Phytohormones and their metabolic engineering for abiotic stress tolerance in crop plants

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THE CROP JOURNAL 4 (2016) 162–176
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Phytohormones and their metabolic engineering for
abiotic stress tolerance in crop plants
Shabir H. Wania,⁎, Vinay Kumarb,⁎, Varsha Shriramc, Saroj Kumar Sahd
aDivisionofGeneticsandPlantBreeding,FacultyofAgricultureWADURA,Sher-e-KashmirUniversityofAgriculturalSciencesandTechnology,
Kashmir191121,India
bDepartmentofBiotechnology,ModernCollegeofArts,ScienceandCommerce,S.P.PuneUniversity,Ganeshkhind,Pune411016,India
cDepartmentofBotany,Prof.RamkrishnaMoreCollege,S.P.PuneUniversity,Akurdi,Pune411044,India
dDepartmentofBiochemistry,MolecularBiology,EntomologyandPlantPathology,MississippiStateUniversity,MS39762,USA
A R T I C L E I N F O A B S T R A C T
Articlehistory: Abioticstressesincludingdrought,salinity,heat,cold,flooding,andultravioletradiation
Received22August2015 causescroplossesworldwide.Inrecenttimes,preventingthesecroplossesandproducing
Receivedinrevisedform more food and feed to meet the demands of ever-increasing human populations have
15January2016 gained unprecedented importance. However, the proportion of agricultural lands facing
Accepted15March2016 multipleabioticstressesisexpectedonlytoriseunderachangingglobalclimatefueledby
Availableonline1April2016 anthropogenicactivities.Identifyingthemechanismsdevelopedanddeployedbyplantsto
counteractabioticstressesandmaintaintheirgrowthandsurvivalunderharshconditions
Keywords: thus holds great significance. Recent investigations have shown that phytohormones,
Phytohormones includingtheclassicalauxins,cytokinins,ethylene,andgibberellins,andnewermembers
Abioticstress including brassinosteroids, jasmonates, and strigolactones may prove to be important
Metabolicengineering metabolic engineering targets for producing abiotic stress-tolerant crop plants. In this
Plantstresses review, we summarize and critically assess the roles that phytohormones play in
plantgrowthanddevelopmentandabioticstresstolerance,besidestheirengineeringfor
conferringabioticstresstoleranceintransgeniccrops.Wealsodescriberecentsuccessesin
identifying the roles of phytohormones under stressful conditions. We conclude by
describingtherecentprogressandfutureprospectsincludinglimitationsandchallenges
ofphytohormoneengineeringforinducingabioticstresstoleranceincropplants.
©2016CropScienceSocietyofChinaandInstituteofCropScience,CAAS.Productionand
hostingbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
2. Abioticstresses:Challengingthechangingworld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
3. Importanceofstudyingplantstressesincombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
⁎ Correspondingauthors.
E-mailaddresses:[email protected](S.H.Wani),[email protected](V.Kumar).
PeerreviewunderresponsibilityofCropScienceSocietyofChinaandInstituteofCropScience,CAAS.
http://dx.doi.org/10.1016/j.cj.2016.01.010
2214-5141/©2016CropScienceSocietyofChinaandInstituteofCropScience,CAAS.ProductionandhostingbyElsevierB.V.Thisisan
openaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
THE CROP JOURNAL 4 (2016) 162–176 163
4. Phytohormones:Keymediatorsofplantresponsestoabioticstresses . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
4.1. Abscisicacid(ABA),theabioticstresshormone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
4.2. Auxins(IAA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
4.3. Cytokinins(CKs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
4.4. Ethylene(ET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
4.5. Gibberellins(GAs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
4.6. Brassinosteroids(BRs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
4.7. Jasmonates(JAs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
4.8. Salicylicacid(SA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
4.9. Strigolactones(SL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
5. Crosstalkbetweenphytohormonessignaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
6. Recentattemptsatdecipheringtheroleofphytohormonesinabioticstresstolerance . . . . . . . . . . . . . . . . 168
6.1. Hormoneshelpinpollendevelopmentundercoldstress . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
6.2. Hormonalbalanceundercoldstress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
6.3. Salicylicacidincreasesgermination,seedlinggrowth,andenzymesactivity . . . . . . . . . . . . . . . . . . 168
6.4. Hormoneresponsiveproteinmediatedstressresponses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
6.5. Putativeauxineffluxcarrierinvolvedinthedroughtstressresponseanddroughttolerance . . . . . . . . . 169
6.6. Phytohormonesalleviatehigh-temperaturestress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
7. Engineeringphytohormonesforproducingabiotic-stresstolerantcropplants . . . . . . . . . . . . . . . . . . . . . 169
7.1. Engineeringtechniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
7.2. Metabolicengineeringofphytohormonesforconferringabioticstresstoleranceoncropplants . . . . . . . 169
8. Conclusionandfutureoutlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
1.Introduction improvecropsnutritionallyandeconomically.Inthisreview,we
present an overview of the phytohormones and their roles in
plant growth, development, and abiotic stress response, in
The human population is rapidly increasing and needs a
additiontotheirmetabolicengineeringtoconferabioticstress
substantial increase in agricultural productivity worldwide.
toleranceincropplantstoincreasefoodquantityandquality.
However,variousbioticandabioticstressesaremajorfactors
Wediscussrecentsuccessesandfutureprospects.
