Chromium(VI) Toxicity in Legume Plants: Modulation Effects

Review ArticleChromium(VI) Toxicity in Legume Plants Modulation Effects ofRhizobial Symbiosis

Uliana Ya Stambulska Maria M Bayliak and Volodymyr I Lushchak

Department of Biochemistry and Biotechnology Vasyl Stefanyk Precarpathian National University 57 Shevchenko StrIvano-Frankivsk 76018 Ukraine

Correspondence should be addressed to Uliana Ya Stambulska ustambulskaukrnet and Maria M Bayliak bayliakukrnet

Received 17 November 2017 Accepted 31 December 2017 Published 14 February 2018

Academic Editor Jesus Munoz-Rojas

Copyright copy 2018 Uliana Ya Stambulska et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Most legume species have the ability to establish a symbiotic relationship with soil nitrogen-fixing rhizobacteria that promoteplant growth and productivity There is an increasing evidence of reactive oxygen species (ROS) important role in formation oflegume-rhizobium symbiosis and nodule functioning Environmental pollutants such as chromium compounds can cause damageto rhizobia legumes and their symbiosis In plants toxic effects of chromium(VI) compounds are associated with the increasedproduction of ROS and oxidative stress development as well as with inhibition of pigment synthesis and modification of virtuallyall cellular components These metabolic changes result in inhibition of seed germination and seedling development as well asreduction of plant biomass and crop yield However if plants establish symbiosis with rhizobia heavy metals are accumulatedpreferentially in nodules decreasing the toxicity of metals to the host plant This review summarizes data on toxic effects ofchromium on legume plants and legume-rhizobium symbiosis In addition we discussed the role of oxidative stress in bothchromium toxicity and formation of rhizobial symbiosis and use of nodule bacteria for minimizing toxic effects of chromiumon plants

1 Introduction

Heavy metals are widespread environmental pollutants andtheir excessive levels in agricultural soils cause serious risksnot only for normal plant growth and crop yield but alsofor the human health Among heavy metals chromium isa highly toxic metal to living organisms with many adverseeffects reported in humans animals plants and microor-ganisms [1ndash4] Chromium belongs to transition metals andit occurs naturally in two predominant valence states hex-avalent chromium (Cr6+) and trivalent chromium (Cr3+)The hexavalent form of the metal Cr6+ is reported tobe more toxic than the relatively less reactive and mobileCr3+ [1] Hexavalent Cr compounds (mainly chromates anddichromates) are extensively used in diverse fields of industryleading to environmental pollution [1 5]

Plants including legumes are able to uptake heavymetals like chromium from soils that result in many adverseeffects such as inhibition of seed germination and seedlingdevelopment reduction in root and shoot biomass quality of

flowers and crop yield [6ndash8] These effects of heavy metalsare connected with inhibition of certain metabolic processesincluding biosynthesis of chlorophylls and proteins [4 9ndash11] As a result progressive chlorosis necrosis and decreasedprotein content are typical signs of heavy metal toxicity toplants [1 12ndash15]

The enhanced production of reactive oxygen species(ROS) is considered to be one of the most importanthallmarks of Cr6+ toxicity (Figure 1) [3 5] ROS such assuperoxide anion radical (O2

∙minus) hydrogen peroxide (H2O2)and hydroxyl radical (OH∙) are highly reactive moleculeswhich can cause oxidative modification of proteins lipidsand nucleic acids [3 12] In response to heavy metal expo-sure plants upregulate various enzymatic or nonenzymaticdefense mechanisms that help to support redox balanceand preventrepair oxidative damage under stress conditions(Table 1) [13 14] If the capacity of protective systems is notsufficient modification of biomolecules can be significantlyincreased leading to the development of oxidative stress withrespective unfavorable effects for plants

HindawiBioMed Research InternationalVolume 2018 Article ID 8031213 13 pageshttpsdoiorg10115520188031213

2 BioMed Research International

Plants

Oxidativestress

ROS

Seed death andinhibitionof seed

Growthinhibition

Reduction ofphotosynthesis

Tissuealterations

Inhibition ofelectrontransportprocesses

Damage ofDNA

Proteininactivation

Changes inenzyme activity

Effects

Ｌ6+

2

∙minus（22

∙（

Figure 1 Toxic effects of Cr(IV) on plants Like other heavy metals Cr can directly inactivate many proteins binding to them or displacingmetals from the active centers of proteins As a transition metal Cr can participate in many cellular redox reactions resulting in generationof reactive oxygen species (ROS) such as O2

∙minus H2O2 and HO∙ When the production of ROS exceeds the capacity of the antioxidant systemcells undergo oxidative stress Both direct protein inactivation and oxidative stress lead to adverse morphological and physiological changesin plants

Symbiotic interaction between legume plants and rhi-zobacteria is a complex physiological process which isregulated by a number of signals produced both macro-and microsymbionts [16 17] Infection by rhizobia increasesROS production intensifying oxidative processes in plants[17] There is a strong evidence that ROS and antioxidantsystem play a key role in the formation and functioning oflegume-rhizobium symbiosis [17 18] Moreover a number ofstudies reported that elevated levels of heavy metals in soilsaffect rhizobial growth and their host legumes [19 20] Undercultivation of legumes on the soils with the high level of heavymetals the root nodules can be the major accumulators ofheavy metals from soil [21] The latter can potentially reducethe toxicity of heavy metals to the plants with simultaneousdecreasing metal content in soils Therefore the use of seedinoculation by symbiotic nitrogen-fixing bacteria is activelydiscussed as one of the possible ways to reduce toxic effectsof heavymetals to legumes and provide an effective approachfor soil bioremediation [21ndash23] At the same time legume-rhizobium symbiosis seems to be also sensitive to heavymetals and its protective effects against metal toxicity are notfully clearThis review summarizes recent data on the toxicityof chromium(VI) to plants especially in legumes with thefocus on ROS involvement and oxidative stress developmentFurthermore we analyze ROS role in the formation ofrhizobium-legume symbiosis with a focus on modulatingeffects of nitrogen-fixing bacteria of the Rhizobium genuson free radical processes in plants exposed to hexavalentchromium

2 Toxic Effects of Chromium in Plants

In the nature chromium exists in two valence forms Cr3+and Cr6+ which chromite minerals are mainly composedof [5 7 12 24 25] Chromates CrO4

2minus and dichromatesCr2O7

2minus are the most abundant anionic forms of chromiumin the environment [26ndash28] Toxicity of chromium for plantsdepends on its valence state with Cr6+ being more toxicand mobile than Cr3+ [12 29ndash32] The hexavalent chromiumis toxic for agricultural plants at concentrations of about05ndash50mgmLminus1 in the nutrient solution and 5ndash100mg gminus1in the soil Under physiological conditions concentration ofchromium ions in plants is less than 1 120583g gminus1 [33 34]

Absorption of chromium by the plant depends on itsform and concentration in soil or water around as well asplant species and their physiological state [1 35] Howeverthe mechanisms of absorption and distribution of chromiumin vegetative and generative plant organs have not beensufficiently studied Chromium is not known as an essentialelement for plants and therefore has no specificmechanismsfor assimilation by plants [36] Both active and passivetransport systems were suggested to participate in absorptionof this element in plants Active transport is proposed tobe responsible for absorption of chromium ions at low con-centrations whereas passive facilitated diffusion promoteschromium intake at its high concentrations [36] It is wellestablished that Cr6+ is absorbed by roots mainly via anenergy-dependent transport while Cr3+ penetrates the plant

BioMed Research International 3

Table1Major

compo

nentso

fantioxidant

defenseinlegumep

lantsa

ndroot

nodu

les

Com

ponents

Locatio

nFu

nctio

nEn

zymaticantio

xidants

Superoxide

dism

utase(SO

D)

Dism

utationof

O2

∙minusO2

∙minus+O2

∙minus+2H+rarr

2H2O2+O2

CuZ

n-SO

DCy

tosolplastid

sMn-SO

DMito

chon

driabacteroides

Fe-SOD

Plastid

ssomeb

acteroides

Catalase

Peroxisomesbacteroides

Dism

utationof

H2O22H2O2rarr

2H2O+O2

Guaiacolp

eroxidase(GPX

)Plasmam

embranevacuoles

Detoxificatio

nof

H2O2H2O2+gu

aiacol

redrarr

H2O+

guaiacol

oxid

Peroxiredo

xins

Cytosolmito

chon

dria

Redu

ctionof

H2O2or

alkylhydroperoxides(RO

OH)toH2Oor

respectiv

ealcoh

ols(RO

H)RO

OH+Trx-(SH) 2rarr

ROHrarr

Trx-S 2

+H2O

Enzymesofascorbate-glu

tathione

cycle

(AA-GS

Hcycle

)

Cytosol(mainly)m

itochon

driaplastids

peroxisomes

Detoxificatio

nof

H2O2in

aserieso

freactions

Ascorbatep

eroxidase(APX

)AP

XH2O2+AArarr

2H2O+MDHA(D

HA)

Mon

odehydroascorbater

eductase

(MR)

MR

2MDHA+NADHrarr

2AA+NAD

Dehydroascorbater

eductase

(DR)

DR

DHA+2G

SHrarr

AA+GSSG

Glutathione

redu

ctase(GR)

GRG

SSG+NADPHrarr

2GSH

+NADP+

EnzymesofGS

Hmetabolism

Glutathione

peroxidase

(GSH

-PX)

Cytosolplastid

sbacteroides

Detoxificatio

nof

H2O2H2O2+GSHrarr

2H2O+GSSG

Glutathione

redu

ctase(GR)

Cytosolmito

chon

driaplastidsbacteroides

Regeneratio

nof

oxidized

glutathion

eGSSG+NADPHrarr

2GSH

+NADP+

120572-G

lutamylcyste

ines

ynthetasea

ndglutathion

esynthetase

Cytosolplastid

sbacteroides

GSH

synthesis

deno

vo

Glutathione-S-tr

ansfe

rase

Cytosol(mainly)plastidsm

itochon

dria

bacteroides

Detoxificatio

nof

xeno

bioticsv

iaconjugationwith

GSH

Nonenzym

aticantio

xidants

Ascorbicacid

(AA)

