Hydrogen peroxide priming modulates abiotic oxidative stress tolerance

19 Pages·2015·1.27 MB·English

REVIEW
published:16June2015
doi:10.3389/fpls.2015.00420
Hydrogen peroxide priming
modulates abiotic oxidative stress
tolerance: insights from ROS
detoxification and scavenging
MohammadA.Hossain1*,SoumenBhattacharjee2,Saed-MoucheshiArmin3,
PingpingQian4,WangXin5,Hong-YuLi6,DavidJ.Burritt7,MasayukiFujita8 and
Editedby:
Lam-SonP.Tran9*
ZuhuaHe,
ShanghaiInstitutesforBiological 1DepartmentofGeneticsandPlantBreeding,BangladeshAgriculturalUniversity,Mymensingh,Bangladesh,2Departmentof
Sciences–ChineseAcademy Botany,UniversityofBurdwan,Bardhaman,India,3DepartmentofCropProductionandPlantBreeding,Collegeof
ofSciences,China Agriculture,ShirazUniversity,Shiraz,Iran,4DepartmentofBiologicalScience,GraduateSchoolofScience,OsakaUniversity,
Reviewedby: Toyonaka,Japan,5SchoolofPharmacy,LanzhouUniversity,Lanzhou,China,6GansuKeyLaboratoryofBiomonitoringand
VasileiosFotopoulos, BioremediationforEnvironmentalPollution,SchoolofLifeSciences,LanzhouUniversity,Lanzhou,China,7Departmentof
CyprusUniversityofTechnology, Botany,UniversityofOtago,Dunedin,NewZealand,8LaboratoryofPlantStressResponses,FacultyofAgriculture,Kagawa
Cyprus University,Takamatsu,Japan,9SignalingPathwayResearchUnit,RIKENCenterforSustainableResourceScience,
NabilI.Elsheery, Yokohama,Japan
TantaUniversity,Egypt
*Correspondence: Plants are constantly challenged by various abiotic stresses that negatively affect
MohammadA.Hossain,
growth and productivity worldwide. During the course of their evolution, plants have
DepartmentofGeneticsandPlant
Breeding,BangladeshAgricultural developed sophisticated mechanisms to recognize external signals allowing them to
University,Mymensingh-2202, respond appropriately to environmental conditions, although thedegree of adjustability
Bangladesh
or tolerance to specific stresses differs from species to species. Overproduction of
[email protected];
Lam-SonP.Tran, reactive oxygen species (ROS; hydrogen peroxide, H2O2; superoxide, O·2−; hydroxyl
SignalingPathwayResearchUnit, · 1
radical, OH and singlet oxygen, O2)isenhanced underabiotic and/orbiotic stresses,
RIKENCenterforSustainable
ResourceScience,1-7-22 which can cause oxidative damage to plant macromolecules and cell structures,
Suehiro-cho,Yokohama230-0045, leading to inhibition of plant growth and development, or to death. Among the various
Japan
[email protected] ROS, freely diffusible and relatively long-lived H2O2 acts as a central player in stress
signal transduction pathways. These pathways can then activate multiple acclamatory
Specialtysection:
responses that reinforce resistance to various abiotic and biotic stressors. To utilize
Thisarticlewassubmittedto
PlantPhysiology, H2O2 as a signaling molecule, non-toxic levels must be maintained in a delicate
asectionofthejournal balancing act between H2O2 production and scavenging. Several recent studies
FrontiersinPlantScience
have demonstrated that the H2O2-priming can enhance abiotic stress tolerance by
Received:12March2015
modulating ROS detoxification and by regulating multiple stress-responsive pathways
Accepted:25May2015
Published:16June2015 and gene expression. Despite the importance of the H2O2-priming, little is known
Citation: about how this process improves the tolerance of plants to stress. Understanding
HossainMA,BhattacharjeeS,Armin the mechanisms of H2O2-priming-induced abiotic stress tolerance will be valuable
S-M,QianP,XinW,LiH-Y,BurrittDJ,
for identifying biotechnological strategies to improve abiotic stress tolerance in crop
FujitaMandTranL-SP(2015)
Hydrogenperoxidepriming plants.Thisreviewisanoverviewofourcurrentknowledgeofthepossiblemechanisms
modulatesabioticoxidativestress
associated with H2O2-induced abiotic oxidative stress tolerance in plants, with special
tolerance:insightsfromROS
detoxificationandscavenging. referencetoantioxidantmetabolism.
Front.PlantSci.6:420.
doi:10.3389/fpls.2015.00420 Keywords:hydrogenperoxide,abioticstress,oxidativestress,priming,stresstolerance
FrontiersinPlantScience|www.frontiersin.org 1 June2015|Volume6|Article420
Hossainetal. H2O2mediatedabioticstresstolerance
Introduction proteins called ‘ROP’s (Rho-related GTPases; Agrawal et al.,
2003).Anotherclassofenzymes,associatedwiththeformationof
In plants the production of reactive oxygen species (ROS) is a H O ,isthecellwall-associatedperoxidases(Bolwelletal.,2002).
2 2
common outcome of various metabolic reactions that occur in Rates of H O accumulation in peroxisomes and chloroplasts
2 2
multiple sites within a plant cell. ROS like hydrogen peroxide maybe30–100timeshighercomparedwithH O formationin
2 2
·− ·
(H O ),superoxide(O ),thehydroxylradical(OH)andsinglet mitochondria.Importantly,ROSformationinmitochondriadoes
2 2 2
oxygen(1O·)arealsoproducedasoneoftheearliestresponsesof not vary significantly in presence or absence of light, since the
2
plant cells to environmental stresses, and these ROS molecules total O consumption is less affected by light than TCA cycle
2
·−
cancausedamagetoavarietyofbiologicalprocesses(Halliwell, activity. However, the formation of O by electron transport
2
2006; Gill and Tuteja, 2010; Das and Roychoudhury, 2014). In systems can be influenced by light, if exposure to light affects
plantssubjectedtovariousabioticstresses,suchassalt,drought, alternative oxidase activity (Dutilleul et al., 2003). Alternative
chilling, heat and metal or metalloid stresses, ROS levels can oxidases have been found to influence ROS generation and
risesignificantly,leadingtoredoximbalanceandoxidativestress to be involved in determining cell survival under stressful
(Hossainetal.,2010;Hasanuzzamanetal.,2011a,b;Hossainand conditions (Maxwell et al., 1999; Robson and Vanlerberghe,
Fujita,2013;Mostofa andFujita,2013;DasandRoychoudhury, 2002).
2014; Mostofa et al., 2014a,b,c; Nahar et al., 2014). High ROS The antioxidant systems that regulate H O levels consist
2 2
levels can result in extensive damage to proteins, DNA, and of both non-enzymatic and enzymatic H O scavengers.