limiting crop productivity [1]. To feed the world population,
productivity must be increased by 70% for an additional 2.3
billion people by 2050 [2]. The mechanisms underlying envi-
ronmentalstressresponseandtoleranceinplantsaredifferent 2.Abioticstresses:Challengingthechangingworld
andmorecomplexthananimals[3].Identifyingtheprinciples
bywhichplantsrespondtovariousenvironmentalstressesis Investigating how abiotic stresses affect plant growth and
oneofthecriticalaspectsforplantbiotechnologists.Amongst developmentatthephysiological,biochemical,andmolecular
variousabioticstresses,drought,salinity,andextremetemper- levels is critical to increasing the productivity of crops,
atures are most widespread and significant [4]. Because of because stresses cause widespread crop losses throughout
thecomplexityofstresstolerancetraits,conventionalbreeding the world [6]. Environmental factors imposing stress on
techniques have met with limited success. They demand plants, including drought, salinity, heat, chilling, freezing,
effectiveadvancestofeedthegrowingworldwidefooddemand. ozone, pathogens, and UV radiation are the major environ-
In this direction, novel and potent approaches should be mental cues that limit crop productivity. The period and
devised.Engineeringofphytohormonescouldbeamethodof development of stress, stages of the plant, and biotic and
choicetoproduceclimate-resilientcropswithhighyields. abiotic factors may influence the stress response [8]. Some
Phytohormones are molecules produced in very low cropsmaybeaffectedatanearlystage,butrecoverandfinally
concentrations but able to regulate a variety of cellular survive. Susceptibility or resistance to stresses may differ
processes in plants. They work as chemical messengers to markedlyamongspeciesorgenotypesofcrops.Amongthese
communicate cellular activities in higher plants [5]. Phyto- stressconditions,droughtisperhapsthemostseverestress,
hormones play key roles and coordinate various signal responsiblefordecreasedagriculturalproductionworldwide.
transductionpathwaysduringabiotic-stressresponse.They It affects plants in many ways: plant growth, membrane
regulate external as well as internal stimuli [6]. Some integrity,pigmentcontent,osmoticadjustments,waterrela-
phytohormones, such as ABA, have been identified as stress tions,andphotosyntheticactivity[9,10].Asrightlypointedout
hormones. ABA plays critical roles in plant development: byPostel[11],waterwillbetheoilofthe21stcentury.Many
maintenance of seed dormancy, inhibition of germination, riversaroundtheglobesaredryingdaily;mosthavenowater
growth regulation, stomatal closure, fruit abscission, besides to discharge to the sea [11]. After drought, salinity is the
mediatingabioticandbioticstressresponses[7].Phytohormone secondmajorstressreducingcropproductivity.Variousplant
engineeringcouldbeaperfectplatformforbiotechnologiststo hormones have shown positive plant-protective functions
164 THE CROP JOURNAL 4 (2016) 162–176
against abiotic stresses and attempts have been made to plays an influential during several plant physiological pro-
assign specific hormone(s) for specific stresses as well as cesses and developmental stages including seed dormancy
combinations of stresses, as some hormones have shown anddevelopment,stomatalopening,embryomorphogenesis,
multiplestress-resistancefunctions. andsynthesisofstorageproteinsandlipids[16].
ABA isconsidered anessential messengerinthe adaptive
response of plants to abiotic stress and its role in stress
3.Importanceofstudyingplantstresses tolerancehasreceivedmuchattention.Inresponsetoenviron-
incombination mental stresses, endogenous ABA levels increase rapidly,
activating specific signaling pathways and modifying gene
Previously,tostudyabioticstressconditions,generallyspecific expressionlevels[17].Nemhauseretal.[18]havereportedthat
stressconditionswereinvestigated,includingdrought,salinity, ABAtranscriptionallyregulatesupto10%ofprotein-encoding
orheat,andthediversemolecularaspectsofplantacclimation genes. ABA also acts as an internal signal enabling plants to
were analyzed. However, conditions differ in the natural surviveunderadverseenvironmentalconditions[19].
environments. In nature, crops face different stresses or Under water-deficit conditions, ABA plays a vital role in
combinations of stresses found in the environment. The providingplantstheabilitytosignaltotheirshootsthattheyare
majority of molecular studies are conducted under con- experiencingstressfulconditionsaroundtheroots,eventually
trolledconditionsinthelaboratoryorgreenhouseanddonot resulting in water-saving antitranspirant activity, notably
reflect the actual conditions that occur in the field. In the stomatalclosureandreducedleafexpansion[20].ABAisalso
laboratory or greenhouse, conditions are controlled so that involvedinrobustrootgrowthandotherarchitecturalmodifi-
one stress is imposed, but in natural conditions there are cationsunderdroughtstress[21]andnitrogendeficiency[22].
manyenvironmentalstressesthatareappliedincombination. ABAregulatestheexpressionofnumerousstress-responsive
Therefore,theexperimentsconductedforevaluatingeffectsof genes and in the synthesis of LEA proteins, dehydrins, and
acombinationofstressfactorsratherthananindividualstress other protective proteins [23,24]. ABA upregulates the pro-
mayproveadvantageous.Droughtandheatstressincombina- cessesinvolvedincellturgormaintenanceandsynthesisof
tion caused losses of $200 billion in the USA (readers are osmoprotectantsandantioxidantenzymesconferringdesic-
encouragedtorefertotheexcellentreviewofSuzukietal.[12]). cationtolerance[25].Zhangetal.[26]reportedaproportional
increase in ABA concentration upon exposure of plants to
salinity.
4.Phytohormones:Keymediatorsofplantresponses
toabioticstresses 4.2.Auxins(IAA)
Plantsmustregulatetheirgrowthanddevelopmenttorespond Although auxin has been studied for over 100 years, its
tovariousinternalandexternalstimuli[13].Phytohormones,a biosynthesis, transport, and signaling pathways are still not
diversegroupofsignalingmoleculesfoundinsmallquantitiesin clear[27].However,someinterconnectingpathwayshavebeen
cells,mediatetheseresponses.Theirpivotalrolesinpromoting proposedso farforbiosynthesisof auxininplants, including
plant acclimatization to ever-changing environments by four tryptophan (Trp)-dependent and one Trp-independent
mediating growth, development, source/sink transitions, pathway [28]. IAA (indole-3-acetic acid) is one of the most
and nutrient allocation have been well established [14]. multi-functionalphytohormonesandisvitalnotonlyforplant
Although plant response to abiotic stresses depends on growthanddevelopmentbutalsoforgoverningand/orcoordi-
various factors, phytohormones are considered the most natingplantgrowthunderstressconditions[29].Thepresence
important endogenous substances for modulating physio- ofanauxinbiosynthesis,signaling,andtransportapparatusin
logical and molecular responses, a critical requirement for single-celledgreenalgaeisclearevidenceoftheevolutionary
plantsurvivalassessileorganisms[14].Phytohormonesact roleplayedbyauxinduringtheadaptationofplantstodiverse
eitherattheirsiteofsynthesisorelsewhereinplantsfollowing land environments [30]. Though there has been a recent
theirtransport [15]. Phytohormonesareofkeyimportancein upsurge in our understanding of auxin regulation of plant
plant development and plastic growth. They include auxin growth and development, its role as a regulator of stress
(IAA), cytokinins (CKs), abscisic acid (ABA), ethylene (ET), responseisstilllittleunderstood[29].
