Circadian regulation of abiotic stress tolerance in plants

15 Pages·2015·1.8 MB·English

by  Isabelle A. Carré

REVIEW
published:27August2015
doi:10.3389/fpls.2015.00648
Circadian regulation of abiotic stress
tolerance in plants
JackGrundy,ClaireStokerandIsabelleA.Carré*
SchoolofLifeSciences,UniversityofWarwick,Coventry,UK
Extremesoftemperatures,droughtandsalinitycausewidespreadcroplossesthroughout
the world and impose severe limitations on the amount of land that can be used for
agriculturalpurposes.Hence,thereisanurgentneedtodevelopcropsthatperformbetter
under such abiotic stress conditions. Here, we discuss intriguing, recent evidence that
circadian clock contributes to plants’ ability to tolerate different types of environmental
stress, and to acclimate to them. The clock controls expression of a large fraction of
abiotic stress-responsive genes, as well as biosynthesis and signaling downstream of
stress response hormones. Conversely, abiotic stress results in altered expression and
differential splicing of the clock genes, leading to altered oscillations of downstream
stress-response pathways. We propose a range of mechanisms by which this intimate
couplingbetweenthecircadianclockandenvironmentalstress-responsepathwaysmay
contributetoplantgrowthandsurvivalunderabioticstress.
Editedby:
Keywords:circadianclock,rhythmicgeneexpression,abioticstress,planthormones
SethJ.Davis,
MaxPlanckInstituteforPlant
BreedingResearch,Germany Introduction
Reviewedby:
ElsebethKolmos, Theworld-widepopulationispredictedtoincreasefrom7.2billiontobetween9.6and12.3billion
BioConsortia,Inc.,USA by2100,increasingthedemandonglobalfoodproductivity(Gerlandetal.,2014).However,nutrient
HeatherKnight, deficiency,mineraltoxicityandsalinityseverelylimittheamountoflandthatisavailableforfood
DurhamUniversity,UK production.Inaddition,world-widecroplossesduetoextremetemperatures,drought,andflooding
*Correspondence: areestimatedtoriseabout50%by2050(Boyer,1982).Waterdeficitwasestimatedtohavecaused
IsabelleA.Carré, €100billionworthofdamageinEuropebetween1977and2007(EuropeanCommission,2007).The
SchoolofLifeSciences,University
severedroughtof2012affected80%ofthecrop-growingterritoryintheUSA(Hoerlingetal.,2013),
ofWarwick,GibbetHillRoad,
andsuchdroughtsarepredictedtobecomemorefrequentasaresultofglobalwarming(Hansen
CoventryCV47AL,UK
etal.,2010;Lawrimoreetal.,2011;LobellandGourdji,2012;Cooketal.,2015).
[email protected]
There is an urgent need for development of crop varieties with improved tolerance to abiotic
stress,butthedevelopmentofsuchimprovedvarietieswillrequireathoroughunderstandingofthe
Specialtysection:
Thisarticlewassubmittedto mechanismsbywhichplantsareaffectedbytheseconditions,andhowtheycantoleratethem.Abiotic
PlantPhysiology, stresscanaffectcellularhomeostasisviaavarietyofmechanisms.Freezingtemperaturespromote
asectionofthejournal intra-andextra-cellularicecrystalformation,leadingtocellulardamage,dehydrationanddisruption
FrontiersinPlantScience oftheplasmamembrane(SteponkusandWebb,1992;Pearce,2001;Yamazakietal.,2008,2009).High
Received:11April2015 temperaturesdamagetheplasmamembranebyincreasingitsfluidity,leadingtoionleakage.Heat
Accepted:04August2015 stresscanalso resultin inhibitionofenzymatic activity andphotosynthesis(SalvucciandCrafts-
Published:27August2015
Brandner,2004;Sharkey,2005;Wahidetal.,2007;Allakhverdievetal.,2008).Hightemperatures,
Citation: salinityanddroughtstressareoftenaccompaniedbyproductionofreactiveoxygenspecies(ROS)
GrundyJ,StokerCandCarréIA
whichcanleadtodenaturationoffunctionalandstructuralproteins(GillandTuteja,2010;Suzuki
(2015)Circadianregulationofabiotic
et al., 2012). Gradual exposure to these different stresses causes plants to become more tolerant,
stresstoleranceinplants.
however, and acclimated plants can survive harsher conditions than non-acclimated plants. The
Front.PlantSci.6:648.
doi:10.3389/fpls.2015.00648 acclimationprocessincludestheexpressionofnewtranscriptsandproteins,andtheaccumulation
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Grundyetal. Circadiancontrolofstressresponses
of metabolites that result in a new homeostatic state that is Webb and Satake, 2015). However, recent data suggest that the
more compatible with the change in environmental conditions. circadianclockmayalsocontributetoplantfitnessbyenhancing
This allows stabilization of proteins and membranes, osmotic theirabilitytotolerateabioticstress.
adjustmentandre-establishmentoftheredoxbalance(Krasensky
andJonak,2012;MittlerandBlumwald,2015).
A number of transgenic lines with improved tolerance to Abnormal Function of the Circadian Clock
environmental stress have been generated in a variety of Results in Altered Abiotic Stress Tolerance
crop species, and a few of these have been approved for
commercialization (Lawlor, 2012; Cabello et al., 2014; Waltz, The molecular mechanism of the plant circadian clock was
2014).Thesetransgeniclinesexploitavarietyoftransgenesthat investigatedusingthegeneticmodelsystemArabidopsisthaliana,
acttomaintainosmoticbalanceunderabioticstressconditions. and was found to be composed of a transcriptional regulatory
Forexample,Monsanto’sDroughtGardmaize(MON87460)was network. The mechanism by which this network oscillates has
transformed with the cold-shock protein CspB from Bacillus been reviewed in detail elsewhere (see, for example Carré and
subtilistoimprovewaterefficiencyunderdroughtstress(Nemali Veflingstad,2013;HsuandHarmer,2014;SeoandMas,2014),and
et al., 2014). The betA gene from the bacterium Rhizobium wewillonlydescribeitminimallyhere,tointroducetherelevant
meliloti was introduced into Indonesian sugar cane in order genetic components. Briefly, the day starts with expression of
to increase production of the osmoprotectant glycine betaine, two closely related transcription factors, LATE ELONGATED
resultinginenhancedresistancetodrought(Marshall,2014).An HYPOCOTYL(LHY)andCIRCADIANCLOCKASSOCIATED
osmoregulatorprotein(osmotin)fromtobaccowasalsousedto 1 (CCA1), which peak in the early morning (Schaffer et al.,
improvethesalinitytoleranceofchilipepper,cotton,tomato,and 1998; Wang and Tobin, 1998). Transcription of the LHY and
strawberryplants(HusainiandAbdin,2008;Parkhietal.,2009; CCA1genesisdown-regulatedintheafternoonandthroughthe
Goeletal.,2010;Subramanyametal.,2010;Patadeetal.,2013). night by a set of sequentially expressed PSEUDO-RESPONSE
Whiletheseapproacheshaveledtosignificantimprovements, REGULATOR proteins known as PRR 9, 7, 5, and 1, where
thegeneticmanipulationofmasterregulatorsofstressresponses PRR1 is also known as TIMING of CAB or TOC1 (Matsushika
would potentially be more powerful because of the broader et al., 2000; Strayer et al., 2000; Alabadi et al., 2001; Nakamichi
spectrumofresistancemechanismsthatcanbetriggeredbysuch et al., 2010). The inhibition of LHY/CCA1 transcription by the
modifications. However, as natural stress responses often lead PRR proteins is released late at night through the action of
to growth arrests, such modifications often cause detrimental an evening complex (EC) composed of three proteins, EARLY
pleiotropic effects, leading to reduced yields under normal FLOWERING (ELF) 3, ELF4, and LUX ARRHYTHMO (LUX).
