Potential Plants Bioremediation Metals Metalloids Radionuclides Biology Essay

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The purpose of the assignment is to review the potential of plants for bioremediation of inorganic compounds, focusing on metals, metalloids, and radionuclides, especially those that pose a greater risk to the environment and human health because of their bio accumulative and carcinogenic effects, such as arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb); and the non-metallic compounds for instance radioactive nuclear waste. (See Table 1) These pollutants are often the by-products of industrial processes such as mining and manufacturing, farming and sewage, either released accidentally or deliberately. Pollutants as those mentioned above are of great concern in regards to environmental degradation as they are extensively used in industry, extremely toxic, and since these elements are non-degradable their accumulation in the environment becomes pervasive. The main sites plagued by the contamination of heavy metals are battery disposal/recycling areas, military sites, industrial sites, sewage waste, landfills, burial pits, chemical disposal areas, electroplating/metal finishing shops, and mining sites. (Singh, kuhad and Ward, 2009). The earth's atmospheric, terrestrial and aquatic systems are not sufficient in absorbing and breaking down these anthropogenic pollutants; in 1993 around 275million tons of hazardous waste was produced in the US alone and in 1996 the US EPA listed approximately 40,000 sites on its inventory of uncontrolled hazardous waste (Glick, 2010). Bioremediation is the term given for the use of living organisms to rectify the contamination of soil and water by organic and non-organic pollutants such as complex organic compounds, oils and heavy metals. Phytoremediation is the term adopted when plants are specifically used for this purpose. This new technology is currently at the centre of research because of the severe contamination to water supplies and soils, creating many negative effects such as loss of function and degradation of ecosystems, impacts on food quality, animal illness and economic loss. Presently most remediation clean-ups use more mechanical methods like excavation and reburial, capping, soil washing and burning, (Lelie et al., 2001) but because of the high cost, adverse effects on microorganisms and difficulty of cleaning up large scale contaminated areas by means of mechanical and chemical remediation, (Scragg, 2004) plants are attracting much attention as an alternative technology of the removal of pollutants, offering many benefits. Indeed, they are cost effective, of minimal disruption to topsoil, offer the possible recovery of metals, can rectify mixed contaminants and are aesthetically pleasing. (Raskin and Ensley, 2000) Phytoremediation techniques are already in practice by means of cleaning up lightly polluted areas by heavy metals such as lead, cadmium, copper and nickel as well as xenobiotics such as pesticides. (See Table 2) However, there is room for improvement and a need to overcome disadvantages to make certain forms of phytoremediation more efficient. For instance, as it takes longer than other types of remediation often needing a number of growing seasons to clean up contamination, cleaning up is restricted to areas only as deep as the plant roots penetrate. With the huge leap in the development of biotechnologies over the past two decades some of these disadvantages and problems can potentially be overcome. Over the past few years funding in Europe and the United States has been substantial, but the way in which the research has been conducted in either place differs. In Europe it has been more research driven especially in the world of academia; in the United States more private enterprises are conducting research, which is more application based and experience driven. This is perhaps because of the United States' more flexible approach to entrepreneurship and relaxed laws on GMO's. (Lelie et al., 2001) The Masons Water Yearbook 2000-2001 reported that the global market for the management, removal and disposal of hazardous waste, brownfield site redevelopment and remediation of contaminated lands was worth one trillion dollars, (Singh, kuhad and Ward, 2009), and the amount of money spent worldwide in 2006 on clean-up of polluted areas was $25 to $50million dollars worldwide (Pilon-Smits and Freeman, 2006). Therefore the potential for Phytoremediation to do well economically and financially is extraordinary and the demand for Bioremediation in developed and the developing nations is growing rapidly. As well as this, the potential to reclaim metals from the harvested biomass of plants is potentially lucrative.

This assignment will look at the various types of phytoremediation and provide examples of current uses; it will focus on key areas of transgenic plant development and take a brief look at the molecular pathways involved in pollutant detoxification and uptake, with mention to identification of key genes and their potential uses, analysis of suitable genes for incorporation into plants and molecular biology techniques for incorporating [these] genes into a host.

