The Key Role of Reactive Oxygen Species and Nitric Oxide in the Plant Responses to Abiotic Stres

15 Pages·2016·0.77 MB·English

by  Fernanda S. Farnese

fpls-07-00471 April11,2016 Time:19:15 #1
REVIEW
published:12April2016
doi:10.3389/fpls.2016.00471
When Bad Guys Become Good Ones:
The Key Role of Reactive Oxygen
Species and Nitric Oxide in the Plant
Responses to Abiotic Stress
FernandaS.Farnese1*,PauloE.Menezes-Silva1,GrasielleS.Gusman2and
JuraciA.Oliveira3
1LaboratoryofPlantEcophysiology,InstitutoFederalGoiano–CampusRioVerde,Goiás,Brazil,2LaboratoryofPlant
Chemistry,Univiçosa–FaculdadedeCiênciasBiológicasedaSaúde,Viçosa,Brazil,3DepartmentofGeneralBiology,
UniversidadeFederaldeViçosa,Viçosa,Brazil
Thenaturalenvironmentofplantsiscomposedofacomplexsetofabioticstressesand
theirabilitytorespondtothesestressesishighlyflexibleandfinelybalancedthroughthe
interactionbetweensignalingmolecules.Inthisreview,wehighlighttheintegratedaction
betweenreactiveoxygenspecies(ROS)andreactivenitrogenspecies(RNS),particularly
nitricoxide(NO),involvedintheacclimationtodifferentabioticstresses.Understressful
Editedby:
conditions, the biosynthesis transport and the metabolism of ROS and NO influence
FrankGaupels,
HelmholtzZentrumMünchen– plant response mechanisms. The enzymes involved in ROS and NO synthesis and
DeutschesForschungszentrumfür scavenging can be found in different cells compartments and their temporal and
GesundheitundUmwelt,Germany
spatial locations are determinant for signaling mechanisms. Both ROS and NO are
Reviewedby:
involved in long distances signaling (ROS wave and GSNO transport), promoting an
EmilioFernandez,
UniversityofCórdoba,Spain acquired systemic acclimation to abiotic stresses. The mechanisms of abiotic stresses
JoseA.Traverso,
response triggered by ROS and NO involve some general steps, as the enhancement
UniversityofGranada,Spain
V.MohanMuraliAchary, of antioxidant systems, but also stress-specific mechanisms, according to the stress
InternationalCentreforGenetic type (drought, hypoxia, heavy metals, etc.), and demand the interaction with other
EngineeringandBiotechnology,India
signalingmolecules,suchasMAPK,planthormones,andcalcium.Thetransductionof
*Correspondence:
ROSandNObioactivityinvolvespost-translationalmodificationsofproteins,particularly
FernandaS.Farnese
[email protected] S-glutathionylation for ROS, and S-nitrosylation for NO. These changes may alter the
activity,stability,andinteractionwithothermoleculesorsubcellularlocationofproteins,
Specialtysection:
changingtheentirecelldynamicsandcontributingtothemaintenanceofhomeostasis.
Thisarticlewassubmittedto
PlantPhysiology, However, despite the recent advances about the roles of ROS and NO in signaling
asectionofthejournal
cascades,manychallengesremain,andfuturestudiesfocusingonthesignalingofthese
FrontiersinPlantScience
moleculesinplantaarestillnecessary.
Received:11December2015
Accepted:24March2016
Keywords: crosstalk, signaling, systemic acquired acclimation, S-nitrosylation, S-glutathionylation, gene
Published:12April2016
expression
Citation:
FarneseFS,Menezes-SilvaPE,
GusmanGSandOliveiraJA(2016) INTRODUCTION
WhenBadGuysBecomeGood
Ones:TheKeyRoleofReactive
Atypical plantcellhas morethan30,000 genesand alargenumber ofproteins,many ofwhich
OxygenSpeciesandNitricOxide
are still unknown, and these proteins may have their activity and/or function altered by several
inthePlantResponsestoAbiotic
typesofpost-translationalmodifications(Crameretal.,2011).Therefore,atthecellularlevel,plant
Stress.Front.PlantSci.7:471.
doi:10.3389/fpls.2016.00471 responsestotheenvironmentareextremelycomplexandinvolveinteractionsandcrosstalkwith
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Farneseetal. ReactiveSpeciesandCellSignalling
manymolecularpathways.Oneofthefirstplantresponsestothe that the changes triggered by these signaling molecules are
environmentinvolvesreactiveoxygenspecies(ROS)andreactive highly variable according to the environmental context. Due
nitrogen species (RNS), which are key signaling molecules and to the high complexity of this process, there is still much
regulatemanydifferentplantprocessesthroughtheactivationof that is unclear about the signaling mechanisms triggered by
secondary messengers, the induction of gene transcription and ROS and NO, the interaction of these molecules with each
changes in enzyme activity (Gaupels et al., 2011; Mengel et al., other and with other components of the signaling pathway,
2013;Lamotteetal.,2015). andthebalancebetweenproductionandeliminationofreactive
Reactive oxygen species is a generic term used to describe species by antioxidants. A growing number of studies have
chemical species formed from the incomplete reduction of sought to answer these questions, and many advances have
molecular oxygen. The best-known ROS include superoxide been made in the field. Thus, considering the central role
•−
anion(O ),hydrogenperoxide(H O )andthehydroxylradical of these molecules in the response and adaptation of plants
2 2 2
(OH−;Table1).ROShavedistinctbiologicalproperties,ashort to changes in the environment, the present review aims to
half-life and high chemical reactivity (OH− has indiscriminate summarize the existing knowledge of the interactions between
•−
reactivity to biological molecules, while O and H O have ROS and NO in the plant response to abiotic stress, focusing
2 2 2
preferred biological targets; del Río, 2015). Similarly, RNS is on the sources and production sites of these molecules,
a term used to collectively refer to nitric oxide (NO) and interactions with other signaling components and molecular
the molecules derived from this radical (Table 1) (Patel et al., aspects.
