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Phytoremediation of Polychlorinated Biphenyls New Trends and Promises

Phytoremediation of Polychlorinated Biphenyls:New Trends and Promises?

B E N O I T V A N A K E N,*,?P A O L A A.

C O R R E A,§A N D

J E R A L D L.S C H N O O R|

Department of Civil and Environmental Engineering,Temple University,

Philadelphia,Pennsylvania,Department of Civil and Environmental Engineering,

West Virginia University,Morgantown,West Virginia,and Department of Civil

and Environmental Engineering,University of Iowa,Iowa City,Iowa

Received August17,2009.Revised manuscript received November13,2009.

Accepted November16,2009.

Transgenic plants and associated bacteria constitute a new generation of genetically modi?ed organisms for ef?cient and environment-friendly treatment of soil and water contaminated with polychlorinated biphenyls(PCBs).This review focuses on recent advances in phytoremediation for the treatment of PCBs,including the development of transgenic plants and associated bacteria.Phytoremediation,or the use of higher plants for rehabilitation of soil and groundwater,is a promising strategy for cost-effective treatment of sites contaminated by toxic compounds,including PCBs.Plants can help mitigate environmental pollution by PCBs through a range of mechanisms: besides uptake from soil(phytoextraction),plants are capable of enzymatic transformation of PCBs(phytotransfor-mation);by releasing a variety of secondary metabolites, plants also enhance the microbial activity in the root zone, improving biodegradation of PCBs(rhizoremediation).However, because of their hydrophobicity and chemical stability,PCBs are only slowly taken up and degraded by plants and associated bacteria,resulting in incomplete treatment and potential release of toxic metabolites into the environment.Moreover, naturally occurring plant-associated bacteria may not possess the enzymatic machinery necessary for PCB degradation.To overcome these limitations,bacterial genes involved in the metabolism of PCBs,such as biphenyl dioxygenases,have been introduced into higher plants,following a strategy similar to the development of transgenic crops.Similarly,bacteria have been genetically modi?ed that exhibit improved biodegradation capabilities and are able to maintain stable relationships

with plants.Transgenic plants and associated bacteria bring hope for a broader and more ef?cient application of phytoremediation for the treatment of PCBs.

Introduction

Phytoremediation is an emerging technology that uses plants and associated bacteria for the treatment of soil and groundwater contaminated by toxic pollutants(1).The concept of using plants for remediation of organic pollutants emerged a few decades ago with the recognition that plants were capable of metabolizing toxic compounds,such as1,1,1-trichloro-2,2-bis-(4′-chlorophenyl)ethane(DDT)and ben-zo[a]pyrene(2,3).Since then,phytoremediation acquired the status of a proven technology for the remediation of soil and groundwater contaminated by a variety of organic compounds,including pesticides,chlorinated solvents, explosives,polyaromatic hydrocarbons(PAHs),dioxins,and polychlorinated biphenyls(PCBs)(1,4–7).It is estimated that the budget invested in phytoremediation programs jumped from50million dollars in1999to300million dollars in2007(8).

Even though phytoremediation has been shown to ef?ciently reduce the chemical hazard associated with various classes of organic and inorganic pollutants,it also suffers serious limitations that prevent large-scale?eld applications (1,7).As autotrophic organisms,plants usually lack the catabolic enzymes necessary to achieve full metabolism of recalcitrant organic compounds,often resulting in slow removal and incomplete degradation(9).Inherent limitations of plants for the metabolism of recalcitrant xenobiotic compounds led to the idea of modifying plants genetically by the introduction of bacterial or mammalian genes involved in the degradation of toxic chemicals,following a strategy that has been applied for decades with transgenic crops (10–12).Similarly,even though rhizoremediation plays a key role in the transformation of organic pollutants,naturally occurring plant-associated bacteria may not harbor the enzymatic machinery necessary for ef?cient degradation of toxic pollutants.To overcome this limitation,genetically modi?ed bacteria have been constructed that exhibit im-proved biodegradation capabilities and are able to maintain a stable relationship with plants.Transgenic plants and associated bacteria for phytoremediation could therefore constitute the new generation of genetically modi?ed(GM) organisms(13).

Although phytoremediation technology has been exten-sively reviewed in the literature(4,6,9,13–24),only very few reviews have been published that focus speci?cally on PCBs (15,25,26).The present article summarizes recent advances in phytoremediation of PCBs,including the use of transgenic plants and plant-associated bacteria. Phytoremediation:Cleaning up Pollution with Plants

Living organisms are commonly exposed to a variety of natural(allelochemicals)or manmade toxic chemicals(xe-nobiotics).As a consequence,they have developed complex

?Temple University.

§West Virginia University.

|University of Iowa.

?Part of the special section“Sources,Exposures,and Toxicities

of PCBs in Humans and the Environment”.

*Corresponding author phone:215-204-7087;fax:215-204-4696;

e-mail:bvanaken@https://www.sodocs.net/doc/d79190917.html,.