The main types of Phytoremediation (See Figure 1)

Phytoextraction is the term used for the extraction of contaminants (usually heavy metals) by the plant from soil into roots, and then from the roots to the above ground parts e.g. shoots and leaves. The idea is that contaminants accumulate in the plant which can then be harvested once a saturation point is reached. In order to easily harvest the plant and the contaminants within, it is desirable that the contaminants accumulate above the ground, although sometimes plants with extensive root systems are used to accumulate pollutants below ground. Phytoextraction can also be enhanced by adding specific chelating agents to the soil that can increase metal solubility in soil water and therefore increasing bioavailability. Plants with desirable qualities are those with a fast growth rate, high yield, high bioconversion factors and large biomass. Plants that can accumulate concentrations of metals at much higher rates, that is 50 to 100 more times than normal plants are known as hyper accumulators . (Scragg, 2004).

Phytodegradation is a method used for the breakdown of various organic chemicals either by the selected plants internal or secreted enzymes or by photosynthetic oxidation/reduction. Pollutants such as oils, solvents and pesticides can be rendered safe this way. One such example is that of the group of enzymes Glutathione S-transferases which are present in all types of aerobic organism. These enzymes detoxify xenobiotics and other hydrophobic compounds by converting them into form inherently more soluble, thus resulting in compounds which can be degraded and secreted more easily. (Metzler, 2001)

Phytovolatisation is a process where plants are used to uptake metal ions from soils and volatise them via foliage. In this process enzymes breakdown specific species of metals such as mercury and selenium and often reduce their toxicity.

Phytostabilisation is a technique used in order to stabilise and immobilize pollutants in soils, especially metals. Plant matter can physically hold pollutants in place, hold soil and restrict leaching by creating a surface barrier which decreases exposure to the elements or physical contact of larger potentially destructive organisms. It also prevents weathering and keeps the toxic soil in place which helps avoid contamination to other environments (such as dust dispersal). The other goal of Phytostabilisation is to transform the toxic type of molecule into a less toxic and less mobile one within the soil. Secretion of agents by roots and enzymes can change soil chemistry leading to the precipitation or immobilization of the contaminants and rendering them insoluble. Plant roots also take up and sequester contaminants in root cells.

Rhizofiltration is a process where plant roots and microorganisms making up the rhizosphere are used to uptake metals or contaminants from flowing water. Generally this process is used to sequester metals and to breakdown organic compounds. This process is especially good for pathogen, nutrient and stabilization. It is a particularly successful method because many of the wetland plants used have very strong microorganism associations with roots plants able to send oxygen from leaves to roots.

(Scragg, 2004).

The potential of Plants for Phytoremediation

Some heavy metals such as sodium (NA), magnesium (Mg), potassium (K), calcium (Ca), chromium (Cr), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), selenium (Se), nickel (Mn) and Molybdenum (Mo) are essential to plants' biochemical functioning: without them they cannot complete their lifecycles, as these elements permit the plant to synthesize the compounds they require for normal growth. (Taiz and Zeiger, 2002) These nutrients are taken up in the form as cations, solubilized and mobilized from the soil by chelating agents produced by the plant which are then hybridized with organic carbon compounds produced; the cations are assimilated through noncovalent bonding, which neutralizes the positive or negative charge. (Taiz and Zeiger, 2002). When greater amounts of these metals are contained within soils they can act as a toxins and can cause harm such as oxidative damage. It has been found that when most plants are grown in metal rich soils they act with either one of two responses. Firstly the plant can either take an approach of exclusion, which is to avoid taking in toxins as far as the shoots; they do this by returning metal ions into the soil or by storing them in the root vacuolar compartments. (Salt, 2006) Another method is for the plant or microorganism in the rhizosphere to release exudates which immobilize the pollutants resulting in reduced bioavailability of pollutants. (Pilon-Smits and Freeman, 2006). Alternatively they can be taken up and dealt with whilst inside the body of the plant lowering toxicity in the rhizosphere. (Kupper et al., 2000) (Inouhe, 2005) This is the least common way, but is a significant trait in regards to Phytoextraction. However, there is also a second hypothesis regarding the reason for hyper accumulation: certain plants may hyper accumulate metals in shoots and leaves in order to defend themselves against pathogens and grazing insects. Whether this adaptation of hyper tolerance is a defense mechanism or is purely for the need to grow on metal rich soils is questionable and perhaps it's possible to make either case depending on the species.