Cytosolmito

chon

driaplastids

peroxisomes

Dire

ctscavenging

ofRO

Sas

ubstrateforA

PX

Glutathione

(GSH

)Cy

tosolmito

chon


Dire

ctscavenging

ofRO

Sac

osub

strateforD

RGSH

-PX

and

glutathion

e-S-transfe

rases

120572-Tocop

herol

Plastid

sDire

ctscavenging

ofRO

Squ

enchingof

lipid

radicalsin

mem

branes

Caroteno

ids

Plastid

sQuenching

ofele

ctron-excitedmoleculesprotectionof

chloroph

yllsfro

mph

otod

amage

Phenolcompo

unds

(pheno

licacidsflavono

idsetc)

Vacuoles

(mainly)cytosolplastidscellwall

Dire

ctscavenging

ofRO

Sandlip

idradicals

metal-chelating

activ

ityFerritin

phytochelatinsandmetallothioneins

Plastid

svacuolescytosol

Metal-binding

activ

ityTrx-(SH) 2Trx-S2reducedoxidizedthioredo

xin

MDHAm

onod

ehydroascorbateDHAdehydroascorbateGSH

GSSGreducedoxidizedglutathion

e


cell by facilitated diffusion [8 29 33 37] The absorption ofchromium ions by roots is facilitated by organic acids whichare present in root excretions and are able to form complexeswith chromiumThe latter makes Cr available for absorptionby the root system [36] After absorption by root hairschromium is poorly transported to other parts of the plantand is stored mainly in roots [33] Plant roots accumulate10ndash100 times more Cr than shoots and other organs [35 38]In particular growth of Pisum sativum L in the mediumcontaining potassium dichromate caused a dose-dependentincrease in Cr content in different plant parts in the followingorder roots gt stem gt leaves gt seed [33 39]

Plants possess certain mechanisms of chromium detox-ification [25] Many plants perform reduction of Cr6+ toCr3+ in the thin lateral roots with further transport of Cr3+to the leaves [40] Soy and garlic plants are known to usethis strategy [37 41] These plants can reduce Cr6+ to theintermediate forms of Cr5+ and Cr4+ which further areconverted to less toxic Cr3+ [37 41] Plants also protectthemselves against Cr toxicity by the immobilization of Crions by the cell wall of roots and isolation of Cr in vacuoles[40 42] In particular Cr3+ can form highly stable complexeswith organic compounds such as peptides (glutathione)carbohydrates (especially pentoses) NADH FADH2 andpossibly also organic acids and these complexes are stored invacuoles of root cells [37 43] Immobilization of heavy metalions in vacuoles helps to remove them from themetabolicallyactive cell compartments [44]

Accumulation of chromium affects metabolic processesin plants which results in different morphological and phys-iological defects [12 29 30 45 46] Seed germination isprimary physiological process which is influenced by toxicmetalsThe treatment with 200120583MCr6+ was shown to reduceby 25 germination of Echinochloa colona L seeds [47] Thepresence of Cr6+ at high concentrations in the soil reducedto 48 seed germination in Phaseolus vulgaris L [48]Decreased seed germination with increasing concentrationof chromium ions was also observed for cowpea Vignasinensis L [45] melon (Cucumis melo L) [49] and wheat(Triticum aestivum L) [50] The seed death and delayed seedgermination may be explained by activation of proteasesor inhibition of amylase activity by chromium with thesubsequent decreased transport of carbohydrates to the germ[8 29]

Among other toxic effects of Cr the inhibition of rootgrowth was widely observed Inhibitory effects of chromiumon the root elongationwere found inwheat and vigna [42 51]Another study showed that the exposure to Cr6+ in the formof potassium dichromate slightly affected pea germinationbut inhibited growth of embryonic root and stem of peaplants [52] The root growth defects under exposure to highlevels of heavy metals can be caused by inhibition of rootcell division andor reduction of cell proliferation in the rootzone of growth [27 29 36 53] In general the toxic effectsof heavy metals on roots include (1) reduction of lengthbiomass and diameter (2) damage of the growth cone (3)destruction of root hairs or decrease in root numbers (4)increase or decrease of lateral roots formation (5) increase

in lignification and (6) changes in the structure of thehypodermis and endoderm [8 29]

The toxicity of heavy metal ions is also manifestedin the inhibition of growth of the aboveground parts ofplants reduction of the size of flowers and fruits [6ndash8]The reduced plant height because of exposure to Cr6+ wasdescribed for Cucumis sativus L Lactuca sativa L andPanicum miliaceum L [54] Besides that potassium dichro-mate reduced the length and mass of roots and shoots ofwheat [50] and pea plants [55] in a concentration- and time-dependent manner Unlike shoots the root system of peaplants was more sensitive to potassium dichromate exposurethat seems to be related to metal accumulation in roots[55]

It has been noted that chromium also affects growthof leaves the main photosynthetic plant organ Increasingchromium concentration leads to a significant reductionin the leaf area and leaf biomass which is accompaniedby decreased photosynthesis and induction of chlorosisand necrosis of leaves [6 8 56] Under Cr exposure manydestructive processes take place in leaves Those includesuppression of chlorophyll synthesis disruption of chloro-plast ultrastructure inhibition of photosynthetic electrontransport and release of magnesium ions from the moleculeof chlorophyll [12 29 30 45] Like other heavy metals Crions can decrease level of carotenoids in some plants [12]However we have previously found that cultivation of Psativum with potassium dichromate at different concentra-tions resulted in higher concentrations of carotenoids andanthocyanins in plant leaves [55] Assuming that carotenoidsand anthocyanins are powerful low-molecular mass plantantioxidants (Table 1) their increased levels may be a result ofan adaptive response of pea plants to Cr6+-induced oxidativestress of moderate intensity [3 55]

Hexavalent chromium was found to inhibit CO2 absorp-tion and photosynthesis [57] leading to plant biomass reduc-tion [58] In pea plants Cr6+ added in the form of potassiumdichromate significantly decreased photosynthesis respira-tion and symbiotic nitrogen fixation [59] In spinach thereduction of photosynthesis induced by Cr6+ was connectedwith inhibition of the electron transport in photosystemsI and II within chloroplasts [59] Cr-induced inhibition ofplant growth was also connected with changes in nitrogenmetabolism In particular chromium treatment adverselyaffected nitrogenase nitrate reductase nitrite reductase glu-tamine synthetase and glutamate dehydrogenase in variousplant organs at different growth stages in cluster bean plants[4]

Heavy metals are able to inactivate directly manyenzymes via replacement of the primary metal in enzymeactive center or causing protein denaturation Chromiuminhibits such enzymes as nitrate reductase [60 61] and Fe3+-reductase in plant roots [62] In plant mitochondria Cr6+can inhibit electron transport by replacing copper and ironions in prosthetic groups of many carrier proteins [8 58 63]In fact Cr6+ at concentrations of 20 and 200 120583M inhibitedcytochrome oxidase in mitochondrial respiratory chain inpea plants [63]


Chromium belongs to transition metals which par-ticipate in cellular redox processes in particular in ROSproduction [12 41 64] The latter are intermediates of partialreduction of molecular oxygen and include free radicalssuch as superoxide anion (O2

∙minus) and hydroxyl radical (HO∙)as well as nonradical reactive species such as hydrogenperoxide (H2O2) and other peroxides [3] Being an ele-ment with changeable valence chromium can enter Haber-WeissFenton-type reactions resulting in generation of HO∙radical In the cell Cr6+ is reduced by cellular reductants suchas glutathione with an assistance of glutathione reductase toCr5+ which can further react with H2O2 in Fenton reactionwith HO∙ formation [3 41 56]

Cr6+GSHenzymes997888997888997888997888997888997888997888997888997888997888rarr

thiolsCr5+complex

H2O

2

997888997888997888997888rarr HO∙ (1)

Cr6+ +O2∙minus 997888rarr Cr5+ +O2 (2)

Cr5+ +H2O2 997888rarr Cr6+ +HO∙ +OHminus (3)

Moreover chromium-induced inactivation electrontransport in pea root mitochondria was accompanied byenhanced O2

∙minus generation [63] It is well established thatROS can interact with virtually all cellular componentsnamely lipids carbohydrates proteins nucleic acids andso on Enhanced ROS production promotes developmentof oxidative stress when oxidants damage biomoleculesprevailing capacity of defensive mechanisms [3] In supportof it treatment with chromium compounds was found tointensify in a dose-dependent manner lipid peroxidation inwheat sorghum moss pea and others [12 61 63 65] Inaddition chromium exposure increased level of carbonylatedproteins which are widely used as a marker of oxidativedamage to proteins [66]

To avoid oxidative damage plant cells have evolved com-plex defense systems including nonenzymatic and enzymaticantioxidants and repair system [67] Antioxidant system ofplants includes (see Table 1) (i) the enzymes that directlyscavenge ROS and other free radicals (superoxide dismu-tase (SOD) catalase and different peroxidases) (ii) thenonenzymatic antioxidant molecules such as ascorbate glu-tathione 120572-tocopherol carotenoids and phenol compounds(iii) the components of ascorbate-glutathione pathway whichscavenge H2O2 in a coupled series of reactions by usingNAD(P)H (iv) the enzymes involved in disulfide reductionthioredoxin and glutaredoxin (v) metal-binding proteinssuch as ferritin phytochelatins and metallothioneins [1568] Capacity of defense systems in plants is largely modu-lated by chromium concentration in the nutrient mediumHexavalent chromium at low concentrations increased theactivity of antioxidant enzymes such as superoxide dismutaseor catalase in pea plants [12 69] seedlings of chicken rice[12 69 70] vigna [57] and basil plants [30] At the sametime chromium at high concentrations reduced activities ofthese enzymes in pea plants [63] and seedlings of chickenrice [69 70]Meanwhile the activation of guaiacol peroxidasewas found in different plants treated with high chromiumconcentrations [19 30 71] One may therefore assume thatchromium at low concentrations induces adaptive response

in plant tissues that allows plant to tolerate this metal withoutsubstantial negative effects However when chromium isavailable at high concentrations the plant defense systems arenot able to cope fully with toxic effects of chromium and evenantioxidant enzymes may be damaged