1999; Rahman et al., 2012). NO is a gaseous, small, reactive
molecule that readily diffuses across the cells and interacts
BIOSYNTHESIS, TRANSPORT, AND
with different cellular compounds, including other radicals
(Correa-Aragunde et al., 2015). Due to their high reactivity METABOLISM OF ROS AND NO
and potential to damage cellular structures under conditions
of redox imbalance, the generation of ROS and RNS in During cell signaling in response to stress, the redox state
cells was originally considered to be a uniquely harmful and of the plant cells is rapidly altered by both the increase in
damagingprocess(Demidchik,2015;Lushchak,2015).Currently, ROS and NO and the inactivation of antioxidant enzymes (del
however, it is known that these molecules are important Río, 2015). As a result, the concentration of these reactive
components of signaling networks in various plant processes, species is suddenly elevated, which is necessary to trigger
whichispossibleduetothedevelopmentofeffectiveantioxidant specific cellular responses. These responses include defense
systems that are capable, in most cases, of containing the mechanismstoabioticstresses,suchasincreasedconcentration
toxicity of ROS and RNS, allowing these molecules to act and activity of antioxidant systems (Shi et al., 2014) or
as efficient signal transducers (del Río et al., 2006; del Río, programmed cell death, which is important to eliminate cells
2015). that have been severely damaged (Yun et al., 2011). The
Nitric oxide and ROS are involved in and interact with enzymes involved in the synthesis of ROS and NO can be
each other in a wide range of cellular processes, which include found in different cellular compartments (Figure 1), and their
response to abiotic stresses (Joudoi et al., 2013), defense temporal and spatial localization is critical for signaling (Groß
against pathogens (Asai et al., 2008) and normal growth and et al., 2013). Indeed, ROS and NO have unique roles based
development processes, such as germination and flowering (El- on their compartment of origin, which is probably due to
Maarouf-Bouteau and Bailly, 2008). It is easy to see, therefore, interactions with local molecules in each organelle (Møller and
Sweetlove, 2010; Shapiguzov et al., 2012; Mur et al., 2013).
It has been observed, for example, that the transcriptional
changes mediated by H O produced in the apoplasts are
TABLE1|Mainreactiveoxygenspecies(ROS)andreactivenitrogen 2 2
species(RNS)foundinplantcells(adaptedfromRahmanetal.,2012). distinct from the gene expression responses triggered by H2O2
produced in the chloroplasts (Gadjev et al., 2006; Sierla
Freeradicals Non-radicals et al., 2013). Similarly, the NO generated from the plasma
Reactiveoxygenspecies membrane is important in hypoxic conditions, whereas the
Superoxide,O•2− Hydrogenperoxide,H2O2 NO generated from the chloroplasts and mitochondria is
Alkoxyl,RO• Hypochlorousacid,HOCl involved in the response to heavy metals (Kumar and Trivedi,
Hydroxyl,OH− Ozone,O3 2016).
Peroxyl,ROO• Peroxynitrite,ONOO− The plasma membrane is the main site of ROS production
Hydroperoxyl,HO2• Singletoxygen,1O2 due to activity from proteins belonging to the NADPH
Reactivenitrogenspecies oxidase family (respiratory burst oxidase homolog, RBOH).
Nitricoxide,NO• Nitrousacid,HNO2 NADPHoxidasesareintegralmembraneproteinsthatpromote
Nitricdioxide,NO2• Nitrosoniumcation,NO+ the transfer of cytoplasmic NADPH electrons to extracellular
•−
Nitrateradical,NO3• Nitrosylanion,NO− oxygen, forming O2 and promoting ROS accumulation in
Peroxynitrite,ONOO− the apoplast (Das and Roychoudhury, 2014). Several studies
Alkylperoxynitrites:ROONO have shown that stressful conditions stimulate the expression
Dinitrogentrioxide,N2O3 and activity of NADPH oxidases, leading to an oxidative burst
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FIGURE1|Mainsourcesofnitricoxide(NO)andreactiveoxygenspecies(ROS)inplantcells(adaptedfromGroßetal.,2013).
(Jajic et al., 2015; Wang X. et al., 2015). Other oxidases and are required to mediate stress-specific SAA (Gilroy et al.,
peroxidases associated with the cell wall are also involved 2014).
in the generation of ROS in the apoplast, although their In addition to the apoplast, various cellular organelles, such
involvement in the response to stressors is not well defined as chloroplasts and mitochondria, also generate ROS. In fact,
(Das and Roychoudhury, 2014). In addition to promoting when illuminated, chloroplasts are important sources of ROS
specific signaling events, which involve interactions with local duetotheintenseelectrontransportduringphotosynthesisand
signals,theRBOH-mediatedoxidativeburstofROSproduction the release of oxygen in PSII (Gupta and Igamberdiev, 2015).
triggers the production of ROS in neighboring cells, initiating In mitochondria, ROS production occurs when the transfer of
a long distance signaling event called a ROS wave. Each electronsexceedsthecapacityofthealternativeoxidaseandthe
cell along the ROS wave activates their own RBOH proteins, cytochrome oxidase to eliminate excess electrons, resulting in
generating a systemic wave of propagation of ROS production, their transfer to molecular oxygen, mainly from complexes I
which travels through the apoplast from the initial tissue to andIII.AnotherorganelleinvolvedinROSsynthesisinstressful
whole plants at rates of up to 8.4 cm min−1, promoting conditions is the peroxisome. Peroxisomes generate O•− and
2
systemic acquired acclimation (SAA; Mittler and Blumwald, H O as a result of their metabolic activity, which involves
2 2
2015). SAA enables all plant cells, not just those who first processes such as photorespiration, the glyoxylate cycle, and
perceived the external stimulus, to alter their gene expression β-oxidation (Tripathy and Oelmüller, 2012). These different
and metabolism in response to the stressor. Although the poolsofROS,producedindistinctcompartments,communicate
ROS wave is necessary for SAA, the response elicited is witheachotherinthecellstoregulatetheplantmetabolism.Itis
not always specific to the stress that initiated the signaling believed,forexample,thatthesignalgeneratedbytheoxidative
process, suggesting that the main function of the ROS wave burstintheapoplastistransducedtochloroplasts,whereasecond
is to prepare the plant for SAA and that other signals wave of ROS generation is initiated (Shapiguzov et al., 2012).
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Thissignaltransductionprobablyinvolvescytosoliccomponents in plants has not been unequivocally demonstrated (Domingos
as well as the transport of ROS through the lipid bilayer (the etal.,2015;GuptaandIgamberdiev,2015).
