Plant hormones

Plant hormones are naturally occurring organic substances that affect physiological processes. This presentation describes about five major groups of plant hormones, such as auxins, gibberellins, cytokinins, abscisic acid and about their biosynthesis, transport, pathways and physiological effects.

1. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page1
Planthormones
Plantgrowthanddevelopmentinvolvestheintegrationofmanyenvironmentaland
endogenoussignalsthat,togetherwiththeintrinsicgeneticprogram,determineplant
form.
Severalgrowthregulatorscollectivelycalledtheplanthormonesorphytohormones.
Accordingtoastandarddefinition,“Planthormonesaresmallorganiccompounds,
synthesizedbyspecificplantcells/tissues,activeinlowconcentrationandpromoteor
inhibitgrowthanddevelopmentalprocesses.”
Planthormonesarenaturallyoccurringorganicsubstancesthataffectphysiological
processesatlowconcentration.
Planthormonesareabletomakeuseoftheiractionlocallyoratadistance(e.g.,
somearetransportedfromoneorgantoanotherorgantoproducetheirphysiological
effect,andsomeothersbringaboutchangesinthesametissue,orwithinthesamecell
wheretheyaresynthesized).
Typesofplanthormones
Theconceptofplanthormonesoriginatesfromaclassicalexperimenton
phototropism,carriedoutbyCharlesDarwinandhissonFrancisin1880,usingoat
seedlingsexposedtoalaterallightsourceresultsbending.
Basedonfunctionorchemicalstructure,therearefivemajorgroupsofplant
hormones.
Thesegroupsare:auxins,gibberellins,cytokinins,abscisicacidandethylene.
Inaddition,thereisavarietyofotherplanthormonesincludingthe
brassinosteroids,polyamines,jasmonicacid,salicylicacidandothers.

2. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page2
Auxin
Auxin,thefirstplanthormonewasdiscoveredbyFritsWentasagrowth
promotingchemicalinthetipofoat(Avenasativa)coleoptiles.
Becauseitpromotedtheelongationofthecoleoptile,itwaseventuallynamed
auxin(derivedfromtheGreekwordauxein,meaningtoincrease).
Biosynthesisandtransport
Variousnaturallyoccurringauxinsareknown,namelyIAA(indole3-aceticacid),
IBA(indole3-butyricacid)andPAA(phenylaceticacid).
Indole-3-aceticacidistheprincipalnaturallyoccurringauxininhigherplants.
Therearemorethan200auxinanalogswithdifferentchemicalstructuresknownto
havethecommonauxinactivity.
Theonlycommonfeaturessharedbythesecompoundsareanunsaturatedplanar
aromaticringstructureandacarboxylsidechain.

3. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page3
Major(primary)sitesforIAAsynthesisaretheshootapicalmeristem,young
leavesanddevelopingfruitsandseeds.
Intracellularlyitisfoundinthechloroplastandcytosol.
IAAoccursinbothconjugatedandfreeforms.
Ithasbeenfoundtobeconjugatedtobothhigh(suchasglycoproteins)andlow-
molecular-weightcompounds(suchasglucose).
IAAconjugatesarelocatedexclusivelyinthecytosol.
TherearemultiplepathwaysforthebiosynthesisofIAA.
However,twomajorroutesfortheproductionofIAAcanbe:
Tryptophan-dependentpathways
Tryptophanservesasprecursorofauxinbiosynthesisbecauseofitsstructural
similarity.
Theindole-3-pyruvicacidpathwayisthemostcommontryptophandependent
pathway.
Overall,theconversionoftryptophantoIAAinvolves:
1.Deaminationoftryptophan,(catalyzedbytrptransaminase).
2.Decarboxylationofindole-3-pyruvicacid(catalyzedbyindole-3-pyruvicacid
decarboxylase).
3.Oxidationofindole-3-acetaldehyde(catalyzedbyindole-3-acetaldehyde
dehydrogenase).

4. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page4
Tryptophan-independentpathway
Inadditiontothetryptophan-dependentpathways,recentgeneticstudieshave
providedevidencethatplantscansynthesizeIAAviaoneormoretryptophan-
independentpathways.
Thisroutedoesn’tinvolvetryptophandirectlyasaprecursortotheformationof
auxin.
Theprecisepathwayfortryptophan-independentIAAsynthesisisnotknown.
Physiologicaleffects
1.Cellelongation:
Auxinsstimulatecellelongation/expansioninstemsandcoleoptiles.
Accordingtotheacidgrowthhypothesis,auxinpromoteselongationgrowthby
increasingcellwallextensibility.
Auxinstimulatesprotonpumpingbyactivating(directlyorindirectly)proton
pump,H+-ATPase,presentontheplasmamembrane.Asaresult,thepHofthecell
wallfallsaslowas4.5.

5. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page5
Thehigherconcentrationofauxinontheshadedsidecausesthecellsonthatside
toelongatemorerapidlyresultinginbendingofthestemtiptowardstheunilateral
light.
Auxincauseselongationprobably:
Byincreasingosmoticsolutes,
Bywallpressure,
Byincreasingpermeabilitytowater,
ByinducingsynthesisofspecificDNAdependentnewm-RNAandspecificenzymic
proteins.
2.Celldifferentiation:
Thedifferentiationofrelativeamountsofxylemandphloemareregulatedby
auxinconcentration.
Ingeneral,highauxinconcentrationsinducethedifferentiationofxylemandlow
auxinconcentrationsinducephloemdifferentiation.
3.Rooting:
Auxininhibitselongationoftheprimaryroot,butstimulatesrootinitiationonstem
cuttings(adventitiousroots)andlateralrootdevelopment.
Lateralrootsarecommonlyfoundabovetheroothairzoneandoriginatefromcells
presentinthepericycle.
Auxinstimulatesthesepericyclecellstodivide.Thedividingcellsgraduallygive
risetothelateralroot.
4.Apicaldominance:
Inmostplants,thegrowingapicalbudinhibitsthegrowthoflateralbuds—a
phenomenoncalledapicaldominance.

6. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page6
Removaloftheshootapexusuallyresultsinthegrowthoflateralbuds.
Inthepresenceofapicalbud,theoutgrowthofthelateralbudisinhibitedbyauxin
thatistransportedbasipetallyfromtheapicalbud.
5.Fruitset:
Auxinpromotesfruitdevelopmentandrapidthegrowthofyoungfruit.
Afterfertilization,fruitgrowthmaydependonauxinproducedindevelopingseeds.
Auxinisproducedintheendospermandtheembryoofdevelopingseeds.
Insomeplantspecies,seedlessfruitsmaybeinducedbytreatmentofthe
unpollinatedflowerswithauxin.Theproductionofsuchseedlessfruitsiscalled
parthenocarpy.
Tropicresponse:
Auxininducesdifferentialgrowthinplantorgansundertheinfluenceofdirectional
stimuli(i.e.light,gravity).Thisistermedastropicresponse.
Gibberellins
Itistetracyclicditerpenoidcompoundsmadeupof
fourisopreneunits.
Itpromotesstemelongation,germination,dormancy,
flowering,sexexpression,enzymeinductionandleaf
andfruitsenescence.
GibberellinswerediscoveredbyKurosawainJapan,whilehisstudiesonfungal
diseasesofrice.

7. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page7
Inparticular,hewasinvestigatingthemechanismbywhichthefungalpathogen
Gibberellafujikuroiledtofoolishseedling(bakanae)diseaseinriceplantsinfected
withthispathogen.Kurosawafoundthattheabnormalgrowthpromotedbythefungi
wasduetoacompoundproducedbyG.fujikuroi.
Thiscompoundwasfirstisolatedin1935byYabutaandSumuki,fromfungal
strains,Gibberellafujikuroi.
Yabutanamedtheisolateasgibberellin.
Allgibberellinsarederivedfromtheent-gibberellaneskeleton.
Therearecurrently136gibberellinsidentifiedfromplants,fungiandbacteria.
ThegibberellinsarenamedGA1...GAninorderofdiscovery.
Theyallhaveeither19carbonatoms(C19-GAs)or20carbonatoms(C20-GAs).
C19-GAsaremorebiologicallyactivethanC20-GAs.
Acarboxylgroupatcarbon-7isafeatureofallbiologicallyactivegibberellins.
Biosynthesisandtrayintheapicaltissuesandyoungleaves.
Itisuncertainwhetherroottissuesalsoproducegibberellins.
Thehighestlevelsofgibberellinsarefoundinimmatureseedsanddeveloping
fruits.
Gibberellinsaresynthesizedbythecondensationoffourisoprenoidssubunits.

8. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page8
Thebasicbiologicalisopreneunitisisopentenylpyrophosphate(IPP).
IPPscondensetoproduce20carbonsgeranylgeranylpyrophosphate(GGPP).
GGPPactsasabiosyntheticprecursorforgibberellins.
Gibberellinsbiosyntheticpathwaycanbedividedintothreestages,eachcompletesin
adifferentcellularcompartment.
Stage1:Geranylgeranylpyrophosphate(a20-carbonlinearmolecule)isconvertedto
ent–kaureneinplastids.
Stage2:Kaureneistransportedfromtheplastidtotheendoplasmicreticulum(ER).In
theER,ent-kaureneisoxidizedtoGA12,thefirstgibberellininthebiosynthetic
pathwayinallplantsandtheprecursorofallothergibberellins.Thehydroxylationof
carbon–13ofGA12givesGA53.
Stage3:Inthecytosol,G12orG53,eachofwhichhas20carbonatomsisconverted
toothergibberellins.
Transport
1.GAsaresynthesizedbytheembryoandreleasedintothestarchyendospermvia
thescutellum.
2.GAsdiffusestothealeuronelayer.
3.Aleuronelayercellsareinducedtosynthesizeandsecreteα-amylaseandother
hydrolasesintotheendosperm.

9. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page9
4.Starchandothermacromoleculesarebrokendowntosmallmolecules.
5.Theendospermsolutesareabsorbedbythescutellumandtransportedtothe
growingembryo.
Physiologicaleffects
1.Stemelongation:
GAspromotesintermodalelongationingeneticallydwarfplantspecies.
DirectapplicationofGAstothedwarfmutantrestoresanormal,tallphenotype.
Elongationofstemsinrosetteplants(suchasspinach,cabbage)afterapplicationof
GAalsosupportsitsroleinstemelongation.

10. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page10
TheeffectofGAsonstemelongationisverydramatic.
2.Floweringandsexdetermination:
RolesofGAsinflowerinductioninreproductivelycompetenthavebeen
establishedinlongdayandbiennialspecies.
Insuchplants,floweringisnon-inductiveconditionscanbeachievedbythe
applicationofGAs
Rosetteplantsgenerallydonotflowerintherosetteformbutitbolt.Boltingcanbe
inducedbyGAsapplication.
Italsoplaysaroleinfloralsexdetermination.
Indicotssuchascucumberandspinach,applicationofGApromotestheformation
ofstaminateflowers.Ontheotherhand,inmaizeitsuppressesstamendevelopment.
Seedgermination:
Gibberellincausesseedsgerminationbyactivatingvegetablegrowthoftheembryo
andmobilizingstoredfoodreservesoftheendospermbypromotingthesynthesisofa
varietyofhydrolyticenzymesthatareinvolvedinthesolubilizationofendosperm
reserves.
Anexample,inembryothescutellumisaspecializedabsorptiveorgan.Itabsorbs
thesolubilizedfoodreservesfromtheendospermandtransmitsthemtothegrowing
embryo.

11. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page11
3.Fruitset:
ApplicationofGAscancausefruitset.
Forexample,stimulationoffruitsetbygibberellinhasbeenobservedinapple
(Malussylvestris).
Cytokinins
CytokininsareN
6
–substitutedadeninederivativesthathavediverseeffectson
importantphysiologicalfunctionsinplants.
Thefirstcytokininwasisolatedfromautoclavedherringspermin1955byMiller
andhisassociates.
Thiscompoundwasnamedkinetinbecauseofitsabilitytopromotecytokinesis.
ThefirstnaturallyoccurringcytokininwasisolatedfromcornbyMillerand
LenthemindependentlyanditwascalledZeatin.

12. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page12
Biosynthesisandtransport
Naturallyoccurringcytokininsareadenines,whichpossesseitheranisoprenoidor
aromaticsidechainattheN
6
position.
ThecommonactivecytokininsareZeatinandisopentenyladenine
Cytokininsaregenerallyfoundinhigherconcentrationsinmeristematicregions
andgrowingtissues.
Theyarebelievedtobesynthesizedintherootsandtranslocationviathexylemto
shoots.
Itsbiosyntheticpathwaytakesplacethroughthemodificationofadenine.
ItstartswiththereactionbetweenisopentenylpyrophosphateandAMPasaresult
isopentenylAMPformationtakesplace.
IsopentenylAMPisconvertedintoisopentenyladenosinebytheremovalofthe
phosphategroup.
Isopentenyladenosineisfurtherconvertedintoisopentenyladeninebyhydrolytic
removalofribosegroup.
Finally,isopentenyladenineisoxidizedtozeatinbyreplacementofonehydrogen
bya-OHinthemethylgroupoftheisopentenylsidechain.

13. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page13
Physiologicaleffects
1.Celldivision:
Cytokininisinvolvedintheformationinthe,maintenanceandgrowthoftheshoot
apicalmeristem.
Theyarepositiveregulatorsofcelldivisionintheshootapicalmeristemand
negativeregulatorsofcelldivisionintherootapicalmeristem.
TheyareinvolvedintheregulationofG1-SandG2-Mtransitions.
2.Morphogenesis:
Cytokininandauxinstimulatemorphogenesis(shootandrootinitiation)inthe
callus.
Whenauxinispresentalone,oriftheratioofauxintocytokininishigh,calluswill
initiaterootformation.Likewiseifcytokininispresentinhighconcentrationpromotes
shootformation.
Equalamountofbothhormonesresultstheproliferationofundifferentiatedcallus.
Example-crowngallformationinAgrobacteriumtumefaciens.
3.Apicaldominance:
Directapplicationofcytokininstoaxillarybudsinhibitsapicaldominanceand
stimulatesthegrowthoflateralbudsinthemanyspecies,dominanttheinhibitory
effectoftheshootapex.

14. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page14
4.Chloroplastsmaturation:
Cytokininspromotetheconversionofetioplastsintochloroplaststhrough
stimulationofchlorophyllsynthesis.
Dark-grownseedlingsaresaidtobeetiolated.
Theinternodesofetiolatedseedlingsaremoreelongatedandchloroplastsdonot
mature.
Insteadofmaturingaschloroplasts,theproplastidsofdark-grownseedlings
developedintoetioplasts,whichcontainprotochlorophyllinsteadofthechloroplast
thyllakoidsystemanddonotsynthesizemostoftheenzymesandstructuralproteins
fortheformationofchloroplastthylakoidsystemandphotosynthesismachinery.
5.Senescence:
Senescenceisanessentialprocessoftheplant’sgrowthanddevelopment.
Itisanenergydependentgeneticallyprogrammedagingprocesswhichleadsto
organdeath.
Hormonesarekeysignalmoleculesthataccelerateordelaythetimingofleaf
senescence.Somehormonesarepositiveregulatorsofsenescence,whileothersactas
negativeregulators.
Samehormonescanalsoactasbothdependingontheageoftheleaf.

15. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page15
Positivesenescenceregulators-Ethylene,ABA,jasmonicacid,brassinosteroids
andsalicylicacidareregardedasasenescencepromotinghormones.
Negativesenescenceregulators–Cytokinins,auxinsandgibberellinshormones
delaysenescence.
Abscisicacid
Abscisicacid(ABA)isastresshormone.
Itistheonlymember.
Itwasfirstidentifiedandchemicallycharacterizedby
FrederickT.Addicottandhisco-workerwhilestudyingabscissionofcottonfruits.
ItwasoriginallycalledabscisinII.
Itplaysamajorroleinabscissionoffruits.
Biosynthesisandtransport
ABAisasesquiterpene(15-carbon).
Itissynthesizedalmostinallcellsthatcontainchloroplastandotherplastids.
Thepathwaystartswithisopentenylpyrophosphate(IPP)andleadstothesynthesis
ofviolaxanthin.
Violaxanthinisconvertedtotheneoxanthin,whichisthencleavedtoformABA
aldehyde.

16. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page16
TransportofABAoccursthroughbothxylemandphloemandalsothrough
parenchymacellsoutsidevascularbundles.
Physiologicaleffects
1.Stomataclosure:
Duringthestress,ABAconcentrationincreasesdramaticallyinplantleaves,which
leadstostomataclosure.
Stomatalclosureisdrivenbyareductioninguardcellturgorpressurecausedbya
largeeffluxofpotassiumionandanions(suchaschlorideandmalateions).
TheeffectofABAonstomatalaperturesunderredandbluelightvaries.
HigherABAconcentrationincellincreasesthecytosoliccalciumconcentrationin
twoways:
Byinfluxofcalciumionsthroughplasmamembrane.
Byreleaseofcalciumionsintothecytosolfrominternalcompartment,suchasER
andvacuole.
2.Rootandshootsgrowth:
ABAhasdifferenteffectonrootandshootsgrowthandtheeffectsarestrongly
dependentonthewaterstatusoftheplant.
Underlowwaterpotential,whenABAlevelishightheendogenoushormone
exertsastrongpositiveeffectonrootgrowthbysuppressingethyleneproduction.

17. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page17
Andnegativeeffectontheshootgrowth.
3.Dormancyandgermination:
HighlevelofABAinmaturingseedsinhibitsgermination.
ManytypesofdormantseedsgerminatewhenABAisremovedorinactivated.
TheratioofABAtogibberellinsdetermineswhethertheseedremainsdormantor
germinates(Hormonebalancetheory).
ABAinhibitstheGA-dependenthydrolyticenzymesynthesis(suchasα-amylase)
thatisessentialforthebreakdownofstoragereservesinseeds.
TheroleofABAinbudsdormancyislessknown.
4.Vivipary:
ABA-deficientembryosmayexhibitprecociousgerminationandvivipary.
Viviparyisthegerminationofmatureseedwinthinthefruitonmaternalplant
(Ex.–Mangroves).
Thephenomenonofseedlingformationwithoutcompletingnormalembryonic
developmentiscalledprecociousgermination.
InactivationABAorlowlevelsofABAcanleadtoprecociousgerminationand
vivivpary.

18. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page18
5.Hydraulicconductivity:
ABAincreasesthehydraulicconductivity(decreasingtheresistancetowater
movementacrossthemembrane)andionfluxofrootinresponsetowaterstress.
Ethylene
Itisagaseoushormone.
Itistheonlymemberlikeabscisicacid.
Ethylenehassimpleststructure.
Itisproducedinallhigherplantsandusuallyassociatedwithfruitripeningandthe
tripleresponse.
Thetripleresponseondark-grownpeaseedlingincludes,reducedstemelongation,
increasedstemthickeningandhorizontalgrowthhabit.
Biosynthesisandtransport
Ethyleneisproducedalmostallpartsofhigherplants.
Meristematicregionsareactivesitesforethylenebiosynthesis.

19. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page19
Therateofproductionalsodependsonthetypeoftissueandthestageof
development.
Ethylenebiosynthesisisincreasedbystressconditionssuchasdrought,flooding,
chillingormechanicalwounding.
Auxinsalsopromoteethylenesynthesis.
Beinggas,ethylenemovesbydiffusionfromitssiteofsynthesis.
Physiologicaleffects
1.Abscission:
Thesheddingofleaves,fruits,flowerandotherplantorgansistermedabscission.
Itoccursataspecializedlayerofcells-theabscissionlayer.
Auxinhasoppositeeffectitpreventsleafabscissionbymaintainingcells.
2.Flowering:
Ethyleneinducesfloweringinmangoandpineapplefamily(Bromelliaceae).
Inmonocotsgenerallymaleandfemaleflowersareseparate,ethylenemaychange
thesexofdevelopingflower.
Thepromotionoffemaleflowerformationincucumberisoneexampleofthis
effect.

20. St.Xavier’sCollege,Mahuadanr(PlantPhysiology)
Dr.EmasushanMinj(AssistantProfessor) Page20
3.Epinasty:
Thedownwardcurvatureofleavesthatoccurswhentheupper(adaxial)sidegrows
fasterthanthelower(abaxial)sideistermedepinasty.
Hence,ethyleneinducesepinasty.
4.Rooting:
Ethyleneinducesadventitiousrootandroothairformation.
Roothairsaretubularprojectionswhichincreasesthesurfaceareaforthe
absorptionofwaterandnutrients.
5.Fruitripening:
Fruitripeningresultscolourdevelopment,softening,starchhydrolysis,sugar
accumulation,productionofaromacompounds.
Thelossofgreenpigmentcausesbythedegradationofchlorophyllandthe
conversionofchloroplaststochromoplasts.
Ethyleneacceleratestheprocessesassociationwithripeninginmanyfleshyfruits.