3 Legume-Rhizobium Symbiosis

Nitrogen (N) is an essential element for plant growth anddevelopment It is a major component of chlorophyllsamino acids nucleotides nucleic acids coenzymes vitaminsamines and other plant constituents [72ndash75] To be providedwith nitrogen for their needs plants absorb N in the forms ofnitrate (NO3

minus) and ammonium (NH4+) from soil but cannot

use free nitrogen gas (N2) from the atmosphere The conver-sion of atmospheric nitrogen into ammonia is provided bymany chemical processes and biological nitrogen fixation isthe most important among them [73 75]

Biological nitrogen fixation is carried out with a largegroup of soil free-living associative or symbiotic prokaryotescalled collectively diazotrophs [72 74 76] They include thenodule bacteria from families Rhizobiaceae Phyllobacte-riaceae Bradyrhizobiaceae and Hyphomicrobiaceae Thesebacteria are able to establish symbiosis with plants fromFabaceae family and accumulate atmospheric nitrogen tocover over 60 of plant needs in nitrogen [73 74] Theformation and functioning of legume-rhizobial symbiosis isa complex process with a few coordinated stages which areregulated by signals from both nodule bacteria and the hostplant The stages include (i) preinfection (ii) infection withnodule formation and (iii) functioning of the mature noduleand its death [74 76]

The symbiotic interaction is initiated when nodule bac-teria infect root hair of the host plant Chemotaxis of soilnodule bacteria to root exudates plays an important role inthe initiation of symbiosis [74 77] Both rhizobia and hostplants exhibit a strong specificity To identify rhizobia asbenefit partners the host plants secrete specific compoundswhich are recognized by the homologous (compatible) bac-teria The main compounds of the root exudates are car-bohydrates organic acids amino acids and phenols [78]Root exudates attract and induce attachment of compatiblerhizobia to walls of the root hair cells [79] Subsequentlyrhizobia adhere to and colonize the root surface Rhizobia areattached to root hairs by the specific surface polysaccharideswhich interact with lectin receptors on root hair cell walls[74 79 80] After attachment the bacteria interact withcertain flavonoids which are produced by root legumesPlant flavonoids trigger the expression of bacterial nodulationgenes (nod-genes) which control the formation of nodulesin plant roots [79 81 82] The products of nod-genes areinvolved in synthesis and export of specific lipochitooligosac-charides called Nod factors Bacterial Nod factors serve assignaling molecules that initiate nodule formation in rootcortex [83 84] Previously it has been assumed that bindingplant lectins to bacterial surface polysaccharides plays a keyrole in the specificity between rhizobia and their legumehosts In accordance with recent data the host specificity oflegume plants is presumably determined by the Nod factors


and to a lesser extent by surface polysaccharides of the nodulebacteria [85] In addition to Nod factors rhizobial surfacepolysaccharides such as exopolysaccharides lipopolysaccha-rides capsular polysaccharides and cyclic glucans are alsoimportant for development of root nodules and modulationof host specificity In particular binding of plant lectins tobacterial polysaccharides can influence adhesion of bacteriato root hairs in pea plants the mutant R leguminosarumstrain defective in synthesis of surface glucomannan hadan impaired ability to attach to root hairs [86] Recentlyexopolysaccharide receptor has been identified in Lotusjaponicus that controls rhizobial infection and distinguishesbetween compatible and incompatible exopolysaccharides[87]

In general the formation of a nodule requires thereprogramming of differentiated root cells to form a pri-mordium which a nodule can develop from The bacteriaenter the developing nodule via formation of infectionthreads Regulation and stages of root nodule formationhave been comprehensively reviewed previously [15 88ndash91]The nodule formation is completed when nodule bacteriaare transformed in nitrogen-fixing bacteroides [92 93] Theformed nodules may be either determinate or indeterminatedepending on the host Determinate nodules have a short-lived meristem and they grow by plant cell expansion anddivision resulting in nodules progressing through well-defined developmental stages Legumes which formed deter-minate nodules include Lotus sp Phaseolus sp and Glycinemax In contrast indeterminate nodules have a persistentmeristem and infection is continuous New nodule cells aresubsequently infected by rhizobia residing in the noduleMedicago sp Vicia sp Trifolium sp and P sativum aretypical legumes with indeterminate nodules [94]

In nodules bacteroides are provided with microaero-bic environment required for expression of enzymes ofthe nitrogenase complex A plant-produced oxygen-bindingprotein called leghemoglobin [15] controls oxygen supplyto bacteroides Nitrogenase complex located on internalmembranes of bacteroides is responsible for ATP-dependentreduction of free nitrogen to ammonia [79 95] Fur-ther ammonia interacts with intracellular keto acids (120572-ketoglutaric pyruvic or oxalic acids) in dehydrogenase-and transaminase-catalyzed reactions forming respectiveamino acids such as glutamine alanine or asparagine [96]In the form of free ammonia amino acids or amidesnitrogen-containing substances are transported fromnodulesto the roots and then to the aboveground parts of plants[97]

Biological nitrogen fixation is closely connected withphotosynthesis since the latter provides assimilates andenergy resources to nodule bacteria and the bacteria in turnprovide photosynthetic apparatus of plants with nitrogencompounds [96 98] The intensity of photosynthesis andammonium inclusion in the plant metabolism depends oncontent and functional activity of chloroplasts the structuralelements of the photosynthetic apparatus [99] At the sametime products of bacterial nitrogen fixation substantiallyaffect the intensity of photosynthesis and transport of pho-toassimilates from plants to nodules [100] Thus this is a real

symbiosis providingmutual benefits for both partners plantsand bacteria

4 Free Radical Processes inLegume-Rhizobium Symbiosis

To date there is much evidence that ROS and antioxidantdefense play an important role in the formation and func-tioning of legume-rhizobium symbiosis [101 102] Similarlyto pathogen invasion response the infection of legumeswith rhizobia causes an intensification of oxidative processesin plant cells promoted by increased production of ROSand nitric oxide (NO∙) However apart from response topathogenesis production of ROS andNO∙may not be a plantdefense response to the rhizobia but rather a process that isneeded for the development of a symbiosis Elevated levels ofROS were found to be necessary for the effective penetrationof bacteria into plant tissues since the decrease of ROS andNO∙ levels prevented formation of bacterial infection threadand delayed nodule formation [101ndash103]

Like jasmonic acid and ethylene ROS particularly O2∙minus

and H2O2 were supposed to act as signaling moleculesregulating formation of senescence of legume-rhizobiumsymbiosis [104ndash106] At the initial stages of symbiosis anoxidative burst occurs in the place of bacterial infection [107]Recent studies suggest that legume NADPH-oxidases play apivotal role in ROS production under oxidative burst andin turn have a crucial role in different stages of nodulation[108] Oxidative burst can have a dual function in legume-rhizobium symbiosis it inhibits the protective reactions ofplants on penetration of compatible bacteria or converselyit can activate the protective mechanisms of plants underadverse conditions for symbiosis [109 110] Accordinglybacterial Nod factors were shown to stimulate oxidative burstby blocking the induction of nod-genes in plants when theinteraction between symbionts is incompatible [110 111]

The free radical processes in plant cells largely depend onmany exogenous and endogenous factors [17] In particularalkalization of the cytoplasmic pH causes membrane depo-larization and increased the interaction of plant cells withrhizobia [76] Plant phenolic compounds which are suscep-tible to rhizobial infection can undergo autoxidation by freeoxygen and thereby increase ROS levels in particular H2O2[17] Production of H2O2 during symbiosis was detected ininfection threads and root nodules of Medicago sativa andP sativum [107 112] Hydrogen peroxide is relatively long-living ROS and can easily diffuse via biological membranesand act at distant places Dependently on the concentrationH2O2 can directly act as an antibacterial agent or as a signalmolecule triggering adaptive response in plants [17 101 113]In addition H2O2 has been shown to be necessary for theoptimal propagation of infectious bacterial threads insideroot hairs and membranes of plant cells [114]

Production of ROS in legume-rhizobium symbiosis alsooccurs during the reductive processes required for nitrogenfixation Many compounds that act as electron donors fornitrogenase (eg ferredoxin) are able to autoxidize withO2∙minus [83] ROS production may be also promoted by leghe-

moglobin which is present in nodules at high levels In the


presence of O2 leghemoglobin can undergo autoxidationand as a result O2

∙minus is generated with further dismutationto H2O2 [83] The interaction of leghemoglobin with H2O2leads to the formation of a highly oxidized ferric-porphyrincation-radical which further can oxidize protein moleculeswith formation of for example tyrosine radicals [83 115]In addition H2O2 can be released from leghemoglobin andpromote HO∙ generation via Fenton reaction [115]

Nitrogenase complex in bacteroides is very sensitiveto ROS therefore it not surprising that legume noduleshave efficient mechanisms to maintain proper redox balanceand low ROS levels Like plants nodules possess a power-ful system of antioxidant defense consisting of antioxidantenzymes (SODs catalase and various peroxidases) enzymesof the ascorbate-glutathione cycle and low-molecular massantioxidant metabolites such as ascorbate glutathione andtocopherols (Table 1) [68 116 117] The capacity of noduleantioxidant system affects largely nitrogen-fixing efficiencyin particular nodules may not function without ascorbate-glutathione cycle [68]

Previous studies have shown that changes in O2∙minus and

H2O2 levels in P sativum roots under symbiosis developmentwith nodule bacteria depend on the efficacy of rhizobialstrains and the ability of peas to form nodules [17] Signif-icantly increased levels of O2

∙minus and H2O2 were found inthe pea roots after inoculation by incompatible strains ofbacteria Rhizobium leguminosarum bv phaseoli This mayindicate ROS involvement in protection against infection ofthe pea roots with rhizobia [17] Differential changes in theactivities of SOD catalase and peroxidase were found inpea roots inoculated by different rhizobial strains [17 101]In response to inoculation with highly effective Rhizobiumstrain the activity of the antioxidant enzymes in pea plantsdid not increase and coincided with the decrease in ROSlevels in the plants roots Such a correlation between ROSlevels and the antioxidant system activity was supposed todetermine interaction of bacteria and plants to promote theireffective symbiosis At the same time enhanced SOD andperoxidase activities in pea plants after inoculation by incom-patible strain R leguminosarum bv phaseoli could result fromincreasedO2