O − producedintheapoplastcanbeconvertedtoH O ,which In addition to biosynthetic processes, another crucial
2 2 2
enters the cell through the aquaporins) or signal detection by factor in NO concentration in the cell is the formation
apoplastic proteins and membrane receptors (de Dios Barajas- of S-nitrosothiols, particularly S-nitrosoglutathione (GSNO),
López et al., 2013). ROS generated in the chloroplasts, in turn, relativelystablemoleculesinsolutionthatmayactasreservoirs
areinvolvedintheretrogradesignalfromthechloroplasttothe ofNO(Leterrieretal.,2011).GSNOisformedbyS-nitrosylation
nucleusandinfluencetheexpressionofmanydefensegenes,in of glutathione (GSH) by NO and can be transported in the
additiontoinhibitingthetranscriptionofgenesassociatedwith phloem, thus contributing to the transport of this signaling
photosynthesis(deDiosBarajas-Lópezetal.,2013).Thishasalso molecule over long distances, which plays an important role in
been observed in other organelles, such as peroxisomes, where SAA (Arasimowicz-Jelonek et al., 2014). GSNO also regulates
ROS accumulation can alter gene transcription (Sandalio and the NO concentration in the cell via inhibition of the nitrogen
Romero-Puertas,2015). assimilationpathways(Fungilloetal.,2014).GSNOturnoveris
ThemaintenanceofROSlevelsalsoinvolvestheparticipation controlled by GSNO reductase (GSNOR), which catalyzes the
of antioxidant mechanisms, which are associated with the deamination of GSNO into glutathione disulfide (GSSG) and
elimination of these reactive species and can be divided into NH .Thus,GSNORregulatesthecellularlevelsofGSNOandis
3
enzymatic and non-enzymatic mechanisms. Among enzymatic importantinmaintaininghomeostasisofNO,whichisessential
antioxidants,superoxidedismutase(SOD)isespeciallyimportant for transient cell signaling (Malik et al., 2011). The levels and
•−
because it catalyzes the removal of O , the first ROS formed activityofGSNORaremodulatedinconditionsofstressandare
2
after exposure to various stressors. Other antioxidant enzymes determined, among other factors, by the balance between ROS
include ascorbate peroxidase (APX), glutathione peroxidase and NO (Cheng et al., 2015; Wang D. et al., 2015; Yang et al.,
(GPX),andcatalase(CAT),whichconvertH O towater(Lázaro 2015).
2 2
etal.,2013).Incombinationwiththeseenzymes,non-enzymatic Degradation of NO is as important as synthesis and
antioxidants, such as glutathione, ascorbate, and tocopherol, transport in determining the final concentration of this signal
also play a crucial role in maintaining ROS levels by acting molecule in plant cells. Recently, Sanz-Luque et al. (2015)
as redox buffers in plant cells. Although the synthesis of these demonstrated that the green alga Chlamydomonas reinhardtii
antioxidant molecules may, at first, be inhibited to allow the has a specific mechanism for the elimination of NO, which
occurrence of the oxidative burst (del Río, 2015), once the involves truncated hemoglobin THB1. The authors verified
signal is initiated, these mechanisms are activated and function that THB1 has NO dioxygenase activity (produces NO − from
3
cooperatively(Viehweger,2014). NO and O ) and maintains in its active form through a
2
In contrast to ROS, the mechanisms of NO synthesis in mechanismthatremoveselectronsfromNRandaltersitsactivity.
plant cells are not yet fully understood, constituting one of Another class of hemoglobins, the non-symbiotic hemoglobins
the major challenges to studies investigating this signaling (nsHb), particularly those belonging to the GLB1 class, have
molecule. However, several biosynthetic pathways for NO have also been reported to have NO dioxygenase activity and to
been proposed (Figure 1), which can be divided into reductive promote the degradation of NO in certain circumstances, such
pathways, including the action of xanthine oxidoreductase in as hypoxia (Perazzolli et al., 2004). The reduction in GLB1
the peroxisomes (Millar et al., 1998) and nitrite:NO reductase expression, moreover, allows NO concentration to increase,
attached to the membrane (Stöhr et al., 2001); and oxidative triggeringdefenseresponsesagainststress(Muretal.,2012).The
•−
pathways, such as the pathways mediated by hydroxylamines interactionbetweenNOandO ,whichgeneratesperoxynitrite
2
(Rümer et al., 2009) and polyamines (Filippou et al., 2013). (ONOO−),is alsoregardedas amechanismofNO elimination
Apparently, the action of nitrate reductase (NR), a cytosolic and involves the modulation of mitochondrial activity (Wullf
enzymeessentialfortheassimilationofnitrogen,alsorepresents etal.,2009).
an important source of NO for plants (Horchani et al., 2011). The biosynthesis and degradation of ROS and NO influence
It has been suggested that NR is involved in the production each other. ROS are well-known inducers of NO synthesis
of NO during a variety of physiological processes, such as in various plant species exposed to abiotic stress, although
bacterialdefense(Modoloetal.,2005;Muretal.,2013),hypoxia the signaling involved in this process is still not completely
(Igamberdiev and Hill, 2004), cold (Zhao et al., 2009), drought understood. NO, in turn, limits the accumulation of ROS by
(Freschi et al., 2010), and various aspects of development, such inhibitionofNADPHoxidases,aswellasbypromotingchanges
as floral transition and the formation of lateral roots (Seligman in the antioxidant systems, suggesting the existence of complex
et al., 2008; Mur et al., 2013). However, under normal growth feedback regulation of both signaling molecules (Groß et al.,
conditions, NR preferentially reduces nitrate to nitrite, and NR 2013). In fact, the activation of antioxidant mechanisms to
isonlyabletogeneratesignificantamountsofNOundercertain maintain ROS homeostasis often involves the participation of
conditions,suchasanaerobicconditionsorhighconcentrations NO(Farneseetal.,2013;Shietal.,2014;Silveiraetal.,2015).It
ofnitrite(Guptaetal.,2011;Muretal.,2013).Therehavebeen has been shown that the addition of NO increased the activity
numerousreportsofanarginine-dependentnitricoxidesynthase of SOD up to 110% in sorghum plants exposed to arsenic
(NOS) in extracts of different plant species (Jasid et al., 2006; (Saxena and Shekhawat, 2013), in addition to the increase in
Zhaoetal.,2007;Gasetal.,2009;delRío,2011),butitspresence CAT and APX and the activation of the ascorbate-glutathione
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cycle (Hasanuzzaman and Fujita, 2013; Shi et al., 2014; Cheng wasdemonstratedthatS-nitrosylationisanegativeregulatorof
etal.,2015).Somestudies,however,havesuggestedthatNOcan transcriptionfactorsfromtheMYBfamily(regulatoroftolerance
inhibit the antioxidant capacity of the cell (Marti et al., 2013). to biotic and abiotic stresses), which may be important for the
These seemingly contradictory results may be due to the dose- inactivation of this regulatory protein after the initial response
dependent effects of NO on cellular redox status. According of plants to stress (Tavares et al., 2014). Several S-nitrosylated
to this hypothesis, low concentrations of NO stimulate the nuclear proteins have also been identified, including histone
antioxidantsystemandpromoteadaptationtostressconditions, deacetylases,whichhighlightstheregulatoryroleofNOinevents
whilehighconcentrationsofNOtriggerseverecelldamageand locatedinthenucleus(Chakietal.,2015).Histonedeacetylases
evencelldeath(Thomasetal.,2008;Großetal.,2013). are responsible for the removal of acetyl groups on histones,
promotingthechromatincondensation,whichmakesthegenes
less accessible to the transcriptional machinery (Mengel et al.,
MOLECULAR BASES OF ROS AND NO
2013). In mammalian cells, S-nitrosylated histone deacetylases
ACTION becomedetachedfromthechromatin,increasingacetylationand
gene expression (Nott et al., 2008). Thus, the S-nitrosylation of
The mode of action of ROS and NO at the molecular level deacetylases suggests that NO participates in the regulation of
was and still is the subject of many studies, both in plants and epigeneticprocessesinplants(Floryszak-Wieczoreketal.,2012;
other organisms, such as mammals and bacteria (Green et al., Chakietal.,2015).
