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iForest - Biogeosciences and Forestry
vol. 4, pp. 7-15
Copyright © 2011 by the Italian Society of Silviculture and Forest Ecology
doi: 10.3832/ifor0555-004

Review Papers

Heavy metals and woody plants - biotechnologies for phytoremediation

Maurizio CapuanaCorresponding author

Introduction 

Pollution of soil and agricultural land is a complex and serious phenomenon that in recent decades has increased its negative effects on the environment. Transfer of toxic elements to human food chain is a concrete danger that has to be faced, taking into account the possibility for plants to accumulate and translocate contaminants to edible and harvested parts ([86], [142], [51], [114], [136]). Traditional technologies for removal of pollutants can be successful in specific situations, but costs associated with these technologies are very high. There is an active effort to develop new, more cost-effective methods to remediate contamination of polluted soils, hence attention is now focusing on innovative biological technologies such as phytoremediation, based on the use of plants to extract, sequester and/or detoxify pollutants ([152]). The development of phytoremediation technologies is continuing, involving transgenic and nontransgenic approaches, as well as different biological, technical, social, and economical aspects. Biotechnologies applied to investigating the remediation capability of woody plants are increasingly showing their efficacy, hence some aspects of their exploitation are presented here.

The exploitation of in vitro culture, the role of endophytic bacteria and mychorrizas, and the efforts to enhance uptake capacity and tolerance to heavy metals by genetic engineering are therefore considered in the following chapters.

Woody plants and phytoremediation 

Twenty-five years ago, studies on phytoremediation techniques were rather scarce; now the scientific and social interest on this subject has increased substantially after the increasing pressure of public opinion.

The use of plants to decontaminate soils and waters has been developed only recently, the first reports appearing in the eighties, followed by more exhaustive articles during the nineties ([24], [10], [36], [151], [138]). Much effort still has to be directed towards an understanding of the basic mechanisms and towards improving knowledge of the applications ([109]), but its usefulness has been demonstrated in many sites and this technology is now used by several environmental companies ([65]).

Phytoremediation is based on the removal of contaminants from the soil by mechanisms such as phytoextraction, phytodegradation, rhizofiltration, phytostabilization and phytovolatilization ([151]), but the mechanisms involved in heavy metal remediation are limited to uptake, adsorption, transport and translocation, sequestration into vacuoles, hyperaccumulation and, in some cases, volatilization ([113]). Within this frame, studies on allocation plasticity and plant-metal partitioning can be of great significance ([9]). When present at increased concentrations, both essential (copper, iron, manganese, molybdenum, zinc) and non-essential metals (e.g., cadmium, chromium, lead, mercury) are toxic. Mercury and selenium can also be converted by plants into a volatile form to release and dilute into the atmosphere. Heavy metals cannot be metabolized, therefore the only possible strategy to apply is their extraction from contaminated soil and transfer to the smaller volume of harvestable plants for their disposal ([151], [94], [138], [123]); biomass can also be used in producing energy and, if economically profitable, metals can be eventually recovered ([188]).

It must be stressed that some processes can limit the efficacy of plants in phytoremediation, such as the availability of the toxic metal ions in the soil for root uptaking, their rate of translocation from roots to shoots and the level of tolerance, the rate of chemical transformation into less toxic compounds ([132]). At the basis, we find the mechanisms implicated in plant metal tolerance and homeostasis (reviewed by [31]). Remediation using plants may take longer than other technologies, but the most relevant limitation is that it is most suited to cases where contaminants are present at shallow levels within the root layer.

Phytoremediation technology has been recently extensively reviewed ([152], [113], [44], [13], [166], [121], [128], [7]) and several species have been classified as hyperaccumulators and extensively investigated ([141], [126]). However, on a large scale, metal uptake by trees can be more effective, mainly because of a deeper root system and a greater yield of biomass ([69], [57]). High productivity and elevated uptake and translocation of pollutants to the harvestable biomass are the basis for efficient in situ restoration by means of vascular plants ([97], [134], [25]).

Some woody species can be advantageously used also for phytoremediation of soils and groundwater from organic pollutants ([35]) and hydrocarbons ([168], [187]). The potential in phytoremediation of metal contaminated soils expressed by forest trees has been assessed for several species in recent years ([3], [130], [91], [112], [131], [90], [147], [133], [59], [115], [172], [21], [47]). Resistance to metals often appears to be clone- or hybrid-specific rather than species-specific ([135]).