2 2
lipids,therebyaffectingnormalcellularfunctions,whichcanlead Enzymes, such as catalase (CAT), ascorbate peroxidase (APX),
to permanent metabolic dysfunction and plant death (Anjum glutathioneperoxidase(GPX),glutathioneS-transferases(GSTs),
etal.,2015).Tocombatoxidativestress,plantshavedevelopedan glutathione reductase (GR), and peroxyredoxin (Prx), and
elaboratesystemtocontrolcellularROStiter(Mittleretal.,2011). non-enzymatic compounds, like ascorbate (AsA), glutathione
Surprisingly, plants have also evolved a way to exploit lower (GSH),α-tocopherol and flavonoids, are constantly involved in
titer of ROS as signaling component to regulate wide variety regulating the concentration of ROS, including H O (Miller
2 2
of plant processes, including cell elongation, differentiation, et al., 2010; Kapoor et al., 2015). In fact, both the production
morphogenesisandresponsestoenvironmentalstress(Datetal., and scavenging of H O in plant cells seem to be integrated
2 2
2000;Foremanetal.,2003;Tsukagoshietal.,2010;Bhattacharjee, in a network and are responsible for the ‘biological effect.’ The
2012). paradox of H O physiology lieswith its opposing activities; at
2 2
In the last decade, H O received considerable interest higher concentrations it causes oxidative damage to important
2 2
among the ROS and other oxygen-derived free radicals. H O , cellularmetabolites, whereasatlower concentrations it initiates
2 2
·−
the result of two electron reduction via O (the first step cellsignaling(GechevandHille,2005;Bhattacharjee,2012).The
2
one electron reduction component), possesses the highest half- redoximbalanceassociatedwithenvironmentalstresses,suchas
life (1 ms) of the ROS. A comparatively long life span and salinity and extremes of temperature, increases the overall rate
the small size of H O molecules permit them to traverse ofmetabolism andeventuallyup-regulatesH O production in
2 2 2 2
through cellularmembranestodifferentcellularcompartments, plantcells(Bhattacharjee,2012,2013).
facilitating signaling functions, including retrograde signaling Priming (pre-treatment of seeds or plants by exposure to
(Apel and Hirt, 2004; Bienert et al., 2006; Maruta et al., stressor or chemical compounds, making them more tolerant
2012; Noctor et al., 2014). The signaling role of H O is well to later stress events) is potentially an important mechanism
2 2
established, particularly with reference to plant processes like of induced resistance in plants against biotic stresses (Beckers
stress acclimation, antioxidative defense,cell wall cross-linking, and Conrath, 2007; Tanou et al., 2012; Borges et al., 2014).
stomatalbehavior,phytoalexinproduction,regulationofthecell Recent studies have shown that priming can also modulate
cycle, and photosynthesis. So, the toxicity or danger associated abioticstresstolerance(Filippouetal.,2012;HossainandFujita,
with H O on one hand and signaling cascades on other 2013; Mostofa and Fujita, 2013; Borges et al., 2014; Mostofa
2 2
make it a versatile molecule whose concentration needs to be etal.,2014a,b,c;Naharetal.,2014;Wangetal.,2014a).Despite
tightlycontrolledwithinplantcells(PetrovandVanBreusegem, the agronomic and ecological importance of priming, in-depth
2012). molecularmechanismsassociatedwithpriminginplantsarestill
There are multiple sources of H O in plant cells, including unknown(Conrath,2011).Mountingevidencesuggeststhatthe
2 2
over-energization of electron transport chains (ETC) or redox initialexposuretochemicalprimingagents(suchasH O ,ABA,
2 2
reactions in chloroplasts or mitochondria, fatty acid oxidation, NO, SA etc.) renders plants more tolerant to abiotic stresses
and photorespiration (Figure 1). Of these sources, the most (Wang et al., 2010a; Hasanuzzaman et al., 2011a; Mostofa and
significant is oxidation of glycolate in the peroxisome during Fujita, 2013; Mostofa et al., 2014a; Sathiyaraj et al., 2014; Teng
the photosynthetic carbon oxidation cycle. In addition, the etal., 2014).A numberof studieson plants havedemonstrated
oxidative burst associated with part of hypersensitive response that the pre-treatment with an appropriate level of H O can
2 2
to pathogens also cause rapid increase in the concentration of enhance abiotic stress tolerance through the modulation of
H O (Milleretal.,2010).OneofthemainsourcesofH O isa multiple physiological processes, such as photosynthesis, and
2 2 2 2
classofcellmembrane-boundNADPH-dependentoxidasesthat by modulating multiple stress-responsive pathways, such asthe
aresimilartotherespiratoryburstoxidasehomologs(RBOH).In ROSandmethylglyoxal(MG)detoxificationpathways(Azevedo-
plants,RBOHareinfactenzymesregulatedbyaclassofRho-like Neto et al., 2005; Chao et al., 2009; Liu et al., 2010a; Xu et al.,
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Hossainetal. H2O2mediatedabioticstresstolerance
FIGURE1|SchematicrepresentationofH2O2generationindifferentintra-andextra-cellularsitesandthesubsequentsignalingassociatedwiththe
regulationofdefensegeneexpressioninplantcells.
2010; Wang et al., 2010a, 2014a; Ishibashi et al., 2011; Gondim tolerance to salinity, drought, chilling, and high temperatures,
etal.,2012,2013;Hossainand Fujita, 2013).Although H O is and heavy metal stress, all of which cause elevated H O
2 2 2 2
knowntoactasasignalingmolecule,activatingmultipledefense production(Gongetal.,2001;Uchidaetal.,2002;Azevedo-Neto
responses that reinforce resistance to various environmental etal.,2005;Chaoetal.,2009;Liuetal.,2010a;Wangetal.,2010a,
stresses in plants (Petrov and Van Breusegem,2012), very little 2014a;Ishibashietal.,2011;Gondimetal.,2012,2013;Hossain
isknownaboutthemechanismsbywhichplantsperceive/sense andFujita,2013).
H O andhowthissensingmechanismiscoordinatedwithinthe
2 2
developmentalprogramofaplant.Inthisreview,wesummarize Exogenous H O andSaltStressTolerance
2 2
ourcurrentunderstandingofthepossiblemechanismsassociated Thesaltstress-inducedoxidativeburstduetouncontrolledROS
withH2O2-inducedenhancedabioticstresstolerancewithspecial accumulation has been well documented in plants. However,
reference to ROS detoxifying/scavenging proteins and gene several recent studies on plants have demonstrated that pre-
expression. treatment with exogenous H O can induce salt tolerance.
2 2
Uchida et al. (2002) studied the effects of H O and nitric
2 2
oxide (NO) pre-treatments on oxidative stress in rice (Oryza
Exogenous H O and Abiotic Stress
2 2 sativa)plantsundersaltorheatstress.Theirresultsshowedthat
Tolerance seedlingstreatedwithlowconcentrations(<10μM)ofH O or
2 2
NOresultedingreenerleavesandahigherphotosyntheticactivity
Many recent studies on plants have demonstrated that H O than that of the control plants under conditions of salt or heat
2 2
is a key player in the signal transduction process associated stress. It was also shown that pre-treatment induced increases
with tolerance to abiotic and biotic stresses, and the induction in ROS scavenging enzyme activities and increased expression
of the stress cross-tolerance phenomena often observed in ofgenesencoding(cid:2)(cid:2)-pyrroline-5-carboxylate synthase,sucrose-
plants. A number of reports, discussed in more detail below, phosphate synthase, and the small heat shock protein 26.
have shown that exogenous application of H O can induce Their findings indicate that NO and H O act as signaling
2 2 2 2
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Hossainetal. H2O2mediatedabioticstresstolerance
molecules that modulate heat and salt stress tolerance by et al., 2012). Up-regulation of the activities of CAT and SOD
regulating the expression of stress-related genes. In addition, following the exogenous application of H O (0.5 mM) was
2 2
Azevedo-Neto et al. (2005) found that supplementation of the also observed in oat (Avena sativa) plants under salt stress (Xu
nutrient solution with H O induced salt tolerance in maize et al., 2008). Similarly, Gondim et al. (2012) found that foliar
2 2
plants,byenhancingantioxidantmetabolismandreducinglipid H O priming was effective in minimizing salt stress in maize
2 2
peroxidation in both leaves and roots. Wahid et al. (2007) and analysis of the antioxidant enzymes CAT, GPOX, APX,
reported that exogenous H O improved salinity tolerance and SOD revealed that the H O foliar spray increased the
2 2 2 2
in Triticum aestivum when seeds were soaked in H O (1– activitiesofalloftheseenzymes.CATwasfoundtobethemost
2 2
120 μM, 8 h) and subsequently grown in saline conditions highly responsive of the above enzymes to H O , with high
2 2
(150mMNaCl).H O levelsintheseedlings,arisingfromH O - activities observed (48 h) after treatment, while GPX and APX
2 2 2 2
treated seeds, were markedly lower when grown under saline respondedmuchlater(240haftertreatment).LowerMDAlevels
conditions than control seedlings from seeds not treated with were also detected in maize plants with higher CAT activities,
H O , and also exhibited better photosynthetic capacity. These which may have resulted from the H O detoxifying function
2 2 2 2
resultssuggestthatseedlingsfromH O -treatedseedshadmore of this enzyme. In addition, Gondim et al. (2013) studied the
2 2
effective antioxidant systems than found in untreated controls. influence of exogenous H O application on AsA and GSH
2 2
Moreover, the H O treatment appeared to improve leaf water metabolism, relative chlorophyll content, relative water content
2 2
+ +
relations,helpedtomaintainturgor,andimprovedtheK :Na (RWC), and gas exchange, in Zea mays grown under salinity.