Environ.Sci.Technol.2010,44,2767–2776

10.1021/es902514d 2010American Chemical Society VOL.44,NO.8,2009/ENVIRONMENTAL SCIENCE&TECHNOLOGY92767 Published on Web12/03/2009

detoxi?cation mechanisms to prevent harmful effects from exposure to these compounds (27–29).Bioremediation exploits the natural capability of living organisms to degrade toxic chemicals.Traditional remediation of PCB-polluted sites requires soil excavation and transport,prior to off-site treatment by solvent extraction,thermal alkaline dechlori-nation,incineration,or land?lling (15).These techniques are costly,damaging for the environment,and,in many cases,practically infeasible due to the range of the contamination (19).There is therefore a considerable interest in developing cost-effective alternatives based on microorganisms or plants.Bioremediation techniques,although requiring more time,are usually considered to represent between 10and 50%of the cost of physical and chemical methods (7).Because of its potential for the sustainable mitigation of environmental pollution,bioremediation has been listed among the “top ten technologies for improving human health”(30).

Phytoremediation encompasses a range of processes beyond direct plant uptake and metabolism,and it is best described as plant-mediated bioremediation (1,4,7,31,32).While de?nitions and terminology vary,the different phy-toremediation processes can be summarized as in Figure 1:pollutants in soil and groundwater are taken up inside plant tissues (phytoextraction )or adsorbed to the roots (rhizo?l-tration );pollutants inside plant tissues are transformed by plant enzymes (phytotransformation )and/or volatilize into the atmosphere (phytovolatilization );and pollutants in soil are degraded by microbes in the root zone (rhizoremediation )orincorporatedtosoilmaterial(phytostabilization )(1,6,10,32).Based on the observation that plants can metabolize pesticides,Sandermann (33)introduced the green liver concept,suggesting a detoxi?cation sequence similar to that which occurs in the liver of mammals (Figure 2)(3,33,34).Phytoremediation offers several advantages over other remediation strategies:low cost because of the absence of energy-consuming equipment and limited maintenance,no or limited negative impact on the environment because of the in situ nature of the process,and large public acceptance as an attractive green technology (19).In addition,phytore-mediation offers potential bene?cial side-effects,such as erosion control,site restoration,carbon sequestration,and feedstock for biofuel production (10,35).As autotrophic

organisms,plants use sunlight and carbon dioxide as energy and carbon sources.From an environmental standpoint,plants can be seen as “natural,solar-powered,pump-and-treat systems”for cleaning up contaminated soils (9).

However,the technology also suffers several limitations:phytoremediation is restrained to shallow contamination of “moderately hydrophobic”compounds susceptible to be ef?ciently absorbed by the roots (36,37).More importantly,remediation by plants is often slow and incomplete:as a corollary to their autotrophic metabolism,plants usually lack the biochemical pathways necessary to achieve total min-eralization of recalcitrant pollutants,such as PAHs and PCBs (7).Phytoremediation can therefore lead to undesirable effects,such as the accumulation of toxic metabolites that may be released to the soil,enter the food chain,or volatilize into in the atmosphere (6,9,14,38,39).In addition,planted trees need several years to reach mature size and,in temperate regions,plants have limited activity during the dormant season (7).Additional constraints to phytoremediation are not of technical order,but are the current regulations,competition with other methods,and proprietary rights (40).An important barrier to the development of transgenic plants for bioremediation is associated with the potential risk of horizontal gene transfer to related wild or cultivated plants (41).There is a critical need for integrated risk assessment of transgenic bioremediation technologies that should lead to community education and reevaluation of current regula-tions (42).Additional research is needed for the development of molecular risk mitigation strategies.It is likely that the next generation of transgenic organisms for phytoremedia-tion will involve systems preventing such a transfer,for instance by the introduction of transgenes into chloroplastic DNA or the use of conditional lethality genes (43).

Even though cleaning up pollution with plants appears to be an ideal remediation technology that has been proven to be effective by extensive laboratory and greenhouse research,a contrasting small number of ?eld applications have been successfully conducted.Although this contradic-tory observation is related to a combination of factors largely shared by most bioremediation systems,phytoremediation is likely victim of its own attractiveness,leading the technol-ogy to be oversold.By its nature,phytoremediation is assorted with speci?c limitations and failure to clearly identify them may lead to ineffectiveness of the remediation process.

FIGURE 1.Phytoremediation of organic pollutants,such as PCBs,may involve several processes:pollutants in soil and groundwater can be taken up inside plant tissues (phytoextraction )or adsorbed to the roots (rhizo?ltration );pollutants inside plant tissues can be transformed by plant enzymes (phytotransformation )or can volatilize into the atmosphere (phytovolatilization );pollutants in soil can be degraded by microbes in the root zone (rhizoremediation )(1,6,9).Adapted from Van Aken (104).

FIGURE 2.Three phases of the green liver model.Hypothetical pathway representing the metabolism of 2,3′-dichlorobiphenyl in plant tissues:Phase I ,activation of the PCB by hydroxylation;Phase II ,conjugation with a plant molecule (sugar);Phase III ,sequestration of the conjugate into the vacuole or cell wall.Adapted from Van Aken (104).

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PCBs:Chemistry,Sources,Transport,and Toxicity

PCBs are xenobiotic chlorinated aromatic compounds that are characterized by high physical and chemical stability and categorized as persistent organic pollutants (POPs)(15).Because of their thermal stability and high dielectric constant,PCBs have been used for a variety of industrial applications,including lubricants,dielectric ?uids,and plasticizers.PCBs were manufactured widely during a half century (from 1929to the 1970s)and an estimated 1.5million tons of PCB have been produced worldwide.Because of their toxicity and persistence in the environment,PCBs have been banned in most countries since 1979.

