Selenium Metabolism in Plants: Molecular Approaches

Selenium (Se) is placed in Group VIA of the Periodic Table. Its chemistry is similar to sulfur (S). Practically all small organic selenium compounds are isologues of corresponding sulfur compounds. With a few exceptions, they are also isologues of sulfur amino acids or derivatives thereof. Selenium plays an indispensable role for humans, animals and microorganisms. It is beneficial for the metabolism at lower concentrations, whereas at higher concentrations it becomes toxic. In other words, the range between deficiency and toxicity is very narrow. Short-term consumption of high levels of Se by human and animals may cause nausea, vomiting, and diarrhea, whereas chronic consumption of high concentrations of Se compounds can result in a disease called selenosis (Goldhaber, 2003). Excess selenium in the environment can be the result of either natural geological processes or industrialization.


Introduction
Selenium (Se) is placed in Group VIA of the Periodic Table .Its chemistry is similar to sulfur (S).Practically all small organic selenium compounds are isologues of corresponding sulfur compounds.With a few exceptions, they are also isologues of sulfur amino acids or derivatives thereof.Selenium plays an indispensable role for humans, animals and microorganisms.It is beneficial for the metabolism at lower concentrations, whereas at higher concentrations it becomes toxic.In other words, the range between deficiency and toxicity is very narrow.Short-term consumption of high levels of Se by human and animals may cause nausea, vomiting, and diarrhea, whereas chronic consumption of high concentrations of Se compounds can result in a disease called selenosis (Goldhaber, 2003).Excess selenium in the environment can be the result of either natural geological processes or industrialization.
Selenium acts as a cancer preventative agent when given in pharmacological amounts.Numerous studies have demonstrated the efficacy of methylselenocysteine (MeSeCys) in preventing mammary cancer in mammalian model systems, and importantly, MeSeCys has been shown to be twice as active as Se-methionine (the primary component of Se-yeast supplements) in preventing the development of mammary tumors (Ip & Ganther, 1992;Lu et al., 1996;Ip et al., 2000;Finley and Davis 2001;Medina et al., 2001;McKenzie et al., 2009).This non-protein seleno amino acid is produced in certain plants including members of the Astragalus, Allium and Brassica genera (Cai et al., 1995;Clark et al., 1996).While the specific mechanism for the anticancer activity of Se has not been fully elucidated, researchers have speculated that the Se could be effect the cell cycle then induce apoptosis in cancer cell lines (Foster et al., 1986;Cai et al., 1995;Andreadou et al., 1996;Ganther & Lawrence, 1997;Combs & Gray, 1998;Ip 1998;Sinha et al., 1999;Lu & Jiang, 2001;Kim et al., 2001;Wang et al., 2002;Ip et al., 2000).There is also evidence that Se may inhibit tumor angiogenesis (Lu & Jiang 2001).The molecular mechanism of cancer prevention by selenium using the genomics approach was studied on the target organs breast, prostate, colon and lung.The results of the microarray analysis indicated that selenium, independent of its form and the target organ, alters several genes in a manner that can account for cancer prevention.Selenium can SMT is the most important enzyme in Se hyperaccumulating plants.SMT catalyses the methylation of SeCys to MeSeCys, and the gene firstly isolated from hyperaccumulator A. bisulcatus (Neuhierl & Bock, 1996;Neuhierl et al., 1999), then isolated some other accumulator and nonaccumulator plant species (Table 2).SMT is constitutively expressed in roots and leaves of A. bisulcatus, and does not induced by Se (Pickering et al., 2003).Heterologous expression of AbSMT in transgenic Arabidopsis thaliana results in the production of MeSeCys and its derivative -glutamyl-Se-MeSeCys, compounds not normally produced in A. thaliana (Ellis et al., 2004).Accumulation of MeSeCys was similarly observed in transgenic Brassica juncea expressing AbSMT (LeDuc et al., 2004).According to these results, only Se-hyperaccumulating species of Astragalus are capable of synthesizing MeSeCys compared with their non-accumulating relatives, and that SMT activity is closely linked with the capacity to hyperaccumulate Se (Sors et al., 2005), it can be hypothesized that Se non-accumulating species do not contain a functional SMT enzyme.

Accumulation capacity Accession number Reference
Astragalus bisulcatus Hyperaccumulator AJ131433.In general, accumulator species likely do not have any Se-specific pathways but take up and metabolize Se and S indiscriminately, also current knowledge demonstrates that Se essentiality in higher plants are still not definitive.The potential health benefits of some Se compounds when combined with the increased application of phytoremediation techniques in contaminated soil were augmented the study of Se biochemistry in plants.Thus, Se metabolism in plants has been reviewed by a number of authors (Terry et al., 2000;Briggers et al., 2002;Germ et al., 2007a;White et al., 2007;Pilon & Quinn, 2010;De Filippis 2010).The present Chapter will focus on basic aspects of molecular selenium metabolism in plants and future perspectives of phytoremediation techniques.