Poplars are particularly suitable for remediation purposes ([46], [22], [155]), having already been considered for trials on metal tolerance in in vivo ([100]) and in vitro observations ([58]). Salicaceae are also reported to grow even in severe soil conditions and to accumulate heavy metals ([133], [14]). Many studies have thus been focused on the use of willows and poplars in phytoextraction ([143], [96], [12], [144], [5], [68], [85], [72], [178], [177], [107], [158], [95], [145], [64], [48], [184], [80]). These species can be advantageously exploited in short rotation coppice cultures (SRC), a strategy whose application in phytoremediation presents interesting and economically promising perspectives ([153], [124], [98], [99], [146], [43], [185]).

Use of in vitro cultures for research on phytoremediation 

The inherent difficulties of experimenting on very large long-lived organisms such as forest trees, motivates the development of model systems. Besides the exploitation of hydroponic cultures, the in vitro model systems using shoot and cell cultures of plants demonstrated to be a useful tool for investigating efficiency of metal uptake and translocation ([23]). Cell and organ culture, in fact, as well as hydroponics, allow very fast accumulation of data in comparison with whole plant experiments under field conditions ([66]), and offer the advantage of testing the effects of contaminants under controlled conditions ([76]). Hydroponic screening is often used to evaluate tolerance, accumulation and translocation in plants. Watson et al. ([183]) demonstrated in Salix that results obtained in hydroponics and in field experiments are comparable. It is always advisable, however, to confirm data obtained by hydroponic tests by field performance trials. Using this technique, several studies have concerned, for instance, the response of willows to a metal cocktail ([182]) and of willows and poplars to the presence of cadmium ([165], [103], [48], [188]), the response of a clone of Populus x euramericana to high concentrations of copper ([19]), the mechanism of resistance to aluminium of Picea abies ([78]), the determination of the role of glutathione reductase metabolism in the defence of poplar (Populus deltoides x P. nigra) against high zinc concentration ([42]).

As stated by Golan-Goldhirsh et al. ([66]), the use of in vitro systems enables dissection of the complex system of plant, soil, and microbial interaction in order to evaluate the effect of stress factors on metabolism, specific enzymes and metabolites involved in plant response to the pollutant. For many woody species, moreover, the application of in vitro propagation techniques, allows for fast plant production and the application of promising genetic engineering programs ([34], [104]).

High concentration of zinc has been found to negatively affect the photosynthetic machinery in poplar: inhibition of adventitious root formation and leaf chlorosis indicated that the clone used was tolerant to external concentrations less than or equal to 1 mM ([23]), while in Eucalyptus globulus moderate concentrations of this metal were shown to either enhance or have no effect on rooting ([157]). Phytoremediation potentials of poplar lines (Populus nigra and transgenic P. canescens) were investigated using in vitro leaf discs cultures and found that Zinc2+ was phytotoxic only at high concentrations (10-2 to 10-1 M) in all P. canescens lines, but P. nigra was more sensitive ([16]). Cadmium added to the culture medium was shown to reduce the fresh and dry weights and the shoot length of white birch, while root length was not affected ([56]). Copper at a concentration of 0.05 mM, manganese at 0.80 mM, and zinc at 0.12 mM showed a negative effect on shoot growth (number of shoots per explant and shoot length) in Ailanthus altissima, considered a fast-growing and contamination-resistant species ([62]). Zinc was found toxic in aspen (Populus tremula x tremuloides) cultures at 0.5 mM concentration, while lead at the same concentration did not show toxic effects and was accumulated at 3500 μg per g of biomass ([81]). In vitro studies were also developed to investigate the effects of high concentrations of zinc and copper on the biosynthesis and accumulation of polyamine in Populus alba ([58]). On the basis of leaf symptoms, rate of adventitious root formation and ethylene production, it was found that Zn at 0.5-1 mM concentration was transiently toxic, while at 2-4 mM was increasingly toxic. Free and conjugated putrescine and spermidine accumulated proportionally to toxicity; also Cu strongly reduced rooting already at 5 μM and caused severe, dose-dependent toxicity symptoms (shoot chlorosis and necrosis) using concentrations up to 500 μM. In in vitro growing microshoots of Populus alba, the effect of high concentrations of cadmium, copper, zinc, and arsenic was investigated, showing differences in the response of different clones ([45]). Axenic poplar tumor cell cultures were tested for demonstrating the capability of taking up trichloroethylene (TCE) and degrading it to several known metabolic products ([120]), while poplar (Populus deltoides×P. nigra) in vitro culture has been used for developing mathematical models to define degradation pathways of nitramine compounds within plant cells ([117]). Metal tolerance was detected in a callus culture established from Acer rubrum seedlings growing in soil contaminated by zinc, cadmium, nickel and arsenic. A positive linear correlation was found between zinc resistance of callus and total Zn in soil beneath sampled trees, while no significant correlations were evidenced with the other metals ([181]). In Acer pseudoplatanus callus culture, Cu-, Zn- and Cd-resistance traits were identified in cell lines originating from trees at a site contaminated by these metals ([180]). In vitro screenings were also used to investigate how several heavy metals affect pollen germination and tube elongation in Pinus resinosa ([26]) and to test the tolerance for Zn and Cu in mycorrhyzal isolates collected in an abandoned Cu mines, in view of their inoculation into Pinus sylvestris seedlings ([1]). Combined micropropagation and hydroponic culture were used to study tolerance to copper and zinc in Betula pendula, finding that a seed-derived clone from a Pb/Zn-contaminated site showed more tolerance to Cu and Zn than bud-derived clones from a Cu/Ni-contaminated site or from an uncontaminated area ([173]).

