmers produce high raw protein by high N application, which inevitablyresults in highenvironmental losses,without anyincrease in grainquality.ThisgeneralpracticeisawasteofN.Thesolutionistochangethepremium paymentssystemsinsuchcountries(Germany,UK)towardsasystemthatquantifiesprotein fractionsthatcontributetograinquality(discussedinthenextsection),ratherthanjustraw proteinconcentration.Thetestingmethodhastobefast,inexpensive,andsensitiveandmay involve ‘lab-on-a-chip’ [35] or asymmetric flow field-flow fractionation methods [36] to quantifytheamountofglutenmacropolymers,butsuitablerapidandcheapspectroscopic methodsshouldalsobedeveloped.
TheProblem: TwoMainFactorsAffectingProteinConcentrationand Composition
EffectofGeneticBackgroundonProteinConcentrationandComposition
NFertilizerApplicationEffectsinWheatProductionandQuality
Nisthemostimportantplantnutrientintermsofyieldformation.Moreover,Nplaysthemost important rolein determiningthe concentration ofstorageprotein ingrains.Studies show controlofgrainN accumulationbythe levelofN fertilizersforwheat[19,48,49],withgrain proteinconcentrationincreasingbyincreasingNfertilizerinput[50,51].
Environmentalconditionssuch as fertilizer management have a notableimpact on baking quality of wheat flour by influencing the concentration and composition of gluten protein subunitsand theirassembly andpolymerization. Weatherconditionsand water availability arealsoimportantfactorsinfluencingstorageproteins[52,53].AtfutureelevatedCO2levels,
grainproteinconcentrationsofcurrentwheatvarietieswillinevitablydecreaseby6–8%[62]A recentFACEexperimentalsoshowedthatNO3 -basedfertilizationwassuperiortoammonium
fertilizers,andevenwithelevatedCO2,Nuptakeefficiencyinthefieldwascloseto100%at
mediumNsupplybutaround75%withluxuryNsupply[54].
Asdiscussedabove,grainNisobtainedviatwo pathways:remobilizedN fromthecanopy (leavesandstems)andupto50%fromsoilafteranthesis[55,56](Figure1).Hence,toincrease ofgrainprotein,ideallybothNsourcesneedtobeboostedatanappropriateratewithasuitable typeoffertilizer.Moreover,theamountofNfertilizerapplicationchangesnotonlyquantitybut alsoquality(composition)ofgrainproteins[57].Itwassuggestedthatthev-gliadinsand HMW-GS,whicharerichinglutamineandprolineandarethemetabolicallymostinexpensiveamino acids,maybegoodsinksforN,whenthereisNsurplus[53].Interestingly,increasingtherateof Nfertilizersin13wheatvarietiesdidnotaffecttheamountandcompositionofalbuminsand globulins,buttheconcentrationofv-gliadinsandHMW-GSincreased[50].In addition,the effectongliadinswasmorepronouncedthanonglutenins,aswellastheeffectonmajorprotein types(a-andg-gliadinsandLMW-GS)incomparisonwithminortypes(v-gliadins,HMW-GS). OtherstudiesshowedthattypeandrateofNfertilizerscanchangetheratioofHMW-GSto LMW-GS [50,58,59]. Genetic downregulation of the gliadin fraction by RNAi was highly efficient,anddifferentNfertilizationhadsubstantialeffectsonotherglutenproteins[60,61]. ThesestudiesdemonstratetheimpactofNfertilizermanagementonthequantityandqualityof wheatgrainprotein.
Gluten (prolamin II) (80–90%)
Wheat grain
Gliadin (50%)
HMW glutenin
LMW glutenin
ω-Gliadin
α-Gliadin
γ-Gliadin
Glutenin (50%) Albumin/globulin
(prolamin I) (10–20%)
Minerals (ash; 3–4%
) Pro
tein
(8
–1
8%
)
Lipid
s (1– 2%)
M oistu
re (10– 15%)
Starch (60–70 %)
Figure3.MainComponentsofWheatKernels and MainStorageProteinFractions.Sulfur-richproteins
(a,g,andLMW)thatarecontributingwithS–Hbridgestotherheologyofthedough-makingprocess.HMW,high
molecularweight;LMW,lowmolecularweight.