Local manufacture,usage,spill,and improper disposal of PCBs have led to extensive environmental contamination.Because of their high volatility and stability,PCBs have been largelydispersedbyatmospherictransport.Theoctanol -water partition coef?cient (log K ow )allows prediction of the mobility of PCBs in the environment:higher-chlorinated PCBs,with log K ow above 6,are associated with particulate matter in the atmosphere,soils,and sediments;lower-chlorinated con-geners,with lower log K ow ,exist in gaseous phase and can be transported over longer distance.As a consequence,soils generally contain a higher proportion of higher-chlorinated congeners,while air is dominated by lower-chlorinated fractions (44).Because higher-chlorinated PCBs are suscep-tible to microbial anaerobic dechlorination,anoxic sediments are often enriched in lower-chlorinated congeners.Today PCBs are still emitted from several sources,such as leaks of existing equipment (e.g.,electrical capacitors and ballasts),volatilization from dredging sediments,and sewage sludge soil application.PCBs have been detected in virtually every compartment of the ecosystem,including air,water,soil,sediment,and living organisms.PCBs are highly hydrophobic,leading to their bioaccumulation in living organisms (bio-magni?cation ).In humans,PCBs are commonly detected in breast milk and blood,with concentrations increasing with age.Plants constitute the major route of entry of PCBs in the food chain (15).

Toxicity of PCBs has been recognized since the 1930s (45):although acute toxicity for adult humans is rather low,chronic exposure to PCBs induces serious neurobehavioral,im-munological,reproductive,and endocrine disorders in children (46,47).According to the Department of Health and Human Services (DHHS),U.S.Environmental Protection Agency (EPA),and International Agency for Research on Cancer (IARC),PCBs are suspected to be carcinogenic in animals and humans (45,48,49).PCBs are listed as EPA Priority Pollutants (https://www.sodocs.net/doc/d79190917.html,/)and are ranked at the ?fth position in the 2007CERCLA (Comprehensive Environmental Response,Compensation,and Liability Act)Priority List of Hazardous Substances (https://www.sodocs.net/doc/d79190917.html,/).

Microbial Degradation of PCBs

The chemical stability of PCBs renders them quite recalcitrant to microbial biodegradation (50).The presence of more chlorine atoms increases the chemical stability and decreases water-solubility of PCBs,making higher-chlorinated con-geners more recalcitrant to biodegradation.In addition,metabolism of PCBs is often energetically unfavorable,requiring an additional carbon source to support their biodegradation (cometabolism).

Although they are classi?ed as POPs,microbial biodeg-radation of PCBs is well documented (45,48,51–53).Two major microbial metabolic pathways are known:anaerobic and aerobic,depending of the degree of chlorination of the PCB congener,the redox conditions,and the type of microorganism involved (48).

Anaerobic Dechlorination of PCBs.Generally speaking,PCB congeners with four or more chlorine atoms undergo anaerobic reductive dechlorination,an energy-yielding pro-cess where PCBs serve as electron acceptor for the oxidation of organic carbon.Chlorine atoms are preferentially removed from the meta-and para -positions on the biphenyl structure,leaving lesser-chlorinated ortho -substituted congeners (54).Microorganisms that reductively dechlorinate PCBs are widespread in contaminated sediments and involve species related to Dehalococcoides (55–59).PCB dechlorination has been mostly attributed to complex bacterial consortia and little is known about metabolic pathways,molecular bases,and enzymes implicated in the process.Only a few bacterial species able to dechlorinate PCBs in pure culture have been identi?ed and their range of activity is limited to a few congeners (45,56).Sequencing the genome of Dehalococ-coides ethenogenes 195,a well-characterized tetrachlorethene degrader,revealed the presence of several reductive deha-logenase genes potentially implicated in PCB transformation (45).However,to date,no enzyme involved in PCB anaerobic dechlorination has been isolated or characterized.

Aerobic Biodegradation of PCBs.Lower-chlorinated PCB congeners s possibly produced by anaerobic dechlorina-tion s undergo cometabolic aerobic oxidation mediated by dioxygenases,resulting in ring-opening and potentially complete mineralization of the molecule (50,53,60).Several bacterial strains are capable of oxidative degradation of PCBs,including mainly members of the genus Pseudomonas ,Burkholderia ,Comamonas ,Rhodococcus ,and Bacillus .The number of chlorine atoms per molecule and placement of chlorine atoms are important factors for the aerobic bio-degradation via oxidative enzymes (53,60,61).Generally,PCB congeners with three or fewer chlorine atoms per molecule are easily degraded,and the ones with ?ve or more are quite recalcitrant (requiring reductive dechlorination prior to aerobic mineralization).However,one of the most ef?cient PCB degraders characterized,Burkholderia xenovorans strain LB400,was shown to metabolize a hexachlorobiphenyl congener (51).Aerobic biodegradation of PCBs typically involves two clusters of genes,the ?rst one responsible for transformation into chlorobenzoates and chlorinated ali-phatic acids (biphenyl upper pathway ),and the second one for further mineralization of chlorobenzoates and aliphatic acids (biphenyl lower pathway )(45,50,51).The upper pathway,which is similar for all described aerobic PCB degraders,involves seven genes grouped into one operon (biphenyl dioxygenase,bph )(Figure 3).A multicomponent dioxygenase (bph A)initiates hydroxylation of two adjacent

FIGURE 3.Bacterial aerobic degradation of lower-chlorinated PCBs is catalyzed by biphenyl dioxygenase (bph )gene cluster (upper pathway).Adapted from Furukawa et al.(61).

