Using Plants To Clean Up Soils
Plant physiologist Leon Kochian
(left) and molecular biologist
David Garvin examine wheat
plants of various genotypes
being studied for aluminum
When it comes to helping clean up
soils contaminated with heavy and toxic metals, nature has
ARS plant physiologist Leon V. Kochian to
During 13 years of research at the U.S. Plant, Soil, and Nutrition
Laboratory at Ithaca, New York, Kochian has become an authority on mechanisms
used by certain plants to take up essential mineral nutrients and toxic heavy
metals from soils. He has also characterized strategies some plants use to
tolerate toxic soil environments.
Kochian is an international expert on plant responses to environmental
stress, plant mineral nutrition, and use of plants to clean up or remediate
soils contaminated with heavy metals and radioisotopes.
Besides providing important new information on how to use plants in this
practical way, Kochian's research may also shed light on an important
nutritional concern: how to prevent toxic metals from entering the food chain.
"One of the primary ways toxic heavy metals, such as cadmium, get in
food is through plant uptakethe metal is taken up by the roots and
deposited in edible portions," he says.
The lack of vegetation in the barren area
above is a result of the soil's high zinc content and low pH. This site in
Pennyslvania, was contaminated by a
zinc smeltry operated from 1890 to 1980.
"Contaminated soils and
waters pose major environmental, agricultural, and human health problems
worldwide," says Kochian. "These problems may be partially solved by
an emerging new technologyphytoremediation."
"Green" Technology: Simple Concept and Cost-Effective
Phytoremediation is the use of green plants to remove pollutants from the
environment or render them harmless.
"Current engineering-based technologies used to clean up
soilslike the removal of contaminated topsoil for storage in
landfillsare very costly," Kochian says, "and dramatically
disturb the landscape."
Kochian's cost-effective "green" technology uses plants to
"vacuum" heavy metals from the soil through their roots. He says,
"Certain plant speciesknown as metal hyperaccumulatorshave the
ability to extract elements from the soil and concentrate them in the easily
harvested plant stems, shoots, and leaves. These plant tissues can be
collected, reduced in volume, and stored for later use."
While acting as vacuum cleaners, the unique plants must be able to tolerate
and survive high levels of heavy metals in soilslike zinc, cadmium, and
Plant physiologist Leon Kochian
(right) and Cornell University
support scientist Jon Shaff
analyze compounds released
from sorghum roots.
"Phytoremediation has been
hampered historically by our inadequate understanding of transport and
tolerance mechanisms," says Kochian. To address this deficit,
Kochianworking with ARS research associate Deborah L. Lethman, Cornell
University postdoctora postdoctoral associates Mitch Lasat and Paul B. Larsen,
and graduate students Nicole S. Pence and Stephen D. Ebbshas been
studying a unique and promising metal hyperaccumulator. The plant is Thlaspi
caerulescens, commonly known as alpine pennycress.
"Thlaspi is a small, weedy member of the broccoli and cabbage
family," Kochian says. "It thrives on soils having high levels of
zinc and cadmium."
His lab has been trying to discover the underlying mechanism that enables
T. caerulescens to accumulate excessive amounts of heavy metals.
How Plants Clean Up
"Hyperaccumulators like Thlaspi are a marvelous model system for
elucidating the fundamental mechanisms ofand ultimately the genes that
controlmetal hyperaccumulation," says Kochian. "These plants
possess genes that regulate the amount of metals taken up from the soil by
roots and deposited at other locations within the plant.
"There are a number of sites in the plant that could be controlled by
different genes contributing to the hyperaccumulation trait," says
Kochian. "These genes govern processes that can increase the solubility of
metals in the soil surrounding the roots as well as the transport proteins that
move metals into root cells. From there, the metals enter the plant's vascular
system for further transport to other parts of the plant and are ultimately
deposited in leaf cells."
Alpine pennycress doesn't just
thrive on soils contaminated
with zinc and cadmium it cleans
them up by removing the excess
Kochian's team has gained insights
into how, at the molecular level, Thlaspi accumulates these metals in
its shoots at astoundingly high levels. "A typical plant may accumulate
about 100 parts per million (ppm) zinc and 1 ppm cadmium. Thlaspi can
accumulate up to 30,000 ppm zinc and 1,500 ppm cadmium in its shoots, while
exhibiting few or no toxicity symptoms," he says. "A normal plant can
be poisoned with as little as 1,000 ppm of zinc or 20 to 50 ppm of cadmium in
The research also suggests an approach for economically recovering these
metals. "Zinc and cadmium are metals that can be removed from contaminated
soil by harvesting the plant's shoots and extracting the metals from
them," he says.
Above, Cornell University research associate
Miguel Pineros (left) and plant physiologist
Leon Kochian study some of these
mechanisms in corn.
