- Email: [email protected]
Genetically engineered phytoremediation: one man's trash is another man's transgene
Update
496
TRENDS in Biotechnology
10 Morris, R.J. and Potten, C.S. (1994) Slowly cycling (label-retaining) epidermal cells behave like clonogenic stem cells in vitro. Cell Prolif. 27, 279–289 11 Liang, L. and Bickenbach, J.R. (2002) Somatic epidermal stem cells can produce multiple cell lineages during development. Stem Cells 20, 21–31 12 Nishimura, E.K. et al. (2002) Dominant role of the niche in melanocyte stem-cell fate determination. Nature 416, 854–860 13 Kumamoto, T. et al. (2003) Hair follicles serve as local reservoirs of skin mast cell precursors. Blood 102, 1654–1660
Vol.22 No.10 October 2004
14 Gharzi, A. et al. (2003) Plasticity of hair follicle dermal cells in wound healing and induction. Exp. Dermatol. 12, 126–136 15 Jahoda, C.A. et al. (2003) Hair follicle dermal cells differentiate into adipogenic and osteogenic lineages. Exp. Dermatol. 12, 849–859
0167-7799/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2004.08.007
Genetically engineered phytoremediation: one man’s trash is another man’s transgene Clayton L. Rugh Michigan State University, Department of Crop and Soil Sciences, 516 Plant and Soil Science Building, East Lansing, Michigan, 48824-1325, USA
Plant-based environmental remediation, or phytoremediation, has been widely pursued in recent years as a favorable clean-up technology and is an area of intensive scientific investigation. For the vast majority of field applications, vegetative ‘phyto-crops’ are selected specifically for their capacity for site decontamination and not for additional concurrent or post-remediation utility. By contrast, a recent publication by Ellis and colleagues highlights potential anti-carcinogenic uses for plants genetically engineered primarily for detoxification of selenium-polluted soils and sediments. Environmental pollution impacts on human health, productivity of agricultural lands and the stability of natural ecosystems. In addition, low-level, large-scale contamination presents monumental economic and logistical barriers to effective and timely treatment. Aggressive engineering-based technologies are typically applied to cleanup of more acutely polluted point sources, though these are often not cost effective or environmentally justified for risk mitigation at marginally impacted sites. Excavation and entombment are also unlikely largely due to diminishing hazardous materials landfill capacity. Subsequently, in situ biological remediation could be the most appropriate corrective option for treatment of widespread, low immediacy contamination. Bioremediation, typically referring to microbial processes, and phytoremediation, or plant-based cleanup, have been implemented against a broad spectrum of hazardous elements and compounds. Engineering-based approaches are usually more rapid, although biological methods can be far less expensive and more ecologically compatible. Phytoremediation provides the ancillary benefits of concurrent site stabilization through erosion mitigation and hydraulic control of solubilized contaminants. Many varied plant taxa possess remarkable natural abilities for metal Corresponding author: Clayton L. Rugh ([email protected]). Available online 28 August 2004 www.sciencedirect.com
phytoextraction or organic compound phytodegradation. Superior phytoremediation species have been characterized at the physiological, biochemical, and molecular genetic levels to identify specific processes for further improvement via genetic manipulation. Toward this goal, laboratory model plants, such as Thale cress (Arabidopsis thaliana) and tobacco (Tabacum nicotiana) have been engineered with non-plant transgenes to enhance phytoremediation effectiveness against such priority pollutants as organomercurials [1], trichloroethylene solvents [2], and nitroaromatic explosives [3]. Since these early reports, additional plant and non-plant genetic resources have been used for biotechnological improvement of phytoremediation capability. Among these, Ellis and colleagues were unique in their identification and transfer of the pivotal trait from a metal-hyperaccumulating species to non-hyperaccumulating Arabidopsis, conferring increased selenium (Se) tolerance and phytoaccumulation [4]. Most notable, however, was their demonstration that transgenic expression of selenocysteine methyltransferase (SMT) not only enhanced