Role of endophytic bacteria and mychorrhizas in phytoremediation 

Bacteria living within plant tissues without causing disease are referred to as endophytes. Some of these have shown the capacity to enhance plant growth and resistance to biotic and abiotic stresses by various mechanisms (e.g., nitrogen fixation, production of phytohormones, solubilisation of minerals, etc.), therefore, recently attention has been focusing on the role of endophytic bacteria in phytoremediation ([159], [121]). Endophytes have been inoculated and studied, e.g., in hybrid spruce ([27]), lodgepole pine ([28]), Douglas-fir ([29]), poplar and willow. Based on their potential for remediation, three Pseudomonas strains were identified and tested in a clone of hybrid cottonwood (Populus trichocarpa x P. deltoides) ([63]). A large part of the research on this subject has been dealt with the activity of endophytes on hydrocarbons. For instance, in hybrid cottonwood a strain of the endophyte Rhizobium tropici was found active in the degradation of explosives ([50]), as well as a Methylobacterium strain isolated from hybrid poplar (Populus deltoides x P. nigra - [174], [175]); poplar endophytic bacteria have been engineered for enhancing thrichloroethylene degradation ([161]) and [167] observed a horizontal gene transfer of a plasmid conferring toluene degradation.

Concerning specifically heavy metals, it has been observed that heavy metal resistant endophytes are present in various hyperaccumulator plants growing on heavy-metal contaminated soil ([137]). Among herbaceous plants, e.g., shoot endophytes of Thlaspi goesingense were found more tolerant to high nickel concentration than the correspondent rhizospheric bacteria ([79]), endophytic bacteria of Nicotiana tabacum could reduce cadmium phytotoxicity ([111]), recombinant heavy-metal resistant endophytic bacteria were studied in Lolium perenne and Lupinus luteus ([102]). Among woody species, some isolates of hybrid cottonwoods have demonstrated tolerance to heavy metals ([118]); bacteria associated with Zn/Cd-accumulating Salix caprea have been studied regarding their potential to support heavy metal phytoextraction ([93]).

Arbuscular mychorrhizas are also well known to be involved in the metal uptake; their presence in the soil may significantly affect the plant response to metal stress ([125]). A vast amount of literature is available on the effects of mycorrhizal colonisation of plants living in heavy metal-polluted soils ([67]), on the protective role of mycorrhizas against heavy-metal induced oxidative stress ([156]), and on their possible role in remediation ([83], [8]). In the hyperaccumulating fern Pteris vittata, for instance, they have been found to increase arsenic uptake ([170]). Adriaensen et al. ([2]) found that Pinus sylvestris seedlings colonized by a Zn-tolerant isolate of Suillus bovinus grew much better and remained physiologically healthier when exposed to elevated Zn concentration than seedlings not inoculated or colonized by a Zn-sensitive isolate. The response to high copper ([169]) and zinc ([101]) concentration was studied on poplar clones inoculated with arbuscular mycorrhizal fungi, while in mycorrhyzed Betula spp. tolerance to zinc ([39]) and Cu and Pb accumulation ([18]) were studied. By X-ray microanalysis of heavy metals, it was found that, in mycorrhized Picea abies seedlings, extracellular complexation of Cd occurred predominantly in the Hartig net hyphae and both extracellular complexation and cytosolic sequestration of Zn occurred in the fungal tissue ([60]). The potential benefits of ectomycorrhizal fungi in protecting their host plants from metal contamination were also investigated by Blaudez et al. ([17]) after testing thirty-nine ectomycorrhizal isolates for their tolerance to cadmium, copper, nickel and zinc. The potential of Salix viminalis and Populus x generosa for the phytoextraction of heavy metals, inoculated or not with the fungus Glomus intraradices, was recently assessed ([15]), while in Eucalyptus globulus grown in Zn-contaminated soil, the improving potential of interactions between saprophytic and arbuscular micorrhizal fungi was investigated ([6]).

