Mehar Fatma，Asim Masood，Tasir S.Per，F(xiàn)aisal Rasheed，Nafees A.Khan*Plant Physiology Section，Department of Botany，Aligarh Muslim University，Aligarh 202002，India

?

Interplay between nitric oxide and sulfur assimilation in salt tolerance in plants

Mehar Fatma，Asim Masood，Tasir S.Per，F(xiàn)aisal Rasheed，Nafees A.Khan*
Plant Physiology Section，Department of Botany，Aligarh Muslim University，Aligarh 202002，India

A R T I C L E I N F OA B S T R A C T

Article history：

Received in revised form

15 January 2016

Accepted 15 March 2016

Available online 1 April 2016

Abiotic stress

Antioxidant system

Glutathione

Plant hormones

Sulfur

Signaling

Nitric oxide（NO），a versatile molecule，plays multiple roles in plant growth and development and is a key signaling molecule in plant response to abiotic stress.Nutrient management strategy is critical for abiotic stress alleviation in plants.Sulfur（S）is important under stress conditions，as its assimilatory products neutralize the imbalances in cells created by excessive generation of reactive oxygen species（ROS）.NO abates the harmful effects of ROS by enhancing antioxidant enzymes，stimulating S assimilation，and reactingwithothertargetmolecules，andregulatestheexpressionofvarious stress-responsive genes under salt stress.This review focuses on the role of NO and S in responses of plants to salt stress，and describes the crosstalk between NO and S assimilation in salt tolerance.The regulation of NO and/or S assimilation using molecular biology tools may help crops to withstand salinity stress.

?2016 Crop Science Society of China and Institute of Crop Science，CAAS.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license

（http://creativecommons.org/licenses/by-nc-nd/4.0/）.

1.Introduction

The exposure of plants to high salt concentration creates ionic toxicity due to the accumulation of Na+and Cl-ions，which impair growth and development of plants.The accumulation of Na+ions in excess is largely responsible for the reductions in growth and yield under salinity.Salt stress disturbs the nutritional homeostasis of minerals［1］，causes membrane damage，inhibits enzyme activity，and alters levels of growth regulators and metabolic activity［2，3］.Salt stress induces reactive oxygen species（ROS）production and causes damage to cellular components，membrane lipids，proteins，and nucleic acids［4］.To prevent the effect of excess ROS production，plants develop multiple detoxification mechanisms.The best-studied mechanism is the induction of antioxidant systems，but the mechanism of enhancement of sulfur（S）-assimilation that induces the production of S compounds via the increased activity of ascorbate-glutathione pathway（AsA-GSH）enzymes has recently been suggested as a response to salttolerance［5-7］.

S is the fourth major essential plant nutrient element after nitrogen（N），phosphorus（P），and potassium（K）［6］.S deficiency substantially limits crop productivity.S is an integral part of major metabolic compounds，such as amino acids，methionine andcysteine（Cys），GSH，F(xiàn)e-Sclusters，sulfolipids，glucosinolates，vitamins（biotinandthiamine），coenzymeA，andthethioredoxin system，which regulate physiological processes and raise salt tolerance［5，6］.A critical concentration of S regulates chlorophyll content，N content，activity of photosynthetic enzymes，protein synthesis，andtheelectrontransportsystem［6］，andappropriate availability of S determines photosynthetic function under optimal and stressful environments and potentially mitigates salt-induced oxidative stress［5，7-9］.Studies of Nazar et al.［7］and Fatma et al.［10］have shown that S supplementation improved the photosynthetic efficiency of plants under salt stress via increased GSH production and activity of enzymes of the AsA-GSH cycle.GSH is a major source of non-protein thiols and acts as an important non-enzymatic antioxidant.The antioxidant system participates in stress resistance and amino acid transport across membranes［11］along with fulfilling other roles associated with redox sensing and signaling and provides protection against salt stress.

Phytohormones are chemical messengers derived from plant biosynthetic pathways that act at the site of their synthesis or aretransportedtosomeothersiteintheplanttomediategrowth and developmental responses under both optimal and stressful environments［12，13］.There are five groups of phytohormones: auxin，gibberellins，ethylene，cytokinin（CK），and abscisic acid （ABA）.There are also other compounds that have important growth-regulating activity and function as phytohormones. These include brassinosteroids，jasmonic acid（JA），and salicylic acid（SA）.Nitric oxide（NO）is considered a new member of this group［14］.It interacts with other signaling molecules to regulate physiological and molecular processes under optimal and stressfulenvironments.NOplaysanimportantroleinresistance to abiotic stresses such as salt，drought，temperature（high and low），UV-B，and heavy metal stress by its antioxidant properties and also by acting as a signal in inducing the activity of ROS-scavenging enzymes to alleviate oxidative stress［15］.It is involved in plant resistance reactions against biotic stresses and potentiatestheinductionofhypersensitivecelldeathinsoybean cells by reactive oxygen intermediates.It functions independentlyofsuch intermediatestoinducegenesforthesynthesisof protective natural products［16］and enhances adaptive responses to drought stress by inducing stomatal closure［17］.NO mediates ABA-induced stomatal closure via regulation of Ca2+fluxes［18］，and closely cooperates with JA，SA，and ethylene in cell responses to different stressors in a complex network［19］. NO functions as a signaling molecule and influences several morphological processes，such as seed germination，root formation，and de-etiolation，and physiological processes through increases in superoxide dismutase（SOD），catalase（CAT），and peroxidase（POD）antioxidant enzymes in Lupinus luteus［20］. Zheng et al.［21］reported that NO provides signals for salt tolerance by increasing the activity of SOD and CAT，decreasing lipid peroxidation and O2-generation rate in the mitochondria.