∙minus generationThe increased antioxidant enzymeactivity can prevent an increase in the level of O2

∙minus to acritical value but complicate the development of symbiosis[17]

The inoculation of pea roots by effective R legumi-nosarum bv viciae strains increased O2

∙minus and H2O2 levelswith simultaneous stimulation of antioxidant enzymes in peaseedling epicotyls However ROS are not directly involvedin the development of infection and subsequent formation ofnodules [118]This suggests that the plants have certainmech-anisms to prevent bacterial infection in organs that cannotform nodules [17] It is supposed that limitation of rhizobialinfection is connected with triggering a reaction similar tothe systemic acquired resistance in phytopathogenesis [101]or systemic induced resistance as in the case of infectionby nonpathogenic microorganisms [17 119] It is known thatROS can upregulate expression of genes encoding hydrolyticenzymes stress-protective proteins enzymes involved insynthesis of phenolic compounds phytotoxins and other

substances required for development of acquired resistanceto pathogens [120]

Thus ROS generation is among key components of theplant response to infection with both compatible and incom-patible bacteria Plants delicately regulate ROS levels usingmechanisms of ROS generation and activation of antioxidantsystem [17] Notably at the later stages of symbiotic forma-tion when the amount of rhizobia in the roots reaches acertain level the host plant may also include mechanisms ofROS generation and activation of the antioxidant system toregulate further nodule formation [117] In addition duringnodule senescence high ROS levels have been detected insenescing symbiosomes suggesting ROS involvement in thisprocess [18]

5 Effects of Legume-Rhizobium Symbiosisand Cr(VI) Toxicity

The ability of nodule bacteria to form a symbiosis withlegume plants depends onmany environment factors such astemperature humidity aeration pH medium soil structurepresence of labile nitrogen forms phosphorus potassiumand magnesium in the soil [121 122] In addition microor-ganisms are very sensitive to the presence of heavy metalions in the soil [123] A number of heavy metals (egCu Ni Zn Cd and As) have been reported to inhibit thegrowth and modify morphology structure and activities ofvarious groups of soil microorganisms including symbioticnitrogen fixators like Mesorhizobium ciceri Rhizobium spBradyrhizobium sp and Sinorhizobium sp [124] Amongmentionedmetal elements a strong inhibitory effect of copperon growth and enzyme activities of Bradyrhizobium BMP1strain was found [124] Effective R leguminosarum bv trifoliipopulation did not survive during long-term incubation insoils containing 71mgCd kgminus1 [125] Heavymetals includingCr inhibit the activity of nitrogenase in nodules leading todecreased intensity of nitrogen fixation [72] Mechanismsof chromium toxicity to nodules are not well studied butone may suggest they include oxidative stress developmentand protein modification Since nitrogenase is very sensitiveto oxidation Cr treatment can lead to inactivation of theenzyme and impair functioning of nodules

Several studies have reported that nitrogen-fixing bac-teria can diminish the toxicity of heavy metals on hostplants [19ndash21 126] The protective effects are proposed to beassociatedwith the accumulation of themetals in the nodulesAccordingly the nodule bacteria are exposed more to heavymetals than the host plant Resistance of the bacteria to heavymetals is both species- and strain-specific [126] One cansuppose that if bacteria have powerful defense mechanismsagainst heavy metal toxicity the protective effect of thesebacteria on the host plant will be more pronounced

Like other bacteria protective mechanisms of rhizobiaagainst chromium toxicity apparently include direct and indi-rect reduction of Cr6+ to Cr3+ metal bindingwith further iso-lation or elimination and upregulation of antioxidant defense[1] In the case of exposure to Cu the rhizobial symbiosiswith Sinorhizobium meliloti CCNWSX0020 also upregulatedexpression of genes encoding components of antioxidant


Without the use ofbacterial treatment

Concentrations(a) Chlorophylls(b) Carotenoids(c) Anthocyanins

(i) CP levelTBARS levelCАТ activity

(ii) GPx activity

length of shootsand roots

wet mass ofshoots and roots ROS


(i) CP level(ii) TBARS level

(iii) CАТ activityGPx activity

length of shootslength of rootswet mass of

shootswet mass of roots

ROS

Strongoxidative stress

Mildoxidative stress

Inoculation of nitrogenfixing bacteria Rhizobium

K2Cr2O7 K2Cr2O7

Figure 2 Effects ofCr(IV) exposure alone and in combinationwith nodule rhizobacteria on selected growthparameters andROShomeostasisin P sativum plants Arrows uarr and darr indicate the increase and decrease in the parameter respectively

defense in both plants and bacteria The results indicatedthat the rhizobial symbiosis with S meliloti CCNWSX0020not only enhanced plant growth and metal uptake but alsoimproved the responses of plant antioxidant defense to Cuexcess [126] Regarding Cr influence we have recently foundthat inoculation with highly effective nitrogen-fixing bacteriadecreased the toxic effects of chromium (IV) on P sativumThe protective effects included improvement of the length ofshoots and mass of the plant roots and enhanced levels ofchlorophylls carotenoids and anthocyanins compared withthe effects of chromium on pea plants without inoculation(Figure 2) [127] In addition treatment with potassiumdichromate did not affect level of oxidized proteins butincreased levels of lipid peroxidation products and decreasedcatalase activity in plants preinoculated with nitrogen-fixingrhizobia but not in noninoculated pea plants [127] Despiteincreased levels of lipid peroxidation products pea plantsgrew better when treated with Cr in the presence of nodulebacteria if compared with the plants treated only with Cr Wesuppose that nodule bacteria are able to decrease Cr toxicityto pea plants and their protective effects could be connectedrather with modulation of synthesis of plant pigments thanwith involvement of enzymatic antioxidant defense Since

carotenoids and anthocyanins have antioxidant propertiesthey might be involved in minimization of negative effectsfrom oxidative stress induced by chromium It is possiblethat accumulation of chromium in root nodules decreased Crtransport to other plant parts allowing the latter to developprotective mechanisms that are more effective

6 Conclusions and Perspectives

The presence of chromium compounds in soils inhibits seedgermination and induces various morphological and physi-ological defects in many plants including legumes Toxicityof chromium in plants is connected with the enhancedROS formation and oxidative stress development resulting inthe intensified protein modification lipid peroxidation andDNA damage In legume-rhizobial symbiosis both host plantand nodule bacteria undergo oxidative stress induced bychromium with rhizobia being more stressed due to prefer-ential Cr accumulation in root nodules Available data suggestthat inoculation with nodule bacteria can be considered asan effective approach to minimize toxic effects of chromiumand other heavy metals on agricultural plants At the sametime the protective efficacy of nodule bacteria depends on


many factors such as type and concentrations of metalscompatibility of partners virulence adaptive capacity andnitrogen-fixing activity of bacteria Therefore the effects ofheavy metal on legume-rhizobium symbiosis and searchof ways to enhance metal resistance of nodule bacteriaare perspective potential research direction It seems thatCr at low concentrations may induce mild oxidative stress[128] which can have beneficial rather than detrimentaleffects on legume-rhizobium symbiosis and plant metabolicprocesses (Figure 2) As known mild oxidative stress mayinduce adaptive response which enhances resistance tomanylethal stresses [128] In addition mild oxidative stress playsan important role in establishment of effective legume-rhizobium symbiosis [103] We propose that using the pre-treatment with nodule bacteria at low levels of oxidantscan aid bacteria to resist high levels of heavy metals in theenvironment The construction of nodule bacterial strainswith higher resistance to environmental stresses may be agreat opportunity to increase the benefit from their use inbioremediation and cultivation at polluted areas

Conflicts of Interest

The authors declare that they have no conflicts of interest

Acknowledgments

The authors express their sincere gratitude to Dr Olga IKubrak (Stockholm University) for English correction of thearticle

References

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[2] O Y Vasylkiv O I Kubrak K B Storey and V I LushchakldquoCytotoxicity of chromium ions may be connected with induc-tion of oxidative stressrdquo Chemosphere vol 80 no 9 pp 1044ndash1049 2010

[3] V I Lushchak ldquoAdaptive response to oxidative stress Bacteriafungi plants and animalsrdquo Comparative Biochemistry and Phys-iology - Part C Toxicology amp Pharmacology vol 153 no 2 pp175ndash190 2011

[4] P Sangwan V Kumar andU N Joshi ldquoEffect of chromium(VI)toxicity on enzymes of nitrogen metabolism in clusterbean(cyamopsis tetragonoloba L)rdquo Enzyme Research vol 2014Article ID 784036 2014

[5] D Bagchi S J Stohs BW DownsM Bagchi andH G PreussldquoCytotoxicity and oxidative mechanisms of different forms ofchromiumrdquo Toxicology vol 180 no 1 pp 5ndash22 2002

[6] R A Gill L Zang B Ali et al ldquoChromium-induced physio-chemical and ultrastructural changes in four cultivars of Bras-sica napus Lrdquo Chemosphere vol 120 pp 154ndash164 2015

[7] R A Gill B Ali P Cui et al ldquoComparative transcriptome pro-filing of two Brassica napus cultivars under chromium toxicityand its alleviation by reduced glutathionerdquo BMC Genomics vol17 no 1 article no 885 2016

[8] G DalCorso ldquoHeavy Metal Toxicity in Plantsrdquo in Plants andHeavy Metals SpringerBriefs in Molecular Science pp 1ndash25Springer Dordrecht Netherlands 2012

[9] M Bibi and M Hussain ldquoEffect of copper and lead onphotosynthesis andplant pigments in black gram [Vignamungo(L) Hepper]rdquo Bulletin of Environmental Contamination andToxicology vol 74 no 6 pp 1126ndash1133 2005

