Relationship of Melatonin and Salicylic Acid in Biotic/Abiotic Plant Stress Responses

16 Pages·2017·1.92 MB·English

agronomy
Review
Relationship of Melatonin and Salicylic Acid
in Biotic/Abiotic Plant Stress Responses
JosefaHernández-RuizandMarinoB.Arnao* ID
DepartmentofPlantBiology(PlantPhysiology),FacultyofBiology,UniversityofMurcia,CampusdeEspinardo,
30100-Murcia,Spain;[email protected]
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:22February2018;Accepted:21March2018;Published:22March2018
Abstract:Melatonin(N-acetyl-5-methoxytryptamine)wasdiscoveredinplantsin1995,whilesalicylic
acidwasthenamegiventotheactiveingredientofwillowin1838. Fromaphysiologicalpointof
view,thesetwomoleculespresentinplantshaveneverbeencompared,eventhoughtheyhavea
greatnumberofsimilarities,asweshallseeinthiswork. Bothmoleculeshavebiosynthesispathways
thatshareacommonprecursorandbothplayarelevantroleinthephysiologyofplants,especially
in aspects related to biotic and abiotic stress. They have also been described as biostimulants of
photosyntheticprocessesandproductivityenhancersinagriculturalcrops. Wereviewthecoincident
aspectsofbothmolecules,andproposeanactionmodel,bywhichtherelationshipbetweenthese
moleculesandotheragentsandplanthormonescanbestudied.
Keywords: antioxidant;nitricoxide(NO);phytomelatonin;planthormones;plantstress;Reactive
oxygenspecies(ROS);salicylicacid
1. MelatoninandSalicylicAcidinPlants
Melatonin(N-acetyl-5-methoxytryptamine)isapleiotropicmoleculewithawiderangeofcellular
andphysiologicalactionsinlivingorganisms, includinganimalsandplants. Itwasdiscoveredin
animals(cow)byLernerandcolleaguesin1958andin1959inhumans[1,2]. Then,in1995,twopapers
simultaneously demonstrated the presence of melatonin in plants, and it is now accepted that
melatoninispresentinallhigherplants[3,4],whereitissometimesreferredtoasphytomelatonin[5,6].
Chemically,melatoninisanindoleaminederivativeoftheaminoacidtryptophan,anditsbiosynthetic
melatonin pathways from tryptophan have been extensively studied in both animals and plants
(Figure1). Tryptophanisconvertedinto5-hydrotryptophaninanimals,whereasinplantstryptophan
is mainly transformed into tryptamine. These last two compounds are converted into serotonin
(5-hydroxytryptamine),whichisfinallyconvertedintomelatonin(N-acetyl-5-methoxytryptamine)
throughthecompoundsN-acetylserotoninor5-methoxytryptamine(seeFigure1). Alltheenzymes
involvedinmelatoninbiosynthesishavebeendescribedandcharacterizedinmanyspeciesofanimals
andplants[7].
Thenamesalicylicacid(SA)(ortho-hydroxybenzoicacid)wasgiventotheactiveingredientof
willow (Salix sp.) bark by Raffaele Piria in 1838. The first commercial production of synthetic SA
beganinGermanyin1874. Aspirin,atradenameforacetylsalicylicacid,wasintroducedbytheBayer
Companyin1898anditrapidlyreplacedtheuseofSAasitproducedlessgastrointestinalirritation
buthadsimilarmedicinalproperties[8]. Salicylicacidisaphenoliccompound. Theshikimicacid
pathwaytakespartinthebiosynthesisofmostplantphenoliccompounds. Themostcommonpathway
inplantsforSAsynthesisisthephenylalaninepathway(Figure1). However,SAbiosynthesismay
alsobecarriedoutbytheisochorismicacidpathway. Thehydroxylationofbenzoicacidcatalyzedby
theenzymebenzoicacid2-hydroxylasesynthesizesSA.Benzoicacidissynthesizedbytrans-cinnamic
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by trans-cinnamic acid (produced from phenylalanine by the action of enzyme phenylalanine
amacmido(npirao dlyuacseed), feriotmherp hveian yβl-aolxaindianteiobny othf efaatctyti oanciodfs eonrz vyima ea pnhoenn-yolxaildaantiinvee apmatmhwonaiya ilnya wseh)i,cehit threarnvs-ia
ciβn-noaxmidiact iaocnido fisf ahttyydarocixdyslaotrevdi atoa fnoornm-o oxritdhaot-icvoeupmaathriwc aayciidn wfohlliocwhetrda nbsy-c oinxnidamatiicona coidf tihseh ysdidreo xcyhlaaitned
(Ftoigfuorrem 1)o r[9th–o1-2c]o. umaricacidfollowedbyoxidationofthesidechain(Figure1)[9–12].

FiFgiugruer e1.1 B.iBoisoysnytnhtehteicti croruotuetse sofo sfaslaicliyclyicli cacaicdi danadn dmmelealtaotnoinni nini npplalnantst.s .