conditions(Hussainetal.,2011).Theseproblemscanbealleviated The EC shuts down the expression of the PRR genes, which
by the use of stress inducible promoters, such that the genetic allowsexpressionofLHYandCCA1toriseagaininthefollowing
modification is only expressed at times when it is beneficial. morning (Hicks et al., 2001; Doyle et al., 2002; Hazen et al.,
Alternatively, the judicial choice of transcription factors that 2005; Onai and Ishiura, 2005; Nusinow et al., 2011; Pokhilko
control specific pathways with minimum unwanted side effects etal.,2012).LHYandCCA1subsequentlybringdownexpression
can minimize such issues (Hussain et al., 2011). Nevertheless, of TOC1 and the cycle restarts (Alabadi et al., 2001). The F-
anidealapproachtoimprovingcropperformanceunderabiotic box proteins ZEITLUPE (ZTL) and LOV KELCH PROTEIN 2
stress conditions would be to exploit a genetic pathway that (LKP2),functionasacomplexwithGIGANTEA(GI)tocontrol
(i) contributes to tolerance to a broad range of environmental thelight-dependentproteolysisoftheTOC1protein,andthisacts
stressesand(ii)cannaturallyadaptintoenhancestresstolerance toincreasetoamplitudeofoscillationsofthegenenetworkunder
mechanisms when plants are exposed to deleterious conditions. diurnallight–darkcycles(Masetal.,2003;Kimetal.,2007).The
Here,wewilldiscussrecentevidencethattheplantcircadianclock functionofthecentraloscillatorissynchronizedtoenvironmental
mayrepresentsuchamechanism. lightdarkcyclesviainputpathwaysmediatedbyphytochromeand
Circadianclocksare24-hbiologicaloscillators,whichgenerally cryptochromephotoreceptors(Somersetal.,1998).Thecircadian
enable organisms to anticipate predictable, daily changes that clockinturncontrolsdownstreamphysiologicalprocessesviaa
are linked to the rotation of the earth. Such clocks have been numberofmechanisms,termedoutputpathways.
found in a wide range of organisms, including cyanobacteria, Mutations in components of the circadian clock not only
fungi, insects, and mammals (Young and Kay, 2001). Having a impacts on the function of the central oscillator but also alter
clockisnotessentialforplantviability,andarrhythmicmutants plants’ ability to both tolerate environmental stress conditions,
are still capable of developing (Green et al., 2002). However, and acclimate to them. For example, plants with mutations
there is convincing evidence that having a clock that is well in CCA1, LHY, ELF3, ELF4, LUX, PRR5, PRR9, and PRR7
tunedtodiurnalcyclesoflightanddarknessmakesanimportant are hypersensitive to ROS-generating agents, whereas CCA1-
contribution to plant fitness. For example, plants whose clocks overexpressingplants(CCA1-ox)aremoretolerantthanwild-type
were a good match to their rhythmic environment performed plants(Laietal.,2012).OverexpressionofLKP2enhancesdrought
bestincompetitionexperiments(Doddetal.,2005).Thismaybe tolerance (Miyazaki et al., 2015). The prr5,7,9 triple mutant is
explained,atleastinpart,bytheroleoftheclocktoensurethe more tolerant to high salinity and drought stresses (Nakamichi
optimumuseofstarchatnightwhileavoidingstarvationbefore etal.,2009).GI-oxplantsshowenhancedsaltsensitivity,whereas
dawn(Grafetal.,2010;Scialdoneetal.,2013;Mugfordetal.,2014; gimutantsshowenhancedsalttoleranceandenhancedsurvival
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Grundyetal. Circadiancontrolofstressresponses
under drought stress and oxidative stress (Kim et al., 2013; be expressed rhythmically in constant light, and 40% of cold-
Fornara et al., 2015). Reduced expression of TOC1 in toc1- regulatedgenes(Covingtonetal.,2008).Rhythmicexpressionof
RNAi plants results in improved drought tolerance, whereas abioticstress-responsivegeneswasalsoreportedforsoybeanand
overexpressionofTOC1causesincreasedwaterlossandreduced barley(Habteetal.,2014;Marcolino-Gomesetal.,2014).
survival under drought conditions (Legnaioli et al., 2009). The Thisraisesthequestionofhowthedailytimingofexpression
prr5,7,9 and toc1 mutants show significantly increased freezing of these genes (i.e., their phase) relates to rhythmic changes in
survival (Nakamichi et al., 2009; Keily et al., 2013), but gi, lux, environmentalconditions.ThiswasexaminedinFigure1,based
lhy, cca1, and lhy cca1 mutants have reduced constitutive and on phase data for Arabidopsis genes retrieved from the Diurnal
acclimated freezing tolerance (Cao et al., 2005; Espinoza et al., database (Blasing et al., 2005; Mockler et al., 2007; Covington
2010;Dongetal.,2011;Chowetal.,2014). et al., 2008) and on lists of cold, heat, drought and mannitol-
Thesefindingsdemonstratethatthecircadianclockcontributes responsivegenesderivedfromexistingdatasets(Leeetal.,2005;
toplants’abilitytotolerateenvironmentalstress.Whiletheeffects Kilian et al., 2007; Wilkins et al., 2010; Barah et al., 2013).
observed may seem relatively small (only a 1–2°C difference in Thisshowedthatalthoughindividualgeneswithinabioticstress-
freezing tolerance for lhy cca1 double mutants, for example), responsepathwayswereexpressedacrossabroadrangeofphases,
this may prove valuable to allow recovery after a frost or after peaks of expression were clustered around specific times of the
extremedroughtforexample(Lawlor,2012).Interestingly,there day. This was true for responses to environmental conditions
isnocorrelationbetweentheeffectofmutationsonthefunction that vary in a diurnal manner, such as cold or heat, but also
of the clock and their effects on abiotic stress tolerance. For forresponsestoconditionsthatprogressoveranumberofdays,
example,mutationswhichcauseashort-periodphenotype(cca1, such as drought, or conditions that are constant, such as high
lhy,prr5),along-periodphenotype(prr9)orcompletearrhythmia osmoticum. This suggests that circadian regulation of abiotic
(elf3, elf4, lux) cause similar hypersensitivity to ROS-generating stressresponsesmayconferanadvantagetotheplantevenunder
agents(GreenandTobin,1999;Mizoguchietal.,2002;Itoetal., conditionsthatdonotchangeinadiurnalfashion.