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biphenyl carbons to form an arene cis-diol.In the second step,a cis-2,3-dihydro-2,3-dihydroxybiphenyl dehydrogenase (bph B)further oxidizes the biphenyl ring to produce a dihydroxychlorobiphenyl.In the third step,a second dioxy-genase,2,3-dihydroxybiphenyl1,2-dioxygenase(bph C),opens the ring in ortho-or meta-position.The four step of the upper pathway involves a hydrolase(bph D)that cleaves the resulting molecule into chlorobenzoate and2-hydroxypenta-2,4-dienoate(48).

Phytoremediation of PCBs

The?rst reports on the potential of plants for bioremediation of PCBs were published in the late1970s s early1980s: Reinholtz and Volpe(62)(aquatic plants),Weber and Mrozek (uptake and translocation)(63),Schwartz and Lehmann(64) (detection of PCBs in plant tissues),and Bacci and Gaggi(65) (translocation and volatilization of PCBs from soil).Since then,signi?cant advances have been made,showing the potential of plants and associated microbes for PCB me-tabolism.Processes recognized to be involved in phytore-mediation of PCBs include rhizoremediation,phytoextraction, and phytotransformation.

Rhizoremediation of PCBs.Due to their high hydro-phobicity,PCBs bind strongly to soil particles and are only poorly taken up inside plant tissues.Therefore,microbes in the rhizosphere play a dominant role in their biodegradation (13).Many reports have shown a signi?cant increase of PCB attenuation in soil planted with a variety of plants,as compared with nonvegetated soils(15,16,19,66).There are many processes by which vegetation can stimulate microbial activity in soil and enhance biodegradation of recalcitrant PCBs:

a.Plant roots release organic compounds,such as sugar,

amino acids,and organic acids,that can be used as electron donors to support aerobic cometabolism or anaerobic dehalogenation of chlorinated compounds.

In some instances,microbial aerobic metabolism will consume oxygen,resulting in anaerobic conditions favorable to PCB dehalogenation(16).

b.Plants secrete extracellular enzymes that can initiate

transformation of PCBs and facilitate further microbial metabolism(67).

c.Plants release inducers that accelerate microbial deg-

radation.Plant phenolic exudates were shown to enhance the activity of the PCB degrader,B.xenovorans LB400(68).

d.Plants increase soil permeability and oxygen diffusion

in the rhizosphere,which potentially enhances micro-bial oxidative transformation by oxygenases(16,69,70).

e.Plant roots are known to secrete diverse microbial

growth factors(15).

f.Plant roots release organic acids and molecules that

can act as surfactants,therefore mobilizing PCBs and rendering them more susceptible to be absorbed inside plant tissues(15).

Several publications have shown the positive effect of root exudates,including phenolic compounds,?avonoids, and terpenes,on microbial activity in soil and on biodeg-radation of PCBs(67,71–73).Vegetation was reported to signi?cantly increase PCB removal from soil,as compared to nonplanted soil,both due to higher microbial degradation of PCBs in the root zone and uptake inside plant tissues. Epuri and Sorensen(74)showed a higher mineralization of hexachlorobiphenyl in Aroclor1260-contaminated soil planted with tall fescue(Festuca arundinacea)as compared with unplanted soil.Singer et al.(75)studied the interactive effects of different treatments on the degradation of Aroclor1242 in soil,including bioaugmentation with PCB-degrading bacteria,biostimulation with inducers and surfactants,and vegetation with Brassica nigra.The authors observed a signi?cantly higher PCB degradation in vegetated soil,as

compared to nonplanted controls,and concluded that plants

enhanced PCB degradation by increasing oxygen diffusion

in soil,amendment in?ltration,and microbial enrichment.

In a phytoremediation experiment using several plant species

(alfalfa,?atpea,Sericea lespedeza,deertongue,reed ca-

narygrass,switchgrass,and tall fescue)for the bioremediation

of PCB-contaminated soil,Aroclor1248was shown to be

removed to a greater extent from all vegetated pots(38%or

less PCB recovery),as compared with nonplanted pots(82%

PCB recovery)(76).In addition,plants increased enzymatic

activity in soil that was shown to correlate with the levels of

PCB biodegradation.Recently,Smith et al.(77)conducted

greenhouse experiments on PCB-contaminated sediments

following different treatments,including addition of organic

amendment(mixture of straw and starch adjusted to a C:N

ratio of10:1)and vegetation with low and high-transpiring

plants(including Scirpus?uviatilis,Tripsacum dactyloides,

Carex aquatalis,and Spartina pectinata).The authors

observed highest PCB removal following the addition of

amendment with low-transpiring plant and no-plant treat-

ments,concluding that organic amendment resulted in

oxygen consumption necessary to achieve anaerobic dechlo-

rination of PCBs.

Molecular biology tools have also been used to locate

PCB degraders in the roots of plants growing in PCB-

contaminated soil:Pseudomonas?uorescens strain was

constructed that expressed a green?uorescent protein(GFP)

under the control of the meta-pathway Pm promoter from

P.putida known to be induced by3-chlorobenzoate,a

product of3-monochlorobiphenyl metabolism.When added

to alfalfa roots(Medicago sativa)growing on3-monochlo-

robiphenyl-contmainated soil,engineered bacteria indicated

the presence of degrading microcolonies on the root surface

and in crevices between root epidermal cells(78).Similarly,

Hogan et al.(79)developed a real-time PCR assay based on

SYBR Green and?uorescence resonance energy transfer

(FRET)probes allowing the sensitive detection of transgenic

P.?uorescens expressing bph operon from the PCB degrader,

B.xenovorans LB400.

