AlkaloidsPart2ChemicalEcologyp. 1

AlkaloidsPart2:ChemicalEcologyExample1.PyrrolizidineAlkaloids(PAs)-Senecionine-N-oxidesBackground -basicstructuraltype,butmanyvariations -occurrenceinplantsinCompositae,Boragincaceae,Orchidaceae -extremelytoxictobothmammalsandinsectsbutcomplexinteractions -documentedco-evolutionwithspecialistinsects,andsophisticateduseofplantdefensesbyinsectsBiosynthesis Senecionine-N-oxidesynthesizedfromtwoseparatepathways/moieties

-necinemoiety,fromarginineviaputrescine(decarboxylate,condense,cyclize)-dicarboxylicacidmoiety,fromisoleucine=>bothformthemacrocyclicdiester=senecionine-keyenzyme:homosperminesynthase.Evolvedindependentlyatleastfourtime-differentplant

families-fromdeoxyhypusinesynthase,andenzymeofprimarymetabolism(activateseukaryotictranslationinitiationfactoreIF5A)

-fivedistinctstructuraltypes,thatvaryinmacrocyclicdiesterbridgecomponent

Pyrrolizidinealkaloidlocalizationandtransport

-rootsynthesisofsenecionine,thentransportedtoshoot-finalaccumulationofalkaloidsisinflowersandshootapex-furtherbiochemicalelaborationandstructuraldiversification(decorationofmacrocycle)occursinshoot-populationdifferentiation(diversemixturesofpyrrolizidinealkaloidsindifferentplantpopulations)->

variabletargetforpests.

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PyrrolizidineAlkaloidToxicityanditsAvoidance -PA-containingplantsaregenerallyavoidedbyherbivores(bittertaste,toxicity) -N-oxideformsarestoredinplant,thesearethelesstoxicform(seefigure) -afteringestion:N-oxidearereducedtotertiaryamineinreducingconditionsingut -inmammals,alivercytochromeP450oxidase(generaldetoxificationenzymes,xenobiotics)actually

activatesthetoxinbyconvertingtopyrroliccompounds(=alkylatingagentsthatdamagebiomolecules, andcauseliverdamage)

-generalistinsectsaresimilarilysusceptibletothis -however,specialistinsectshaveaspecificsenecionineN-oxidasethatkeepsPAinN-oxideformand

preventsthetoxiceffectsInterestingly,guineapigsareimmunetothepyrrolizidinealkaloids.Itturnsouttheyhavenon-specificoxidase

whichmaintainsthePAastheN-oxideform,anditcanbeexcretedwithoutcausingdamage.

Additionalspecialistinsectadaptationstothepyrrolizidinealkaloids

i)tolerance(seeabove-senecionineN-oxidase)

ii)uptakeandsequestration: -cinnabarmothstolerateandaccumulatePAs(keepintheN-oxideform) -specializedstorageandsecretionbyOreinabeetles(spraypredatorswithPAs) -inAmericanarctiidmoths,thePAsaresequestered,andretainedthroughmetamorphosis(larvaeto

adultmoth),andultimatelyincorporatedintoeggsandsperm.Theyprotecttheeggsandlarvae. -alkaloid-sequesteringspeciesarebrightlycoloredtowarnpredators

iii)derivativesusedaspheromones

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-insomesequesteringmoths,thePAsareconvertedtomatingpheromones(hydroxydanaidal] -hydroxydanaidalisplacedonspecializedorgans(hairpencils)ofmaleasacourtshippheromone (=PAcontentofmalesignaledviahydroxydanaidal)

iv)pharmacophagy(Danainaebutterflies)-takeuppyrrolizidinealkaloidsfromnon-foodsource(stemsornectar).Adultssecretesalivaandabsorthechemicalsviamouth->obtainalkaloidsforprotection

Example2.NicotinealkaloidsfromNicotiana(tobacco)Background -structuralfamily:nicotine,nornicotine,anabasine -occurrence:Nicotianaspp.(flowers,seeds,youngleaves) -highlytoxictomammalsandinsects,bindsandstimulatescholinergicsynapses -inhumansathigherdosescauses:"nausea,vomiting,diarrhea,mentalconfusion,convulsions,respiratory

paralysis" -highlyaddictive(interactsw.dopamine...)