Uptake and transport
Selenium could be occurs in following oxidation states: -2 (selenide), 0 (elemental Se), +4 (selenite), and +6 (selenate).Selenate is accumulated in plant cells via the process of active transport (Brown & Shrift, 1982).Unlike selenate, there is no evidence that the uptake of selenite is mediated by membrane transporters.Alternatively, plants can take up organic forms of Se such as selenomethionine (SeMet) actively, but not effectively (Abrams et al., 1990) (Figure 2).Selenate directly competes with sulfate for uptake by plants.It has been proposed that both anions are taken up via a sulfate transporter in the root plasma membrane.Selenate uptake in other organisms, including Escherichia coli (Lindblow-Kull et al., 1985) and yeast (Cherest et al., 1997), is also mediated by a sulfate transporter.The expression of the sulfate transporter genes is regulated by the S status of the plant, as well as by the regulators, glutathione (GSH) and O-acetylserine.While high levels of sulfate and GSH decrease transcription, high levels of O-acetylserine increase transcription of the high-affinity transporter genes as well as sulfate uptake.Thus, increasing O-acetylserine levels can potentially increase selenate uptake (Davidian et al., 2000).Terry and colleagues reported that application of O-acetylserine increased selenate accumulation in Indian mustard almost two-fold compared to untreated plants, and they speculate that Oacetylserine, a precursor of cysteine (Cys) and a product of the nitrate assimilation pathway, might pivotal importance as a coregulator of the S and nitrogen metabolic pathways.
Overexpression studies on the sulfate transporter genes increased selenate accumulation up to two-fold in transgenic plants compared to wild type.These information show that sulfate transporter is involved in selenate uptake (Terry et al., 2000).
Translocation of the Selenium in the plant parts depend on the form of how it is supplied.Zayed et al (1998) showed that the shoot/root ratio of the Se concentrations ranged from 1.4 to 17.2 when selenate was supplied but was only 0.6 to 1 for plants supplied with SeMet and less than 0.5 for plants supplied with selenite.Time-dependent kinetics of Se uptake by Indian mustard showed that only 10% of the selenite taken up was transported from root to shoot, whereas selenate was rapidly transported into shoots (De Souza et al., 1998).Thus, plants transport and accumulate substantial amounts of selenate in leaves but much less selenite or SeMet.Selenite is rapidly converted to organic forms of Se such as SeMet which are retained in the roots (Zayed et al., 1998), this helps to explain why selenite is poorly translocated to shoots.In addition, partitioning of Se in various plant parts is species specific, also depends on the stage of development, and on physiological condition of the plant.In the accumulators, Se is gathered in young leaves during the early vegetative and reproductive stage of growth, and 3,5 fold high levels of Se are found in seeds while the Se content in leaves is drastically reduced.Non accumulating cereal crop plants, often show about the same Se content in grain and in roots, but smaller amounts in the stems and leaves.Distribution of Se in plants also depends on the form and concentration of Se supplied to the roots and on the nature and concentration of other substances, especially sulfates, accompanying the Se (Zayed et al., 1998).Plants can also absorb volatile Se from the atmosphere via the leaf surface.The Se absorbed by the leaves is accumulated in roots as inorganic selenite, selenoglutathione (SeGSH), SeMet, and protein-bound SeMet..

