Environmental and Experimental Botany 68 (2010) 1425

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Environmental and Experimental Botany

journa l homepage: www.e lsev ier .com/

Review

Effect of exogenous salicylic acid under changing

Qaiser Ha Plant Physiolob Department o

a r t i c l

Article history:Received 30 AReceived in reAccepted 18 A

Keywords:Salicylic acidGrowthStressPhotosynthesiAntioxidants

Contents

1. Introd2. Biosyn3. Signal4. Effect of exogenous salicylic acid on growth and bio-productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165. Effect of exogenous SA on photosynthesis and plant water relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166. Effect of exogenous SA on Rhizobium-legume symbiosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177. Relationship of SA with antioxidant system and its impact on the plants exposed to stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7.1. Biotic stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187.2.

8. Concl9. Future

Refer

1. Introdu

Salicylictributed in1878, whenmany (Rask

CorresponE-mail add

0098-8472/$ doi:10.1016/j.Abiotic stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187.2.1. Effect of exogenous SA on plants exposed to heavy metal stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187.2.2. Effect of exogenous SA on plants grown under salinity stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197.2.3. Effect of exogenous SA on plants grown under temperature stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197.2.4. Effect of exogenous SA on the plants exposed to UV radiation or ozone stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207.2.5. Effect of exogenous SA on plants exposed to water stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

usions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

ences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

ction

acid or ortho-hydroxy benzoic acid is ubiquitously dis-the whole plant kingdom and its history dates back toit was worlds largest selling drug synthesized in Ger-in et al., 1990). Theword salicylic acidwas derived from

ding author. Tel.: +91 9412328593.ress: shayat@lycos.com (S. Hayat).

a latin word salix meaning willow tree and the name was givenby Rafacle Piria in 1938. SA has been characterized in 36 plants,belonging to diverse groups (Raskin et al., 1990). In the plants, suchas rice, crabgrass, barley and soybean the level of salicylic acid isapproximately 1 microgram g1 fresh mass. Floral parts of sevenspecies and the leaves of 27 thermogenic species exhibited sub-stantial variation in the level of SA (Raskin et al., 1990). Salicylicacid is considered to be a potent plant hormone (Raskin, 1992a)because of its diverse regulatory roles in plantmetabolism (Popovaet al., 1997). Salicylic acid is an endogenous plant growth regulator

see front matter 2009 Elsevier B.V. All rights reserved.envexpbot.2009.08.005ayata, Shamsul Hayata,, Mohd. Irfana, Aqil Ahmadb

gy Section, Department of Botany, Aligarh Muslim University, Aligarh 202002, U.P., Indiaf Applied Sciences, Higher College of Technology, Al-Khuwair, Oman

e i n f o

pril 2009vised form 17 August 2009ugust 2009

s

a b s t r a c t

Salicylic acid (SA), an endogenous plant growth regulator has been found to generate a wide range ofmetabolic and physiological responses in plants thereby affecting their growth and development. In thepresent review, we have focused on various intrinsic biosynthetic pathways, interplay of SA and MeSA,its long distance transport and signaling. The effect of exogenous application of SA on bio-productivity,growth, photosynthesis, plant water relations, various enzyme activities and its effect on the plantsexposed to various biotic and abiotic stresses has also been discussed.

2009 Elsevier B.V. All rights reserved.

uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14thesis and metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15ing and transport of salicylic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15locate /envexpbot

environment: A review

Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425 15

of phenolic nature that possesses an aromatic ring with a hydroxylgroup or its functional derivative. In free state, SA is found in a crys-talline powder state having amelting point of 157159 C and a pHof2.4 (Raskin, 1992b). Salicylic acidhasbeen found toplayakey rolein the reguother organ(Raskin, 19ther, its rolowering inuptakeandstomatal co

Salicylicmolecule wtance in plaet al., 1992to the planof oxidativdiverse phyconstraintsinvolvemenon bio-prodimpact on p

2. Biosynth

In earlysized in plOne pathwcinnamic achydroxylatisalicylic aciand in ricetransformaed in QuerRanjeva, 19are yet to bethesis of sato o-coumaand the reahydroxylatewhich wasHowever, thand Ranjeva(Gestetnerpathway isradiolabeleolabeled safurther strefrom cinnarecently, geacidwas alseither inhibexperimentsalicylic acirismate synby this patacquired re

SA hasof moleculeby glycosylconjugatedsion culturein the rootYalpani etof salicylic a

glucosyltransferase (Gtase) (Balke and Schulz, 1987; Yalpani et al.,1992). SA may also be metabolized to 2,3-dihydrobenzoic acid or2,5-dihydrobenzoic acid as was identied in the leaves of Astilbesinensis and Lycopersicon esculentum after administering the radio-

cinn

nalin

is wea keyt vari1997ts. Tindre ints toRasmurtheevetcan

no et2+ (Ccco ccid rraneof aer, ed nondenalsoin sigsenele apovaopismg (Sshias trouslxperper leanding tto

t boto in

ticlets (Ortherar ladiffuMetlecuh phn bed. SA inse balsoicallauthd toMindue baP2),lation of plant growth, development, interaction withisms and in the responses to environmental stresses92a,b; Yalpani et al., 1994; Senaratna et al., 2000). Fur-e is evident in seed germination, fruit yield, glycolysis,thermogenic plants (Klessig and Malamy, 1994), ion

transport (HarperandBalke, 1981), photosynthetic rate,nductance and transpiration (Khan et al., 2003).acid is considered to be an important signalinghich is involved in local and endemic disease resis-nts in response to various pathogenic attacks (Enyedi; Alverez, 2000). Besides providing disease resistancets, SA can modulate plant responses to a wide rangee stresses (Shirasu et al., 1997). Keeping in view thesiological roles of SA in plants, and the necessary space, we restrict our coverage to its biosynthesis, transport,t in signaling and the effects of exogenous salicylic aciductivity, growth, activities of various enzyme and itslants, exposed to various biotic and abiotic stresses.