Biosynthesis Tworingstructurewithtwocomponents: -nicotinicacid(asinNAD/NADH)(fromaspartate) -pyrrole(furanring),synthesizedfromarginine(ornithine) -note:decarboxylationfirst -twokeyenzymestoremember:putrescinemethyltransferase(PMT),nicotinesynthase NB:lownicotinemutantsoftobaccohavelowPMT -synthesisoccursrootandnicotineistransportedtoshootinphloem

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Nicotinesynthesisisinducedbywounding -thisissomewhatunusualforanalkaloid -'topping'ofcommercialtobaccoplantationsisknowntoincreasenicotinesynthesis -putrescinemethyltransferaseiswound-inducedinroot.Thissupportstheideaofadefensivefunction -agoodexampleofsystemicsignaling.Itisnowknowntorequiremethyljasmonate)Directexperimentalevidenceofnicotinefordefensefunctionagainstinsectpests (Nicotianaattenuatasystem,IanBaldwin) -RNAi-mediatedsuppressionofputrescinemethyltransferasetoareductionofnicotinelevelsforbioassays -bothconstitutiveandinducednicotinelevelsareaffected. -insectsbioassayeffectsseenonlyafterthe -lowernicotineleadstofastergrowthofinsectpests(Steppuhnetal.data,PLoS2:e217(2004).Interestingcounteradaptationbyspecialistinsects i)Manducasexta(tobaccohornworm)toleratesotherwiselethaldosesofnicotine(metabolism/excretion) -larvaecanatleastpartiallysuppressesnicotineinductionduringfeeding -ontheotherhand,nicotineinlarvaehelpsdefendagainstparasitoidwasppredatorsofManduca(lower

ratesofemergenceonhigh-nicotineManduca.) ii)Nicotineinnectarpreventsoverconsumptionandpromotesshorterfeedingtimes iii)Nicotineisusedbytobaccohornwormtorepelwolfspiders(Kumaretal.,PNAS111:1245(2014)

-larvaefeedingonlownicotineN.attenuata(PMT-RNAisilenced)arepreferredbynocturnalspiders -observationsandlossofnicotinesuggestthatnicotineisemittedfromspiraclesoflarvae

("defensivehalitosis") -larvaefeedingonplantsexpressingaCYPB46RNAigeneshowreducednicotineemission(iethe

larvaecannotemitnicotine).thesearemorevulnerabletopredation.

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Keycontrolexperiments-injectlarvaewithnicotinetoshowthisisthekeyfactorofmanipulations)

Example3.Quinolizidinealkaloids(lupines-Lupinussp.)[lupanine]Lupines(legumefamily)arecommonnativespecies,highlytoxicduetoalkaloids,onlyeatenbyherbivoresif

starving.-strongbiologicaleffectsinmammals:"lossofcoordination,convulsions,liverdamage,..crookedcalfdisease

(teratogen)"-cattlepoisoning,especiallyinautumn,fromhighalkaloidseedsoflupinesonrange.-wildhareslearntoavoidhigh-alkaloidlupine(feedingdeterrent)Otherplantspeciescontainingquinolizidinealkaloid,butmaynotsynthesizethem:-IndianPaintBrush(Castillejaspp):doesnotsynthesize,butisarootparasiteoflupineroots-stealsalkaloids!-othercommonplants(Laburnumanagyroides:cytisin;Cytisusscoparius:spartein)-Gorse(likeScotchbroom) Whyaresomanyinvasivespecieshighlytoxic??Isitaninvasivespeciesadvantage?[see

Naturwissenschaften(2012)99:883].-lupanineappeartobeeffectiveindefenseagainstweevilsandrust(thoughdatalookspreliminary)

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4.SolanineandPotatoSteroidAlkaloids(Humanchemicalecology)Biologicaleffects: -canleadtorespiratoryfailureinhumans(mammaliandefense?) -antifungalagentinpotatoandtomato,alsotoxictoColoradopotatobeetle(anti-herbivory)Implicationsforhumandiet: -solanidinisfoundinediblepotatoesatverylevels,soitdoesn'thavediscernableeffects.However,higher

levelscanbefoundinotherorgans(leaves,flowers,fruit) -synthesisintubersisinducedbygreening(triggeredbylight)-storepotatoesinthedark! -wildpotatospecies(Peru,Bolivia)containhighlevelsofthesecompounds.Presumablycommercialvarieties

wereselectedforlowerlevels(attheriskofgreaterdiseaseincidenceorherbivory)Geophagy(eatingofsoilorclay)=bindingofalkaloidsbyspecialclays -detoxificationandremovalofalkaloids. -effectivenesscanbeshowninlabexperiments(bindingtoclay)Concludingthoughtsonalkaloids-therearemanyverytoxicplantchemicals(iealkaloids)incommonplantspecies-toxicchemicalsarepresentinmanyfoodplants,andinnature-planttoxinsareeffectiveagainstgeneralistinsectsorherbivores,butspecialistshaveevolvedcounter-

adaptations-alkaloidsarepartofevolvedchemicalwarfaresystemsbetweenplantsandinsects(ofdifferenttrophiclevels)

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StudyQuestionDescribetheevidence(usingspecificexamplesandexperiments)thatdemonstratestheimportanceofalkaloids

inplant-insectinteractionsortheireffectivenessindefense[Thinkofdifferenttypesofargumentsyoucoulduse,intermsofdifferentadaptations,thatdemonstratethe

efffectivenessofalkaloidsasprotectiveagents]