[10] J Ma C Lv M Xu G Chen C Lv and Z Gao ldquoPhotosynthesisperformance antioxidant enzymes and ultrastructural analysesof rice seedlings under chromium stressrdquoEnvironmental Scienceand Pollution Research vol 23 no 2 pp 1768ndash1778 2016

[11] Z Xue H Gao and S Zhao ldquoEffects of cadmium on thephotosynthetic activity in mature and young leaves of soybeanplantsrdquo Environmental Science and Pollution Research vol 21no 6 pp 4656ndash4664 2014

[12] S K Panda and S Choudhury ldquoChromium stress in plantsrdquoBrazilian Journal of Plant Physiology vol 17 no 1 pp 95ndash1022005

[13] S Maiti N Ghosh C Mandal K Das N Dey and M KAdak ldquoResponses of the maize plant to chromium stress withreference to antioxidation activityrdquo Brazilian Journal of PlantPhysiology vol 24 no 3 pp 203ndash212 2012

[14] Z Olteanu L Oprica and E Truta ldquoChanges induced by twochromium-containing compounds in antioxidative responsesoluble protein level and amylase activity in barley seedlingsrdquoAnalele Stiintifice ale Universitatii ldquoAlexandru Ioan Cuzardquo dinIasi Sec II a Genetica si Biologie Moleculara vol 13 no 3 pp41ndash47 2012

[15] A Baryla P Carrier F Franck C Coulomb C Sahut and MHavaux ldquoLeaf chlorosis in oilseed rape plants (Brassica napus)grown on cadmium-polluted soil Causes and consequences forphotosynthesis and growthrdquo Planta vol 212 no 5-6 pp 696ndash709 2001

[16] G M Drozdenko P M Momenko and Y S Kots ldquoThe activityof guaiacol and ascorbate peroxidase and protein compositionof soybean roots during the formation and early functioning ofthe symbiotic systems Glycine max - Bradyrhizobium japon-icum Serrdquo Biology vol 1 pp 18ndash26 2013

[17] A K Glyanrsquoko G P Akimova L E Makarova M G Sokolovaand G G VasilrsquoEva ldquoOxidative processes at initial stages ofinteraction of nodule bacteria (Rhizobium leguminosarum)andpea (Pisum sativumL) A reviewrdquoApplied Biochemistry andMicrobiology vol 43 no 5 pp 516ndash522 2007

[18] C Chang I Damiani A Puppo and P Frendo ldquoRedox changesduring the legume-rhizobium symbiosisrdquoMolecular Plant vol2 no 3 pp 370ndash377 2009

[19] A W B Johnston K H Yeoman and M Wexler ldquoMetalsand the rhizobial-legume symbiosis - Uptake utilization andsignallingrdquo Advances in Microbial Physiology vol 45 pp 113ndash156 2001

[20] K Broos J Mertens and E Smolders ldquoToxicity of heavymetalsin soil assessed with various soil microbial and plant growthassays A comparative studyrdquo Environmental Toxicology andChemistry vol 24 no 3 pp 634ndash640 2005

[21] X Hao S Taghavi P Xie et al ldquoPhytoremediation of Heavyand Transition Metals Aided by Legume-Rhizobia SymbiosisrdquoInternational Journal of Phytoremediation vol 16 no 2 pp 179ndash202 2014

[22] A Ike R Sriprang H Ono Y Murooka and M YamashitaldquoBioremediation of cadmium contaminated soil using symbio-sis between leguminous plant and recombinant rhizobia with


the MTL4 and the PCS genesrdquo Chemosphere vol 66 no 9 pp1670ndash1676 2007

[23] P AWani M S Khan and A Zaidi ldquoChromium-reducing andplant growth-promoting Mesorhizobium improves chickpeagrowth in chromium-amended soilrdquo Biotechnology Letters vol30 no 1 pp 159ndash163 2008

[24] A Kumar and S K Maiti ldquoAvailability of chromium nickeland other associated heavy metals of ultramafic and serpentinesoilrock and in plantsrdquo International Journal of emergingTechnology andAdvanced Engineering vol 3 no 3 pp 256ndash2682013

[25] A M Zayed and N Terry ldquoChromium in the environmentfactors affecting biological remediationrdquoPlant and Soil vol 249no 1 pp 139ndash156 2003

[26] O I Kubrak O V Lushchak J V Lushchak et al ldquoChromiumeffects on free radical processes in goldfish tissues Comparisonof Cr(III) and Cr(VI) exposures on oxidative stress markersglutathione status and antioxidant enzymesrdquo Comparative Bio-chemistry and Physiology - C Toxicology and Pharmacology vol152 no 3 pp 360ndash370 2010

[27] J Liu Z Sun M Lavoie X Fan X Bai and H QianldquoAmmonium reduces chromium toxicity in the freshwater algaChlorella vulgarisrdquoAppliedMicrobiology andBiotechnology vol99 no 7 pp 3249ndash3258 2015

[28] O V Lushchak O I Kubrak I M Torous T Y NazarchukK B Storey and V I Lushchak ldquoTrivalent chromium inducesoxidative stress in goldfish brainrdquo Chemosphere vol 75 no 1pp 56ndash62 2009

[29] F U R Shah N Ahmad K R Masood J R Peralta-Videaand F U D Ahmad ldquoHeavy metal toxicity in plantsrdquo PlantAdaptation and Phytoremediation pp 71ndash97 2010

[30] V Rai P Vajpayee S N Singh and S Mehrotra ldquoEffect ofchromium accumulation on photosynthetic pigments oxida-tive stress defense system nitrate reduction proline level andeugenol content of Ocimum tenuiflorum Lrdquo Journal of PlantSciences vol 167 no 5 pp 1159ndash1169 2004

[31] E M M Marwa A A Meharg and C M Rice ldquoRiskassessment of potentially toxic elements in agricultural soils andmaize tissues from selected districts in Tanzaniardquo Science of theTotal Environment vol 416 pp 180ndash186 2012

[32] M Valko H Morris andM T D Cronin ldquoMetals toxicity andoxidative stressrdquoCurrentMedicinal Chemistry vol 12 no 10 pp1161ndash1208 2005

[33] HOliveira ldquoChromiumas an environmental pollutant insightson induced plant toxicityrdquo Journal of Botany vol 2012 ArticleID 375843 8 pages 2012

[34] S Ali S A Bharwana M Rizwan et al ldquoFulvic acid mediateschromium (Cr) tolerance in wheat (Triticum aestivum L)through lowering of Cr uptake and improved antioxidantdefense systemrdquo Environmental Science and Pollution Researchvol 22 no 14 pp 10601ndash10609 2015

[35] A Zayed C M Lytle J-H Qian and N Terry ldquoChromiumaccumulation translocation and chemical speciation in veg-etable cropsrdquo Planta vol 206 no 2 pp 293ndash299 1998

[36] S Hayat G Khalique M Irfan A S Wani B N Tripathi andA Ahmad ldquoPhysiological changes induced by chromium stressin plants An overviewrdquoProtoplasma vol 249 no 3 pp 599ndash6112012

[37] P Babula V Adam R Opatrilova J Zehnalek L Havel andR Kizek ldquoUncommon heavy metals metalloids and their planttoxicity A reviewrdquo Environmental Chemistry Letters vol 6 no4 pp 189ndash213 2008

[38] R A Skeffington P R Shewry and P J Peterson ldquoChromiumuptake and transport in barley seedlings (Hordeum vulgare L)rdquoPlanta vol 132 no 3 pp 209ndash214 1976

[39] K K Tiwari S Dwivedi N K Singh U N Rai and R DTripathi ldquoChromium (VI) induced phytotoxicity and oxidativestress in pea (Pisum sativum L) Biochemical changes andtranslocation of essential nutrientsrdquo Journal of EnvironmentalBiology vol 30 no 3 pp 389ndash394 2009

[40] F X Han B B M Sridhar D L Monts and Y Su ldquoPhytoavail-ability and toxicity of trivalent and hexavalent chromium toBrassica junceardquo New Phytologist vol 162 no 2 pp 489ndash4992004

[41] A Levina R Codd C T Dillon and P A Lay ldquoChromiumin biology Toxicology and nutritional aspectsrdquo Progress inInorganic Chemistry vol 51 pp 145ndash250 2003

[42] A K Shanker M Djanaguiraman R Sudhagar C N Chan-drashekar and G Pathmanabhan ldquoDifferential antioxida-tive response of ascorbate glutathione pathway enzymes andmetabolites to chromium speciation stress in green gram (Vignaradiata (L) RWilczek cv CO 4) rootsrdquo Journal of Plant Sciencesvol 166 no 4 pp 1035ndash1043 2004

[43] V I Lushchak ldquoGlutathione homeostasis and functions poten-tial targets for medical interventionsrdquo Journal of Amino Acidsvol 2012 Article ID 736837 26 pages 2012

[44] P Carrier A Baryla and M Havaux ldquoCadmium distributionand microlocalization in oilseed rape (Brassica napus) afterlong-term growth on cadmium-contaminated soilrdquo Planta vol216 no 6 pp 939ndash950 2003

[45] K Nath D Singh S Shyam and Y K Sharma ldquoEffect ofchromium and tannery effluent toxicity on metabolism andgrowth in cowpea (vigna sinensis l saviex hassk) seedlingrdquoResearch in Environment and Life Sciences vol 1 no 3 pp 91ndash94 2008

[46] M Adrees S Ali M Iqbal et al ldquoMannitol alleviates chromiumtoxicity in wheat plants in relation to growth yield stimulationof anti-oxidative enzymes oxidative stress and Cr uptake insand and soil mediardquo Ecotoxicology and Environmental Safetyvol 122 pp 1ndash8 2015

[47] G R Rout S Samantaray and P Das ldquoEffects of chromium andnickel on germination and growth in tolerant and non-tolerantpopulations of Echinochloa colona (L) linkrdquo Chemosphere vol40 no 8 pp 855ndash859 2000

[48] P Dreyer Parr and F G Taylor Jr ldquoGermination and growtheffects of hexavalent chromium in Orocol TL (a corrosioninhibitor) on Phaseolus vulgarisrdquo Environment Internationalvol 7 no 3 pp 197ndash202 1982

