Use of genetically engineered plants to decontaminate soils

Use of Indian mustard to decontaminate agricultural soils: Indian mustard is an example of a plant that is being used to remediate many different types of contaminated sites. It is particularly useful for phytoremediation because it is fast-growing, produces a large biomass, tolerates fairly toxic soils, and can be grown several times in one growing season. We have increased the capacity of this plant for phytoremediation by genetically engineering it to remove selenium, cadmium and other toxic trace elements from contaminated soil. We have developed over one hundred lines of transgenic plants that overexpress key enzymes responsible for metal and/or metalloid sequestration and volatilization by plants. Specifically, we developed transgenic plants with a superior capacity for the phytoremediation of selenium and/or heavy metals by over expressing the enzymes ATP sulfurylase (APS), g-glutamyl cysteine synthetase (ECS), or glutathione synthetase (GS). In fact, our laboratory was the first laboratory worldwide to develop and successfully field test genetically engineered plants with an enhanced capacity for phytoremediation - specifically, we enhanced the ability of Indian mustard plants for the removal of Se from contaminated soil. The field research showed that the APS transgenics accumulated almost five-fold more Se in their aboveground tissues than wildtype while ECS and GS transgenic plants accumulated 3- and 2.5-fold more Se than WT, respectively. This research has since been followed up with a second field study that showed that transgenic plants overexpressing selenocysteine methyltransferase and selenocysteine lyase were also successful in improving phytoremediation of Se under field conditions. The APS, ECS and GS plants were also successful in promoting the increased accumulation of toxic heavy metals.

Selenium phytoremediation using genes from hyperaccumulator plants: A quite different approach to genetically engineering plants for improved phytoremediation is to introduce genetic traits from slow-growing, hyperaccumulator species into a fast-growing, high biomass species. Several researchers have attempted to combine genes in this way in order to develop the ideal plant for phytoremediation, i.e., one with a high growth rate and biomass which can also hyperaccumulate the pollutant. Our laboratory was the first to achieve this using a gene from the selenium hyperaccumulator, Astragalus bisulcatus, a plant able to accumulate Se to concentrations in excess of 4000 parts per million. A. bisulcatus contains a gene that encodes the enzyme selenocysteine methyltransferase (SMT). This enzyme confers tolerance to Se by methylating the toxic amino acid, selenocysteine, to non-toxic methylselenocysteine. This prevents selenocysteine from being incorporated into proteins and thereby altering their structure and function. Our research showed that the fast-growing and high biomass Indian mustard transgenics overexpressing SMT were able to accumulate and volatilize Se at substantially higher rates than wildtype (unaltered) plants.

Cadmium phytoremediation: Metal-tolerant plants are known to accumulate and store high concentrations of heavy metals by binding them to peptides called phytochelatins. We have over-expressed the genes for two important enzymes involved in phytochelatin synthesis, i.e. g-glutamyl cysteine synthase (ECS) and glutathione synthetase (GS) in Indian mustard plants. The resulting transgenic plants had an increased ability to take up cadmium (Cd) compared to wildtype plants. The transgenic ECS and GS plants were also more tolerant to high Cd levels compared to wildtype plants.

Use of chloroplast engineered tobacco to cleanup mercury: This research (in collaboration with Prof. Henry Daniell at the University of Central Florida) showed that chloroplast engineered plants considerably enhanced the phytoremediation of mercury-contaminated soil. In contrast to engineering the plant nucleus, chloroplast engineering prevents the escape of potentially dangerous foreign genes via pollen to related weeds or crops. Our research showed that genetically engineered tobacco plants containing genes encoding two important bacterial enzymes (mercuric ion reductase, merA, and organomercurial lyase, merB) could efficiently remove and detoxify mercury (Hg) when the mercury was supplied as 400 uM phenylmercuric acetate (which mimics highly toxic methyl mercury). The transgenic tobacco plants showed an approximate 100-fold increase in the efficiency of Hg accumulation in shoots compared to wild-type plants. The transgenic plants were able to grow well with root concentrations of Hg up to three-fold higher, depending on the form supplied, and were also able to volatilize elemental mercury to permanently remove the contaminant from the system.