Avidin: An Egg-Citing
Insecticidal Protein in Corn
Chemist Karl Kramer (left) examines the molecular interactions between biotin
and avidin, a biopesticide protein in the corn. Meanwhile, technician Thomas
Morgan examines insects feeding on avidin corn.
|"Smart" is the operative
word for plants and seeds that can fight off insect pestseven when stored
in grain bins for months.
Among the newest crop of "smarter" plants are genetically modified
corn plants containing a protein found in egg whites.
Agricultural Research Service chemist
Karl J. Kramer and colleagues at the Grain Marketing and Production Research
Center in Manhattan, Kansas, have shown that the protein, called avidin, is a
powerful growth inhibitor of insects. Their studies are now being used by a
Texas-based biotechnology company to help find uses for corn containing avidin.
Such plants may become an alternative to methyl bromidethe closely
regulated pesticide scheduled for phaseout in 2005.
Maize weevil, Sitophilus zeamaise.
|In 1994, the U.S. Department of
Agriculture launched a national integrated pest management (IPM) initiative in
which the ARS role has been to develop biologically based alternatives for
controlling insect pests, weeds, and crop diseases. Plant resistance and
biological control are key parts of the IPM program.
Each year, stored-grain insects cause multimillion-dollar losses for U.S.
producers of stored commodities such as corn, wheat, rice, and grain sorghum.
Biotina common vitaminis essential for insect growth and
development. Avidin restricts the availability of biotin, so the insect stops
developing and dies. Avidin may have a similar role in chicken egg
whitesto protect chicken embryos from disease-causing organisms that
require biotin to grow.
"As a biopesticide, avidin is better than Bacillus thuringiensis
(Bt) in corn because it has a knockout punch that hits a broader range of
insects," says Kramer. Insertion of the genes of Bt, a bacterial
biocontrol agent, allows corn plants to resist primarily moths and only a few
species of beetles.
Kramer and his collaborators first evaluated avidin as a biopesticide in the
early 1990s. He, ARS chemist Brenda Oppert, ARS biological research technician
Thomas D. Morgan, and Thomas Czapla, an entomologist at Pioneer Hi-Bred
International in Johnston, Iowa, studied the protein in artificial insect
diets. More recently, they studied transgenic corn (corn in which the genes
were modified) that contained avidinsupplied by ProdiGene of College
Technician Thomas Morgan
aspirates maize weevils into
a glass vial. Adult females will
be placed on kernels of avidin
maize to test the kernel's
resistance to the pest.
| When kernels of avidin corn were
infested with Angoumois grain moths or maize weevils, most of the larvae died
inside kernels that contained at least 20 parts per million of avidin. Cornmeal
obtained from the avidin corn was resistant to all common U.S. storage pests.
Meal with more than 100 ppm avidin killed larvae of lesser grain borers, red
flour beetles, confused flour beetles, sawtoothed grain beetles, flat grain
beetles, warehouse beetles, Indianmeal moths, and Mediterranean flour moths.
While avidin was lethal to many stored-grain pests, one species escaped death
in the ARS studies. Prostephanus truncatus, the larger grain
borerrarely found in the United Stateswas not affected. This
insect's tolerance could be from bacteria living within its cells that may
synthesize biotin or from an enzyme in its gut that may inactivate avidin.
Overall, effectiveness of avidin corn hinged on the amount eaten and how long
it was consumed. Most of the insects fed corn with 100 ppm avidin were unable
to grow and develop, and about half were killed by concentrations as low as 30
"Avidin corn can benefit farmers by offering grain buyers a higher quality
corn that can be stored longer without pesticides," says Kramer. "It
also means that products made from avidin corn will have a longer shelf life.
Because the corn has this property, one proposed use of it is as a background
germplasm for production of other valuable recombinant proteins in corn."
Avidin corn is now grown by U.S. farmers under contract with Stauffer Biotech
in Aurora, Nebraska. The avidin compound is currently used in medical and
biochemical kits as a diagnostic protein. When produced from chicken eggs,
avidin sells for about $3,000 per gram. But the cost is lower when the chemical
is produced from cornnow being done by the Sigma-Aldrich chemical company
in St. Louis, Missouri.
Although avidin is already a common protein in human diets, the Food and Drug
Administration will require a thorough risk assessment of avidin corn before
approving it as a food or feed product.
One challenge researchers encountered was determining the concentration of
avidin in corn kernels. Only half of the kernels in the corn varieties
contained avidin because male sterility occurs in avidin-positive plants. An
immunoassay can measure avidin concentration, but this method requires
extracting the protein from ground-up kernels.
Kramer asked ARS agricultural engineer Floyd E. Dowell and entomologist James
E. Throne to use less destructive near-infrared technology to analyze kernels.
This helped solve the problem of distinguishing kernels with high avidin
"We want to do more work to ensure that every kernel has a consistent
amount of avidin," says Kramer.
The only insects affected are those that actually feed on plants because, in
this case, the plant is the sole source of the biopesticide. That means
beneficial insects and others that don't feed on the crop would not be
affected. Groundwater, too, is protected from contamination because avidin is a
Avidin imparts to corn a kind of host plant resistance that can last from the
field to the storage bin. Unlike chemical insecticidal sprays that can be
washed off by rain or inactivated by ultraviolet rays, avidin works regardless
of the weather.
To avoid any problems that could result from avidin being expressed in corn
pollen, Kramer says that molecular techniques will use specific promoters that
place avidin not in the pollenbut in the seed, roots, stems, or
leaves.By Linda McGraw,
Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Karl J. Kramer,
Brenda Oppert, and
James E. Throne are in the USDA-ARS
Biological Research Unit, Grain Marketing
and Production Research Center (GMPRC), 1515 College Ave., Manhattan, KS
66502; phone (785) 776-2711, fax (785) 537-5584.
Floyd E. Dowell is in the GMPRC
Engineering Research Unit; phone (785) 776-2753, fax (785) 776-2792.
"Avidin: An Egg-Citing Insecticidal Protein in
Corn" was published in the
issue of Agricultural Research magazine.