Plants for Phytoextraction. Naturally occurring metal hyper accumulating plants are the perfect start for the search to use plants to remediate soils polluted with heavy metals. Four hundred species of plant with this unique ability have been discovered, which have been found to accumulate metals including arsenic, cadmium, cobalt, manganese, nickel, selenium and zinc (Kupper et al., 2000). As previously mentioned, in some plants hyper accumulation appears to be an adapted defence mechanism to protect against pathogen attack and predation by grazing insects. A study conducted by Boyd & Jhee (2005) grew the Ni hyper accumulator Streptanthus polygaloides on two treatments of soil, one with a high and another with a low Ni concentration. After 10 days this led to the high concentration plant acquiring 3,800 μg Ni/g and the low plant acquiring just53 μg Ni/g; slugs were then left to eat either plan. It turned out that the plant growing on the low Ni concentration was heavily grazed, the one grown with the high Ni content was left almost unscathed. With this result we can assume that hyper accumulation of Ni acts as a deterrent. Another similar research paper conducted by Boyd (1994) again used Streptanthus polygaloides on two treatments of soil, again one with a high and one with a low Ni concentration. This time the plants were inoculated with a strain of bacterial pathogen. After an incubation time of 72 hours, the plants were analysed and the bacterial pathogens were only found to have reproduced in the plants that were grown in low concentrations of Ni; the bacteria were unable to survive and reproduce in the plant hyper accumulating high levels of Ni. There are also many other papers with similar evidence supporting these theories and with other types of metals. In order to achieve hyper accumulation of metals, plants have gained internal hypertolerance mechanisms to avoid toxicological effects and have evolved mechanisms which help utilize these potentially toxic elements from the soil and distribute them within the plants modular body. To develop plants for the phytoremediation of sites contaminated with heavy metals, it is essential to understand the molecular and genetic basis of these mechanisms. Over recent years this understanding has come a long way and the potential for plants is gradually growing. All hyper accumulator plants appear to have certain key features in common, that is vacuolar compartmentalisation, chelation, ability to detoxify, transport and sequester heavy metals. (Mukhopadhyay and Maiti, 2010). Essential elements or those chemically similar are taken up by transporters, detoxified by chelation and compartmented for safe storage in a place such as the vacuole. This whole process of shipping these metals seems to be done through active transport and is of cost to the plants' energy reserves. Even though there are very few species of hyperaccumulators, some have been selected for their potential to elevate soils from pollution. Singha et al (2008) recently examined the possible panacea for phytoremediation purposes Vetiver grass (Vetiveria zizanoides) to extract radionuclides. This plant is known to have many other useful attributes (such as its broad root system) that make it ideal to remedy other types of contamination. In this study roots Vetiver grass (Vetiveria zizanoides) plantlets were immersed in 3 types of solutions containing separate radionuclides 90Sr and 137Cs and both together. The study found that the Vetiver grass had removed 94% of 90Sr and 61% of 137Cs from solutions after 168 h. When 90Sr and 137Cs were both added to the solution, 91% of 90Sr and 59% of 137Cs were removed at the end of 168 h. The study also found that V. zizanoides removed radioactive elements from a low-level nuclear waste solution, after 15 days of exposure the level of radioactivity was undetectable This paper clearly shows that the grass is an excellent candidate for the removals of 90Sr and 137Cs and potentially other forms of nuclear waste.