ratio of salt stressed seedlings. H O treatment also enhanced Photosynthesis and transpiration, stomatal conductance, and
2 2
membrane properties, with greatly reduced relative membrane intercellularCO concentrationsalldeclinedinplantsundersalt
2
permeability (RMP) and lower ion leakage. Surprisingly, the stress;however,thenegativeimpactofsaltstresswasnotasgreat
expression of two heat-stable proteins (32 and 52 kDa) was inplantssprayed with H O .Inaddition,H O -sprayed plants
2 2 2 2
also observed in H O pre-treated seedlings. Fedina et al. had higher RWCs, relative chlorophyll contents and lower leaf
2 2
(2009)reportedthatHordeumvulgareseedlingspre-treatedwith H O accumulation, which correlated positively with improved
2 2
H O (1 and 5 mM) had higher rates of CO fixation and gas exchange, compared with control plants under conditions
2 2 2
lower malondialdehyde (MDA) and H O contents, following of NaCl stress. The non-enzymatic antioxidants AsA and GSH
2 2
exposure to 150 mM NaCl for 4 and 7 days, when compared did not appear to play any obvious roles as ROS scavengers
with seedlings subjected to NaCl stress only. In addition, the in this study. The authors of the above study concluded that
−
leaf Cl content of NaCl treated plants was considerably less salt tolerance of maize plants, brought by pretreatment of
in H O pre-treated plants. The above findings indicate that leaves with H O , was due to less H O accumulation and to
2 2 2 2 2 2
H O metabolism might be important for the induction of salt maintenance of the leaf RWC and chlorophyll contents. These
2 2
tolerance. characteristics allowed higher photosynthesis and improved
Gondim et al. (2010) evaluated the roles of H O on the growth of maize plants under salt stress. In addition to these
2 2
growth and acclimation of maize (Zea mays) triple hybrid findings, Ashfaque et al. (2014) conducted an experiment to
(BRS3003) seedlings exposed to salinity stress, with three study the role H O played in mitigating salt stress in wheat
2 2
consecutive studies. In the first studies, H O accelerated the (Triticum aestivum L.) plants. Treatment of plants with H O
2 2 2 2
percentagegerminationofseedsat100mM,butnotat500mM positively influenced plant growth under saline and non-saline
H O .Insecondstudy,pre-treatmentofseedswithH O caused conditions.Theapplicationof50or100μMH O reducedthe
2 2 2 2 2 2
+ −
an up-regulation of APX and CAT activities after 30 h. In severityofsaltstress,withreductionsinbothNa andCl ion
contrast, GPX activity was lower in seeds primed with H O levelsand anincrease inproline content andinN assimilation.
2 2
for 12, 24, 30, 36, and 42 h as compared with the seeds Improved water relations, increased levels of photosynthetic
primed with water only. The activity of SOD was not affected pigments and greater growth rateswere alsoobserved in H O
2 2
by pre-treatment of seeds with H O , except for the 24 h pre- under salt stress when compared with untreated plants. Under
2 2
treatment. In the third experiment, seeds were pre-treated by non-saline conditions application of H O also improved all
2 2
soaking in 100 mM H O for 36 h, or in distilled water (DW), the parameters detailed above. Treatment with 100 μM H O
2 2 2 2
and then grown in a culture solution with or without salt providedmaximalprotectionforwheatplantsgrownundernon-
stress (80 mM NaCl). Their findings showed that priming of saline conditions and also alleviated the effects of salt stress
seeds with H O increased seedling tolerance to salinity, with in plants grown under saline conditions. Recently, Sathiyaraj
2 2
seedlingsdemonstratingimprovedgrowthrates.Thedifferences et al. (2014) found that Panax ginseng seedlings treated with
inthelevelsofantioxidantenzymeactivitiesdetailedabovemay 100μMH O for2daysshowedenhancedsalinitytoleranceand
2 2
explain the higher salinity tolerance of seedlings from seeds increasedactivitiesofAPX,CAT,andguaiacolperoxidase.Other
pre-treated with H O . In addition, Li et al. (2011) reported oxidativeparameterssuchasMDAlevelsandendogenousH O
2 2 2 2
that exogenously applied H O (0.05 μM) reduced the MDA andO·−levelswerelowerinH O treatedsalt-stressedseedlings.
2 2 2 2 2
content, enhancedtheGSHcontentandincreased theactivities Seedling dry weight, and chlorophyll and carotenoid contents
ofAPX,CAT,SOD,andPODinwheatseedlingsundersaltstress. were also greater in H O treated seedlings than in untreated
2 2
A similar response in Suaeda fruticosa (a halophyte) was also controls,whenseedlingsweresubjectedtosaltstress.Theabove
found, indicating that cellular defense antioxidant mechanisms findings demonstrate that H O priming can induce tolerance
2 2
are enhanced by the exogenous application of H O (Hameed to salinity in plants by modulating physiological and metabolic
2 2
FrontiersinPlantScience|www.frontiersin.org 4 June2015|Volume6|Article420
Hossainetal. H2O2mediatedabioticstresstolerance
processes such as photosynthesis, proline accumulation and reducing starch contents. These findings demonstrate that NO
ROS detoxification, and that this ultimately leads to better orH O canprotectmesophyllcellultrastructurefromdamage,
2 2
growth and development. Importantly, ROS metabolism also improve the photosynthetic performance of leavesandmitigate
playsapivotalroleinthedevelopmentofstressandcrossstress the negative effects of drought stress, by enhancing nitrogen
tolerance. and carbohydrate accumulation. Recently, Hossain and Fujita
(2013) examined the potential biochemical mechanisms of
Exogenous H2O2 andDrought Stress H2O2 priming-induced drought tolerance in mustard (Brassica
Tolerance juncea L.) seedlings by investigating ROS scavenging and MG
Drought stress is widely thought to induce oxidative stress metabolism. Eight-day-old seedlings were pre-treated with a
by increasing the levels of H O and singlet oxygen (de low concentration (50 μM) of H O for 24 h prior to the
2 2 2 2
Carvalho, 2013). However, Jing et al. (2009) investigated the imposition of drought stressfor 48 h. H O priming enhanced
2 2
capacity of H O priming to promote drought tolerance in cell membrane stability in leaf tissues under drought stress, by
2 2
Cucumber plants. Drought stress resulted in cucumber plants reducingtissueMDAcontents.ThelevelsofendogenousH O ,
2 2
with round chloroplasts, and indistinct chloroplast membranes in H O pre-treated, drought stressed-seedlings were markedly
2 2
andthylakoids.WhileH O primingdidnotchangechloroplast lowerthanthatofseedlingssubjectedtodroughtstresswithout
2 2
ultrastructure, priming did increase the activities of the H O pre-treatment. Lower activities of APX, CAT, and Gly II
2 2
antioxidant enzymes SOD, CAT, GPOX, APX, DHAR, DHAR, were observed in response to drought stress, whereas DHAR,
GR, and the levels of AsA and GSH, resulting in lower levels GPX, and Gly I activities significantly increased. AsA, GSH,
·−
of MDA, H O and O . The authors of this study concluded andGSSGlevelsincreasedsignificantly,whereastheGSH/GSSG
2 2 2
that by increasing antioxidant capacity H O priming reduced ratiodecreasedindrought-stressedseedlings.Surprisingly,H O
2 2 2 2
theaccumulationofROSintreatedplants,andalleviatedsomeof pre-treated drought-stressed seedlings maintained significantly
themembranedamagefoundinthechloroplastsofplantsunder higher APX, GR, CAT, GST, and Gly II activities, as well as a
