Perspective on Phytoremediation for Improving Heavy Metal Contaminated Soils

Hong-Bo Shao, Li-Ye Chu, Fu-Tai Ni, Dong-Gang Guo, Hua Li, and Wei-Xiang Li

Abstract Heavy metal pollution of soil is a significant environmental problem and has its negative potential impact on human health and agriculture. Phytoremediation strategies with appropriate heavy metal-adapted rhizobacteria (for example, myc-orrhizae) have received more and more attention. Some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, and they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration, which is manifested by an interaction between a genotype and its environment. A coordinated network of molecular processes provides plants with multiple metal-detoxifying mechanisms and repair capabilities, which allow plants to survive under metal-containing soil environments. The growing application of

Institute for Life Sciences, Qingdao University of Science & Technology (QUST), Qingdao 266042, China; Yantai Institute of Costal Zone Research, Chinese Academy of Sciences (CAS), Yantai 264003, China e-mail: [email protected]

Institute for Life Sciences, Qingdao University of Science & Technology (QUST), Qingdao 266042, China e-mail: [email protected] F.-T. Ni (B)

College of Life Sciences, Jilin Normal University, Siping 136000, China e-mail: [email protected]

College of Environment and Resources, Shanxi University, Taiyuan 030006, China e-mail: [email protected]

College of Environment and Resources, Shanxi University, Taiyuan 030006, China e-mail: [email protected]

Shanxi Agricultural University, Taigu 030801, China e-mail: [email protected]

M. Ashraf et al. (eds.), Plant Adaptation and Phytoremediation, 227

DOI 10.1007/978-90-481-9370-7_11, © Springer Science+Business Media B.V. 2010

molecular genetic technologies has led to an increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance, as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. This article reviews advantages, disadvantages, possible mechanisms, current status and future directions of phytoremediation for heavy metal contaminated soils and environments.

Keywords Phytoremediation ■ Heavy metals ■ Soil ■ Mechanisms ■ Signal transduction ■ Phytohormones ■ Transcription factors ■ Biotechnology ■ Hyperaccumulator ■ Gene expression

Contents

1 Introduction 228

2 Understanding Mechanisms of Phytoremediation for Improving Heavy

Metal Contaminated Soils 229

2.1 Heavy Metal Accumulation in Plants 229

2.2 Genes Involved in Heavy Metal Perception and Signal Transduction 230

3 Important Standards for Heavy Metal Hyperaccumulator Plants 235

4 Biotechnology and Phytoremediation of Heavy Metal Contaminated Soils 236

5 Conclusion 240

References 241

1 Introduction

Phytoremediation of metals is being developed as an effectiveand environment-friendly solution for heavy-metal-contaminatedsoils (Barcelo and Poschenrieder 2003; Banuelos et al. 2007; Aina et al. 2007). In recent years, major scientific strides have been takenin understanding the soil chemical and plant molecular-geneticmechanisms that drive metal hyperaccumulation in plants. Becausehyperaccumulators are mostly low biomass and slow-growing plants,current research is focused mainly on designing transgenic plantsthat can overcome this deficiency. The complexity of plant-metal interactions and influences of the environment, andspecific matrix factors that control the chemical specia-tionof the metal, and interactions of other toxicants that may bepresent at the site all add to the strategy of phytoremediation (Bassirirad 2000; Bauer and Bereczky 2003). Extensive progress has been made in characterizingand modifying the soil chemistry of the contaminated sites topromote/accelerate metal phytoremediation. However, extensivefield deployment of this technique on a large scale is stillbeing hampered by a lack of specific understanding of the complex interactions between metal, soil, and plant systems that are instrumental in metal uptake, translocation, and storage in plants. A multidisciplinary research effort that integrates the work of plant biologists, soil chemists, microbiologists, and environmental engineers is essential for the success of phytoremediation as a viable soil cleanup technique in metal-contaminated sites (Brewer et al. 1999; Bennett et al. 2003).

Phytoremediation is the use of a plant's natural ability to contain, degrade, or remove toxic chemicals and pollutants from soil or water. It can be used to clean up metals, pesticides, solvents, explosives, crude oil, and contaminants that may leak from landfill sites. The term phytoremediation is a combination of two words -phyto, which means plants, and remediation, which means to remedy (Clemens 2006; Denton 2007; Shao et al. 2008a, b, c, d, e).