Melatonin and SA biosynthesis pathways share a final common precursor—chorismic acid—
Melatonin and SA biosynthesis pathways share a final common precursor—chorismic
which is generated from shikimic acid (a condensation product of phosphoenolpyruvic acid from
acid—whichisgeneratedfromshikimicacid(acondensationproductofphosphoenolpyruvicacid
glycolysis and erythrose 4-phosphate from the pentose phosphate pathway (Figure 1). Chorismic acid
fromglycolysisanderythrose4-phosphatefromthepentosephosphatepathway(Figure1). Chorismic
is the precursor of the synthesis of three aromatic amino acids—phenylalanine, tryptophan and
acidistheprecursorofthesynthesisofthreearomaticaminoacids—phenylalanine,tryptophanand
tyrosine. Melatonin is synthetized from tryptophan through the anthranilate/indole pathway and SA
tyrosine. Melatoninissynthetizedfromtryptophanthroughtheanthranilate/indolepathwayandSA
from phenylalanine, in addition to the direct isochorismic acid route (Figure 1).
fromphenylalanine,inadditiontothedirectisochorismicacidroute(Figure1).
With respect to catabolism, melatonin is usually hydroxylated in different positions of the indole
With respect to catabolism, melatonin is usually hydroxylated in different positions of the
ring, with 2-hydroxymelatonin being the major catabolite [13]. In the case of SA, several conjugates
indole ring, with 2-hydroxymelatonin being the major catabolite [13]. In the case of SA, several
hcaovnej ubgeaetne sdheascvreibbeede,n sduecshc raibs emd,estuhychl saaslimcyeltihcy alcsiadl,i csyalliiccaycloidy,l-sLa-laicsyplaorytilc-L a-acsidp,a rstailcicaycliidc, asacildic y2l-iOca-βc-id
glucoside and salicyloyl-glucose ester [9].
2-O-β-glucosideandsalicyloyl-glucoseester[9].
2.2 C. Comommmonon EEfffefcetcst sofo fMMelealtaotnoninin anandd SSAA inin AAbbioiotitci cPPlalnant tSStrteresss s
MMelealtaotnoninin hhasa sbebeenen cocnonsisdidereerded a ammuultlitriergeguulaltaotoryry mmoolelceuculel einin hhigighhere rpplalnantst sbbeceacuauses eofo fththe ewwidide e
anandd ddiviveresres erarnangge eofo cfeclellululalra ranandd pphhysyisoiolologgiciacal lacatcitoionns saatttrtirbibuutetedd toto iti.t I.nIn 2020040,4 t,hteh eacatciotino nofo mfmelealtaotnoinnin
asa sa aggrorowwthth pprorommootetre rwwaas sddeemmoonnstsrtartaetded inin eetitoiolaltaetded LLuupipninuus salablbuus s[1[41]4.] .AAlslos,o m,melealtaotnoinni nisi sabalbel etot o
inindduucec erorooto ptrpimrimorodridailas lfsrofrmom pepriecryiccylec lceelclse,l lgs,engeernaetrinagti nngewn eawdvaednvtietniotuitsio aunsda lnadterlaatle rroaoltrso [o1t5s].[ 1A5 ].
siAgnsifigicnainfitc arnotler oolfe mofemlateolnationn iangaaignastin astbiaobtiico tisctrsetsrse swsaws aaslsaols opopsotustlualtaetde,d u, suinsign ga acocoldld-i-nindduucecded
apapopoptotsoissi smmodoedl einl icnarcroart rcoetllsc e[l1l6s].[ 1T6h]u.s,T mheulsa,tomneinla htoans ibneehna asttbreibeunteadtt rwibituht ethde wcaipthactihtye tcoa rpeagcuitlaytet o
cerlelguulalar taencde lplulalnart garnodwptlha;n ptrgormowotteh s;eperdo mgeortmeisneaetdiogne arnmdi nroatoitoinnga;n odptriomotizineg p;hoopttoimsyinztehpethioc teofsfiycnietnhceyti c
anefdfi ciwenatceyr/aCnOd2w faotleira/r COex2chfoalniagre;e xrcehgaunlgaete; retghuel aitnettehrenainl tebrinoalolgbiicoallo gcicloaclkc loacnkda nfdloflwoewrienrgin ganandd
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ripenirnipge/nsiennge/ssceennecsecepnrcoec epsrsoecse;sasneds; fiannadl lfyi,ntaollayc, ttaos aacnt aesn dano geenndoougsebniooussti bmiouslatitmorualagtaoirn astgaabiniostt iacboirotic or
biotic stressors [17,18].
bioticstressors[17,18].
Many physiological functions have also been assigned to SA. In 1989, Carswell and colleagues
ManyphysiologicalfunctionshavealsobeenassignedtoSA.In1989,Carswellandcolleagues
reported that acetyl SA can promote colony formation in maize protoplasts, suggesting a role in the
reported that acetyl SA can promote colony formation in maize protoplasts, suggesting a role
regulation of the cell cycle [19]. The first indication of a physiological effect on the part of SA was the
in the regulation of the cell cycle [19]. The first indication of a physiological effect on the part
discovery of it role in flowering induction and bud formation in tobacco cell cultures [20]. The
of SA was the discovery of it role in flowering induction and bud formation in tobacco cell
pioneering works of Malamy et al. (1990) on the effect of SA in Tobacco Mosaic Virus [21] and of
cultures [20]. The pioneering works of Malamy et al. (1990) on the effect of SA in Tobacco
Métraux et al. (1990) on the role of SA as signaling in systemic acquired resistance (SAR) [22] clearly
Mosaic Virus [21] and of Métraux et al. (1990) on the role of SA as signaling in systemic acquired
demonstrated the implications of SA in plant pathogen responses. Also, SA influences seed
resistance (SAR) [22] clearly demonstrated the implications of SA in plant pathogen responses.