Independently conducted studies of the roles of NO and S have shown their involvement in salt stress tolerance by interaction with other signaling molecules.There could be interplay between NO and S assimilation in salt tolerance.The present review explains the action of NO and S in salt resistance and describes the potential interplay between NO and S assimilation in salt tolerance.

2.Nitric oxide：Its role in salt tolerance

NO was recognized during the last decade of the 20th century as a signaling molecule with multifaceted physiological roles in plants［22］.It is converted to one of three different species:the radical（NO），the nitrosonium cation，or the nitroxyl anion，showing high reactivity and tendency to bind with reduced heme proteins［23］.Exogenous NO donors constitute a powerful way to supplement plants with NO.Most of the NO donors are organic compounds that form NO complexes such as sodium nitroprusside（SNP）［24］.SNP is the most widely studied compound of the iron nitrosyl family.Studies of Velikova et al. ［25］and Courtois et al.［26］have shown a cytoprotective role of NOin photosynthesisbyaction asan antioxidantmolecule or by regulation of stomatal closure and interaction with Ca2+signals. Exogenous application of NO protected cells from oxidative damage under stress by enhancing antioxidant enzymes［27］. Plants emit NO from leaves and herbicide or NO2treatment enhances the release of NO［28］.In vivo nitrate reductase（NR）assays release NO［29］.Plant mitochondria also make NO from nitrite［30］.However，NO synthesis in plants appears more complex.Major sites of NO biosynthesis in plants are protoplasts，chloroplasts，mitochondria，and peroxisomes［31］.Fig.1 shows different sources of NO biosynthesis in plants.

TherelationshipofArabidopsisthalianaNITRICOXIDE SYNTHASE 1（AtNOS1）with NOS is debatable.It was previously recognized as a potential NO synthase（NOS）in A.thaliana and was shown to be involved in plant development and phytohormones action［32］.Based on similarity to a hypothetical snail NOS or NOS partner that cross-reacted with mammalian NOS antibody，potential NOS was identified in A.thaliana［32，33］. Upon knockout of the AtNOS1 gene in A.thaliana，reductions in root NO accumulation and NOS activity in leaf extracts were observed.Further，overexpression of AtNOS1 resulted in higher levels of NOS activity in leaf extracts.However，study ofrecombinant AtNOS1 revealed no production of the originally reportedNOSactivity［33］.RecentreportsconfirmthatAtNOS1is not a NOS and accordingly，it has been renamed NITRIC OXIDE ASSOCIATED PROTEIN1（AtNOA1）.It is now recognized to be a member of the circularly permuted GTPase family（cGTPase）［34，35］.Moreau et al.［35］reported that AtNOS1 was unable to bind and oxidize arginine to NO but specifically bound and hydrolyzedGTP.GTPaseactivitywasnecessarybutnotsufficient for its function in planta.cGTPases appeared to be RNA-binding proteins，and the closest homolog of AtNOA1，the Bacillus subtilis YqeH，and was shown to participate in ribosome assembly/ stability.Moreover，YqeH and AtNOA1 act as G-proteins that regulate nucleic acid recognition and not as NOS［34］.Zhao et al. ［36］reported that AtNOA1 functions in root waving.They reported that AtNOA1 modulated SA-induced root waving by affecting cytosolic Ca2+signaling and the PIN-FORMED2 based polar auxin transport pathway，giving new insight into the mechanisms that control root growth behavior.In Nicotiana benthamiana，gain-of-functionandloss-of-functionstudies showed that mitogen-activated protein kinase（MAPK）cascade MEK2-SIPK/NTF4 controls NO and ROS generation induced by elicitor INF1，with the latter also modulated by the MEK1-NTF6 cascade［30］.MPK6，the A.thaliana ortholog of SIPK，interacts in vitro and in vivo with NIA2 and phosphorylates NIA2 at a specific serine residue，leading to an increase in NR activity and NO biosynthesis in response to H2O2during A.thaliana root development［37］.

Fig.1-Sources of nitric oxide（NO）synthesis in plants.

The protective functions of NO under stressful environments have received attention in recent years.Several studies have shown the protective role of NO in salt-induced damage.The application of 50 μmol L-1SNP stimulated ROS-scavenging enzymes and reduced accumulation of H2O2induced by NaCl in mitochondriaofCucumissativusroots［38］.NOservesasasignalin inducingsaltresistancebyincreasingtheK+toNa+ratio，whichis dependent on increased PM H+-ATPase activity in Phragmites communis［39］.The lower concentration of 0.2 mmol L-1SNP was more effective in increasing CAT and glutathione reductase（GR）activity，whereas the higher concentration of 1 mmol L-1SNP was more effective in increasing SOD activity and decreasing membrane injury and lipid peroxidation levels under salt-stress conditions in Cicer arietinum［40］.A protective effect of NO on relative water content under salt stress has been reported in Zea maysleaves by increase in the activity of vacuolar H+-ATPase and H+-PPase，which provide the driving force for Na+/H+exchange ［41］.Exogenous NO application markedly decreased membrane permeability，rate of O2-production，contents of MDA and H2O2，and intercellular CO2concentration under 50 mmol L-1NaCl stress by increasing the activities of SOD，POD，CAT，and ascorbate peroxidase（APX），and the contents of photosynthetic pigments and proline［42］.The study of Lopez-Carrion et al.［43］focused on the possible relationship between NO and the induction of proline in response to salt stress and suggested that NO could mitigate the damage associated with salt stress. NO confers salt tolerance on Kosteletzkya virginica by preventing both oxidative membrane damage and translocation of Na+from roots to shoots［44］.NO produced under salt stress served as a second messenger for the induction of PM H+-ATPS expression ［39］.NOeffectivelyprotectedseedlingsagainstsaltstressdamage by enhancing activity of antioxidant enzymes to quench excess ROS caused by salt stress and promoting the increase of ferritin accumulation to chelate larger numbers of ferrous ions［45］. Exogenous application of NO to salt-grown plants reduced lipid peroxidation and ROS accumulation.Salinity-inhibited growth，measured as reduced leaf area and dry weight，was restored by NO in Zea mays［41］and Oryza sativa［46］.Pretreatment with NO effectively contributed to better balance between carbon and N metabolism by increasing total soluble protein and enhancing the activity of endopeptidase and carboxypeptidase in plants undersaltstress［47］.Zhang etal.［48］reported thatNOenhanced salt tolerance in Populus euphratica callus under salinity by increasing the K+/Na+ratio，where H2O2was involved in the increase of（PM）H+-ATPase activity.Liu et al.［49］showed that the glucose-6-phosphate dehydrogenase enzyme played an important role in NR-dependent NO production and in establishingtoleranceofPhaseolusvulgarisrootstosaltstress.Ruanet al.［50］reported thatNOstimulatedproline accumulation under salt stress，owing to NO-induced increase in K+in Triticum aestivum seedling roots under salt stress conditions.Several morpho-physiological parameters，growth，biomass attributes，photosynthetic rate，photosystem II activity，and gas exchange characteristics decreased under salt stress.NO alleviated the decrease of photosynthetic rate induced by non-stomatal factors and damage by photoinhibition to the photosynthetic system.Corpasetal.［51］suggestedthatadding appropriate SNP alleviated salt toxicity and improved net photosynthesis in Pisum sativum.Important studies describing the action of NO in salt tolerance are listed in Table 1，and Fig.2 shows the mechanisms of NO action in salt tolerance involving various processes in plant cells.