Researchers are investigating phytoremediation potential by using plants such as sunflower, ragweed, cabbage, geranium, Thlaspi caerulescens, Arabidopsis thaliana, Lycopersicon esculentum, Zea mays, Hordeum vulgare, Oryza sativa, Pisum sativum, Lotus japonicas, Brassica, Sedum alfredii, Cannabis sativa, as well as other less known species. The plants are often used in combination with other traditional technologies for cleaning up contaminated sites because of the phytoremediation limitations (Cobbett 2002; Curie and Briat 2003; Citterio et al. 2003; Czako et al. 2006) There are many advantages of phytoremediation for heavy metal-contaminated soils (Table 11.1).

Table 11.1 Advantages of phytoremediation

Advantages

Disadvantages

1. Environment friendly, cost-effective, and aesthetically pleasing

2. Metals absorbed by the plants may be extracted from harvested plant biomass and then recycled

3. Phytoremediation can be used to clean up a large variety of contaminants;

4. May reduce the entry of contaminants into the environment by preventing their leakage into the groundwater systems

1. Relies on natural cycle of plants and therefore takes time

2. Phytoremediation works best when the contamination is within reach of the plant roots, typically three to six feet underground for herbaceous plants and 10 to 15 feet for trees

3. Some plants absorb a lot of poisonous metals, making them a potential risk to the food chain if animals feed upon them

2 Understanding Mechanisms of Phytoremediation for Improving Heavy Metal Contaminated Soils

2.1 Heavy Metal Accumulation in Plants

Heavy metals can be accumulated in various plant organs, which belong to the long-term effects of heavy metal action (Cunningham et al. 1995; Datta and Sarkar 2004). Their presence was detected in roots, stems, leaves, seeds and fruits. The cell wall is suggested to be the main accumulation site of Cd and other heavy metals. A similar accumulation site was found in vacuoles, especially in the case of Zn. In stems, Zn accumulated along the walls of vascular bundles, and in roots along cell walls. Its deposition occurred either in the form of simple Zn salts or proteins and carbohydrates complexes with Zn. Irons of heavy metals are detoxificated in the cytosol by high-affinity ligands like amino acids, organic acids and two types of peptides: PCs (phytochelins) and MTs (metallothioneins) (Deckert 2008; Doty 2008). It is generally assumed that the major sites of metal sequestration are vacuoles of root cells. PC-Cd complexes are transported into the vacuole, where heavy metal complexes are formed. Accumulation of heavy metals in chloroplasts is still controversial (Eide et al. 1996; Dhankher et al. 2002).

Ni was found to accumulate in seeds of Raphanus sativus, its level being maximal after 10 h of treatment (Elizabeth 2005). In wheat leaves, most of Ni accumulated up to the 3rd day after the application because of a fast and long distance transport of this metal (Fox and Guerinot 1998; Fayiga et al. 2004). Roots and shoots of Pisum sativum showed different metal accumulation capabilities. Ni amount in roots increased as a function of metal supply and was markedly higher than in shoots. In maize, Ni accumulated in chloroplasts of the bundle sheath cells and in the root apex. In chloroplasts, Ni was found to be more associated with their lamellar fraction than with the stroma and envelope (Gleba et al. 1999; Ghosh and Singh 2005; Huang and Cunningham 1996).

The content of Hg in tomato seedlings increased concurrently with Hg concentration and exposure time. More Hg was accumulated in roots than in above ground plant parts. Mature tomato leaves contained the greatest, whereas younger ones the smallest Hg content (Savenstrad and Strid 2004).

In rice seedlings growing at increasing lead concentration, Pb was distributed in an organ-dependent specific manner, which was greater in roots than in shoots. Pb was unevenly distributed in roots, where different tissues act as barriers to apoplastic and symplastic Pb transport, restricting its transport to shoots (Rugh et al. 1998; Hartley-Whitaker et al. 2001, 2002; Kramer 2005; Haydon and Cobbett 2007).

2.2 Genes Involved in Heavy Metal Perception and Signal Transduction

2.2.1 Heavy Metal Sensors

There are limited data on metal perception and signal transduction pathways in plants. The perception of extracellular signals is thought to be mediated by receptorlike protein kinases. The receptor-like kinase involved in heavy metal stress in plants has been reported very recently. The gene coding for lysine motif receptor-like kinase in barley was shown to be induced by Cr, Cd, Cu during leaf senescence (Fusco et al. 2006). The proteomic study on Cd-treated rice roots indicated the induction of putative receptor protein kinase. However, more detailed study on the function of this putative receptor has not been published so far.