germination, seedling establishment, cell growth, respiration, stomatal closure, senescence-
Also,SAinfluencesseedgermination,seedlingestablishment,cellgrowth,respiration,stomatalclosure,
associated gene expression, basal thermotolerance, nodulation in legumes, and fruit yield, among
senescence-associatedgeneexpression,basalthermotolerance,nodulationinlegumes,andfruityield,
others [8,9,23,24]. In both cases (melatonin and SA), the role in some of these processes may be
amongothers[8,9,23,24]. Inbothcases(melatoninandSA),theroleinsomeoftheseprocessesmay
indirect because they modulate the synthesis and/or signaling of other plant hormones [25,26] (see
beindirectbecausetheymodulatethesynthesisand/orsignalingofotherplanthormones[25,26]
below). Table 1 presents a list of the physiological effects in which both molecules (melatonin and
(seebelow). Table1presentsalistofthephysiologicaleffectsinwhichbothmolecules(melatonin
SA) seem to play a relevant role. As can be seen, there are many coincidences between both molecules,
and SA) seem to play a relevant role. As can be seen, there are many coincidences between both
but undoubtedly the aspects that have aroused most interest are those related to their actions in
molecules, but undoubtedly the aspects that have aroused most interest are those related to their
improving resistance to stress situations.
actionsinimprovingresistancetostresssituations.
TableT1.aCbloem 1m. Coonmpmhyosnio plohgyisciaolloegffieccatls edffeescctrsi bdeedscfroirbemde floatro mnienlaatnodniSnA anind pSlAan itns .plants.
Physiological Effect
PhysiologicalEffect
Vegetative development
Vegetativedevelopment Seed germination
SePedlanget rgmroinwatthio n
PPhloatnotsygnrothwetshis
Photosynthesis
Mineral nutrition
Mineralnutrition
Crop yield
Cropyield
Carbohydrate metabolism
Carbohydratemetabolism
Nitrate metabolism
Nitratemetabolism
AAnnttiiooxxiiddaanntt nneettwwoorrkk
SSeenneesscceennccee
PPllaanntt ppaatthhooggeenn rreessppoonnssee
Reproductive development
Reproductivedevelopment
FFlloowweerriinngg
SSeeeedd ffoorrmmaattiioonn
Abiotic stresss
Abioticstresss
Water stress: drought, flooding
Waterstress:drought,flooding
Salinity Salinity
Metals Metals
UV,extremeligUhVt , extreme light
Greater Resistance
TemperaturestTreesms:pceorladt,uhreea sttress: cold, heat
BioticstressorsBiotic stressors
Fungi
Fungi
Bacteria
Bacteria
TablesTa2balnesd 23 asnhdo w3 sshoomwe sroepmree sreenptraetsiveneteaxtiavme pelxeasminplwesh iinch wthhiechp rtohtee cptrivoetercotilvees oroflmese olaft omneilnatonin
and SaAnda gSaAin asgtadinifsfte dreinffteraebnito atibciosttrice sssteresshseasv heabveee bnesetnu sdtiueddi.edA. sAcsa cnanb ebes eseenen,,m maannyy aassppeeccttss sshhooww close
closessiimmiillaarriittiieess bbeettwweeeenn bbooththm moloelceucluelse.sI. nIng egneenraelr,abl,o tbhotahr eairnev oinlvveodlviendr einsp roensspeosntsoeasb tioo tiacbsiotrteics sstress
situatsioitnusa,tiionnclsu, dininclgudaimnga rak emdairmkepdr oimvepmroevnetminentht einw tahtee rwsatatetur sstiantudsr oiun gdhrtosuigtuhat tisoitnusa,teionnhsa, necnehdanced
biosynbtiohseysinsthoefspish ootfo psyhnottohseytinctahlleytiacactlliyv eapctiigvme epnigtsmaesnwtse alls awseolfl tahse opf hthoeto psyhnotthoseytinctrhaettei,c arnatien, carnea isnecrease
inmeitna bmoelittaebsoalnitdesa anntdio axnidtiaonxtideannzty emnezsymtoebsa tloa nbcaelatnhcee rtehdeo rxedstoaxt ustsa,toussm, oostmicoatdicj uasdtjmusetnmtetnotr teod ruecdeuce of
of memmebmrabnreaninej uinryjuuryn duenrdsetrr esstsrecsosn dcoitniodnitsi,oanns,d ainnds oinm esocmasee sc,agsreos,w gthropwrtohm portoiomnoatniodn eannhda necnehdanced
produpcrtoivdituyctainvdityy iaenldd [y8i,e1l7d, 1[88,,2177–,1380,]2.7–30].
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Table2.Effectsofmelatonininabioticstressresponses.