Unfortunately most of the plants found to be hyper accumulators are in fact rather slow growing and small. The use of selective breeding and more recently genetic engineering to enable, modify and enhance a given plant's ability and efficiency at sequestering, degrading and transporting toxic metals is now at the centre of research. Thanks to advances in genetic engineering over the past 30 years such as the discovery of Ti plasmid (tumour inducing plasmid) of agrobacterium tumefaciens (Chilton and Matzke, 1981) and the possibility of introducing foreign genes into a plant chromosomes or chloroplasts . (Hussein et al., 2007) It is now possible to isolate, remove and incorporate genes from any species (including those for the animal and bacteria kingdoms etc.) to certain plant species with the goal of altering its existing enzyme activity by up-regulating (overexpression) or by down-regulating (knockdown) or creating entirely new enzymatic activity. (Pilon-Smits and Freeman, 2006). As well as this gene expression can be controlled by using various promoters, and the product of a certain gene can be switched on and off at different times for example responses to night and day, proteins can also be directed to different cellular compartments such as the cell wall, vacuole or chloroplast, by using different targeting sequences that are kind of similar to address labels. (Bizily et al., 1999) Presently, the model plants species used for transgenic testing in lab and green house experiments for phytoremediation are Arabidopsist, tobacco (Nicotiana tabacum) and Indian mustard (Brassica juncea). These plants have been selected because of their ease of transformation and short generation time. B juncea and Hybrid Poplar (Populus sp) are widespread species for used in phytoremediation research/development. B juncea for its efficient accumulator abilities, quick growth and high biomass and Hybrid Poplar for its tolerance to life in a range of environments, along with its transpiration rate, and its ability to translocate metals to shoots. (Pilon-Smits and Freeman, 2006). There are many gene sites, mechanisms and gene controls that are possibly fundamental for hyper accumulation. The genes under scrutiny are responsible for processes that increase the solubility of metals in the soil surrounding the rhizosphere and for the transport proteins that aid shipment of metals into the root cells (Inouhe, 2005) and extensive genetic studies have shown that hyper accumulation and hyper tolerance are independent genetic traits. (Salt, 2006) When heavy metals enter the plant vascular system, it has been found that they are further transported into the vacuoles of specific cells of the plant. Studies conducted by Leon Kochian of the U.S. Plant, Soil, and Nutrition Laboratory (Becker, 2000) found that in normal plants zinc transporter genes are regulated and fixed to zinc concentrations within roots, whereas a hyper accumulator of zinc Alpine Pennycress (Thlaspi caerulescens) was found to be expressing its zinc transporter protein to a maximum level. Plants, and indeed all living organisms, have evolved to deal with the uptake and accumulation of essential and non-essential heavy metal ions which can potentially be toxic to the organism. Mechanisms include the chelation and sequestering of these elements by certain ligands. The two most investigated are Phytochelatins (PCs) and Metallthioneins (MTs). (Cobbett and Goldsbrough, 2002) MTs were discovered sometime before PCs and have been extensively researched in their ability to detoxify metals. They are found in all kingdoms of life whether it be animal, fungi or plant. They are synthesised on the ribosome and are encoded by a family of genes. (Inouhe, 2005). In animals they have found to protect against cadmium toxicity but for plants evidence supports that they increase copper tolerance and homeostasis. Some plant MTs have been found to bind to copper and expression of MT genes is induced by copper levels. (Cobbett and Goldsbrough, 2002). PCs were discovered some year later and have been found to be rather different from MTs in terms of structure and biosynthetic pathways. (Inouhe, 2005). PC synthesis is stimulated at the cellular level when the plant is exposed to metal ions and the production of PC proteins is the result of a series of enzymatic reactions under the control of 3 genes in the pathway. A number of genes have been isolated which play a role in the PC synthesis pathway; for example those isolated for the final stage of production such as the plant gene CAD1 (found in S.Pombe) and animal gene PCS1 (found in Caenorhabditis). (Inouhe, 2005) See Figure 4. The same genes have also been found in other organisms of quite different animal kingdoms which can tell us something about their age. The manipulation and control of expression of PC using genetic engineering is one of the much researched areas and demonstrates the potential of Phytoextraction. (Gong, Lee and Schroeder, 2003). Research by Gong (2003) examined if long-distance root-to-shoot transport of cadmium was aided by PCs. Some researchers proposed that it travels into shoots via the xylem independent of PC. Gongs experiment used a normal Arabidopsis strain (WT) and cad1-3 a mutant Arabidopsis recessive featuring a loss-of-function of AtPCS1 gene (a key gene in PC production). Wheat TaPCS1 cDNA was incorporated into the genome of the mutant strain, in one individual expression was targeted to the roots using the Arabidopsis alcoholdehydrogenase (Adh) promoter (Adh::TaPCS1cad1-3) and in another TaPCS1 was ectopically expressed using the cauliflower mosaic virus 35S promoter (35S::TaPCS1cad1-3). The plants were grown in a hydroponic growth solution. The study found that Cad1-3 (with no PC production) accumulated Cd2+ into roots only; it did not transport Cd2+ to roots or shoots. There were also signs of severe stress with limited growth compared to other individuals. The incorporation of TaPCS1 into the two altered transgenic mutant individuals accumulated less Cd2+ in roots but significantly enhanced long distance transport of Cd2+ into rosette leaves and stems. The transgenic expression of TaPCS1 was found to limit the heavy-metal sensitivity of cad1-3 and regulate PCs by provide a major mechanism for regulating long-distance Cd2 transport in Arabidopsis, because of TaPCS1 protein and mRNA are root specific in one of the transgenic plants, it demonstrates that PCs can be transported from roots to shoots, and that transgenic expression of the TaPCS1 gene improves long-distance root-to-shoot Cd2+ transport and reduces Cd2+ accumulation in roots. See Figure 5. The researchers also proposed an ''overflow protection mechanism'' which contributed to maintaining a low Cd2+ in roots.