phytoextraction of a hazardous pollutant, but also biochemically transformed toxic mineral selenite into a seleno-compound with potent anti-carcinogenic nutritional properties. Phytomining of toxic and nutrient metals The initial publications of metal-hyperaccumulating plants recognized their historical utility in ‘phytoprospecting’ for identification of sought-after metal deposits [5]. These observations led to applied re-vegetation of barren, industrially impacted soils and metal recovery (‘phytomining’) of useful metallic materials, which was for all practical purposes the ‘invention’ of phytoremediation technology [6]. Effective recycling of phytoextracted metals has been achieved in pilot-scale demonstrations for nickel [7] and thallium [8]. Plant-facilitated recovery and reuse of soil metal is likely to be achievable and desirable for most metal pollutants except those having little economic value
Update
TRENDS in Biotechnology
or industrial utility (e.g. mercury, lead, and arsenic) which will need to be safely processed as hazardous wastes instead. Similarly, only biologically beneficial minerals are candidates for combined phytoextraction and use as dietary supplements, unlike harmful, non-essential metals such as the nutrient elemental periodic table ‘cousins’, cadmium and cesium. This notion was summarized in a treatise promoting the coordination of trace element phytoextraction processes and crop fortification for such essential nutrients as iron, selenium and zinc [9]. This article detailed different physiological processes – soil-metal decomplexation, root uptake, plant tolerance, and shoot accumulation – that are each components of effective phytoremediation and production of mineralenriched vegetation. Ongoing research projects using high-throughput trace metal profiling of mutant Arabidopsis lines [10] and bioinformatics analysis of metal transporter gene sequence data [11] have tremendously advanced our capacity for rational engineering of improved trace element phytosequestration. Indian mustard (Brassica juncea) possesses many of the qualities desired for an effective phytoremediation species, including rapid growth, high biomass, and appreciable trace metal extraction rates. B. juncea selenium metabolism has been systematically analyzed to elucidate major steps in transport, methylation, volatization, and detoxification [12,13]. These studies identified opportunities for improved Se phytoremediation through analysis of target-gene expression [14] or complementation with foreign genes [15]. In addition to environmental cleanup, Se-enriched vegetation has been shown to serve as an effective nutritional supplement for livestock. Biological transfer studies showed that laboratory rats displayed lower frequency of pre-cancerous lesions when supplied with Se-laden vegetation compared with those fed mineral Se supplements [16]. GM methylselenocysteine plants Selenium hyperaccumulation has been extensively studied among Astragalus species, particularly A. bisulcatus [17]. The primary mode of Se toxicity in non-tolerant species is incorporation of Se-analogs of cysteine and methionine into proteins impairing structural or enzymatic function. This pathway is effectively avoided in Astragalus hyperaccumulators by formation of non-toxic methylselenocysteine (MeSeCys), which is excluded from proteins [17]. To block Se-Cys incorporation into proteins, A. bisulcatus expresses seleno-Cys methyltransferase, an enzyme unique to the Astragalus spp. [18]. In addition to its reduced cellular toxicity, MeSeCys has been shown to possess potent anticarcinogen properties in laboratory animal studies and as a human dietary supplement [19,20]. The ability of A. bisulcatus to transform toxic forms of selenium to compounds with chemopreventative qualities led to its consideration as a genetic resource for improvement of more agronomically compatible plant species [21]. Ellis and colleagues advanced this concept by isolating the A. bisulcatus SMT1 sequence encoding the SMT enzyme [22] and transforming Arabidopsis plants with SMT1 plant expression constructs [4]. Transgenic plants displayed greatly increased tolerance to selenite relative to www.sciencedirect.com
Vol.22 No.10 October 2004
497
