DELICATE yellow flowers unique to the Greek island of Lesbos have provided vital clues that could lead to the development of plants that will clean up contaminated soil. Andrew Smith and Ute Krämer from the Department of Plant Sciences at the University of Oxford have discovered how the plant, Alyssum lesbiacum, mops up nickel. Armed with this knowledge, they hope to design plants that draw metals out of soil much more cheaply and cleanly than is possible with existing techniques.
A. lesbiacum absorbs nickel very effectively from the soil, shuttling the metal upwards into its green tissues above ground. Like other “hyperaccumulators”, it absorbs levels of metal that would kill most other plants. No one knows why some plants accumulate metals instead of keeping them out. “One theory is that the metals keep insect pests at bay by deterring them from feeding,” says Smith.
But whatever the reason, hyperaccumulators could be very useful in cleanup operations, because they take metals out of the soil and store them in parts of the plant above ground. Metal-rich shoots and leaves could simply be harvested and disposed of, in landfill sites for example. “It might even be possible to extract and recycle the metals,” says Smith.
In research that could accelerate the development of these techniques, Smith and Krämer have discovered that A. lesbiacum mops up nickel using the amino acid histidine. Nitrogen atoms in the amino acid carry spare electrons which they donate to the electron-hungry nickel ion, forming a strong bond as they do so. The nickel is then imprisoned inside the histidine’s molecular cage. To their surprise, the researchers also found that the histidine roves freely in the plant’s roots. Usually, histidine only functions as part of much larger biological molecules such as peptides or proteins.
Smith and Krämer, who report their findings in this week’s Nature, are now trying to track down the genes that orchestrate hyperaccumulation. “The problem with all these hyperaccumulators is that they are slow-growing and could take many years to decontaminate a site,” says Smith. The process might be speeded up if the genes could be made to work in brassicas, including sprouts and cauliflowers, that grow fast and have plenty of foliage to soak up the metal. If the researchers find the genes responsible, they will try to transfer them into fast-growing plants that will decontaminate soils more quickly.
Alan Baker, an ecologist from the University of Sheffield, is collaborating with Smith’s team, tracking down plant species that hyperaccumulate metals other than nickel. The team is to begin work soon on plants from the copper-mining belt in southern Zaire that accumulate copper and cobalt. Another candidate is alpine penny cress (Thlaspi caerulescens), a plant native to Britain that hyperaccumulates zinc.
One problem that the researchers face is that polluted land is usually contaminated with several metals. Plants tolerant to only one will die through exposure to the rest. But by understanding the genetic machinery controlling hyperaccumulation, Smith’s team hopes to genetically engineer plants to survive this exposure, or to mop up several metals at once.
Current decontaminating methods involve trapping the metals in the soil to make them harmless, or removing soil from the original site and exposing it to strong acids that leach out the metals. But the acids kill microorganisms, leaving the soil barren and sterile. Also, the process costs as much as £1 million per hectare. “We think decontamination with plants could be much cheaper as well as being more environmentally friendly,” Smith says.