The scientists were able to insert two genes from the common bacterium Escherichia coli that allow a member of the mustard family called Arabidopsis to tolerate arsenic, which is usually lethal to plants. Arabidopsis can then remove arsenic from the soil and transport it to the plant's leaves in a form which is far less biologically available in the environment.
"Our data demonstrate the first significant increase in arsenic tolerance and what we call 'hyperaccumulation' by genetically engineered plants," said Dr. Richard Meagher of UGA. "This new system is a major step in developing methods of cleaning up the environment using plants."
The study was published today in the journal Nature Biotechnology. Co-authors include postdoctoral associates Om Parkash Dhankher and Yujing Li of UGA; Julie Senecoff and Nupur Sashti, formerly students at UGA; Barry Rosen and Jin Shi of Wayne State University in Detroit, Mich.; and David Salt of Purdue University. Dr. Rosen was the first to characterize these genes in bacterial and fungal systems, making this plant strategy possible.
Arsenic contamination is an enormous worldwide problem. While soils are contaminated both through natural occurrences of arsenic and spills and drainage from chemical and manufacturing plants, by far the most serious problems involved drinking water. In the Indian state of West Bengal and in Bangladesh, the problem is a crisis, and researchers estimate that more than 112 million people are afflicted with various levels of arsenic poisoning. In these places, the arsenic levels in water far exceed World Health Organization maximum permissible levels.
Worse, these arsenic-laden waters are causing sickness in millions of people--far outstripping the damage caused by the radiation leak in Chernobyl or the chemical catastrophe in Bhopal, India. And yet the problems have received relatively little publicity internationally. Scientists and policy-makers in the United States have been discussing American standards for some years, and a National Academy of Sciences panel found in the fall of 2001 that the risks of cancer from high levels of arsenic in drinking water was even greater than previously thought.
The new strategy for what researchers call phytoremediation--the cleaning of polluted soils through the use of plants that sequester poisons, make them less harmful, and then can be harvested--has the potential to be of use on millions of acres of arsenic-polluted lands worldwide.
The problems facing the research team were daunting. First of all, arsenic is highly toxic to most plants, so the idea of using a plant to withdraw arsenic from the soil seemed counterintuitive. Still, Meagher knew from other experiments that certain genes can make plants tolerate substances that normally sicken or kill them.
"Our working hypothesis was that controlling the electrochemical state of arsenic in the aboveground tissues and increasing organic sulfur 'sinks' throughout the plant would result in both resistance and increased accumulation of arsenic," said Meagher.
What the team discovered was that inserting two unrelated genes from E. coli called arsC and ECS into the model plant Arabidopsis did precisely what they wished. When grown on arsenic, the transgenic plants accumulated 17 times greater fresh shoot weight and two to three times more arsenic per gram of tissue that common or "wild type" plants.
"One of the most important aspects of the research is that this new system should be applicable to a wide variety of plant species," said Meagher. "My colleague Scott Merkle, in UGA's Warnell School of Forest Resources, is already working on putting the genes into cottonwood trees, which have a large root system and could be useful in the phytoremediation of arsenic."
Most arsenic in surface soil and water exists primarily in its oxidized form, arsenate. Plants actively take up arsenate, mistaking it for phosphate and transfer it to their leaves. The team engineered the arsC gene to be turned on strongly by light, which falls naturally on leaves and stems. (Light-induced gene expression as a tool for genetics, also pioneered by the Meagher lab, has been around for at least two decades.)
The arsC gene reduces arsenate to a more toxic compound called arsenite, but only in leaves. Thus, the new system allows plants to remove arsenic from the soil, concentrate it, and then send it to the leaves. This is where the second gene, ECS, comes in. ECS creates more sulfur sinks to bind tightly to arsenate, making it less biologically available. And instead of dying from exposure, the new plants absolutely thrive on the arsenic exposure, and when the "healthy" plants are harvested, much of the arsenic pollution, once in the soil, can be removed from the site.
In tests, 96 percent to 100 percent of arsenic in leaves was reduced to arsenite and bound by sulfur--making the system high effective.
Inorganic arsenic species are classified as Group A human carcinogens and cause skin lesions, lung, kidney and liver cancers, and also damage the central nervous system. The paper notes that hundreds of U. S. Superfund sites are listed on the National Priority List as having unacceptably high levels of arsenic and are recommended for cleaning. In most cases, arsenic-polluted sites have not been cleaned up at all, because the cost of digging up the soils and removing them to storage sites is prohibitive and environmentally destructive.
The entire idea of phytoremediation is relatively new. Other researchers have already found that a fern native to the southern U. S. can hyperaccumulate arsenic to very high levels, but the genetic basis for this activity is unknown, and the narrow growing conditions for most fern species make these plants less likely candidates for phytoremediation.
One of the main problems in India is that new techniques of growing rice in flooded fields during the so-called "Green Revolution" of the 1960s brought to the surface water highly contaminated with naturally occurring arsenic. As villagers walked in these areas, they became exposed to the arsenic, and millions developed illnesses from it.
Om Dhankher, a native of India, said health officials in that country consider arsenic pollution, especially in West Bengal to be a catastrophe.
"In all, this is several fold worse than Chernobyl and Bhopal, and it is getting little attention," said Dhankher. "There has been much more attention to the problem in Bangladesh, but in India, the situation is extremely serious."
The World Health Organization, Dhankher said, estimates that in West Bengal (India) and Bangladesh alone, more than 112 million people are drinking water contaminated with arsenic that exceed the WHO maximum permissible level of 50 micrograms per liter. An estimated 200,000-300,000 people in India already have arsenic-induced skin lesions and cancer, and an estimated 200,000-270,000 cancer caused deaths in Bangladesh will be due to high levels of arsenic in drinking water.
The scientists say the plants genetically engineered to remove arsenic could be used now, but they expect dramatic improvements in the amount of arsenic they can extract as this current strategy is expanded in future experiments.