control plants with close correlation of relative biomass accumulation on selenite-media and SMT expression levels, supporting the hypothesis that MeSeCys biosynthesis is the principal mechanism for Se detoxification observed in A. bisulcatus. SMT1-Arabidopsis plants were not tolerant to selenate, however, which was expected given the inefficient selenate reduction observed in this species. Comprehensive spectroscopic analysis revealed that SMT1-Arabidopsis plants accumulate methylated Se compounds, MeSeCys and gGluMeSeCys, and total Se at foliar concentrations approaching those of naturally occurring Se hyperaccumulating species. SMT expression was shown to alter the metabolic fate of accumulated selenite conferring Se-resistance to transgenic lines by conversion to anti-carcinogenic methylseleno compounds. Such biotechnological coupling of Se phytoextraction and chemo-preventative enrichment advances the concept of dual-purpose environmental cleanup and nutritionally enhanced crop production on sites that otherwise have limited utility. Para-phytoremediation Perhaps such mixed-benefit strategies could be considered ‘para-phytoremediation’, drawing on the meanings ‘beside’, ‘alongside’, and ‘beyond’ for the prefix ‘para’. This etymological construction recognizes concurrent or post-remediation uses for phytoremediation plants in addition to environmental detoxification. As previously described, nutritional enhancement is one such worthwhile outcome, although this strategy is restricted to biologically essential trace elements. Similarly, metal recovery and recycling are limited to materials with commercial value. What products could be obtained from plant tissues loaded with hazardous levels of valueless metals? In addition, what para-phytoremediation benefits could be derived from trees, shrubs, and grasses used for phytodegradation or rhizosphere-assisted biodegradation of organic pollutants? Energy is one commodity that could be obtained via the same procedure – incineration – that must be performed for post-harvest treatment of hyperaccumulator plant biomass to realize the reduced waste volume or metal recovery benefits. Energy production strategies have been implemented for fast-growing, shortrotation trees used to treat wastewater and biosludges [23] and have great potential as a sustainable biofuel provided certain economic breakpoints are achieved [24]. Interestingly, wood biomass can provide comparable energy production to coal and, in the case of organic phytoremediation, burns more cleanly and cheaply owing to the low sulfur content and reduced need for emission controls. Perhaps environmental remediation projects could also serve as biofiber resources for any number of industries, including lumber, cellulose products, and agri-ethanol feedstock. One attractive benefit of this proposition is that these ‘crops’ could be grown on otherwise non-productive lands to offset cost-intensive remediation activities. Conclusions The pace of published reports of genetically improved phyto-crops has increased steadily in recent years demonstrating the feasibility of mixed-use strategies [25,26]. Given the vast sum of plant bioscience funding
Update
498
TRENDS in Biotechnology
directed towards improvement of agricultural qualities, perhaps these goals can be combined to increase tolerance to temperature extremes, salinity, flooding, or insect pests in plants capable of pollutant detoxification or, more importantly for value-enhancement – transfer of phytoremediative traits to elite plant cultivars having the highest biomass or agricultural productivity. Obviously, concerns about contaminant uptake and accumulation will limit the use of phyto-crops for food or human contact products, so every effort must be made to identify parent compound fate and toxicity for these applications. However, as observed with the development of chemopreventative enriched, Se-hyperaccumulating plants, opportunities exist to combine pollutant decontamination capabilities with beneficial human and ecological health qualities in engineered plants. References 1 Bizily, S.P. et al. (1999) Phytoremediation of methylmercury pollution: merB expression in Arabidopsis thaliana confers resistance to organomercurials. Proc. Natl. Acad. Sci. U. S. A. 96, 6808–6813 2 Doty, S.L. et al. (2000) Enhanced metabolism of halogenated hydrocarbons in transgenic plants containing mammalian cytochrome P450 2E1. Proc. Natl. Acad. Sci. U. S. A. 97, 6287–6291 3 French, C.E. et al. (1999) Biodegradation of explosives by transgenic plants expressing pentaerythritol tetranitrate reductase. Nat. Biotechnol. 