AbioticStress PlantSpecies MelatoninTreatment(µM) EffectsObserved Reference
Cold Arabidopsis 10–30 ↑freshweight,shootheightandprimaryroots,survival [31,32]
Cold Cucumber 50–500 ↑GSHpool,↓ROSburst [33]
Cold Rhodiolacrenulata 0.1 ↑cryopreservationofcallus [34]
Cold Americanelm 0.1–0.5 ↑regrowthfrozenshoots [35]
Cold Watermelon 150 ↑photosynthesis,↓cold-relatedmicroRNA [36]
Cold Wheat 1mM ↑redoxbalance,Chls,osmorregulation,↓ROSburst [37]
Cold Cabbage 10–1000 ↑anthocyanins,proline,redoxbalance,↓ROSburst [38]
Cold,salt,drought Bermudagrass 20–100 ↑freshweight,osmorregulation,↓ROSburst,celldamage [39,40]
Cold,salt,drought Arabidopsis 50 ↑sucrose,survivalrate [41]
Cold,drought Barley 1mM ↑photosynthesisefficiency,ABA,watercontent,↓ROSburst [42]
Heat Phacelia 0.3–90 ↑germination [43]
Heat Arabidopsis 5–20 ↑thermotolerance [44]
Heat Tomato 10 ↑thermotoleranceandcellprotection [45]
Metal-Cu Pea 5 ↑plantsurvival [46]
Metal-Cu Redcabbage 1–100 ↑freshweight,germination,↓membraneperoxidation [47]
Metal-Cd Tomato 25–500 ↑Cdtolerance,phytochelatins,ATPaseactivity [48]
Metal-Pb Tobacco 0.2 ↑cellculturegrowth,↓mortalitycells,ROSburst [49]
Oxidative Arabidopsis 5–10 ↑plantsurvival,autophagy,↓oxidizedproteins [50]
Oxidative Pisumsativum 50–200 ↑photosynthesisefficiency,pigments,watercontent,↓ROSburst [51]
Salinity Malus 0.1 ↑shootheight,leafnumber,chlorophylls,↓electrolyteleakage [52]
Salinity Malus 0.1 ↑shootheight,K+channels,K+level,↓ROSburst [53]
Salinity Citrus 1 ↑osmorregulation,Chls,↓ROSburst,membraneperoxidation [54]
Salinity Sunflower 15 ↑rootandhypocotylgrowth,antioxidantpotential [55]
Salinity Cucumber 1 ↑germination,GA4,↓ROSburst,membraneperoxidation,ABA [56]
Salinity Viciafaba 100–500 ↑plantheight,RWC,photosyntheticpigments,osmolites,phenolic [57]
Salinity Tomato 50–150 ↑photosynthesis,PSIIefficiency,D1proteinturnover,↓ROSburst [58]
Alkalinity Tomato 0.25–1 ↑seedlinggrowth,photosynthesis,ionhomeostasis,↓ROSburst [59]
Salinity,drought Soybean 50–100 ↑seedlinggrowth,leafsize,biomass,seedyield [60]
Drought Cucumber 100 ↑germination,rootgrowth [61]
Drought Grape 0.05–0.2 ↑seedlinggrowth,osmorregulation,photosynthesis,↓ROSburst [62]
Drought Malus 100 ↑waterstatus,Chls,photosynthesisefficiency,↓ROSburst [63]
Leaf-senescence Barley 0.01–1 ↓senescence,↑Chls [64]
Leaf-senescence Malus 10mM ↓senescence,ROSburst,↑Chls,photosynthesisefficiency [65,66]
Leaf-senescence Arabidopsis 20–125 ↓senescence,ROSburst,↑Chls,photosynthesisefficiency [67]
Leaf-senescence Rice 10–20 ↓senescence,ROSburst,celldeath,↑Chls [68]
Leaf-senescence Perennialryegrass 20–100 ↓senescence,ROSburst,↑Chls,photosynthesisefficiency [69]
↑,Increasedcontentorincreasedaction.↓Decreasedcontentordecreasedaction.
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Table3.EffectsofSAinabioticstressresponses.
AbioticStress PlantSpecies SATreatment(µM) EffectsObserved Reference
Cold Hordeumvulgare 100 ↑antioxidativeenzymes,icenucleationactivity [70]
Cold Musaacuminata 500 ↑chloroplastandmitochondriaultrastructure [71]
Cold Lycopersiconesculentum 100 ↑resistance,antioxidativeenzymes,PRproteins [72]
Cold Prunuspersica 1mM ↑antioxidativeenzymes,antioxidantmetabolites,firmness [73]
Heat Triticumaestivum 500 ↑prolinecontent,waterpotential,gasexchange,glutamylkinaseactivity [74]
Heat Arabidopsisthaliana 10 ↑survival,thermotolerance,↓oxidativedamage [75]
Metal-Cd Brassicajuncea 1mM ↑mineralnutrients [76]
Metal-Cd Cucumismelo 100 ↑photosynthesisefficiency,wateruseefficiency [77]
Metal-Cd Glycinemax 120mM ↑Chls,photosynthesisefficiency,antioxidativeenzymes,GSH [78]
Metal-Cd Ricinuscommunis 500 ↓gasexchange,Chls [79]
Salinity Glycinemax 500 ↑antioxidativeenzymes,ascorbate [80]
Salinity Vignaradiata 500 ↑photosynthesisefficiency,plantdrymass,glycinebetaine [81]
Salinity Torreyagrandis 500 ↑photosynthesisefficiency,netCO2assimilationrates,Chls [82]
Drought Zeamays 1 ↑netdryweight,waterpotential,leafrolling [83]
Drought Simaroubaglauca 50 ↑polyphenols,alkaloids [84]
Drought Triticumaestivum 1mM ↑moisturecontent,drymass,Rubisco,SOD,Chls [85]
UV-B Poapratensis 150mg/m2 α-tocopherol,SOD,CAT,anthocyanins [86]
↑,Increasedcontentorincreasedaction.↓,Decreasedcontentordecreasedaction.