[49] I E Akinci and S Akinci ldquoEffect of chromium toxicity ongermination and early seedling growth in melon (Cucumismelo L)rdquo African Journal of Biotechnology vol 9 no 29 pp4589ndash4594 2010

[50] J K Datta A Bandhyopadhyay A Banerjee andN KMondalldquoPhytotoxic effect of chromium on the germination seedlinggrowth of some wheat (Triticum aestivum L) cultivars underlaboratory conditionrdquo Journal of Agricultural Technology vol 7no 2 pp 395ndash402 2011

[51] H Amin B A Arain F Amin et al ldquoAnalysis of growthresponse and tolerance index of Glycine max (L) Merr underhexavalent chromium stressrdquo Advanced life sciences vol 1 no4 pp 231ndash241 2014

[52] N R Bishnoi A Dua V K Gupta and S K Sawhney ldquoEffectof chromium on seed germination seedling growth and yield of


peasrdquo Agriculture Ecosystems amp Environment vol 47 no 1 pp47ndash57 1993

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[55] U Y Stambulska ldquoEffect of potassium dichromate on somephyziological and biochemical parameters of pea plantsrdquo inProblems of physiological functions regulation vol 18 pp 64ndash68Bulletin of Taras Shevchenko National University of Kyiv 2015

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[58] H P Singh P Mahajan S Kaur D R Batish and R K KohlildquoChromium toxicity and tolerance in plantsrdquo EnvironmentalChemistry Letters vol 11 no 3 pp 229ndash254 2013

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[60] S K Panda andH K Patra ldquoNitrate and ammonium ions effecton the chromium toxicity in developing wheat seedlingsrdquo inProceedings of the National Academy of Sciences India vol 70pp 75ndash80 2000

[61] J Zou K Yu Z Zhang W Jiang and D Liu ldquoAntioxidantresponse system and chlorophyll fluorescence in chromium(VI)- treated Zea Mays L seedlingsrdquo Acta Biologica Cracovien-sia Series Botanica vol 51 no 1 pp 23ndash33 2009

[62] L L Barton G V Johnson A G OrsquoNan and B M WagenerldquoInhibition of ferric chelate reductase in alfalfa roots by CobaltNickel Chromium and Copperrdquo Journal of Plant Nutrition vol23 no 11-12 pp 1833ndash1845 2000

[63] V Dixit V Pandey and R Shyam ldquoChromium ions inactivateelectron transport and enhance superoxide generation in vivoin pea (Pisum sativum L cv Azad) root mitochondriardquo PlantCell amp Environment vol 25 no 5 pp 687ndash693 2002

[64] N-N Trinh T-L Huang W-C Chi S-F Fu C-C Chen andH-J Huang ldquoChromium stress response effect on signal trans-duction and expression of signaling genes in ricerdquo PhysiologiaPlantarum vol 150 no 2 pp 205ndash224 2014

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[66] S Gangwar and V P Singh ldquoIndole acetic acid differentlychanges growth and nitrogen metabolism in Pisum sativum Lseedlings under chromium (VI) phytotoxicity Implication ofoxidative stressrdquo Scientia Horticulturae vol 129 no 2 pp 321ndash328 2011

[67] N M Semchuk O V Lushchak J Falk K Krupinska andV I Lushchak ldquoInactivation of genes encoding tocopherolbiosynthetic pathway enzymes results in oxidative stress inoutdoor grown Arabidopsis thalianardquo Plant Physiology andBiochemistry vol 47 no 5 pp 384ndash390 2009

[68] M Becana M A Matamoros M Udvardi and D A DaltonldquoRecent insights into antioxidant defenses of legume rootnodulesrdquo New Phytologist vol 188 no 4 pp 960ndash976 2010

[69] S K Panda and M H Khann ldquoAntioxidant efficiency in rice(Oryza sativa L) leaves under heavy metal toxicityrdquo Journal ofPlant Biology vol 30 no 1 pp 23ndash30 2003

[70] S K Panda ldquoChromium-mediated oxidative stress and ultra-structural changes in root cells of developing rice seedlingsrdquoJournal of Plant Physiology vol 164 no 11 pp 1419ndash1428 2007

[71] U Y Stambulska Effect oF Potassium Dichromate in the Envi-ronment on Biochemical Parameters of Pea Plants vol 20 ofBiology Visnyk of Odesa National University 37 edition 2015

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[73] E Menendez P Martınez-Hidalgo L R Silva et al ldquoRecentadvances in the active biomolecules involved in Rhizobia-Legume symbiosisrdquo in Microbes for Legume Improvement pp45ndash74 Springer Cham Germany 2017

[74] F Mus M B Crook K Garcia et al ldquoSymbiotic nitrogenfixation and the challenges to its extension to nonlegumesrdquoApplied and Environmental Microbiology vol 82 no 13 pp3698ndash3710 2016

[75] M Janczarek K Rachwał A Marzec J Grzadziel and MPalusinska-Szysz ldquoSignal molecules and cell-surface compo-nents involved in early stages of the legume-rhizobium inter-actionsrdquo Applied Soil Ecology vol 85 pp 94ndash113 2014

[76] V V Volkogon ldquoAssociative nitrogen-fixing microorganismsrdquoMikrobiologichny Zhurnal vol 62 no 2 pp 51ndash68 2000

[77] C K Yost K T Clark K L Del Bel and M F HynesldquoCharacterization of the nodulation plasmid encoded chemore-ceptor gene mcpG from Rhizobium leguminosarumrdquo BMCMicrobiology vol 3 article no 1 pp 1ndash10 2003

[78] H P Bais T L Weir L G Perry S Gilroy and J M VivancoldquoThe role of root exudates in rhizosphere interactions withplants and other organismsrdquo Annual Review of Plant Biologyvol 57 no 1 pp 233ndash266 2006

[79] C M Halbleib and P W Ludden ldquoRegulation of biologicalnitrogen fixationrdquo Journal of Nutrition vol 130 no 5 pp 1081ndash1084 2000

[80] A Skorupska M Janczarek MMarczak A Mazur and J KrolldquoRhizobial exopolysaccharides Genetic control and symbioticfunctionsrdquoMicrobial Cell Factories vol 5 article no 7 2006

[81] C W Ribeiro G Alloing K Mandon and P Frendo ldquoRedoxregulation of differentiation in symbiotic nitrogen fixationrdquoBiochimica et Biophysica Acta (BBA) - General Subjects vol1850 no 8 pp 1469ndash1478 2015

[82] D L Foreman E M Vanderlinde D C Bay and C K YostldquoCharacterization of a gene family of outer membrane proteins(ropB) in Rhizobium leguminosarum bv viciae VF39SM andthe role of the sensor kinase ChvG in their regulationrdquo Journalof Bacteriology vol 192 no 4 pp 975ndash983 2010

[83] B Gourion F Berrabah P Ratet and G Stacey ldquoRhizobium-legume symbioses The crucial role of plant immunityrdquo Trendsin Plant Science vol 20 no 3 pp 186ndash194 2015


[84] L V Rinaudi and J E Gonzalez ldquoThe low-molecular-weightfraction of exopolysaccharide II from Sinorhizobium melilotiis a crucial determinant of biofilm formationrdquo Journal ofBacteriology vol 191 no 23 pp 7216ndash7224 2009

[85] J A Downie ldquoThe roles of extracellular proteins polysaccha-rides and signals in the interactions of rhizobia with legumerootsrdquo FEMS Microbiology Reviews vol 34 no 2 pp 150ndash1702010

[86] A Williams A Wilkinson M Krehenbrink D M RussoA Zorreguieta and J A Downie ldquoGlucomannan-mediatedattachment of Rhizobium leguminosarum to pea root hairs isrequired for competitive nodule infectionrdquo Journal of Bacteriol-ogy vol 190 no 13 pp 4706ndash4715 2008

[87] Y Kawaharada S Kelly M W Nielsen et al ldquoReceptor-mediated exopolysaccharide perception controls bacterialinfectionrdquo Nature vol 523 no 7560 pp 308ndash312 2015

[88] M Schultze and A Kondorosi ldquoRegulation of symbiotic rootnodule developmentrdquo Annual Review of Genetics vol 32 pp33ndash57 1998

[89] R Geurts and T Bisseling ldquoRhizobium nod factor perceptionand signallingrdquoThe Plant Cell vol 14 pp S239ndashS249 2002

[90] M Fauvart and J Michiels ldquoRhizobial secreted proteins asdeterminants of host specificity in the rhizobium-legume sym-biosisrdquo FEMSMicrobiology Letters vol 285 no 1 pp 1ndash9 2008

[91] C-W Liu and JDMurray ldquoThe role of flavonoids in nodulationhost-range specificity An updaterdquoPlants vol 5 no 3 pp 4045ndash4053 2016

[92] R Karunakaran V K Ramachandran J C Seaman et alldquoTranscriptomic analysis of Rhizobium leguminosarum biovarviciae in symbiosis with host plants Pisum sativum and Viciacraccardquo Journal of Bacteriology vol 191 no 12 pp 4002ndash40142009

[93] PMergaert T Uchiumi B Alunni et al ldquoEukaryotic control onbacterial cell cycle and differentiation in the Rhizobium-legumesymbiosisrdquo Proceedings of the National Acadamy of Sciences ofthe United States of America vol 103 no 13 pp 5230ndash52352006

[94] A Muszynski C Heiss C T Hjuler et al ldquoStructures Ofexopolysaccharides involved in receptor-mediated perceptionof Mesorhizobium loti by Lotus japonicusrdquo The Journal ofBiological Chemistry vol 291 no 40 pp 20946ndash20961 2016

[95] B M Hoffman D R Dean and L C Seefeldt ldquoClimbing nitro-genase Toward a mechanism of enzymatic nitrogen fixationrdquoAccounts of Chemical Research vol 42 no 5 pp 609ndash619 2009

[96] J P White J Prell V K Ramachandran and P S PooleldquoCharacterization of a 120574-aminobutyric acid transport system ofRhizobium leguminosarum bv viciae 3841rdquo Journal of Bacteri-ology vol 191 no 5 pp 1547ndash1555 2009