Molecular biology and genetic engineering for phytoremediation 

Molecular biology and genetic engineering are being increasingly considered as effective tools for better understanding and improving the phytoremediation capability of plants, whose biological functions can now be analyzed in detail and partly modified. The metal resistance systems are better known in microorganisms ([162], [71], [163]); in plants only a few systems of metal tolerance and/or sequestration are sufficiently characterized ([82]). In recent years, several key steps have been identified at the molecular level, allowing an increasing application of molecular-genetic technologies and a transgenic approach to a better understanding of mechanisms involved in heavy metal tolerance and accumulation in plants ([32], [186]). It has been demonstrated in classic genetic studies that only a few genes are responsible for metal tolerance ([106]). Transfer and/or overexpression of genes responsible for metal uptake, translocation and sequestration may allow for the production of plants which, depending on the strategy, can be successfully exploited in phytoremediation ([92], [129], [32], [148], [53], [30], [49]). In this case, special attention must be paid to problems related to the introduction of genetically modified trees, particularly concerning their legal and social acceptance ([88]). Promotion of growth and biomass production is a correlated task accomplished, for instance, by increased gibberellin biosynthesis ([55]).

The improvement of the phytoremediation properties of plants can be achieved by the modification of their primary and secondary metabolism and the introduction of new phenotypic and genotypic characters ([38]). Even if most of the genes involved in metal uptake, transport and sequestration have been studied on the herbaceous model plant Arabidopsis, it must be considered that this species, as well as the most common species defined as hyperaccumulators, have a limited phytoremediation capacity due to their small size or slow growth rate. On the contrary, large, fast-growing plants, like some woody plants, are an important tool for this purpose; on poplars, for instance, reasonable transformation frequencies have been achieved ([73]).

Studies on Arabidopsis and other species (hyperaccumulators), however, open the way to a transfer and application to high-biomass plants. Among these, Populus species (poplars, cottonwoods, aspens) and hybrids have become a model system in forest tree biotechnology ([20]), due to several useful features: short rotation cycle, rapid growth rate and ease of vegetative propagation and in vitro multiplication ([34]). Moreover, the poplar genome has been entirely sequenced ([171]). It is always important, however, to take into account the risks associated with the biotechnological applications and carefully evaluate the field performances of transgenic plants ([34]).

Hairy roots induction by Agrobacterium rhizogenes is probably the easiest method for enhancing the root biomass and, consequently, improving metal uptake. This has been demonstrated for some hyperaccumulator plants ([108], [119], [52], [53]).

Transgenic white poplar has been recently obtained expressing the PsMTa1 gene from Pisum sativum for a metallothionein-like protein. Transformed plants showed enhanced resistance to heavy metal, surviving high concentrations of CuCl2 in in vitro culture, which strongly affected the nontransgenic plants. Rooting capacity of microshoots was maintained in transgenic lines exposed to 0.1 mM CuCl2, while it was totally destroyed in nontransgenic shoots. In the presence of 1 mM ZnSO4, nontransformed shoots rooted abundantly, while different rooting rates were observed in transgenic lines ([11]).