With this advancement in the genetic engineering of plants over the years much research has gone into trying to develop something to remedy mercury pollution. To date no naturally occurring plants have been found to tolerate and accumulate mercury. Research conducted by Bizily et al (1999) engineered Arabidopsis thaliana to express a modified bacterial gene, merBpe, which encoded for organomercurials lyase (MerB). This gene is known to give immunity to certain bacteria against methyl mercury by breaking it down to a simpler and less mobile form. Transgenic plants expressing this gene were found to grow successfully when roots were exposed to methyl mercury, whereas the control plants lacking the gene died. This was one of the early papers which suggested that trees, shrubs and grasses inserted with this gene could potentially be used for phytoremediation of sites polluted with toxic forms of mercury. A more recent study by Hussein et al (2007) used two native bacterial genes merA and merB and successfully incorporated them into the chloroplast DNA of a tobacco plant. Various types of Hg (mercury) were supplied to the plants via the roots. Firstly these genes allowed the transgenic plant to take up high levels of Hg into the root system, Up to 1500 μg g−1 more than that of the control. The transgenic plant was also found to have 100 times the levels of control of Hg in the shoots and leaves. The plants were also found to break down molecules of Hg2+ to elemental mercury (Hg0) which was then volatised (a process known as Phytovolatisation). This paper provides a good indication of improvements which can allow plants to tolerate and detoxify mercury. In order for a plant to perform Phytoextraction, pollutants must be soluble in the rhizosphere for the plant to assimilate. One way to improve this is to apply chemical chelating agents to soils, which increases bioavailability, uptake and translocation to plants. One commonly tested chelator is EDTA (ethylenediaminetetracetic acid). This seems to work well in lab experiments, but is perhaps dubious in the field as it can enhance the problem of pollution by mobilizing a surplus of metals; those not taken up by the plant are prone leaching (potentially increasing contamination of ground water) as well as causing toxic effects on microbial communities. (Surridge, Wehner and Cloete, 2009).

However, researchers are experimenting with new types of chemical chelators such as EDDS (ethylenediaminedisuccinate) which has been found to have a higher biodegradability, to be less harmful to soil microbes and whose rate of chelation more closely matched to the plant's rapidity of uptake. Of course different types of pollutants have more suitable chelators. In order to assess the potenetial of the two biodegradable chelants citric acid and NTA as an alternative to EDDS A study by Carmo and Williams (2010) carried out experiments using maize to extract lead (Pb) with the enhancment of the 3 chelators. The soil used to grow the maize was taken from a battery recycling centre with high levels of Pb. Chelating agents were applied 60 days after growth. The results showed that The Pb concentration in maize roots and shoots increased as a function of chelant rates applied to soil, a dose of of 3, 5, 7, and 10 mmol dm−3 of EDTA stimulated a 25-, 37-, 59-, and 73-fold increase in shoots Pb concentration. The citric acid doses of 5, 10, 15, and 30 mmol dm−3 promoted a 1, 3, 9, and 19-fold the Pb concentration in shoots. NTA doses of 3, 5, 7, and 10 mmol dm−3 provoked a 4.4-, 7.2-, 9.4-, and 10.5-fold increase in Pb concentration in the maize shoots. The EDTA plants performed rather better than the citric acid and NTA (See Figure 2 ) but as mentioned previously it perhaps performed too well since an excess in soil could lead to leaching(in the field). This makes the biodegradable chelates potentially more desirable since they could offer more control, and present lower toxicity to microorganisms. Unfortunatly a far larger proportion of Pb in plants used along with the two biodegradable chelates was found in the root systems (See Figure 3) EDTA results show a far greater amount of Pb allocated to the shoots and this study concludes that it perhaps enhances the mobility of Pb within plants. However the study shows that citric acid and maize are certainly good candidates for the clean up of sites contaminated with lead. The need to find ways in which roots or microorganisms in the rhizosphere exude larger amounts of natural chelators has been proposed and would be more appropriate. But in contradiction many microorganisms aid the plant by excluding heavy metals. (Lelie et al., 2001). In regards to phytoremediation technology as a whole, it's essential not to neglect the importance of rhizospheric microorganisms and their association to plant wellbeing. This has sometimes been overlooked and it's important to note that around 90% of terrestrial plants are mycorrhizal (Gerhardt et al., 2009), an understanding of beneficial microbes is paramount in order to successfully produce a combined package for bioremediation (Phytoremediation and Mycorrhizoremediation) (Glick, 2010) When plant roots exude nutrients, microorganisms have been found to colonise the rhizosphere, their numbers have been found to be 10 to 1000 greater than in bulk soil. Microbes facilitate plants by improving growth and health, enhancing root development and increasing plant tolerance to various environmental stresses. (Glick, 2010) They do this by acting as bio control agents guarding roots against pathogen attack, fix and provide roots with nitrogen, solubilize minerals and limit stress (some bacteria have found to lower the amount of stress hormone ethylene by producing an enzyme ACC deaminase which cleaves the precursor to ACC (Glick, 2010)) recently researchers have genetically engineered bacteria that produce various metal-binding peptides improving bioavailability. However incorporation into field studies is limited due to legal restriction. When microbes have been implemented in phytoremediation field studies, they have mostly failed because of the introduced microbes inability to compete with existing micro flora and micro fauna, suffer from nutrients deficiencies (can be low in contaminated soils) and excess of toxins inhibits growth. (Gerhardt et al., 2009) (See Table 3 for some examples of Plant-bacteria combinations.)