2014; Lamotte et al., 2015; Morales et al., 2015). The available The S-nitrosylation state of any protein is determined by
data indicate that the effects of NO, as well as certain species thebalancebetweennitrosylationanddenitrosylationreactions.
derived from this molecule, depend on chemical changes in In fact, denitrosylation, which involves the removal of NO
proteins,whichcanoccurbythreedifferentmechanisms:metal from cysteine residues, is essential for the reversibility of
nitrosylation, tyrosine nitration, and S-nitrosylation (Lamotte S-nitrosylation and influences the enzyme activity, protein–
et al., 2015). Metal nitrosylation consists of NO binding to proteininteractionsandmanyotheraspectsofsignaling(Sevilla
transition metals in metalloproteins. Soluble guanylate cyclase et al., 2015). Although this process has been more extensively
is an example of an enzyme that is modulated by this type of studied in mammals, recent evidence has shown that it occurs
post-translationalmodification.Tyrosinenitrationistheaddition in plant cells (Kneeshaw et al., 2014; Benhar, 2015). Among
of a nitro group to tyrosine residues. Tyrosine nitration is the molecules which may be involved in the denitrosylation
carried out mainly by peroxynitrite (ONOO−), the product of process, the GSNO/GSNOR (discussed earlier in this review)
•−
thereactionbetweenNOandO .Althoughtyrosinenitration andthethioredoxin/thioredoxinreductase(Trx/TR)systemsare
2
was originally considered indicative of stress, recent evidence essentialforthemaintenanceofhomeostasisofnitrosothiolsin
suggests its involvement in cell signaling (Mengel et al., 2013). plants(Lamotteetal.,2015).IntheTrx/TRsystem,Trxreduces
Finally,S-nitrosylation,whichconsistsofNObindingtocysteine nitrosothiols through its dithiol moiety, generating free thiol
residuesintargetproteins,isapparentlytheprincipalmechanism groupsandoxidizedTrx.RegenerationofTrxoccursthroughthe
for the transduction of the NO bioactivity. S-nitrosylation can action of TR and NADPH (Benhar, 2015). Thus, the control of
also occur via trans-nitrosylation, that is, by the transfer of theredoxstatusofthiolgroupsdependsontheirinteractionwith
NO from an S-nitrosylated residue to another thiol group NO and with the denitrosylation systems, which influences the
through the action of low-molecular weight nitrosothiols, such intensityanddurationofthesignalingevents(Benhar,2015).
as GSNO (Lamotte et al., 2015). Regardless of the mechanism As observed for NO, ROS also transmit signals via post-
involved,S-nitrosylationisapost-translationalmodificationthat translationalmodificationsinproteinsand,oncemore,cysteine
can alter the activity, stability, conformation, interactions with residues are the main targets. However, while NO promotes
othermoleculesorsubcellularlocalizationofthetargetprotein, S-nitrosylation,ROScantriggeradiverserangeofoxidativepost-
regulatingawiderangeofcellularfunctionsandsignalingevents translationalmodifications(Ox-PTM),reversibleorirreversible,
(Sevillaetal.,2015). including S-glutathionylation, disulfide bond formation, and
S-nitrosylationisanimportantprocessinplantresponsesto sulfhydration(Akteretal.,2015).Asingleproteincanundergo
abioticstress.Exposuretosaltstress,forexample,resultsinthe differenttypesofOx-PTMs,andthereisevidencethateachOx-
S-nitrosylation of enzymes involved in different physiological PTMmayhaveadistinctbiologicalrole(Couturieretal.,2013).
processes,suchasrespiration,photorespiration,andantioxidant The first step in ROS-dependent signaling involves the reverse
pathways (Camejo et al., 2013), while in plants exposed to oxidation of a cysteine residue, forming sulfenic acid (R-SOH).
low temperatures, the enzymes involved in carbon metabolism Thismodificationishighlyunstableandwillleadtosubsequent
were the main group of S-nitrosylated proteins (Puyaubert changes;themajoronesarethereactionwithfreeproteinthiolsto
et al., 2014). S-nitrosylation of proteins that participate in formdisulfidebondsorthecovalentattachmentoflow-molecular
central processes in the plant cell presumably contributes to weight thiols, such as GSH, promoting S-glutathionylation, a
themetabolicreprogrammingrequiredtomaintainhomeostasis process that is important in signaling and protein protection
understressconditions.Inadditiontochangesincellularenzyme against superoxide. The reduction of disulfide bonds and
dynamics, S-nitrosylation may also trigger changes in gene deglutathionylation interrupt the signal that initiated with the
expressionasaresultofS-nitrosylationoftranscriptionfactors, Ox-PTMsandarecontrolledbyglutaredoxinsandthioredoxin,
affecting their affinity for DNA or their location. Recently, it respectively(Waszczaketal.,2014,2015).
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TheOx-PTMs,particularlyS-glutathionylation,playacentral et al., 2015). The exact signaling mechanisms involved in this
role in the response to abiotic stresses and can modulate process, however, have not been clarified, and many questions
numerous cellular processes affecting proteins, transcription remainunanswered.Currently,itisknownthatincreasesinROS
factors, and chromatin structure. These mechanisms, however, are one of the first cellular signals in response to excess heavy
have mostly been studied in animals and bacteria, and many metals.Infact,heavymetalscanactivatetheproductionofROS
aspects of the Ox-PTM-mediated responses are unknown in intheapoplast,stimulatingNADPHoxidases,andinorganelles
plants (Zagorchev et al., 2013). One example of Ox-PTMs suchaschloroplastsandmitochondria(Chmielowska-Baketal.,
mediating changes in cellular dynamics is the transcription 2014).HeavymetalsenhancethesynthesisofNObymechanisms
factor ERFVII (ethylene-responsive group factor VII), which is thatvaryaccordingtothechemicalcharacteristicsofthemetal.
importantinalteringgeneexpressionunderhypoxicconditions. Lead, for example, apparently increases the activity of cytosolic
ERFVIIisboundtotheplasmamembraneandisonlyreleasedin NR (Yu et al., 2012), while increases in NO levels mediated by
low-oxygenconditions.Itstranslocationtothenucleusactivates cadmium are related to iron-induced deficiency (Besson-Bard
the expression of hypoxia-responsive genes. In the presence et al., 2009). Interestingly, some metalloids, such as arsenic,
of oxygen, however, ERFVII cysteine residues are oxidized stimulateboththesynthesisofNOandtheactivityofGSNOR,
to sulfenic acid, conjugated with arginine and directed to andthebalancebetweenthesetwoprocesseswilldeterminethe
degradation,down-regulatingtheexpressionofgenesthatareno final concentration of the signaling molecule (Leterrier et al.,
longerneeded(Dietz,2014).TheOx-PTMscanalsobepositive 2012).Inaddition,theincreaseinNOmayresultfromsignaling
regulators of gene transcription, as in the case of transcription triggered by excessive ROS following exposure to metals (Yun
factorsofheatshockproteins(HSF),whoseoxidationbyH O etal.,2011).