drought stress.InasimilarstudyIshibashietal.(2011)showed higherGSH/GSSGratiocomparedwithseedlingsunderdrought
that spraying plants with H O could alleviate the symptoms only. These results show that H O priming can activate both
2 2 2 2
ofdroughtstressinsoybean.TheRWCcontent,photosynthetic ROSandMGdetoxificationpathwaysandmodulatethetolerance
rate and stomatal conductance of drought-stressed leaves in of seedlings to water deficit (Hossain and Fujita, 2013). Ashraf
plantssprayedwithH O wereallhigherthaninleavessprayed etal.(2014)investigatedthebeneficialrolesofexogenousH O
2 2 2 2
with DW. In contrast to spraying with DW, spraying with ondroughtstresstolerance inmaize.Maizeseedlingswerepre-
H O causedanincreaseintheexpressionofgalactinolsynthase treatedwithdifferentconcentrations ofH O andgrownunder
2 2 2 2
(GolS) and d-myo-inositol 3-phosphate synthase 2 (GmMIPS2) conditionsofwaterstress.Highergerminationpercentageswere
genes,whichareresponsibleforthesynthesisofoligosaccharides. foundinseedssoakedin140mMH O .Droughtledtoasharp
2 2
These findings indicated that H O spraying enabled soybean decreaseinphotosyntheticpigments,whereasthelevelsofH O ,
2 2 2 2
plants to avoid drought stress by helping to maintain leaf lipid peroxidation and AsA increased. The activities of CAT,
water levels, and that leaf water retention was probably due to SOD,andPOXrapidlyincreased.Importantly,the140mMH O
2 2
increasedoligosaccharidebiosynthesisratherthanrapidstomatal treatmentreducedphotosyntheticpigmentdegradationandlipid
closure. Abass and Mohamed (2011) also studied the effects of peroxidationandincreasedtheactivitiesofantioxidantenzymes
priming seedswithH O onthe droughttolerance ofcommon and AsA levels. The beneficial influence of exogenous H O
2 2 2 2
bean seedlings (Phaseolus vulgaris L.). A significant decrease treatments have also been observed in plants under osmotic
in plant growth parameters, photosynthetic pigments, and the stress. Liu et al. (2010a) studied the effects of exogenous H O
2 2
totalcarbohydrate contentwasobservedinresponsetodrought on osmotic stress-induced alterations in the ultra-structures of
stress. In contrast, a significant increase in compatible solutes, chloroplastsandmitochondriaintwocucumber(Cucumissativus
polyamine and antioxidant levels, and abscisic acid (ABA) L.)varieties.Osmoticstresscausedthedegradationofchloroplast
contents were observed in plants in response to drought stress. andmitochondrialmembranesinbothcucumbergenotypesand
H O -primingofseedsenhancedalloftheaboveparametersin increased MDA levels. Osmotic stress and exogenous H O
2 2 2 2
seedlingsgrownunderdroughtconditionswhencomparedwith bothincreasedMnSOD,GPX,CAT,GPOX,APX,GR,MDHAR,
the seedlingsofwater-treated seeds.Theabovefindingssuggest DAHR activities and levels of the antioxidants AsA and GSH.
that H O could trigger the activation of defense mechanisms, The combined effects of osmotic stress and exogenous H O
2 2 2 2
including increasedlevelsofantioxidants, whichthenpersist in resulted in the highest antioxidant levels in both cucumber
developing seedlingsand helpto alleviatedamageand improve ecotypes. Liu et al. (2010a) proposed that pre-treatment with
plantgrowthandperformanceunderdrought.Liaoetal.(2012) H O increased antioxidant levels in the leaves of cucumbers,
2 2
studied the beneficial roles of exogenous NO and H O in therebydecreasingMDAlevels,andprotectingtheultrastructure
2 2
marigold (Tagetes erecta L.) adventitious root formation in ofmostchloroplasts andmitochondria inplantsunderosmotic
responsetodrought.NOorH O treatmentreducedthedamage stress. Terzi et al. (2014) also found that exogenous H O
2 2 2 2
to mesophyll cell ultrastructure caused by drought stress. NO (10 mM) pre-treatment induced osmotic stress tolerance in
or H O treatment also increased leaf chlorophyll contents, maize (Zea mays L.) seedlings. H O treatment caused a
2 2 2 2
chlorophyll fluorescence parameters (Fv/Fm, (cid:3)PS II, and qP), decrease in MDA levels and stomatal conductance, whereas
andhypocotyl solublecarbohydrate andproteincontents,while an increase in endogenous H O , leaf water potential, ABA
2 2
FrontiersinPlantScience|www.frontiersin.org 5 June2015|Volume6|Article420
Hossainetal. H2O2mediatedabioticstresstolerance
concentration, and metabolite levels, including soluble sugars, withH O inmodulatingchillingstresstoleranceofmascarene
2 2
proline, and polyamines, were observed. Osmotic stress caused grass(Zoysiatenuifolia)andmanilagrass(Zoysiamatrella).Pre-
a decline in leaf water potential and stomatal conductance, treatment with H O (10 mM) was found to modulate chilling
2 2
◦ ◦
but the levels of MDA, H O , metabolite levels and the (7 C/2 C, day/night) stress tolerance as indicated by lower
2 2
ABAcontentincreased.Importantly,H O pretreatedosmotical MDAand ELlevelsand higher protein contents. Pre-treatment
2 2
stressed seedlings showed improved water status and stomatal significantly increased the activities of APX, GPX, and CAT in
conductance,aswellasaccumulationofMDA,H O ,ABA,and Zoysia matrella and APX, GR, and POD activities in Zoysia
2 2
metabolites.TheseresultsdemonstratethatH O pre-treatment tenuifolia, indicating that H O acts as a signaling molecule
2 2 2 2
induces osmotic stress tolerance by increasing soluble sugar, and modulates the metabolic responses associated with ROS-
proline,andpolyaminelevels. induced damage caused by chilling. Importantly, optimal pre-
treatments reduced any increases in H O levels, improved
2 2
Exogenous H2O2 andChillingStressTolerance chilling tolerance, and increased CAT, POD, APX, GR, and
The positive role of exogenous H O in modulating low GPX activities. Therefore, antioxidative enzymes are likely to
2 2
temperature stress tolerance has been well documented. Prasad be important factors for the acquisition of chilling tolerance
et al. (1994a,b) reported that addition of H O modulated in both Zoysia cultivars. Moussa and Mohamed (2011) showed
2 2
chillingtolerance,duetoatransientincreaseinH O -activated that priming of pea seeds with H O or NO significantly
2 2 2 2
acclimation mechanisms. The authors suggested that H O has enhanced drought induced oxidative stress tolerance. Seeds
2 2
dual effects on maize plants during acclimation to chilling; it were pre-treated with 70 mM H O or 10 μM sodium
2 2
serves as a signal to induce the synthesis of ROS-scavenging nitroprusside (a NO donor). Seeds pre-treated pea seedlings
enzymes, and in non-acclimated seedlings it accumulates to havelessROS-induceddamage,acceleratedprolinesynthesisand
higher levels and acts as a destructive agent. Additionally, enhanced total chlorophyll and carotenoid contents, increased
it was reported that both H O and SA could mediate the photosynthetic activity, and increased growth when subjected
2 2
induction of protective mechanisms againstabiotic stresses.SA to osmotic stress. Drought stressreduced the activities of APX,
·−
pre-treatmentinducedanincrease inH O concentrations that GPX, and CAT, and caused an overproduction of O in the
2 2 2
in turn triggers an increase in antioxidant enzyme activities leaves of pea plants, which in turn increased MDA levels
and eventually leads to higher tolerance to chilling stress in and reduced photosynthetic performance. Pre-treatment with
maize seedlings (Janda et al., 1999). Likewise, H O and SA SNP or H O modulated the activities of antioxidant enzymes,
