UGA Scientists Engineer Yellow Poplar Trees That May Help Clean Up Toxic Mercury Pollution

University of Georgia researchers have genetically engineered yellow poplar trees giving them the ability to absorb toxic mercury from soil, convert the toxin to a relatively inert form, and release the converted matter as a vapor into the atmosphere. The research, the cover story in the October issue of the journal, Nature Biotechnology, suggests that trees are particularly promising for phytoremediation, the use of plants (phyto) to remedy (remediate) environmental pollution. The research was funded by the U.S. Department of Energy and UGA's Warnell School of Forest Resources.

"The yellow poplar is fast-growing, has an extensive root system and large leaves that provide plenty of surface area to release processed contaminants," said Scott Merkle, a forest biotechnologist at UGA. "All these make it attractive for remediation."

The UGA researchers, who include Merkle and geneticists Rich Meagher, Clayton Rugh and Julie Senecoff, fitted the poplars with a gene, merA, borrowed from a mercury-resistant bacteria. The bacteria are soil-borne and thrive at sites polluted with heavy metals. They also live in the guts of people with mercury amalgam fillings in their teeth. The fillings release trace amounts of mercury, "and the bacteria in our guts developed the ability to deal with this," said Merkle.

The bacteria detoxify metals on a small scale but, alone, can't possibly clean up the estimated $200 billion worth of heavy metal pollution in the U.S. Early attempts to insert the gene in plants were only marginally successful, so researchers had to extensively modify the gene for expression in plants, and finally trees.

"It was Rich's idea a dozen or more years ago to try to put this gene into plants," said Merkle. "It's taken a long time, but thanks to Rich, Clayton [Rugh] and many others, we've finally got a successful, quantifiable product with the yellow poplar. And this is just one example of the many possible uses of transgenic trees."

In laboratory trials, yellow poplars with the gene showed a 10-fold increase over control trees in their ability to absorb toxic mercury ions and convert them to a vapor. In fact, Merkle said that merA trees not grown in mercury containing medium, actually performed poorly and looked weak and sickly compared to those grown in the mercury-spiked medium.

"This is a weird effect of merA on plants," said Merkle. "They actually grow worse when they're not 'on' the mercury. One theory is that the merA gene product may be scavenging other cations like magnesium that the plant really needs. We're really not sure yet."

Mercury's effect in human and wildlife populations can be serious. Several years ago, mercury-contaminated fish in Japan caused a major outbreak of neurological illnesses. When fish ingest mercury and then are eaten by birds and humans, the metal accumulates in tissues. Wildlife research shows mercury and other heavy metals take a significant toll on the reproductive success of many shore nesting birds, which feed on the contaminated fish.

The next step, said Merkle, is to test the yellow poplars in the greenhouse and finally, on contaminated sites in the field.