2 2
induces translocation from the cytosol to the nucleus (Habibi, According to their concentrations, ROS and NO can cause
2014).Despitethegrowingnumberofstudies,however,thereis oxidative/nitrosative stress in cells or may act as signaling
stilllittleinformationabouttheeffectsofS-nitrosylationandOx- molecules. At low concentrations, NO contributes to increased
PTMsongeneexpressionandtheconsequencesofthesechanges tolerance of the plants to metals in various ways, for example,
on plant metabolism in stress conditions. Thus, the molecular by promoting metal binding to the cell wall, preventing
mechanismsinvolvedincellsignalingmediatedbyROSandNO their entry into the cell (Singh et al., 2011) or promoting
arestillfarfrombeingfullyunderstood. their compartmentalization in the vacuole, either by increasing
phytochelatinsynthesis(DeMicheleetal.,2009)orbyalteringthe
activityofprotonpumpsinthevacuolarmembranetocreatean
CROSSTALK BETWEEN ROS AND NO IN
electrochemicalgradientthatfavorstheabsorptionofmetals(Cui
THE RESPONSE TO ABIOTIC STRESS
etal.,2010).Inadditiontotheseeffects,NOcanalsoreprogram
plant physiological processes and stimulate the synthesis and
Heavy Metals
activity of antioxidant systems, which is essential to limit the
Traditionally, heavy metals are considered those chemicals that oxidativestressinducedbymetals(Chengetal.,2015;Andrade
haveadensityhigherthan5gcm−3oranatomicnumberhigher etal.,2016).Finally,post-translationalchangestriggeredbyNO
than20.Inplantphysiology,however,thetermheavymetalhas candecreasetheactivityofenzymesinvolvedinROSmetabolism,
been used generically to refer to any metal or metalloid that is suchasglycolateoxidaseandNADPHoxidase,allowingthecell
toxictoplants,evenwhenpresentatlowconcentrations(Singh tore-establishredoxhomeostasis(Yunetal.,2011;Quiangetal.,
et al., 2011; Oz et al., 2015). Although some of the members 2012).
ofthisgrouparenecessaryforgrowthanddevelopment,others
Drought
have no known function in plant cells and, regardless of their
physiological role, the accumulation of metals usually results Water deficits are the main environmental factor limiting
in severe cell damage, which can lead to the death of the the growth and productivity of plants worldwide. Indeed, the
plant (Besson-Bard et al., 2009). Heavy metals enter plant cells damage triggered by drought may be greater than the damage
by transporters present in the plasma membrane and may be caused by other biotic and abiotic factors combined (Chaves
retained in the roots or transported to the shoots, according to et al., 2009). Plants tolerant to drought usually have a strict
thecellulardetoxificationmechanismsineachspecies(Tangahu control of stomatal movements and a fine balance of cellular
et al., 2011). In general, the defense mechanisms of plants metabolism, and both ROS and NO are important in these
to heavy metals can be divided into two groups, which may processes (Figure 2) (Osakabe et al., 2014). Drought stress-
occursimultaneously:(i)regulationoftheconcentrationoffree inducedNOisfoundinawidevarietyofplantspecies,including
metal in the cytosol through metal exclusion, metal binding to vegetables, horticultural plants and epiphytes, suggesting the
the cell wall or compartmentalization in the vacuole; and (ii) universal requirement of NO during drought stress signaling
physiological,biochemical,andmolecularchangesthatallowthe (Santisreeetal.,2015).Themetabolicpathwaysinvolvedinthis
reprogramming of plant metabolism and the maintenance of process, however, are still unclear, although evidence suggests
cellularhomeostasis. the involvement of NR (Arasimowicz-Jelonek et al., 2009) and
RecentstudieshaveindicatedthattheinteractionbetweenNO xanthine oxidoreductase (Yu et al., 2014). The generation of
and ROS is essential for tolerance to heavy metals (Figure 2) ROS during drought, on the other hand, is well known and
(Wang et al., 2014; Feigl et al., 2015; Silveira et al., 2015; Thao commonly involves changes in plant metabolic processes. For
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FIGURE2|SchematicrepresentationshowingtheinterplaybetweenNO,ROS,andothersignalingmoleculesinresponsetoabioticstress.The
signalingmoleculesincludehormones,mitogen-activatedproteinkinases(MAPKs),cyclicguanosinemonophosphate(cGMP),cyclicadenosinediphosphoribose
(cADPR)andcalcium(Ca2+).Besidestheinteractionwithcomponentsofsignalingpathways,NOandROSalsotransmitsignalsviapost-translationalmodifications
inproteins[metalnitrosylation,tyrosinenitration,S-nitrosylation,andoxidativepost-translationalmodifications(Ox-PTM)].ROSdegradationbyantioxidantsandNO
storage(GSNO,S-nitrosoglutathione)anddegradationbyGSNOR(GSNOreductase)andTHB(truncatedhemoglobin)aredemonstrated.ArrowsandT-bars
indicateactivationandinhibition,respectively(adaptedfromOzetal.,2015).
example, drought may reduce the activity of Rubisco (Parry to water stress, acts as an upstream regulator, inducing the
et al., 2002), which compromises the fixation of CO and the synthesis of NO. NO and ROS, in turn, act synergistically to
2
regeneration of NADP+ via the Calvin cycle. This results in an mediatestomatalclosurethroughtheformationof8-nitro-cGMP
over-reductionoftheelectrontransportchaininthechloroplasts (Joudoietal.,2013).ABAisalsoinvolvedinseveralotherplant
and,consequently,theleakageofelectronstoO ,mainlybythe responses to water stress, for example, the induction of gene
2
Mehlerreactionandphotorespiration,resultinginthegeneration expression and the synthesis of defense compounds, such as
of ROS (Carvalho, 2008). In parallel with the physiological proline.IncreasingonlyABA,however,isnotsufficienttoinduce
changes, NADPH oxidases are also involved in the generation the synthesis of these compounds, as observed after exogenous
of ROS in water stress conditions, as they are essential for the application of ABA to Arabidopsis thaliana (Verslues and Bray,
activationofdefenseresponsesagainstdrought(WangX.etal., 2006), suggesting that factors other than ABA are required to
2015). modulate the response to drought, possibly the cellular redox
One of the first and most important physiological responses status.Thus,theproductionofROSandthesubsequentchangein
inducedbydroughtisthereductionofthestomataopening(Neill theredoxstateofthecellhavebeensuggestedasfactorsrequired
et al., 2008). In water stress conditions, the complex dynamics toinitiatesignaltransductionmediatedbyABA(Carvalho,2008).
