Genetic Engineering PossibilitiesTo determine possible mechanisms for genetically engineering a plant to increase its remediation ability, it is best to begin with the mechanism for arsenic uptake and storage. It is believed that arsenate is taken up from the soil by the phosphate transporter. This has been suggested by exclusion experiments in which inorganic arsenate competed with inorganic phosphate for transport by a phosphate transporter. (Csanaky and Gregus 2001) It is believed that the high affinity phosphate uptake system is involved, because it has a lower selectivity than the low affinity system, and suppression of the high affinity system results in smaller arsenic uptake and accumulation. (Fitz and Wenzel 2002) One mechanism that could be useful would be to overexpress the phosphate ion transporter gene in plant roots. This would allow for more arsenic uptake by the plant, and could therefore lead to greater arsenic sensitivity, due to increased concentration of arsenate in the plant, and accumulation.
Once inside the plant, the arsenate travels with the transpiration flow through the xylem and into the stems and leaves. (Tu et.al 2002) Here it is reduced, possibly by an arsenate reductase enzyme (ArsC) into its more toxic form, arsenite. (Dhankher et.al 2002) Another possible mechanism would be to overexpress the arsenate reductase gene in plant shoots. Studies have coupled the arsenate reductase gene to a soybean ribulose bisphosphate carboxylase small-subunit gene promoter, which will give it strong light-induced expression, insuring that the arsenate is reduced in the shoots, not roots. (Dhankher et.al 2002) Since arsenite is more toxic than arsenate, all the transgenic plants were more sensitive to arsenic in the growing medium. (Dhankher et.al 2002)
Arsenite has a great affinity for thiol groups and consequently binds to peptide-thiol molecules, also known as phytochelatins. (Dhankher et.al 2002) Phytochelatin production appears to be activated by the presence of heavy metals and arsenic. (Sauge-Merle et.al 2003) One enzyme critical in the pathway for the formation of these peptide-thiol molecules is y-glutamylcysteine synthetase (y-ECS). (Dhankher et.al 2002) This enzyme produces the first, and possibly limiting enzyme in the phytochelatin producing pathway. (Dhankher et.al 2002) Studies have coupled this gene to a constitutively expressed actin promoter, giving a constant high level of phytochelatins in the plant and providing a sink for reduced arsenite within the cells. (Dhankher et.al 2002) Plants transformed with both ArsC and y-ECS produced four times more biomass than the control when grown four weeks on a 200 micromolar arsenate medium, showing greater tolerance as well as accumulation. (Dhankher et.al 2002)
Arsenic has been shown to have deleterious effects on seed germination, as well as root length and mass, which can detract from the effectiveness of a phytoremediation treatment. (Nie et.al 2002) Some bacteria, such as Enterobacter cloacae promote plant growth and minimize arsenic stress on the roots. (Nie et.al 2002) The Enterobacter cloacae UW4 1-aminocyclopropane-1-carboxylate deaminase gene in plants has been shown to increase the interaction between plants and these growth promoting bacteria. (Nie et.al 2002) In addition, transformed plants accumulated four times as much arsenic as non-transformed plants. (Nie et.al 2002) Overexpression of this gene is another possible mechanism to increase plant growth and phytoremediation of arsenic-contaminated soils.
A combination of these transformations and others could create a plant that was capable of high levels of arsenic accumulation and tolerance, when used to transform a species with high biomass that is adapted to living in the environment in which the arsenic-contaminated site exists. A tree such as cottonwood would be ideal for this purpose, and researchers are concentrating their efforts on performing this transformation. (www.edie.net) Further studies must be performed on the unique hyperaccumulating species, such as Pteris vittata and Pityrogramma calomelanos, to determine if they possess a novel mechanism or just an abnormally high expression of a given enzyme in the accumulation pathway. These hyperaccumulators or the transgenic accumulators can then be used with various phytoremediation strategies.Phytoremediation Strategies
Phytostabilization is a possible strategy for control of arsenic, where arsenic tolerant plants, such as Spergularia grandis and Agrostis catellana are used to keep Arsenic contaminated soil in place to prevent erosion and leaching of the contaminant. (Fitz and Wenzel 2002)
Phytovolatilization is the use of plants to turn a pollutant into a gas to remove it from a site. Although this does occur to some extent in nature, volatile arsenic is still toxic, so phytovolatilization is not really a feasible option. (Fitz and Wenzel 2002)
One positive phytoremediation option is rhizofiltration, the removal of pollutants from water using a hydroponic system. Arsenic contaminated runoff could be run through a set-up with Lepidium sativum, which has been shown to remove arsenic from anaerobic flooded environments. (Robinson et.al 2003) This can also be used in conjunction with bioleaching, which would produce a heavily arsenic-contaminated runoff from soil with initially low arsenic bioavailability. (Seidel et.al 2002)
One positive phytoremediation option is rhizofiltration, the removal of pollutants from water using a hydroponic system. Arsenic contaminated runoff could be run through a set-up with Lepidium sativum, which has been shown to remove arsenic from anaerobic flooded environments. (Robinson et.al 2003) This can also be used in conjunction with bioleaching, which would produce a heavily arsenic-contaminated runoff from soil with initially low arsenic bioavailability. (Seidel et.al 2002)
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