esis and metabolism

1960s, it was suggested that salicylic acid is synthe-ants from cinnamic acid by two possible pathways.ay involves the decarboxylation of the side chain ofid to form benzoic acid, which inturn undergoes a 2-on to form salicylic acid. Such biosynthetic pathway ofd has been reported in tobacco (Yalpani et al., 1993)(Silverman et al., 1995). The enzyme that catalyzes thetion of cinnamic acid to benzoic acid has been identi-cus pedunculata (Alibert and Ranjeva, 1971; Alibert and72). However, other enzymes involved in the pathwayexplored. The other pathway proposed for the biosyn-

licylic acid involves a 2-hydroxylation of cinnamic acidric acid which is then decarboxylated to salicylic acidction is catalyzed by an enzyme trans-cinnamate-4-(Alibert and Ranjeva, 1971; Alibert and Ranjeva, 1972)

rst detected in pea seedlings (Russell and Conn, 1967).is enzymewas also identied inQ. pedunculata (Alibert, 1971; Alibert and Ranjeva, 1972) and inMelilotus albaand Conn, 1974). However, the exact mechanism of thestill an anomaly. Ellis andAmichein (1971) incorporatedd benzoic acid or cinnamic acid and recovered the radi-licylic acid in Gaultheria procumbens. This observationngthened the belief that salicylic acid is synthesizedmic acid via the formation of benzoic acid. However,netic studies in Arabidopsis have shown that salicylico producedwhen thepathwaysmentioned earlierwereited or the specic activity of radiolabeled SA in feedings was lower than expected. According to this pathway,d is synthesized from chorismate by means of isocho-thase in chloroplasts and the salicylic acid synthesizedhway is responsible for providing local and systemicsistance in plants (Wildermuth et al., 2001)got a property of forming conjugates with a varietys (Ibrahim and Towers, 1959; Grifths, 1959) eitheration or by esterication (Popova et al., 1997). Theform of SA as -glucoside-SA was reported in suspen-s of Mellotus japonicus (Tanaka et al., 1990) and alsos of Avena sativa seedlings (Balke and Schulz, 1987;al., 1992). The enzyme that catalyzes the metabolismcid to -glucoside-SA was identied and named as SA-

labeled

3. Sig

SAplaysagainset al.,in plansome kliteratuof plan1990;were f(Shulain cells(Kawaand Caof tobaicylic amembvationHowevacid diindepeIt haspatedof leafmolecuandPogravitrripenin

Ohaacid wexogentheir eing up10minindicatenoughthe plaerancethe cuin planwas fucuticulble of1998).ing mothrougthat carequirefrom SrespontissuessystemTheseresponon thetrol th2 (SABamic acid or benzoic acid (Billek and Schmook, 1967).

g and transport of salicylic acid

ll known naturally occurring signaling molecule thatrole in establishing and signaling a defense responseous pathogenic infections (Malamy et al., 1990; Durner) and also induces systemic acquired resistance (SAR)he induction of SAR, after a localized infection, requiresof long distance communication mediator. A survey ofdicates that salicylic acid moves from infected organsthe non-infected ones through phloem (Metraux et al.,ussen et al., 1991; Yalpani et al., 1991). These ndingsr conrmed by using radiolabeled SA or its analoguesal., 1995;Molders et al., 1996). Salicylic acid synthesizedmove freely in and out of the cells, tissues and organsal., 2004) and thismovement is nely regulated by ROShen and Kuc, 1999; Chen et al., 2001). Supplementationell suspension culturewith higher concentration of sal-esulted in a de novo induction of SA excretion across thewhich wasmediated by the generation of ROS and acti-cascade of Ca2+ signaling and protein phosphorylation.xogenous supply of lower concentrations of salicylict require a de novo synthesis of proteins and was foundt of ROS, Ca2+ and protein kinases (Chen et al., 2001).been reported by Morris et al. (2000) that SA partici-naling and regulation of gene expression in the coursescence in Arabidopsis. Salicylic acid acts as a signalingnd regulates the biogenesis of chloroplasts (Uzunova, 2000), photosynthetic activity (Fariduddinet al., 2003),(Medvedev andMarkova, 1991) and inhibition of fruit

rivastava and Dwivedi, 2000).et al. (2004) reported that the radiolabeled salicylicanslocated at an unexpectedly rapid rate when appliedy at cut end of petiole in tobacco plants. The results ofiment revealed that the signal reached to 6 neighbor-aves and three adjacent lower leaves with in a span ofaccumulated throughout the plant body within 50minhat the transport of salicylic acid is rapid and smoothallow a systemic distribution of SA signal throughoutdy with in a short span of time, thereby providing tol-fections. Further, it is also cited in the literature that

hardly allows the entry of surface applied salicylic acidhashi et al., 2004; Niederl et al., 1998). However, itreported that salicylic acid can pass through the toughyer in its methylated (MeSA) formwhichmakes it capa-sing across cuticle independent of pH (Niederl et al.,hyl salicylate (MeSA) is a volatile long distance signal-le that moves from infected to the non-infected tissuesloem. MeSA represents an inactive precursor of SAtranslocated and converted to salicylic acid wheneverhulaev et al. (1997) reported that MeSA was producedtobacco plants, after infection and induced the defensey reverting back to SA. Further, MeSA levels in plantparallel the increase in SA concentration locally and

y after viral or bacterial infections (Seskar et al., 1998).ors also reported that NahG mutants were unable toeSA, indicating that this compoundhasnodirect effect

ction of defense response. In tobacco, two enzymes con-lance between SA and MeSA: the SA binding proteinwhich converts biologically inactive MeSA into active

16 Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425

SA (Forouhar et al., 2005), and SA methyl transferase 1 (SAMT1),which catalyses the formation of MeSA from SA (Ross et al., 1999).Recently the research onMeSA reached a breakthrough, when Parket al. (2007) demonstrated that MeSA functions as a crucial longdistance SAesterase acttion in the dof SABP2 anPark et al. (for the devunclear wh

4. Effect ofbio-produc

Salicylicphysiologickey role in rSA and itsproductionmination angrainswereicylic acid (the drymatjuncea, wheHowever, h

In anothand dry masoaked in losignicantlyin barley s1996). Khocharacteristsprayed withe carbohy(2007) in thwheat plantan enhancecharacteristleaves, stemplant as awSA had mor

It is westressful enin their megrowth (Raof growth ication of sacarried outapplied saliity of carroboron toxiccylic acid sisulphur cona concomitand that ofline accumuboth in shoreported aof leaves anwhen salicyinhibition imaticawhe2002). Furthshift the nuand potassi

dent on pH, suggesting a higher activity of protonated form of SA(Hayat and Ahmad, 2007).