of stomatal movement are directly related to the concentration Similar to ABA and ROS, NO has been shown to alter gene
of abscisic acid (ABA), ROS, and NO. In this process, ABA, an expressioninresponsetowaterdeficits.TransgenicArabidopsis
importantplanthormonetraditionallyassociatedwithresponses lines that constitutively express rat neuronal NO synthase
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showed changes in gene expression relative to wild plants of binding to NO, and at low O concentrations catalyze its
2
when subjected to drought, with 184 genes up-regulated and conversion to nitrate, which can be converted to nitrite by NR
263 down-regulated. The main transcriptional changes were (Dordas,2015).Nitrite,inturn,canbeconvertedagaintoNOin
observed in the genes involved in redox metabolism and sugar themitochondria,closingthecycle(Guptaetal.,2011).
metabolism and in transcription factors. These transcriptional
Salt Stress
changes were accompanied by higher survival rates and high
biomass production, indicating a protective effect of NO (Shi Salinity is an ever-present threat to crop yields, especially in
et al., 2014). Moreover, it has been reported that NO may places where irrigation is required. In fact, studies show that
trigger epigenetic modifications, such as DNA methylation, in approximately 20% of all cultivated areas in the world are
responsetowaterstressconditions.Indeed,itwasobservedthat affected by this type of stress, and this percentage is likely
theexogenousapplicationofSNP(anNOdonor)decreasesthe to increase due to inadequate irrigation practices (Muns and
overallmethylationlevelsinDendrobiumhuoshanense,leadingto Tester, 2008). Salt stress compromises the intracellular ion
increasesintheactivityofantioxidantenzymes(Fanetal.,2012). homeostasis, which leads to membrane dysfunction, alteration
of metabolic activities, growth inhibition, and even cell death
Flooding
(Zhang et al., 2006). Plants tolerant to salt stress have many
Climate projections for the next decades, besides pointing diversestrategiestotoleratehighconcentrationsofsolutes.The
out an increase in drought intensity and frequency for some principal mechanisms include, but are not limited to, (i) ion
regions, also highlight a significant increase in rainfall for homeostasis and compartmentalization, (ii) ion transport and
others,especiallythoseregionswithtropicalclimate(Hirabayashi uptake, (iii) biosynthesis of osmoprotectants and compatible
etal.,2013).Ifthesepredictionsareconfirmed,productivityfor solutes, (iv) activation of antioxidant enzymes and synthesis of
severalcropareasandecosystemscanbesignificantlyimpacted antioxidantcompounds,and(v)synthesisofpolyamines(Gupta
due to flooding events (Bailey-Serres and Voesenek, 2008). andHuang,2014).Alltheseeventsaretriggeredandintegrated
Despite the fact that flooding has been studied to a lesser into the plant cell through the action of signaling molecules,
extentthandrought,inrecentyears,researchesaboutmolecular especially ROS, NO, and plant hormones (Figure 2) (Filippou
mechanisms that promote flooding tolerance have progressed etal.,2014).
rapidly (Voesenek and Bailey-Serres, 2015). In general, the Nitric oxide is essential for the tolerance of plants to high
mechanismsthatconfertolerancetosuchstresscanbegrouped salt concentrations. Examples of the role of NO in this process
into two categories: “escape,” which involves anatomical and canbeobservedinmutantplantsofArabidopsis(Atnoa1)which
morphologicalmodificationsthatallowaccessofthesubmerged show deficiency in NO synthesis and hypersensitivity to salt
cells to O and CO , and “quiescence,” which promotes a stress(Zhaoetal.,2007).Additionally,studiesusingNOdonors
2 2
profound change in metabolism and growth of flooded plants and inhibitors showed that the ability of NO to alleviate stress
(VoesenekandBailey-Serres,2015). was related to the change in the Na+/K+ ratio in the cytosol
Several evidences have suggested that plants can sense duetotheincreaseinH+-ATPaseandH+-PPaseactivityinthe
the reduction of O availability caused by flooding through plasmaandvacuolarmembranes(Zhaoetal.,2004;Zhangetal.,
2
different ways. Some of these mechanisms of perception may 2006;Wangetal.,2009).Inaddition,NOisalsoabletoinduce
involve changes in the dynamics of ROS and NO production the expression of defense genes against stress and significantly
and consumption, because of mitochondrial electron transport increasetheactivityofenzymesoftheantioxidantsystem(Uchida
inhibition (Rhoads and Subbaiah, 2007). In fact, studies in et al., 2002). H O also alters the expression of stress response
2 2
Arabidopsis thaliana showed that oxygen deprivation induces genesinplantssubjectedtohighsaltconcentrations,particularly
ROS production in the complex III, which triggers a transient in root cells, in addition to increasing the activity of specific
activation of MAPK signaling cascade (Chang et al., 2012). In enzymes (Miller et al., 2010). It is also important to note that
addition,regulatoryresponsesinducedbyROSinvolve,atleastin the changes triggered by ROS and NO are not restricted to the
part,theinteractionwithethylene,animportantplanthormone tissueswherethesemoleculeswereproducedbecauseadditionof
involved in plant response to various stresses. Indeed, in rice NOandH O totherootsreducedthephysiologicalimbalances
2 2
plants, ROS and ethylene were involved in the production of caused by NaCl in leaves of Citrus plants (Tanou et al., 2009).
adventitiousrootinstemnodes(Steffensetal.,2012). ThisprocessispossibleduetomechanismssuchastheROSwave
The origins of NO in hypoxic conditions are not clearly andthetransportofNObecausebothNOandGSNOhavebeen
understood, although nitrite and ascorbate are apparently observed in vascular tissues of plants exposed to salinity stress
involvedinthisprocess,aswellasmitochondrialreactions(Wang (Valderramaetal.,2007).