3.S assimilation：Its role in salt tolerance

The management of mineral nutrients plays a key role in augmenting the growth and development of economically important crop plants under varied environmental conditions［63，64］.Supplementation with S has significant role in protection against salt-induced oxidative damage［9］.Enzymes of the S-assimilatory pathway were induced by application of S under salt stress and helped in neutralizing or scavenging ROS ［5，7-10］.

Table 1-Studies showing response of plants to NO under salt stress.

S assimilation is highly regulated in a demand-driven manner［6，9，10，65，66］.S is taken up by roots in the form of sulfate.The uptake of sulfate by roots and transport to shoot are strictly controlled and appear as primary points of regulation of S assimilation.Sulfate reduction takes place in leaf chloroplasts and produces sulfide.The key regulatory steps of sulfate assimilation are the activation of sulfate in cells by ATP-sulfurylase（ATPS）and the reduction of adenosine 5′-phosphosulfate（APS）to sulfite by APS reductase（APR）［67］.Sulfite is reduced by sulfite reductase with ferredoxin as a reductant and the sulfide formed is further incorporated into Cys by coupling to O-acetyl serine（OAS）.The process is controlled by the enzyme OAS thiol lyase，also called Cys synthase.Cys is used for the production of GSH.

GSH is an important S-containing compound associated with the exclusion of ROS［4］.The S-containing group thiol is strongly nucleophilicandsuitableforbiologicalredoxreactionsandplays an important role in protection against salt stress-induced oxidative damage［9］.Astolfi et al.［68］reported that salt-stress affected root thiol content through its effects on the rate of S assimilation.External S supply improves salinity tolerance by meeting the demand for GSH synthesis via increased Cys synthesis［9］.Plants grown with salt showed increased S assimilation，resulting in higher Cys biosynthesis required for increased GSH production and defense responses to salt stress ［69-72］.Thus，the regulation of synthesis of S-containing compounds using genetic tools offers potential option for increasing salt tolerance.The upregulation of Cys synthesis in A.thaliana in response to salt stress［73］augments GSH content，which in turn plays a protective role against salt stress in the plant.In addition，transgenic approaches have been employed successfullytoincreasethecapacityofsalttoleranceinplantsby manipulation of S assimilation and metabolism.In plants，S-containing compounds such as methionine，thioredoxins，vitamins，and coenzyme A play an important role in salt stress responses in addition to Cys and GSH［72］.Methionine acts as a regulatory molecule as part of S-adenosyl methionine（SAM）. The level of SAM synthase increases significantly under salt stress，suggesting the sensitivity of the methionine pathwayto salt stress，and its supplementation has been reported to increase salt tolerance［5，72］.Thioredoxins are small （12-13 kDa）ubiquitous，heat-stable proteins involved in responses of plants to salt stress［5］.They participate in ROS metabolism and reduce H2O2production by acting as a hydrogen donor and signal for plant salt stress responses ［4，72］.El-Shintinawy et al.［74］reported that the addition of thiamine to medium alleviated salt stress by increasing the contents of Cys and methionine.Table 2 lists studies of the role of S assimilation in salt tolerance.

Fig.2-Nitric oxide signaling in plant cells under stressful environments.Salt stress induces reactive nitrogen species（RNS）and reactive oxygen species（ROS）and causes oxidative damage in plant cells.NO induces S-nitrosylation and formation of nitrosothiols，thereby inducing the expression of stress-associated proteins.NO also influences the antioxidant system and increases the activity of antioxidant enzymes that subsequently inhibit ROS generation and limit oxidative damage.NO influences signaling of abscisic acid（ABA），jasmonic acid（JA），salicylic acid（SA），and ethylene under stress.These phytohormones induce antioxidant system in plants resulting in induced stress tolerance.

