Ethylene is a plant hormone that controls certain physiological functions including Ripening

1. Ethylene Biosynthesis in Plants (With Diagram)
Contents :
1. Discovery of Ethylene,
2. Chemical Nature of Ethylene,
3. Detection & Estimation of Ethylene,
4. Biosynthesis of Ethylene,
5. Inhibitors of Ethylene Biosynthesis,
6. Antagonists of Ethylene Action
7. Inactivation of Ethylene in Plants,
8. Occurrence, Distribution & Transport of Ethylene in Plants, and
9. Mechanism of Ethylene Action.
Discovery of Ethylene:
A unique feature aboutdiscoveryof ethylene asplantgrowthhormone isthatexperimentersfirstknew
ethylene asexogenouschemical affectingplantgrowthandthereafteronlygraduallyandoveraperiod
of overhalf a century,itbecame evidentthatitisin-facta natural plantgrowthhormone.
The discoveryof ethyleneasa plantgrowthregulatormayindirectlybe tracedbackto 19th century
whenstreetlightswere usedtobe lightedwithilluminatingcoal gas.It hadbeena commonobservation
at that time that the treesin the vicinityof the streetlampsdefoliatedmore extensivelythanother
trees.EvenancientChineseknewthattheirharvestedfruitswouldripenmuchfasterinburningincense.
(Ethylene waslateridentifiedasimportantcomponentof coal gas or burning incense whichwas
probablyresponsibleforsucheffectsonplants).
However,the creditforfirstestablishingthe factthat ethyleneaffectsplantgrowthgoestoaRussian
physiologistDimitryN.Neljubowwhoin1901 identifiedethylene inlaboratoryair fromilluminatingcoal
gas whichcausedtypical symptomsinetiolatedpeaseedlingsgrownindarkinthe lab.viz.,
(i) Inhibitionof stemelongation,
(ii) Stimulationof radial swellingof stemsand
(iii) Horizontal growthof stemswithrespecttogravity.
These symptomswere latertermedas‘triple response’andwere notobservedinetiolatedpea
seedlingsgrowninnormal airfree fromcoal gas.‘The firstindicationthatethylenemightbe anatural
productof planttissuescame throughan annual report byH. H. Cousinsin1910 to JamaicanAgriculture
Departmentwhereinhe mentionedthat“bananasshouldnotbe storedwithorangesinshipsbecause
some emanationsfromorangescausedbananastoripenprematurely”.

2. (However,since orangesproducerelativelyverylittleethylene incomparisontootherfruitssuchas
apples,itwasprobablythe fungi suchas Penicilliumoninfectedorangeswhichproducedsubstantial
amountof ethyleneasemanations).But,itwasR .Gane (1934) who clearlyestablishedthatethylene is
actuallya natural product of ripeningfruitsandisresponsible forfasterripeningprocess.
Meanwhile,several otherexperimentersfoundevidenceof ethylene beingproducednotonlyby
ripeningfruitsbutalsobyflowers,seeds,leavesandevenrootsandhavingprofoundregulatoryactivity
inplants.But,theirproposalstoconsiderethyleneasnatural planthormone metwithstrongcriticism
by otherwell knownphysiologistsof thattime especiallyWentandThimann(1937) and were rejected.
(The ideawasthenprevalentthatauxinwasthe mainplanthormone andthe effectsof ethyleneon
plantgrowthwere consideredtobe due toauxinand thatethylene playedonlyinsignificantandindirect
physiological role.The problemwascompoundeddue tolackof suitable techniquesforquantificationof
ethylene inplanttissuesatthattime).
For furtheralmosttwoand a half decades,the importance of ethyleneasnatural planthormone
remainedsubdued.Itwasonlyafterthe adventof gas chromatography(GC) and itsuse inethylene
research,thatimportance of ethylene asimportanthormonal regulatorof physiological processeswas
realised(BurgandThimann,1959, 60). Soon,thiswasfollowedbyanavalanche of experimental
researchworkon ethyleneandfinallyethylene emergedasanacceptednatural plantgrowthhormone
(Prattand Goeschal,1969).
Chemical Nature of Ethylene:
Ethylene (C2H4) withamoleculeweightof 28, isa well knownandsimplestolefmicgasandhas the
followingstructural formula,
Ethylene isflammable andhighlyvolatile substance thatreadilyundergoesoxidationtoproduce
ethylene oxide.Inmanyplanttissues,ethylenecanbe fullyoxidizedtoCO2 throughethylene oxide.Itis
colorless,lighterthanairat room temp, andunderphysiological conditionsandissparinglysoluble in
water.Ethylene isreadilyabsorbedbypotassiumpermanganate(KMnO4).The latterisfrequentlyused
to remove excessethylene fromthe storage chambers.
Detection& Estimationof Ethylene:
Previously,bioassaymethodsbasedonetiolateddicotseedlings‘triple response’wereusedtodetect
and estimate ethylene concentrations.But,these methodshave now beenreplacedwithmostaccurate
and sensitivegaschromatographictechnique alongwiththe flameionizationdetector.Thistechnique is
so sensitiveandaccurate that itis possible todetectaslow as3ppb (3 parts per billioni.e.,3pLL–
(3pico
literperliter);pico= 1012
) of ethylene withinafew (1-5) minutesonly.More recently,laserdriven
photo-acousticdetectorhasbeeninuse whichcandetectaslow as 50ppt (50 partsper trillion) i.e.,0.05
pLL–
of ethylene.
BiosynthesisofEthylene:

3. Ethylene isknowntobe synthesizedinplanttissuesfromthe aminoacidmethionine.A non-protein
aminoacid,1-aminocyclopropane-l-carboxylicacid(ACC) isanimportantintermediate andalso
immediate precursorof ethylene biosynthesis.The twocarbonsof ethylenemoleculeare derivedfrom
carbon no.3 and 4 of methionine.
Whole processof ethylene biosynthesis(Fig.17.27) is a three stepspathway and is aerobic:
(i) First Step:
In the firststep,an adenosine group(i.e.,adenine +ribose) istransferredtomethionine byATPtoform
S-adenosylmethionine (SAM).Thisreactioniscatalyzedbythe enzyme SAM-synthetase(methionine
adenosyl transferase).
(ii) SecondStep:
In the secondstep,SAMis cleavedtoform1-aminocyclopropane-l- carboxylicacid(ACC) and5′-
methylthioadenosine(MTA) bythe enzyme ACC-synthase.
i.Synthesisof ACCisrate limitingstepinethylene biosynthesisinplanttissues.
ii.ExogenouslysuppliedACCgreatlyenhancesproductionof ethylene inplanttissues.
(iii) ThirdStep:

4. In the thirdand laststepof ethylenebiosynthesis,ACCisoxidisedbythe enzyme ACC-oxidase
(previouslycalledethylene formingenzyme i.e.,EFE) toformethylene.Twomolecules,one eachof HCN
and H2O are eliminated.
i.ACC oxidase activitycanbe rate limitingstepinethylene biosynthesisinplanttissueswhichshowhigh
rate of ethylene productionsuchasripeningfruit.
ii.The enzyme ACCoxidase requiresferrousiron(Fe2+
) andascorbate ascofactors.
iii.ACCcan be conjugatedtogive N-malonyl ACC(Fig.17.27) andthus, mayplayan importantrole is
regulationof ethylene biosynthesis.
Yang Cycle:
There isonlylimitedamountof free methionine (whichisasulphurcontainingaminoacid) inplant
tissues.Therefore,tosustainnormal rate of ethylene biosynthesis,the sulphurreleasedduringethylene
biosynthesisisrecycledtomethionine againthroughmethioninecycle orYang cycle (sonamedafterthe
pioneerworkerS.F.Yangonethylene biosynthesis).The CH3-Sgroupissalvagedandreappearin
methionineasa unit.The remaining4Catomsof methionine are suppliedfromribosemoietyof ATP
whichwasoriginallyusedtoformSAM.A transaminationreactionprovidesthe aminogroup(Fig.
17.27).
Factors Stimulating Ethylene Biosynthesis:
Ethylene biosynthesisisknowntobe stimulatedbyanumberof factors suchas IAA,cytokinins,fruit
ripening,stressconditions(drought,flooding,chilling,exposure toozone etc.) andmechanical
wounding.Inall these cases,ethylene biosynthesisisstimulatedbyinductionof ACCsynthase.In
climactericfruits,ethylene itselfpromotesbiosynthesisof ethylene byautocatalysis.
Inhibitorsof Ethylene Biosynthesis:
There are twopotentinhibitorsof ethylene biosynthesisviz.,aminoethoxy-vinyl- glycine(AVG) and
aminooxyaceticacid(AOA) whichblockconversionof SAMtoACC (Because AVGandAOA are well
knowninhibitorsof enzyme thatrequirespyridoxal phosphate ascofactor,the enzyme ACCsynthase isa
pyridoxal phosphatedependentenzyme.
Cobaltions(CO2+
) are alsoknowntoinhibitethylenebiosynthesisbyblockingthe conversionof ACCto
ethylene byACCoxidase.
Antagonists ofEthylene Action:
A numberof inhibitorsare knownwhichinhibitethyleneactioninplants.
A briefaccount of these follows:
(i) CO2:
At highconcentration(5to 10%), CO2 inhibitsmanyeffectsof ethylene suchasinductionof fruit
ripening.CO2 probablyactsascompetitive inhibitorof ethyleneaction.AccordingtoBurg(1965), the
relative affinityof active site forCO2 andethyleneis1:100,000.