gibberellins (GAs), salicylic acid (SA), brassinosteroids (BRs), Interestingly, there is growing evidence that IAA plays an
andjasmonates(JAs).Thestrigolactone(SL)arerelativelynew integral part in plant adaptation to salinity stress [14,31]. It
phytohormones.Fig.1showsthechemicalstructuresofmajor increasesrootandshootgrowthofplantsgrowingundersalinity
phytohormones. or heavy metal stresses [32,33]. Salinity reduced IAA levels in
maizeplants,butsalicylicacidapplicationeffectivelyincreased
4.1.Abscisicacid(ABA),theabioticstresshormone them [34], indicating that hormonal balance and crosstalk
arecriticaltosignalperception,transduction,andmediationof
Abscisicacid(ABA)owesitsnametoitsroleinabscissionof stressresponse[14].Auxinstimulatesthetranscriptionofalarge
plantleavesandisperhapsthemoststudiedphytohormone numberofgenescalledprimaryauxinresponsegenes,andthese
for its response and distinct role in plant adaptation to genes have been identified and characterized inseveral plant
abiotic stresses, and is accordingly termed a “stress hor- species including rice, Arabidopsis, and soybean [35]. Auxin is
mone.” It is an isoprenoid plant hormone produced in the regarded as an influential constituent of defense responses
plastidal2-Cmethyl-D-erythritol-4-phosphatepathway.ABA via regulation of numerous genes and mediation of crosstalk
THE CROP JOURNAL 4 (2016) 162–176 165
Abscisic Acid IAA (auxin) Zeatin (cytokinin) Ethylene Gibberellic acid
Brassinolide (brassinosteroid) Jasmonic acid Salicylic acid (+)-Strigol (strigolactone)
Fig.1–Majorclassesofphytohormonesinvolvedinabioticstressresponseandtoleranceinplants.
between abiotic and biotic stress responses [36]. However, AdoMettoACC,whileACCoxidasecatalyzestheconversionof
identificationofnovelgenesinvolvedinstressresponsesmay ACCtoethylene.
provetobeavitaltargetforengineeringabioticstresstolerance Abiotic stresses including low temperature and salinity
inprincipalcrops. alterendogenousETlevelsinplants.Enhancedtolerancewas
accordingly achieved with higher ET concentrations [44]. ET
4.3.Cytokinins(CKs) also plays a major role in the defense response of plants to
heat stress[45]. Environmental stressinducesET accumula-
CKsplayinfluentialrolesinmanyplantgrowthanddevelop- tion which increases plant survival chances under these
mental processes and are considered as master regulators adverse conditions [42]. ET has been proposed to function
during plant growth and development [37,38]. Alteration of viamodulationofgeneexpressionconsideredastheeffectors
endogenouslevelsofCKsinresponsetostressindicatestheir ofethylenesignal[46].
involvementinabioticstress[17],includingdrought[37]and ET incombination withotherphytohormonessuchas JA
salinity[38].Mutantsandtransgeniccells/tissueswithaltered and SA often acts cooperatively. These are considered the
activity of cytokinin metabolic enzymes or perception ma- main players involved in regulating plant defense against
chinery points toward their crucial involvement in several pests and pathogens [6]. The biosynthesis, transport, and
crop traits including productivity and increased stress toler- accumulationofthesehormonestriggeracascadeofsignal-
ance[39]. ingpathwaysinvolvedinplantdefense[47].Asconcludedby
Although plant responses to CKs have been evaluated Yin et al. [48], ET and ABA seem to act synergistically or
mostoftenviatheirexternalapplication,stressfulconditions antagonisticallytocontrolplantgrowthanddevelopment.
are also known to enhance their endogenous levels via
uptake and enhanced biosynthesis [40]. In contrast to the 4.5.Gibberellins(GAs)
ABA inhibition of seed germination, they also release seeds
from dormancy [36]. CKs are often considered ABA antago- The gibberellins (GAs) are a large group of tetracyclic
nists[41].Inwater-stressedplants,decreasedCKcontentand diterpenoidcarboxylicacids,butonlyafewofthemfunction
accumulation of ABA lead to an increased ABA/CK ratio. asgrowthhormonesinhigherplants,predominantformsbeing
The reduced CK levels enhance apical dominance, which, GA and GA [49]. The GAs show positive effects on seed
1 4
togetherwiththeABAregulationofstomatalaperture,aidsin germination, leaf expansion, stem elongation, flower and
adaptationtodroughtstress[17]. trichome initiation, and flower and fruit development [50].
They are essential for plants throughout their life cycle for
4.4.Ethylene(ET) growth-stimulatoryfunctions.Theyalsopromotedevelopmen-
tal phase transitions [51]. Interestingly, there is increasing
ET, a gaseous phytohormone, is involved in several phases of evidence for their vital roles in abiotic stress response and
plant growth and development, notably fruit ripening, flower adaptation[51].Recently,experimentshavebeenperformedto
senescence, and leaf and petal abscission, besides being an investigate the role of GAs in osmotic stress response in
essentialregulatorofstressresponses[42,43].Itisbiosynthesized Arabidopsisthalianaseedlings[52,53].GAsareknowntointeract
frommethionineviaS-adenosyl-L-methionine(AdoMet)andthe withallotherphytohormonesinnumerousdevelopmentaland
cyclic non-protein amino acid ACC. ACC synthase convert stimulus-responseprocesses[54].TheinteractionsbetweenGA
166 THE CROP JOURNAL 4 (2016) 162–176
and ET include both negative and positive mutual regulation thaliana plants against Cu and Cd stress via accumulation of
dependingonthetissueandsignalingcontext[54]. phytochelatins[76].