2003; Yamamoto et al., 2003; Lai et al., 2012). Furthermore, Themajorityofcold-induciblegenespeakedintheafternoon,
plantsthatshowsimilararrhythmicphenotypescanhaveopposite a few hours ahead of cooler temperatures at night (Figure 2).
phenotypes with regard to ROS sensitivity (elf3, elf4, and lux Thisincludedthecold-responsivetranscriptionfactorsCRE/DRE-
mutants vs CCA1-ox), drought tolerance (LKP2-ox vs prr5,7,9 BINDING FACTORs 1,2, and 3, which function as master
mutants) or freezing tolerance (lux vs prr5,7,9 mutants; Hicks regulatorsofthecoldacclimationresponse(CBF1,2,3,alsoknown
et al., 1996; Wang and Tobin, 1998; Schultz et al., 2001; Doyle asDehydration-ResponsiveElementBinding orDREB1–B,C,and
etal.,2002;Hazenetal.,2005;Nakamichietal.,2005,2009;Chow A,respectively),andthecircadianandcold-regulatedgenesCCR1,
etal.,2014;Miyazakietal.,2015).Thissuggeststhattheeffectof CCR2, and RD29A (Carpenter et al., 1994; Harmer et al., 2000;
thesemutationsoncircadianperiodorontheabilityoftheclock Dodd et al., 2006). On the other hand, genes that are down-
tooscillateisnottheprimarycauseofthealteredstresstolerance regulated by cold peaked around dawn (Figure 2). Thus, the
phenotype.Amorelikelyhypothesisisthatthealteredexpression timing of expression of cold-responsive genes was consistent
leveloftheclockgenesinthemutantshasadirectimpacttoeither with a role in anticipation of predictable, diurnal changes in
up-ordown-regulateabioticstressresponses,asdiscussedinthe temperature. In cotton and soybean plants, maximum low-
followingsections. temperatureresistancewasobservedatthebeginningofthenight,
correspondingtothetimewhenmostcold-responsivegenesare
Circadian Regulation of Stress-Responsive expressed in unstressed conditions (Couderchet and Koukkari,
1987;Rikinetal.,1993).Thissuggeststhatrhythmictranscription
Genes
ofcold-responsivegenesmayformthebasisofrhythmicchanges
The effects of the clock on plant physiology and development inchillingtolerance.
are largely mediated through the regulation of gene expression. In Arabidopsis, the majority of heat-inducible genes were
Approximately 80% of transcripts cycle in Arabidopsis plants expressedduringtheday,andheat-repressedgeneswerebroadly
under diurnal light–dark or temperature cycles, and 30–40% of expressed at night (Figure 2). Similarly, in spinach, five genes
these oscillations persist upon transfer to constant conditions encoding heat shock proteins (HSP70s) were found to exhibit
(Harmeretal.,2000;Edwardsetal.,2006;Covingtonetal.,2008; maximumexpressioninthemiddleoftheday.Thiswasassociated
Michael et al., 2008). Similar observations have been made in a with a diurnal variation in thermo-tolerance. Plants exhibited
number of crop plants and tree species, including rice, poplar, minimumheat-inducedelectrolyteleakagefollowingtreatments
maize,tomato,andsoybean(Facellaetal.,2008;Khanetal.,2010; in the afternoon and early evening, which was consistent with
Filichkinetal.,2011;Marcolino-Gomesetal.,2014).Remarkably, the hypothesis, that the rhythmic expression of HSP70s confers
the phasing of expression for the majority of genes was found heat tolerance at the end of the day (Li and Guy, 2001). The
to be conserved across monocots and dicots (Filichkin et al., production of ROS-scavenging enzymes also peaks mid-day in
2011).Oneparticularlyinterestingoutcomefromtranscriptomic Arabidopsisandmaycontributetoheattolerancebyminimizing
analyseswasthatmanygenesthatareresponsivetoabioticstress ofheat-inducedoxidativestress(Laietal.,2012).
werefoundtobeunderthecontrolofthecircadianclock.Thus, Themajorityofdrought-induciblegenespeakedarounddawn
in Arabidopsis, approximately 50% of genes that are responsive (Figure2).Plantsexperienceawaterdeficitinthedaytime,dueto
toheat,osmoticum,salinityorwaterdeprivationwerefoundto theprolongedopeningofstomataforphotosyntheticgasexchange
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Grundyetal. Circadiancontrolofstressresponses
FIGURE1|Circadianregulationof(A-D)cold,(E-H)heat,(I-L)drought etal.,2013),waterlimitationfor4days(Wilkinsetal.,2010),andgrowthon
and(M-P)osmoticum-responsivegenesinArabidopsis.Histogramsshow 300mMmannitol(Kilianetal.,2007).PhasedataforArabidopsisgeneswere
thetimingofpeakmRNAlevelsin12L12Dorfollowingtransfertoconstantlight, obtainedfromexperimentsLDHH-StittandLL12intheDiurnaldatabase
forthe50mostsignificantlystress-induced(up)orrepressed(down)genesthat (Blasingetal.,2005;Mockleretal.,2007;Covingtonetal.,2008).Solidwhite
alsoexhibitcircadianregulation.Listsofstress-responsivegeneswerebasedon andblackbarsindicateintervalsoflightanddarkness,respectively,whereas
thefollowingdatasets:24hat0°C(Leeetal.,2005),3hofat38°C(Barah hatchedbarsindicatesubjectivenights.