and Hargrove, 2013). NO generation in mitochondria is an Another effect of NO and H O in plants is the acquisition
2 2
importantprocessatlowoxygenconditions.Inthisprocess,the of immunity against salinity. This process, known as priming,
nitriteactsasanelectronacceptorincomplexIV,complexIIIand describesthephenomenoninwhichplantspreviouslysubjected
inthealternativeoxidase,keepingalimitedproductionofATP toaparticularstressfactoraccelerateandpotentiatetheirdefense
when oxygen is not available and, thus, preventing the collapse responseswhensubjectedtothatsamestressor(Zhu,2003;Zhang
ofcellularenergystatus(Guptaetal.,2011).TheproducedNO et al., 2004; Molassiotis et al., 2010). In fact, plants treated
diffuses into the cytosol, where it is converted to nitrate by with low concentrations of NO and H O had their metabolic
2 2
hemoglobin action. Class 1 hemoglobins are proteins capable and physiological responses potentiated when exposed to salt
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stress. Likewise, NO-associated salt priming could be observed shown to be involved in signal transduction that culminates in
in halophytes and glycophytes that have increased tolerance to increasedexpressionofheatshockgenes,whichencodeproteins
salinitywhenpreviouslyexposedtoNOdonors(Lietal.,2005). thatplaycriticalrolesinthemaintenanceofcellularhomeostasis
These data show that NO and H O are priming agents that instressfulconditions(Königshoferetal.,2008).
2 2
promoteincreasedtoleranceinthewholeplant,minimizingthe As observed in high temperatures, exposure to cold can
deleteriouseffectsofsubsequentexposuretosalinity. also promote a rapid increase in the endogenous levels of
As noted in this review, different abiotic stresses have the NO and ROS (Zhao et al., 2009; Cantrel et al., 2011). The
same convergence point: they induce the production of ROS increased production of NO in these conditions most likely
andNO.Thus,althougheachtypeofstresshasitsownspecific involves an increase in NR because the nia1nia2 mutants (NR-
characteristics,nearlyallabioticstressorsalterthecellularredox defective double mutants) showed lower concentrations of NO
state, and therefore, the participation of antioxidants in the and increased susceptibility to cold stress (Cantrel et al., 2011).
plant response is essential. The activity and gene expression of Tolerance to low temperatures mediated by NO involves the
antioxidants are strongly influenced, directly or indirectly, by reprogramming of gene expression because it has been shown
changesintheconcentrationsofROSandNO(Shaoetal.,2008; thatNOproduction,inducedbyNR,promotesthetranscription
Shi et al., 2014). In addition to the well-established role of the of the P5CS1 and ProDH genes, along with accumulation of
antioxidants in removing ROS, which has been discussed here, proline and an increased tolerance to cold (Zhao et al., 2009).
these molecules are important in signaling, providing essential In addition to the effects mediated by NO, the role of H O
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information about the cellular redox status and influencing the in cold acclimation has also been widely documented (Prasad
expression of defense genes against biotic and abiotic stresses etal.,1994;Yuetal.,2003;Hungetal.,2007;Wangetal.,2010).
(Pastori et al., 2003; Foyer, 2005). Moreover, antioxidants are Several studies indicate that exogenous application of H O
2 2
also needed to interrupt the signal transduction cascades when reducescellulardamagecausedbylowtemperatures,increasing
they are no longer needed, as is the case of glutaredoxin and thesurvivalrates(Neilletal.,2003).Again,thisbeneficialeffect
thioredoxin(Waszczaketal.,2015). of H O appears to involve an increase in the effectiveness of
2 2
both the enzymatic (increased activity of the enzymes APX,
Temperature Stress
GPX, and CAT) and non-enzymatic antioxidant mechanisms
Temperature,incombinationwithwaterdeficiency,isoneofthe (increasesinGSHlevels;˙Iserietal.,2013;Wangetal.,2013).The
mainabioticfactorsthatdeterminethesurvivalanddistribution signaling cascade responsible for the attenuation of cold stress
of species worldwide. There are several reports in the literature involves, at least partially, transient increases in the cytosolic
indicating the adverse effects of high and low temperatures on Ca2+ concentration due to H O -mediated activation of Ca2+
2 2
the molecular, biochemical, and physiological characteristics of channelsintheplasmamembrane(Knightetal.,1996).However,
plants(SuzukiandMittler,2006).Althoughexposuretohighand althoughtheseresultsclearlyindicatethatROSandNOarekey
lowtemperaturestriggersverydistinctmetabolicdisorders,there molecules in the tolerance to high and low temperatures, few
isacommonresponsebetweenthesetwostressors:theincrease studieshavefocusedonthemolecularaspectsandthesignaling
in ROS, which damages cellular structures (Figure 2) (Potters cascadesresponsiblefortheseprocesses.
etal.,2007;Zhouetal.,2012;BitaandGerats,2013).Similarto
otherabioticstresses,increasedlevelsofROSfollowingextreme
INTERACTIONS BETWEEN ROS, NO,
temperaturesappeartoinvolveanimbalancebetweencapturing
AND OTHER SIGNALING MOLECULES
and processing energy (Hasanuzzaman et al., 2013). However,
overthepastfewyears,severalstudieshaveshownthatROSand IN THE RESPONSE TO ABIOTIC STRESS
RNS, especially the interaction between them, are essential for
acclimatization to high and low temperatures (Yu et al., 2014; The signaling pathways that respond to environmental stresses
Hossain et al., 2015). In fact, plants treated with low doses of constituteintricatemolecularnetworksthatinvolvemanyother
NOandH O performbetterunderconditionsofthermalstress componentsbesidesROSandNO,suchascalcium(Ca2+),cyclic
2 2
(Neill et al., 2002; Uchida et al., 2002; Abat and Deswal, 2009; nucleotides, plant hormones, and mitogen-activated protein
Cantreletal.,2011). kinases (MAPKs; Figure 2). The MAPK cascades, for example,
Exposure to high temperatures usually results in increased are activated by ROS, NO, and hormones, representing a
productionofNO,aresponsethatisimportantforacclimationto convergence point for these signaling molecules. MAPK signal
thistypeofstress(Leshem,2000;Neilletal.,2003;Bouchardand transduction involves a phosphorylation cascade comprising
Yamasaki,2008;Yuetal.,2014).Indeed,additionofexogenous MAPK kinase kinase (MAPKKK), MAPK kinase (MAPKK),
NO to plants subjected to high temperatures promotes the andfinally,MAPK,whichisactivatedafterphosphorylationsof
activation of enzymatic and non-enzymatic defense systems threonine and tyrosine residues in the conserved motif T-X-Y
against ROS, reducing cellular damage (Song et al., 2006; Zhao (Rodriguezetal.,2010).TheinteractionbetweenROS,NO,and
et al., 2009; Hasanuzzaman et al., 2013). In addition, treatment MAPKshasbeendemonstratedintheplantresponsetovarious
withNOscavengers,suchascPTIO,reversesthebeneficialeffects stressors, such as heavy metals (Chmielowska-Bak et al., 2014),
caused by NO, which further reinforces the importance of this drought(Wangetal.,2014),andosmoticstress(Xuetal.,2011).