Accumulation in plants
Hyperaccumulation is the ability of certain plants to accumulate extraordinarily high concentrations of metals and trace elements, even when grown in soil with low concentrations (Baker & Brooks, 1989).This ability for certain elements gives some selective advantage to the hyperaccumulators plants.A selective benefit of hyperaccumulation is predominant occurrence of this kind of plant species on soils that are enriched in the elements.Hypothesis for the ecological significance of hyperaccumulation include drought tolerance, allelopathy, and chemical defense against herbivores and/or pathogens (Boyd & Martens, 1993;Jhee et al., 1999, Galeas et al., 2007).Some plant species are known to hyperaccumulate more than one metals or trace elements.At least 400 plant species in 45 plant families are hyperaccumulators, and these have been found in many different geographic locations (Reeves & Baker, 2000).Hyperaccumulation of Se has been observed in the plant families Asteraceae, Brassicaceae, Chenopodiaceae, Lecythidaceae, Fabaceae, Rubiaceae and Scrophulariaceae, are only found on seleniferous soils (Beath et al., 1934;Cannon, 1960;Reeves & Baker, 2000).The Se accumulator species can tolerate Se in the field up to 10.000 mg kg -1 DW (Pilon-Smits & Quinn, 2010).
Plants differ in their ability to accumulate Se when they grow on seleniferous soils, and divided into three groups according to Se accumulation capacity: primary accumulators (hyperaccumulators), secondary accumulators, and non-accumulators.Accumulator plants can accumulate from hundreds to several thousand milligrams of Se kg −1 dry weight in their tissues, without any negative effects.That ability is mainly due to the reduction of the intracellular Se concentration of Se-Cys and Se-Met which are normally incorporated into proteins (Pilon-Smits & Quinn, 2010).
Special attention should be paid to A. bisulcatus, since this is the best-characterized Se accumulator.This species grows on naturally Se-rich soils in the southwestern part of the USA.Typical for these plants is their strong Se (sweet) odor.In its natural habitat, that species can take up to 0.65% Se dry weight in their shoots (Dumont et al., 2006).When the plants are grown on a selenate rich soil, the older leaves contain mainly inorganic Se (91%), whereas in the young leaves 90-95% of the Se is organic.The roots show the lowest Se level when compared with others tissues.Although, at root level, the Se is mainly organic (92%).
There is a presumption that the MeSeCys in the young leaves is metabolized and that the Se is reoxidized to form selenate as the leaves become older.Another explanation would be that the MeSeCys is exported from the young shoots as it ages and accumulates in the even younger shoots.An alternative explanation is the metabolization of MeSeCys to DMSe, which would explain the malodorous nature of the plant used for protection against insect attack.In these plants, the main compound found is MeSeCys, one of the common species found in Se accumulators (Dumont et al., 2006;Pickering et al., 2003).
On the nonaccumulators side, most forage and crop plants, as well as grasses, contain less than 25 mg Se kg −1 dry weight and do not accumulate Se much (Brown & Shrift, 1982).Although Se accumulators grow on seleniferous soils, not all plant species on seleniferous soils are Se accumulators: some plants accumulate only a few milligrams of Se kg −1 dry weight.For example, the genus Astragalus contains both Se accumulating species and nonaccumulating species, and they can grow next to each other on the same soil (Duckart et al., 1992).
Primary accumulators have discrimination coefficients (DC i = [Se/S] plant /[Se/S] solution ) of more than one in solution culture, and have concentrations of Se in the range of thousands of mg per kg dry weight (Ellis & Salt, 2003).Primary accumulators include various Astragalus species, which are members of the Fabaceae, as well as Stanleya pinnata, a member of the Brassicaceae (Feist & Parker, 2001).Secondary accumulators take up Se in proportion to the amount of Se available in the soil, they have a DC i of less than one, and tissue concentrations of Se in the hundreds of mg kg -1 (Bell et al., 1992).Members of this group include species of Astragalus, Aster, Atriplex and Melilotus, as well as Brassica juncea (Indian mustard) (Banuelos & Meek, 1990;Guo & Wu, 1998;Ellis & Salt, 2003).Recently, Ari and her colleagues has identified an Astragalus species, A. chrysochlorus, as a new secondary Seaccumulator plant with a typical Se concentration of more than several hundred milligrams of Se kg −1 dry weight in tissues (DC i =0.95) when grown on tissue culture media containing sodium selenate (Ari et al., 2010).
Selenium hyperaccumulations may increase the surrounding soil Se concentrations (phytoenrichment).The enhanced soil Se contents around hyperaccumulators can impair the growth of Se-sensitive plant species, pointing to a possible role of Se hyperaccumulation in elemental allelopathy (El Mehdawi et al., 2011).Selenium also may increase the tolerance of the plants to drought-induced oxidative damage and high temparature stress by enhancing their antioxidant defense and methylglyoxal detoxification system (Hasanuzzaman & Fujita, 2011;Djanaguiraman et al., 2010).