(Jarvis,1976),andtherhythmicexpressionofdrought-inducible maximalinductionobservedinthemorning(Fowleretal.,2005;
genes may allow plants to minimize water loss at this crucial Thomashow,2010).ConsistentwiththeroleofCBFtranscription
time.However,manydrought-repressiblegeneswerealsofound factors as master regulators of the chilling response, about 75%
topeakduringtheday,whentheywouldhavebeenexpectedto ofcold-responsivegenesexhibitsimilargatingofcoldresponses
peak at night. The physiological significance of this is unclear. withmaximumresponsivenessinthemorning.Asimilartiming
Insummary,whilethetimingofexpressionofstress-responsive of maximal cold-responses was observed in peach trees. Out of
genesismostlyconsistentwiththetimingofdailyenvironmental fiveCBF homologs,fourshowedmaximalcold-inductioninthe
changes,thisisnotalwaysthecase.Onepossibleexplanationis morning,andthiswasassociatedwithinductionofdownstream
that,geneswhichplayaroleintolerancetoagiventypeofabiotic targetsincludingDEHYDRIN(DHN)1and3(Artlipetal.,2013).
stressmayalsocontributetootherpathways,andthattheremaybe The conserved gating of low temperature responses across
agreaterselectivepressureforoptimaltimingoftheirexpression distant plant species strongly suggests that the circadian gating
relativetotheseotherprocesses. of chilling responses confers a selective advantage. Little is
known,however,abouthowthegatingoftemperatureresponses
contributes to plant fitness. One hypothesis is that, while the
Circadian Gating of Abiotic Stress rhythmicexpressionofcold-responsivegenesallowsanticipation
Responses ofpredictablecoldtemperaturesatnight,gatedresponsesmaybe
importanttoallowrapidchangesingeneexpressioninresponse
In addition to driving the rhythmic, basal expression of stress- tounexpectedlycoldmornings.
responsive genes, the clock controls the amplitude of their Responsestowaterdeficitarealsogatedbythecircadianclock,
responses to environmental signals, in a process called gating. and maximal drought-induced changes in gene expression are
For example, cold treatments at different times of the day observed at dusk (Wilkins et al., 2010; Kiełbowicz-Matuk et al.,
result in differential induction of CBF1,2,3 transcription, with 2014).Interestingly,differentsetsofgenesareinducedbyidentical
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Grundyetal. Circadiancontrolofstressresponses
FIGURE2|Timingofabioticstressresponsesacrosstheday-night majorityofstress-responsivegenesisindicatedinblue,thetimeofmaximum
cycle.Thetimesofthedayatwhichdifferenttypesofabioticstressare accumulationofstressresponsehormonesisshowninblack,andthetime
mostprominentinthenaturalenvironmentareindicatedinred.Foreach ofmaximumresponsivenesstodifferentenvironmentalsignalsisshownin
typeofenvironmentalstress,thetimeofpeakbasalexpressionforthe green.
cold or drought treatments given at different time of the day experiments(Harmeretal.,2000;Dongetal.,2011),andlossof
(Bieniawskaetal.,2008;Wilkinsetal.,2010).Similarly,significant LHYandCCA1functioninthelhycca1doublemutantresulted
differences in heat stress responsive pathways were observed inarrhythmicbasalexpressionofCBF1,2,and3.Thisprovided
in grapevine, depending on whether the treatment was applied strongevidencethatLHYandCCA1playanessentialroleinthe
duringthedayorduringthenight(Rienthetal.,2014),andthe rhythmicexpressionoftheCBF genesandofthecoldresponses
physiological implications of these time of the day-differences thattheyregulate.TheTOC1andLUXproteinswerealsofoundto
remaintobeinvestigated. bindthepromoterofCBF3,andmathematicalmodelingshowed
that the regulation of CBF3 transcription by TOC1 and LUX
Regulation of Abiotic Stress Responses by was important for the correct gating of CBF3 responses (Keily
Core Oscillator Components etal.,2013).Accurategatingpatternscouldonlybereproduced
insimulationswhen(i)expressionofCBF3wasdown-regulated
A number of components of the circadian clock encode by TOC1 and the EC; (ii) it was up-regulated by LHY/CCA1,
transcriptionfactors,whichhaveadualrole,bothtoensurethe and (iii) cold was assumed to increase the level of LHY/CCA1
function of the central oscillator and to control the rhythmic protein. In addition to controlling master regulators of the low
expressionofdownstreamgenes. temperatureresponse,LHYandCCA1controlexpressionofcold-
Thus, a number of abiotic stress-responsive genes are under responsive genes in a direct manner. Thus, cold-responsiveness
the control of core components of the circadian oscillator. For of the cold-inducible genes CONSTANS-LIKE 1 (COL1) and
example,theLHYandCCA1transcriptionfactorsplayakeyrole COLDREGULATEDGENE27(COR27)wasshowntoresultfrom
toregulatelowtemperatureresponsesbycontrollingexpression a synergy between EEs and ABA-responsive elements (ABREs,
of the CBF genes. The promoters of CBF1, 2, and 3 contain CACGTG) in their promoters (Mikkelsen and Thomashow,
LHY and CCA1 binding sites, also known as Evening Elements 2009).
or EEs (AAATATCT). A CCA1:GFP fusion protein was found LKP2wasrecentlysuggestedtoplayaroletocontrolresponses
to bind the CBF1-3 locus in chromatin immunoprecipitation to drought. LKP2-overexpressing plants exhibited a smaller
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TABLE1|DirectregulationofstresshormonebiosynthesisandsignalingbycorecomponentsoftheArabidopsiscircadianclock.
Hormone GeneID Genename Role TOC1target PRR5target PRR7target
JA AT1G17420 LOX3 JAbiosynthesis Y N N
JA AT1G72520 LOX4 JAbiosynthesis Y N N
JA AT3G45140 LOX2 JAbiosynthesis Y N N
JA AT1G19180 JAZ1 RepressorofJA-responsivegenes Y N N
JA AT1G70700 JAZ9 RepressorofJA-responsivegenes Y N N
JA AT5G11270 OCP3 JAsignaling Y N Y
JA AT1G25540 PFT1 RegulatorofJAresponses N Y N
JA AT1G80840 WRKY40 JA-responsivetranscriptionfactor N Y N
JA AT3G06490 MYB108 JA-responsivetranscriptionfactor N Y N
JA AT1G52890 ANAC019 JA-responsivetranscriptionfactor N Y N
ET AT4G11280 ACS6 ETbiosynthesis Y N N
ET AT5G03280 EIN2 ETsignaling Y N N
ET AT5G47230 ERF5 ET-responsivetranscriptionfactor Y N N
ET AT1G53170 ERF8 ET-responsivetranscriptionfactor Y N N
ET AT4G39403 PLS NegativeregulatorofETsignaling N Y N
ABA AT5G13630 ABAR PutativeABAreceptor Y Y N
ABA AT5G59220 PP2C:HAI1 CatalyticsubunitofABAreceptor N Y N
ABA AT3G11410 PP2CA/AHG3 CatalyticsubunitofABAreceptor Y Y N
SA AT2G41010 CAMBP25 RegulationofSAbiosynthesis Y N N
SA AT1G20020 LFNR2 RegulationofSAbiosynthesis N Y N
SA AT3G56710 SIB1 RegulationofSAbiosynthesis N Y N
EvidencefordirectregulationofgenesintheJA,SA,ET,orABApathwayswasobtainedfromgenome-wideanalysesofbindingsitesfortheTOC1,PRR5,andPRR7proteins(Huang
etal.,2012;Nakamichietal.,2012;Liuetal.,2013).