2 2 2 2
·−
were involved in the signal transduction pathway leading to limited O production, and inhibited membrane peroxidation
2
acclimation during heatstressinmustard (Dat etal.,1998).Yu underdroughtstress,whichindicatedanenhancedoperationof
et al. (2003) showed that a transient oxidative shock, induced antioxidantsystems.Moreover,afterH O orSNPpre-treatment
2 2
by exogenous H2O2, effectively increased chilling tolerance in seedlingshadenhancedmembranestabilityasrevealedbyalower
mung bean (Vigna radiata L. cv. V3327) seedlings. Seedlings MDA contents. The increased production of antioxidants in
pre-treated with 200 mM H O had increased survival rates seedlingsfromseedspre-treatedwithH O orSNPpersistedfor
2 2 2 2
(from 30 to 70%) and lowered EL (86 to 21%). Importantly, sometime,alleviatingROS-inducedimpairmentandmodulating
the endogenous level of H O was not affected by exogenous the physiological characters associated with drought tolerance
2 2
applicationofH O .Surprisingly,exogenousH O repressedthe of seedlings. Recently, ˙I¸seri et al. (2013) investigated whether
2 2 2 2
stimulationofROSdetoxifyingenzymesAPXandCAT;however, exogenous H O application could influence the short-term
2 2
GSHlevelsincreasedsignificantlyunderbothchillingandcontrol cold responses of tomato plants and induce acclimation. Pre-
conditions. Pre-treatment of mung bean plants with both ABA treatments were performed by immersing roots into 1 mM
and H O showed no synergistic effect on GSH content. The H O solution for 1 h and then transferring the seedlings
2 2 2 2
◦
authors concluded that H O -mediated chilling tolerance in to the soil (acclimated group). Cold stress (3 C for 16 h)
2 2
mung bean plants might be mediated by an increase in GSH caused a significant reduction in the RWC of control and
content that is independent of ABA. Supporting this finding, non-acclimated groups when compared with unstressed plants.
Hung et al. (2007) showed that H O pre-treatment induced H O promotedthemaintenanceofahigherRWCunderstress.
2 2 2 2
chilling tolerance in chilling sensitive mung bean seedlings Anthocyaninlevelsintheleavesofacclimatedplantsundercold
(V. radiata L. Cv Tainan Number 5). Seedlings pre-treated stressweresignificantly higher thanthose ofunstressedcontrol
◦
with 200 mM H O or cold-acclimated (10 C for 48 h in andnon-acclimatedplants.HighMDAlevelsdemonstratedlow
2 2
the light) showed lower electrolyte leakage (EL) compared to temperature induced oxidative damage in control and non-
◦
seedlings subjected to chilling stress (4 C for 36 h) without acclimated plants. MDA levels in acclimated plants remained
H O treatmentorcold-acclimation.Chillingtoleranceinduced similar to those of unstressed plants, which demonstrated
2 2
by H O appeared to depend on the accumulation of GSH, as that the H O acclimation process protected the cells against
2 2 2 2
tolerance could be reversed by pretreatment with buthionine cold induced lipid peroxidation. In addition, H O acclimation
2 2
sulfoximine (BSO). In contrast, tolerance induced by cold- caused proline accumulation in roots under cold stress and
acclimation was neither accompanied by the accumulation of APX activity in the roots of cold-stressed and -unstressed
GSH nor reversed by BSO, suggesting that there are at least H O -acclimatedplantsincreasedwhencomparedwithcontrol
2 2
two independentmechanismsfor developingchilling tolerance. and non-acclimated plants, with the highest increase in the
Wang et al. (2010a) studied the effects of foliar pre-treatment roots of acclimated plants under cold stress. CAT levels in
FrontiersinPlantScience|www.frontiersin.org 6 June2015|Volume6|Article420
Hossainetal. H2O2mediatedabioticstresstolerance
the roots of acclimated plants also increased, whereas levels antioxidant defense systems that ultimately lead to improved
remained unchanged in unstressed plants. Endogenous H O thermotoleranceinturfgrassspecies.
2 2
levels increased significantly in the roots of control and non-
acclimatedplantsundercoldstress.Incontrast,theH2O2content Exogenous H2O2 andHeavyMetal Stress
oftherootsofacclimatedplantswassignificantlylowerthanthat Excessive production of ROS, especially H O , in response
2 2
of control and non-acclimated plants under cold stress. These to heavy metal exposure has been widely observed in plants
results demonstrate that H O significantly enhances oxidative (Hossain et al., 2010; Mostofa and Fujita, 2013; Mostofa et al.,
2 2
stress responses by elevating the antioxidant status of tomato 2014a).H O priminghasalsobeenfoundtoincreasetolerance
2 2
plants. of plants to heavy metals. Hu et al. (2009) showed H O pre-
2 2
treatmentinducedCdtoleranceinrice(O.sativa).Cdstressled
Exogenous H2O2 andHeat Tolerance toasignificantdecreaseinboththelengthandbiomassofroots
Like other abiotic stresses endogenous levels of H O increase andshoots.However,pre-treatmentwith100μMH O for1day
2 2 2 2
in heat stressed plants (Hossain et al., 2013a,b; Mostofa et al., mitigated Cd stress and increased the levels of the antioxidant
2014b) and exogenous pre-treatments haves been found to enzymes (SOD, CAT,GPX,APX,and GST), as well as elevated
increasetheheattoleranceofplants.Kangetal.(2009)reported thelevelsofGSHandAsA.Consequently,thelevelsofMDAand
that H O pre-treatments increased the activities of APX and H O declined andthe growth ratesofthe seedlingsimproved.
2 2 2 2
glucose-6-phosphated dehydrogenase (G6PDH) in cucumber H O pre-treatmentalsodecreasedtheCdconcentrationfound
2 2
and tomato seedlings, and induced tolerance to heat stress. intheshoots,andloweredtheshoot:rootCdratio,indicatingthat
Bhattacharjee (2012) reported that H O enhanced tolerance H O mayaffectCddistributioninriceseedlings.ImprovedCd
2 2 2 2
of two rice cultivars differing in salt tolerance (SR 26B, salt- tolerance wasthought tobepartlyduetoenhancedantioxidant
tolerant; Ratna, salt-sensitive cultivar) to heat- or chilling- metabolismthateffectivelypreventedtheincreaseinROSlevels
induced oxidative stress. Salt or drought stress results in under Cd stress. Higher Cd sequestration in root tissue may
significantincreasesinlipidperoxidationandproteinoxidation, also contribute to the decline in Cd translocation. Chao et al.
along with concomitant increases in the accumulation of ROS (2009) investigated the role of GSHin modulating heavymetal
·−
(O andH O )andareductioninantioxidantdefenses(assessed (Cd) stress tolerance of rice seedlings. Seedlings treated with
2 2 2
in terms of total thiol content and the activities of SOD, either a heat shock or H O showed a significant increase in
2 2
CAT, APX, and GR) in both the seedlings of salt-sensitive leaf GSH levels. Treatment with exogenous GSH under non-
Ratnaandsalt-tolerantSR26Bcultivars.Imbibitionaltreatment heat stress conditions, which also resulted in an increase in
with low concentrations of H O reduced oxidative damage GSH levels in leaves, also enhanced the Cd tolerance of rice
2 2
to newly assembled membrane systems caused by heat and seedlings. Pre-treatment of seedlings with an inhibitor of GSH
chilling stress in the seedlings of both cultivars of rice (Ratna synthesis inhibited the increase in GSH levels caused by heat
and SR 26B). Imbibitional H O treatment also caused an shock orH O treatmentandcaused areduction Cdtolerance.
