Plant Physiol Chapter 23 - Abscisic Acid - A Seed

AbscisicAcid:ASeedMaturationandAntistressSignal23ChapterTHEEXTENTANDTIMINGOFPLANTGROWTHarecontrolledbythecoordinatedactionsofpositiveandnegativeregulators.Someofthemostobviousexamplesofregulatednongrowthareseedandbuddor-mancy,adaptivefeaturesthatdelaygrowthuntilenvironmentalcon-ditionsarefavorable.Formanyyears,plantphysiologistssuspectedthatthephenomenaofseedandbuddormancywerecausedbyinhibitorycompounds,andtheyattemptedtoextractandisolatesuchcompoundsfromavarietyofplanttissues,especiallydormantbuds.Earlyexperimentsusedpaperchromatographyfortheseparationofplantextracts,aswellasbioassaysbasedonoatcoleoptilegrowth.Theseearlyexperimentsledtotheidentificationofagroupofgrowth-inhibit-ingcompounds,includingasubstanceknownasdorminpurifiedfromsycamoreleavescollectedinearlyautumn,whenthetreeswereenter-ingdormancy.Upondiscoverythatdorminwaschemicallyidenticaltoasubstancethatpromotestheabscissionofcottonfruits,abscisinII,thecompoundwasrenamedabscisicacid(ABA)(seeFigure23.1),toreflectitssupposedinvolvementintheabscissionprocess.ItisnowknownthatethyleneisthehormonethattriggersabscissionandthatABA-inducedabscissionofcottonfruitsisduetoABA’sabilitytostimulateethyleneproduction.Aswillbediscussedinthischapter,ABAisnowrecognizedasanimportantplanthormoneinitsownright.Itinhibitsgrowthandstomatalopening,particularlywhentheplantisunderenvironmentalstress.Anotherimportantfunctionisitsregulationofseedmaturationanddormancy.Inretrospect,dorminwouldhavebeenamoreappropriatenameforthishormone,butthenameabscisicacidisfirmlyentrenchedintheliterature.OCCURRENCE,CHEMICALSTRUCTURE,ANDMEASUREMENTOFABAAbscisicacidhasbeenfoundtobeaubiquitousplanthormoneinvas-cularplants.Ithasbeendetectedinmossesbutappearstobeabsentinliverworts(seeWebTopic23.1).SeveralgeneraoffungimakeABAasasecondarymetabolite(Milborrow2001).Withintheplant,ABAhasbeendetectedineverymajororganorlivingtissuefromtherootcaptotheapicalbud.ABAissynthesizedinalmostallcellsthatcontainchloro-plastsoramyloplasts.TheChemicalStructureofABADeterminesItsPhysiologicalActivityABAisa15-carboncompoundthatresemblestheterminalportionofsomecarotenoidmolecules(Figure23.1).Theorientationofthecarboxylgroupatcarbon2determinesthecisandtransisomersofABA.NearlyallthenaturallyoccurringABAisinthecisform,andbyconventionthenameabscisicacidreferstothatisomer.ABAalsohasanasymmetriccarbonatomatposition1′inthering,resultingintheSandR(or+and–,respec-tively)enantiomers.TheSenantiomeristhenaturalform;commerciallyavailablesyntheticABAisamixtureofapproximatelyequalamountsoftheSandRforms.TheSenantiomeristheonlyonethatisactiveinfastresponsestoABA,suchasstomatalclosure.Inlong-termresponses,suchasseedmaturation,bothenantiomersareactive.Incontrasttothecisandtransisomers,theSandRformscan-notbeinterconvertedintheplanttissue.StudiesofthestructuralrequirementsforbiologicalactivityofABAhaveshownthatalmostanychangeinthemoleculeresultsinlossofactivity(seeWebTopic23.2).ABAIsAssayedbyBiological,Physical,andChemicalMethodsAvarietyofbioassayshavebeenusedforABA,includinginhibitionofcoleoptilegrowth,germination,orGA-inducedα-amylasesynthesis.Alternatively,promotionofstomatalclosureandgeneexpressionareexamplesofrapidinductiveresponses(seeWebTopic23.3).Physicalmethodsofdetectionaremuchmorereliablethanbioassaysbecauseoftheirspecificityandsuitabilityforquantitativeanalysis.Themostwidelyusedtechniquesarethosebasedongaschromatographyorhigh-perfor-manceliquidchromatography(HPLC).Gaschromatogra-phyallowsdetectionofaslittleas10–13gABA,butitrequiresseveralpreliminarypurificationsteps,includingthin-layerchromatography.Immunoassaysarealsohighlysensitiveandspecific.BIOSYNTHESIS,METABOLISM,ANDTRANSPORTOFABAAswiththeotherhormones,theresponsetoABAdependsonitsconcentrationwithinthetissueandonthesensitiv-ityofthetissuetothehormone.Theprocessesofbiosyn-thesis,catabolism,compartmentation,andtransportallcontributetotheconcentrationofactivehormoneinthetis-sueatanygivenstageofdevelopment.ThecompletebiosyntheticpathwayofABAwaselucidatedwiththeaidofABA-deficientmutantsblockedatspecificstepsinthepathway.ABAIsSynthesizedfromaCarotenoidIntermediateABAbiosynthesistakesplaceinchloroplastsandotherplastidsviathepathwaydepictedinFigure23.2.SeveralABA-deficientmutantshavebeenidentifiedwithlesionsatspecificstepsofthepathway.Thesemutantsexhibitabnormalphenotypesthatcanbecorrectedbytheappli-cationofexogenousABA.Forexample,flacca(flc)andsitiens(sit)are“wiltymutants”oftomatoinwhichtheten-dencyoftheleavestowilt(duetoaninabilitytoclosetheirstomata)canbepreventedbytheapplicationofexogenousABA.TheabamutantsofArabidopsisalsoexhibitawiltyphenotype.Theseandothermutantshavebeenusefulinelucidatingthedetailsofthepathway(Milborrow2001).Thepathwaybeginswithisopentenyldiphosphate(IPP),thebiologicalisopreneunit,andleadstothesynthesisoftheC40xanthophyll(i.e.,oxygenatedcarotenoid)violaxanthin(seeFigure23.2).Synthesisofviolaxanthiniscatalyzedbyzeaxanthinepoxidase(ZEP),theenzymeencodedbytheABA1locusofArabidopsis.Thisdiscoveryprovidedconclu-siveevidencethatABAsynthesisoccursviathe“indirect”orca