The soil nutrient solution enters the plant body through its rootsand besides some other factors a healthy root system plays a key

enh69) ofolloLarqspirin. Sinf exotiviter cocta (Se wi98),ed tdomant,ial tobless (Arimilathe pf salival-Yto wantlerinto y

ed tod anl planuntrf salwerlicylra-Tuof asla (Kna antion1987tlywylic ationd foret aKinea synomber iner, Onfersing. Tent ohis han eoduc

ct orelat

a wewideR signal in tobacco. The authors reported that MeSAivityof enzymeSABP2 is essential for SARsignal percep-istal tissues. The fact was further conrmed by the used/or SAMT1 silenced plants, where SAR was blocked.2009) conrmed the importance of SABP2 and MeSAelopment of SAR in tobacco. However, it still remainsether MeSA plays a similar role in other plant species.

exogenous salicylic acid on growth andtivity

acid and other salicylates are known to affect variousal and biochemical activities of plants and may play aegulating their growth and productivity (Arberg, 1981).close analogues enhanced the leaf area and dry massin corn and soybean (Khan et al., 2003). Enhanced ger-d seedling growth were recorded in wheat, when thesubjected topre-sowing seed-soaking treatment in sal-Shakirova, 2007). Fariduddin et al. (2003) reported thatter accumulationwas signicantly enhanced in Brassican lower concentrations of salicylic acid were sprayed.igher concentrations of SA had an inhibitory effect.er study, Hayat et al. (2005), the leaf number, freshss per plant of wheat seedlings raised from the grainswer concentration (105 M) of salicylic acid, increased. Similar growth promoting response was generated

eedlings sprayed with salicylic acid (Pancheva et al.,dary (2004) observed a signicant increase in growthics, pigment contents andphotosynthetic rate inmaize,th SA. The exogenous SA application also enhanceddrate content in maize (Khodary, 2004). Hussein et al.eir pot experiment sprayed salicylic acid to the foliageofs, irrigatedwithMediterranean seawater and reportedd productivity due to an improvement in all growthics including plant height, number and area of greendiameter and dry weight of stem, leaves and of the

hole.Moreover, the plants that received treatmentwithe proline content.ll documented that the plans on being exposed tovironments such as high salinity, result in a declinetabolic activity, thereby leading to retarded overallmagopal, 1987). However, salinity induced retardationn wheat was to a great extent alleviated by the appli-licylic acid (Shakirova, 2007). Eraslan et al. (2007) alsoan experiment to elucidate the effect of exogenouslycylic acid on growth, physiology and antioxidant activ-t plants grown under combined stress of salinity andity. The results of their experiment revealed that sali-gnicantly enhanced the overall growth, root dry mass,centration, carotenoids and anthocyanin contents withant enhancement of total antioxidant activity of shootstorage root. The SA application also regulated the pro-lation and decreased the toxic ion (Cl, B) accumulation,ot and storage root. However, Pancheva et al. (1996)delayed leaf emergence and a decrease in the growthd roots of barley plants in a dose-dependent manner,lic acid was applied exogenously. A dose-dependentn bud formation was also observed in Funaria hygro-nSAwas supplied exogenously (Christianson andDuffy,er, exogenous application of SA has also been found to

trient status leading to a decreased uptake of phosphateum by roots and this decrease was found to be depen-

role inal. (19plants,out bywith arootingeffect oproduc

Lowtus erein tunal. (19increasmotingimportpotentvegetavulgari

A swhentions o(Sandonouslysignic

Flowrelatedreport(Clelanmentato thespray oresultswith sa(HerrecationSpirode(Khuraassocia(Hew,nicanof salicapplicaand poKumarof GA,foundother cof owHowevacid coowertreatmports tacts asand pr

5. Effewater

It isates aancing the growth and productivity of plants. Basu etbserved that the rooting was enhanced in mungbeanwing the treatment of salicylates. In a study carriedue-Saavedra et al. (1975), treatment of bean explants, which is a close analogue of salicylic acid, enhancedce then a lot of work was carried out to elucidate thegenous SA and other salicylates on rooting and therebyy in plants.ncentrations of salicylic acid enhanced rooting in Tage-andoval-Yapiz, 2004) and these ndings were strictlyth the observations made by Gutierrez-Coronado etwhere foliar application of salicylic acid signicantlyhe length of roots in soybean. This root growth pro-ain of salicylic acid has now made it one of the most

effective and cost benecial phytohormone that has theenhance the root growth in economically important

and salads like Daucus carota, Raphanus sativus and Betaisteo-Cortes, 1998).r promotion was generated in shoot system as well,lants of T. erecta were treated with lower concentra-cylic acid, thereby enhancing the productivity of plantsapiz, 2004). Further, salicylic acidwhen applied exoge-heat seedlings increased the size and mass of plantletsy, compared to the untreated control (Shakirova, 2007).g is another important parameter that is directlyield and productivity of plants. Salicylic acid has beeninduceowering in anumberof plants, including Lemnad Ajami, 1974). Different plant species including orna-t Sinningia speciosa oweredmuch earlier as comparedeated control, when they received an exogenous foliaricylic acid (Martin-Mex et al., 2003, 2005a). Promisinge obtained when plants of Carica papaya were treatedic acid which showed a signicantly higher fruit settingz, 2004; Martin-Mex et al., 2005b). Exogenous appli-pirin (a close analogue of SA) enhanced owering inhurana andMaheshwari, 1980) andWolamicroscopicadMaheshwari, 1987; Tomot et al., 1987). Moreover, inwith sucrose, SA enhanced ower opening in Oncidium). In cucumber and tomato, the fruit yield enhanced sig-hen the plantswere sprayedwith lower concentrationscid (Larque-Saavedra andMartin-Mex, 2007). The foliarof salicylic acid to soybean also enhanced the oweringmation (Kumar et al., 1999). In a comparative analysis,l. (2000), studied the cumulative effect of SA with thattin, NAA, ethral and chloro chloro chloride (CCC), andergistic effect of SA and GA on owering compared toinations of hormones. However, the exact mechanismducing property of salicylic acid is yet to be explored.ota (1975), hypothesized that o-hydroxyl of salicylicthe metal chelating property that favours induction ofhe induction of owering in Lamnaceae, following thef chelating agents (Seth et al., 1970; Oota, 1972), sup-ypothesis. Thus, it may be concluded that salicylic acidndogenous regulator that potentially affects the growthtivity in plants.

f exogenous SA on photosynthesis and plantions

ll-established fact that salicylic acid potentially gener-array of metabolic responses in plants and also affects

Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425 17

the photosynthetic parameters and plant water relations. Hayatet al. (2005) reported that the pigment content was signicantlyenhanced in wheat seedlings, raised from the grains pre-treatedwith lower concentration (105 M)of salicylic acid,whereas, higherconcentratitreatment,fruitful in inal., 2002). Siwere sprayechlorophyllconcentratiHowever, cphyll conte(Anandhi anet al. (2003carotenoidsepoxidationand chloropcation of Sinternal COtance and tFurther, Khrate and stoSA and otheried out inthe water ucentration (the transpirand Commethis decreasalicylic ac1978, 1979enhanced,sprayed toplied exogegrains (Hayconcentratidecrease inal. (1996), wlase/oxygenconcentraticoncomitanboxylase (Pwhich wasHayat et al.