2.2.2 Signaling Involved in Calcium, Reactive Oxygen Species (ROS) and Mitogen-Activated Protein Kinases (MAPK)

The heavy metal stress signaling in plants involves calcium changes, MAPK cascades and transcriptional activation of the stress-responsive genes (Gasic and Korban 2007; Li et al. 2005; 2006). The expression of metal-induced barley receptor-like kinase is also mediated by Ca level. It was suggested that certain metals (Cd, Ni, Co) may cause perturbation in intracellular calcium level and interfere with calcium signaling by substituting Ca in calmodulin regulation (Kim et al. 2007). By using calcium indicator, it was recently proved that metals such as Cd and Cu induce calcium accumulation in rice roots (Yeh et al. 2007). The treatment of tobacco cells and Scots pine roots with Cd and lupine roots with Pb caused the generation of H2O2(Meda et al. 2007). The Cd-producing oxidative burst in tobacco is mediated by calmodulin and/or calmodulin-dependent proteins. Thus, available data suggest the involvement of Ca/calmodulin pathway in signaling of metal response in plants (Sunkar and Zhu 2004).

MAPK pathway is involved in the transduction of extracellular signals to intra-cellular targets in all eukaryotes (McCully 1999; Pence et al. 2000; Shao et al. 2008). It was recently indicated that Cd and Cu activate four different MAPKs (SIMK, MMK2, MMK3 and SAMK) in alfalfa, whereas Cd induces one such kinase (ATMEKK1) in Arabidopsis and one (OsMAPK2) in rice (Persans et al. 2001; Sasaki et al. 2006; Kassis et al. 2007). However, it is not clear if activation of MAPKs occurs by direct action of these metals or through ROS, which also activates MAPK cascade in Arabidopsis or it occurs via action of other mediators (Wawrzynski et al. 2006). Recent information shows that Cd- and Cu-induced MAPK activation requires the involvement of calcium-dependent protein kinase (CDPK) and phosphatidyl-inositol 3-kinase (PI3 kinase) (Yazaki et al. 2006). Therefore, the current model for Cd and Cu signal transduction pathway states that both metals induce ROS production and calcium accumulation. The CDPK and PI13 kinase may be involved in metal-induced MAPK activities. However, both of these metals induce MAPK activation via distinct ROS-generating systems, therefore the MAP responsiveness may differ depending on the type of metals and ROS involved. MAPKs usually link the cytoplasmic signal to nucleus, where they activate other protein kinases, specific transcription factors and regulatory proteins (Sunkar et al. 2006; Shao et al. 2008).

2.2.3 Phytohormone Signaling

The signaling pathways involving abscisic acid (ABA), salicylic acid (SA) and auxin (IAA) also participate in the response to heavy metals, as respective cis-DNA regulatory elements were detected in heavy metal-induced genes. The auxin-responsive mRNA was detected in Cd-treated Brassica juncea plants (Lindblom et al. 2006). Proteomic analysis of Cd-treated Arabidopsis thaliana showed the induction of nitrilase protein, which is involved in auxin biosythesis (Roth et al. 2006). The transcription activation of the gene (SAMT) involved in biosynthesis of SA was detected in pea treated with Hg. It is known that Cd induces the biosynthesis of ABA and ethylene, which in turn evoke various stress responses. All these data confirm that phytohormones play a role in plant responses to heavy metals. However, it is not clear if they play the signaling role in activation of heavy metal-responsive genes, or serve as effectors of certain heavy metal-imposed reactions to participate in both processes.