Phytostabilization of soils polluted with heavy metals is a far simpler and already a proven technology. This can be used with or without the addition of metal inactivating chemical additives such as coal fly ash or zeolites; sometimes a mixture of organic and non-organics substances is added. The idea is to use these additives in conjunction with re revegetation, using plants relatively tolerant of the contaminants and using additives that help to control pH, decrease the amount of metal bioavailability (by the formation of insoluble metal organic complexes) and nutrients to increase soil fertility. Creating balances that also aid microbial biodiversity are very important in this case since they play a crucial role in maintenance of soil fertility, functioning and resilience. (Surridge, Wehner and Cloete, 2009). Plants help to stabilise the contaminants in the rhizosphere through absorption and accumulation in roots, onto roots or by precipitation through the root area and covering the ground as well as shoring up the earth. All together these processes decrease metal leaching to ground water and prevent dispersal of the polluted dust through erosion from wind and rain. (Surridge, Wehner and Cloete, 2009).

Conclusions/Closing remarks

Phytoremediation is a multidisciplinary subject and requires research and project teams' expert in plant physiology, agronomy, soil science and environmental engineering. The criteria for development of plants and projects should take the following pre harvest parameters into consideration: the type and degree of pollution, treatability, agronomic techniques, groundwater capture zone, uptake rate, transpiration rate, clean up time; and also the following post-harvest parameters of collection of residues, waste disposal and treatment of contaminated materials (Singh, kuhad and Ward, 2009). Many plants with the unusual ability to take up and store ionic metals seem to be those with a low biomass and slow growth rate, which currently limits large-scale exploitation of the technology because of the long time necessary for decontamination of the soil. In order to make phytoremediation more viable for commercialisation and improve function, the need for genetic enhancement exists. Creation of genetically modified plants to create heavy metal hyper accumulators with broad root systems, high biomass and heightened translocation capabilities is essential, but the safety must be carefully considered in order to avoid contamination into natural populations. The new 'super-strain' which escapes into wild environments could possibly act invasively in other plant communities and ecosystems, as well as contaminating the gene pool of non GMO counterparts. Other dangers are the potential contamination of air if plant matter catches fire, can be hazardous to wildlife and has the potential to enter the food chain. However phytoremediation presents us with part of the solution in answering our quest to cure environmental degradation. It also presents investors with a large economic opportunity when sized and scoped to environmental problems associated with metal-contaminated soils (Chaney et al. 1997). Because research on phytoremediation only commenced some twenty years or so, the majority of the work has taken place in place in the laboratory and not the field. Many of the genetic based solutions are currently in the very early stages of development, and are achieving excellent results. However achieving this success in the field is a far more complicated task and will certainly prove more difficult, especially making it appeal economically and in terms of site remediation time scale. Considerable time and effort is being dedicated to making phytoremediation a commercial success, but to alleviate failures plant stress factor need to be present in lab testing and green house studies. Also the need to establish reliable monitoring methods seems to be appropriate as mentioned by Gerhardt et al. (2009) It seems that the genetic development of hyper accumulating trees with high biomass and fast growth rates is the next step to take in terms of developing plants for phytoremediation. Also more emphasis should be put on the role of microbes and their interactions with plants in the soil, such as the roles of fungi/bacteria that can facilitate the plant sin ways such biodegrade pollutants, growth-promote and defend against pathogens.