6. Effect o

SA is repsymbiosis.in responseendogenounodulationgrowth of Ralso delayeber of noduanother stulevel in theof Rhizobia,However, Mstrain of Rhroots of hosble strains owhich are ption of SA in

Van Spronsen et al. (2003) reported that the exogenous applica-tion of SA at lower concentration strongly inhibited indeterminatenodule formation in Vicia sativa and pea thereby decreasing thenodulation, nitrogenxationandultimatelygrowthofplants.How-

e sams, Lotid nod thory ey maotost ofulaticompid diestabtingexogthe

ermitionalsodev

ed aswthningng thsizegenorogenium she a. Theleaveatms anivityppliccreasbe d(Kumate rraise) of

sprayy (Farr10

withnanrove

ationt on

ssfueciede (Hstattteri989;vel ins, pr(Smouslcienons did not prove to be benecial. Besides seed-soakingthe foliar application of SA also proved to be equallycreasing thepigment contents inBrassicanapus (Ghai etmilar resultswere obtainedwhen theplants ofB. junceadwith lower concentrations (105 M) of SA, where, thecontent was signicantly enhanced, whereas, higherons proved to be inhibitory (Fariduddin et al., 2003).ontrary to these observations, a reduction in chloro-ntwas observed in plants pre-treatedwith salicylic acidd Ramanujam, 1997; Pancheva et al., 1996).Moharekar) reported that salicylic acid activated the synthesis ofand xanthophylls and also enhanced the rate of de-with a concomitant decrease in chlorophyll pigmentshyll a/b ratio in wheat and moong. Exogenous appli-

A was found to enhance the net photosynthetic rate,2 concentration, water use efciency, stomatal conduc-ranspiration rate in B. juncea (Fariduddin et al., 2003).an et al. (2003) reported an increase in transpirationmatal conductance in response to foliar application ofr salicylates in corn and soybean. In another study car-soybean, foliar application of salicylic acid enhancedse efciency, transpiration rate and internal CO2 con-Kumar et al., 2000). However, contrary to these results,ation rate decreased signicantly in Phaseolus vulgarislina communis after the foliar application of SA andse in transpiration rate was attributed to the fact thatid induced the closure of stomata (Larque-Saavedra,). The leaf carbonic anhydrase activity was signicantlywhen SA at lower concentration (105 M) was eitherthe foliage of Brassica (Fariduddin et al., 2003) or sup-nously as pre-sowing seed-soaking treatment to wheatat et al., 2005). However, the treatment with higherons of SA decreased the activity of the enzyme. Such athe enzyme activity was also observed by Pancheva ethere the activity of ribulose-1,5-biphosphate carboxy-ase (RuBPCO) in barley decreased with the increasingon of SA and this decrease was accompanied by at increase in the activity of phosphoenol pyruvate car-EPCase) resulting in a decline in photosynthetic ratecontrary to the results of Fariduddin et al. (2003) and(2005).

f exogenous SA on Rhizobium-legume symbiosis

orted to affect the early stages of Rhizobium-legumeThe nod factors produced by the colonizing Rhizobiato avonoids released by the legume, changed the

s SA content of the host plant during the early stages of(Mabood and Smith, 2007). Exogenous SA inhibited thehizobia and the production of nod factors by them andd the nodule formation, thereby decreasing the num-les per plant (Mabood and Smith, 2007). However, indy, Martinez-Abarca et al. (1998), observed that the SAroots of Medicago sativa, inoculated with specic straineither decreased or remained close to the basal levels.. sativa plants when inoculated with an incompatibleizobia, resulted in a marked accumulation of SA in thet plant. It was therefore, concluded that the compati-f Rhizobia produce certain signals (specic nod factors)erceived by the host plant that suppress the accumula-the roots (Martinez-Abarca et al., 1998).

ever, thvulgariules, drevealeinhibitand drand phcontento inoccantlyaforesarole ininfestaply ofcheckssis is tnodulasis arenoduleregardanygrone tulowerisyntheit endo

NitRhizobaffect taswellin theThe treteinaseNR actnous awas inmightmentof nitrplants(105 Mwhenactivit(103 oplantsnitrogewere p

7. Relimpac

Stregen spperoxiating aand Guet al., 1ROS leas lipidostasisexogenthe efe concentrationof SAwhensprayed toplants suchasP.us japonicus and soybean, producing determinate nod-t inhibit nodulation. The results of Lian et al. (2000)at higher concentrations (5 and 1mM) of SA had anffect on nodulation, thereby decreasing nodule numberss in soybean, thereby lowering the nitrogen xationynthesis. The nodule number, N2 xation and proteinVigna mungo, raised from the seeds soaked in SA prioron with specic strain of Rhizobium, decreased signi-ared to unsoaked control (Ramanujan et al., 1998). Thescussion clearly indicates that SA has crucial regulatorylishing early stages of nodulation and is providedby theRhizobia. The relation is terminated under initial sup-enous SA, particularly higher concentrations severelysymbiotic relation. Once the establishment of symbio-nated the upcoming benets of well known benets ofviz. nitrogenxation, protein content andphotosynthe-hampered. However, SA did not affect the subsequentelopment, if supplied after inoculation. This could bea good example of spatial and temporal regulation ofregulator. Furthermore, there appears an adjustment orof internal releaseof SA in responseof exogenous supplye overburden on tissue-specicmetabolicmachinery toit especially under stress. Conversely, plant synthesizesusly as in case of inoculation with incompatible strain.metabolism is an important aspect of legume-

ymbiosis and exogenous application of SAwas found toctivities of the enzymes of nitrate/nitrogen metabolismactivity of enzymenitrate reductase (NR)wasenhanceds of wheat following the exogenous application of SA.ent also protected the enzyme from the action of pro-d trypsin (Rane et al., 1995). Lead induced decline inwas revived in the maize plants following the exoge-ation of SA (Sinha et al., 1994). The total protein contented in soybean plants sprayed with SA and this increaseue to enhanced activity of NR following the SA treat-ar et al., 1999). A signicant increase in the activityeductase was observed both in roots and leaves of thed from the wheat grains soaked in lower concentrationSA (Hayat et al., 2005). Such a lower concentration of SAed to the foliage of mustard plants enhanced their NRiduddin et al., 2003).However, at higher concentrations4 M), SAproved tobe inhibitory. The treatmentofmaizelower concentrations of SA also enhanced the uptake ofdactivityof enzymeNR,whereas, higher concentrationsd to be inhibitory (Jain and Srivastava, 1981).

ship of SA with antioxidant system and itsthe plants exposed to stress

l environments induce the generation of reactive oxy-s (ROS) such as superoxide radicals (O2), hydrogen2O2), hydroxyl radicals (OH) etc. in plants thereby cre-e of oxidative stress in them (Elstner, 1982; Halliwelldge, 1988; Asada, 1994; Gille and Singler, 1995; MonkPrasad et al., 1999; Panda et al., 2003a,b). This increasedplants cause oxidative damage to biomolecules such

oteins and nucleic acids, thus altering the redox home-irnoff, 1993; Gille and Singler, 1995). When appliedy at suitable concentrations, SA was found to enhancecy of antioxidant system in plants (Knorzer et al., 1999).