After investigating the molecular
physiology of zinc hyperaccumulation in Thlaspi, Kochian's group found
that several key sites for zinc transport were greatly stimulated in this
plant. To get at the mechanism underlying the stimulation, they cloned a zinc
transport geneone of the first such accomplishments achieved with any
plant. This breakthrough enabled the researchers to discover that zinc
transport is regulated differently in normal and hyperaccumulator plants.
"In normal plants, the activity of zinc transporter genes is regulated
by the zinc levels in the plant," he says. "In Thlaspi,
however, these genes are maximally active at all timesindependent of
plant zinc levelsuntil you raise the tissue zinc levelsto very high
concentrations. This results in very high rates of zinc transport from the soil
and movement of this metal to the leaves."
It Even Works With Uranium
For soil contaminated with uranium, Kochian found that adding the organic
acid citrate to soils greatly increases both the solubility of uranium and its
bioavailability for plant uptake and translocation. Citrate does this by
binding to insoluble uranium in the soil.
Leon Kochian and ARS research
associate Deborah Lethman study electrophoresis films to identify
Thlaspi caerulescens genes
responsible for heavy-metal transport. (K8785-1)
"With the citrate treatment,
shoots of test plants increased their uranium concentration to over 2,000
ppm100 times higher than the control plants," he says. This
demonstrates the possibility of using citratean inexpensive soil
amendmentto help plants reduce uranium contamination.
Recently, Kochian, with colleagues Lasat and Ebbs, identified specific
agronomic practices and plant species to remediate soils contaminated with
radioactive cesium or cesium-137.
"Although the cause of cesium-137 contaminationaboveground
nuclear testinghas been reduced, large land areas are still polluted with
radiocesium," Kochian says. "Cesium is a long-lived radioisotope with
a half-life of 32.2 years. It contaminates soils at several U.S. Department of
Energy (DOE) sites in the United States. Projected costs of cleaning up these
soils is very highover $300 billion."
Phytoremediation is an attractive alternative to current cleanup methods
that are energy intensive and very expensive.
Leon Kochian (left) and
molecular biologist David Garvin
check wheat plants for aluminum
tolerance. Some wheat and corn
plants can tolerate aluminum
by excluding the metal from
the root tip.
In initial lab and greenhouse
studies, Kochian's team showed that the primary limitation to removing cesium
from soils with plants was its bioavailability. The form of the element made it
unavailable to the plants for uptake.
In a series of soil extraction studies, Kochian's team found the ammonium
ion was most effective in dissolving cesium-137 in soils. This treatment
increased the availability of cesium-137 for root uptake and significantly
stimulated radioactive cesium accumulation in plant shoots.
Later, Kochian did field studies with six different plant species in
collaboration with Mark Fuhrmann, a DOE scientist at Brookhaven National
Laboratory in Upton, New York. They found significant variation in the
effectiveness of plant species for cleaning up contaminated sites.
"One species, a pigweed called Amaranthus retroflexus, was up to
40 times more effective than others tested in removing radiocesium from soil.
We were able to remove 3 percent of the total amount in just one 3-month
growing season," says Kochian. "With two or three yearly crops, the
plant could clean up the contaminated site in less than 15 years."
As a result of Kochian's findings, DOE is performing pilot studies at
Brookhaven using this technology.
Hyperaccumulators like Thlaspi
possess genes that regulate
the amount of metals taken
up from the soil by roots and
deposited at other locations
within the plant.
Aluminum Hurts Crops
Kochian's lab is also working on finding ways to grow crops on marginal
lands such as acid soils, where toxic levels of aluminum limit crop production.
Aluminum is the third most abundant element in the Earth's crust; it is a major
component of clays in soil.
At neutral or alkaline pH values, aluminum is not a problem for plants.
However, in acid soils a form of aluminumAl+3is solubilized into a
soil solution that is quite toxic to plant roots.
For years, scientists have been baffled by the causes of aluminum toxicity
"Aluminum toxicity limits crop production on acid soils, which cover well
over half of the world's 8 billion acres of otherwise arable land, including
about 86 million acres in the United States," Kochian says. "When
soils become acid, the toxic aluminum damages plant root systems, which greatly
Kochian's research in collaboration with ARS plant
molecular biologist David F. Garvin uses an interdisciplinary approach
integrating molecular, genetic, and physiological research to provide insights
into how particular genetic types of some plant speciesincluding wheat,
corn, and sorghumcan tolerate high levels of the metal in acid soils.
"We found that the root tip is the key site of injury, leading to
inhibited root growth, a stunted root system, and reduced yields or crop
failures from decreased uptake of water and nutrients," Kochian says.
"Aluminum triggers the release of protective organic acids,
specifically from the root tip into adjacent soil. When released, these acids
form a complex with the toxic aluminum, preventing the metal s entry into the
root. Wheat and corn tolerate aluminum by excluding the metal from the root
tip," Kochian says.
