In the early 2000s, homeowners in Washington, DC’s leafy Spring Valley neighborhood received some unwelcome news. The Army Corps of Engineers had discovered that the soil on 177 of the Spring Valley properties contained unsafe levels of arsenic, a remnant of World War I-era weapons testing in the area (1).

Some plants naturally detoxify soil. Here, the fern known as Edenfern extracts arsenic from a contaminated backyard in Spring Valley, a residential neighborhood in Washington, DC. Image courtesy of Michael Blaylock (Edenspace Systems Corporation, Purcellville, VA).

On highly contaminated properties, the Corps dug up yards and hauled soil away. On properties with lower arsenic levels, the Corps gave homeowners another option: the Edenfern. Edenfern is the trademarked name for the brake fern, a group of ferns in the genus Pteris that researchers discovered naturally draw arsenic from soil.

Twenty-two homeowners chose these plants to clean up their properties. Edenspace Systems Corporation, the crop biotechnology company that markets the Edenfern, planted one fern for every square foot of contaminated space. The ferns extracted arsenic over the course of their growing season, about five months, and then Edenspace harvested the fronds, leaving purified soil in place. On 16 properties, remediation lasted a single growing season. Other more contaminated sites required repeat plantings and harvests, but all homeowners saw arsenic drop to safe levels within five years. “We were able to save a lot of money in restoration cost because we didn’t have to come back in and restore the landscape,” says Michael Blaylock, president and CEO of Edenspace.

Removing arsenic with ferns is just one example of phytoremediation, using plants to purify land or water. By putting plants to work, remediation practitioners can save money on excavation costs and preserve soil structure. Ilya Raskin of Rutgers University coined the term phytoremediation in a 1991 grant proposal to the Superfund Program of the US Environmental Protection Agency (EPA). He used the grant to explore the potential for plants to purify soil and water contaminated with heavy metals (2). Since then, scientists have steadily advanced phytoremediation technologies, coaxing plants to detoxify a range of pollutants ranging from lead in abandoned mining areas, to pesticides on old orchards, to petroleum hydrocarbons resulting from gasoline leaks (3).

In addition, advances in genomics, transcriptomics, and proteomics are elucidating the genetic mechanisms that make some plants better equipped for phytoremediation than others (4). Novel genetic engineering techniques, meanwhile, enable scientists to enhance phytoremediation capabilities by inserting transgenes (5). But despite academic success, commercial examples like the Edenfern remain relatively rare. Phytoremediation is still struggling to make the leap from lab to field.

Plants as Purifiers Phytoremediation works by using plants to immobilize or extract toxins from contaminated soil and water. In ideal cases, the plants also convert toxins into less harmful substances. Some plants break down “We could have some great success technically, but the challenge of making that work as a business seems really hard.” —Michael Blaylock organic contaminants by releasing toxin-degrading enzymes into soil or by extracting soil contaminants and degrading them inside their own tissues. In other cases, plants facilitate degradation by providing nutrients for soil microorganisms that do the job. Metals and metalloid contaminants cannot be broken down, but plants can make them less toxic by changing their valency and storing them in roots or leaf tissues. “Phytoremediation has some real potential,” says Bill Suk, director of the Superfund Research Program at the National Institute of Environmental Health Sciences. The EPA currently lists over 1,300 Superfund sites: areas that may require remediation of hazardous substances, pollutants, or contaminants (6). Suk and his institute are charged with finding innovative ways to limit human exposure to toxins at these sites. “If you can reduce exposure using phytoremediation, you can reduce the burden of disease,” he says. The Superfund Research Program has funded phytoremediation research from the start of the field, including the pioneering work of biochemist Milton Gordon of the University of Washington, who showed in the 1990s that hybrid poplar trees can extract and detoxify the organic solvent trichloroethene from groundwater (7).

Boosting Nature’s Detoxifiers Some plants like the brake fern are excellent for phytoremediation because they naturally tolerate contaminants and grow quickly. Biogeochemist Lena Ma of the University of Florida first identified the brake fern’s potential when she discovered it growing on an abandoned, arsenic-laden wood preservation site (8). In other cases, researchers screen mutants to identify lines with strong phytoremediation abilities or they genetically engineer plants to tolerate and extract toxins better. Biologist Neil Bruce at the University of York in the United Kingdom uses a range of genetic tools to develop plants that could detoxify the explosives 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX). The US Department of Defense’s Strategic Environmental Research and Development Program funds his research in hopes of finding a safe, low-cost mechanism for remediating the 10 million hectares of military land polluted with these toxins (9). “We wanted to find out how TNT is toxic to plants,” says Bruce. “If we could understand that, then maybe we could make plants more resistant to TNT toxicity so that they could be used in the field to remove TNT at higher concentrations.” His team screened mutant lines of Arabidopsis until they found one that grew well in TNT-contaminated soil. They then traced this enhanced tolerance to a mutation in a gene known as MDHAR6, which codes for an enzyme found in mitochondria. This enzyme reacts with TNT and sets off a chain of chemical reactions resulting in cell-damaging free radicals. The mutant line lacked a functional MDHAR6 enzyme, so it could absorb TNT without triggering those free radicals (10). Arabidopsis itself does not have enough biomass to be useful for phytoremediation on a large scale. But now that scientists know the molecular mechanism for TNT toxicity in plants, they have the potential to identify lines that are similarly deficient in MDHAR6 in other plant species that are better suited for phytoremediation. Bruce and his team are also introducing novel genes that improve plants’ abilities to take up, transport, or break down specific contaminants. Together with researchers at the University of Washington, Bruce inserted bacterial genes that enhance RDX degradation capabilities into two perennial grass species, switchgrass (Panicum virgatum) and creeping bentgrass (Agrostis stolonifera) (11). In another study, Bruce and colleagues introduced a Drosophila gene into Arabidopsis, which enhanced the plant’s ability to detoxify TNT (9).