Incorporation of Se into protein
In most of the selenoproteins discovered so far, selenium is present as a selenocysteine residue that is integrated into the main chain of amino acids, as was first demonstrated for formate dehydrogenase and glutathione peroxidase (Birrigger et al., 2002).Whenever investigated, the selenocysteine residue was shown to be of pivotal importance for the catalytic efficiency of such proteins.The incorporation of selenocysteine into these selenoproteins is directed by a specific tRNA that recognizes a UGA codon.Normally, the UGA codon acts to terminate translation.In combination with a selenocysteine insertion sequence (SECIS), however, the UGA codon is recognized by the selenocysteine tRNA, which directs the insertion of selenocysteine (Gladyshev & Kryukov, 1999;Low and Berry, 1996;Ellis & Salt, 2003).
Organisms that require Se for normal cellular function contain essential selenoproteins, such as formate dehydrogenase, glutathione peroxidase, and selenophosphate synthase.
Reports have suggested the presence of selenoproteins in plants, but there is no direct evidence for the specific incorporation of selenocysteine in vascular plants.However, plants are thought to assimilate SeCys, where SeCys is metabolized to SeMet, which are both nonspecifically incorporated into proteins.SeCys is formed by the action of Cys synthase, which couples selenide with O-acetylserine (Ng & Anderson, 1978) (Figure 3).GS-Se − may be the physiological substrate of Cys synthase rather than free Se 2− (Tsang & Schiff, 1978).Kinetic studies of in vitro enzymes were showed that cystathionine-synthase exhibited a preference for SeCys: It had a higher affinity for SeCys (K m =70 µM) than for Cys (K m =240 µM) (Dawson & Anderson, 1988).Cystathionine-lyase did not differentiate between the Se and S forms of cystathionine, since the enzyme had a similar affinity for cystathionine (K m =0.31 mM) and selenocystathionine (K m =0.35 mM) (McCluskey et al., 1986).The most likely enzyme for the synthesis of SeMet from SeHomoCys is the cytosolic enzyme, Met synthase (Figure 3).Selenium is readily incorporated into proteins in nonaccumulator plants treated with Se (Brown & Shrift, 1982).The incorporation into proteins occurs through the nonspecific substitution of SeCys and SeMet in place of Cys and Met, respectively (Figure 3).Studies showed that both Met and SeMet are substrates for the methionyl t-RNA synthetase (Terry et al., 2000).SeCys lyase is a Se-specific enzyme, whereas all the other enzymes shown recognize both S and Se.The only exception is Met synthase, which is involved in Met synthesis, and is very likely to be involved in SeMet synthesis although there is no evidence supporting this (from Terry et al., 2000).

Health benefits of Selenium
Severe selenium deficiency of human and animals have been observed in isolated seleniumpoor areas.Although Se deficiency is rare in the US, it does occur in several parts of the world, such as China, where concentrations of Se in the soil are low.Consumption of food containing less than 0.1 mg Se kg −1 results in deficiency.Regular consumption of food containing more than 1 mg Se kg −1 results in only toxicity, but 1000 mg Se kg −1 DW can lead to acute Se poisoning and death for humans and animals (Wilber, 1980).
In the 1960s, selenium was proposed to be an essential trace element as a consequence of human and animal studies (Birringer et al., 2002).Since that time, scientists have showed growing interest on Se studies.The toxic effects of excess Se have been known for some time but, in the past decade, it has become more evident that Se has many potential health benefits beyond meeting basic nutritional requirements.In the seventies, Chinese scientists reported that severe selenium deficiency causes diseases in humans: Keshan disease, which is a fatal cardiomyopathy, and Kashin-Beck disease, a disabling chondronecrosis (Birringer et al., 2002).In addition, Se deficiency can lead to heart disease, hypothyroidism and a weakened immune system (Combs, 1980).Concerns about the health hazards from overexposure now tend to become overwhelmed by a bewildering discussion of the benefits.Adequate alimentary selenium supply is claimed to delay the onset of ageing, cardiovascular diseases and cancer, to enable an optimum immune response, to guarantee an appropriate function of the endocrine system, and to be indispensable for male reproduction.For example, in a long-term double-blind studies, supplemental Se was associated with significant reductions in lung, colorectal and prostate cancers (Ip & Ganther, 1992).In 1996, Clark and co-workers reported that supplementation of people with selenized yeast is capable of reducing the overall cancer morbidity by nearly 50%.The possible anti-cancer effect of Se might be summarize according to critical reviews on this area that appear to be widely accepted (Birringer et al., 2002): optimize somehow glutathione peroxidases (GPx) activities; provides optimum selenoprotein expression; the nature of the Se compound has critical importance; although, synthetic selenium compounds do not support selenoprotein synthesis, but also found to be anticarcinogenic."Need to be proved" advances have led to several mechanisms being proposed for the anticancer activity of Se: antioxidant protection (via selenoproteins); altered carcinogen metabolism; enhanced immune surveillance; regulation of cell proliferation and tumor cell invasion and inhibition of neoangiogenesis (Zeng & Combs, 2008).

Molecular approaches to alter Se metabolism in plants
Several different transgenics have been obtained so far.They were showed enhanced Se tolerance, accumulation, and assimilation from inorganic to organic Se, and volatilization.Selenium accumulation was up to nine-fold higher than wild type and volatilization up to three-fold faster, under laboratory conditions.These findings may be useful for cleaning up of excess levels of Se in the environment and also as fortified foods to prevent Se deficiency related diseases.For example, accumulators of MeSeCys would be especially useful for the anticarcinogenic purpose (Unni et al. 2005).In a first step to assess the transgenics' potential for phytoremediation or as Se-fortified food, they were tested for their capacity to accumulate Se from naturally seleniferous soil and from Se-contaminated sediment.
The genetic engineering strategcy for biofortification and phytoremediation are both the same: higher Se levels in harvestable plant parts are purpose.A significant difference between genetic engineering for biofortification and phytoremediation objectives that Se in tissues of renewable plants should not get through toxic concentrations.In biofortification case, some Se compounds have more powerful anti-carcinogenic properties than the others.For example, MeSeCys is the best form of Se to use in biofortified foods, and according to that reason overexpression of SMT may be the best purpose of biofortification.It may also be possible to overexpress some targeted gene(s) in specific plant tissues, such as in the grain, or to overexpress these targeted genes so that anticarcinogenic Se compounds can be readily extracted for production.