4.Interplay between NO and S assimilation in regulation of salt tolerance

Coordination between phytohormones and nutritional signaling playsanimportantroleinsalttolerance.Phytohormonessuchas JA，ABA，SA，and ethylene play important roles in the accumulation of GSH under stressful conditions［66］.Phytohormones such as ethylene and SA affect salt tolerance by regulating S metabolism［8，85，86］.Moreover，theinteractionbetweenROSand the AsA-GSH cycle triggers the synthesis of JA and SA［87］，which further help in the upregulation of nutrient uptake under nutrient deficit conditions.It has been suggested that NO is involved in reducing oxidative stress through GSH formation under salt stress［88］.Supplementation with S-regulated GSH contentinplantsthroughinducedSassimilation［10］.Thus，there is a possibility of interaction between NO and S assimilation. Barroso et al.［89］showed that both NO and NO-derived peroxynitrite react with GSH to generate S-nitrosoglutathione （GSNO），which behaves as an NO donor in plant tissues.The enzyme GSNO reductase（GSNOR）involved in the formation of GSNO played an important role in alleviating and limiting NO in plant cells under stress conditions［89，90］.The reduction of the metabolite GSNO to oxidized glutathione（GSSG）and NH3is catalyzedbyGSNOR.TheresultingGSSGisthenreduced againto GSH in an NADPH-dependent reaction catalyzed by GR［14］. Figure 3 shows that application of both NO and S reduces NaCl-induced oxidative stress by regulating NO generation in plants receiving S.An improvement in GSH content and redox stateresulted inenhanced salttolerancethroughanincrease in the antioxidant system and efficiency of AsA-GSH cycle［91］. NO triggers expression of redox-regulated defense-associated genes directly or indirectly to establish stress tolerance［92］.NO enhances the activity of SOD and CAT，which separately contribute to a delay in H2O2accumulation and elevate the proportions of GSH/GSSG and AsA/DHA in T.aestivum leaves to protect from oxidative damage caused by salt stress［93］. Plants with impaired GSNO metabolism are compromised in several physiological processes.Both GSNO and GSNOR were downregulated under cadmium stress in P.sativum［89］，indicating that fine control of S-nitrothiol homoeostasis was important for growth，development，and resistance to abiotic stresses［94］.However，the peroxynitrite formed from the reaction of NO with ROS has been found to be one of the major cell antioxidants that are important in the control of ROS metabolism.Baudouin and Hancock［95］summarized GSNO metabolism and suggested the occurrence and function of GSNO in plant cells，along with other NO reaction products. These are important issues for future understanding of the role of NO in plant development and stress responses.Sehrawat et al.［96］suggested that the depletion of Rubisco from samples improved proteome coverage of cold-responsive S-nitrosylated targets inBrassicajuncea.Recently，ithas been proposedthatNO negatively regulates CK signaling by limiting phosphorelay activity via S-nitrosylation［97］.NO also activates different biochemical pathways and interacts with metals to produce metal proteins，and with sulfhydryl groups and nitro groups in the process of nitration to provide resistance against salt stress ［14，98］.

Table 2-Studies showing response of some plants to S assimilation under salt stress.

5.Conclusion and future prospects

NO plays an important role in tolerance to saltstress inplants.It acts as a signal molecule and induces salt tolerance in plants by enhancing S assimilation and synthesis of S compounds and modulating the activity of antioxidant enzymes.Interaction between NO and S assimilation regulates GSH synthesis for the adaptation of plants to stressful environments.The regulatory interaction between NO and S can be manipulated for adjusting plants to the changing environment for sustainable agricultural development.However，studiesareneededtotracethesignaling pathways of NO action and its biosynthesis in response to environmental cues.Moreover，the physiological and molecular mechanisms by which NO induces S assimilation，and how it interacts with other plant hormones and nutrients to achieve plant salt tolerance，await investigation.

Acknowledgments

INSPIRE Fellowship to the first author by the Department of Science&Technology，New Delhi and research facilities in lab of NAK in the DBT-BUILDER programme（No.BT/PR4872/INF/ 22/150/2012）of Department of Biotechnology，New Delhi，are gratefully acknowledged.

R E F E R E N C E S

［1］R.Munns，M.Tester，Mechanisms of salinity tolerance，Annu. Rev.Plant Biol.59（2008）651-681.

［2］S.Mahajan，N.Tuteja，Cold，salinity and drought stresses:an overview，Arch.Biochem.Biophys.444（2005）139-158.

［3］M.Hasanuzzaman，K.Nahar，M.Fujita，Plant response to salt stress and role of exogenous protectants to mitigate salt-induced damages，in:P.Ahmad，M.M.Azooz，M.N.V. Prasad（Eds.），Ecophysiology and Responses of Plants under Salt Stress，Springer，New York 2013，pp.25-87.

［4］G.Noctor，A.Mhamdi，S.Chaouch，Y.Han，J.Neukermans，B. Marquez-Garcia，G.Queval，C.H.Foyer，Glutathione in plants: an integrated overview，Plant Cell Environ.35（2012）454-484.

［5］M.Fatma，M.I.R.Khan，A.Masood，N.A.Khan，Coordinate changes in assimilatory sulfate reduction are correlated to salt tolerance:involvement of phytohormones，Annu.Rev. Res.Biol.3（2013）267-295.

［6］H.Marschner，Mineral Nutrition in Higher Plants，Academic Press，London，1995 405-435.

［7］R.Nazar，S.Umar，N.A.Khan，Exogenous salicylic acid improves photosynthesis and growth through increase in ascorbateglutathione metabolism and S assimilation in mustard under salt stress，Plant Signal.Behav.10（2015），e1003751.

［8］R.Nazar，S.Umar，N.A.Khan，Involvement of salicylic acid in sulfur induced salinity tolerance:a role of glutathione，Annu. Res.Rev.Biol.4（2013）3875-3893.

［9］R.Nazar，N.Iqbal，A.Masood，S.Syeed，N.A.Khan，Understanding the significance of sulfur in improving salinity tolerance in plants，Environ.Exp.Bot.70（2011）80-87.