4.6.Brassinosteroids(BRs) 4.8.Salicylicacid(SA)
Brassinosteroids (BRs) comprise a relatively new group of Salicylicacid(SA)isanaturallyoccurringphenoliccompound
polyhydroxysteroidalplanthormoneswithstronggrowthand involved intheregulation ofpathogenesis-associatedprotein
development-promoting potential. They were first isolated expression[77].Inadditiontodefenseresponses,itplaysavital
and characterizedinpollenof the rapeplant (Brassicanapus). role in the regulation of plant growth, ripening and develop-
More than 70 BRs have been isolated from plants. However, ment, as well as responses to abiotic stresses [78,79]. The
brassinolide,28-homobrassinolide,and24-epibrassinolidecon- synthesisofSAoccursviatwopathways:theisochorismate(IC)
stitute the three most bioactive BRs and are widely used in andthephenylalanineammonia-lyase(PAL)pathway.Ofthese,
physiologicalandexperimentalstudies[55].Theyarepresentin themajorpathwayistheICpathwayinNicotianabenthamiana
almosteverypartofplantsincludingpollen,flowerbuds,fruits, [80]andtomato[81].
seeds,vascularcambium,leaves,shoots,androots[56].They An interesting and noteworthy general belief is that low
playacriticalroleinnumerousdevelopmentalprocessessuch concentrations of SA enhance the antioxidant capacity of
asstemandrootgrowth,floralinitiation,anddevelopmentof plants, but high concentrations of SA cause cell death or
flowersandfruits[56]. susceptibilitytoabioticstresses[82].Mostgenesthatrespond
However,recentfindingssuggeststress-impactmitigating positively to acute SA treatment are associated with stress
roles of BRs and associated compounds in various plants andsignalingpathwaysthateventuallyledtocelldeath.SA
subjected to various abiotic stresses. These abiotic stresses consistsofgenesencodingchaperones,heatshockproteins,
arehightemperature[57],chilling[58],soilsalinity[59],light antioxidants, and genes involved in the biosynthesis of
[60], drought [61], flooding [62], metals/metalloids [63], and secondary metabolites, such as sinapyl alcohol dehydroge-
organic pollutants [64]. Recent research has shown tremen- nase,cinnamylalcoholdehydrogenase,andcytochromeP450
dous potential of BRs and associated compounds in the [82].
modulation of components of antioxidant defense system SAisinvolvedinplantresponsetoabioticstressessuchas
in-response-to and to counteract the abiotic stress-induced drought [83], salinity [34,84], chilling [85], and heat [86]. SA
oxidative burst, reviewed by Vardhini and Anjum [65]. along with ABA is involved in the regulation of drought
However, there is tremendous scope for further research response[77]. Drought stress induced a five-fold increase in
focused on the sites, pathways, and enzymology of their the endogenous levels of SA in Phillyrea angustifolia[87]. The
biosynthesis, source–sink relationships, developmental and SA content in barley roots was increased approximately
stress physiology, their interactions with microorganisms, twofoldbywaterdeficit[88].TheSA-induciblegenesPR1and
fungi, and animals, and the realization of their powerful PR2 (pathogenesis-related genes) are induced by drought
applications[14]. stress [83]. However, the detailed molecular mechanisms of
SA’srolesinabioticstresstoleranceremainlargelyunknown
4.7.Jasmonates(JAs) and more comprehensive investigations are needed in this
direction.
Thecyclopentanonephytohormonesderivedfromthemetab-
olism of membrane fatty acids including primarily methyl 4.9.Strigolactones(SL)
jasmonate (MeJA) and its free acid jasmonic acid (JA) are
collectivelycalledjasmonates(JAs)andarewidespreadinthe Strigolactones (SLs) constitutes a small class of carotenoid-
plantkingdom.Thesemultifunctionalcompoundsareinvolved derived compounds, first characterized more than 45years
in crucial processes associated with plant development and ago as seed germination stimulants in root parasitic plants
survival including reproductive processes, flowering, fruiting, such as Striga, Orobanche, and Phelipanche species [89,90].
senescence, secondary metabolism, and direct and indirect Several types of SLs can be synthesized by single plant
defenseresponses[66,67].JAisthemostabundant,bestknown, species, whereas mixtures of different types and quantities
andbestcharacterizedoftheJAs.Inadditiontodevelopmental ofSLmoleculesareproducedinintraspecificvarieties[89,91].
functions of plants, JA activates plant defense responses to Although they are produced and exuded in small amounts
pathogenicattackaswellasenvironmentalstressesincluding primarilyinroots,otherplantpartscanalsosynthesizethem
drought, salinity, and low temperature [68,69]. JAs are vital [92]. A comparative study involving wild-type and mutant
signalingmoleculesinducedbyvariousenvironmentalstresses Arabidopsis plants indicatestheir role in the development of
includingsalinity[68],drought[69,70],andUVirradiation[71]. rootsystemarchitecture[93].ApplicationofGR24,asynthetic
Theyhavegreatpotentialtomitigateanarrayofthreatening and biologically active SL [94,95], repressed lateral root
environmentalstresses[72].TheexogenousapplicationofMeJA formation in wild-type seedlings and SL-synthesis mutants
effectivelyreducedsalinitystresssymptomsinsoybeanseed- (max3andmax4)butnotinthestrigolactone-responsemutant
lings [73]. Remarkably, endogenous levels of JA increased in (max2), suggesting the MAX2-dependent negative effect of
rice roots under salinity stress and reported to counteract strigolactoneonlateralrootformation[93,96].Apparently,SLs
the deleterious effects of salinity stress [74]. JAs applications are involved in plant responses to environmental stimuli
alleviate heavy metal stress in plants by activating the fromtheirearlyevolution.Inhigherplants,theyparticipatein
antioxidant machinery [75]. MeJA confers tolerance in A. both shoot and root architecture in response to nutritional
THE CROP JOURNAL 4 (2016) 162–176 167
conditions[97].SLs alsoactassignalingmoleculesfor plant We now discuss stomatal closure as a rapid response
interactions with microbes. They stimulate nodulationin to water deficit conditions and phytohormonal crosstalk in
the legume–rhizobium interaction process [98,99]. Overall, it its regulation. Stomatal closure is regulated by a complex
can be concluded that SLs constitute an important group of networkofsignalingpathways,andasashort-termresponse