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3. MelatoninandSAinBioticStress(PlantPathogenResponse)
Tables 4 and 5 provide a list of the papers related to the positive effect of melatonin and SA
on plant pathogen responses. In the first paper, related to melatonin and fungus plant-pathogen
infection,melatonin-treatedappletreesusingrootirrigationimprovedtheresistanceofMalusprunifolia
againstthefungusDiplocarponmali(Marssoninaappleblotch). At20days,thetreatedtreesshoweda
lowernumberofdamagedleaves,higherchlorophyllcontent,amoreefficientPhotosystemII,andless
defoliationthaninfecteduntreatedtrees.Ingeneral,melatoninhelpedplantswithresistancetofungal
infection,reducinglesions,inhibitingpathogenexpansion,andgenerallyalleviatingdiseasedamage[87].
Also,insomeinvitroassays,differentconcentrationsofmelatoninshowedgrowthinhibitionactivities
against plant fungal pathogens such as Alternaria spp., Botrytis spp., and Fusarium spp. The same
occurredinplant-pathogenattacksbyPenicilliumspp.innon-sterilizedLupinusalbusseeds[17].Table4
showsfivepapersthatusedtheArabidopsis/Pseudomonassyringaeasamodelofplant–bacterialpathogen
interaction. Melatonininducedpathogen-relatedgenesinArabidopsis(alsointobaccoplants), which
isinaccordancewiththepossibleroleofthismethoxyindoleasadefencesignallingmoleculeagainst
pathogensinplants.Inarecentandsignificantpaper,Zhangetal.(2017)demonstratedthatmelatonin
attenuatesseverepotatolateblightcausedbyPhytophthorainfestans. Melatonininducedplantinnate
immunityagainstfungalinfection,inhibitingmycelialgrowthandchangingexpressionofmanygenes
associatedwithstressandvirulence[88].Insum,melatoninup-regulatespathogenesis-related,SAand
ethylene-dependent genes, an effect that was suppressed in mutants defective in SA and ethylene
signalling.Also,melatoninincreasednitricoxide(NO)andSA-relatedgenes,accompaniedbyreduced
susceptibility to the pathogen, leading to an increase in both melatonin and NO. SNAT knockout
mutantsnotonlyexhibitedreducedlevelsofmelatonin,butalsolowerlevelsofSA,alongwithagreater
susceptibilitytothepathogen[89].Nostudiesonplantvirusesandmelatoninhavebeenpublishedto
date.Nevertheless,inanimals,melatoninisagoodtherapeuticalternativeforfightingbacterial,viraland
parasiticinfections[90].Also,duringsepsis,melatoninhasbeenreportedtoblocktheoverproductionof
pro-inflammatorycytokinesandincreaseinterleukin-10levels.Withrespecttoviralinfection,Venezuelan
equineencephalomyelitis(VEE)isanimportanthumanandequinediseasecausedbytheVEEvirus.
Reactiveoxygenspecies(ROS)havebeenimplicatedinthedisseminationoftheresponsiblevirus,and
its deleterious effects may be diminished by melatonin treatment. The administration of melatonin
significantlydecreasedtheviruslevelinthebloodandbraincomparedwiththelevelsseenininfected
controlmice[90].
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Table4.Effectsofmelatonininbioticstressresponses.
Melatonin
PlantSpecies BioticStressor EffectsObserved Reference
Treatment(µM)
↑Resistancetofungalinfection
Malusprunifolia Diplocarponmali 50–500 ↓Leaflesions,celldeath [87]
↓Pathogenexpansion
↑Defencerelatedgenes
Arabidopsisandtobacco PseudomonassyringaeDC3000 10 [91]
↑Resistance(10-foldvs.mock)
↓Melatonin(50%),SA
Arabidopsis PseudomonassyringaeDC3000 SEofSNAT ↓Defencerelatedgenes [89]
↓Resistancetoinfection
↑NOandmelatonin
Arabidopsis PseudomonassyringaeDC3000 20 ↑Defencerelatedgenes [92]
↑Resistance
Arabidopsisandtobacco PseudomonassyringaeDC3000 1 ↑MAPkinasescascade [93]
↑CBF/DREB1(stressfactors)
Arabidopsis PseudomonassyringaeDC3000 50 ↑CCA1(internalclockfactors) [41,94]
↑Defencerelatedgenes
Lupinusalbus Penicilliumspp. 20–70 ↑Resistancetofungalinfection [17]
Xanthomonasoryzae,XooMagnaporthe Changesinmelatoninbiosynthesis
Rice – [95]
oryzae,blastfungus enzymestranscripts
↑Resistancetofungalinfection
Solanumtuberosum Phytophthorainfestans 10mM [88]
↑fungicideeffects,↓virulence
MeRAV1/2(AP2/ERF)upregulate7melatonin
Manihotesculenta Xanthomonasaxonopodis [96]
biosynthesisgenes
↑,Increasedcontentorincreasedaction.↓,Decreasedcontentordecreasedaction.