stomatal opening and showed elevated expression of several enzymes responsible for biosynthesis of carotenoid precursors
drought-responsivegenes(Miyazakietal.,2015).LKP2’sprotein and the rate-limiting enzyme NINE-CIS-EPOXYCAROTENOID
partner, GI was also found to regulate the expression of genes DIOXYGENASE (NCED) peak in the morning (Thompson
involved in drought and cold stress responses, including a et al., 2000; Covington et al., 2008). In maize, the rhythmic
number of EARLY RESPONSIVE TO DEHYDRATION genes expressionofABAbiosyntheticenzymeswasalsoconsistentwith
(ERD10, ERD7, and AT4G04340) and cold-regulated genes ABA synthesis taking place in the early morning (Khan et al.,
(COR78, KIN2, COR15a, COR15b, COR314, and COR413; 2010). In sugar cane, the temporal pattern of ABA biosynthesis
Fornara et al., 2015). This effect of GI was dependent on may be more complex as one gene involved in ABA synthesis
CYCLING DOF FACTORs (CDFs), a family of rhythmically peakedintheearlymorning,andtwootherspeakedduringthe
expressed transcriptional repressors known for their roles in night(Hottaetal.,2013).TherhythmicproductionofABAwas
thephotoperiodicregulationoffloweringtime(Imaizumietal., proposedtobecontrolledbythePRR5,7,and9proteins,because
2005). As LKP2 and its homolog FKF1 interact with GI to analysis of a triple mutant (prr5,7,9) revealed increased ABA
control the light-dependent degradation of CDF proteins, these levels (Fukushima et al., 2009). However, this effect is likely to
observations provide independent evidence for a role of the be indirect because ChIP-seq analyses of genome-wide binding
LKP2/GI/CDFregulatorymoduleindroughttolerance. sitesforPRR5,PRR7,andTOC1didnotrevealbindingofthese
proteinstothepromotersofanycomponentofABAbiosynthesis
pathways(Table1;Nakamichietal.,2010;Huangetal.,2012;Liu
Circadian Control of Hormone etal.,2013).
Biosynthesis and Signaling Plants’abilitytorespondtoABAalsovarieswiththetimeof
theday.Forexample,stomatalclosureismoresensitivetoABA
Many plant responses to abiotic stress are mediated by intheafternoon,allowingforamoresensitivecontrolofstomatal
phytohormones. For example, abscisic acid (ABA) accumulates aperture(Correiaetal.,1995).TOC1-overexpressingplantshad
rapidly upon exposure to water stress or salinity and controls widelyopenstomatathroughoutthediurnalcycleandexhibited
manystress-adaptationresponses(AgarwalandJha,2010).ABA increasedsensitivitytodrought.Incontrast,plantswithreduced
levels are under circadian control. Peak levels were observed expressionofTOC1hadtheoppositephenotype,suggestingthat
during the day in the angiosperms Senecio minimus, Arbutus TOC1mayacttolimitstomatalaperture(Legnaiolietal.,2009).
unedo,andArabidopsisthaliana,andinthegymnospermCallitris This effect of TOC1 was proposed to be mediated through the
rhomboidea (Burschka et al., 1983; Lee et al., 2006; McAdam ABA-bindingproteinABAR(alsoknownasCHLHorGUN5)as
etal.,2011).Morecomplextemporalpatternshavebeenobserved expression of the ABAR gene was directly controlled by TOC1
in Nicotiana tabacum, where ABA levels showed a first peak binding to its promoter (Legnaioli et al., 2009; Huang et al.,
immediately after dawn, a second in the afternoon and a third 2012).However,thefunctionofABARinABAsignalingremains
at night (Novakova et al., 2005). Circadian regulation takes controversial as the mechanisms by which it regulates ABA
place at the level of ABA biosynthesis. In Arabidopsis, the responses are not known (Hubbard et al., 2010). TOC1 may
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neverthelessregulateABAresponsesthroughadifferentsignaling or JAZ proteins) and several JA-induced transcription factors
pathway,asChIP-seqdataindicatethatitbindstothepromoters (Huang et al., 2012; Nakamichi et al., 2012; Liu et al., 2013;
of two class A phosphatases 2C, HAI1 and PP2CA/AHG3, that Table 1). However, the gating of JA responses seems to be
functionascatalyticsubunitsofanotherABAreceptor[regulatory primarily mediated by the clock-associated protein TIME FOR
componentsofABAreceptor(RCAR),alsoknownaspyrabactin COFFEE (TIC; Shin et al., 2012). TIC interacts with a positive
resistance1(PYR)orPYR1-like(PYL);Huangetal.,2012]. regulator of JA signaling, MYC2, leading to its proteasomal
The clock also regulates the expression of ABA-responsive degradationandtothedown-regulationofJAresponses(Kazan
genesinadirectmanner.InArabidopsis,40%ofABAresponsive and Manners, 2013). MYC2 mRNA accumulation is also under
geneswerefoundtoalsobeundercircadiancontrol(Covington circadiancontrol,providinganotherlayerofrhythmicregulation
et al., 2008). Analysis of motifs in the promoters of these genes ofJAsignaling.Whileticmutantsretainsomelevelofgatingof
revealed over-representation for LHY and CCA1 binding sites, JAresponses,thisiscompletelyabolishedinmyc2mutants(Shin
suggesting that LHY and CCA1 control expression of large etal.,2012).