signaling molecules and are key regulators of plants’ devel- istriggeredprimarilybyABA.ABAcontrolslong-termgrowth
opmentaladaptationstochangingenvironmentalconditions. responses via the regulation of gene expression that favors
Theyhavethepotentialtobeusedinagricultureforvarious maintenance of root growth, which optimizes water uptake
purposesincluding as inducers of suicidal seed germination [26].However,followingtherulesofdroughtstressresponse,
ofparasiticplants[100]. ABA apparently interacts with other plant hormones and
signaling molecules such as JA and nitric oxide (NO) to
stimulate stomatal closure, whereas its regulation of gene
expression includes the induction of genes associated with
5.Crosstalkbetweenphytohormonessignaling response to ethylene, cytokinin, or auxin [101]. It has been
proposed by the authors that stress-induced JA production
Environmentalstressesrequireplantstoperceiveandreactto interactswithABA-mediatedstomatalclosurebystimulating
thesesignalsinahighlycoordinatedandinteractivemanner. theinfluxofextracellularCa2+and/orbyactivatingH O /NO
2 2
Plantsbeingsessileorganismsneedtomaintainplasticityin signaling [101]. In contrast, Desikan et al. [106] provided
growthandabilitytoadapttoharshchangingenvironmental evidence that stomatal closure by ethylene is regulated via
conditions, and this adaptation is mediated by elaborate its signal transduction pathway, which both stimulates
signalingnetworks.Signaltransductioncascadesthatinteract production and requires H O synthesis. This crosstalk also
2 2
with the baseline pathways transduced by phytohormones involves JA, whose biosynthesis is induced by stress condi-
get triggered by the perception of abiotic stresses [101]. tionsincludingherbivory[107], whereasmanyJA-associated
The fluctuations of stress-responsive hormones help alter signaling genes are regulated by drought stress [108]. JA
cellulardynamicsandthusplayacentralroleincoordinately interacts with ABA-regulated stomatal closure by increasing
regulating growth responses under stress conditions [102]. Ca2+ influx, which ultimately stimulates calcium-dependent
Theconvergencepointsamonghormonesignaltransduction protein kinase (CDPK) production and the resulting signal
cascadesare considered crosstalk, and together they form a cascade [101]. A 10-min treatment with either ABA or MeJA
signaling network [101]. In this way, hormones seemingly resulted in a reduction of stomatal aperture in turgid and
interactbyactivatingeitheracommonsecondmessengerora excised leaves of Arabidopsis [109]. The authors postulated
phosphorylation cascade. In the last few decades, insights thatbothABAandMeJAinteractinguardcellsandinducethe
intothebiosyntheticandcoresignalingcomponentsofmajor formation of reactive oxygen species and NO and that both
phytohormones including ABA, IAA, BRs, GAs, JA, and ET are present at reduced concentrations in MeJA-insensitive
have been revealed [103]. However, owing to the extreme plants. In another experiment, Kim et al. [110] found that
complexityofresponsestodifferentstressthresholds,lackof droughtstressinduceda19-foldincreaseinMeJAlevelsanda
knowledge about tissue-specific stress response, and inade- 2-foldincreaseinABAlevels,whichwasassociatedwithsevere
quateunderstandingofgeneticplasticityanditsadaptability yield loss due mainly to poor seed set. This phenotype was
to environments, the mechanistic basis of abiotic stress mimickedbyconcentrationsofMeJAinyoungpaniclesofrice
tolerance remains largely unclear and confusing [102]. Con- through the ubiquitous expression of AtJMT transgenically,
sequently, all these factors collectively contribute to more which also led to a dramatic reduction in grain yield and a
confusion than resolution [104]. Still, perturbed phytohor- substantialincreaseinABA,suggestingcrosstalkbetweenMeJA
monefluxesandthesubsequentsignaltransductioncascade andABAbiosynthesis[110].Similarly,long-termphysiological
have been revealed as one of the primary stress responses responses to abiotic stress conditions are caused and influ-
evolvedbyplants. encedbyalteredABA-mediatedgeneregulationoftranscription
Variousplanthormonesinteracttogetherforsignaldefense factors(TFs)thatbindtoABA-responsiveelements(ABREs)on
networking to fine-tune the defense against environmental ABA-regulated genes. Notably, phosphorylation cascades like
challenges. Among them, SA, JA, and ABA hold particular those signaling stomatal closure also lead to changes in
importanceandareregardedaskeyplayersintheregulation ABA-regulatedTFs.Forinstance,theABA-responsiveTFsABF1
ofsignalingpathways.Inrecentyears,decipheringtheinterac- and ABF4 are activated upon their phosphorylation by
tions between phytohormones and their coordinated roles ABA-inducible kinases CPK4 or CPK11 [111], as reviewed by
in counteracting abiotic stresses has gained new attention. Harrisonetal[101].
Experimentalfindingsunequivocallyshowthattheinteractions Onasimilarline,preferentialupregulationofJApathway
amongphytohormonesaretheruleratherthantheexception genesundersalinitystressinbarleyplantshasbeenreported
inintegratingdiverseinputsignalsandreadjustinggrowthand [112].AlthoughthepreciseroleofJAindroughtandsalinity
acquiring stress tolerance in plants [102]. The presence of remains unknown; it could be implicated as a molecule
multiple and frequent redundant signaling intermediates for signaling cell death [102]. Nishiyama et al. [38], studying
eachhormonehintsattheirapparentrolesinsuchcrosstalk. gene expression, concluded that exogenous ABA treatment
Understanding the crosstalk between phytohormonal and strongly downregulated isopentenyltransferase (IPT), a key
defensesignalingpathwaysisthusimportant,asitmayreveal cytokininbiosynthetic pathwaygene,butupregulatedgenes
new potential targets for the development of host resistance encodingcytokininoxidasesanddehydrogenases.Further,in
mechanismsandphytohormones[105]. addition to its well-documented role in controlling plant
168 THE CROP JOURNAL 4 (2016) 162–176
growth and development in close associations with auxins genetic male sterile line by using transcription regulation of
and BRs [113], GA is also involved in cross-talk of hormonal pollendevelopment;fordetails,seereviewbySharma[123].
interactions in signaling environmental inputs [114,115].