［10］M.Fatma，M.Asgher，A.Masood，N.A.Khan，Excess sulfur supplementation improves photosynthesis and growth in mustard under salt stress through increased production of glutathione，Environ.Exp.Bot.107（2014）55-63.

［11］J.Wang，P.P.Sun，C.L.Chen，Y.Wang，X.Z.Fu，J.H.Liu，An arginine decarboxylase gene PtADC from Poncirus trifoliate confers abiotic stress tolerance and promotes primary root growth in Arabidopsis，J.Exp.Bot.62（2011）2899-2914.

［12］L.Ozturk，M.A.Yazici，C.Yucel，C.A.Aorun，C.Cekic，A.Bagci，I.Cakmak，Concentration and localization of zinc during seed development and germination in wheat，Physiol.Plant.128 （2006）144-152.

［13］Z.Peleg，E.Blumwald，Hormone balance and abiotic stress toleranceincropplants，Curr.Opin.PlantBiol.14（2011）290-295.

［14］M.Leterrier，M.Airaki，J.M.Palma，M.Chaki，J.B.Barroso，F(xiàn).J. Corpas，Arsenic triggers the nitric oxide（NO）and S-nitrosoglutathione（GSNO）metabolism in Arabidopsis，Environ.Pollut.166（2012）136-143.

［15］M.H.Siddiqui，M.H.Al-Whaibi，M.O.Basalah，Role of nitric oxide in tolerance of plants to abiotic stress，Protoplasma 248 （2011）447-455.

［16］M.Delledonne，Y.Xia，R.A.Dixon，C.Lamb，Nitric oxide functions as a signal in plant disease resistance，Nature 394 （1998）585-588.

［17］C.Garc??a-Mata，L.Lamattina，Nitric oxide induces stomatal closure and enhances the adaptive plant responses against drought stress，Plant Physiol.126（2001）1196-1204.

［18］S.Neill，R.Barros，J.Bright，R.Desikan，J.Hancock，J.Harrison，I.Wilson，Nitric oxide，stomatal closure，and abiotic stress，J. Exp.Bot.59（2008）165-176.

［19］M.Arasimowicz，J.Floryszak-Wieczorek，Nitric oxide as a bioactive signaling molecule in plant stress responses，Plant Sci.172（2007）876-887.

［20］M.Kopyra，E.A.Gwozdz，Nitric oxide stimulates seed germination and counteracts the inhibitory effect of heavy metal and salinity on root growth of Lupinus luteus，Plant Physiol.Biochem.41（2003）1011-1017.

［21］C.Zheng，D.Jiang，F(xiàn).Liu，T.Dai，W.Liu，Q.Jing，W.Cao，Exogenous nitric oxide improves seed germination in wheat against mitochondrial oxidative damage induced by high salinity，Environ.Exp.Bot.67（2009）222-227.

［22］L.A.Mur，J.Mandon，S.Persijn，S.M.Cristescu，I.E.Moshkov，G.V.Novikova，K.J.Gupta，Nitric oxide in plants:an assessment of the current state of knowledge，AoB Plants 5 （2013），pls052.

［23］D.D.Thomas，M.G.Espey，M.P.Vitek，K.M.Miranda，D.A. Wink，Protein nitration is mediated by heme and free metals through Fenton-type chemistry:an alternative to the NO/O reaction，Proc.Natl.Acad.Sci.U.S.A.99（2002）12691-12696.

［24］Y.C.Hou，A.Janczuk，P.G.Wang，Current trends in the development of nitric oxide donors，Curr.Pharm.Des.5（1999）417-442.

［25］V.Velikova，S.Fares，F(xiàn).Loreto，Isoprene and nitric oxide reduce damages in leaves exposed to oxidative stress，Plant Cell Environ.31（2008）1882-1894.

［26］C.Courtois，A.Besson，J.Dahan，S.Bourque，G.Dobrowolska，A.Pugin，D.Wendehenne，Nitric oxide signaling in plants: interplays with Ca2+and protein ?eli?k kinases，J.Exp.Bot.59 （2008）155-163.

［27］X.Wu，W.Zhu，H.Zhang，H.Ding，H.J.Zhang，Exogenous nitric oxide protects against salt-induced oxidative stress in the leaves from two genotypes of tomato（Lycopersicom esculentum Mill.），Acta Physiol.Plant.33（2011）1199-1209.

［28］H.Nishimura，T.Hayamizu，Y.Yanagisawa，Reduction of NO2to NO by rush and other plants，Environ.Sci.Technol.204 （1986）13-416.

［29］L.A.Klepper，Nitric oxide emissions from soybean leaves during in vivo nitrate reductase assays，Plant Physiol.85 （1987）96-99.

［30］E.Planchet，J.K.Gupta，M.Sonoda，W.M.Kaiser，Nitric oxide emission from tobacco leaves and cell suspensions:rate limiting factors and evidence for the involvement of mitochondrial electron transport，Plant J.41（2005）732-743.

［31］T.Roszer，The Biology of Subcellular Nitric Oxide，Springer，Heidelberg/New York，2012 49-66.

［32］F.Q.Guo，M.Okamoto，N.M.Crawford，Identification of a plant nitric oxide synthase gene involved in hormonal signaling，Science 302（2003）100-103.

［33］N.M.Crawford，F(xiàn).Q.Guo，New insights into nitric oxide metabolism and regulatory functions，Trends Plant Sci.10 （2005）195-200.

［34］J.Sudhamsu，G.I.Lee，D.F.Klessig，B.R.Crane，The structure of YqeH an AtNOS1/AtNOA1 ortholog that couples GTP hydrolysis to molecular recognition，J.Biol.Chem.283（2008）32968-32976.