fraction of ABA-responsive genes by binding directly to their Salicylic acid is increased in response to most biotic stresses
promoters(Huangetal.,2007). andwasproposedtohaveanacclimation-likeeffect,dueprimarily
While ABA is the main hormone involved in responses to toenhancedanti-oxidativecapacity(Horváthetal.,2007;Miura
droughtandtoosmoticstress,salicylicacid(SA),jasmonicacid and Tada, 2014). SA is generally thought to act antagonistically
(JA),ethylene(ET)alsoplayaroleinresponsestoothertypesof withJA,atleastwithregardtodefensesagainstpathogens(Spoel
abiotic stress (Yang et al., 2004; Cheng et al., 2013; Wasternack et al., 2007). As might be expected, SA levels peaks at night in
andHause,2013;Kazan,2015),andreductionofGAlevelsand Arabidopsis, 180° out of phase with JA levels (Goodspeed et al.,
signalingcontributestotheresultinggrowtharrests(Colebrook 2012, 2013; Zhang et al., 2013). PRR5 and TOC1 may play a
etal.,2014). role to up-regulate the production of SA at night, as they bind
EthyleneandJAactcooperativelyintheregulationofresponses to the promoters of several genes encoding regulators of SA
to cold, salinity, drought and heat. The production of ET peaks biosynthesis(Huangetal.,2012;Nakamichietal.,2012;Liuetal.,
during the subjective day in Arabidopsis and sorghum, and at 2013; Table 1). SA accumulation is negatively influenced by a
the end of the day in potato (Finlayson et al., 1999; Thain rhythmicallyexpressedphosphatetransportergene(PHT4;1)and
et al., 2004; Chincinska et al., 2013). In Arabidopsis, rhythmic this has also been proposed to contribute to oscillations in SA
expressionoftherate-limitingenzymeACCSYNTHASE8(ACS8) levels(Wangetal.,2014).
is key to the rhythmicity of ET biosynthesis, and a knock-out Altogether, these studies reveal a pervasive effect of the
mutation abolished circadian ET accumulation (Thain et al., clock on stress-responsive hormonal pathways. Remarkably,
2004). Another ACS gene (ACS6) may also be under circadian circadianregulationoccursatmultiplelevels,includinghormone
control, as TOC1 was found to bind its promoter (Huang biosynthesis, signaling and rhythmic expression of hormone-
et al., 2012; Table 1). Two putative ACC OXIDASE enzymes responsive genes. ChIP-seq analyses have revealed the direct
may also play a role since they exhibit rhythmic expression regulation of key components of hormone signaling pathways
with a similar phase as ACS8 (Covington et al., 2008). Sucrose by PRR5, PRR7, and TOC1 (Table 1) and it is likely that
transport from source to sink tissue may contribute to the further genome-wide analyses will reveal additional roles for
rhythmic production of ET, as mutation of the rhythmically othercomponentsoftheclock.Interestingly,dataavailablesofar
expressedsucrosetransporterSUT4abolisheddiurnalchangesin revealadivisionoflaborbetweenthePRRproteins.Forexample
ET levels in potatoes (Chincinska et al., 2013). The clock may TOC1 was found at the promoter of several JA biosynthesis
also influence signaling downstream of ET, since the ethylene- genes(LOX2,3,4)andoftworepressorsofJAresponses(JAZ1,9)
responsivetranscriptionfactor(ERF)ETHYLENE-INSENSITIVE whereas PRR5 was found at a distinct set of genes encoding
3 (EIN3) is expressed rhythmically in Arabidopsis under the regulatorsofJAresponsesandJA-responsivetranscriptionfactors
controloftheclock-associatedXAP5CIRCADIANTIMEKEEPER (PFT1, WRKY40, MYB108: ANAC019; see Table 1) and PRR7
(XCT; Ellison et al., 2011). Furthermore, ChIP-seq analyses only regulated one JA signaling component (OCP3). This may
revealed the binding of TOC1 and PRR5 proteins to genes reflect a need for sequential expression of these genes, or
encodingERFs8and5,andtoregulatorsofETsignalingsuchas alternatively, for independent control of their expression in
EIN2 and POLARIS (PLS; Huang et al., 2012; Nakamichi et al., response to changes in the environment. Understanding how
2012;Table1). regulationbythedifferentclockproteinsisintegratedtoregulate
Jasmonic acid levels also peak in the middle of the day flux through the different signaling pathways will be a major
(Goodspeed et al., 2012; Zhang et al., 2013). Rhythmic changes challenge, which can only be addressed through mathematical
in JA levels may be controlled by the TOC1 protein, as TOC1 modeling.
was found at the promoters of several genes encoding 13-
lipoxygenase enzymes (13-LOX) which initiate the biosynthesis
of JA (Table 1). Responses to JA may also be regulated by Effects of Abiotic Stress on the
TOC1,PRR5,andPRR7,asChIP-seqanalysesrevealedbinding Transcription of Clock Genes
of these proteins to the promoters of several genes encoding
regulatorsofJAresponses.Thisincludedtwonegativeregulators While the clock has a clear impact on how plants respond to
of the JA signaling pathway (JASMONATE ZIM DOMAIN environmental stress, thereis also evidencethat the functionof
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Grundyetal. Circadiancontrolofstressresponses
the clock is altered in response to abiotic stress conditions. For Differential Splicing of Clock Genes in
example, high salinity resulted in lengthening of the circadian Response to Environmental Stress
periodinwheat,andosmoticstressincreasedtheexpressionlevel
of barley clock genes and advanced their phase of expression The plant clock is also regulated by alternative splicing, and
(Erdei et al., 1998; Habte et al., 2014). Similarly, drought stress mutation of a spliceosomal component (SPLICEOSOMAL
reduced the expression of evening-specific components of the TIMEKEEPER LOCUS1 or STIPL1) produced a long-period
clock in soybean (TOC1, LUX, ELF4, and PRR-like genes), phenotype in Arabidopsis (Jones et al., 2012). Many clock-
and this led to disruption of the circadian system (Marcolino- associated genes including CCA1, LHY, PRR7, PRR9, TOC1,
Gomes et al., 2014). Although the period of the clock varies ELF3, and ZTL undergo extensive alternative splicing through
by less than 1 h in Arabidopsis over the physiological range diverse mechanisms including intron retention, exon skipping
′
of temperatures (Gould et al., 2006; Salome et al., 2010), and selection of alternative 5 splice sites (Filichkin et al., 2010;
the expression level of oscillator components is altered. Peak FilichkinandMockler,2012;Jamesetal.,2012;Seoetal.,2012;
expression levels of the TOC1 and LUX transcripts increase Kwonetal.,2014).Manyofthesealternativesplicingeventsoccur
as temperatures get warmer, whereas CCA1 and LHY RNA in response to abiotic stress conditions (Filichkin et al., 2010,
rhythms increase in amplitude and peak expression level as 2015;FilichkinandMockler,2012;Jamesetal.,2012;Seoetal.,
temperatures get colder, from 17 to 12°C (Gould et al., 2006; 2012;Kwonetal.,2014).Forexample,lowtemperaturessuppress
Mizuno et al., 2014). On the other hand, heat stress was alternativesplicingofELF3butinducethatofTOC1.Incontrast,
reportedtoshortenthecircadianperiod(Kusakinaetal.,2014), heatinducesthealternativesplicingofCCA1,PRR7,TOC1,and
whereas cold stress leads to a loss of rhythmicity. At 4°C, ELF3 genes, and high salinity suppresses that of ELF3 (Kwon
most clock-associated genes oscillate with reduced amplitudes etal.,2014).