17, 491–494 4 Ellis, D.R. et al. (2004) Production of Se-methylselenocysteine in transgenic plants expressing selenocysteine methyltransferase. BMC Plant Biol. 4, 1 5 Baker, A.J.M. and Brooks, R.R. (1989) Terrestrial higher plants which hyper-accumulate metallic elements: a review of their distribution, ecology and phytochemistry. Biorecovery 1, 81–126 6 Baker, A.J.M. et al. (1994) The possibility of in situ heavy metal decontamination of polluted soils using crops of metal-accumulating plants. Res. Conserv. Recyc. 11, 41–49 7 Li, Y.M. et al. (2003) Development of the technology for commercial phytoextraction of nickel: economic and technical considerations. Plant Soil 249, 107–115 8 LaCoste, C. et al. (2001) Uptake of thallium by vegetables: Its significance for human health, phytoremediation, and phytomining. J. Plant Nutr. 24, 1205–1215 9 Guerinot, M.L. and Salt, D.E. (2001) Fortified foods and phytoremediation. Two sides of the same coin. Plant Physiol. 125, 164–167
Vol.22 No.10 October 2004
10 Lahner, B. et al. (2003) Genomic scale profiling of nutrient and trace elements in Arabidopsis thaliana. Nat. Biotechnol. 21, 1215–1221 11 Maser, P. et al. (2001) Phylogenetic relationships within cation transporter families of Arabidopsis. Plant Physiol. 126, 1646–1667 12 de Souza, M.P. et al. (1998) Rate-limiting steps in selenium assimilation and volatilization by Indian mustard. Plant Physiol. 117, 1487–1494 13 Singh, M. and Singh, N. (1977) Effect of sulfur and selenium on sulfurcontaining amino-acids and quality of oil in Raya (Brassica juncea Coss) in normal and sodic soil. Ind. J. Plant Physiol. 20, 56–62 14 Garifullina, G.F. et al. (2003) Expression of a mouse selenocysteine lyase in Brassica juncea chloroplasts affects selenium tolerance and accumulation. Physiol. Plant. 118, 538–544 15 Berken, A. et al. (2002) Genetic engineering of plants to enhance selenium phytoremediation. Crit. Rev. Plant Sci. 21, 567–582 16 Banuelos, G.S. et al. (2002) Biotransfer possibilities of selenium form plants used in phytoremediation. Int. J. Phytoremediation 4, 315–329 17 Brown, T.A. and Shrift, A. (1981) Exclusion of selenium from proteins in selenium-tolerant Astragalus species. Plant Physiol. 67, 1951–1953 18 Neuhierl, B. and Bock, A. (1996) On the mechanism of selenium tolerance in selenium-accumulating plants: purification and characterization of a specific selenocysteine methyltransferase from cultured cells of Astragalus bisulcatus. Eur. J. Biochem. 239, 235–238 19 Combs, G.F. and Gray, W.P. (1998) Chemopreventative agents. Selenium. Pharmacol. Ther. 79, 179–192 20 Medina, D. et al. (2001) Se-methylselenocysteine: a new compound for chemoprevention of breast cancer. Nutr. Cancer 40, 12–17 21 Orser, C.S. et al. (1999) Brassica plants to provide enhanced human mineral nutrition: selenium phytoenrichment and metabolic transformation. J. Med. Food 1, 253–261 22 Pickering, I.J. et al. (2003) Chemical form and distribution of selenium and sulfur in the selenium hyperaccumulator Astragalus bisulcatus. Plant Physiol. 131, 1–8 23 Pulford, I.D. et al. (2002) Heavy metal uptake by willow clones from sewage sludge-treated soil: the potential for phytoremediation. Int. J. Phytoremediation 4, 59–72 24 Bungay, H. (2004) Confessions of a bioenergy advocate. Trends Biotechnol. 22, 67–71 25 Pilon-Smits, E. and Pilon, M. (2002) Phytoremediation of metals using transgenic plants. Crit. Rev. Plant Sci. 21, 439–456 26 Rugh, C.L. (2001) Mercury detoxification with transgenic plants and other biotechnological br eakthroughs for phytoremediation. In Vitro Cell. Dev. Biol. Plant 37, 321–325
0167-7799/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2004.08.003
Exploring the post-transcriptional RNA world with DNA microarrays Vishwanath R. Iyer Center for Systems and Synthetic Biology, and Institute for Cellular and Molecular Biology, University of Texas at Austin, 1 University Station A4800, Austin, TX 78712-0159, USA
Genomic approaches are valuable for understanding the complex layer of gene regulation that involves the control of RNA processing, localization and stability. Recent work Corresponding author: Vishwanath R. Iyer ([email protected]).
www.sciencedirect.com
provides a prime example of the power of unbiased microarray-based methods to discover unexpected functions for proteins in the RNA world. The challenges ahead relate to extending such approaches to larger genomes and to integrating this type of information with that generated by standard expression profiling.