WithrespecttoSA,theexogenousapplicationofSAatnon-toxicconcentrationstosusceptible
fruitsandvegetablescouldenhanceresistancetopathogensandhelpcontrolpost-harvestdecay[97].
SA effectively reduced fungal decay in a concentration-dependent manner, as can be seen in the
examplesofTable5. InthecaseofSA,somestudiesindicatethatitinhibitsviralreplication[98–100].
Table5.EffectsofSAinbioticstressresponses.
SATreatment
PlantSpecies BioticStressor EffectsObserved Reference
(mM)
Fragariaananassa Botrytiscinerea 1–2 ↓ethylene,fungaldisease,↑fruitquality [101]
↓ethylene,lycopene,fungaldisease,↑fruit
Lycopersiconesculentum Botrytiscinerea 5 [97]
quality
Mangiferaindica Collectotrichumgloeosporioides 2 ↑colour,firmness,↓diseaseseverity [102]
Malusdomestica Penicilliumexpansum 0.07–0.7 ↑efficacyofantagonistC.laurentii [103]
↑antioxidativeenzymes,chitinase,glucanase,
Prunusavium Penicilliumexpansum 2 [104]
fungalresistance
↑antioxidativeenzymes,PAL,chitinase,
Pyrusbretschneideri Penicilliumexpansum 2.5 [105]
glucanase,↓diseaseseverity
↑,Increasedcontentorincreasedaction.↓,Decreasedcontentordecreasedaction.
4. Melatonin,SAandROS/RNSNetwork
A relevant role for NO in melatonin responses is proposed, mainly for auxin-like and plant
immuneresponses. NOandotherradicalnitrogenspecies(RNS)andROSarekeysignalsthatincrease
under abiotic/biotic stress [8,106]. Generally, RNS and ROS signals tend to act in a coordinated
way. NO levels are self-regulated and also regulate the ROS network through NO-dependent,
post-translationalmodifications[107]. NOmodulatesseveralfunctionsthroughproteinmodifications
by nitration, S-nitrosylation and the ligation of NO to transition metals, but also through the
modificationoflipids(nitro-fattyacids)andDNA(8-nitroguanine)[108–112]. Moreover,NOtriggers
a set of responses to alleviate stress and cellular damage, which includes transient metabolic
reprogramminginbothprimaryandsecondarymetabolicpathways[107].
Abioticandbioticstressinduceanincreaseinendogenousmelatoninthroughtheupregulationof
melatoninbiosyntheticgenes[96,113]. MelatoninalsoincreasesNOlevelsthroughtheupregulation
of nitrate reductase (which usually reduces nitrate to nitrite, but can also reduce nitrite to NO
using NADPH as a cofactor). Also melatonin induces the NO synthase-like pathway, in iron
Agronomy2018,8,33 8of16
deficiency-induced NO in rice [114]. Thus, melatonin can also act as an NO and ROS scavenger,
andcuriously,inanNOfeedbackmechanism,NOinducesmelatoninbiosynthesis[113].
Although some data point to the possibility that melatonin might act as an upstream signal,
thecomplexityofthemelatonin–NOinteractionmakesitdifficulttoelucidatewhethermelatoninis
upstreamordownstreamofNO.Also,H O (animportantsignalmoleculeinstresssituations),seems
2 2
tobedecisiveintheupregulationofmelatoninbiosynthesisenzymes,takingasaresponseanincrease
inmelatoninlevelsinstressedplants. Inshort,mitogen-activatedproteinkinase(MAPK)signalling,
SA,NO,andH O ,aswellastheircross-talk,arerequiredformelatonin-mediatedinnateimmunity
2 2
inArabidopsis. Also, melatoninandNOchangetheexpressionofseveraltranscriptionfactorsand
hormonesignallingelements,whichdeterminestheoverallanti-stressresponse. Also,someplant
Agronomy 2h0o1r8m, 8o,n xe sFOsuRc hPEaEsRIA RAE,VCIEKWsa ndABAcanstimulateNOproduction[25,26]. 8 of 15
This complex relationship between ROS, NO and melatonin is pictured in Figure 2. In the
case of abiotic stress, no model including the signalling cascade for melatonin and SA has been
to date. Fleta-Soriano et al. (2017) studied the role of melatonin in plant response to drought stress
proposedtodate. Fleta-Sorianoetal. (2017)studiedtheroleofmelatonininplantresponsetodrought
and recovery in maize plants [115]. Furthermore, in that study, the endogenous contents of melatonin
stress and recovery in maize plants [115]. Furthermore, in that study, the endogenous contents of
positively correlated with those of stress-related phytohormones, particularly with those of SA,
melatoninpositivelycorrelatedwiththoseofstress-relatedphytohormones,particularlywiththoseof
althoughS Aex,aolgtheonuoguhse xaopgpenliocuastiaopnp loicfa tmioenloaftomneilnat odniidn dniodtn aolttealrt etrhteh ecoconntteennttss ooffa annyyp hpyhtoyhtoorhmoornme.one.