Uptake of PCBs inside Plant Tissues.To predict uptake

of organic pollutants by plants,Briggs et al.(36)and Burken

and Schnoor(37)developed experimental relationships based

on log K ow.Based on their models,only“moderately

hydrophobic”compounds(0.5

The ef?ciency of plant uptake of PCBs s with log K ow ranging

from4.5(2-monochlorobiphenyl)to8.2(decachlorobiphenyl)s is

expected to decrease fast with the degree of chlorination.

Studying phytoextraction of Aroclor1260-contaminated soil

from three sites in Canada by nine plant species,Zeeb et al.

(80)detected variable concentrations of PCBs in root tissues,

and,to a lesser level,in shoot tissues.The authors observed

higher concentration of tetrachloro-to hexachlorobiphenyls

in shoots,although heptachloro-and nonachlorobiphenyls

were also present in detectable amounts.These results

suggest that,despite the predictions based on log K ow,higher-

chlorinated congeners would be susceptible to be taken up

inside plant tissues.On the other hand,using hydroponic

hybrid poplars,Liu and Schnoor(81)observed that selected

mono-to tetrachlorinated PCBs adsorbed on plant roots,

but only lower-chlorinated PCBs were translocated to aerial

parts(mono-,di-,and trichlorinated PCBs to upper stems

and mono-and dichlorinated PCBs to shoots).In a?eld trial

on Aroclor-contaminated soil,Aslund et al.(82)showed an

increase of PCB concentration in the stems and leaves of

pumpkin plants with time of exposure,while root concen-

tration remained unchanged.The authors suggested that

PCB transfer in plants occurs primarily via uptake and

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translocation,while other potential mechanisms,such as volatilization and deposition,have negligible contribution.

Plant Metabolism of PCBs.Plant metabolism of xeno-biotic compounds is conceptually represented as a three-phase process known as the green liver model(Figure2) (33,34):Phase I,the initial activation,consists of oxidation of PCBs to produce various hydroxylated products,charac-terized by a higher solubility and reactivity.Phase II involves conjugation of phase I-activated compounds with molecules of plant origin(e.g.,glutathione or aminoacids)forming adducts less toxic and more soluble than parent PCBs.Phase III involves sequestration of the conjugates in plant organelles (e.g.,vacuole)or incorporation into plant structures(e.g., cell wall)(3,33,34).

Although plants were shown to contribute to PCB at-tenuation in soil since the1970s,it was not until the1990s that the capability of plants to metabolize PCBs was demonstrated.In pioneer work studying the transformation of19PCB congeners in plant cell cultures of Rosa spp.(Paul’s Scarlet rose),Lee and Fletcher(83)observed that11individual congeners had been metabolized by more than10%.Wilken et al.(84)studied the metabolism of10PCB congeners in12 plant species and detected various mono-and dihydroxylated metabolites.Mackova et al.(85)used in vitro cell cultures of a variety of plant species(Armoracia rusticana,Solanum aviculare,Atropa belladonna,and Solanum nigrum)to characterize the metabolism of a commercial mixture(Delor 103)and observed that PCB transformation capability greatly differed from strain to https://www.sodocs.net/doc/d79190917.html,ing in vitro hairy root culture of S.nigrum(black nightshade),Kucerova et al.(86)showed that plant cells were capable of oxidizing mono-and dichlorinated PCBs into mono-and dihydroxylated biphenyls. Different laboratory experiments conducted with plant cell cultures showed that all mono-and dichlorobiphenyls were slightly hydroxylated,with the exception of4,4′-dichloro-biphenyl,hypothesized to be sterically protected from enzymatic attack(86–88).Further studies using plant cell cultures showed that more persistent dichloro-,trichloro-, and tetrachlorobiphenyl congeners could also be metabolized by plant cells:Harms et al.(88)demonstrated that3,3′,4,4′-tetrachlorobiphenyl could be oxidized to several monohy-droxylated intermediates by plant cell cultures of Rosa spp. and Lactuca sativa(lettuce).Following a similar pathway, 2,2′,5,5′-tetrachlorobiphenyl was transformed to3,4-dihy-droxy-2,2′,5,5′-tetrachlorobiphenyl.In experiments using black nightshade hairy root cultures exposed to several dichlorinated,trichlorinated,tetrachlorinated,and pen-tachlorinated PCB congeners,Rezek et al.(89)observed the formation of hydroxylated PCB metabolites from dichloro-and trichlorobiphenyl congeners,while tetrachloro-and pentachlorobiphenyl congeners were not metabolized.

In summary,plant metabolism of PCBs varies according to the plant species and degree of chlorination and substitu-tion pattern.Initial steps in plant metabolism of PCBs involve oxidation of the biphenyl core,which is discouraged by the presence of electron-withdrawing chlorine atoms.Plant metabolism of PCBs appears therefore limited to tetra-chlorinated and lower congeners.In some instances,lower-chlorinated congeners are more recalcitrant than higher ones, suggesting the importance of substitution pattern.For instance,in the study cited above,Lee and Fletcher(83) observed that4,4′-dichlorobiphenyl was not hydroxylated but2,4,4′-trichlorobiphenyl was.