2 2 2 2
increase in antioxidant defenses (activities of SOD, CAT, APX, Importantly, the negative effects of BSO could be reversed by
GR, and total thiol content) in the heat and chilling stressed the addition of GSH. A time-course analyses of heat stress
seedlings and caused a significant improvement in the early in rice seedlings demonstrated that the accumulation of H O
2 2
growthperformancesofbothcultivars.BetterresponsestoH O - preceded the increase in GSH. This finding suggests that early
2 2
mediated acclamatory performances and restoration of redox- augmentationofH O levelsduringheatshocktreatmentsactsas
2 2
homeostasis under extremes of temperature were noted for asignaltomodulateGSHbiosynthesisandtoprotectriceplants
the salt-sensitive rice cultivar Ratna compared with the salt- from Cd-induced damage. Xuet al. (2010) reported the H O -
2 2
tolerant SR 26B. In general, these results suggest a significant inducedup-regulationofAsAandGSHmetabolism-inducedAl
·−
role for an ‘inductive pulse’ of H O in acclimatizing plants to toleranceinwheatseedlings.AlstressresultedinhigherO and
2 2 2
adversetemperatures,byhelpingtomaintainredox-homeostasis H O contents,greaterMDAlevels,programmedcelldeath,and
2 2
and mitigating oxidative membrane, protein and lipid damage inhibited root growth in both rice genotypes. The activities of
duringtherecoveryphaseofthepost-germinationevent.Wang CAT,POD,SOD,GPX,GR,MDHAR,andDHARandthelevels
etal.(2014a)studiedthe beneficialrolesof exogenousH O in of GSH and AsA increased in response to Al stress. However,
2 2
modulating heat stress tolerance in turfgrass species. Ryegrass H O -primed Al-stressed seedlings showed higher activities of
2 2
(Loliumperennecv.Accent)andtallfescue(Festucaarundinacea GPX, CAT, POD, MDHAR, DHAR, and GR, and higher AsA
cv. Barlexas)were sprayed with 10 mMH O before they were and GSH contents as well as a more favorable redox state than
2 2
◦
exposed to heatstress(38/30 C,day/night) and compared with seedlings subjected to Al-stress only. Notably, a large increase
◦
plantsmaintainedatcontroltemperatures(26/15 C,day/night). in ROS detoxifying enzyme activities was observed in the Al-
Before being subjected to heat stress seedlings treated with susceptible genotype as compared with the resistant genotype.
H O were found to have increased activities of POD, CAT, H O pre-treatment increased the tolerance of plants to Al-
2 2 2 2
APX, GR, and GPX, as well as larger AsA and GSH pools. induced oxidative stress by increasing level of GSH and AsA
Importantly theratioofGSH/GSSGwasalsolower.Underheat and the activities of enzymes involved in their metabolism.
stressH O pre-treatedseedlingsshowedloweroxidativedamage Bai et al. (2011) studied the effects of H O pretreatment on
2 2 2 2
andH O levels,andincreasedactivitiesofAPX,GR,GST,and Cd tolerance and translocation by utilizing two rice genotypes
2 2
GPX. These results indicated that H O could up-regulate the (N07-6 and N07-63) with contrasting Cd tolerance. Cd stress
2 2
FrontiersinPlantScience|www.frontiersin.org 7 June2015|Volume6|Article420
Hossainetal. H2O2mediatedabioticstresstolerance
(50μmol/L)ledasharpdeclineinseedlinggrowthandincreased stressalonegroup.ThesefindingssuggestthatexogenousH O
2 2
productionofMDA,GSH,NPT,andphytochelatins(PCs),aswell canincreasedrymatterproductionandmineraliondistribution
asGSTactivity.H O pre-treatmentfurtherimprovedCdstress in maize seedlings. Additionally, osmotic regulation might be
2 2
tolerancebyincreasingthelevelsofNPT,GSH,andPCs,andthe involvedinthealleviationofCutoxicity ofmaizeleavescaused
activity of GST in roots. The increase wasgreater in N07-63 as bypre-treatmentofH O .
2 2
comparedwiththeN07-6.
Chou et al. (2012) investigated the involvement of H2O2 in ExogenousH2O2 andMultipleStressTolerance
heat-shock induced Cdtolerance, in relation tothe activities of The possible involvement of H2O2 in heat-induced cross-
ROS detoxifying enzymes (APX and GR) in rice plants. Heat- adaptation to salinity, drought, chilling and heat stress was
shocktreatmentsincreasedthecontentofH O beforeincreases studiedbyGongetal.(2001)intwocultivarsofmaizediffering
2 2
◦
in the activities of APX and GR were observed in rice leaves. intheirstresstolerance.Aheat-shockpre-treatment(42 C,4h)
◦
Importantly, heat-shock induced OsAPX2 gene expression was following 4-h recovery at a temperature of 28 C considerably
associated with heat shock induced increases in APX activity. enhanced the survival of seedlings, reduced the leakage of
UponimpositionofCdstresstheH O contentandtheactivities electrolyte from the roots and the loss of coleoptile vitality
2 2
of APX and GR increased, but the increase was less than that of seedlings after stress was imposed. Importantly, the heat-
observed in seedlings subjected to Cd-stress without heat pre- shock pre-treatment produced an H2O2 peak in the maize
treatment. The authors concluded that H2O2 is involved in seedlings. The accumulation of H2O2 lead to the generation
the regulation of heat shock and Cd-induced increases in the of cross-tolerance, indicating that an early short-lived increase
activitiesofGRandAPXinriceleaves,andthuscross-tolerance in endogenous H2O2 is essential for the induction of cross-
inriceplants.Yildizetal.(2013)studiedtheamelioratingeffects adaptation,bytriggeringincreasedexpressionofgenesencoding
of H O on chromium (Cr) toxicity in canola (B. napus L.) ROS detoxification and therefore increasing the activities of
2 2
plantsinrelationtothiolcontent,lipidperoxidation,antioxidant antioxidantenzymes.HenceH2O2couldhaveasignalingrolein
enzyme activities and the growth and chlorophyll content, as inducingcross-toleranceinmaizeseedlings.