Phytoremediation and biofortification
Selenium accumulator plants can convert inorganic selenate and selenite to SeCys and other organic selenocompounds, including volatile forms.Se hyperaccumulators may have special metabolic pathways for methylation of SeCys and the conversion of methyl-SeCys to volatile DMDSe.Transgenic approaches have been used to further enhance plant Se accumulation, tolerance, and volatilization (Table 3).
Selenate is translocated without chemical modification through the xylem to the leaves after its root absorption via the sulfate transporter (De Souza et al. 1998, Zayed et al. 1998).
Afterwards, selenate is metabolized by the enzymes responsible of sulfate assimilation when it enters chloroplasts.ATP sulfurylase catalyzes the key step in the reduction of selenate by activating it to adenosine phosphoselenate (APSe), an activated form of selenate.In vitro ATP sulfurylase has been shown to activate selenate, as well as sulfate (Burnell 1981;Dilworth & Bandurski, 1977;Shaw& Anderson, 1972).A gene construct containing the A. thaliana aps1 gene (Leustek et al. 1997), with its own chloroplast transit sequence, fused to the Cauliflower Mosaic Virus 35S promoter cloned into indian mustard plants to overexpress ATP sulfurylase.Molecular studies provided in vivo evidence that ATP sulfurylase is responsible for selenate reduction, and that this enzyme is rate limiting for selenate reduction and Se accumulation (Pilon-Smits et al. 1999).X-ray absorption spectroscopy (XAS) analysis of wild-type Indian mustard plants supplied with selenate showed that selenate was accumulated in both roots and shoots, but when selenite was supplied, an organo-Se compound (similar to SeMet) accumulated (De Souza et al. 1998).It is concluded that the reduction of selenate was rate limiting to selenate assimilation.This rate-limiting step was overcome in transgenic plants overexpressing ATP sulfurylase because these plants accumulated a SeMet-like compound when supplied with selenate (Pilon-Smits et al. 1999).In another study, transgenic A. thaliana overexpressing both ATP sulfurylase and APR (APS reductase) had a significant enhancement of selenate reduction as a proportion of total Se, whereas SAT (serine acetyl transferase) overexpression resulted in only a slight increase in selenate reduction to organic forms.In general, total Se accumulation in shoots was lower in the transgenic plants overexpressing ATPS, PaAPR (P.aeruginosa APR), and SAT.Root growth was adversely affected by selenate treatment in both ATPS and SAT overexpressors and less so in the PaAPR transgenic plants.It is concluded that ATPS and APR are major contributors of selenate reduction in planta.However, Se hyperaccumulation in Astragalus is not driven by an overall increase in the capacity of these enzymes, but rather by either an increased Se flux through the S assimilatory pathway, generated by the biosynthesis of the sink metabolites MeCys or MeSeCys (Sors et al., 2005).
The dominant adenosine 5'-phosphosulfate reductase (APR2) in A. thaliana converts activated sulfate to sulfite, a key reaction in the sulfate reduction pathway.apr2-1 transgenic plants had decreased selenate tolerance and photosynthetic efficiency.Sulfur metabolism was perturbed in apr2-1 plants grown on selenate, as observed by an increase in total sulfur and sulfate, and a 2-fold decrease in glutathione concentration.Knockout of APR2 also increased the accumulation of total selenium and selenate.However, the accumulation of selenite and selenium incorporation in protein was decreased in apr2-1 mutants.Decreased incorporation of selenium in protein is typically associated with increased selenium tolerance in plants.However, because the apr2-1 mutant exhibited decreased tolerance to selenate, Grant et al. (2011) proposed that selenium toxicity can also be caused by selenate's disruption of glutathione biosynthesis leading to enhanced levels of damaging reactive oxygen species.
As described above, selenium can be assimilated and volatilized via the sulfate assimilation pathway.Cystathionine--synthase (CgS) is the enzyme which catalyzes the synthesis of Se-cystathionine from Se-cysteine, the first step in the conversion of Secysteine to volatile dimethylselenide.Overexpression of CgS in B. juncea, the first enzyme in the conversion of SeCys to SeMet, resulted in two to threefold higher volatilization rates compared to untransformed control plants (Van Huysen et al., 2003).The CgS transgenics accumulated 40% less Se in their tissues than wild type probably as a result of their enhanced volatilization.Probably due to their lower tissue Se levels the CgS transgenics were also more Se tolerant than wildtype plants.Van Huysen et al. (2003) studied APS and CgS transgenics to evaluate for their capacity to accumulate Se from soil that is naturally rich in Se.In that study, wild-type Indian mustard and the Se hyperaccumulator S. pinnata were used for comparison.After growing 10 weeks on Se soil, similar to those of S. pinnata, the APS transgenics contained 2,5-fold higher shoot Se levels than wild type Indian mustard.The CgS transgenics contained 40% lower shoot Se levels than wild type.These findings were very significant that they are the first report on the performance of transgenic plants on Se in soil and they showed the potential of genetic engineering for phytoremediation.
Selenocysteine lyase (SL) catalyzes the removal of selenium from L-selenocysteine to yield L-alanine.This enzyme is proposed to have a role in the recycling of the micronutrient selenium from degraded selenoproteins which contain selenocysteine residue.Selenocysteine lyase has a strict substrate specificity for L-selenocysteine and no activity for L-cysteine.However, it is unknown how the enzyme distinguishes between selenocysteine and cysteine.To manipulate plant Se metabolism, another genetic engineering approach is the prevention of the toxic process of its nonspecific incorporation into proteins.A mouse SL was expressed in A. thaliana and B. juncea (Pilon et al. 2003;Garifullina et al. 2003).Selenocysteine lyase enzyme specifically breaks down SeCys into alanine and elemental Se.The SL transgenics showed reduced Se incorporation into proteins.Se tolerance increased when mouse SL was expressed in the cytosol of A. thaliana, but decreased when it was expressed in the chloroplast (Pilon et al. 2003).All the transgenic SL plants showed enhanced Se accumulation, up to twofold compared to wildtype plants.Similar results were obtained when an A. thaliana homologue of the mouse SL (called CpNifS) was discovered and overexpressed: the CpNifS transgenics showed less Se incorporation in proteins, twofold enhanced Se accumulation, as well as enhanced Se tolerance (Van Hoewyk et al., 2005).This enzyme has been cloned from A. thaliana and expression of this gene in B. juncea originally appeared to reduce selenate toxicity, and Banuelos et al. (2007) attributed this to a reduction in incorporation of Se into proteins.The gene used in this study may be similar to the AtCpNifS chloroplast gene used by Van Hoewyk et al. (2005).
