Smart corn
By Linda McGraw

Look out Bt corn. Here comes avidin. Research on this egg white protein is showing that when inserted into corn plants, it can kill a broader range of insects when ingested by the corn pests compared to the Bt protein. The egg protein may be the answer to the replacement of methyl bromide.

Smart is the operative word for plants and seeds that can fight off insect pests--even when those seeds have been 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 (ARS) chemist Karl J. Kramer and colleagues at the Grain Marketing and Production Research Center in Manhattan, Kan., have shown that a protein called avidin is a powerful growth inhibitor of insects. The 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 bromide--the closely regulated pesticide scheduled to be phased out by 2005.

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, insects cause multimillion-dollar losses for U.S. producers of stored commodities such as corn, wheat, rice and grain sorghum.

Biotin--a common vitamin--is 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 whites--to 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," said 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.

Biopesticide tests
Kramer and his collaborators first evaluated avidin as a biopesticide in the early 1990s. He and his team studied the protein in artificial insect diets. More recently, they studied transgenic corn (corn in which the genes were modified) that contained avidin--supplied by ProdiGene of College Station, Texas.

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 (ppm) of avidin. Cornmeal obtained from the avidin corn was resistant to all common U.S. storage pests. Meal with more than 100 ppm of 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 borer--rarely found in the United States--was 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 ppm.

"Avidin corn can benefit farmers by offering grain buyers a higher-quality corn that can be stored longer without pesticides," said 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 grown by U.S. farmers under contract with Stauffer Biotech in Aurora, Neb. The avidin compound is 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 corn at the Sigma-Aldrich chemical company in St. Louis, Mo.

Safety
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.

A challenge researchers encountered was determining the concentration of avidin in corn kernels. Only half of the kernels in the corn 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 kernels.

Kramer asked an ARS agricultural engineer and an ARS entomologist to use a less destructive near-infrared technology to analyze kernels. This helped solve the problem of distinguishing kernels with a high-avidin content.

"We want to do more work to ensure that every kernel has a consistent amount of avidin," said 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 biodegradable protein.

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 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 pollen but in the seed, roots, stems or leaves.

Linda McGraw works for USDA's Agricultural Research Service.