18 Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425

SA treatment was found to alleviate the oxidative stress gener-ated by paraquat (one of the most widely used herbicides, whichis quick-acting and non-selective, killing green plant tissue on con-tact) in tobacco and cucumber (Strobel and Kuc, 1995). Further,the treatmeof catalase2003) whicet al., 1993et al., 2002antioxidantmutase (SOplants of L. eplants of B. jthe exogenties of antiowith a concThe priminging, lowereand also en(CAT, guaiacsativa, therandPatra, 2and Panda (idant enzymfollowing tacid.

7.1. Biotic s

Plants coof a varietythemselvesstitutive anthe accumuis supportelarge amouresistant toinoculationwas observColletotrichusis virus (M1991).

These napplicationagainst vardefense sigmany dicotyacid and acetobaccomoFurther, salnously induand also cobacterial, o(MalamyanShah and K1994; KogePasquer et a

Singh etcade of evetheir cell-toconcentratisition of caplant defenreported thtion of H2Ohypersensit

microbes. Salicylic acid is found to alter the activity of a mitochon-drial enzyme, alternative oxidase, which mediates the oxidation ofubiquinol/ubiquinone pool and reduction of oxygen towater,with-out the synthesis of ATP inmitochondria and this altered activity of

e altturn004)cylicconitessigof thhoma de

Durrhich998ens (

iotic

Effect

ongvy mce fodmiusily aolic pandt upamanried(Qu985ase ad hinchiiso affo anStibole ofhas bobseduceic aciarleyThetol

as matedotecthereof SAulatially egenohe iercured deincrexicistude toxva froed thy annt with salicylic acid resulted in temporary reduction(CAT) activity and increased H2O2 level (Janda et al.,h possibly played a key role in providing the SAR (Chen) and tolerance against the oxidative stress (Gechev) in plants. SA was found to enhance the activities ofenzymes, CAT, peroxidase (POX) and superoxide dis-D), when sprayed exogenously to the drought stressedsculentum (Hayat et al., 2008) or to the salinity stresseduncea (Yusuf et al., 2008). Krantev et al. (2008) reportedous application of salicylic acid enhanced the activi-xidant enzymes ascorbate peroxidase (APX) and SODomitant decline in the activity of CAT in maize plants.of seeds with lower concentrations of SA, before sow-

d the elevated levels of ROS due to cadmium exposurehanced the activities of various antioxidant enzymesol peroxidase, glutathione reductase and SOD) inOryzaeby protecting the plants from oxidative burst (Panda007).However, contrary to this observation, Choudhury2004) reported a decline in the activities of the antiox-es CAT, POX, SOD and glutathione reductase in rice

he pre-sowing seed-soaking treatment with salicylic

tress

ntinuously remain exposed to the challenging threatsof pathogenic attacks. However, in order to defendagainst these attacks, plants have evolved various con-d inducible mechanisms, one such mechanism beinglation of large quantities of salicylic acid. This notiond by the observations of Malamy et al. (1990), wherents of salicylic acid accumulated in the leaves of TMV-bacco variety Nicotiana tabaccum cv. Xanthi nc, uponwithTMV.A similar increase in theendogenousSA leveled in the phloem sap of cucumber plants, infected withm lagenarium, Pseudomonas syringae or tobacco necro-etraux et al., 1990; Rasmussen et al., 1991; Smith et al.,

dings open a new window for the role to exogenousof salicylic acid in providing tolerance to the plantsious pathogens. The involvement of exogenous SA innaling has been characterized and well documented inledonous plants. The exogenous application of salicylictyl salicylic acidwas found to induce resistance againstsaic virus (TMV) in tobacco (Antoniw andWhite, 1980).icylic acid or acetyl salicylic acid when applied exoge-ced the expression of PR (pathogenesis related) genesnferred resistance against various pathogens of viral,omycete and fungal origin in a variety of dicot plantsdKlessig, 1992;Silvermanetal., 1995;Ryals et al., 1996;lessig, 1999) and in monocot plants (Wasternack et al.,l et al., 1994; Gorlach et al., 1996; Morris et al., 1998;l., 2005; Makandar et al., 2006).al. (2004) reported that salicylic acid activated a cas-nts resulting in the inhibition of viral replication and-cell and long distance transmission in plants. Lowerons of salicylic acid were found to enhance the depo-llose plugs in Arabidopsis which contributed to these system (Kohler et al., 2002). Lamb and Dixon (1997)at salicylic acid causes an increase in the accumula-2 in plant tissues which plays a key role in initiatingive responses and providing SAR against pathogenic

enzymand inet al., 2

SaliAPX, aand Klsomeredoxtion of2002;tion,wet al., 1pathog

7.2. Ab

7.2.1.stress

Ambe heanicanlike caare eametabin rootnutrien(BoussPoscheuidityet al., 1of ATPards CMerakand alsRubisc1998;

A roplants(1999)cury insalicylCd in b2002).miniumthat wthe treacid prstress,effectup-reg(Metw

Exoviate tand mreportto ancury torecentated thM. satireportated bernative oxidase affects the ROS levels in mitochondriainduces an antiviral defense response in plants (Singh.acid has an afnity to bind with the enzymes like CAT,ase and carbonic anhydrase (Chen et al., 1993; Durner, 1995; Ruffer et al., 1999; Slaymaker et al., 2002) andese enzymes are involved in ROS metabolism and ineostasis. Alteration in this homeostasis leads to induc-fense response in plants (Mittler, 2002; Torres et al.,ant and Dong, 2004). SA also affects the lipid peroxida-playsakey role in initiatingdefense response (Anderson) and induction of SAR in plants when challenged withMaldonado et al., 2002; Nandi et al., 2004; Shah, 2005).

stress

of exogenous SA on plants exposed to heavy metal

the naturally occurring elements, 53 are considered toetals and a few of them have got some biological sig-r plants (Weast, 1984). However, the heavy metalsm, if present in elevated levels in agricultural soils,ssimilated by plants and induce serious visible anderturbations e.g. leaf roll, chlorosis, growth reductionshoot, browning of leaf tips (Kahle, 1993), decrease intake (Sandalio et al., 2001), alterednitrogenmetabolismet al., 1999), inhibition of stomatal opening (Barcelo ander, 1990), disruption of membrane composition andariti et al., 1997), decrease photosynthetic rate (Stobort; Padmaja et al., 1990; Gadallah, 1995) and disruptionctivity (Fodor et al., 1995). In addition to these haz-ders the development of chloroplasts (Stoyanova andka-Nikolova, 1992; Stoyanova and Tchakalova, 1997)ects the activities of twomain photosynthetic enzymesd phosphoenol pyruvate carboxylase (Siedlecka et al.,rova, 1998; Malik et al., 1992).salicylic acid in alleviating the heavy metal toxicity ineen reported by many workers. Mishra and Choudhurirved that SApre-treatment alleviated the lead andmer-d membrane disruptions in rice. Further, exogenousd was found to alleviate the toxic effects generated by(Metwally et al., 2003) and in maize plants (Pal et al.,

application of Salicylic acid exogenously, conferred alu-erance to the plants of Cassia tora, exposed to Al toxicityediated by an increase in citrate efux in the roots ofplants (Yang et al., 2003). Similarly, exogenous salicylicted barley plants from lipid peroxidation, induced by Cdby increased the freshmass of roots and shoots and thiswas mediated by suppressing the cadmium-induced

on of H2O2 metabolizing enzymes such as CAT and APXt al., 2003).us application of salicylic acid was also found to alle-ll effects generated by other heavy metals like leady in rice (Mishra and Choudhuri, 1999). These authorsterioration of the membranes in the leaves of rice dueased lipoxigenase activity, induced by lead and mer-ty which was mitigated by exogenous SA. In a morey, Zhou et al. (2009) reported that salicylic acid allevi-icity generated by mercury and protected the roots ofm oxidative damage induced by mercury. The authorsat this protection from oxidative damage was medi-increased activity of various antioxidant enzymes. A

Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425 19

similar ameliorative role of salicylic acid was observed in soybeanseedlings exposed to cadmium toxicity (Drazic and Mihailovic,2005). In a study carried out by Drazic et al. (2006), the pre-sowingseed-soaking treatment with lower concentrations of salicylic acidenhanced thinhibited byto maintain(alfalfa). Prthe inhibitocarboxylaseof antioxidation in theal., 2008). Arecorded winduced lipraised fromSimilarly, Ctive role ofsativa. Theielongationmulation ofin plants. Hsalicylic aciand was mdation, lesssuperoxideShi and Zhuicity generaresponse woxidation. Tfollowing thwhereas, SOactivities w

A declintathione recomparedPanda, 200SOD, glutatin the plancylic acid. Tthiobarbiturice, thereboxidative st2007).

7.2.2. EffectA high

plants, as itin favour ofmay damagtion (Smirnthat exogencan potentienhanced tseedlings raand Al-Haktomato planpresumed tviz. aldose rlation of ceSzepesi et a

Accumuadaptive re(Rai, 2002).under salinacid was ap

effects of salinity (Shakirova et al., 2003). The exogenous applica-tion of salicylic acid prevented the lowering of IAA and cytokininlevels in salinity stressed wheat plants resulting in the better-ment of cell division in root apical meristem, thereby increasing

ands alscumuaptasis otionf actX wrovantioic acibutdhany ofssedity w

opsised thafferatiod calined thrase)salicyr theerea reing e

Effect

. Heaous pe duetion

evet temylicent,O2 leentilar reltureic acpoins stotioned thact

l. Hoate pDwaatmet byat stum wpar

ent awite growth of root and shoot of alfalfa plants, which wascadmium exposure. Further, the treatment was foundthe ionic homeostasis in the seedlings of M. sativa

e-sowing seed treatment with salicylic acid alleviatedry effects of cadmiumon the activities of enzymesRUBPand PEP carboxylase and also enhanced the activitiestive enzymes APX and SOD with a concomitant reduc-activities of enzyme CAT in maize plants (Krantev etsignicant improvement in growth parameters was

ith a concomitant reduction in the rate of cadmium-id peroxidation and electrolyte leakage in maize plants,the seeds soaked in salicylic acid (Krantev et al., 2008).houdhury and Panda (2004) investigated the ameliora-SA on cadmium-induced oxidative stress in roots of O.r study revealed that Cd toxicity resulted in the loss ofgrowth and biomass of roots with a concomitant accu-cadmium in them, thereby, generating oxidative stressowever, the pre-sowing seed-soaking treatment withd, decreased the toxic effects, generated by cadmiumanifested in the form of lowered level of lipid peroxi-er production of H2O2, reduction in the generation ofradicals and maintaining the stability of membranes.(2008) reported that exogenous SA alleviated the tox-ted in Cucumis sativus by manganese exposure and theas mediated by reduction in ROS level and lipid per-he antioxidant enzymes also showed varied responseeSA treatmente.g. CATandAPXactivitieswere reducedD, POX, dehydroascorbate reductase (DHAR) and GRere enhanced.e in the activities of enzymes CAT, POX, SOD and glu-ductase was observed in the plants treated with SAto the untreated plants of O. sativa (Choudhury and4). However, contrary to this, higher activities of CAT,hione reductase and guaiacol peroxidasewere observedts of O. sativa, raised from the seeds primed with sali-he treatment of salicylic acid also lowered the level ofric acid reactive substances (TBARS), H2O2 and O2 iny provided additional tolerance to the plants againstress generated by cadmium exposure (Panda and Patra,

of exogenous SA on plants grown under salinity stresssalinity induces serious metabolic perturbations ingenerates ROS which disturb the cellular redox systemoxidized forms thereby creating an oxidative stress thate DNA, inactivate enzymes and cause lipid peroxida-off, 1993). However, a large body of literature indicatesous application of salicylic acid to the stressed plantsally alleviate the toxic effects, generated by salinity. Anolerance against salinity stress was observed in wheatised from the grains soaked in salicylic acid (Hamadaimi, 2001). Similar observations were also made ints raised from the seeds soaked in salicylic acid andwaso be due to the enhanced activation of some enzymeseductase and ascorbate peroxidase and to the accumu-rtain osmolytes such as proline (Tari et al., 2002, 2004;l., 2005).lation of large amounts of osmolytes (proline) is ansponse in plants exposed to stressful environmentsWheat seedlings accumulated large amounts of prolineity stress which was further increased when salicylicplied exogenously, thereby alleviating the deleterious

growthauthorthe acpre-adsyntheproteclevel oand PO(Shakithese asalicyl(Sakhaacid enstabilitity streof salinArabidobservitivelyK+/Na+

a, b annon-saters ananhydwhenFurtheSOD) wtent asprovid2008).

7.2.3.stress7.2.3.1in serimay bgenera2007).