［35］M.Moreau，G.I.Lee，Y.Wang，B.R.Crane，D.F.Klessig，AtNOS/ AtNOA1 is a functional Arabidopsis thaliana cGTPase and not a nitric-oxide synthase，J.Biol.Chem.283（2008）32957-32967.

［36］X.Zhao，J.Wang，J.Yuan，X.L.Wang，Q.P.Zhao，P.T.Kong，X. Zhang，NITRIC OXIDE-ASSOCIATED PROTEIN1（AtNOA1）is essential for salicylic acid-induced root waving in Arabidopsis thaliana，New Phytol.207（2015）211-224.

［37］P.Wang，Y.Du，Y.Li，D.Ren，C.P.Song，Hydrogen peroxidemediated activation of MAP kinase 6 modulates nitric oxide biosynthesis and signal transduction in Arabidopsis，Plant Cell 22（2010）2981-2998.

［38］Q.Shi，F(xiàn).Ding，X.Wang，M.Wei，Exogenous nitric oxide protect cucumber roots against oxidative stress induced by salt stress，Plant Physiol.Biochem.45（2007）542-550.

［39］L.Zhao，F(xiàn).Zhang，J.Guo，Y.Yang，B.Li，L.Zhang，Nitric oxide functions as a signal in salt resistance in the calluses from two ecotypes of reed，Plant Physiol.134（2004）849-857.

［40］S.Sheokand，V.Bhankar，V.Sawhney，Ameliorative effect of exogenous nitric oxide on oxidative metabolism in NaCl treated chickpea plants，Braz.J.Plant Physiol.22（2010）81-90. ［41］Y.Zhang，L.Wang，Y.Liu，Q.Zhang，Q.Wei，W.Zhang，Nitric oxide enhances salt tolerance in maize seedlings through increasing activities of proton-pump and Na+/H+antiport in the tonoplast，Planta 224（2006）545-555.

［42］H.Fan，S.Guo，Y.Jiao，R.Zhang，J.Li，Effects of exogenous nitric oxide on growth，active oxygen species metabolism，and photosynthetic characteristics in cucumber seedlings under NaCl stress，F(xiàn)ront.Agric.China 1（2007）308-314.

［43］A.I.Lopez-Carrion，R.Castellano，M.A.Rosales，J.M.Ruiz，L. Romero，Role of nitric oxide under saline stress:implications on proline metabolism，Biol.Plant.52（2008）587-591.

［44］Y.Guo，Z.Tian，D.Yan，J.Zhang，P.Qin，Effects of nitric oxide on salt stress tolerance in Kosteletzkya virginica，Life Sci.J.6 （2009）67-75.

［45］Q.Y.Li，H.B.Niu，J.Yin，M.B.Wang，H.B.Shao，D.Z.Deng，Y.C. Li，Protective role of exogenous nitric oxide against oxidativestress induced by salt stress in barley（Hordeum vulgare），Colloids Surf.B:Biointerfaces 65（2008）220-225.

［46］A.Uchida，A.T.Jagendorf，T.Hibino，T.Takabe，T.Takabe，Effects of hydrogen peroxide and nitric oxide on both salt and heat stress tolerance in rice，Plant Sci.163（2002）515-523.

［47］J.Song，G.Shi，S.Xing，M.Chen，B.Wang，Effects of nitric oxide and nitrogen on seedling emergence，ion accumulation，and seedling growth under salinity in the euhalophyte Suaeda salsa，J.Plant Nutr.Soil Sci.172（2009）544-549.

［48］F.Zhang，Y.Wang，Y.Yang，H.A.O.Wu，D.I.Wang，J.Liu，Involvement of hydrogen peroxide and nitric oxide in salt resistance in the calluses from Populus euphratica，Plant Cell Environ.30（2007）775-785.

［49］Y.Liu，R.Wu，Q.Wan，G.Xie，Y.Bi，Glucose-6-phosphate dehydrogenase plays a pivotal role in nitric oxide-involved defense against oxidative stress under salt stress in red kidney bean roots，Plant Cell Physiol.48（2007）511-522.

［50］H.Ruan，W.Shen，M.Ye，L.Xu，Protective effects of nitric oxide on salt stress-induced oxidative damage to wheat （Triticum aestivum L.）leaves，Chin.Sci.Bull.47（2002）677-681.

［51］F.J.Corpas，J.B.Barroso，A.Carreras，R.Valderrama，J.M. Palma，A.M.León，L.A.Del Río，Constitutive argininedependent nitric oxide synthase activity in different organs of pea seedlings during plant development，Planta 224（2006）246-254.

［52］A.?eli?k，F(xiàn).Eraslan，Effects of exogenous nitric oxide on mineral nutrition and some physiological parameters of maize grown under salinity stress，Ziraat Fak.Dergisi-Süleyman Demirel üniv.10（2015）55-64.

［53］W.Liu，R.J.Li，T.T.Han，W.Cai，Z.W.Fu，Y.T.Lu，Salt stress reduces root meristem size by nitric oxide-mediated modulation of auxin accumulation and signaling in Arabidopsis，Plant Physiol.168（2015）343-356.

［54］J.Manai，H.Gouia，F(xiàn).J.Corpas，Redox and nitric oxide homeostasis are affected in tomato（Solanum lycopersicum）roots under salinity-induced oxidative stress，J.Plant Physiol. 171（2014）1028-1035.

［55］S.Liu，Y.Dong，L.Xu，J.Kong，Effects of foliar applications of nitric oxide and salicylic acid on salt-induced changes in photosynthesis and antioxidative metabolism of cotton seedlings，Plant Growth Regul.73（2014）67-78.

［56］J.Chen，Q.Xiao，C.Wang，W.H.Wang，F(xiàn).H.Wu，B.Y.He，Z. Zhu，L.L.Ru，L.L.Zhang，H.L.Zheng，Nitric oxide alleviates oxidative stress caused by salt in leaves of a mangrove species，Aegiceras corniculatum，Aquat.Bot.117（2014）41-47.