5. Because of thiseffect,CO2 at higherconcs.isoftenusedtodelayripeningof pickedfruitsandsome
vegetables.However,the highconc.of CO2 requiredforinhibitionmakesitunlikelythatCO2 oftenacts
as antagonistof ethylene actioninvivo.
(ii) SilverIons:
Silverions(Ag+
) inthe formof silver-nitrate(AgNO3) andespeciallyassilverthissulphate (Ag(S2O3)3-
2 are
potentandmuch more effective inhibitorsof ethylene actionthanCO2.Theyare knowntoinhibittriple
response of etiolatedpeaseedlings,enhancingabscissionof leaves,flowersandfruitsof cottonand
inductionof senescence inorchidflowers.Silverthiosulphateismuchmore effective indelaying
senescence of cutflowersthansilvernitrate.
(iii) SyntheticVolatile OlefinCompounds:
In recentyears,manysyntheticvolatile olefiniccompoundshave beenfound tobe strongcompetitive
inhibitorsof ethylenereceptorsandthuspreventingethyleneaction.Some of these are,trans-
cyclooctene,1-methylcyclopropene(MCP) and2, 5-norbornadiene.
Inactivation of Ethylene inPlants:
Due to its highdiffusivity,excess ethylene canreadilybe flushedoutof planttissues.Therefore,
catabolismof ethyleneisnotof much significance inplants.Nevertheless,radioisotopicstudiesdone
with14
Clabelledethylenehave shownCO2,ethylene oxide andethylene glycol tobe the major
breakdownproductsof ethylene inplanttissues.But,these breakdownproductsare knowntoplay
hardlyany role inregulatinglevel of thisgaseoushormone(ethylene) inplants.
Duringbiosynthesisof ethyleneinplanttissues,notall ACCisconvertedintoethyleneandsome of itis
divertedtoformitsconjugatedformas N-malonylACCthataccumulatesinplanttissuesandmayplay
importantrole inregulatinglevel of ethylene inplanttissues.
Occurrence,Distribution & Transport of Ethylene in Plants:
Ethylene isproducedbyall groupsof plantsincludingbacteria,fungi,some blue-greenalgae,
bryophytes,pteridophytes,gymnospermsandangiosperms.Inhigherplants,ethylenecaneasilybe
synthesizedinall plantorganssuchas roots,stems,leaves,tubers,bulbs,fruitsandseeds.But,its
productionvariesdependingonthe type of tissue andstage of development.Itishighestinsenescing
tissuesandripeningfruits.
Withinthe plantorgans,ethylene formationismainlylocatedinperipheral tissues.
Ethylene isbiologicallyactive atverylowconcentration(<1ppm).AccordingtoKidd&West (1945), the
minimumthresholdvalue formanyethylenedependentresponsesinplantswasabout0.1 ppm.
As a by-productof hydrocarboncombustion,ethylene isalsoacommonenvironmental pollutantthat
can playhavoc withgreenhouse culturesand/orlaboratoryexperiments.
Due to its hydrophobicnature,ethylene caneasilypassthroughplasma-membraneintothe cell,easily
diffuse withinthe plantorplant part andflushedoutof planttissuesthroughintercellularspaces.
Cuticle onplantsurfacesacts as resistantbarrier,whilestomata,lenticelsandcutplacesserve asexit
pointsforthe gaseoushormone.