Various recent reports make it clear that interplay between 6.2.Hormonalbalanceundercoldstress
environmentally activated ABA, ET, GA, and CK signals is
crucialindeterminingplantstressresponses[3,116,117]. Kolaksazov et al. [124] reported that stress phytohormones
Thereisevidencethatsomephytohormonesalsocounteract such as ABA, JA, and SA triggerphosphoprotein cascade
adverse conditions. For example, a signal peptide system pathways, leading to expression of genes associated with
coupled with JA is reported to be involved in wound-induced cold stress tolerance. They reported high contents of JA in
salt-stressadaptationintomato[118],indicatingcross-tolerance the three different populations at a controlled temperature
signaling.Thepossiblerolesofphytohormonesinabioticstress of 22°C, with a 10-fold reduction in sensitive plants but no
tolerance and crosstalk between phytohormone signaling are changeintolerantplantsat4°C.
illustratedinFig.2.
6.3.Salicylicacidincreasesgermination,seedlinggrowth,and
enzymesactivity
6. Recent attempts at deciphering the role of
phytohormonesinabioticstresstolerance Gharib and Hegazi [125] showed that SA stimulated various
growthaspectsofbeanseedlings,wasresponsibleforbiosyn-
6.1.Hormoneshelpinpollendevelopmentundercoldstress thesis of growth-promoting and -inhibiting substances, and
reducedtheadverseeffectofcoldstressincommonbean.They
Abiotic stress can affect any developmental stages, but the conductedanexperimentonsixcommonbeanvarietieswith
reproductive stage is perhaps most critical. If stresses are optimal temperature 25°C and chilling stress at 15°C. They
applied at the reproductive stage, it may damage the whole foundthatgerminationandseedlinggrowthofthesixvarieties
plantandultimatelyreducethecropyield.Further,amongthe were slowed under chilling stress. Seeds treated with SA
various reproductive stages, pollen development is crucial showedincreasedgerminationgerminationratecomparedto
andisaffectedbyabioticstressessuchasdrought,cold,and untreated(control)seedsundercontrolaswellaschillingstress.
heat, leading to reduced crop yield [119]. In pollen develop-
ment,pollenmeiosisisthestagemostsensitivetocold,which 6.4.Hormoneresponsiveproteinmediatedstressresponses
causes pollen sterility and also reduces anther dehiscence,
pollen load to the stigma, pollen germination, and pollen Inplantstressresponse,phytohormonesplayavitalrole,and
tube growth [120,121]. Hormones such as GA and ABA are someproteinscanbeusedforcommunication.Bhaskaretal.
consideredmajorsignalsforcold-inducedpollensterilityand [126]reportedthatC1-(cysteinerichproteinfamily)domain-
anunderstandingofmolecularmechanismofpollendevelop- containing proteins play a part in plant hormone-mediated
mentunderstressandnon-stressconditionscanbebeneficial stress responses. Authors identified 72 other proteins in
for hybrid seed production by producing novel sterile plants. Arabidopsis that contained all three unique signature do-
In rice, Zhang and co-workers [122] developed a hybrid seed mains.Transgeniclinesalsoshoweddifferentialregulationof
production line in the form of a new photoperiod-sensitive manyabioticstress-responsivedistinctgenes,indicatingthe
High light Drought
CK Aux SA
ht C
g o
ou ld
Dr
alt Drought
s
h ABA ET
g
Hi High salt
E
alt xce
s s
BR High s wa
te
r
SL GA
Fig.2–Thepossiblerolesofphytohormonesinabioticstresstoleranceandcrosstalkbetweenphytohormonesignaling
THE CROP JOURNAL 4 (2016) 162–176 169
involvementofC1-clanproteinfamilyinhormone-mediated [131,132].Theuseofbiolisticsorparticlebombardmentisbyfar
stressremediation. themostwidelyuseddirectgenetransfermethod.
Withitsever-increasinghostrange,Agrobacteriumtumefaciens
6.5.Putativeauxineffluxcarrierinvolvedinthedroughtstress hasbecometheclearmethodofchoiceforgenetransfertoallof
responseanddroughttolerance themaincropplants.Thissoilbacteriumpossessesthenatural
abilitytodeliveradistinctpartofitsplasmidDNA(transferor
Auxinactsasanecessarysignalinreactiontoabioticstresses. T-DNA)intothenucleargenomeofitshostplant.Thiscondition
Zhang et al. [127] identified a putative auxin efflux carrier has arisen also partly because although direct gene transfer
gene (OsPIN3t) in rice that acts in polar auxin transport methodsareusefulforstabletransformationaswellastransient
involved in the drought response in rice. They also found expression, there are still some problems: alow frequency of
that knockdown of the OsPIN3t gene leads to crown root stabletransformation,unwantedgeneticrearrangementsdueto
abnormalities in the seedling stage and that its overexpres- thehighcopynumberofgenes,andthelongperiodrequiredto
sion increased drought tolerance. Under 20% polyethylene regeneratewholetransgenicplants.InAgrobacterium,bacterial
glycolstress,GUSactivitysignificantlyincreasedunderNAA genescanbesuccessfullyreplacedwithgene(s)ofinterest,and
treatment.ThestudyshowedthatOsPIN3tisinvolvedinauxin interestingly,thisreplacementdoesnotaffectthetransforma-
transportandthedroughtstressresponseinplants. tionprocessorfrequency[133,134].
For efficient transformation and subsequent regenera-
6.6.Phytohormonesalleviatehigh-temperaturestress tion, Agrobacterium-mediated methods are dependent on
severalfactors.Theyarethechoiceofexplant,thehormonal
Owingtohightemperatures,cropproductivityisdecreasingin composition of the medium used, nutritional supplements,
many parts of the world. Chhabra et al. [128] performed an cultureconditionsbeforeandduringinoculation,durationof
experimenttotesttheeffectofvarioushormonalconcentrations co-cultivation, virulence of the Agrobacterium strain, and
onheat-stresseffectsandobservedthatbothgrowth-promoting concentrationandcompositionofthebacteriostaticagentused.
and growth-retarding hormones mitigated heat-stress effects. The length of selection and concentration of the antibiotic
They also reported that the most effective concentration of selectionmarkerarealsoimportant.Otherimportantfactorsare
kinetin was 100μmolL−1 followed by 50μmolL−1 and that the plantcultivar and various conditions of tissue culture,
soaking seeds in ABA delayed 50% seedling mortality by 1h includingarobustsystemofplantregeneration[135].However,
and50minat0.5and1.0μmolL−1concentrations. through extensive efforts, remarkable progress has been
achievedduringthelast2–3decadesinplantgeneticengineering.