［57］H.F.Fan，C.X.Du，S.R.Guo，Nitric oxide enhances salt tolerance in cucumber seedlings by regulating free polyamine content，Environ.Exp.Bot.86（2013）52-59.

［58］M.Keyster，A.Klein，N.Ludidi，Caspase-like enzymatic activity and the ascorbate-glutathione cycle participate in salt stress tolerance of maize conferred by exogenously applied nitric oxide，Plant Signal.Behav.7（2012）349-360.

［59］M.Hasanuzzaman，M.A.Hossain，M.Fujita，Nitric oxide modulates antioxidant defense and the methylglyoxal detoxification system and reduces salinity-induced damage of wheat seedlings，Plant Biotechnol.Rep.5（2011）353-365.

［60］C.Zheng，D.Jiang，T.Dai，Q.Jing，W.Cao，Effects nitroprusside，a nitric oxide donor，on carbon and nitrogen metabolism and the activity of the antioxidation system in wheat seedlings under salt stress，Acta Ecol.Sin.30（2010）1174-1183（in Chinese with English abstract）.

［61］S.Sheokand，A.Kumari，V.Sawhney，Effect of nitric oxide and putrescine on antioxidative responses under NaCl stress in chickpea plants，Physiol.Mol.Biol.Plant 14（2008）355-362.

［62］H.H.Ruan，W.B.Shen，L.L.Xu，Nitric oxide modulates the activities of plasma membrane H+-ATPase and PPase in wheat seedling roots and promotes the salt tolerance against salt stress，Acta Bot.Sin.46（2004）415-422.

［63］O.G.Polesskaya，E.I.Kashirina，N.D.Alekhina，Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition，Russ.J.Plant Physiol.53（2006）186-192.

［64］A.Anjum，S.Umar，A.Ahmad，M.Iqbal，N.A.Khan，Sulfur protects mustard（Brassica campestris L.）from cadmium toxicity by improving leaf ascorbate and glutathione，Plant Growth Regul.54（2008）271-279.

［65］A.G.Lappartient，B.Touraine，Demand-driven control of root ATP-sulfurylase activity and SO42-uptake in intact canola，Plant Physiol.111（1996）147-157.

［66］S.Kopriva，S.H.Rennenberg，Control of sulfate assimilation and glutathione synthesis:interaction with N and C metabolism，J. Exp.Bot.55（2004）1831-1842.

［67］P.Vauclare，S.Kopriva，D.Fell，M.Suter，L.Sticher，P.Von Ballmoos，U.Kr?henbühl，R.P.denCamp，C.Brunold，F(xiàn)luxcontrol of sulphate assimilation in Arabidopsis thaliana:adenosine 5'-phosphosulphate reductase is more susceptible than ATP sulfurylasetonegativecontrolbythiols，PlantJ.31（2002）729-740.

［68］S.Astolfi，S.Zuchi，H.M.Hubberten，R.Pinton，R.Hoefgen，Supply of sulfur to S-deficient young barley seedlings restores their capability to cope with iron shortage，J.Exp. Bot.61（2010）799-806.

［69］L.Rais，A.Masood，A.Inam，N.Khan，Sulfur and nitrogen coordinately improve photosynthetic efficiency，growth andproline accumulation in two cultivars of mustard under salt stress，J.Plant Biochem.Physiol.1（2013）101.

［70］C.Lopez-Berenguera，M.Carvajala，C.Garcea-Viguerab，C.F. Alcaraz，Nitrogen，phosphorus，and sulfur nutrition in Broccoli plants grown under salinity，J.Plant Nutr.30（2007）1855-1870.

［71］N.A.Khan，R.Nazar，N.A.Anjum，Growth，photosynthesis and antioxidant metabolism in mustard（Brassica juncea L.）cultivars differing in ATP-sulfurylase activity under salinity stress，Sci.Hortic.122（2009）455-460.

［72］M.I.R.Khan，M.Asgher，N.Iqbal，N.A.Khan，Potentiality of sulfur-containing compounds in salt stress tolerance，in:P. Ahmad，M.M.Azooz，M.N.V.Prasad（Eds.），Ecophysiology and Responses of Plants Under Salt Stress，Springer，New York 2013，pp.443-472.

［73］L.C.Romero，J.R.Domínguez-Solís，G.Gutiérrez-Alcalá，C. Gotor，Salt regulation of O-acetylserine（thiol）lyase in Arabidopsis thaliana and increased tolerance in yeast，Plant Physiol.Biochem.39（2001）643-647.

［74］F.El-Shintinawy，M.N.El-Shourbagy，Alleviation of changes in protein metabolism in NaCl-stressed wheat seedlings by thiamine，Biol.Plant.44（2001）541-545.

［75］S.Astolfi，S.Zuchi，Adequate S supply protects barley plants from adverse effects of salinity stress by increasing thiol contents，Acta Physiol.Plant.35（2013）175-181.

［76］M.I.R.Khan，N.Iqbal，D.A.Masood，N.A.Khan，Variation in salt tolerance of wheat cultivars:role of glycinebetaine and ethylene，Pedosphere 22（2012）（2012）746-754.

［77］R.Nazar，N.Iqbal，S.Syeed，N.A.Khan，Salicylic acid alleviates decreases in photosynthesis under salt stress by enhancing nitrogen and sulfur assimilation and antioxidant metabolism differentially in two mungbean cultivars，J.Plant Physiol.168 （2011）807-815.

［78］C.Herschbach，M.Teuber，M.Eiblmeier，P.Ache，A.Polle，J.P. Schnitzler，H.Rennenberg，Changes in sulphur metabolism of grey poplar（Populus×canescens）leaves during salt stress:a metabolic link to photorespiration，Tree Physiol.30（2010）1161-1173.

