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New GMO technique really cuts the mustard
Engineered to absorb high levels of selenium, and tested with great caution, these plants may aid in toxic cleanup

| 02 February 2005

In the first field trial of plants genetically tweaked to absorb more contaminants, researchers found that the transgenic plants handily beat out their wild-type counterparts. The results raised hopes that the plants might someday be used to clean up polluted soil.

The new research findings, published Tuesday in the journal Environmental Science and Technology, show that three transgenic lines of the Indian mustard plant, Brassica juncea, absorbed two to four times more selenium from contaminated soil than did the genetically unaltered, wild-type plants.

Researchers from Berkeley and the USDA’s Agricultural Research Service teamed up for the six-week trial to see if they could replicate in field conditions the results of prior studies in laboratory greenhouses. Those previous tests showed that transgenic plants performed up to three times as well as wild plants in cleaning up selenium-polluted soil.

“Field conditions involve a million different variables, from weather to soil conditions, so results can be radically different than those in the lab,” said Norman Terry, professor of plant and microbial biology at the College of Natural Resources and co-lead author of the study. “It turns out that our field-test results were better than those from the greenhouse, and that was a surprise.”

As much as 100,000 cubic meters of sediment contaminated with selenium, salt, and boron remain in the San Luis Drain, a concrete-lined canal originally intended to channel irrigation wastewater from Central Valley farms to the Sacramento River Delta near Antioch. Selenium is considered an essential trace mineral for both humans and animals, but it becomes toxic at high doses. The dangers of selenium toxicity came to light in the 1980s when biologists discovered that irrigation drain water held at the Kesterson Reservoir in the San Joaquin Valley was causing serious deformities in birds.

The researchers say cleaning up the sediment in the San Luis Drain could cost millions of dollars using conventional methods, including soil washing, excavation, and reburial. In contrast, they say that using plants to remove contaminants — a process called phytoremediation — provides one of the most cost-effective methods of cleaning polluted soil available.

“The thing that’s holding people up from using plants more widely is that, by and large, they work slowly,” says Terry. “We want to see if it’s possible to increase this plant’s ability to absorb selenium and other pollutants tenfold, a hundredfold, even a thousandfold,” says Terry. “This field test is the proof-of-concept that shows we’re heading in the right direction.”

Researchers like using the Indian mustard plant because it is very efficient at absorbing selenate, the bioavailable form of selenium in the soil. The plant is tricked into absorbing selenate because it is chemically similar to sulfate, an essential nutrient for the plant.
The three types of transgenic plants and the wild-type control plants were transplanted into four 33-by-1 meter field plots, two that contained contaminated sediment from the San Luis Drain and two that contained clean soil.

One line of Indian mustard plants was engineered to produce more of the enzyme adenosine triphosphate sulfurylase (APS), which enables the plant to convert selenate into a non-toxic form of selenium; this allows the plant to accumulate more of the contaminant without being harmed. In the field trial, the APS plant line absorbed 4.3 times more selenium than the wild-type plants.

The other two transgenic lines were engineered to produce more of the enzymes gamma-glutamyl cysteine synthetase (ECS) and glutathione synthetase (GS), both of which play key roles in the production of glutathione. (Part of the plant’s antioxidant system, glutathione may be buffering the impact of the contaminants, say the researchers.) The ECS and GS lines absorbed 2.8 times and 2.3 times more selenium respectively than did the wild plants. The GS plants seemed particularly tolerant of the contaminated soil, growing 80 percent as well as the GS plants planted in clean soil.

The researchers took great care to minimize the transfer of genes through pollen. Aerial surveys were conducted to ensure that no related plant species were grown in the vicinity. In addition, trained workers swept through the fields every morning to literally nip any flowers in the bud, and netting and buried chicken wire were used to keep wildlife away from the plants.

Terry says that techniques now being developed by plant geneticists will eventually reduce the need for such constant monitoring. It’s worth pursuing methods of improving phytoremediation, he believes. “Mustard plants can metabolize selenium into a gas called dimethyl selenide,” he explains. “This is something we've been working to enhance in our lab. Getting inorganic selenium into gas form will allow it to just dissipate harmlessly into the atmosphere. No other form of remediation can do that.”

Once the plants have soaked up their share of selenium, they can be harvested, dried, and carefully added to animal feed or used as a soil amendment in areas where selenium is in short supply.

“There are many areas where selenium is deficient, so farmers actually pay for animal feed that is supplemented with selenium,” says Terry. “Farmers would love to have this source of selenium.”