Plant Enzymes Involved in PCB Transformation.Several studies suggest that different oxygenases may be implicated in the initial metabolism of PCBs in plants(Phase I of the green liver model),including cytochrome P-450monooxy-genases(88)and peroxidases(85,87,90).Studying the metabolism of PCBs in rose cell cultures,Lee and Fletcher (83)observed a decrease of PCB metabolism by cytochrome P-450inhibitors,while peroxidase inhibitors did not produce signi?cant effect,suggesting the intervention of cytochrome P-450s.On the other hand,Koller et al.(91)reported extensive transformation and dechlorination of dichloro-and tetra-chlorobiphenyl by commercial horseradish peroxidase(HRP). Also,using various in vitro plant cell cultures,Chroma et al. (87,92,93)observed a correlation between PCB transforma-tion and various catabolic enzymes including peroxidases, Remazol Brillant Blue R(RBBR)oxidases,and cytochrome P-450s,suggesting the implication of these three enzymes in PCB metabolism in plants.

Although very little is known about conjugative enzymes involved in PCB metabolism(Phase II of the green liver model),knowledge gained from the degradation of other nucleophilic xenobiotics suggests that various transferases, such as glutathione S-transferases(e.g.,conjugation of glutathione with several pesticides)and glycosyltransferases (e.g.,conjugation of glucose with chlorophenols and DDT) are likely to be involved in the conjugation and compart-mentation of PCB adducts in plant tissues(33,94).Plant tolerance(Brassica juncea)to several chlorinated pollutants, such as atrazine,metolachlor,and1-chloro-2,4-dinitroben-zene(CDNB),was enhanced by the overexpression of enzymes involved in glutathione synthesis,including γ-glutamylcysteine synthetase(ECS)and glutathione syn-thetase(GS),further suggesting the potential implication of glutathione in PCB metabolism(95).

Our knowledge of plant metabolism of xenobiotics is still fragmentary and other enzymes are likely to be involved in PCB transformation.For instance,Magee et al.(96)reported recently dechlorination of2,2′,4,4′,5,5′-hexachlorobiphenyl by crude extract of nitrate reductase from Medicago sativa and a pure commercial nitrate reductase from Zea mays. Also,plant dehalogenation of chlorinated solvents has been reported following a mechanism similar to microbial anaer-obic dechlorination(97,98).Although no plant dehalogenase has been identi?ed or characterized,such a mechanism could potentially lead to PCB dechlorination in plant tissues. Transgenic Plants for Phytoremediation of PCBs Although PCBs have been shown to be removed by plants, only rather slow biodegradation rates have been achieved in ?eld trials,potentially leading to accumulation and volatil-ization of toxic compounds.Genetic transformation of plants for enhanced phytoremediation capabilities is typically achieved by the introduction of external genes whose products are involved in various detoxi?cation processes (17,99).Microbes and mammals are heterotrophic organisms that possess the metabolic enzymes to achieve a near-complete mineralization of organic molecules.Microbial and mammalian catabolic genes can therefore be used to complement the metabolic capabilities of plants(9).

Transgenic plants have been produced for phytoreme-diation of both heavy metals and organic pollutants(9,99). Early examples include tobacco plants expressing a yeast metallothionein gene and showing a higher tolerance to cadmium(100),Arabidopsis thaliana overexpressing a zinc transporter protein and showing a2-fold higher accumulation of zinc in roots(101),and tobacco plants expressing a human cytochrome P-450for enhanced metabolism of trichloro-ethylene(11).The use of transgenic plants for phytoreme-diation applications has been reviewed recently in several articles(9,17,18,24,26,99,102–105).Table1presents a nonexhaustive list of transgenic plants and bacteria con-structed for phytoremediation of PCBs.

Plant cytochrome P-450-mediated metabolism of PCBs produces toxic epoxide intermediates and trans-diol me-tabolites not easily further biodegraded.Unlike cytochrome P-450s,bacterial biphenyl dioxygenases produce cis-diol VOL.44,NO.8,2009/ENVIRONMENTAL SCIENCE&TECHNOLOGY92771

intermediates susceptible to ring cleavage and complete mineralization(26).In an attempt to overcome this limitation, components of bacterial biphenyl dioxygenase operon,bph, were introduced into plants.In pioneering work,Francova et al.(106)genetically modi?ed tobacco plants(Nicotiana tobacum)by insertion of the gene responsible for2,3-dihydroxybiphenyl ring cleavage,bph C,from the PCB degrader Comamonas testosteroni.bph C gene was cloned into plasmid p B1121under the control of the strong CaMV 35S promoter and introduced into the“natural genetic engineer”Agrobacterium tumefaciens.Successful transfor-mation was con?rmed by ampli?cation of bph C using PCR. Although the engineered plants were not tested for their capability to metabolize PCBs,this work constitutes a milestone in the development of transgenic plants for the transformation of PCBs.In a similar study,Mohammadi et al.(107)inserted bph genes from B.xenovorans LB400,one of the most ef?cient PCB degraders,into tobacco plants. Three components of the bph operon necessary for dioxy-genation of the biphenyl ring,bph AE,bph F,and bph G,were individually cloned and expressed in transgenic plants.The authors showed that puri?ed enzymes from the plants were capable of oxidizing4-chlorobiphenyl into2,3-dihydro-2,3-dihydroxy-4′-chlorobiphenyl.Recently,Novakova et al.(108) constructed transgenic tobacco plants expressing bph C from the PCB degrader,P.testosteroni B-356.When grown in the presence of2,3-dihydroxybiphenyl(0.5mM),one transgenic line,H2,exhibited higher resistance to the toxic compound, as compared to wild-type plants.Although successful ap-plication of this revolutionary strategy will require more development,such as engineering-improved PCB-degrading enzymes and coordinated expression of different genes,these results suggest that transgenic plants expressing the complete bacterial PCB metabolic pathway could help overcome inherent limitations of phytoremediation(26). Transgenic Plant-Associated Bacteria for Rhizoremediation of PCBs