496
TRENDS in Biotechnology
10 Morris, R.J. and Potten, C.S. (1994) Slowly cycling (label-retaining) epidermal cells behave like clonogenic stem cells in vitro. Cell Prolif. 27, 279–289 11 Liang, L. and Bickenbach, J.R. (2002) Somatic epidermal stem cells can produce multiple cell lineages during development. Stem Cells 20, 21–31 12 Nishimura, E.K. et al. (2002) Dominant role of the niche in melanocyte stem-cell fate determination. Nature 416, 854–860 13 Kumamoto, T. et al. (2003) Hair follicles serve as local reservoirs of skin mast cell precursors. Blood 102, 1654–1660
Vol.22 No.10 October 2004
14 Gharzi, A. et al. (2003) Plasticity of hair follicle dermal cells in wound healing and induction. Exp. Dermatol. 12, 126–136 15 Jahoda, C.A. et al. (2003) Hair follicle dermal cells differentiate into adipogenic and osteogenic lineages. Exp. Dermatol. 12, 849–859
0167-7799/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2004.08.007
Genetically engineered phytoremediation: one man’s trash is another man’s transgene Clayton L. Rugh Michigan State University, Department of Crop and Soil Sciences, 516 Plant and Soil Science Building, East Lansing, Michigan, 48824-1325, USA
Plant-based environmental remediation, or phytoremediation, has been widely pursued in recent years as a favorable clean-up technology and is an area of intensive scientific investigation. For the vast majority of field applications, vegetative ‘phyto-crops’ are selected specifically for their capacity for site decontamination and not for additional concurrent or post-remediation utility. By contrast, a recent publication by Ellis and colleagues highlights potential anti-carcinogenic uses for plants genetically engineered primarily for detoxification of selenium-polluted soils and sediments. Environmental pollution impacts on human health, productivity of agricultural lands and the stability of natural ecosystems. In addition, low-level, large-scale contamination presents monumental economic and logistical barriers to effective and timely treatment. Aggressive engineering-based technologies are typically applied to cleanup of more acutely polluted point sources, though these are often not cost effective or environmentally justified for risk mitigation at marginally impacted sites. Excavation and entombment are also unlikely largely due to diminishing hazardous materials landfill capacity. Subsequently, in situ biological remediation could be the most appropriate corrective option for treatment of widespread, low immediacy contamination. Bioremediation, typically referring to microbial processes, and phytoremediation, or plant-based cleanup, have been implemented against a broad spectrum of hazardous elements and compounds. Engineering-based approaches are usually more rapid, although biological methods can be far less expensive and more ecologically compatible. Phytoremediation provides the ancillary benefits of concurrent site stabilization through erosion mitigation and hydraulic control of solubilized contaminants. Many varied plant taxa possess remarkable natural abilities for metal Corresponding author: Clayton L. Rugh ([email protected]). Available online 28 August 2004 www.sciencedirect.com
phytoextraction or organic compound phytodegradation. Superior phytoremediation species have been characterized at the physiological, biochemical, and molecular genetic levels to identify specific processes for further improvement via genetic manipulation. Toward this goal, laboratory model plants, such as Thale cress (Arabidopsis thaliana) and tobacco (Tabacum nicotiana) have been engineered with non-plant transgenes to enhance phytoremediation effectiveness against such priority pollutants as organomercurials [1], trichloroethylene solvents [2], and nitroaromatic explosives [3]. Since these early reports, additional plant and non-plant genetic resources have been used for biotechnological improvement of phytoremediation capability. Among these, Ellis and colleagues were unique in their identification and transfer of the pivotal trait from a metal-hyperaccumulating species to non-hyperaccumulating Arabidopsis, conferring increased selenium (Se) tolerance and phytoaccumulation [4]. Most notable, however, was their demonstration that transgenic expression of selenocysteine methyltransferase (SMT) not only enhanced phytoextraction of a hazardous pollutant, but also biochemically transformed toxic mineral