under day-night cycles and become arrhythmic upon transfer Alternatively spliced isoforms often contain a premature
to constant light. Only a small proportion of the genes that termination codon that can give rise to a shorter but still
cycleat20°Cmaintaincleardiurnaloscillationsat4°C(Martino- functional protein, or to a non-functional protein that will
Catt and Ort, 1992; Bieniawska et al., 2008; Espinoza et al., be degraded by the non-sense-mediated decay pathway. For
2010). example, the CCA1 and LHY transcripts are both alternatively
These effects of abiotic stress on the function of the clock spliced to form either the full-length protein or a truncated
may be mediated by changes in the level of stress response proteinlackingtheMYBDNA-bindingregion(Jamesetal.,2012;
hormones. In Arabidopsis, SA application did not influence Seo et al., 2012). Both truncated proteins, produced through
circadianperiodlength,butapplicationoftheETprecursorACC intron retention, suppress the formation of functional homo-
(1-aminocyclopropane-1-carboxylicacid) shortened the period and heterodimers (CCA1-CCA1, CCA1-LHY, and LHY-LHY)
length (Hanano et al., 2006). In contrast, ABA treatment throughcompetitiveinhibition.ThisimpairstheabilityofLHY
lengthened the period of the Arabidopsis clock (Hanano et al., and CCA1 proteins to bind DNA and results in dampened
2006).ThiswasassociatedwithareductioninCCA1mRNAlevels, rhythmicity. Interestingly, the production of the truncated
which is surprising because loss of function of CCA1 causes a isoformofCCA1(CCA1-b)decreasesatlowtemperature,while
shortperiodphenotype(GreenandTobin,1999).However,ABA that of truncated isoform of LHY increases. This results in a
may affect the oscillator mechanism by altering expression of relatively constant level of functional homo- and heterodimers
otherclockgenes,which would resultin lengtheningof period. andenablestheclocktomaintainaconstantperiodlengthacross
Evolutionary conserved ABREs are present in the promoters the physiological range of temperatures (James et al., 2012).
of TOC1, LHY, and CCA1, and may represent multiple points Overexpression of the full-length form of CCA1 (35S::CCA1-
of entry for ABA signals into the clock (Bieniawska et al., a) enhanced freezing tolerance whereas overexpression of the
2008; Spensley et al., 2009; Picot et al., 2010; Habte et al., truncated variant (35S::CCA1-b) decreased it (Seo et al., 2012).
2014). ThiswastakentosuggestthattheregulationofCCA1alternative
Severalcoreclockcomponentsareregulatedbystress-related splicingbycoldtemperaturesisimportantforfreezingtolerance.
transcription factors. For example, the heat shock transcription However, the effect of these transgenes is not an accurate
factor HsfB2b represses transcription from the PRR7 promoter reflection of what happens in response to temperature in wild-
athightemperaturesandinresponsetodrought(Kolmosetal., type plants, as any change in the splicing of CCA1 is normally
2014), and binding of CBF1 to the promoter of the LUX compensated for by an opposite change in the splicing of
gene plays a role to maintain robust transcriptional oscillations LHY. An alternative and perhaps simpler interpretation for the
at low temperatures (Chow and Kay, 2013). In addition, the effects of CCA1-a and CCA1-b overexpression is that reduced
transcriptional repressor FLOWERING BASIC HELIX-LOOP- functionofbothLHYandCCA1proteinsin35S::CCA1-bplants
HELIX1(FBH1)bindstotheCCA1promoter.FBH1expression may result in a cold-sensitive phenotype, whereas increased
is up-regulated by heat and down-regulated by cold, and its function of CCA1 in 35S::CCA1-a may result in enhanced
overexpressionintransgenicplantsleadstoreducedexpressionof chillingresistance.Moreworkisrequiredtofullyunderstandthe
CCA1andshorteningofcircadianperiodbyapproximately1h. importanceofCCA1alternativesplicingforplanttolerancetocold
Thissuggestthat,inwild-typeplants,FBH1playsaroletodown- temperatures.
regulate CCA1 expression at warm temperatures (Nagel et al., The molecular mechanisms linking the plant circadian clock
2014). to the control of RNA processing are largely unknown, but
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Grundyetal. Circadiancontrolofstressresponses
PROTEIN ARGININE METHYLTRANSFERASE 5 (PRMT5) Conclusions and Perspectives
is thought to play a key role in this process. This rhythmically
expressed protein acts to transfer methyl groups to arginine Thepastfewyearsofresearchhaverevealedawebofregulatory
residues present in histones and Sm spliceosomal proteins, connections between abiotic stress response pathways and the
and contributes to the regulation of many pre-messenger-RNA plantcircadianclock.Thecurrentknowledgeoftheseinteractions
splicing events (Sanchez et al., 2010). In particular, prmt5 is summarized in Figure 3, but the picture is unlikely to be
mutant plants produce an alternatively spliced form of the core complete at this stage. Many aspects of abiotic stress responses
clock component PRR9, resulting in a long period phenotype are regulated directly by components of the central oscillator.
(Hong et al., 2010; Sanchez et al., 2010; Hernando et al., 2015). The clock modulates the activity of stress response pathways at
The rhythmic expression of PRMT5 was responsive to both multiplelevelsandthrougharangeofmechanisms,includingthe
light and temperature cues, but interestingly the transcript transcriptionalregulationofkeyregulatorsofthestressresponse,
accumulated at different phases in different environmental thedirectregulationofstress-responsivegenesandsignalingby
conditions. Expression of PRMT5 peaked at dusk under stressresponsehormones.Mostintriguingly,abioticstresscauses
diurnal light–dark cycles, but peaked just before dawn under alteredtranscriptionand/orsplicingoftheclockgenes,suggesting
thermocycles(Hongetal.,2010).Thisalteredphaseinresponse thatabioticstressresponsepathwaysfeedbackontotheoscillator
to thermocycles was also seen when plants were exposed to toregulateitsfunction.
photocyclessuggestingthatthePRMT5rhythmispreferentially How does circadian regulation of abiotic stress responses
synchronized to temperature cycles. Thus, PRMT5 may play an provideanadvantagetoplants,whentheirconstitutiveexpression
importantroletoensurechangesinsplicingoftheclockgenesin would provide a high level of stress tolerance at all times?