Plants are known to increase both microbial numbers and activity in soil,which can result in an increase of biodeg-radation activity(69,109).However,endogenous or rhizo-sphere bacteria capable of maintaining a stable relationship with plants may not harbor the metabolic enzymes necessary for the ef?cient catabolism of persistent pollutants(110,111). In an attempt to improve rhizoremediation performances, several research groups have cloned key catabolic genes of known xenobiotic degraders into speci?c rhizosphere bac-teria(16,19,29,49,66,112,113).In a pioneering study, Brazil et al.(110)introduced the genetically engineered transposon,TnPCB,containing bph genes from the PCB degrader,B.xenovorans LB400,into P.?uorescens F113,a bacterium colonizing the roots of many plants.The recom-binant bacterium,strain F113pcb,expressed heterologous bph genes,as it was con?rmed by its ability to utilize biphenyl as sole carbon source.Rhizosphere competence of strain F113pcb was identical to wild-type P.?uorescens,as con-?rmed by colonization experiments of sugar beet seedling roots.This study demonstrated that rhizosphere-adapted microbes can be genetically engineered to metabolize recalcitrant xenobiotics without affecting their ecological competence.Following a similar approach,bph operon from B.xenovorans strain LB400,was inserted into strain F113 under the control of the strong promoter,nodbox4,from Sinorhizobium meliloti(111).The constructed strain, F113::1180,expressed a high level of biphenyl dioxygenase and was capable of metabolizing biphenyl and several mono-, di-,and trichlorinated PCB congeners at a much higher rate than strain F113pcb.In addition,the transgenic strain, F113::1180,was able to metabolize Delor103better than the initial bph donor strain,B.xenovorans LB400.Recently, another group reported higher PCB metabolization rates with transgenic P.?uorescens F113::1180and B.xenovorans LB400, as compared to strain F113pcb(114).Using mesocosm experiments with PCB-contaminated soil,the authors re-ported a good survival ability of F113strains in willow plant rhizosphere,suggesting that association of transgenic rhizo-sphere bacteria with plants constitutes a promising approach for the treatment of PCB-contaminated soils.

Nitrogen-?xing bacterium,S.meliloti,lives in symbiotic association with roots of the leguminous alfalfa plants,M. sativa,providing its host with reduced nitrogen and increas-ing soil fertility.In an attempt to increase rhizoremediation performance,S.melitoti was transformed by introduction of a PCB-degrading plasmid containing the oxygenolytic ortho-dechlorination gene,ohb(115).Transformant strains were able to grow on100mg L-12′,3,4-trichlorobiphenyl and

TABLE1.Summary of Publications about Transgenic Plants and Bacteria for the Phytoremediation of PCBs compound gene source host organism reference

PCBs Biphenyl dioxygenase,bph,

located on transposon,TnPCB Pseudomonas

sp.strain LB400

Sugar beet seeds(cv.Rex)(110)

PCBs Biphenyl dioxygenase,bph C Comamonas

testosteroni B-356

Tobacco(Nicotiana tabacum)(106)

2′,3,4-Trichloro-biphenyl Oxygenolytic ortho-dechlorination

(ohb)gene Pseudomonas

aeruginosa strain142

Sinorhizobium meliloti colonizing

Alfalfa(Medicago sativa)

(115)

3-Chloro-biphenyl Biphenyl dioxygenase,bph

Green?uorescent protein,gfp Burkholderia

xenovorans LB400

Alfalfa roots(Medicago sativa,

var.Resis,DLF Trifolium)

(78)

PCBs Biphenyl dioxygenase,

bph C on mini-transposon,Tn5Burkholderia

xenovorans LB400

Pseudomonas?uorescens

F113colonizing

alfalfa roots

(79)

2′,3,4-Trichloro-biphenyl PCB-degrading genes,

ortho-halobenzoate

1,2-dioxygenase(ohb)genes Pseudomonas

aeruginosa strain142

Sinorhizobium meliloti colonizing

Alfalfa(Medicago sativa)

(116)

Individual PCBs congeners Biphenyl dioxygenase,bph Burkholderia

xenovorans LB400Pseudomonas?uorescens F113

colonizing Alfalfa(Medicago sativa) rhizosphere

(111)

PCB-contaminated soil Biphenyl dioxygenase,bph Burkholderia

xenovorans LB400Pseudomonas?uorescens colonizing rhizosphere of willow(Salix sp.)