Figures and Tables

Table 1: Top 10 US toxic substances and causes of environmental related disease for 2007

Rank

Substance

1

2

3

4

5

6

7

8

9

10

Arsenic

Lead

Mercury

Vinyl chloride

Polychlorinated biphenyls

Benzene

Cadmium

Polycyclic aromatic hydrocarbons

Benzo(A)Pyrene

Benzo(B)Fluoranthene

Table 2: Successful European phytoremediation field projects. (Lelie et al., 2001)

Table 3: Plant-bacteria combinations and effects in various metal phytoremediation studies (Glick, 2010)

Bacterium

Plant

Metal

Effect/Mechanism Reference

10 different rhizosphere bacteria

Goat willow (Salix caprea) Cadmium,

zinc

Increased metal uptake; IAA,

siderophores, ACC deaminase all not

involved

Achromobacter xylosoxidans Ax10

Indian mustard

Copper

Increased root and shoot length and

biomass; ACC deaminase, phosphate

solubilization, IAA

Bacillus edaphicus NBT

Indian mustard

Lead

Increased biomass; IAA, siderophores,

ACC deaminase

Bacillus sp. J119

Canola corn sudan grass Sorghum vulgare var.

sudanense), tomato

Cadmium

Some increased biomass and cadmium

uptake, IAA, siderophores, biosurfactant

production

Enterobacter aerogenes, Rahnella aquatilis

Indian mustard

Nickel,

chromium

Increased biomass and metal uptake; IAA,

siderophores, ACC deaminase, phosphate

solubilization

Enterobacter sp. NBRI K28

Indian mustard

Nickel,

zinc, chromium

Increased biomass and metal uptake; IAA,

siderophores, ACC deaminase, phosphate

solubilization

Flavobacterium sp.

Orychophragmus violaceus

Zinc

Increased root length, biomass, metal

uptake; mechanism unknown

P. aeruginosa MKRh3

Black gram (Vigna mungo)

Cadmium

Increased biomass and rooting, and decreased cadmium uptake; IAA,

siderophores, ACC deaminase, phosphate

solubilization

P. fluorescens G10, Microbacterium sp. G16

Canola

Lead

Increased biomass and metal uptake; IAA,

siderophores, ACC deaminase, both

strains are endophytes

P. putida HS-2

Canola

Nickel

Increased seed germination and biomass;

siderophores, IAA, ACC deaminase

Proteus vulgaris KNP3

Pigeon pea (Cajanus cajan)

Copper

mechanism unknown

Increased germination, biomass and

chlorophyll, and decreased metal uptake;

Pseudomonas sp.

Chickpea (Cicer arietinum)

Nickel

Increased biomass and decreased metal

uptake; siderophores

Pseudomonas sp.

RJ10, Bacillus sp. RJ16 Tomato

Cadmium,

lead

Increased root length, above ground

biomass and above ground metal;

siderophores, IAA, ACC deaminase

Pseudomonas sp. 29C, Bacillus sp. 4C

Indian mustard

Nickel

Increased biomass; IAA, siderophores,

ACC deaminase, phosphate solubilization

Pseudomonas sp. M6, Pseudomonas jessenii M15

Castor bean (Ricinus

communis)

Nickel,

copper,

zinc

Increased biomass; IAA, ACC deaminase,

phosphate solubilization

Streptomyces tendae F4

Sunflower

Cadmium

Decreased metal uptake and increased

iron content; siderophores

Figure 1: Diagram of phytoremediation occurrence

Figure 2: Regressions between Pb concentrations in maize roots and shoots as a function of chelates (EDTA, NTA, and citric acid) added to the soil. (Carmo and Williams, 2010)

Figure 3: Regressions between the net removal of Pb (mg pot−1) by maize shoots and crescent doses of EDTA, citric acid, and NTA added to the soil. (Carmo and Williams, 2010)

Figure 4: Biosynthesis of PCs in higher plants. (Inouhe, 2005)

Figure 5: Enhanced long-distance Cd2 transport and reduced Cd2 accumulation in roots by transgenic expression of TaPCS1. Accumulation in stems (A), rosette leaves (B), and roots (C) by (Gong, 2003)

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