Engineered plants demolish toxic waste

Greenery may one day clean up the chemical fallout of oil spills and air pollution.

Wielding the metabolic machinery of microbes, plants can now digest polycyclic aromatic hydrocarbons, the ubiquitous chemicals known as PAHs that ooze from oil spills and settle out from smog. The vegetation is still in early stages of development, but scientists are hopeful that it may act as green cleanup crews in future dirty environments. Plant-based scrubbings could be around one-tenth the cost of current methods to clean up contamination, such as harmful PAHs, researchers say.

The United States spends billions of dollars each year cleaning up dangerous waste sites. Global costs are estimated to reach up to $50 billion. The expense of the work — which often covers excavating contaminated land or pumping in chemical treatments — often results in waste sites being deserted without any cleanup.

For a cheaper fix, scientists have eyed biological cleansers. Certain microbes are natural waste-gobblers, sucking in chemical pollutants and snapping them apart to make harmless molecules. Some of these bacteria have been used to clean up oil spills in seas, rivers and even soils. But getting microbes to live in toxic dumps and monitoring their progress can be difficult. Many plants also take up pollution, but they tend to be vulnerable to poisoning because most are unable to break down the harmful chemicals.

Agricultural researcher Quan-Hong Yao and colleagues at the Shanghai Academy of Agricultural Sciences got around the problem by arming plants with pollutant-cracking enzymes from bacteria. The proteins they chose are encoded in a cluster of four genes found in Pseudomonas putida, a soil microbe that breaks down PAHs.  

Other researchers have put these genes into plants before, including biochemist Michel Sylvestre of the Centre INRS – Institut Armand-Frappier in Laval, Canada.  But there’s a sticking point, Sylvestre says: If protein production isn’t synchronized, those enzymes can’t work together to digest the PAHs. “All those who have tried have never succeeded,” Sylvestre says.

Yao and colleagues pulled it off with careful assembly of the four genes into one neat genetic package. For the receiving plants, the researchers chose rice and a flowering plant called Arabidopsis thaliana, both of which are commonly used like guinea pigs in plant research.

The plants took in the genes, coordinated their enzyme manufacturing and broke down phenanthrene, a common PAH pollutant. After 30 days, the plants digested up to 50 percent of the phenanthrene in spiked soil, Yao and colleagues report October 9 in Environmental Science & Technology.

“This is a big deal,” says environmental chemist Christopher Reddy of the Woods Hole Oceanographic Institution in Massachusetts, who studies oil spills. The genetic feat and the success in a lab cleanup are promising, but he cautions, that it’s still unclear whether the plants can digest dangerous waste outside the lab.

Some of the pollutants, particularly those from soot, form hard-to-break chemical complexes, he says. How well the plants will grow in contaminated sites is also unknown.

Yao agrees: “There is still a long way to go to use these transgenic plants.” He is planning to test the plants’ scrubbing potential in real-world conditions.

However, strict regulations on genetically engineered plants in the United States and elsewhere may make these green cleaners difficult to use, says environmental scientist Sharon Doty of the University of Washington in Seattle. In her research on PAH remediation, Doty introduces whole bacteria instead of injecting bacterial genes. Called endophytes, these microbial partners live inside a plant, breaking down PAHs and other pollutants, she says.

With the same bacteria Yao’s group used, willow shrubs and grass could chew up phenanthrene in contaminated soil, Doty and her colleagues report in the Oct. 21 Environmental Science & Technology.

Collectively the new data, she says, shows that plants could one day solve the pollution problem.