By Sean Henahan, Access Excellence

ATHENS, Ga. (4/22/96)- A genetically altered plant with a healthy appetite for heavy metal pollutants could prove a valuable envirnmental clean-up tool, report researchers.

Heavy metal contamination is a serious and growing problem worldwide. Metals such as mercury and lead have reached toxic levels from pollution in the air, land and water. In other areas soils are contaminated with naturally occurring toxic metals.

Now, scientists from the University of Georgia have designed a gene that, when inserted in test plants, can remove heavy metal pollutants from the soil and render them harmless. The plants show a dramatic ability to remove toxic mercury and convert it to a relatively inert form.

"The results were just astounding, and far better than what we had expected," said Dr. Rich Meagher, a professor of genetics at the University of Georgia. "This could have a huge environmental impact on any site contaminated with heavy metals."

The results in all laboratory tests so far have been dramatic. Meagher and his colleagues inserted a bacterial gene called merA into the commonly used test plant Arabidopsis. The new transgenic plants not only grow on toxic mercury, they thrive. The product of the merA gene, mercuric ion reductase, catalyzes the detoxification of ionic mercury and reduces it to a less-toxic form.

The laboratory plants are now producing seeds that retain the ability of the parent to consume and change mercury.

The gene, found in soil-borne bacteria, has been long known. These bacteria are found at every site on earth polluted by heavy metals. While they help detoxify the metals in a minor way, they can do nothing for serious problems because they aren't effective at large-scale sequestration of the metals, nor are they active more than a few inches into the soil.

"The problems with heavy metals are really widespread, and they don't ever convert into something that is easy to break down," said Meagher. "They will probably be leaching out of the soil in some places for the next 100 years."

The genetic engineering behind the breakthrough has been long and difficult. One of Meagher's graduate students first inserted the gene in a petunia in 1989, but the gene did virtually nothing. An undergraduate student, Nicole Stack, "rebuilt" the genetic sequence of merA, post-doctoral associate Dayton Wilde inserted the gene into arabidopsis and Forest Resources student Clayton Rugh characterized the plants.

Meagher considered the newly engineered gene an important step forward, but it was only when the tiny arabidopsis plants began to grow in culture on petri dishes that the success was obvious. Control plants on the mercury rich medium died, or in some cases, never grew beyond a few feeble sprouts. The plants with the merA gene not only grew, they flourished.

While tests so far have been done only with arabidopsis and yellow poplar on mercury, Meagher believes the gene could be inserted successfully in many different plants, which could then help remove heavy metals from the environment. For instance, the gene might be inserted in a marsh grass called spartina to help clean up pollution in fragile salt marshes caused by paper mills.

Already, the team has inserted merA in the yellow poplar (often called the "tulip poplar"), a rapidly growing species. The idea is to develop a tree to help clean soil while developing usable biomass. Similar experiments can be expected in the sweet gum. It's not year clear if transgenic cypress would be possible, but the idea intrigues Meagher.

"There is a considerable problem out there that this gene might help," said Meagher. "There is agricultural land in Florida, for example, that is heavily contaminated with mercury that was part of formerly used fungicides and bactericides. This land is in orange groves that are often right next to wetlands."

A fringe of transgenic plants might be planted around such areas to prevent any contaminated runoff from reaching the wetlands. Currently, the problem can become systemic when fish ingest the mercury, then birds eat the fish.

Some years ago, mercury-contaminated fish in Japan caused a major outbreak of associated neurological illnesses. And the neurological damage caused by heavy metals is usually irreversible.

While human health problems are worrisome, environmental damage is by far the most serious threat. Evidence of mercury has been found as far as 50 miles downstream from pulp bleaching facilities, Meagher said, and "significant" pollution with heavy metals may extend for five miles or more. Dredging of shipping channels sometimes brings up metals that have settled there and puts them back into the ecosystem.

Theoretically, the gene could be altered to sequester any heavy metal, from cadmium to lead, though experiments have not yet been attempted on those metals. While the success in the lab is encouraging, it will take some years before plants are available for planting. Meagher said that different soil varieties and growing conditions may necessitate adjustments in the gene's design, but he no longer doubts that it is possible.

The research was published in the 4/16/96 issue of the Proceedings of the National Academy of Sciences.

Related information on the Internet

Biological Control Virtual Information Center

Arabidopsis Database

Dr. Yanofsky's Homepage

Gene Therapy for Plants