Figure F2i.g uErfefe2c.tE offfe catboifoatbicio/tbici/obtiico tsictrsetrsessosorrss oonn tthheea anntiotixoidxaidntannett wneortkwRoOrkS/ RNOOS//mNeOlat/omnienl.atonin.
A modeAl omfo mdeelloaftomneilnat/oSnAin//NSOA//RNOOS/ RaOctSioanct iionn biniobtiioct isctrstersess srreessppoonnsseess (p(patahtohgoegnerne srisetsainsctea)nce) has
been prohpaossbeede n[9p,2r5o]p (oFseigdu[r9e,2 35]). (PFaigtuhroeg3e)n. aPtatathcokg ienncraettaasceksi NncOre,a sSeAs aNnOd, mSAelaantodnmine llaetvoneilns tlhevroelusgh ROS.
through ROS. In the well-known model of Arabidopsis/Pseudomonas syringae DC3000 (avrRpt2),
In the well-known model of Arabidopsis/Pseudomonas syringae DC3000 (avrRpt2), plant–pathogen
plant–pathogeninteractionsrevealedthatthemitogen-activatedproteinkinasecascade(MAPKKK3)
interactions revealed that the mitogen-activated protein kinase cascade (MAPKKK3) and OXI1
and OXI1 (oxidative signal-inducible1) kinases are responsible for triggering melatonin-induced
(oxidative signal-inducible1) kinases are responsible for triggering melatonin-induced defence
defencesignallingpathways[93,116]. ThekeyenzymeinSAbiosynthesis—isochorismatesynthase-1
signallin(gIC pS-a1t)h—wwaayssu [p9r3eg,1u1la6t]e.d Tbhyem keelayt oenninz,yimncere ainsi nSgAS Abiloesvyelnstahnedsitsr—iggiesroinchgoarpisamthaotgee ns-yinndtuhcaesde-1 (ICS-
1)—was reusppornesgeu. laAtelsdo , bmye lamtoenliantoanndinN, Oinwcreeraesainblge tSoAin dleuvceeljsa smanondi ctarcigidge(JrAin)gb ioas ynptahtehsiosgaennd-induced
increase several sugar and glycerol levels, all of which activate pathogen-related gene expression.
response. Also, melatonin and NO were able to induce jasmonic acid (JA) biosynthesis and increase
Themelatonin-inductionofethylenebiosynthesis,throughACCsynthase(ACS6),collaboratesinthe
several sugar and glycerol levels, all of which activate pathogen-related gene expression. The
melatonin-induction of ethylene biosynthesis, through ACC synthase (ACS6), collaborates in the
induction of pathogenesis-related genes (PR), whereby ethylene insensitive (EIN), enhanced disease
susceptibility 1 (EDS1), phytoalexin deficient 4 (PAD4) and NPR1 factors are key signalling
components in the plant SA- and ethylene-mediated defence responses [17,41,91,92,96,116–119].
More recently, it has been demonstrated that MeRAV1 and MeRAV2 factors (apetala2/ethylene
response factor, AP2/ERF) are essential for plant disease resistance against bacterial blight in cassava
through the upstream of transcription factors of melatonin biosynthesis genes [96].
Agronomy2018,8,33 9of16
inductionofpathogenesis-relatedgenes(PR),wherebyethyleneinsensitive(EIN),enhanceddisease
susceptibility1(EDS1),phytoalexindeficient4(PAD4)andNPR1factorsarekeysignallingcomponents
in the plant SA- and ethylene-mediated defence responses [17,41,91,92,96,116–119]. More recently,
it has been demonstrated that MeRAV1 and MeRAV2 factors (apetala2/ethylene response factor,
AP2/ERF) are essential for plant disease resistance against bacterial blight in cassava through the
Augprsontroemaym 201o8f, t8r, axn FsOcRr iPpEtEioRn RfEaVcItEoWrs ofmelatoninbiosynthesisgenes[96]. 9 of 15

FFiigguurree 33.. MMooddeell ooff RROOSS//NNOO//mmeelalatotonninin aacctitoionn oonn bbioiotitcic sstrteressss rreessppoonnsseess (p(paaththooggeenn rreessisisttaannccee)). .
55.. FFuuttuurree PPeerrssppeeccttiivveess ffoorr MMeellaattoonniinn aanndd SSAA iinn AAggrroonnoommiicc AApppplliiccaattiioonnss
BBootthh mmoolleeccuulleess aarree ooff ggrreeaatt iinntteerreesstt aass mmoodduullaattiinngg aaggeennttss ooff aannttii--ssttrreessss rreessppoonnsseess iinn ppllaannttss.. AAllssoo,,
bbootthh hhaavvee tthhee ccaappaacciittyy ttoo ddiirreeccttllyy oorr iinnddiirreeccttllyy iinntteerraacctt wwiitthh RROOSS aanndd RRNNSS.. KKnnoowwlleeddggee ooff tthhee ppoossssiibbllee
rreellaattiioonnsshhiippss wwiitthh ootthheerr hhoorrmmoonneess iinn aassppeeccttss rreellaatteedd ttoo ppaatthhooggeenn rreessiissttaannccee aanndd tthhee rreessppoonnssee ttoo
aabbiioottiicc ssttrreesssseess iiss ooff ggrreeaatt rreelleevvaannccee.. TThhee ppoossssiibbiilliittyy ooff ““sseennssiittiizziinngg”” ppllaannttss ttoo aabbiioottiicc aaggeennttss tthhrroouugghh
pprriimmiinngg oorr ootthheerr mmeetthhooddss mmiigghhtt bbee ooff iinntteerreesstt iinn oorrddeerr ttoo iinnccrreeaassee tthhee rreessiissttaannccee ooff ccrrooppss.. SSAA-- oorr
mmeellaattoonniinn--iinndduucceedd acatcitviavtaiotinonto rtoed urecedudcaem adgaemcaaugsee dcabuysweda tebryd ewficaittesr( ddroeufigchitts) ,w(dhrioleugmhati)n, tawinhiinleg
maintaining the proper metabolism in the face of this stress situation is clearly an essential objective
for increasing plant/crop development and yield. Obtaining transgenic plants that overexpressed SA
and melatonin biosynthesis genes could be another interesting approach, both for research per se and
for application in crops, provided that the limitations to the use of transgenic plants are not
transgressed. In both cases, the over-accumulation of SA and melatonin in plant tissues increase the
resistance response against stressors. Figure 4 shows a schematic model in which both molecules
studied in this paper present agonist behaviour to reduce or moderate the harm caused by stressors
with the final aim of improving plant development and crop yield.