well as the levels of a metallothionein protein (BnMP1). Cr
stress (50 μM) significantly reduced plant growth, which was
H O -Induced Abiotic Stress Tolerance
accompanied by increased lipid peroxidation and a decrease 2 2
and a Possible Biochemical and
in the chlorophyll content of the leaves. H O pre-treatment,
2 2
enhanced plant growth reduced the MDA levelsand promoted Molecular Basis
higher levels of pigments. Additionally, the accumulation of
Cr was higher in the aerial parts of the H O -pretreated Recent studies have shown that H O originating in the same
2 2 2 2
seedlings. Increased thiol levels were observed under Cr stress cell can induce two dissimilar kinds of responses: one that
and were further enhanced by H O pre-treatment. POD and depends on the subcellular sites of H O production and the
2 2 2 2
SOD activities increased in response to Cr stress, whereas the otherintegrating H O signalsindependentlyofthe subcellular
2 2
activitiesofCATandAPXdecreased.H O pre-treatmentcaused site of production. Several studies have shown that H O is a
2 2 2 2
an increase in the activities of APX and POD in response to part of the retrogate signaling mechanism, from mitochondria
Cr stress, but CAT and SOD activities remained unchanged. or chloroplasts that activate the expression of nuclear-encoded
BnMP1 expression analysis showed enhanced expression after stress-responsive genes. The aforementioned evidence clearly
1 day of treatment, and then a decrease after 7 days exposure demonstrates that H O priming modulates abiotic stress
2 2
to Cr. In contrast, after 7 days exposure to Cr, H O -pre- tolerance in plants by modulating ROS and MG detoxification
2 2
treated seedlings showed a less decrease in BnMP1 expression and scavenging(Gongetal.,2001;Uchida etal., 2002;Yu etal.,
as compared with the seedlings subjected to Cr stress only. 2003; Xu et al., 2008, 2010; Hu et al., 2009; Jing et al., 2009;
These findings indicate that H O may act as a signal that Gondimetal.,2010,2012;Liuetal.,2010a;Wangetal.,2010a;Li
2 2
triggers defense mechanisms that in turn protect plants from etal.,2011;Bhattacharjee,2012;Chouetal.,2012;Zhangetal.,
ROS-induced damage caused by exposure to Cr. In addition, 2012; Hossain and Fujita, 2013; Ashraf et al., 2014; Sathiyaraj
the roles of exogenous H O on plant growth, water status, et al., 2014; Wang et al., 2014a), by enhancing the expression
2 2
mineralioncontent,prolinecontent,andtotalsugarandsoluble of heatshockproteins (Uchida etal.,2002;Wahid etal.,2007),
protein contents were evaluated in maize leaves exposed to byenhancingGSHandAsAbiosynthesis(Yuetal.,2003;Hung
copper (Cu) stress (0.5 mM Cu) by Guzel and Terzi (2013). et al., 2007; Chao et al., 2009; Xu et al., 2010; Bai et al., 2011;
Cu stress resulted in a decrease in leaf water potential, ionic Wangetal.,2014a),by enhancing proline biosynthesis (Uchida
concentration (Na+, K+, Ca+, Mg2+) and protein levels, but et al., 2002; Guzel and Terzi, 2013; ˙I¸seri et al., 2013; Ashfaque
increases in the content of proline content and total soluble etal.,2014;Sathiyarajetal.,2014;Terzietal.,2014),byenhancing
sugars, when compared with controls. Importantly H O -pre- photosynthesis(Wahidetal.,2007;Ishibashietal.,2011;Moussa
2 2
treated seedlings showed an increase in growth, water content, and Mohamed, 2011; Liao et al., 2012; Gondim et al., 2013),
mineral concentration, proline content, total soluble sugar and by enhancing ABA biosynthesis (Abass and Mohamed, 2011;
soluble protein contents when compared with control plants. Terzi et al., 2014) and by regulating multiple stress responsive
Agreaterincreasewasalsoobservedintheprolineandtotalsugar pathways and genes.Basedon the above findings we propose a
contentsoftheCu+H O groupofplantscomparedwiththeCu hypotheticalmodelthatsummarizesthepossiblemodeofaction
2 2
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Hossainetal. H2O2mediatedabioticstresstolerance
ofH O inplanta(Figure2).ItisspeculatedthatH O priming 2004) and are distributed both in the cytosol (mainly in the
2 2 2 2
induces a mild oxidative stress by disruption of cellular ROS inactiveform) andinthe nucleus.Once Hsfsenterthe nucleus,
homeostasis, from which develops a ROS-dependent signaling they bind to the heat shock elements of the promoters of
networkandinducestheaccumulationoflatentdefenseproteins ROS-sensitive genes, such as the gene encoding APX. There
suchasROS-scavengingenzymesandtranscriptionfactors(TFs), is evidence that certain Hsfs directly sense ROS and control
among others, resulting in a primed state and enhanced stress gene expression during oxidative stress (Miller and Mittler,
responses. Another notable effect of H O is its capacity to 2006). For example, the transcription of genes encoding the
2 2
inducetheexpressionofTFsandgenesresponsibleforosmolyte cytosolicperoxidasesAPX1andAPX2canberegulatedbyHsfs,
synthesis,e.g.,prolineandbetaine,andactivatephosphorylation which in turn can be modulated by ROS (Mazars et al., 2009).
cascades using mitogen-activated protein kinases (MAPKs). In Increased expression of Hsfs caused by exogenous H O was
2 2
the following section we willcritically discussthe perception of also found to modulate salinity tolerance (Uchida et al., 2002;
extracellularandintracellularH O byplantsandhowthissignal Wahidetal.,2007).TransgenicB.napusplantsover-expressing
2 2
transducesandmodulatestheabioticstresstoleranceofplants. theArabidopsisHEATSHOCKTRANSCRIPTIONFACTORA1b
showedenhancedresistancetodrought(Bechtoldetal.,2013).
H O asaSignalingMoleculeInvolvedin
2 2
Stress Tolerance SignalTransduction,StressToleranceandStress
SensingofH O Cross-Tolerance
2 2
H O can act as a signaling molecule via chemical reactions H O signaling appears to be integrated with many different
2 2 2 2
with targeted amino acids that can lead to peptide/protein signaling networks in plant cells, like Ca2+-signaling, protein
modifications. It is now well established that GSH and protein kinase networks, cellular metabolic networks etc. In some
cysteine (Cys) residuesare particularly wellsuited for reactions cases, H O and ROS accumulation was found to precede
2 2
with oxidants such as H O (D’Autréaux and Toledano, 2007; the activation of signaling, whereas in other cases H O
2 2 2 2
Munné-Bosch et al., 2013). Cys residues in proteins are one of accumulation was found to be a consequence of signaling.
the most sensitive targets for ROS-mediated posttranslational Signaltransductioncomponentsincludingproteinkinases,such
modifications,andhavebecomethefocusofmanyROSsignaling as calcium-dependent protein kinases (CDPKs) and mitogen-
studies. The electron-rich sulfur atom makes Cys residues the activated protein (MAP) kinases have been implicated in stress
major sites of oxidative modifications within proteins (Akter tolerance as well as cross-tolerance between biotic and abiotic
et al., 2015). It is a well-recognized concept that profiling of stress responses (Wurzinger et al., 2011). MAPK cascades are
ROS/RNS-modified proteins containing Cys can be used to important pathways in abiotic stress responses and enable
help identify key redox sensors involved in signal transduction extracellular stimuli to be transduced into intracellular changes
pathways (Couturier et al., 2013). Heat stress transcription (Zhou et al., 2014). A number of cellular stimuli that induce
factors (Hsfs) may also function as ROS-dependent redox- ROS (H O ) production can also activate MAPK pathways
2 2
sensors. Hsf proteins contain a DNA binding domain, control in multiple cell types (Torres and Forman, 2003; McCubrey
the transcription of heat stress associated genes (Baniwal et al., et al., 2006). H O - and ROS-responsive MAPKKK, MAPK1,
2 2
FIGURE2|AhypotheticalmodeloftheinfluenceofH2O2onplant network,therebyenhancingtheaccumulationoflatentdefenseproteins,such
defensemechanismsassociatedwithabioticstresses.H2O2treatmentis asROS-scavengingenzymesandtranscriptionfactors(TFs),resultingina
capableofinducingabioticstresstolerancethroughthedevelopmentofasmall primedstateandanenhancedstressresponse(modifiedfromBorgesetal.,
oxidativeburst.ThisburstsubsequentlyactivatesaROS-dependentsignaling 2014).