OptimizedprotocolshavebeendevelopedfortheAgrobacterium-
7. Engineering phytohormones for producing mediatedgenetictransformationofabroadrangeofcropplants
abiotic-stresstolerantcropplants including monocots, previously considered to lie outside the
Agrobacteriumhostrange.Recently,successhasbeenachievedin
7.1.Engineeringtechniques developinginplantatransformationmethods.However,inspired
by the complex patent landscape of Agrobacterium technology
Geneticengineeringhasopenednewavenuesforintroducing and in search of a perfect (open source) platform for plant
abioticstresstoleranceinnumerouseconomicallyimportant biotechnology, Broothaerts et al. [136] have identified three
crop species. Recent success has made genetic engineering non-Agrobacteriumspecies:Rhizobiumsp.NGR234,Sinorhizobium
approachesasoneofthemostpotentsolutionsforimproving meliloti, and Mesorhizobium loti as being capable of successful
cropproductivity underchallengingenvironments.The suc- genetic transformation of different plant species. A recent
cess of transgenic research, however, depends largely on additiontothislistofbiologicalvectorsforgenetictransforma-
effectiveplanttransformationmethodsforstableintegration tionofplantsisvirus-basedvectors.
and functional expression of foreign genes in the plant
genome. Since initial reports in tobacco [129,130], rapid 7.2. Metabolic engineering of phytohormones for conferring
developmentsintransformationtechnologyhaveresultedin abioticstresstoleranceoncropplants
the genetic modification of large number of plant species
[131]. Considering that phytohormones are key regulators of plant
Twogeneralapproachesareavailablefortransferringgene(s) growthanddevelopmentaswellasmediatorsoftheresponse
to plants: one vector-mediated, by a biological vector such as toenvironmentalstress[24],hormonemetabolismandsignal-
Agrobacterium, and the other direct gene transfer approach, ing processes are excellent targets of manipulation to obtain
whereDNAisintroducedintocellsbyphysical,chemical,oreven enhanced abiotic stress tolerance. However, maintenance of
electricalmeans.Directornonbiologicalgenetransfermethods hormonal balance to minimize possible adverse effects on
includeparticlebombardment,DNAuptakeintoprotoplastsin growthanddevelopmentiscritical[15,137].
the presence of polyvalent cations, protoplast fusion with Among various phytohormones, ABA is perhaps the most
bacterialspheroplastsandwithliposomescontainingforeign sought-afterhormoneforengineeringabioticstresstolerancein
DNA(lipofection),electrotransfection,polymer-basedtrans- crop plants owing to its identity as a stress hormone and its
fection (polyfection), silicon carbide fiber-mediated DNA up- vastarrayoffunctionsunderenvironmentalstressconditions,
take, injection-based methods (micro- and macro-injection), particularlydrought.Asaresult,manyofthekeyABAbiosyn-
wave and beam-mediated transformation, desiccation-based theticpathwayenzymeshavebeeninvestigatedtransgenically
transformation,andexogenousDNAapplicationandimbibition for improved abiotic stress tolerance [138]. Park et al. [139]
170 THE CROP JOURNAL 4 (2016) 162–176
reportedenhancedosmoticstresstolerancebyoverexpressing DREB1B from Arabidopsis improved tolerance to water-deficit
anABA-responsivestress-relatedgeneinArabidopsis.C-repeat stress in tomato [140]. Likewise, Al-Abed et al. [141] found
binding factor (CBF) and/or dehydration-responsive element- expressionofCBF/DREBgenesunderstress-induciblepromoters
binding (DREB) genes have been manipulated to confer im- in transgenic plants that do not express detectable levels of
proveddroughttolerance.Forexample,overexpressionofCBF1/ these genes under non-stress conditions, minimizing growth
Table1–Listofattemptsmadeatengineeringphytohormonesforenhancedabioticstresstoleranceofplants.