(117)

4-Chloro-biphenyl Biphenyl dioxygenases,bph A,

bph E,bph F,bph G Burkholderia

xenovorans LB400

Nicotiana tabacum,

Nicotiana benthamiana

(107)

Individual PCBs congeners Biphenyl dioxygenase,bph Burkholderia

xenovorans LB400Pseudomonas?uorescens

F113colonizing willow

(Salix sp.)rhizosphere

(114)

2,3-Dihydroxy-biphenyl Biphenyl dioxygenase,bph C Pseudomonas

testosteroni B-356

Tobacco(Nicotiana tabacum)(108) 27729ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.44,NO.8,2009

dechlorinate100%of PCBs,as compared to15%achieved by wild-type bacteria.In another study,S.meliloti was trans-formed by introduction of a PCB-degrading plasmid harbor-ing the bph operon.Transgenic S.meliloti was shown to degrade2′,3,4-trichlorobiphenyl.Plant chamber tests re-vealed that alfalfa plants inoculated with transgenic bacteria were capable of2-fold higher dechlorination of2′,3,4-trichlorobiphenyl,as compared to control alfalfa inoculated with wild-type S.meliloti(116).More recently,to improve bioremediation of the commercial mixture,Delor103,in contaminated soil,de Carcer et al.(117)inoculated the roots of willows(Salix viminali s×schwerinii)with two genetically modi?ed(GM)P.?uorescens strains:class1GM strain modi?ed with a single chromosomal insertion of bph operon and class2GM strain with insertion of bph operon under the control of the nod regulatory system of S.meliloti.After about 6months,analysis of PCBs showed a statistically signi?cant increase of the degradation rate in rhizosphere soil inoculated with class GM1and GM2P.?uorescens strains,as compared to control soil inoculated with wild-type strain.In addition, the presence of transgenic bacteria did not affect the microbial community in bulk soil.

An interesting approach to enhance rhizoremediation of PCBs is based on the concept of rhizoengineering.Rhizoengi-neering consists of using transgenic plants or metabolic mutants exuding modi?ed patterns of plant secondary metabolites,therefore promoting the growth of speci?c bacterial groups capable of xenobiotic https://www.sodocs.net/doc/d79190917.html,ing a“rhizosphere metabolomic”approach,Narasimhan et al. (118)identi?ed a large majority of phenylpropanoids, including?avonoids,in plant exudates.Different near-isogenic lines of Arabidopsis mutants overproducing?a-vonoids were then used to promote root colonization by P. putida PML2.P.putida PML2is a rhizospheric bacterium that has the capability of metabolizing both?avonoids and PCBs.Results obtained showed that Pseudomonas PML2 colonized the roots of Arabidopsis?avonoid-overproducing mutants at higher levels.In addition,Pseudomonas PML2 wasabletoreachsigni?cantlyhigherdepletionof2-monochlo-ro-and4-monochlorobipheynls(90%)when growing in association with?avonoid-expressing Arabidopsis,as com-pared to?avonoid null mutant.The authors concluded that this approach complements the use of transgenic plants for bioremediation applications.

Conclusions

Transgenic bacteria have been used for industrial production of pharmaceuticals and human proteins(e.g.,insulin)and transgenic plants have been used for the expression of insect or pesticide resistance(e.g.,Bt-maize).From an environ-mental standpoint,agricultural plants expressing genes involved in the biodegradation of pesticides are the?rst transgenic organisms used for phytoremediation applica-tions.Recently,nonagricultural plants and associated bacteria have been developed to mitigate pollution of soil and groundwater by toxic agrochemicals and other xenobiotic pollutants,including PCBs(17,99).

To date,only genes involved in Phase I of the green liver model have been introduced into transgenic plants for PCB degradation.Further developments may involve the intro-duction of multiple transgenes involved in different phases of the green liver model,which would help overcome a major limitation inherent to phytoremediation,i.e.,the threat that accumulated toxic compounds would volatilize or otherwise contaminate the food chain(6,38,39,99).As an illustration of transgenic plants expressing enzymes involved in Phase II of the green liver model,Indian mustard(Brassica juncea) was modi?ed to overexpress enzymes involved in glutathione metabolism(ECS and GS),resulting in enhanced tolerance to atrazine,metolachlor,and CDNB(95).

Another interesting approach to enhance phytoremeda-tion ef?ciency would consist of engineering plants to secrete microbial enzymes being released into the environment to achieve ex-planta bioremediation,such as transgenic tobacco expressing extracellular fungal peroxidases for the removal of pentachlorophenol(PCP)(119).

Although it has not been used for the treatment of PCBs, an alternative strategy may involve the genetic transformation of endophytic bacteria.Unlike rhizospheric bacteria,endo-phytic bacteria colonize the internal tissues of plants(35,120). Many endophytes have been shown to play a role in the metabolism of toxic xenobiotic pollutants,therefore poten-tially enhancing phytoremediation(121).Barac et al.(122) described the conjugative transformation of natural endo-phytes harboring a toluene-degradation plasmid(p TOM)for improved in planta degradation of toluene.As suggested in a recent review by Weyens et al.(105),metal-tolerant endophytes equipped with enzymes capable of biodegrada-tion of organic compounds would allow phytoremediation of sites cocontaminated with mixture of toxic metals and organic pollutants.

Finally,an important barrier to the?eld application of transgenic trees for bioremediation is associated with the true or perceived risk of horizontal gene transfer to wild or cultivated plants.There is therefore a critical need for further risk-bene?t analysis and risk mitigation strategies to ensure that transgenic biotechnologies would result in wider ac-ceptance and application of phytoremediation(42,43). Acknowledgments

We are grateful to the University of Iowa Superfund Basic Research Program(NIEHS;award number P42ES05605). Literature Cited

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How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.

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