2.2.4 Heavy Metal - Induced Transcription Factors and Heavy Metal Responsive Elements

Little is known about transcriptional processes in plants in response to heavy metals as well as functional link between signaling pathways and responses at transcription level. The transcriptional profiling of plants treated with various heavy metals indicated that they can induce into heavy metal-induced transcription factors (LeDuc et al. 2006). The Cd-induction of transcripts for basic region leucine zipper (bZIP) and zinc finger transcription factors has been detected in Arabidopsis thanliana and Brassica juncea (Ramos et al. 2007). Screening of Cd-responsive genes in Arabidopsis thanliana indicated that DREB2A gene is up-regulated by Cd. The DREB proteins bind to dehydration response element and in Cd-treated Arabidopsis thaliana, DREB2A preferentially activates the rd29A gene, which is thought to play an important role under cold, high-salt and dehydration (Rosen 2002; Srivastava et al. 2005; Shao et al. 2008). On the other hand, one of the Cd-induced bZIP transcription factor (OBF5) in Arabidopsis thaliana binds to promoter region of glutathione transferase gene (GST6), which is known to be induced by auxin, SA and oxidative stress (Qi et al. 2007). The Zn treatment of Arabidopsis thaliana caused the induction of one type of transcription factor (bHLH), whereas the expression of two others (WRKY and zinc-finger, GATA-type) was decreased in the presence of excess of Zn (Ouelhadj et al. 2007). Despite existing data on the heavy metal-induction of different transcription factors, it is still not clear if these activations are specific to particular heavy metal ,common to most of the metals, related to oxidative stress (caused directly or indirectly by most of the heavy metals), mediated by phytohormones or connected with the general plant stress response (Sun and Zhou 2005). The process of ROS-mediated transcription activation of factors is thought to be a common link in different stress responses in plants. Therefore, among all possible pathways, ROS seems to play a key, but not the only one, role in activation of heavy metal-induced transcription factors in plants. Other organisms, such as yeast and animals, contain specific heavy metal-induced transcription factors which bind to heavy metal responsive element present in promoters of heavy metal-responsive genes (Cobbett 2002). The cis-acting elements related to heavy metal responsive elements have been found within promoters of a few plant genes, including metallothionein-like genes, however there is no evidence that these sequences confer heavy metal responsiveness of these genes. So far only two types of cis-DNA elements, which may be functional in heavy metal response, have been described in plants (Deckert 2008). One type is iron-dependent regulatory sequences (IDRS), which are responsible for the iron-regulated transcription of genes involved in Fe acquisition. The second one has been recently identified within the promoter region of PvSR2 gene from Phaseolus vulgaris. PvSR2 gene encodes a heavy metal stress related protein, whose expression is strongly stimulated by Hg, Cd, As and Cu, but not by other environmental stresses such as UV radiation, high temperature or pathogens. The heavy metal-responsive elements were localized within two regions of PvSR2 gene promoter. Region I contains a motif similar to the consensus metal-regulatory element of the animal metallothionein genes, whereas the region II represents a novel heavy metal-responsive element in plants and has no similarity to previously identified cis-acting DNA elements involved in heavy metal induction.

According to the above concerning the activation of various transcription factors, which also confer the response to other stimuli, the lack of specific heavy metal-induced transcription factors and very limited data on the function of cis-acting and metal-specific DNA elements indicate that plants employ a wide array of mechanisms to activate the genes required to cope with the excess of heavy metals in their environment (Rocovich and West 1975; Ma et al. 2001; Rupali and Sarkar 2004). Possible molecular mechanisms of phytoremediation for heavy metal-contaminated soils, in combination with signaling pathways and transcription regulation, has been summarized in Fig. 11.1.

2.2.5 Phospholipid Signaling

Phospholipid signaling plays a crucial role in serving as a second messenger in plant responses to heavy metal stress (Shao et al. 2008). Phospholipds are rapidly produced in response to a variety of stimuli by the activation of lipid kinases or phosphatases. The expression of phospholipase D was shown to be induced by ABA, cold, drought, high salinity, wound and pathogen interactions (Bergmann and Munnik 2006). Some results indicate that this pathway may also be involved in plant response to heavy metals as the increased level of phospholipases transcripts were observed in cadmium-treated plants and phosphatidyl-inositol 3-kinase was shown to take part in cadmium and copper activation of MAPKs in rice roots (Yeh et al. 2007). The growing evidence suggests that plant signaling consists of network of pathways operating during various stress situations and that the crosstalk exists among stress responses, phytohormones and ROS signaling (see Fig. 11.1) (Sunkar and Zhu 2004; Sunkar et al. 2006; Fujita et al. 2006; Shao et al. 2008).

2.2.6 Posttranscriptional Regulation of Heavy Metal-Dependent Genes By MicoRNAs

MicroRNAs (miRNA) and short interfering RNAs (siRNAs) are small noncod-ing RNAs that have recently come out as a global important regulator of mRNA degradation, translational repression and chromatin modification (Sunkar and Zhu 2004). MicroRNAs are small, 21-22 nucleotides long, RNA molecules that can contribute to the regulation of gene expression in plants by directing an endo-ribonuclease complex to degrade the target mRNAs.The involvement of miRNAs in regulation of gene expression is mostly known for various developmental processes

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