Agronomy2018,8,33 10of16
thepropermetabolisminthefaceofthisstresssituationisclearlyanessentialobjectiveforincreasing
plant/cropdevelopmentandyield. ObtainingtransgenicplantsthatoverexpressedSAandmelatonin
biosynthesisgenescouldbeanotherinterestingapproach,bothforresearchperseandforapplication
incrops,providedthatthelimitationstotheuseoftransgenicplantsarenottransgressed. Inboth
cases,theover-accumulationofSAandmelatonininplanttissuesincreasetheresistanceresponse
againststressors. Figure4showsaschematicmodelinwhichbothmoleculesstudiedinthispaper
presentagonistbehaviourtoreduceormoderatetheharmcausedbystressorswiththefinalaimof
improvingplantdevelopmentandcropyield.
Agronomy 2018, 8, x FOR PEER REVIEW 10 of 15

Figure 4. Melatonin/SA model of biostimulating activity in plants to enhance resistance to stress
Figure4. Melatonin/SAmodelofbiostimulatingactivityinplantstoenhanceresistancetostress
situations and increase crop yield.
situationsandincreasecropyield.
Acknowledgments: Thanks to P. Díaz Vivancos for his generous invitation.
Acknowledgments:ThankstoP.DíazVivancosforhisgenerousinvitation.
Author Contributions: Marino B. Arnao conceived and designed work. Marino B. Arnao and Josefa Hernández-
RAuuitzh aonraClyoznedtr itbhue tdioantas.: MMarairnion oB.B A.rAnaron aworoctoen tcheeiv medanaunsdcridpet.s iMgnaeridnow Bo. rAk.rnaMo aarnidn oJoBse.faA HrnearonáannddezJ-oRsueifza
Hernández-Ruiz analyzed the data. Marino B. Arnao wrote the manuscript. Marino B. Arnao
critically revised the final version of manuscript. Marino B. Arnao and Josefa Hernández-Ruiz read and
and Josefa Hernández-Ruiz critically revised the final version of manuscript. Marino B. Arnao and
approved the final article.
JosefaHernández-Ruizreadandapprovedthefinalarticle.
CCoonnfflliiccttss ooff IInntteerreesstt:: TThhee aauutthhoorrss ddeeccllaarree nnoo ccoonnfflliicctt ooff iinntteerreesstt..
RReeffeerreenncceess
11.. LLeerrnneerr,, AA..BB..;; CCaassee,,J .JD.D.;.T; aTkaakhaahsahsih,Yi,. ;YL.e; eL,eTe.H, T.;.MH.o; rMi,Wor.i,I sWola. tIisoonlaotfiomne loaft omneinla,taopniinne, aal fpaicntoearlt hfaacttloigrh ttheants
lmigehltaennosc myteelsa.nJo.cAymte.sC. Jh.e Amm.S. oCch.e1m9.5 S8,oc8.0 1,925588,7 8.0[,C 2r5o8s7sR. ef]
22.. LLeerrnneerr,, AA..;; CCaasese,,J .;JM.; oMrio,rWi, .;WW.r; igWhtr,igMh.t,M Mela. toMneinlatinonpiner iipnh eprearlinpehrevrea.l Nnaetruvree.1 9N5a9t,u1r8e3 ,11985291, .1[C83r,o s1s8R2e1f,]
d[Poiu:1b0M.1e0d3]8/1831821a0.
33.. HHaattttoorrii, ,AA.;. ;MMiiggititaakkaa, ,HH.;. ;IIigigoo, ,MM.;. ;YYaammaammoottoo, ,KK.;. ;OOhhtatanni-iK-Kaanneekkoo, ,RR.;. ;HHaarara, ,MM.;. ;SSuuzzuukki,i ,TT.;. ;RReeitieter,r ,RR.J..J .
IIddeennttiiffiiccaattiioonn ooff mmeellaattoonninin iinn ppllaannttss aanndd iittss eeffffeeccttss oonn ppllaassmmaa mmeellaattoonniinn lleevveellss aanndd bbiinnddiinngg ttoo mmeellaattoonniinn
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