selenite into a seleno-compound with potent anti-carcinogenic nutritional properties. Phytomining of toxic and nutrient metals The initial publications of metal-hyperaccumulating plants recognized their historical utility in ‘phytoprospecting’ for identification of sought-after metal deposits [5]. These observations led to applied re-vegetation of barren, industrially impacted soils and metal recovery (‘phytomining’) of useful metallic materials, which was for all practical purposes the ‘invention’ of phytoremediation technology [6]. Effective recycling of phytoextracted metals has been achieved in pilot-scale demonstrations for nickel [7] and thallium [8]. Plant-facilitated recovery and reuse of soil metal is likely to be achievable and desirable for most metal pollutants except those having little economic value
Update
TRENDS in Biotechnology
or industrial utility (e.g. mercury, lead, and arsenic) which will need to be safely processed as hazardous wastes instead. Similarly, only biologically beneficial minerals are candidates for combined phytoextraction and use as dietary supplements, unlike harmful, non-essential metals such as the nutrient elemental periodic table ‘cousins’, cadmium and cesium. This notion was summarized in a treatise promoting the coordination of trace element phytoextraction processes and crop fortification for such essential nutrients as iron, selenium and zinc [9]. This article detailed different physiological processes – soil-metal decomplexation, root uptake, plant tolerance, and shoot accumulation – that are each components of effective phytoremediation and production of mineralenriched vegetation. Ongoing research projects using high-throughput trace metal profiling of mutant Arabidopsis lines [10] and bioinformatics analysis of metal transporter gene sequence data [11] have tremendously advanced our capacity for rational engineering of improved trace element phytosequestration. Indian mustard (Brassica juncea) possesses many of the qualities desired for an effective phytoremediation species, including rapid growth, high biomass, and appreciable trace metal extraction rates. B. juncea selenium metabolism has been systematically analyzed to elucidate major steps in transport, methylation, volatization, and detoxification [12,13]. These studies identified opportunities for improved Se phytoremediation through analysis of target-gene expression [14] or complementation with foreign genes [15]. In addition to environmental cleanup, Se-enriched vegetation has been shown to serve as an effective nutritional supplement for livestock. Biological transfer studies showed that laboratory rats displayed lower frequency of pre-cancerous lesions when supplied with Se-laden vegetation compared with those fed mineral Se supplements [16]. GM methylselenocysteine plants Selenium hyperaccumulation has been extensively studied among Astragalus species, particularly A. bisulcatus [17]. The primary mode of Se toxicity in non-tolerant species is incorporation of Se-analogs of cysteine and methionine into proteins impairing structural or enzymatic function. This pathway is effectively avoided in Astragalus hyperaccumulators by formation of non-toxic methylselenocysteine (MeSeCys), which is excluded from proteins [17]. To block Se-Cys incorporation into proteins, A. bisulcatus expresses seleno-Cys methyltransferase, an enzyme unique to the Astragalus spp. [18]. In addition to its reduced cellular toxicity, MeSeCys has been shown to possess potent anticarcinogen properties in laboratory animal studies and as a human dietary supplement [19,20]. The ability of A. bisulcatus to transform toxic forms of selenium to compounds with chemopreventative qualities led to its consideration as a genetic resource for improvement of more agronomically compatible plant species [21]. Ellis and colleagues advanced this concept by isolating the A. bisulcatus SMT1 sequence encoding the SMT enzyme [22] and transforming Arabidopsis plants with SMT1 plant expression constructs [4]. Transgenic plants displayed greatly increased tolerance to selenite relative to www.sciencedirect.com
Vol.22 No.10 October 2004
497