responsetodiurnalchangesintemperature.Interestingly,prmt5 As stress responses in plants are frequently associated with a
mutants also exhibit reduced tolerance to salinity (Hernando growtharrest,wesuggestthatcircadianregulationmayprovide
et al., 2015), but it is not clear at this point whether this is an advantage by ensuring that plants can grow during part
related to the altered function of their circadian clock, or of the diurnal cycle, while only expressing stress tolerance
whether this is due to direct effects on abiotic stress response mechanisms at times of the day when they are required most
pathways. critically. However, this does not explain the contribution of
Alternative splicing is known to mediate clock responses theclocktosalinityandosmoticstresstolerance,asthesestress
to environmental changes in other organisms. In Neurospora, conditionsdonotvaryinadiurnalfashion.Therefore,theclock
temperature-dependentalternativesplicingoftheFREQUENCY may also contribute to abiotic stress tolerance via some other
transcript allows adaptation of the circadian clock to seasonal mechanisms.
variations of photoperiod and temperature (Diernfellner et al., Astheclockallowsplantstoperceivechangesinseason(Carré,
′
2005, 2007). In Drosophila, thermosensitive splicing in the 3- 2001; Yanovsky and Kay, 2002), it may act to modulate abiotic
UTR of the clock gene period (per) leads to phase advanced stress responses depending on the time of the year. This would
accumulation of per RNA on cold days, which is crucial for ensurethatastrongresponseisonlytriggeredatthosetimesof
seasonal adaptation of the circadian locomotor activity rhythm theyearwhenastressislikelytobepronouncedandpersistent.
(Majercak et al., 1999). Interestingly, the dart5-1 mutant, which Thus,plantresponsestocoldwererecentlyfoundtoberegulated
is affected in the Drosophila PRMT5 homolog, also exhibited by seasonal changes in photoperiod (Lee and Thomashow,
an abnormal period of locomotor activity (Sanchez et al., 2012). Under warm temperatures, the CBF2 gene oscillated
2010). This suggested that PRMT5 may also play a role to with higher amplitude under short day conditions (8L16D)
regulate the alternative splicing of one or more clock genes in than under long day conditions (16L8D). This resulted in up-
flies, and may identify a conserved mechanism for alternative regulationoftheCBFpathwayandincreasedfreezingtolerance
splicing of clock-associated genes in response to environmental in the winter. Another study suggested that CDFs, a family of
signals. transcription factors known for their roles in the photoperiodic
Abioticstressmayalsoaffecttheabundanceofclock-associated regulation of flowering, may mediate improved cold tolerance
proteins at the post-translational level. For example, salt stress undershort-dayphotoperiods.Shortdaysleadtothestabilization
induces the proteasomal degradation of the GI protein, thus of CDF proteins, which act to up-regulate expression of cold-
reducing the amplitude of its oscillations (Kim et al., 2013). responsive genes, resulting in enhanced cold tolerance in the
ReducedGIproteinlevelscontributetosaltacclimationbyfreeing winter (Fornara et al., 2015). Future work may reveal similar
the SOS2 kinase to activate the Na+/H+ antiporter SOS1. It is seasonal regulation of other types of abiotic stress responses in
likelythatfurtherexamplesofsuchpost-translationalregulation plants.
willemergeinthenearfuture. As the function of the clock is altered in response to
These findings are interesting because they show that the abiotic stress, circadian regulation may also provide a form
function of the circadian clock is altered in response to abiotic of memory of the signal, which may contribute to the stress
stress. Furthermore, the presence of evolutionarily conserved acclimation process. Just like perturbing the oscillation of a
stress-responsive promoter elements in the promoters of clock pendulum results in lasting changes in its swinging pattern,
genesfromdistantlyrelatedspeciessuggeststhattheseeffectsof acute up- or down-regulation of the clock genes in response to
abiotic stress on the clock mechanism may provide a selective abiotic stress would be expected to result in persistent changes
advantage. in the phase or amplitude of their oscillation. This may, in
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Grundyetal. Circadiancontrolofstressresponses
FIGURE3|Roleofthecircadianclockinintegrationofabioticstress componentsonabioticstressresponsepathways.Genesareindicatedinboxes
signals.Thewhiteandshadedrectanglesillustratethedayandnightintervals andhormonesinovals.Solidbluntandpointedarrowsindicatetranscriptional
ofthecircadiancycle.Oscillatorcomponentsarelistedwithintheseboxesand activationandrepression,respectively.Dottedlinesindicateinductionof
theirpositionintherectanglesgivesanindicationofthetimingoftheir alternativesplicing.Dashedlinesindicateregulationofhormonalpathways.
expressionrelativetoeachotherandtodawnanddusk.Thetoppartofthe Theselineshavenosignbecauseregulationofoppositesignmayoccurat
diagramillustratestheeffectsofdifferenttypesofabioticstressonexpressionof differentlevelsofthehormonebiosynthesisandsignalingpathwayandthenet
theclockgenes.Thebottompartofthediagramshowstheeffectsofoscillator effectofthisregulationisnotknown.
turn, result in long-term changes in the expression of the represses expression of GI, which promotes the activity of the
stress response pathways that they regulate. For example, water Na+/H+ transporter SOS1. However, as plants in their natural
deficit causes an increase in the amplitude of the rhythmic environmentareoftenexposedtomultipletypesofabioticstresses
expression of plasma membrane aquaporins, and this was simultaneously, the level and temporal pattern of expression
proposed to benefit the plant by decreasing transpiration and of the clock genes is more likely to reflect the integration of
byavoidingexcessivedehydrationoftherhizospherethatwould multiple signals. For example, drought and heat stress, which
dramatically decrease its hydraulic conductivity (Caldeira et al., are often experienced in combination, have opposite effects on
2014). TOC1andLUX transcription,butactsynergisticallytoenhance
Figure 3 suggests a number of mechanisms by which the PRR7 expression by promoting that of HSFB2b. Therefore the
clockmightensurethatstressresponsesaresustainedfollowing predictedcombinedeffectwouldbeelevatedexpressionofPRR7
initial exposure. For example, cold temperatures increase the andintermediatelevelsofTOC1andLUX transcription.Onthe
expression of LHY and CCA1, and this would in turn act to otherhand,coldpromotestheexpressionofLUXandCCA1but
enhance expression of cold-responsive genes (CBFs and CORs) hasnoknowneffectonTOC1.Therefore,thepredictedcombined
over multiple circadian cycles. Heat stress, on the other hand, effectofcoldanddroughtwouldagainbeintermediatelevelsof
down-regulates LHY and CCA1 expression while increasing LUX expression,butthiswouldbeassociatedwithlowlevelsof
that of TOC1, and this would be expected to reduce the peak TOC1andhighlevelsofCCA1.Thus,differentcombinationsof
expression of CBF genes. Drought stress represses expression environmentalstressesmayleadtodistinctprofilesofexpression
of TOC1, resulting in reduced stomatal opening, while salinity of the clock genes and to distinct effects on downstream stress
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