[97] A Munoz P Piedras M Aguilar and M Pineda ldquoUrea isa product of ureidoglycolate degradation in chickpea Purifi-cation and characterization of the ureidoglycolate urea-lyaserdquoPlant Physiology vol 125 no 2 pp 828ndash834 2001

[98] A S Voisin C Salon C Jeudy and F R WarembourgldquoSymbiotic N2 fixation activity in relation to C economy ofPisum sativum L as a function of plant phenologyrdquo Journal ofExperimental Botany vol 54 no 393 pp 2733ndash2744 2003

[99] Z Wu X Zhang B He et al ldquoA chlorophyll-deficient ricemutant with impaired chlorophyllide esterification in chloro-phyll biosynthesisrdquo Plant Physiology vol 145 no 1 pp 29ndash402007

[100] P C Sehnke H-J Chung K Wu and R J Ferl ldquoRegulationof starch accumulation by granule-associated plant 14-3-3 pro-teinsrdquo Proceedings of the National Acadamy of Sciences of theUnited States of America vol 98 no 2 pp 765ndash770 2001

[101] A K Glyanrsquoko and G G Vasilrsquoeva ldquoReactive oxygen and nitro-gen species in legume-rhizobial symbiosis a reviewrdquo AppliedBiochemistry and Microbiology vol 46 no 1 pp 15ndash22 2010

[102] R Mittler ldquoOxidative stress antioxidants and stress tolerancerdquoTrends in Plant Science vol 7 no 9 pp 405ndash410 2002

[103] I Damiani N Pauly A Puppo R Brouquisse and A BoscarildquoReactive oxygen species and nitric oxide control early stepsof the legume ndash Rhizobium symbiotic interactionrdquo Frontiers inPlant Science vol 7 no 2016 article no 454 2016

[104] A Puppo N Pauly A Boscari K Mandon and R BrouquisseldquoHydrogen peroxide and nitric oxide Key regulators of thelegume - Rhizobium and mycorrhizal symbiosesrdquo Antioxidantsamp Redox Signaling vol 18 no 16 pp 2202ndash2219 2013

[105] G P Bolwell and A Daudi ldquoReactive Oxygen Species inPlantndashPathogen Interactionsrdquo in Reactive Oxygen Species inPlant Signaling vol 49 of Signaling and Communication inPlants pp 113ndash133 Springer Berlin Heidelberg 320 edition2009

[106] B J Ferguson and U Mathesius ldquoSignaling Interactions duringNodule Developmentrdquo Journal of Plant Growth Regulation vol22 no 1 pp 47ndash72 2003

[107] R Santos D Herouart S Sigaud D Touati and A PuppoldquoOxidative burst in alfalfa-Sinorhizobium meliloti symbioticinteractionrdquo Molecular Plant-Microbe Interactions vol 14 no1 pp 86ndash89 2001

[108] J Montiel M-K Arthikala L Cardenas and C QuintoldquoLegume NADPH oxidases have crucial roles at different stagesof nodulationrdquo International Journal of Molecular Sciences vol17 no 5 article no 680 2016

[109] D Herouart E Baudouin P Frendo et al ldquoReactive oxygenspecies nitric oxide and glutathione A key role in the establish-ment of the legume-Rhizobium symbiosisrdquo Plant Physiologyand Biochemistry vol 40 no 6-8 pp 619ndash624 2002

[110] S L Shaw and S R Long ldquoNod factor inhibition of reactiveoxygen efflux in a host legumerdquo Plant Physiology vol 132 no 4pp 2196ndash2204 2003

[111] V Munoz F Ibanez M Tordable M Megıas and A FabraldquoRole of reactive oxygen species generation and Nod factorsduring the early symbiotic interaction between bradyrhizobiaand peanut a legume infected by crack entryrdquo Journal of AppliedMicrobiology vol 118 no 1 pp 182ndash192 2015

[112] M C Rubio E K James M R Clemente et al ldquoLocalization ofsuperoxide dismutases and hydrogen peroxide in legume rootnodulesrdquo Molecular Plant-Microbe Interactions vol 17 no 12pp 1294ndash1305 2004

[113] M Iwano F-S Che K Goto N Tanaka S Takayama andA Isogai ldquoElectron microscopic analysis of the H2O2 accu-mulation preceding hypersensitive cell death induced by anincompatible strain of Pseudomonas avenae in cultured ricecellsrdquoMolecular Plant Pathology vol 3 no 1 pp 1ndash8 2002

[114] A Jamet K Mandon A Puppo and D Herouart ldquoH2O2is required for optimal establishment of the Medicago sativaSinorhizobium meliloti symbiosisrdquo Journal of Bacteriology vol189 no 23 pp 8741ndash8745 2007

[115] M J Davies andA Puppo ldquoDirect detection of a globin-derivedradical in leghaemoglobin treated with peroxidesrdquo BiochemicalJournal vol 281 no 1 pp 197ndash201 1992


[116] M A Matamoros A Saiz M Penuelas et al ldquoFunction ofglutathione peroxidases in legume root nodulesrdquo Journal ofExperimental Botany vol 66 no 10 pp 2979ndash2990 2015

[117] M A Matamores D A Dalton J Ramos M R ClementeM C Rubio and M Becana ldquoBiochemistry and MolecularBiology of Antioxidants in the Rhizobia-Legume SymbiosisrdquoPlant Physiology vol 133 no 2 pp 499ndash509 2003

[118] G G Vasilrsquoeva A K Glyanrsquoko and N V Mironova ldquoHydrogenperoxide content and catalase activity on inoculation withroot nodule bacteria of pea seedlings with different ability fornodulationrdquo Applied Biochemistry and Microbiology vol 41 no6 pp 547ndash550 2005

[119] C M Pieterse J A Van Pelt S C Van Wees et alldquoRhizobacteria-mediated induced systemic resistance trigger-ing signalling and expressionrdquo European Journal of PlantPathology vol 107 no 1 pp 51ndash61 2001

[120] Y E Kolupaev Y V Karpets and L IMusatenko ldquoParticipationof active forms of oxygen in induction of salt tolerance ofseedlings of wheat with salicylic acidrdquo Reports of the NationalAcademy Sciences of Ukraine vol 6 pp 154ndash158 2007

[121] C Fernandez-Aunian T B Hamouda F Iglesias-Guerra etal ldquoBiosynthesis of compatible solutes in rhizobial strainsisolated from Phaseolus vulgaris nodules in Tunisian fieldsrdquoBMCMicrobiology vol 10 article no 192 2010

[122] G Hernandez O Valdes-Lopez M Ramırez et al ldquoGlobalchanges in the transcript and metabolic profiles during sym-biotic nitrogen fixation in phosphorus-stressed common beanplantsrdquo Plant Physiology vol 151 no 3 pp 1221ndash1238 2009

[123] J P Obbard and K C Jones ldquoMeasurement of symbioticnitrogen-fixation in leguminous host-plants grown in heavymetal-contaminated soils amended with sewage sludgerdquo Envi-ronmental Pollution vol 111 no 2 pp 311ndash320 2000

[124] A Zaidi P A Wani and M S Khan Toxicity of Heavy Metalsto Legumes and Bioremediation Springer Science and BusinessMedia 2012

[125] M T Gomez-Sagasti and D Marino ldquoPGPRs and nitrogen-fixing legumes A perfect team for efficient Cd phytoremedia-tionrdquo Frontiers in Plant Science vol 6 article no 81 2015

[126] Z Kong O A Mohamad Z Deng X Liu B R Glick and GWei ldquoRhizobial symbiosis effect on the growth metal uptakeand antioxidant responses of Medicago lupulina under copperstressrdquo Environmental Science and Pollution Research vol 22no 16 pp 12479ndash12489 2015

[127] U Y Stambulska ldquoEffect of symbiotic nitrogen-fixing bacteriaand xenobiotics on physiological and biochemical parametersof pea plantsrdquo Dissertation Chernivtsi National Unisersitynamed after Yu Fedrsquokovych Chernivtsi pp 190 2017

[128] V I Lushchak ldquoFree radicals reactive oxygen species oxidativestress and its classificationrdquoChemico-Biological Interactions vol224 pp 164ndash175 2014

Hindawiwwwhindawicom

International Journal of

Volume 2018

Zoology

Hindawiwwwhindawicom Volume 2018

Anatomy Research International

PeptidesInternational Journal of



Journal of Parasitology Research

GenomicsInternational Journal of


Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018


BioinformaticsAdvances in

Marine BiologyJournal of



Neuroscience Journal


BioMed Research International

Cell BiologyInternational Journal of



Biochemistry Research International

ArchaeaHindawiwwwhindawicom Volume 2018


Genetics Research International


Advances in

Virolog y Stem Cells International



Enzyme Research



MicrobiologyHindawiwwwhindawicom

Nucleic AcidsJournal of

Volume 2018

Submit your manuscripts atwwwhindawicom


Plants

Oxidativestress

ROS

Seed death andinhibitionof seed

Growthinhibition

Reduction ofphotosynthesis

Tissuealterations

Inhibition ofelectrontransportprocesses

Damage ofDNA

Proteininactivation

Changes inenzyme activity

Effects

Ｌ6+

2

∙minus（22

∙（

Figure 1 Toxic effects of Cr(IV) on plants Like other heavy metals Cr can directly inactivate many proteins binding to them or displacingmetals from the active centers of proteins As a transition metal Cr can participate in many cellular redox reactions resulting in generationof reactive oxygen species (ROS) such as O2

∙minus H2O2 and HO∙ When the production of ROS exceeds the capacity of the antioxidant systemcells undergo oxidative stress Both direct protein inactivation and oxidative stress lead to adverse morphological and physiological changesin plants

