Engineered Plants Soak Up Arsenic
WASHINGTON, DC, October 7, 2002 (ENS) - A team
of researchers has developed the first transgenic system
for removing arsenic from the soil by using genetically
modified plants. The new system could help remove the
toxic metal from naturally and artificially polluted soil
and water, reducing their threat to the environment and
to human and animal health around the world.
The scientists inserted two genes from the common bacterium
Escherichia coli (E. coli) that allow the test plant,
a member of the mustard family called thale cress, to
tolerate arsenic, which is normally lethal to plants.
The plant removes arsenic from the soil, storing it in
its leaves in a form that is less available to the environment,
and easier to remove and eliminate.
"Our data demonstrate the first significant increase in
arsenic tolerance and what we call 'hyperaccumulation' by
genetically engineered plants," said Dr. Richard Meagher
of the University of Georgia, who led the research effort.
"This new system is a major step in developing methods of
cleaning up the environment using plants."
Phytoremediation - the cleaning of polluted soils through
the use of plants - has the potential to be of use on
millions of acres of arsenic polluted lands. After plants
absorb toxic materials, they store them above ground,
away from soils and groundwater, and where they can be
harvested and destroyed in a safe manner.
Still, the research team faced some daunting problems.
Arsenic is toxic to most plants, so the idea of using
a plant to withdraw arsenic from the soil seemed counterintuitive.
But the team knew from other experiments that certain
genes can make plants tolerate substances that would normally
sicken or kill them.
Barry Rosen of Wayne State University in Detroit, Michigan,
a coauthor of a report that appears today in the journal
"Nature Biotechnology," was the first to characterize
the genes for arsenic resistance in bacterial and fungal
systems, making phytoremediation of arsenic possible.
That knowledge was combined with the special expertise
of the team's other members, including postdoctoral associates
Om Parkash Dhankher and Yujing Li of the University of
Georgia (UGA); former UGA students Julie Senecoff and
Nupur Sashti; Jin Shi of Wayne State University; and David
Salt of Purdue University.
"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 UGA's Meagher.
Most arsenic in surface soil and water exists in its oxidized
form, arsenate. Plants actively take up arsenate - mistaking
it for the nutrient phosphate - and transfer it to their
The team was able to insert two unrelated genes from
E. coli called arsC and ECS into the model plant, thale
cress or Arabidopsis thaliana. The team engineered the
arsC gene to be activated by exposure to light, a technique
that has been around for at least two decades.
The arsC gene reduces arsenate to a more toxic compound
called arsenite, but only in the plant's leaves. The second
gene, ECS, creates compounds in the plant that bind tightly
to arsenate, making it less available to poison either
the plant or to leach back into soil or water.
In essence, the altered plants remove arsenic from the
soil, concentrate it, and then send it to their leaves.
Instead of dying from exposure, the new plants thrive
on the arsenic exposure, and when the plants are harvested,
much of the arsenic pollution, once in the soil, can be
removed from the site.
When grown on arsenic, the transgenic plants accumulated
17 times more weight in fresh shoots, and two to three times
more arsenic per gram of tissue, than in common or wild
type plants. Laboratory tests showed that 96 percent to
100 percent of the arsenic in the plants' leaves was reduced
to arsenite and bound by sulfur.
"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
Other researchers have already found that a fern native
to the southern U. S. can accumulate arsenic at 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.
Plants genetically engineered to remove arsenic could
be used now, the study's authors say, but they expect
dramatic improvements in the amount of arsenic they can
extract as this current strategy is expanded in future
Using plants to remove arsenic from contaminated soil
could be useful to almost every nation. Inorganic arsenic
compounds are classified as Group A human carcinogens,
and last year, a U.S. National Academy of Sciences panel
found that the risks of cancer from high levels of arsenic
in drinking water was even greater than previously thought.
Exposure to arsenic can cause skin lesions, lung, kidney
and liver cancers, and damage the central nervous system.
While soil in some areas is contaminated by natural
arsenic deposits, many other sites are contaminated by
spills and drainage from chemical and manufacturing plants.
Today's research paper notes that hundreds of polluted
sites in the United States are listed on the National
Priority List, or Superfund list, because they contain
high levels of arsenic. Although these sites are recommended
for cleaning, most have not been cleaned yet because digging
up the soils and removing them to storage sites is both
expensive and environmentally destructive.
But the most serious human health problems from arsenic
involve drinking water. In the Indian state of West Bengal
and in Bangladesh, where naturally occurring arsenic contaminates
water at concentrations far above the recommended safe
levels set by the World Health Organization (WHO), researchers
estimate that more than 112 million people are afflicted
with various levels of arsenic poisoning.
WHO estimates that 200,000-300,000 people in India have
arsenic induced skin lesions and cancer, and an estimated
200,000 to 270,000 cancer deaths in Bangladesh will be
due to high levels of arsenic in drinking water.
Study author Om Dhankher, a native of India, said health
officials in that country consider arsenic pollution,
particularly 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 problems of arsenic contamination have received
relatively little international publicity. While WHO and
the European Union have adopted a drinking water standard
of just 10 parts per billion (ppb) of arsenic, standards
in other nations are mixed.
The United States is scheduled to replace its 60 year
old standard of 50 ppb with a new, 10 ppb standard, effective
January 23, 2006.