In Arabidopsis genome, there are three highly conserved homologues of the mammalian 56-kD selenium-binding protein (SBP).A transgenic approach is used to study the function of SBP in this model plant by constitutively overexpressing and down-regulating the endogenous Atsbp1 gene.It was employed both a conventional antisense method and gene silencing by intron-containing hairpin RNAs.Atsbp1-overexpressing and silenced plants were phenotypically normal, under standard growth conditions, when compared with wild type plants.Transgenic plants exhibited different growth responses to exogenously supplied selenite, which correlated with the expression levels of Atsbp1.
Plants with increased Atsbp1 transcript levels showed enhanced tolerance to selenite, while plants with reduced levels were more sensitive.Results indicate that Atsbp1 appears to be involved in processes controlling tolerance of Arabidopsis to selenium toxicity (Agalou et al., 2005).A more distant related family of genes that well studied in A. thaliana, induce higher levels of binding polypeptides and proteins.It was recently found by Dutilleul et al. (2008) that expression of specific binding proteins for Se also delivered tolerance to cadmium (Cd), most likely also by binding this heavy metal (Dutilleul et al. 2008).
The Sultr 123 gene family orchestrate sulphate transporters, and by co-operation may also regulate Se transportation.Lydiate et al. (2007) used 'knock-down' technology in A. thaliana, determined that Sultr 123 genes reduced high affinity sulphate transporters transportation of Se and stated that reduced, but had little effect on selenite transportation (Table 3).The Sultr gene family are similar to the SHST family of sulphate transporter genes.
Se upregulates transcripts that regulate the synthesis and signaling of ethylene and jasmonic acid.Arabidopsis mutants which are defective in ethylene or jasmonate response pathways exhibited reduced tolerance to Se, therefore, it suggests an important role for these hormones in Se tolerance.Selenate upregulated a variety of transcripts that were also induced in stress conditions.Selenate seemed to repress plant development, as suggested by the downregulation of genes involved in cell wall synthesis and auxin-regulated proteins.By discovering the Se-responsive genes plants could be created that can better tolerate and accumulate Se, which may enhance the effectiveness of Se phytoremediation or serve as Sefortified food (Van Hoewyk et al. 2008).
MeSeCys is produced from selenocysteine and S-methylmethionine by SMT enzme.Neuhierl et al. (1999) were cloned successfully the gene encoding SMT from A. bisulcatus (AbSMT).This enzyme belongs to a class of methyltransferases involved in metabolism of Smethylmethionine.It shares significant sequence homology with homocysteine Smethyltransferases (HMT).Despite the fact that both SMT and HMT enzymes catalyze methyl transfer using S-methylmethionine as the methyl donor, they exhibit significant Secontaining (for SMT) and S-containing (for HMT) substrate choice as a methyl acceptor in vitro (Neuhierl and Bock, 1996;Ranocha et al., 2000).SMT was found to be constitutively expressed in roots and leaves of A. bisulcatus, and appear to be not affected by Se induction (Pickering et al., 2003).Heterologous expression of AbSMT in transgenic A. thaliana results in the synthesis of MeSeCys and its derivative -glutamyl-Se-MeSeCys, these are the compounds not natively produced in A. thaliana (Ellis et al., 2004).In transgenic Brassica juncea expressing AbSMT accumulated MeSeCys similarly (Le Duc et al., 2004).The SMT transgenics showed increased Se accumulation, in the form of methyl-SeCys, as well as increased Se tolerance.Se volatilization rates also enhanced with the expression of SMT, with more volatile Se synthesized in the form of DMDSe.In SMT expressing trangenics, Se tolerance, accumulation, and volatilization drawed the attention when the plants were supplied with selenite as opposed to selenate.In this manner, the conversion of selenate to selenite were thought to be a rate-limiting step for the production of SeCys.APS and SMT transgenics were hybridized to create double-transgenic plants that overexpress both APS and SMT (APSxSMT plants) to deal with this rate-limitation.The APS x SMT double transgenics accumulated up to nine times higher Se levels than wild type (LeDuc et al. 2006).
The predominant form of the Se compounds in the double transgenics was methyl-SeCys.
The APSxSMT double transgenics accumulated up to eightfold more methyl-SeCys than wild type and almost two fold more than the only SMT transgenics.Se tolerance was similar in the single and double transgenics.On the other hand, McKenzie et al (2009) concluded that while the SMT gene from Se hyperaccumulators can probably be utilised universally to increase the metabolism of Se into MeSeCys, the effects of enhancing ATP sulfurylase activity could vary depending on the species involved.
The APS enzyme seems to be rate-limiting for the assimilation of selenate to organic Se compounds, and CgS enzyme is also rate-limiting for DMSe volatilization.Increased APS expression also appears to induce selenate uptake and Se and S accumulation, probably depending on upregulation of sulfate transporter expression.The results from the SL and CpNifS transgenics indicate that SeCys breakdown can decrease nonspecific incorporation of Se into proteins.This situation enhances Se tolerance because elemental Se does not involve with cellular processes.As mentioned above, in plants CpNifS functions in Se tolerance in nature is unknown; it's most serious function is in synthesis of iron-sulfur clusters (Van Hoewyk et al. 2007).The results from the SMT transgenics show that SMT is a key enzyme for Se hyperaccumulation, offering increased Se tolerance and accumulation when expressed in nonaccumulators.Nevertheless, for Se assimilation and detoxification, APS also needs to be overexpressed with SMT.APS x SMT double transgenics link the ability to reduce selenate to selenite and SeCys with the competence to methylate SeCys and thus to detoxify the internal Se.These studies suggest that through genetic manipulation of high biomass, fast-growing plants, Se phytoremediation and biofortification can be improved into a viable option, while producing crops with better nutritional quality.