Howagainsof salictreatmtheH2the potA simithe cusalicyl(2004)Agrostiprotecreporton POXcontroascorband SOthe treried outhe hearietinSA, comtreatmicantlyproductivity of plants (Shakirova et al., 2003). Theseo reported that the pre-treatment with SA resulted inlation of ABA which might have contributed to the

tion of seedlings to salinity stress as ABA induces thef a wide range of anti-stress proteins, thereby providingto the plants. Further, the treatment also lowered theive oxygen species and therefore the activities of SODere also lowered in the roots of young wheat seedlingset al., 2003). These ndings indicate that the activities ofxidant enzymes are directly or indirectly regulated byd, thereby providing protection against salinity stressinova et al., 2004). Exogenous application of salicylicced the photosynthetic rate and also maintained themembranes, thereby improved the growth of salin-barley plants (El Tayeb, 2005). The damaging effectsere also alleviated by exogenous application of SA inseedlings (Borsani et al., 2001). Kaydan et al. (2007)at pre-sowing soaking treatment of seeds with SA pos-cted the osmotic potential, shoot and root dry mass,and contents of photosynthetic pigments (chlorophyll

rotenoids) in wheat seedlings, under both saline andconditions. The loss of growth, photosynthetic parame-e activities of enzymes (nitrate reductase and carbonicas a result of salinity stress in B. juncea was revivedlic acid was sprayed to the foliage, at 30 days stage.activities of various antioxidant enzymes (CAT, POXandincreased with a concomitant increase in proline con-sult of salinity exposure and/or SA treatment, therebynhanced tolerance against salinity stress (Yusuf et al.,

of exogenous SA on plants grown under temperature

t stress. Deviation from optimum temperature resultserturbations in plant growth and development whichto membrane disruptions, metabolic alterations and

of oxidative stress (Mittler, 2002; Posmyk and Janas,

r, salicylic acid plays a key role in providing toleranceperature stress. A foliar spray of lower concentrationsacid conferred heat tolerance to mustard. Further thisaccompanied with hardening at 45 C for 1h enhancedvel and also reduced theCATactivity, thereby increasingal of plants towithstand the heat stress (Dat et al., 1998).sponse was observed in potato plantlets, raised froms, supplemented with lower concentrations of acetylid (Lopez-Delgado et al., 1998). Larkindale and Huangted out that the enhanced heat tolerance in plants oflonifera, pre-treated with salicylic acid was due to theof plants from oxidative damage. These authors furtherat the pre-treatment with salicylic acid had no effectivity, whereas, the CAT activity declined, compared towever, the treatment enhanced the activity of enzymeeroxidase. Contrary to this, an enhanced activity of CATs observed in heat stressed plants of Poa pratensis, afternt with salicylic acid (He et al., 2005). In a study car-Chakraborty and Tongden (2005), it was reported thatress induced membrane injury in the plants of Cicerhich was signicantly reduced by the application ofed to the heat acclimatized and untreated control. Thelso enhanced the protein and proline contents signif-h a concomitant induction of various stress enzymes

20 Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425

viz. POX and APX. However, the CAT activity was found to bereduced.

7.2.3.2. Cold stress. Besides providing tolerance to the plantsagainst heatance towareported anhydroponicThe treatmcence anddecline in Cactivities oalso observhyde aspirichilling street al., 2002)analogues mgrowth conconductancafter 1 daynormal groinjury mansignicantlytions of saliSaltveit, 200excised radsignicantlythe additionsevere damgenerated b

Exogenoeffects of loTasgin et alet al., 2003antioxidant(Kang et al.in the activlowing SA tchilling, the2003b). Preto affect thenhanced thet al., 2002tures (KorkmSA not onlybut was equ(Frost) inju

7.2.4. Effectradiation or

The leveby day andsis are unabon photosyand Ramanaging air pbetween nreleased duand Wang,crops. Prolobition of phpartitioning(Black et al.Sandermantect the plastress are o

plants accumulated large amounts of salicylic acid when exposedto ozone or UV radiations (Yalpani et al., 1994; Sharma et al., 1996).The role of salicylic acid in counteracting the damaging effectsof ozone was best demonstrated in Arabidopsis thaliana, where

utadeteroveenhaopsisportis incy of tthesi2004magilue gent eidantB extocopic aciallevnce.

Effectosurd bio, phlar curveyprovht orstresappd a stabiredter ce acer, ttionsetersHighntssalicumuwerprovato aruitfregued foul coght-drou-Bosulatenes.resueat sic acidryd tortheenzyf vat shock, exogenous salicylic acid also generates resis-rds chilling or cold stress. Janda et al. (1997, 1999)enhanced cold tolerance in maize plants, grown insolutions, supplemented with 0.5mM of salicylic acid.ent positively affected various parameters of uores-lowered those associated with electrolyte leakage. AAT activity with a concomitant enhancement in the

f glutathione reductase and guaiacol peroxidase wased. Besides, salicylic acid, its analogues like benzalde-n or coumaric acid also had a protective role againstss in maize plants (Janda et al., 1998, 2000; Horvath. However, it should be underlined here, that SA or itsay exert deleterious effects on plants under normal

ditions. A decline in net photosynthetic rate, stomatale and transpiration rate was observed in maize plantsof SA, benzaldehyde (BA) or aspirin treatment underwth conditions (Janda et al., 1998, 2000). The chillingifested in the form of electrolyte leakage in leaves wasreduced following the application of lower concentra-

cylic acid to maize, cucumber and rice plants (Kang and2). However, the extent of electrolyte leakage from theicals of cold stressed maize seedlings was not alteredby SA pre-treatment. Other studies have shown thatof salicylic acid to the hydroponic solution may cause

age to roots (Pal et al., 2002) indicating a toxic effecty SA.us salicylic acid potentially alleviates the damagingw temperatures in rice and wheat (Szalai et al., 2002;., 2003), bean (Senaratna et al., 2000) and banana (Kanga). Pre-treatment with salicylic acid activated variousenzymes inmaize (Janda et al., 1999, 2000) and banana, 2003b) exposed to chilling stress. Further the increaseities of antioxidant enzymes, SOD, CAT and APX fol-reatment was related to H2O2 metabolism produced byreby providing tolerance against the stress (Kang et al.,-treatmentwith salicylic acidor its analogueswas founde seed germination as well. SA or acetyl salicylic acide germination percentage of carrot seeds (Rajasekaran

) and in the seeds of Capsicum annum at low tempera-az, 2005). Tasgin et al. (2003) reported that exogenousprovided protection against heat and cold stresses,ally benecial in providing tolerance against freezing

ry to winter wheat.

of exogenous SA on the plants exposed to UVozone stressl of UV radiations in the environment is increasing daythe plants, which use direct sunlight for photosynthe-le to avoid UV radiations which imparts adverse effectsnthesis and other physiological processes (Rajendiranujam, 2003). Similarly, ozone is the other most dam-ollutant generated through photochemical reactionsitrogen oxides, carbon monoxide and hydrocarbons,ring the burning of fossil fuels in urban areas (Mauzerall2001) and is responsible for tremendous loses to ournged chronic exposure to ozone results in the inhi-otosynthesis, premature senescence, altered biomassultimately reducing the growth and yield of plants

, 2000; Pell et al., 1997; Saitanis and Karandinos, 2002;n, 1996). Therefore, the mechanisms which may pro-nts from the harmful effects of UV-exposure or ozonef particular concern. It has been reported earlier that

NahG mto theSA implead toArabidalso reand thactivitbiosynet al. (the datuky btreatmantioxby UV-and-salicylrole inirradia

7.2.5.Exp

ical angrowthof cellu

A srole in(drougwaternouslyrevealebrane snitratetive waand thHowevcentraparamstress.the plaacetylter accThe lonouslyin tomshow fhighlyof storstressfof drouunderMunneSA reghormo