Item Gene Functionofgene Expression/ Phenotypeoftransgenics Reference
knock-out
Planthormone MoCo Regulationofthelaststep ↑ Transgenicsoybeanshowedhigherbiomass, [150]
sulfurase ofABAbiosynthesis yield,andoverallenhanceddroughttolerance
ABA LOS5 KeyregulatorofABA ↑ TransgenicmaizewithenhancedABA [142]
biosynthesis accumulationandincreaseddroughttolerance
AtLOS5 KeyregulatorofABA ↑ Increasedsalinitytoleranceattributedto [151]
biosynthesis enhancedNa+effluxandH+influxand
consequent
NCED Importantroleinrate ↑ IncreasedlevelsofendogenousABA,decreased [152]
limitingstepofABA stomatalconductanceandincreaseddrought
biosynthesisforfeedback tolerance
control
MsZEP ImportantroleinABA ↑ Heterologousexpressionofgeneresultedin [153]
biosynthesis bettersaltanddroughttolerance
SnRK2.4 Importantserine/ ↑ TransgenicArabidopsisexhibitedenhanced [154]
threonineproteinkinase tolerancetodrought,salt,andfreezingstress
inABAsignalingnetwork associatedwithdecreasedwaterloss,improved
photosynthesis,andosmoticpotential
OsPIN3t Auxineffluxcarrier, ↑ Increaseddroughttoleranceinrice [127]
importantinpolarauxin
transport
Auxin YUCCA6 ImportantgeneinAuxin/ ↑ Overproductionofauxinattributedforincreased [27]
IPAbiosynthesis tolerancetodroughtandoxidativestress
YUCCA6 ImportantgeneinAuxin/ ↑ Phenotypesofpotatowithhigherauxincontent [155]
IPAbiosynthesis andenhanceddroughttolerance
OsIAA6 Amemberofriceauxin/ ↑ Betterdroughttoleranceoftransgenicrice [156]
IAAgenefamily plantsviaauxinbiosynthesisregulation
IPT Cytokininbiosynthesis ↑ Transgenictomatoshowedenhancedgrowth [157]
andyieldundersaltstress
Cytokinins ↑ Transgenictobaccoshowedenhancedsaltstress [158]
tolerance
SlIPT3 ↑ Transgenictomatoshowedenhancedsalinity [159]
stresstolerance
CKX Cytokinindehydrogenase ↑ TransgenicArabidopsisplantsoverexpressing [160]
cytokininoxidase/dehydrogenasegeneshowed
enhanceddroughttolerance
AtCKX1 Cytokinindehydrogenase ↑ Transgenicbarleyplantsshowedbetterdrought [161]
toleranceviabetterdehydrationavoidance
ERF-1(JERF1) Responsefactorsfor ↑ Riceplantsshowedincreaseddroughttolerance [162]
Ethyleneaswellas
Jasmonates
Ethylene ETOL1 ↑ Increasedtolerancetodroughtand [163]
submergence
ACC-Synthase Catalyzesrate-limiting Gene-silencing Reducedethylenelevelswithbetterdrought [164]
stepinethylene toleranceintransgenicmaizeplants
biosynthesis
ZmARGOS Negativeregulatorsof ↑ Improveddroughttoleranceoftransgenic [165]
ethylenesignal Arabidopsisandmaizeplants
transduction
OsGSK1 BRnegativeregulator Knockoutof Increasedtoleranceofknockoutmutantstocold, [166]
OsGSK1 heat,saltanddroughtstresses
Brassinosteroids AtHSD1 RoleinBRbiosynthesis ↑ OverproductionofBRincreasedgrowthrateand [167]
seedyield,increasedsalinitytolerance
BdBRI1 BR-receptorgene Downregulation Improveddroughttolerancewithdwarf [168]
phenotypesofpurplefalsebrome
↑:Overexpression
THE CROP JOURNAL 4 (2016) 162–176 171
retardationandotheradverseeffects.Lietal.[142]overexpressed
8.Conclusionandfutureoutlook
theMoCosulfurasegeneinsoybean,resultinginhigherbiomass
productionandyieldwithoverallenhanceddroughttolerance
Overall, phytohormone engineering represents an important
attributed to higher ABA accumulation, reduced water loss
platformforabioticstresstolerance,providingnewopportunities
throughsmalleropeningsofstomata,andinducedantioxidant
tomaintainsustainablecropproductiontofeedtheworldunder
enzymaticmachinery.
changingenvironmentalconditions.Ithasamajorapplicationin
However, on some occasions, overexpression of gene(s)
plant stress tolerance and adaptation to a variety of stresses,
involvedinABAbiosynthesisorcatabolicpathwaysresulted
owing to the involvement of multiple stress responsive genes.
in increased drought tolerance, but led to impaired growth
During the past few years, with the rapid development of
due to pleiotropic effects even with the use of inducible
genomictechnology,muchresearchhasbeenperformedtowards
promoters[143].Tooffsettheseunwantedgrowthanomalies,
understanding of plant abiotic stress response. Still many
Zhang et al. [122] overexpressed CRK45, a stress-inducible
challenges are lying ahead to uncover and understand the
kinase involved in ABA signaling, and the resulting trans-
complexity hidden in stress signal-transduction pathways. For
genics showed enhanced drought tolerance but with tighter
example, to acquire wide-ranging understanding of plant re-
control of ABA levels and signaling, indicating the role
sponsestoabioticstressincomingyears,moreextensivework
of CRK45 in fine-tuning of ABA levels. Similarly, IPT was
should be performed at the genetic level of the biosynthetic
expressedunderthecontrolofstress-induciblepromotersto
pathwayof hormones such as IAA. Research to date indicates
avoid pleiotropic effects, leading to increased CK content,
thatphytohormoneengineeringhasbeeninitiated.Therolesof
antioxidant scavenging, and better root growth with overall
planthormonesinresponsestofluctuatingenvironmentshave
improved grain yield under drought conditions in Agrostis
been demonstrated, and hormones play a primary role in
stolonifera[144,145].
determining plant stress responses. In this review, we have
Recently, transgenic poplars were produced via overex-
summarized the role of phytohormones, crosstalk among
pression of the Arabidopsis YUCCA6 gene (a member of the
hormones,theimportanceofstudyingplantstressesincombi-
YUCCA family of flavinmonooxygenase-like proteins), which
nation,andengineeringtechniquesandstrategiesfordeveloping
is involvedin tryptophan-dependent IAA biosynthesis path-
stress tolerance in plants. In a wide range of stresses, plant
way and known to respond to environmental cues, under
hormones are involved directly or indirectly, and it is clear
thecontrolofthestress-inducibleSWPA2promoter[27].The
that plant hormones play roles in plant defense and plant–
transgeniclinesdisplayedauxin-overproductionmorpholog-
environmentinteractions.
ical phenotypes, including rapid shoot growth and retarded
In conclusion, although phytohormone engineering is
main root development with increased root hair formation.
promising for plant biologists, there isstill along wayto go
Also, SY plants had higher levels of free IAA and early
before the technology can reach its full potential. Among
auxinresponsegenetranscripts.Thetransgeniclinesshowed
the greatest challenges that remain to be addressed is the
tolerancetodroughtstress,associatedwithreducedlevelsof
development of stable phytohormone-engineered crops that
reactiveoxygenspecies[27].ThericemutantCONSTITUTIVELY
produce main staple foods such as rice, wheat, and corn.
WILTED1 was deficient in the YUCCA homolog [146], and an
Towardsthisgoal,studyshouldbefocusedonacombination
activationtaglineofYUCCA7inArabidopsisshowedenhanced
of stress responses such as those in field environments,
droughttolerance[147].Droughttolerancebyoverexpressionof
because different stresses are most likely to occur simulta-
AtYUC6 in potato was also observed in 4-month-old potted
neouslyunderfieldconditions.
plantsingreenhousesbymonitoringrecoveryafterwithholding
waterfor18days[148].
Sakamotoet al. [149]modified GA levelsbyoverexpres-
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