Problems and future aspects
The possible transfer of undesirable traits to elite plants and crop cultivars for agriculture is an obvious concern over phytoremediation techniques, especially in using genetically modified plants (Hanson et al. 1997;Terry et al. 2000).The use of phyto-crops for food or animal consumption may be affected by hyperaccumulation and high levels of some elements, for example Se, into plants' part.However technology exists to identify the fate of most of these toxic compounds, and their toxicity as demonstrated by the development of chemo preventitive enriched Se accumulating (fortified) edible crop plants (e.g.potato, radish and other vegetables) in Australia, UK, USA and other parts of the world (Broadley et al. 2004;Lefsrud et al. 2006;Pedrero et al. 2006;Haug et al. 2007;Zhao et al. 2007).
To clean-up Se from constructed wetlands and their waters is the major environmental problem.An affective solution seems to be to use of 'artificially constructed wetlands'.For wetland efficiency for removal of Se the most suitable plant species should be planted and some species like cattail grass (high biomass) and widgeon grass (high amounts hyperaccummulated) removed the most Se in trials (Banuelos 2006;Nyberg 1991).The world Se resources need to be managed so that this non-renewable vulnerable resource is not squandered.Selenium uptake, mobilisation and assimilation are quite well understood and are similar to sulphur, however there are some steps not well understood, especially enzymatic and non-enzymatic steps about to the reduction of intermediates to selenide.
New genes and proteins will be discovered to improve Se tolerance, accumulation, and volatilization with the arrival of the genomic era.Also, comparative studies of Se hyperaccumulators and related nonaccumulators or of Se-tolerant ecotypes and non-tolerant ecotypes of the same species may reveal new genes that upregulate Se uptake, accumulation, and volatilization.Such new genes may not be involved in the commonly studied sulfur metabolism.For example, a Se-binding protein (SBP) homolog, when overexpressed in A. thaliana, increased tolerance to Se as well as cadmium (Agalou et al. 2005).SBP's function is unknown so far, but it has been hypothesized to be similar to glutathione.Moreover, recent genetic and genomic studies (Zhang et al. 2006;Tamaoki et al. 2008;Van Hoewyk et al 2008) have identified new quantitative trait loci (QTL) and genes involved in Se tolerance.The plant hormones jasmonic acid (JA) and ethylene are emerging as important players in plant responses to Se tolerance, possibly via their influence on S and Se assimilation.Further studies may reveal key genes that induce the responses that together provide Se tolerance and accumulation in model plants and hyperaccumulators.
These key genes could be the candidates for overexpression, producing the complete Se hyperaccumulation in plants.It is desirable to study the potential ecological implications of growing Se accumulating or volatilizing plants before existing and future transgenics are used at a large scale in the field for phytoremediation or as fortified foods.Additional considerations for the use of transgenics for phytoremediation are the same as those involved with growing transgenics for other purposes and should also be evaluated and weighed against the risks of alternative remediation methods.
Many molecular studies have been reported the overexpression of genes encoding proteins involved in Se uptake, transport and assimilation.In this way further strategies for genetic engineering of Se accumulation, transformation and toxicity will become evident, and the use of transgenic plants for use in a variety of ways could be evaluated.Phytoremediation offers a cost effective and environmentally friendly alternative or complementary technology to conventional bioremediation techniques.However the biological processes of phytoremediation are still largely unknown in many cases, and plant-microbe interactions, mechanisms of degradation and transformation, volatilisation, chelation, binding and detoxification need more detailed investigations.In this point of view there is value in enhancement of traits in plants useful in phytoremediation such as high biomass and growth potential in seleniferous soils, which might otherwise be considered agriculturally non-productive land.Se-hyperaccumulating plants (wether naturally occurring or transgenic plants) have possibilities in that they combine pollutant decontamination with production of a product with beneficial properties to humans and animals.