6. Mechanismof Ethylene Action:
As mentionedearlier,ethyleneexhibitswiderange of physiological effectsinplants.However,earlier
stepsare assumedto be similarinall casesthat include,
(i) Bindingof ethylene toareceptor,
(ii) Activationof one ormore signal transductionpathwaysand
(iii) Modulationof gene expressionleadingtocellularresponse.
i.In Arabidopsisthalianaandotherplants,ethyleneresponsesare negativelyregulatedbyareceptor
gene family.
ii.Anethylene bindingreceptorproteinhasbeenidentifiedinA .thaliana, tomatoandotherplants
whichisknownas ETR1. Fourotherproteinsare alsoknownthat serve asethylene receptorswhichare
calledasETR2, ERS1, ERS2 and EIN4.
iii.Like cytokine receptor,the ethylene receptor(ETR1&others) isalsosimilartobacterial two
componentsensorhistidineKinases.Itisadimerof twopolypeptideswhichare heldtogetherby
disulphide (-S-S-) bondsandislocatedonER (endoplasmicreticulum).
iv.Each polypeptideof the dimerreceptorproteinconsistsof three domains,
(i) Aminoterminal domainwhichspansthe ERmembrane atleastthrice and containsthe ethylenebind-
ingsite,
(ii) A middle histidine kinase domainand
(iii) A receiverdomaintowardsthe carboxyl terminusof the polypeptide (Fig.17.28).
v. Ethylene binds totrans-membrane domainof the receptorthroughacoppercofactor. The latteris
incorporatedintothe receptorproteinbyRANIprotein.
vi.Bindingof ethylenetoreceptorinactivatesaproteinkinase suchasCTR1 (amemberof RAF familyof
proteinkinases)inthe cytosol.
vii.Inactivationof CTR1allowsa trans-membrane protein(onER) calledEIN2tofunction.
viii.EIN2nowactivatesa cascade of transcriptionfactorsinthe nucleussuchas EIN3, ERF1 etc.
ix.These transcriptionfactorsinturnmodulate gene expressionandultimatelyethylene response
occurs.
(Inthe absence of ethylene,CTR1becomesactive whichinhibitsdownstreamsignal transduction
componentsandtherefore,ethylene response doesnotoccur).
Variousstepsof ethylenesignallingare showninFig.17.28.

7. Commercial Uses of Ethylene in Plants
The belowmentionedarticle providesa study note on the commercial usesof ethylene inplants.
Ethylene isone of the mostwidelyusedplantgrowthhormonesinagriculture.But,due toits gaseous
nature and highdiffusionrate,ethylenecannotbe administeredtoplantswithoutconfiningthemin
closedchambersandit isverydifficulttobe appliedingaseousforminthe field.
However,thisproblemhasbeenovercome due toavailabilityof some syntheticchemicalcompounds
whichwhensprayedonplantsinaqueoussolution are readilyabsorbedandtranslocatedwithinthem
and breakdowntorelease ethylene.
One such mostcommonlyusedchemical compoundisethephon(2-chIoroethylphosphonicacid) which
isknownby varioustrade namessuchas ethrel.Ethephonisstable atlow pH,but slowlybreaksdownat
pH 4 or more.Since,the pH of plantcellsislessacidic(about6); ethephonbreaksdowninplantcells
and releasesethylene whichexertsits hormonal effect.
Conversionofethephoninto ethylene isnon-enzymaticandis a simple base catalysed reactionin
which phosphoricacid and chloride ionsare the byproducts:

8. Breakdownof ethephonintoethylene isaveryslow processandmaycontinue for several daysinplant
cells.
(a) Aqueoussolutionof ethephonissprayedonplantsindesiredconcentrationstohastenfruitripening,
intomato and apple andde-greeningof citrusfruits.Itisalsoeffectivelyusedinsynchronizingflowering
and fruits setinpineapple andhasteningabscissionof flowersandfruits.
Other commercial uses ofethephon(ethylene) are:
(i) To induce fruitthinning(fruitdrop) incotton,cherryandwalnut.
(ii) Toinhibitterminalbudgrowthinsome plantssotheirfloweringstemsare made more compact.
(iii) Topromote formation(expression) of female flowersincucumber,avoidself pollinationand
increase yield.
(b) Sometimes,promotersof ethylene biosynthesissuchasauxinsandACCare alsousedinagriculture
practice, whichtriggernatural biosynthesisof ethylene inplants.
(c) Contrary to the above,reverse measuresare oftenemployedoncommercial scale toreduce rate of
ripening,preventingoverripeningtoenhance post-harvestpreservationof fruitsandtoincrease
longevityof cutcarnationsandotherflowersbyinhibitingorreducingthe natural biosynthesisof
ethylene inplanttissuesorremovingethylenefromstorage chambers.
This can be accomplishedin various ways:
(i) By ControllingStorage Atmosphere:
Low O2 conc. and lowtemp,inhibitbiosynthesisof ethylene.Low atmosphericpressure isusedto
remove ethylene andO2 fromthe storage chambersthat reducesrate of fruitripening.CO2 athigher
concentrations(5-10%) actsas antagonistof ethylene actionandhelpsinpreventingover-ripening.
Potassiumpermanganate (KMnO4) isaveryeffectiveabsorbentof ethylene andisusedinapple storage
chambersto delayripeningandextendingshelf life of the fruits.
(ii) By UsingInhibitorsof Ethylene Biosynthesis:
AVG,a potentinhibitorof ethylenebiosynthesiscanbe usedto retardfruitripeningandflowerfading.
(iii) By UsingAntagonistsof Ethylene Action:
Besideshigherconc.of CO2, silverionsespeciallyassilverthiosulphate are potentandmuchmore
effectiveinhibitorsof ethyleneactionandare extensivelyusedindelayingsenescence of cutcarnations
and otherflowers.1-Methyl cyclopropene (MCP),asyntheticvolatile olefmiccompoundisemergingas
yetanotherantagonistof ethyleneactionforuse inmanypost-harvestagricultural practices.
(iv) Through Biotechnology:

9. By makingexpressionof anantisense versionof ACCsynthase andACCoxidase intomato,the
biosynthesisof ethylene canbe blockedandfruit-ripeningcompletelyinhibited.Fruitripeninginsuch
geneticallymodifiedortransgenictomatoescanbe restoredbyexternallyappliedethylene onlywhen
needed.Ethylenebiosynthesiscanbe blockedinmanyotherplantssuchas Petuniaalsothrough
biotechnologyorgeneticengineeringtoincrease longevityof cutflowerstoseveral weeks.
(vi) Morphactins:
In 1960s (1964-65 onwards) a newgroupof syntheticgrowthregulatorscalledasmorphactins(meaning
morphologicallyactive substances)hadcome into prominence andarousedgreatinterestamongplant
physiologistsbecauseof theirpolyvalentaction(i.e.,widerange of action) onthe natural regulation
mechanismof plants.Theiractionisusuallyinhibitorytodevelopmentandgrowth.
The morphactinswhichare syntheticderivativesof fluorene-9-carboxyIicacidare absorbedviaseeds,
leavesorrootsand are distributedinthe plantnotstrictlypolarly(asIAA) butbasipetallyand
acropetally.
The morphactins are peculiarin that:
(i) Theyare non-toxicoverawide range of concentrationsand
(ii) Theyaffectthe organswhichdevelopaftertheirapplication(i.e.,the new growth).
Characteristic Physiological andMorphological Effects:
(i) General inhibitionof internodeselongation.
(ii) Reductioninlaminararea.
(iii) Reductionof apical dominance of the mainshoots(promotionof branching).
(iv) Stronginhibitionof lateral rootformations(i.e.,reinforcementof apical dominance of the taproots).
(v) Abolitionof phototropismof shootsandgeotropismof roots.
ESHFAQ BHATT
M.SC FOOD SCIENCE & TECHNOLOGY (KU)
Mob. : 9596406283/7006384308
Email: beingsahilishfaq@gmail.com ; bhatishfaq.sp890@gmail.com