Genes encoding enzymes changing the oxidation state of heavy metals can be introduced into plants ([150], [74]). For instance, compared to wild type, transgenic Populus deltoides overexpressed mer-A9 and mer-A18 genes, when grown in soil with high mercury concentration, developing higher biomass and higher amount of Hg(0), which evaporates through the cell surface ([29]). Increased tolerance to ionic mercury was first obtained in yellow poplar (Lyriodendron tulipifera) transformed with mer-A gene ([149]). For the remediation of Hg, Populus deltoides has been engineered with the bacterial mer-A (mercuric ion reductase) and mer-B (organomercurial lyase - [29]); transgenic trees expressing both genes showed tolerance up to 10 μM of phenylmercuric acetate (PMA - [105]). Significant results on tolerance to mercury and related remediation capacity were obtained also in Oryza sativa ([77]) and in Spartina alterniflora ([37]). In Salix spp. it was proved that the majority of the mercury is accumulated and retained in the cell wall of the roots and only a very small part is transferred to the shoots ([179]).

Genetic engineering for arsenate reduction, increased translocation from root to shoot, and volatilisation has been recently illustrated and discussed ([189]). Arabidopsis has been transformed in order to better control the mobility and sequestration of arsenic ([40]). In Pteris vittata, genes have been identified that encode enzymes with arsenate reductive activity ([40], [54], [139]).

For selenium, a strategy to protect protein synthesis from the activity of this metal was applied in transformed Arabidopsis by the expression of a mammalian selenocysteine lyase ([127]) and could be now tested on woody species.

Transgenic poplar, with increased glutathione peroxidase activity, showed increased tolerance to zinc, probably due to an enhanced ability to detoxify the active oxygen species generated by the pollutant ([16]). Alterations in photosynthetic parameters and reduction in growth have been reported for a Populus x euramericana clone after treatment with high concentrations of zinc ([41]).

Cadmium in the environment derives from industrial processes, urban pollution (heating systems and traffic), fertilizers and mineralization of rocks ([140]). Sensitivity to and accumulation of cadmium in some woody species have been recently studied in Sweden ([122]). In a relatively new strategy, aimed to compartmentalize the metals, tolerance to lead and cadmium was enhanced in Arabidopsis by the overexpression of the yeast vacuolar transporter protein YCF1 ([164]), demonstrating the possibility to engineer phytoremediators for increasing their ability to sequester heavy metals. Poplars overexpressing a bacterial glutamylcysteine synthetase in the cytosol reached a 30-fold increase in its foliar activity compared to untransformed controls; this allowed greater tissue cadmium accumulation but had only a marginal effect on cadmium tolerance ([4]).

Plant roots are able to release into the rizosphere chelating agents with binding ability for metals ([84]). These metal chelators or other molecules within plant cells that have a high affinity for metals can help in the metal sequestering ([70], [116], [154], [160], [61]). Plants may also be engineered to enhance the synthesis of metal chelators ([82], [32]). Metal chelators include phytochelatins, metallothioneins, organic acids and amino acids. In vitro experiments have shown that cadmium in the form of phytochelatin complex is much better tolerated by plant enzyme than its free radical ion ([87]). In Nicotiana glauca transformed with a gene encoding a phytochelatin synthase, more metals were accumulated when grown in mine soils compared with non-transformed plants ([110]). Attempts have been made to increase the formation of phytochelatins by overexpressing genes encoding enzymes stimulating the synthesis of cysteine and glutathione ([75]). Metallothioneins, a category of remarkable interest, are defined as low molecular mass cysteine-rich proteins that can bind heavy metals and may play a role in their intracellular sequestration. In the hybrid poplar genome, they form a multigene family and it has been hypothesised that they participate in the process of metal homeostasis and possibly in the process of tolerance ([89]). Advances in understanding the regulation of phytochelatins biosynthesis and metallothioneins gene expression and their possible roles in heavy metal detoxification and homeostasis have been recently reviewed ([33]).

Conclusions 

Phytoremediation of metal-polluted soil by plant phytoextraction is a technique attracting the interest of an increasing scientific community and the use of woody species, in particular, presents some aspects of relevance. Biotechnologies are surely powerful tools allowing to investigate and evaluate the potential of phytoremediation. As described in this paper, many fields of study are contributing to a rapid increase of our knowledge on the mechanisms involved. However, despite of the intensive research carried out in the last years on this topic, very few field trials demonstrated the technical feasibility and economic workability of the described approaches ([176]). Indeed, most of the literature rarely provides information on the practical application of phytoremediation techniques.

Specialisation and fragmentation of research is probably real, but it should not be seen as a limit: every progress can contribute and converge to increase the possibility of an advantageous exploitation of woody plants for phytoremediation.

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Paper ID# ifor0555-004
Title Heavy metals and woody plants - biotechnologies for phytoremediation
Authors Capuana M
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