Conclusion
Building on the genomic and biochemical studies described above, follow-up research may reveal key genes that trigger the cascade of responses that provide Se tolerance and accumulation in model plants and hyperaccumulators.Also, genes may be found that encode specific transporters of selenocompounds into and within hyperaccumulators.Such key genes will be the ultimate candidates for overexpression studies, with the potential of transferring the complete Se hyperaccumulator profile into high-biomass species.Recent research has elucidated many important ecological interactions involving Se in plants.In this chapter, it has been focussed some important areas for future research.Particularly, more research is desirable on the role of soil microbes in plant Se uptake and volatilization, and the movement of Se through the food chain via Se hyperaccumulators or Se-fortified crop plants.The role of Se in below-ground ecological interactions with microbes and other organisms is also a fairly unexplored area.In addition to effects of Se on root-microbe interactions, Se may protect plants from root feeding herbivores, and selenocompounds released from hyperaccumulator roots may be toxic to surrounding vegetation.Similarly, the effects of Se on pollination ecology will be an interesting field of further study.Better knowledge of the processes involved in plant metabolism, the limiting factors involved, the contributions of ecological partners and the effects of Se on ecological partners are all useful for minimizing potential harmful effects of Se while benefiting from the positive effects of plant Se on animal and human health.
The capacity of plants to accumulate and volatilize Se will be very useful for the phytoremediation of Se-contaminated soils and waters (Banuelos and Meek 1990).When plant Se accumulation is well managed, this offers an efficient and cost-effective way to remove Se from the environment.Since plants are an effective source of dietary Se, Seenriched plant material from phytoremediation or other sources can be considered fortified food.After being grown on Se-contaminated soil or being irrigated with Se-contaminated water, the Se-laden plant material may be used as a feed supplement for livestock, or as a biofuel.If successful, the potential of this strategy may be further enhanced by the use of selected transgenic lines.Of course, any use of Se-accumulating wildtype or transgenic plants will need to be accompanied by careful risk assessment, to avoid escape of transgenes and any adverse ecological effects of the accumulated Se.

Fig. 2 .
Fig. 2. Schematic representation of the main steps of Se metabolism in plants (fromGerm et al., 2007b)

Fig. 3 .
Fig. 3. Schematic pathway of the incorporation of selenide into SeCys, SeMet, and proteins.SeCys lyase is a Se-specific enzyme, whereas all the other enzymes shown recognize both S and Se.The only exception is Met synthase, which is involved in Met synthesis, and is very likely to be involved in SeMet synthesis although there is no evidence supporting this (fromTerry et al., 2000).

Table 2 .
Plants that SMT gene isolated and characterized so far

Table 3 .
Molecular genetic studies on selenium tolerance and accumulation, including the origin of the genes (modified from De Filippis, 2010).