Arsenic ranks first on the U.S. government's Priority List of Hazardous Substances, both because of its toxicity and its prevalence in the environment. Wayne State University researchers have been collaborating with others for years to genetically create "arsenic-eating" plants that could be grown in polluted soil for clean-up. Until now, however, the arsenic sequestered from soil remained largely in the roots of the plant, making it difficult to harvest for safe disposal.
For the first time, researchers including Barry Rosen, Ph.D., professor and chair of the Department of Biochemistry and Molecular Biology at the Wayne State University School of Medicine, have discovered a way to move the arsenic from roots to shoots, the next step in their quest for plants that can clean up arsenic. The payoff could be a new and effective tool in cleaning up thousands of sites where arsenic presents serious dangers to human health.
Dr. Rosen and his colleagues have published their findings in the April 4 issue of the Proceedings of the National Academy of Sciences. This and related studies were carried out in collaboration with Richard Meagher, Om Parkash Dhankher and Elizabeth McKinney from the department of genetics at the University of Georgia.
"We are continually exposed to arsenic, both from natural sources and from the use of man-made arsenical compounds in herbicides and pesticides. Exposure to arsenic produces a wide range of health effects, the most serious of which is as a carcinogen. There are high levels of arsenic in soil and in drinking water in the United States and elsewhere in the world, and cleaning up arsenic contamination is very expensive," Dr. Rosen said.
The most serious problems are found in West Bengal, India, and in Bangladesh. During the so-called "green revolution" of the '60s and '70s, the cultivation of rice in flooded fields became pervasive in those areas, and workers dug open wells through soil and rocks with naturally occurring arsenic. The result was widespread arsenic pollution in contaminated water. The World Health Organization predicts that long-term exposure to arsenic could reach epidemic proportions, the PNAS paper reports. The WHO says a staggering 1 in 10 people in northern India and Bangladesh may ultimately die of diseases resulting from arsenic-related poisoning.
The new strategy employing genetic engineering is part of what researchers call phytoremediation, the cleaning of polluted soils through the use of plants that sequester poisons and make them less harmful. This has the potential to be used on millions of acres of arsenic-polluted lands worldwide. In their research reported in 2002 in Nature Biotechnology, Dr. Rosen and his collaborators inserted two unrelated genes from the bacterium E. coli called arsC and gshI into Arabidopsis , a model lab plant and small member of the mustard family. This allowed the plants to resist the toxic effects of arsenic and sequester three-fold more arsenic in their shoots than normal plants. Still this was too ineffective to allow planting of the transgenic plants on arsenic-polluted sites, since far more arsenic needed to be moved into the plant leaves for safe harvesting and disposal.
In the recent PNAS report, the team identified a single gene, ACR2 , in the Arabidopsis genome as one that allows the plants to move sequestered arsenic in roots. By engineering plant lines with a silenced ACR2 gene, they discovered they could get 16-fold more arsenic in shoots than in natural wild-type Arabidopsis. This experiment identified the active mechanism for sequestering arsenic in roots. It appears possible to create arsenic-eating plants using this new knowledge.
The PNAS article is titled "Hyperaccumulation of Arsenic in the Shoots of Arabidopsis Silenced for Arsenate Reductase 2." Full text is available online at: http://www.pnas.org/cgi/content/short/103/14/5413?rss=1.