FrontiersinPlantScience|www.frontiersin.org 9 June2015|Volume6|Article420
Hossainetal. H2O2mediatedabioticstresstolerance
MAPK4, and MAPK6 usually remain highly active under titer in plant tissues. A rapid increase in cellular Ca2+ is
oxidative stress and redox regulation of environmental stress consideredasoneoftheearliestresponsesassociatedwithH O
2 2
responses. Direct exposure of cells to exogenous H2O2 leadsto signaling. Wu et al. (2010) showed that spermidine oxidase-
activation of MAPK pathways (Dabrowski et al., 2000; Ruffels derived H O regulates pollen membrane hyper-polarization-
2 2
et al., 2004). The MEKK1 pathway, which was found to be activatedCa2+ channelsinordertoinducepollentubegrowth.
highlyactiveunderoxidativestressandinducedbyunfavorable In contrast, cytoplasmic Ca2+ is able to trigger changes in
environmental conditions seems to be the activator of two H O levels. It is also evident that H O synthesis requires a
2 2 2 2
MAPKKs(MKK1andMKK2),thatinturnactivateother MAP continuouscytoplasmicinfluxofCa2+,whichactivatesNADPH
kinases (Mittler et al., 2011). MEKK1 is thought to be needed oxidases located at the plasma membrane (Lamb and Dixon,
for the activation of MAPK4 by H2O2 (Mittler et al., 2011). 1997).TheCa–CaMsignaling pathwayalsoregulatesanumber
Similarly, MAPK12 was found to be up regulated in response of different target proteins in signaling cascades, including
to ABA and H2O2 application (Mittler et al., 2011). MKP2 is MAPkinases.MAPkinasepathways inturnnegativelyregulate
a key regulator of the MPK3 and MPK6 networks that are H O synthesis by up-regulating the expression of RbohD.
2 2
involved in controlling both abiotic and biotic stress responses Thus, CDPKs are involved in tolerance to abiotic stresses(Wei
(Jammes et al., 2009; Mittler et al., 2011). Zhou et al. (2012) etal., 2014). Treatment of tomato and wheat leaveswith H O
2 2
reported the regulatory role of H2O2 in cold acclimation- increased theexpression ofCDPKs(Chico etal.,2002;Lietal.,
inducedchillingtoleranceintomato.Coldacclimationinducesa 2008). Costa et al. (2010) showed that CAT scavenging of
modestincreaseinH2O2,RBOH1geneexpressionandNADPH H2O2 is regulated through a Ca2+-dependent pathway in the
oxidase activity that modulates the expression and activity of peroxisomesofArabidopsisguardcells.Recentevidencesuggests
ROS detoxifying enzymes and ensures stress cross-tolerance. that RBOH-dependent H O production might be mediated
2 2
Zhouetal.(2014)further provedtheinvolvementofapoplastic by Ca2+ homeostasis in Arabidopsis (Suzuki et al., 2011). In
H2O2 in modulating stress cross-tolerance. Tomato plants pre- this case, cytoplasmic Ca+2 was shown to bind to the EF-
treatedwithmildcold,paraquat(PQ),ordroughtpre-treatment hands of the N-terminal region of RBOH and thus promote
modulatedabioticstresstolerancebyincreasingtheendogenous the activation of RBOH and the production H O (Takeda
2 2
level of H2O2 in the apoplast, which is well correlated with et al., 2008). The Ca2+ channels and transporters activated
RBOH1 transcription. An enhanced H2O2 level was found to by these stimuli form specific Ca2+ signatures and changes
modulate the expression of stress and defense-related genes, in these Ca2+ signatures are transmitted by protein sensors
increase the activities of SOD, APX, CAT, and GR, maintain that preferentially bind Ca2+. The binding of Ca2+ results in
higher GSH/GSSG ratio and activate MPK1/2. Their findings conformational changes in these protein sensors that modulate
supporttheinvolvementofH2O2andMPK1/2incross-tolerance their activities or their ability to interact with other proteins,
inplants.TheexogenousapplicationofH2O2 toA.thalianacan andactivatetheexpressionofdownstreamsalt-responsivegenes
activate the MAPK cascades that regulate ROS production and through a Ca2+ signaling cascade (Rentel and Knight, 2004;
detoxification(Panetal.,2012).Insomecases,MPK3/6responses Dodd et al., 2010; Kudla et al., 2010; Batistic and Kudla,
tocadmium(Cd)treatmentaremediatedbytheH2O2-signaling 2012).
pathway,whereMPK3/6isupregulatedafteranaccumulationof WRKY and zinc finger TFs are both widely involved in the
H O (Liuetal.,2010b).H O mayalsobeinvolvedintheMAP regulationofROS-relateddefensegenes.Itwasobservedthatthe
2 2 2 2
kinase 8 (MPK8) pathway, since expression of RBOHD rapidly ZAT7andZAT12zincfingerproteinsofArabidopsisarestrongly
decreases via MPK8, resulting in negative regulation of H O up regulated by oxidative stress in apx knockout mutants in
2 2
synthesis(Takahashietal.,2011). responsetoH O andmethylviologen(MV)treatment(Rizhsky,
2 2
Ca2+ one of the most important second messenger in 2004).ZAT10hasadualrolebothasaninducerandasarepressor
the sophisticated network of plant abiotic stress signaling ofROS-responsivegenesundersalt,droughtandosmoticstresses
(Dodd et al., 2010; Wei et al., 2014) and regulation of Ca2+ (Sakamotoetal.,2004;Mittler,2006).ZAT6positivelyregulates
homeostasis is one of the main targets of H2O2 signaling tolerancetodrought,salt,andchillingstress,aswellasresistance
(Petrov and Van Breusegem, 2012). Regulation of guard cells tobacterial infection, bymodulatingROS levelsand SA-related
during stomatal opening is one of the most widely studied gene expression (Shi et al., 2014). Accumulation of NO under
processes involving H2O2-mediated Ca2+ signaling and Ca2+ stressful conditions was found to initiate defense responses
is a central signaling component in guard cell responses to similar to those seen following H O production (Wang et al.,
2 2
stimulilikeABA,ROSandNO(Allenetal.,2000;Youngetal., 2014b). NO and H O are also involved in the stimulation
2 2
2006).Ca2+ homeostasisalsoregulatesantioxidativedefensesin of stomatal closure in Arabidopsis in response to ultraviolet-B
plants. An increase in the concentration of intracellular Ca2+ exposure(Heetal.,2005).RemovalofH O withantioxidantsor
2 2
causes efficient detoxification of H2O2 and involves increased inhibitionofitssynthesisbyinhibitingNADPHoxidaseactivity
levelsofdetoxificationenzymes,includingCa2+-sensitiveCAT3. prevents NO generation and stomatal closure. Recent evidence
Application of the Ca2+ channel blocker LaCl3, Ca2+ chelator supports theideathatH2O2-inducedsynthesisofNOmightbe
(EGTA)orthecalmodulin(CaM)inhibitor(trifluroperazine)to mediatedbyMPK6inArabidopsis(Wangetal.,2010b).
germinatingAmaranthusseeds,causesasignificantreductionin The links between H O signaling and other signaling
2 2
the levels of H O -scavenging enzymes (Bhattacharjee, 2008), pathways are summarized in Figure 3. These demonstrate a
2 2
strongly supporting the role of Ca2+ as a regulator of H O complexinteractionbetweenH O signaling,environmentaland
2 2 2 2
FrontiersinPlantScience|www.frontiersin.org 10 June2015|Volume6|Article420