molecule in the tolerance to high temperatures (Song et al., In Arabidopsis, high levels of ROS, especially H O , induce the
2 2
2006).Similarly,accumulationofROS,especiallyH O ,hasbeen transcription of the genes OXI1 (Rentel et al., 2004) and ANP1
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(Kotvun et al., 2000), which encode protein kinases required andwith othercellular messengers, suchas Ca2+, andthe final
for full activation of the MAPKs. NO acts simultaneously with response is dependent on the fine balance between all of these
ROS in the activation of MAPKs, although the mechanisms components(Xiaetal.,2015).
involved in this process are not completely understood. In Inanimals,NO-mediatedsignalingislargelydependentonthe
mammals, the activation of MAPK cascades by NO occurs synthesis of cyclic nucleotides, especially cyclic GMP (cGMP).
indirectly and involves an increase in the synthesis of cGMP In these organisms, NO binding to a soluble guanylate cyclase
(Francis et al., 2010). It is possible that a similar mechanism increases the activity of this enzyme up to 200 times, with
is present in plants because it has been demonstrated that NO a consequent increase in the formation of cGMP (Francis
triggers the increases in the concentration of cyclic nucleotides et al., 2010). In plants, both NO and H O increase the
2 2
(Pasqualinietal.,2009).Onceactive,MAPKscanphosphorylate concentration of cyclic nucleotides, which apparently involves
many target molecules, both in the cytosol and in the nucleus, the binding of these molecules to enzymes with guanylate
including enzymes or transcription factors (Rodriguez et al., cyclaseactivity(Dubovskayaetal.,2011;Mulaudzietal.,2011).
2010). The responses mediated by MAPKs involve induction In Chlamydomonas, an NO-dependent guanylate cyclase that
of antioxidative or pro-oxidative enzymes and may attenuate participates in the transcriptional repression of NR has been
or amplify the original signal triggered by ROS and NO (Asai identified, suggesting a model that integrates, among other
et al., 2008; Opdenakker et al., 2012). In addition, MAPKs components,NO,cGMPandthenitrogenassimilationpathways
may also interfere in the signaling and in the biosynthesis (de Montaigu et al., 2010). Two different types of responses
of hormones, leading to the activation of downstream stress are associated with cGMP, depending on the time between
responses. The signal is interrupted by phosphatases, which the perception of the stimulus and the peak in nucleotide
promote the dephosphorylation of MAPK (Opdenakker et al., concentration: fast responses, which involve the modulation of
2012). ion channels, such as Ca2+ channels; and long-term adaptive
Plant hormones are also critical messengers in plant responses, which result in changes in the transcriptome and in
developmentandstresstolerance.Thesignalingcascadesofmost theproteome(Pasqualinietal.,2009).Inadditiontoincreasing
hormonesincludeinteractionswithROSandNO,asisthecase the concentration of cGMP (Durner et al., 1998; Dubovskaya
ofABA,mentionedabove,andauxin.Auxinisanessentialplant etal.,2011),ROSandNOalsopromotenitrationofthismolecule,
hormonethatparticipatesinvariouscellularprocesses.Incertain changingitsstructureandfunction,andthestomatalclosureis
situations,auxininducesthesynthesisofROSandNO,which,in oneoftheclearestexamplesoftheinteractionofNO,ROS,and
turn,influencethehormone-mediatedsignaling(Jooetal.,2001; cGMP(Joudoietal.,2013).
Yadavetal.,2013).ROSaccumulationinstressfulsituationscan Ca2+ is a highly versatile signaling molecule that plays a
triggeroxidativeinactivationordegradationofauxin,aswellas central role in the response to environmental stressors (Dodd
a decrease in the expression of genes involved in its transport et al., 2010; Schulz et al., 2013; Chmielowska-Bak et al., 2014).
andsignaling,throughspecificMAPKcascades(Xiaetal.,2015). Theexposuretostressorsactivatescalciumchannels,pumpsand
GSNO accumulation also compromises the polar transport of transporters in the plasma membrane or in the membranes of
auxinandreducesitseffectsviaS-nitrosylationofcomponentsof organelles,whichresultsintherapidinfluxofthecationintothe
itssignalingpathway(Shietal.,2015).Theattenuationofauxin cytosol,increasingtheconcentrationofcytosolicCa2+.Transient
signaling leads to changes in plant growth and acclimatization calcium levels in the cytosol are detected by calcium-binding
tonewenvironmentalconditions.Insomecases,however,ROS proteins such as calmodulin, Ca2+-dependent protein kinases
andNOcanactaspositiveregulatorsofauxin(Woodwardand (CDPKs), phosphatases regulated by Ca2+ and by changes in
Bartel,2005;Terrileetal.,2012).Anotherhormoneinvolvedin Ca2+ channels and pumps (Gilroy et al., 2014), and these
the acclimation of plants to stress is salicylic acid (SA), which signalsarethentransmitted.OneexampleistheCa2+-permeable
is responsible for transcriptional reprogramming during the channelsintheplasmamembrane,whicharerelativelyinactive
defenseagainstabioticstress.TheinteractionbetweenSA,ROS, under normal conditions but can be activated by ROS in the
andNOiscomplex,withROSandNOactingbothupstreamand guardcellsinresponsetodrought(Peietal.,2000;Doddetal.,
downstreamofSA.NOandROSinducethesynthesisofSA,and 2010). Similar to ROS, NO synthesis is induced by Ca2+, and
NOalsoactivatestranscriptionfactorsthatinitiateSA-dependent it activates intracellular Ca2+-permeable channels and CDPKs
geneexpressionbyinducingthesynthesisofvariousmolecules, (Astieretal.,2010).
such as pro-oxidants and antioxidants (Mur et al., 2013). The
synthesisofpro-oxidantsandantioxidantsduringSA-mediated
signaling presents biphasic redox dynamics. In the first phase, FUTURE PERSPECTIVES
the oxidative phase, transient increases in ROS levels trigger
signaling events that are dependent on the cell redox state, and It is widely recognized that ROS and NO interact with each
this is followed by a reductive phase characterized by increased other and are key molecules in the plant response to various
antioxidantsanddecreasedROS(Herrera-Vásquezetal.,2015). types of abiotic stresses. Additionally, a large body of evidence
InadditiontoABA,auxin,andSA,severalotherhormones,such has shown that cellular redox signaling contributes to the
asethylene,gibberellins,andbrassinosteroids,interactwithROS developmentofSAAinplantsand,insomecases,priming,which
andNOintheprocessofacclimationtostress(Bartolietal.,2013; can involve other signaling networks, such as hormones and
Xia et al., 2015). These hormones also interact with each other MAPK cascades. However, although there has been significant
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