Transgenic plants remove more selenium from polluted soil than wild plants

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 become a viable alternative for cleaning up polluted soil.

The new research findings, published today (Tuesday, Feb. 1) 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 the genetically unaltered, wild-type plants.

Researchers from the University of California, Berkeley, and the Agricultural Research Service (ARS) of the U.S. Department of Agriculture 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 UC Berkeley’s 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.”

In California, 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.

“Phytoremediation can help clean up the selenium, but the thing that’s holding people up from using plants more widely is that, by and large, they work slowly,” said Terry. “So what we want to do is take a plant like Indian mustard, because it can grow quickly — up to six to seven feet tall — and it is tolerant to many toxic conditions. It’s a good plant for remediation, but we want to see if it’s possible to increase its ability to absorb selenium and other pollutants ten-, one hundred-, or even one thousand- fold. This field test is the proof-of-concept showing that we are 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.

Gary S. Bañuelos, a soil scientist with the USDA’s ARS and co-lead author of the paper, directed the field tests, which were carried out in Fresno County.

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). The enzyme is key to the plant’s ability to convert selenate into a non-toxic form of selenium, allowing the plant to accumulate more of the contaminant without incurring harm. 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. Glutathione, part of the plant’s antioxidant system, may be buffering the impact of the contaminants, said the researchers.

The ECS and GS lines absorbed 2.8 times and 2.3 times more selenium respectively than the wild plants. Moreover, the GS plants seemed particularly tolerant of the contaminated soil, growing 80 percent as well as the GS plants planted in clean soil.

Because USDA regulators are scrupulous about experiments involving genetically modified plants in the field, the researchers took great care to minimize the transfer of genes through pollen. “Before we started the study, we took aerial surveys to ensure that no other mustard-related plant species were being grown in the vicinity,” said Bañuelos.

He noted that every morning trained workers swept through the fields to literally nip any flowers in the bud, and that netting and buried chicken wire were used to keep wildlife away from the plants. “One of the challenges in developing transgenic plants for remediation is engineering them in such a way that the risk of gene transfer is reduced or eliminated,” said Bañuelos.

Terry said that techniques now being developed by plant geneticists — such as modifying chloroplast DNA rather than nuclear DNA — will eventually reduce the need for such constant monitoring. Since chloroplast DNA is maternally inherited, there is little risk of pollen transfer, said Terry.

Terry said it’s worth pursuing methods of improving phytoremediation because there are benefits unique to plants. “A particularly promising aspect about mustard plants is that they can metabolize selenium into a gas called dimethyl selenide,” said Terry. “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.”

And what happens to the plants after they’ve soaked up their share of selenium? The researchers say that the plants 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,” said Terry. “Farmers would love to have this source of selenium.”

Other co-authors of the study include Danika L. LeDuc, a post-doctoral researcher at UC Berkeley’s Department of Plant and Microbial Biology, Elizabeth A.H. Pilon-Smits, associate professor of biology at Colorado State University, and Bruce E. Mackey, a biostatistician at the USDA’s ARS.

The study was funded by the Electric Power Research Institute and the Agriculture Research Initiative of California State University.