Thethe whsalicylhigherSOD antrol. Futo thelevel onts, decient in SA biosynthesis were more sensitiveriorating effects of ozone (Sharma et al., 1996). Since,d the activity of antioxidant enzyme system, therefore,nced tolerance against ozone stress inNahGmutants of(Rao and Davis, 1999). Like ozone, UV radiations wereed to induce the accumulation of SA in tobacco plantsreased accumulation of SA was probably due to higherhe enzyme BAZ-hydroxylase, which is involved in SAs (Yalpani et al., 1994). In a study carried out by Ervin), the exogenous application of salicylic acid alleviatedng effects induced by UV-B radiation exposure in Ken-rass and tall fescue sod. These studies revealed that thenhanced photochemical efciency and the activities ofenzymes CAT and SOD which were greatly reduced

posure. The treatment also increased the anthocyaninherol contents in the UV-B stressed plants treatedwithd. Thus, it may be concluded that SA plays promotingiating the damaging effects of ozone and/or ultraviolet

of exogenous SA on plants exposed to water stresse of plants to water stress leads to serious physiolog-chemical dysfunctions including reduction in turgor,otosynthetic rate, stomatal conductance and damagesomponents (reviewed by Janda et al., 2007).of literature indicates that salicylic acid plays a keyiding tolerance to the plants, exposed to water stressooding). Hayat et al. (2008) studied the growth of

sed L. esculentum (tomato) plants in response to exoge-lied salicylic acid. The results of their experimentsignicant decline in photosynthetic parameters, mem-lity index, leaf water potential, activities of the enzymesuctase and carbonic anhydrase, chlorophyll and rela-ontents with a concomitant increase in proline contenttivities of antioxidant enzymes (CAT, POX and SOD).he treatment of these stressed plants with lower con-of salicylic acid signicantly enhanced the aforesaidthereby improved tolerance of the plants to droughter tolerance to drought stress was also observed inraised from the grains soaked in aqueous solution ofylic acid and the treatment also enhanced dry mat-lation (Hamada, 1998; Hamada and Al-Hakimi, 2001).concentrations of salicylic acid, when applied exoge-ided tolerance against the damaging effects of droughtnd bean plants, whereas, higher concentrations did notul results (Senaratna et al., 2000). Leaf senescence is alatedphysiological process, allowing the remobilizationod from the older leaves to the rest of the plant, duringnditions and salicylic acid is involved in the promotioninduced leaf senescence in Salvia ofcinalisplants grownght stress in Mediterranean eld conditions (Abreu andch, 2008). However, the authors also pointed out thats the leaf senescence in association with other phyto-

lts reported by Singh and Usha (2003) revealed thateedlings subjected to drought stress when treated withd, generally exhibited higher moisture content and alsomatter accumulation, carboxylase activity of Rubisco,tal chlorophyll content compared to the untreated con-r, the treatment also provided a considerable protectionme nitrate reductase thereby maintained the normalrious proteins in the leaves (Singh and Usha, 2003).

Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425 21

nduct

Exogenousing effects oconcomitanhave contrscarcity (Baance to plaof SA was aplants againsuspensionstapanus (

8. Conclus

Itmaybeacts as a potvarious plan

Exogenouproductiv

The owephytohormental pl

Exogenouthereby pstress.

Besides pattacks, Sthe plants

SA effectito the expperature,

Exogenouacid provgrowth aacteristic

everof stenous enzenouboliserferts,wh

ure pFig. 1. Model of the biosynthesis and action of salicylic acid on the i

application of salicylic acid also alleviated the damag-f water decit on cell membranes of barley plants andtly increased the ABA content in leaves, which mightibuted to the enhanced tolerance of plants to waterndurska and Stroinski, 2005). Besides providing toler-nts against drought stress, the exogenous applicationlso found to be effective in providing resistance to thest the excessive water stress as was observed in cell

s prepared from the fully turgid leaves of SporobcdusGhasempour et al., 2001).

Howlevel

Exogdant

Exogmeta

It intplan

9. Futions

concluded from the abovediscussion that salicylic acident plan growth regulator that can effectivelymodulatet growth responses.

s application of salicylic acid enhances the growth andity of plants.r inducing domain of salicylic acid makes it an importmone that can enhance owering in a variety of orna-ants.s application of salicylic acid induces the SAR in plants,rovides a considerable protection against various biotic

roviding protection against infections and pathogenA imparts tolerance against various abiotic stresses to.vely alleviated the toxic effects generated in plants dueosure to various abiotic stresses viz. Heavymetals, tem-water, ozone, UV irradiance and salinity stress etc.s application of the lower concentrations of salicyliced to be benecial in enhancing the photosynthesisnd various other physiological and biochemical char-s of plants.

Hence, iabove thatand potentvarious bioduced phytout to elucmajor or mnism of actiplayed by swork is alsbeing regulishedphytrange (auxi(NO, jasmoH2O2). Onerange phytotation andbroad rangrole of aforand growthchemical inmight incidconcentratigenes or racern. In fution of biotic and abiotic stress tolerance.

, at higher concentrations, SA itself may cause a highress in plants.s application of SA enhances the activities of antioxi-yme system.s SA can protect and enhance the enzymes of nitratem under stressful environments.es the indeterminate nodule formation in leguminousereas, subsequentnoduledevelopment isnothindered.

erspectivest may be resolved from the survey of literature citedsalicylic acid plays diverse physiological roles in plantsially alleviates the devastating effects generated bytic and abiotic stresses. However, this recently intro-ohormone still demands a lot of work to be carriedidate the exact pathways of its biosynthesis; weatherinor, key regulatory points of biosynthesis, mecha-on and other specic and collaborative regulatory rolesalicylic acid that have remained elusive till date. Theo needed on how this plant hormone interacts andlated by the cross-talk in harmony with other estab-ohormones andplant growth regulatorsworking at longns, cytokinins, gibberellins, ethylene etc.), short rangenates, brassinosteroids etc.) and very short range (ROS,could also argue how the regulated doses of these shorthormones mostly produced in-vicinity to biotic infes-

then transported systemically to play their role duringe abiotic stresses. It is also worthwhile to elucidate theesaid phytohormone in tissue-specic differentiationof plant parts during growth and development. Bio-

hibitors of key enzymes of pathways and mutant studyent some light on such aspects. Locating tissue-specicons during seedling development fusing with reporterdioactive molecules could pave the way in this con-ure, the exogenous application of this phytohormone

22 Q. Hayat et al. / Environmental and Experimental Botany 68 (2010) 1425

might act as a powerful tool in enhancing the growth, produc-tivity and also in combating the ill effects generated by variousabiotic stresses in plants (Fig. 1). The future applications of thisplant hormone holds a great promise as a management tool forproviding tconstrains cnear future

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