Symbiotic interaction between legume plants and rhi-zobacteria is a complex physiological process which isregulated by a number of signals produced both macro-and microsymbionts [16 17] Infection by rhizobia increasesROS production intensifying oxidative processes in plants[17] There is a strong evidence that ROS and antioxidantsystem play a key role in the formation and functioning oflegume-rhizobium symbiosis [17 18] Moreover a number ofstudies reported that elevated levels of heavy metals in soilsaffect rhizobial growth and their host legumes [19 20] Undercultivation of legumes on the soils with the high level of heavymetals the root nodules can be the major accumulators ofheavy metals from soil [21] The latter can potentially reducethe toxicity of heavy metals to the plants with simultaneousdecreasing metal content in soils Therefore the use of seedinoculation by symbiotic nitrogen-fixing bacteria is activelydiscussed as one of the possible ways to reduce toxic effectsof heavymetals to legumes and provide an effective approachfor soil bioremediation [21ndash23] At the same time legume-rhizobium symbiosis seems to be also sensitive to heavymetals and its protective effects against metal toxicity are notfully clearThis review summarizes recent data on the toxicityof chromium(VI) to plants especially in legumes with thefocus on ROS involvement and oxidative stress developmentFurthermore we analyze ROS role in the formation ofrhizobium-legume symbiosis with a focus on modulatingeffects of nitrogen-fixing bacteria of the Rhizobium genuson free radical processes in plants exposed to hexavalentchromium

2 Toxic Effects of Chromium in Plants

In the nature chromium exists in two valence forms Cr3+and Cr6+ which chromite minerals are mainly composedof [5 7 12 24 25] Chromates CrO4

2minus and dichromatesCr2O7

2minus are the most abundant anionic forms of chromiumin the environment [26ndash28] Toxicity of chromium for plantsdepends on its valence state with Cr6+ being more toxicand mobile than Cr3+ [12 29ndash32] The hexavalent chromiumis toxic for agricultural plants at concentrations of about05ndash50mgmLminus1 in the nutrient solution and 5ndash100mg gminus1in the soil Under physiological conditions concentration ofchromium ions in plants is less than 1 120583g gminus1 [33 34]

Absorption of chromium by the plant depends on itsform and concentration in soil or water around as well asplant species and their physiological state [1 35] Howeverthe mechanisms of absorption and distribution of chromiumin vegetative and generative plant organs have not beensufficiently studied Chromium is not known as an essentialelement for plants and therefore has no specificmechanismsfor assimilation by plants [36] Both active and passivetransport systems were suggested to participate in absorptionof this element in plants Active transport is proposed tobe responsible for absorption of chromium ions at low con-centrations whereas passive facilitated diffusion promoteschromium intake at its high concentrations [36] It is wellestablished that Cr6+ is absorbed by roots mainly via anenergy-dependent transport while Cr3+ penetrates the plant


Table1Major

compo

nentso

fantioxidant

defenseinlegumep

lantsa

ndroot

nodu

les

Com

ponents

Locatio

nFu

nctio

nEn

zymaticantio

xidants

Superoxide

dism

utase(SO

D)

Dism

utationof

O2

∙minusO2

∙minus+O2

∙minus+2H+rarr

2H2O2+O2

CuZ

n-SO

DCy

tosolplastid

sMn-SO

DMito

chon

driabacteroides

Fe-SOD

Plastid

ssomeb

acteroides

Catalase


Dism

utationof

H2O22H2O2rarr

2H2O+O2

Guaiacolp

eroxidase(GPX

)Plasmam

embranevacuoles

Detoxificatio

nof

H2O2H2O2+gu

aiacol

redrarr

H2O+

guaiacol

oxid

Peroxiredo

xins

Cytosolmito

chon

dria

Redu

ctionof

H2O2or


OH)toH2Oor

respectiv

ealcoh

ols(RO

H)RO

OH+Trx-(SH) 2rarr

ROHrarr

Trx-S 2

+H2O


tathione

cycle

(AA-GS

Hcycle

)

Cytosol(mainly)m

itochon

driaplastids

peroxisomes

Detoxificatio

nof

H2O2in

aserieso

freactions

Ascorbatep

eroxidase(APX

)AP

XH2O2+AArarr

2H2O+MDHA(D

HA)

Mon

odehydroascorbater

eductase

(MR)

MR

2MDHA+NADHrarr

2AA+NAD

Dehydroascorbater

eductase

(DR)

DR

DHA+2G

SHrarr

AA+GSSG

Glutathione

redu

ctase(GR)

GRG

SSG+NADPHrarr

2GSH

+NADP+

EnzymesofGS

Hmetabolism

Glutathione

peroxidase

(GSH

-PX)

Cytosolplastid

sbacteroides

Detoxificatio

nof

H2O2H2O2+GSHrarr

2H2O+GSSG

Glutathione

redu

ctase(GR)

Cytosolmito

chon


Regeneratio

nof

oxidized

glutathion

eGSSG+NADPHrarr

2GSH

+NADP+

120572-G

lutamylcyste

ines

ynthetasea

ndglutathion

esynthetase

Cytosolplastid

sbacteroides

GSH

synthesis

deno

vo

Glutathione-S-tr

ansfe

rase


itochon

dria

bacteroides

Detoxificatio

nof

xeno

bioticsv

iaconjugationwith

GSH

Nonenzym

aticantio

xidants

Ascorbicacid

(AA)

Cytosolmito

chon

driaplastids

peroxisomes

Dire

ctscavenging

ofRO

Sas

ubstrateforA

PX

Glutathione

(GSH

)Cy

tosolmito

chon


Dire

ctscavenging

ofRO

Sac

osub

strateforD

RGSH

-PX

and

glutathion

e-S-transfe

rases

120572-Tocop

herol

Plastid

sDire

ctscavenging

ofRO

Squ

enchingof

lipid

radicalsin

mem

branes

Caroteno

ids

Plastid

sQuenching

ofele


chloroph

yllsfro

mph

otod

amage

Phenolcompo

unds

(pheno

licacidsflavono

idsetc)

Vacuoles


Dire

ctscavenging

ofRO

Sandlip

idradicals

metal-chelating

activ

ityFerritin


Plastid

svacuolescytosol

Metal-binding

activ


xin

MDHAm

onod



e















thiolsCr5+complex

H2O

2

997888997888997888997888rarr HO∙ (1)















































(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018



Table1Major

compo

nentso

fantioxidant

defenseinlegumep

lantsa

ndroot

nodu

les

Com

ponents

Locatio

nFu

nctio

nEn

zymaticantio

xidants

Superoxide

dism

utase(SO

D)

Dism

utationof

O2

∙minusO2

∙minus+O2

∙minus+2H+rarr

2H2O2+O2

CuZ

n-SO

DCy

tosolplastid

sMn-SO

DMito

chon

driabacteroides

Fe-SOD

Plastid

ssomeb

acteroides

Catalase


Dism

utationof

H2O22H2O2rarr

2H2O+O2

Guaiacolp

eroxidase(GPX

)Plasmam

embranevacuoles

Detoxificatio

nof

H2O2H2O2+gu

aiacol

redrarr

H2O+

guaiacol

oxid

Peroxiredo

xins

Cytosolmito

chon

dria

Redu

ctionof

H2O2or


OH)toH2Oor

respectiv

ealcoh

ols(RO

H)RO

OH+Trx-(SH) 2rarr

ROHrarr

Trx-S 2

+H2O


tathione

cycle

(AA-GS

Hcycle

)

Cytosol(mainly)m

itochon

driaplastids

peroxisomes

Detoxificatio

nof

H2O2in

aserieso

freactions

Ascorbatep

eroxidase(APX

)AP

XH2O2+AArarr

2H2O+MDHA(D

HA)

Mon

odehydroascorbater

eductase

(MR)

MR

2MDHA+NADHrarr

2AA+NAD

Dehydroascorbater

eductase

(DR)

DR

DHA+2G

SHrarr

AA+GSSG

Glutathione

redu

ctase(GR)

GRG

SSG+NADPHrarr

2GSH

+NADP+

EnzymesofGS

Hmetabolism

Glutathione

peroxidase

(GSH

-PX)

Cytosolplastid

sbacteroides

Detoxificatio

nof

H2O2H2O2+GSHrarr

2H2O+GSSG

Glutathione

redu

ctase(GR)

Cytosolmito

chon


Regeneratio

nof

oxidized

glutathion

eGSSG+NADPHrarr

2GSH

+NADP+

120572-G

lutamylcyste

ines

ynthetasea

ndglutathion

esynthetase

Cytosolplastid

sbacteroides

GSH

synthesis

deno

vo

Glutathione-S-tr

ansfe

rase


itochon

dria

bacteroides

Detoxificatio

nof

xeno

bioticsv

iaconjugationwith

GSH

Nonenzym

aticantio

xidants

Ascorbicacid

(AA)

Cytosolmito

chon

driaplastids

peroxisomes

Dire

ctscavenging

ofRO

Sas

ubstrateforA

PX

Glutathione

(GSH

)Cy

tosolmito

chon


Dire

ctscavenging

ofRO

Sac

osub

strateforD

RGSH

-PX

and

glutathion

e-S-transfe

rases

120572-Tocop

herol

Plastid

sDire

ctscavenging

ofRO

Squ

enchingof

lipid

radicalsin

mem

branes

Caroteno

ids

Plastid

sQuenching

ofele


chloroph

yllsfro

mph

otod

amage

Phenolcompo

unds

(pheno

licacidsflavono

idsetc)

Vacuoles


Dire

ctscavenging

ofRO

Sandlip

idradicals

metal-chelating

activ

ityFerritin


Plastid

svacuolescytosol

Metal-binding

activ


xin

MDHAm

onod



e















thiolsCr5+complex

H2O

2

997888997888997888997888rarr HO∙ (1)















































(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018
















thiolsCr5+complex

H2O

2

997888997888997888997888rarr HO∙ (1)















































(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018






thiolsCr5+complex

H2O

2

997888997888997888997888rarr HO∙ (1)















































(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018




































(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018























(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018






(ii) GPx activity








ROS




K2Cr2O7 K2Cr2O7










Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018






Acknowledgments


References









































































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018




















































































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018




















































































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018



















































Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018


















Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018




Volume 2018

Zoology











Volume 2018

















Advances in




Enzyme Research





Volume 2018


Documents

Chromium(VI) Toxicity in Legume Plants: Modulation Effects