［79］Q.S.Wu，Y.N.Zou，Adaptive responses of birch-leaved pear （Pyrus betulaefolia）seedlings to salinity stress，Not.Bot.Horti. Agrobot.Cluj-Napoca 37（2009）133-138.

［80］C.A.Jaleel，P.Manivannan，G.M.A.Lakshmanan，R.Sridharan，R.Panneerselvam，NaCl as a physiological modulator of proline metabolism and antioxidant potential in Phyllanthus amarus，C.R.Biol.330（2007）806-813.

［81］E.Fediuca，S.H.Lipsa，L.Erdei，O-acetylserine（thiol）lyase activity in Phragmites and Typha plants under cadmium and NaCl stress conditions and the involvement of ABA in the stress response，J.Plant Physiol.162（2005）865-872.

［82］D.A.Meloni，M.A.Oliva，C.A.Martinez，J.Cambraia，Photosynthesis and activity of superoxide dismutase，peroxidase and glutathione reductase in cotton under salt stress，Environ.Exp.Bot.49（2003）69-76.

［83］V.Mittova，F(xiàn).L.Theodoulou，G.Kiddle，L.Gómez，M.Volokita，M. Tal，M.Guy，Coordinate induction of glutathione biosynthesis and glutathione-metabolizing enzymes is correlated with salt tolerance in tomato，F(xiàn)EBS Lett.554（2003）417-421.

［84］A.Shalata，V.Mittova，M.Volokita，M.Guy，M.Tal，Response of the cultivated tomato and its wild salt-tolerant relative Lycopersicon pennellii to salt-dependent oxidative stress:the root antioxidative system，Physiol.Plant.112（2001）487-494.

［85］N.Iqbal，A.Masood，M.I.R.Khan，M.Asgher，M.Fatma，N.A. Khan，Crosstalk between sulfur assimilation and ethylene signaling in plants，Plant Signal.Behav.8（2013），e22478.

［86］M.Asgher，N.A.Khan，M.I.R.Khan，M.Fatma，A.Masood，Ethylene production is associated with alleviation of cadmium-induced oxidative stress by sulfur in mustard types differing in ethylene sensitivity，Ecotoxicol.Environ. Saf.106（2014）54-61.

［87］A.Mhamdi，J.Hager，S.Chaouch，Arabidopsis glutathione reductase 1 plays a crucial role in leaf responses to intracellular H2O2and in ensuring appropriate gene expression through both salicylic acid and jasmonic acid signaling pathways，Plant Physiol.153（2010）1144-1160.

［88］M.Fatma，N.A.Khan，Nitric oxide protects photosynthetic capacity inhibition by salinity in Indian mustard，J.Funct. Environ.Bot.4（2014）106-116.

［89］J.B.Barroso，F(xiàn).J.Corpas，A.Carreras，M.Rodríguez-Serrano，F(xiàn).J. Esteban，A.Fernández-Oca?a，L.A.del Río，Localization of S-nitrosoglutathione and expression of S-nitrosoglutathione reductase in pea plants under cadmium stress，J.Exp.Bot.57 （2006）1785-1793.

［90］U.Lee，C.Wie，B.O.Fernandez，M.Feelisch，E.Vierling，Modulation of nitrosative stress by S-nitrosoglutathione reductase is critical for thermotolerance and plant growth in Arabidopsis，Plant Cell 20（2008）786-802.

［91］M.Dr??kiewicz，E.Skórzyńska-Polit，Z.Krupa，Response of the ascorbate-glutathione cycle to excess copper in Arabidopsis thaliana（L.），Plant Sci.164（2003）195-202.

［92］C.H.Sung，J.K.Hong，Sodium nitroprusside mediates seedling seedling development and attenuation of oxidative stresses in Chinese cabbage，Plant Biotechnol.Rep.4（2010）243-251.

［93］H.H.Ruan，W.B.Shen，K.L.Liu，L.L.Xu，Effect of exogenous NO donor on glutathione dependent antioxidative system in wheat seedling leaf under salt stress，Acta Agron.Sin.31 （2005）1144-1149（in Chinese with English abstract）.

［94］F.J.Corpas，J.D.Alche，J.B.Barroso，Current overview of S-nitrosoglutathione（GSNO）in higher plants，F(xiàn)ront.Plant Sci.4 （2013）1-3.

［95］E.Baudouin，J.T.Hancock，Nitric oxide signaling in plants，F(xiàn)ront.Plant Sci.4（2014）533.

［96］A.Sehrawat，J.K.Abat，R.Deswal，Rubisco depletion improved proteome coverage of cold responsive S-nitrosylated targets in Brassica juncea，F(xiàn)ront.Plant Sci.4（2013）342.

［97］J.Feng，C.Wang，Q.Chen，H.Chen，B.Ren，X.Li，J.Zuo，S-nitrosylation of phosphotransfer proteins represses cytokinin signaling，Nat.Commun.4（2013）1529.

［98］M.Leterrier，M.Chaki，M.Airaki，R.Valderrama，J.M.Palma，J.B.Barroso，F(xiàn).J.Corpas，F(xiàn)unction of S-nitrosoglutathione reductase（GSNOR）in plant development and under biotic/ abiotic stress，Plant Signal.Behav.6（2011）789-793.

18 September 2015

*Corresponding author.

E-mail address:naf9@lycos.com（N.A.Khan）.

Peer review under responsibility of Crop Science Society of China and Institute of Crop Science，CAAS.

http://dx.doi.org/10.1016/j.cj.2016.01.009

2214-5141/?2016 Crop Science Society of China and Institute of Crop Science，CAAS.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license（http://creativecommons.org/licenses/by-nc-nd/4.0/）.