BioEd Online

Genetically modified Indian mustard plants have successfully cleaned up excessive selenium in a California field.

This is the first field trial for a pollution-busting transgenic plant, and it proves that the technology can work outside the laboratory, say the researchers who carried out the test.

Farmland in certain parts of California is heavily irrigated, and the water dissolves selenium in shale found in the region. As the water evaporates on the surface soil, selenium is concentrated to levels that are toxic to plants. But Indian mustard (Brassica juncea) has a natural resistance to the element, and absorbs it as it takes in water through its roots.

"Indian mustard is able to grow fast and attain a high biomass even under environmentally stressful conditions," says Norman Terry, a plant biologist at the University of California, Berkeley, who led the study. The researchers boosted Indian mustard's abilities by adding extra genes that produce selenium-hungry enzymes.

They found that the transgenic plants could accumulate up to 4.3 times as much selenium as conventional, wild-type Indian mustard. The research is published online in Environmental Science & Technology¹.

Take the strain

The researchers created three different strains of the transgenic mustard plants, each producing different enzymes to soak up selenium, and tested them in selenium-contaminated soils alongside wild-type Indian mustard.

The transgenic plants showed up to 80% of the growth expected in uncontaminated soil, whereas the wild-type plants had their growth halved by the selenium. They were harvested after 45 days in the field, but the researchers expect that longer growth periods could remove more selenium, and estimate that the most effective plants removed about 4.4% of the element in the top 25 centimetres of soil.

"It's important to test these plants in the field, because the results can often vary from [those of] greenhouse or laboratory surveys," says Clayton Rugh, a plant biologist at Michigan State University in East Lansing.

Expensive burial

Dealing with toxic chemicals in soil is still a decidedly low-tech process. "The major way is to dig up the soil and bury it somewhere else," says Rugh. Chemical treatments are also used for soil washing, but all of these options are expensive and labour-intensive. "They're also quite environmentally damaging," adds Rugh. "The soil they leave is of very poor quality, and it takes a long time for the site to regenerate."

The science of using plants to remove unwanted chemicals from soils, known as phytoremediation, has the potential to be much cheaper, but it can take many years. Although non-transgenic plants such as Chinese brake fern (Pteris vittata) are already used to soak up arsenic from soil, genetic modification might help to speed up the cleaning process, says Rugh.

The possibility of the transgenic plants crossbreeding with food crops is a worry, admits Rugh. "If you're going to engineer a plant to take up high quantities of metals, you must ensure it doesn't get into food crops," he says. "They would have to be carefully contained with measures above and beyond those for genetically modified food crops," he says.

Terry says that the field trial was carefully planned to ensure that there were no close relatives of Indian mustard nearby, and that flowers were picked as soon as they appeared. Ideally, the plants would be allowed to grow for much longer, so further genetic engineering may be required to ensure that the pollen does not contain the transgene, he adds.

In a useful spin-off, the Indian mustard plants could eventually be used as feed for cattle with insufficient selenium in their diet, says Terry. The team is now trying to boost the plants' power even more. "We'd like to see increases in accumulation of 10 to 100 times that possible with wild-type plants," says Terry. "This research is a great start."