control plants with close correlation of relative biomass accumulation on selenite-media and SMT expression levels, supporting the hypothesis that MeSeCys biosynthesis is the principal mechanism for Se detoxification observed in A. bisulcatus. SMT1-Arabidopsis plants were not tolerant to selenate, however, which was expected given the inefficient selenate reduction observed in this species. Comprehensive spectroscopic analysis revealed that SMT1-Arabidopsis plants accumulate methylated Se compounds, MeSeCys and gGluMeSeCys, and total Se at foliar concentrations approaching those of naturally occurring Se hyperaccumulating species. SMT expression was shown to alter the metabolic fate of accumulated selenite conferring Se-resistance to transgenic lines by conversion to anti-carcinogenic methylseleno compounds. Such biotechnological coupling of Se phytoextraction and chemo-preventative enrichment advances the concept of dual-purpose environmental cleanup and nutritionally enhanced crop production on sites that otherwise have limited utility. Para-phytoremediation Perhaps such mixed-benefit strategies could be considered ‘para-phytoremediation’, drawing on the meanings ‘beside’, ‘alongside’, and ‘beyond’ for the prefix ‘para’. This etymological construction recognizes concurrent or post-remediation uses for phytoremediation plants in addition to environmental detoxification. As previously described, nutritional enhancement is one such worthwhile outcome, although this strategy is restricted to biologically essential trace elements. Similarly, metal recovery and recycling are limited to materials with commercial value. What products could be obtained from plant tissues loaded with hazardous levels of valueless metals? In addition, what para-phytoremediation benefits could be derived from trees, shrubs, and grasses used for phytodegradation or rhizosphere-assisted biodegradation of organic pollutants? Energy is one commodity that could be obtained via the same procedure – incineration – that must be performed for post-harvest treatment of hyperaccumulator plant biomass to realize the reduced waste volume or metal recovery benefits. Energy production strategies have been implemented for fast-growing, shortrotation trees used to treat wastewater and biosludges [23] and have great potential as a sustainable biofuel provided certain economic breakpoints are achieved [24]. Interestingly, wood biomass can provide comparable energy production to coal and, in the case of organic phytoremediation, burns more cleanly and cheaply owing to the low sulfur content and reduced need for emission controls. Perhaps environmental remediation projects could also serve as biofiber resources for any number of industries, including lumber, cellulose products, and agri-ethanol feedstock. One attractive benefit of this proposition is that these ‘crops’ could be grown on otherwise non-productive lands to offset cost-intensive remediation activities. Conclusions The pace of published reports of genetically improved phyto-crops has increased steadily in recent years demonstrating the feasibility of mixed-use strategies [25,26]. Given the vast sum of plant bioscience funding
Update
498
TRENDS in Biotechnology
directed towards improvement of agricultural qualities, perhaps these goals can be combined to increase tolerance to temperature extremes, salinity, flooding, or insect pests in plants capable of pollutant detoxification or, more importantly for value-enhancement – transfer of phytoremediative traits to elite plant cultivars having the highest biomass or agricultural productivity. Obviously, concerns about contaminant uptake and accumulation will limit the use of phyto-crops for food or human contact products, so every effort must be made to identify parent compound fate and toxicity for these applications. However, as observed with the development of chemopreventative enriched, Se-hyperaccumulating plants, opportunities exist to combine pollutant decontamination capabilities with beneficial human and ecological health qualities in engineered plants. References 1 Bizily, S.P. et al. (1999) Phytoremediation of methylmercury pollution: merB expression in Arabidopsis thaliana confers resistance to organomercurials. Proc. Natl. Acad. Sci. U. S. A. 96, 6808–6813 2 Doty, S.L. et al. (2000) Enhanced metabolism of halogenated hydrocarbons in transgenic plants containing mammalian cytochrome P450 2E1. Proc. Natl. Acad. Sci. U. S. A. 97, 6287–6291 3 French, C.E. et al. (1999) Biodegradation of explosives by transgenic plants expressing pentaerythritol tetranitrate reductase. Nat. Biotechnol. 