By Gillian Klucas, IANR News Service

For nearly 40 years, farmers have turned to an effective, environmentally friendly herbicide to kill broadleaf weeds in grassy crops, such as wheat and corn. But it has been off-limits for broadleaf crops.

Dicamba-based herbicides, sold under trade names such as Banvil and Clarity, are relatively inexpensive and easy on the environment because the chemical disappears quickly in plants and soil. But like all broadleaf herbicides, dicamba can't distinguish broadleaf crops from their weedy cousins so it can't be used to kill weeds in soybeans, cotton, tobacco and vegetables.

Instead of trying to develop a smarter herbicide, University of Nebraska-Lincoln scientists decided to help broadleaf crops resist dicamba. They went to the source that makes the herbicide environmentally friendly -- a soil microorganism that easily breaks down this synthetic chemical.

"With this new technology we feel relatively confident that we will be able to produce soybeans, cotton, canola, certain vegetable crops and even certain trees that could be sprayed with dicamba with little or no effect on their productivity," said Don Weeks, the Institute of Agriculture and Natural Resources biochemist heading the research.

Researchers chose one of several thousand bacterial species they found thriving in soils at a dicamba manufacturing plant. These bacteria grow on dicamba by breaking it down and using its carbons as a sole energy source.

The team's goal: identify and isolate the gene responsible for dicamba inactivation, then insert that gene into a broadleaf plant, thereby transferring dicamba resistance.

To find the gene, the team worked backward, tracking down the enzyme doing the gene's work. Researchers discovered a complex system consisting of three enzymes. By decoding the sequence of amino acid building blocks that make up the three components, they deduced the genetic codes for the genes that make the enzymes.

The next challenge was to genetically engineer these genes and get them into a plant.

Plant Scientist Tom Clemente, head of the university's Plant Transformation Core Facility, helped the team insert these genes into a plant's chromosomes.

That's when the team discovered that the plant's own ferredoxin can easily substitute for the bacteria's ferredoxin. That meant they needed only one of the genes to create dicamba-resistant transgenic plants.

They also discovered they could modify the gene to target the DNA of chloroplast, where photosynthesis takes place.

Using the chloroplast has two benefits, Weeks said. Ferredoxin is most abundant in the chloroplast of cells so it creates greater dicamba resistance. Also, chloroplast genes are inherited through the maternal side, not through male pollen. This has practical implications in the field. When the foreign gene is inserted into the chloroplast DNA, the genetically modified crop can't spread resistance to other plants through pollen carried by wind or insects.

"In the long term, this approach may help to calm people's fear that there could be gene drift through the pollen," Weeks said.

Scientists know how to insert DNA into the chloroplasts of only a few plant species, so the IANR researchers created all but a few plants by altering the nuclear DNA. But, Weeks said, it's just a matter of time before chloroplast DNA insertions are possible for major crops.

So far, the team has grown dicamba-resistant tomatoes and tobacco in the greenhouse. Tobacco plants sprayed with the equivalent of 25 pounds of dicamba per acre -- 50 times the typical field application -- show little or no damage.

Now, they're concentrating on producing dicamba-resistant soybeans, and plan to create resistant canola and cotton. They hope to field test their dicamba-resistant soybeans in the next two years.

The university is patenting this new technology. Weeks estimates dicamba-resistant crop seed might be commercially available within seven years.