Wild Plant Puts the Bite on Heavy Metals

Wild Plant Puts the Bite on Heavy Metals

A wild mustard plant that grows only in the Austrian Alps naturally accumulates large amounts of metals in its tissues. This tiny plant may be the key to cleaning up large areas contaminated by heavy metals from industrial production.

The plant's genes involved in metal accumulation have been identified and cloned by David Salt, professor of plant molecular physiology at Purdue University.

The plant, Thlaspi goesingense, is similar to the nonmetal-accumulating plant Arabidopsis thaliana, the first plant to have its genome sequence completed, in December 2000.

Salt says his work is expected to lead to new transgenic crop plants with genes from the wild mustard that can clean up industrial contamination by absorbing metals from the soil.

"This is really one of the first tools that we've got to manipulate this process of metal hyperaccumulation," Salt says. "So what we're going to do now is to start expressing these genes in nonaccumulating plants to see if we can turn them into metal-accumulating plants."

Scientists are interested in using metal hyperaccumulating plants to clean up contaminated brownfield sites. Researchers believe that soil polluted with heavy metal or radioactive materials could be cleaned up by using crop plants to absorb the material, a process called phytoremediation.

Salt says more than 350 species of plants are known to accumulate metal such as nickel, zinc, copper, cadmium, selenium or manganese in high levels.

"The plant species that we're interested in can accumulate one percent of their dry biomass as nickel. In a normal plant you might expect to find 10 to 100 parts per million of nickel in their tissue, and these plants can accumulate 10,000 parts per million," he says.

Salt says, "They do this in the wild without any interference from man. They just do this for a living."

Hyperaccumulating plants store the metal in microscopic structures in their cells called vacuoles. The vacuoles are membrane-lined structures that protect the rest of the cell from the toxic effects of the metal. The protective membranes that surround the vacuoles closely resemble cell membranes in the human liver that serve a similar function.

Scientists aren't completely sure why some rare plants try to grab as much metal as they can, but studies indicate that they do this to stop insects and other creatures from eating them.

Just as people hate to bite down on a piece of aluminum foil, insects tend to avoid eating plants that taste like metal.

"You can imagine if you're a bug and you bite down on a plant and it's got 10,000 parts per million of nickel in its leaf, it's not going to taste too good," Salt says.

Scientists around the world worked on phytoremediation techniques throughout the 1990s. Cultivating plants that take up metal on industrial waste sites can provide a clean, cheap alternative to the current "suck, muck and truck" approach to cleaning heavy metal contaminated soils.

Small scale field trials with such plants, collected from naturally contaminated soils, have demonstrated the feasibility of the phytoremediation approach, says a paper presented at Environment97, an conference and exhibition organised by the Institution of Chemical Engineers on behalf of the UK Engineering Council.

"Imagine if you have a site contaminated with cadmium. Right now your options are to put a fence around it and put a sign up telling people to stay out, build a parking lot over it, or dig up all of the soil and truck it to a landfill, which is very expensive," Salt says.

"You could essentially farm the metal out of the ground. Over five or 10 years, by growing crop rotations there, you could remove the metal from the site. The nice thing is that it's cheap, and you're left with a soil at the end of it which could be used for other things," he says.

The metal hyperaccumulating plants found in nature would not be used for phytoremediation because they are all small and slow growing. Instead, scientists could move the genes Salt and his colleagues have identified into fast growing, large plants, such as grasses.

Researchers are already planning to build on Salt's work to genetically engineer foods to contain micronutrients missing from diets in certain areas. Metals are essential nutrients in small doses, but some regions of the world lack foods that contain enough of these metals for optimum human health. Using the genetic tools Salt and his colleagues have identified, scientists could begin to bioengineer foods that contain these essential micronutrients.

The research could also be applied to improved crop nutrition. "Instead of adding zinc to the soil because you live in a zinc-deficient region, why not have the wheat plant itself be more zinc-efficient so that you can reduce agricultural inputs?" Salt asks.

Salt's research is published in the August 14 issue of the "Proceedings of the National Academy of Sciences."