17, 491–494 4 Ellis, D.R. et al. (2004) Production of Se-methylselenocysteine in transgenic plants expressing selenocysteine methyltransferase. BMC Plant Biol. 4, 1 5 Baker, A.J.M. and Brooks, R.R. (1989) Terrestrial higher plants which hyper-accumulate metallic elements: a review of their distribution, ecology and phytochemistry. Biorecovery 1, 81–126 6 Baker, A.J.M. et al. (1994) The possibility of in situ heavy metal decontamination of polluted soils using crops of metal-accumulating plants. Res. Conserv. Recyc. 11, 41–49 7 Li, Y.M. et al. (2003) Development of the technology for commercial phytoextraction of nickel: economic and technical considerations. Plant Soil 249, 107–115 8 LaCoste, C. et al. (2001) Uptake of thallium by vegetables: Its significance for human health, phytoremediation, and phytomining. J. Plant Nutr. 24, 1205–1215 9 Guerinot, M.L. and Salt, D.E. (2001) Fortified foods and phytoremediation. Two sides of the same coin. Plant Physiol. 125, 164–167
Vol.22 No.10 October 2004
10 Lahner, B. et al. (2003) Genomic scale profiling of nutrient and trace elements in Arabidopsis thaliana. Nat. Biotechnol. 21, 1215–1221 11 Maser, P. et al. (2001) Phylogenetic relationships within cation transporter families of Arabidopsis. Plant Physiol. 126, 1646–1667 12 de Souza, M.P. et al. (1998) Rate-limiting steps in selenium assimilation and volatilization by Indian mustard. Plant Physiol. 117, 1487–1494 13 Singh, M. and Singh, N. (1977) Effect of sulfur and selenium on sulfurcontaining amino-acids and quality of oil in Raya (Brassica juncea Coss) in normal and sodic soil. Ind. J. Plant Physiol. 20, 56–62 14 Garifullina, G.F. et al. (2003) Expression of a mouse selenocysteine lyase in Brassica juncea chloroplasts affects selenium tolerance and accumulation. Physiol. Plant. 118, 538–544 15 Berken, A. et al. (2002) Genetic engineering of plants to enhance selenium phytoremediation. Crit. Rev. Plant Sci. 21, 567–582 16 Banuelos, G.S. et al. (2002) Biotransfer possibilities of selenium form plants used in phytoremediation. Int. J. Phytoremediation 4, 315–329 17 Brown, T.A. and Shrift, A. (1981) Exclusion of selenium from proteins in selenium-tolerant Astragalus species. Plant Physiol. 67, 1951–1953 18 Neuhierl, B. and Bock, A. (1996) On the mechanism of selenium tolerance in selenium-accumulating plants: purification and characterization of a specific selenocysteine methyltransferase from cultured cells of Astragalus bisulcatus. Eur. J. Biochem. 239, 235–238 19 Combs, G.F. and Gray, W.P. (1998) Chemopreventative agents. Selenium. Pharmacol. Ther. 79, 179–192 20 Medina, D. et al. (2001) Se-methylselenocysteine: a new compound for chemoprevention of breast cancer. Nutr. Cancer 40, 12–17 21 Orser, C.S. et al. (1999) Brassica plants to provide enhanced human mineral nutrition: selenium phytoenrichment and metabolic transformation. J. Med. Food 1, 253–261 22 Pickering, I.J. et al. (2003) Chemical form and distribution of selenium and sulfur in the selenium hyperaccumulator Astragalus bisulcatus. Plant Physiol. 131, 1–8 23 Pulford, I.D. et al. (2002) Heavy metal uptake by willow clones from sewage sludge-treated soil: the potential for phytoremediation. Int. J. Phytoremediation 4, 59–72 24 Bungay, H. (2004) Confessions of a bioenergy advocate. Trends Biotechnol. 22, 67–71 25 Pilon-Smits, E. and Pilon, M. (2002) Phytoremediation of metals using transgenic plants. Crit. Rev. Plant Sci. 21, 439–456 26 Rugh, C.L. (2001) Mercury detoxification with transgenic plants and other biotechnological br eakthroughs for phytoremediation. In Vitro Cell. Dev. Biol. Plant 37, 321–325
0167-7799/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2004.08.003
Exploring the post-transcriptional RNA world with DNA microarrays Vishwanath R. Iyer Center for Systems and Synthetic Biology, and Institute for Cellular and Molecular Biology, University of Texas at Austin, 1 University Station A4800, Austin, TX 78712-0159, USA
Genomic approaches are valuable for understanding the complex layer of gene regulation that involves the control of RNA processing, localization and stability. Recent work Corresponding author: Vishwanath R. Iyer ([email protected]).
www.sciencedirect.com
provides a prime example of the power of unbiased microarray-based methods to discover unexpected functions for proteins in the RNA world. The challenges ahead relate to extending such approaches to larger genomes and to integrating this type of information with that generated by standard expression profiling.