ConcludingRemarksandFuturePerspectives
Thefuture(seeOutstandingQuestions)ofwheatproductionneedstobecoordinatedwitha considerablereduction of environmental N losses. This requires a reduction inN fertilizer application,andseveralsolutionsareproposed:
(i) ThereductionofuseofN-basedfertilizers(upto20–40%)withoutqualityandquantityloss mightbeachievable.Thereductionhasseveralbeneficialeffects,suchasreducingenergy forfertilizerproductionmostlybyHaber–Boschmethod,reducingcostsforfarmers,and reducingnegativeenvironmentaleffectsofleachedandgaseousN,whichwasnottaken upbyplants.Anotheraspectto reducetheamountN fertilizeristoadapt thefertilizer applicationmanagementortooptimizethetimingandratesofNfertilizersontopofcurrent largesoilNdepositsresultingfromdecadesofhighNfertilizerratesandhighmineralization potential,withafocusduringdevelopmentoftheears(seeabove)[51,54].
(ii) Usingwheat geneticsfor the deletion of excess storage protein genes that may not contributetobakingqualityorareevenharmfulforconsumerswithwheatallergyorceliac disease,butconsumeNresources.TargetsoftheseN-consuming proteinsare many proteinsfromthealbumin/globulinfraction,aswellastoalesseramounttheprolamins. Suchgenes,whichdonotdirectlycontributetoproductquality,havetobeidentifiedand reduced.Thiscan alreadybedoneby classicalwheatbreeding.By contrast,deleting geneswithouttraceoftransgenicgenesequencesintheproductispossiblebygenome editing techniques such as CRISPR-Cas. Although mutants produced by targeted genome editing thatdo not contain any residualtransgenein thegenome should be acceptablefor thepublic, thesehavejustrecently beenclassifiedas beinggenetically modifiedbytheEuropeanCourtofJustice,sostrongregulationsapply.However,thereis noglobalconsensus,andtheremaybedifferentviewsonthesegenomeediting techni-quesinAmericaandotherpartsoftheworld.Aprerequisiteofthisstrategy,todeletethese genes,isthatthesestoragegenesarereallydispensable.Asmentionedabove,certain genes(1Dx2, 1Dx5, etc.)and theirfunctionsare well known, butmany others ofthe prolaminfraction,aswellasthoseofthegliadin/gluteninfraction,stillhavetobe charac-terized.Onecouldarguethatgrainsneedthese(general)storageproteinstomaintaintheir germinationvigor.However, it is alreadytechnically possibleto delete a wholeset of storageproteinssuchasa-gliadinswhilemaintainingthevigorofwheatgrowth[58,59].In thiscase,noeffectsonflourfunctionalityandslightlydetrimentaleffectsonbakingquality werenoted;however,moresubstantialalterationswouldbeexpectedtoseriouslymodify bakingproperties.Asuccessfulcommercialstrategyusingthisapproachwouldneedto balancereductionsinstorageproteinswithrequirementsforenduseand,forexample, maybebestappliedtofeedwheatsratherthanbread-makingwheats.
(iii) Themosteffectiveapproachwillbetousestrategiessimultaneously:theuseofwheat varietieswithout unnecessarystorageprotein genesand useofan optimized fertilizer applicationstrategy.
(iv) Finally,thefocusonmoreyield-stablevarietieswillpreventNlossesinunfavorableyears forallstrategies,withorwithoutmodificationstostorageproteincomposition.
Insummary,thereispotentialbyusingtheseagronomicandbiotechnologicalstrategiesto avoidunnecessaryNfertilizeruseinthefuture,todecreaseenvironmentalimpacts,savemoney andenergy,andstillproducequalitywheat.
Acknowledgments
C.Z.waskindlygrantedasabbaticalatUWAPerthbythe‘home’UniversityofHohenheim.ProfessorTimComer(School
ofAgriculture,UniversityofWesternAustralia)isacknowledgedforhelpfuldiscussionandcriticalreadingofthe
manu-script.RothamstedResearchreceivesstrategicfundingfromtheBiotechnologicalandBiologicalSciencesResearch
OutstandingQuestions
Isthereapotentialtodecrease
nitro-gen fertilizer in wheat production
worldwide?
Can we optimize nitrogen fertilizer
applicationstrategy?
Whichwheatstorageproteins
contrib-utetowheat(baking)quality?
Whichwheatstorageproteinsdonot
contributetoquality?
Can we improve plants bydeleting
suchstorageproteinsthatdonot
con-tributetoqualityandtherebyreduce
nitrogen supply inwheat cultivation
Council(BBSRC)aspartofthe20:20Wheat(BBS/E/C/00005202)andDesigningFutureWheatprojects(BB/P016855/1)
andbytheDefrasponsoredWheatGeneticImprovementNetworkproject.
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