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High-Tech Castor Plants May
Open Door to Domestic
Production
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Castor beans.
(K9200-2)
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Inside the beans of the castor plant
is a toxin seven times more deadly than cobra venom. Known as ricin, the
compound's toxicity is one reason why American farmers no longer grow this crop
extensively—even though a lucrative market exists for the castor bean's
unique oil.
Components of the oil, known as hydroxy fatty acids, are essential for making
high-quality lubricants for heavy equipment or jet engines, for example. Castor
oil is also used in paints, coatings, plastics, antifungal compounds, shampoo,
and cosmetics. |
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Plant physiologist Grace Chen removes castor bean
pods to test for genetic transformation.
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Allergens Pose Health Hazard
Besides the ricin toxin, there's another compelling reason why this crop has
fallen out of favor with U.S. growers. The shiny, beetle-shaped seeds contain
powerful allergens. People who work with the off-white meal ground from castor
beans may develop allergic reactions, such as hives or asthma. In severe cases,
they may go into anaphylactic shock, which can be fatal.
Conventional breeding to rid castor of lethal ricin and troublesome allergens
hasn't solved the problem. But biotechnology might, according to Thomas A.
McKeon of ARS' Western Regional Research
Center in Albany, California. He and colleague Grace Q. Chen, both in the Crop
Improvement and Utilization Research Unit, are the first in the world to
genetically engineer castor plants.
In preliminary experiments, McKeon and Chen used marker genes to determine
whether their tactics for shuttling new genes into plants actually worked. Now
the scientists want to give the plants other genes—ones that could, among
other things, block production of ricin poison and the powerful allergens.
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A castor bean pod.
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Biotech Strategies
Scientists elsewhere have already isolated and copied a gene critical to ricin
production, as well as a gene that produces the key allergen proteins in
castor. McKeon and Chen aim to build and insert slightly different versions of
those genes into the castor plant, to block the action of the ricin and
allergen genes. For example, they want to construct antisense genes, which are
genes that make nonsense copies of the authentic ricin or allergen genes.
"Antisense genes," McKeon says, "can interfere with the gene
expression needed for producing ricin and allergens. That may leave the plant
unable to form these compounds."
Castor plants that are free of ricin and allergens could renew interest in
farming this crop. That could happen not only in the southern United States,
where it was grown until the early 1970s, but also in the arid Southwest, where
it could grow well if irrigated.
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Chemist Thomas McKeon and Grace Chen remove leaf
disk samples from genetically transformed castor plants to test
for enzyme activity.
(K9198-2)
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"Castor is a semitropical plant
that thrives in sunny climates," McKeon says. Although some types of
castor grow to be 30- to 40-foot-tall trees in the Tropics, in the United
States castor can be harvested annually when it is only about 4 to 5 feet high.
In the past, U.S. production has reached 1,000 pounds of oil per acre.
"That's an impressive feat for any oilseed crop," notes McKeon.
Production of a U.S. castor crop could ensure a more reliable supply of the oil
for American industries and for defense. This country depends on imports of
castor oil, primarily from India. In 1999, America imported nearly 103 million
pounds of castor oil, worth about $41 million. The world demand for castor oil
is about 1 billion pounds annually, valued at more than $400 million.
Although some other plants, like lesquerella, can produce oil that contains
hydroxy fatty acids similar to the ones in castor, these alternative crops are
not yet widely grown commercially. Another approach? Synthesize hydroxy fatty
acids in chemical factories. Although the technology exists to do that, growing
castor plants outdoors in the sunshine is a more economical approach, McKeon
says.
Epoxy Oil—A Possibility?
In addition to reviving production of castor, genetic engineering might someday
be used to tweak its oil-producing mechanism so that it could yield another
valued oil, known as epoxy.
Says McKeon, "There is a potential U.S. market of about $300 million a
year for epoxy oil. An epoxy-based paint, for example, offers all the
advantages of a premium, oil-based paint, yet does not give off certain
volatile chemicals that pollute the atmosphere." That's unlike the
solvents in oil-based paints, which can be an environmental hazard.
"We think that production of epoxy oil by castor plants is possible,"
says McKeon, "because the chemical structure of epoxy oil is very similar
to that of castor oil. The modification that's needed to cue the castor plant
to make epoxy oil instead of castor oil is minor. That's very different than
trying to genetically engineer a corn plant or a soybean plant to make epoxy
oil. The oils that those plants make are very unlike epoxy oil."
McKeon and Chen have produced about a dozen genetically engineered castor
seedlings in their laboratory and greenhouse. They are applying for a patent
for their discoveries (U.S. Patent Application No. 60/167,360,
"Transformation of Ricinus communis, The Castor
Plant").—By Marcia Wood,
Agricultural Research Service Information Staff.
This research is part of New Uses, Quality, and Marketability of Plant and
Animal Products, an ARS National Program (#306) described on the World Wide Web
at http://www.nps.ars.usda.gov.
Thomas A. McKeon and
Grace Q. Chen are in the USDA-ARS
Crop Improvement and
Utilization Research Unit, Western Regional Research Center, 800 Buchanan
St., Albany, CA 94710; phone (510) 559-5754, fax (510) 559-5768. |
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"High-Tech Castor Plants May Open Door to Domestic
Production" was published in the
January 2001
issue of Agricultural Research magazine.
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