BioCycle August 2005, Vol. 46, No. 8, p. 35
Engineered by researchers to have their own detergents to strip harmful chemicals from soil, transgenic plants will soon be used in field trials.
PLANTS that secrete detergents accelerate the breakdown of environmentally persistent organic pollutants, according to scientists speaking at the Association for Advancement of Science annual meeting in Washington D.C. last February. “There’s been a burst of new developments in this field,” says Donald Cheney of Northeastern University, “in particular with the identification of new native hyperaccumulators, as well as the use of genetically engineered plants.”
Until now phytoremediation, in which contaminants are taken up by plants and sometimes broken down to less harmful constituents, has not been effective with persistent organic pollutants (POPs). These environmentally persistent pollutants, including polychlorinated biphenyls, pesticides, polyaromatic hydrocarbons and dioxin, are insoluble in water. They cling to soil particles and, because of their hydrophobicity, are not readily absorbed by plants.
The current removal method for POPs involves large-scale excavation of contaminated soil, which is then buried in landfills or treated with chemical surfactants to flush out pollutants. “Essentially you’re increasing the solubility of the hydrophobic compounds by adding dishwashing detergent or much more aggressive surfactants,” explains Clayton Rugh, assistant professor of crop and soil science at Michigan State University. Not only is this method costly, but it can leave the soil sterile since many of the chemical surfactants are toxic to bacteria.
But transgenic plants engineered by researchers come with their own detergent, which strips harmful chemicals from the soil and exposes them to biodegrading microbes around the plant roots. These transgenic plants represent a low risk method of phytoremediation because the contaminants are not taken up by the plants, as for instance in the phytoremediation of arsenic by ferns. The plants also turn out to be healthier due to antifungal properties associated with rhamnolipids. “In addition to phytoremediation of these contaminants, the plants also have enhanced tolerance to plant diseases,” adds Rugh.
Naturally occurring biosurfactants, such as rhamnolipids, which are synthesized by the bacterium Pseudomonas aeruginosa, have been used since the 1950s to bioremediate soils contaminated with oil and heavy metals. The naturally occurring biodegradable detergents are effective over a wide range of contaminants. Bacteria make rhamnolipids by combining two rhamnose sugar molecules and one or two beta decahydoxydecanoic acid molecules. Plants contain these same building blocks but lack the gene to combine them together in this particular way.
The researchers’ idea was to engineer transgenic plants that would produce rhamnolipids by expressing genes derived from Pseudomonas aeruginosa. Rugh’s team in Michigan is collaborating with scientists at the Institute of Genetics and Cytology at Minsk, Belarus; the University of York in England; and the Sainsbury Laboratory at Norwich Research Park, U.K.
In research trials conducted in Belarus, the transgenic plants significantly increased the biodegradation of fuel oil in contaminated soil. The transgenic plants flourished in soil spiked with 2.4 percent Number 4 fuel oil, breaking down between 25 and 35 percent of the oil in 45 days, as shown by gas chromatography/mass spectrometry soil analysis. Similar nonengineered plants were stunted and accumulated oil. Chemicals in the oil, once freed from soil particles, are broken down by plant and soil microbes in the area close to the plant roots known as the rhizosphere. “These plants are expressing this bacterial biosynthesis pathway; they’re secreting biosurfactants into the soil, enhancing biodegradation,” notes Rugh.
The next steps are more greenhouse and then field trials. “You have to try this stuff under field circumstances to get a really true picture of the technology,” he adds. But if successful, Rugh thinks the rhamnolipid phytoremediation technology could be used commercially as a cleanup tool within three to five years.
“There’s a lot of industrial interest if for no other reason than there’s a lot of community interest,” sums up Rugh. “The public and government regulatory offices are very interested in phytoremediation simply because it has the ability to save a great deal of money and treat sites that are otherwise just not feasible. The vast majority of contaminated sites are just too large or too remote to really justify urgent expensive action. I think phytoremediation is going to be realized to be a very effective cleanup alternative.”
Pat Hemminger is a freelance science and environmental writer and associate editor of Pollution A-Z published by Macmillan in 2003.