Kochian is also conducting research on an aluminum tolerance mechanism in
collaboration with plant molecular biologist Steve H. Howell of Boyce Thompson
Institute at Cornell, using thale cress, Arabidopsis thaliana, a
diminutive, weedy member of the mustard family.
He and colleagues have successfully identified Arabidopsis mutants
that are aluminum tolerant. Kochian is studying differences between these
mutants and a wild type of Arabidopsis to identify the molecular basis
The ultimate goal of this research is to isolate the genes conferring
aluminum tolerance. It should then be possible to improve the tolerance of
relatively aluminum-sensitive crop species, such as barley, or to further
enhance the tolerance of existing al aluminum-tolerant germplasm.
"One of the major goals for agricultural scientists for the immediate
future is to increase food production to keep up with an ever-growing world
population," Kochian says. "As much of the world's best agricultural
land is already under cultivation or is being lost to industrialization, there
is increasing pressure for farmers to cultivate marginal lands such as the huge
expanses of acid soils that are not currently used for production."
He continues, "Research aimed at producing crop genotypes that tolerate
the suboptimal conditions of these marginal lands is one way global food
production can be increased significantly. Being able to produce a wider range
of crop species with increased aluminum tolerance will make a major
contribution to these efforts to cultivate marginal, stressed soil
Besides helping farmers who grow crops on acid soils, Kochian's
phytoremediation research findings are used by other scientists in government
and academia and by environmental consultants, government, and industry groups
complying with cleanup of contaminated sites.
For his landmark phytoremediation research, Kochian has received two awards:
in 1999, the U.S. Department of Agriculture Secretary's Honor Award for
Environmental Protection and an award as ARS 1999 Outstanding Senior Scientist
of the Year.By Hank
Becker, Agricultural Research Service Information Staff.
This research is part of Plant Biological and Molecular Processes, an ARS
National Program (#302) described on the World Wide Web at
Leon V. Kochian is with the
USDA-ARS Plant, Soil, and
Nutrition Laboratory, Cornell University, Tower Rd., Room 121, Ithaca, NY
14853-2901; phone (607) 255-2454, fax (607) 255-2459.
Agronomist Rufus Chaney examines
the roots of a metal-accumulating
Thlaspi plant in a growth chamber.
"Phyto-miners" Rush to the Cry of "There's Metals in Them Thar
Gold rush miners might have been better off using plants to find gold rather
than panning streams for the precious metal.
Early prospectors in Europe used certain weeds as indicator plants that
signaled the presence of metal ore. These weeds are the only plants that can
thrive on soils with a high content of heavy metals. One such plant is alpine
pennycress, Thlaspi caerulescens, a wild perennial herb found on zinc-
and nickel-rich soils in many countries. This plant occurs in alpine areas of
Central Europe as well as in our Rocky Mountains. Most varieties grow only 8 to
12 inches high and have small, white flowers.
In 1998, ARS agronomist Rufus L. Chaney and colleagues in ARS, at the
University of Maryland, and in England patented a method to use such plants to
"phyto-mine" nickel, cobalt, and other metals.
Chaney says biomining is the use of plants to mine valuable heavy-metal
minerals from contaminated or mineralized soils, as opposed to decontaminating
"The crops would be grown as hay. The plants would be cut and baled
after they'd taken in enough minerals," Chaney says. "Then they'd be
burned and the ash sold as ore. Ashes of alpine pennycress grown on a high-zinc
soil in Pennsylvania yielded 30 to 40 percent zincwhich is as high as
high-grade ore. Electricity generated by the burning could partially offset
USDA has signed a cooperative research and development agreement with
Viridian Environmental, a technology company based in Houston, Texas. The CRADA
involves Scott Angle at the University of Maryland; Alan J.M. Baker at the
University of Sheffield, United Kingdom; plant breeder Yin-Ming Li with
Viridian; and a cooperator at Oregon State University. Viridian is funding the
CRADA's phyto-mining research and development to the tune of $1 million over 5
Chaney says that to make phyto-mining as well as phytoremediation worthwhile
requires, at a minimum, a plant with very high annual intake of minerals, such
as the high-cadmium-accumulating pennycress variety for which they have filed a
"Better still, the traits of plants like pennycress could be
incorporated into a high-yielding commercial crop like canola grown for
hay," Chaney says.
His idea of the best hyperaccumulators? "They'd have all the
characteristics of a hay crop: They should be tall, high yielding, fast
growing, easy to harvest, and deep rooted. And they should hold onto their
mineral-rich leaves so they can be harvested along with the plant
stems."By Don Comis,
Agricultural Research Service Information Staff.
Rufus L. Chaney is at the
USDA-ARS Environmental Chemistry
Laboratory, Bldg. 007, 10300 Baltimore Ave., Beltsville, MD 20705-2350;
phone (301) 504-8324, fax (301) 504-5048.
"Phytoremediation: Using Plants To Clean Up
Soils" was published in the June
2000 issue of Agricultural Research magazine.