In This Issue

Research Uses Biotechnology
In Search for Nematode Resistance

Soybean cyst nematode (SCN) ranks as the number one pest of soybeans with yearly losses in Illinois alone estimated as high as $120 million.

The SCN problem has persisted across much of the Midwest despite significant efforts to develop resistant varieties from soybean germplasm using classical breeding techniques.

But, in research underway at the University of Illinois, scientists are now expanding the search for SCN-resistant genes to a wide range of other sources by applying the techniques of biotechnology.

"The traditional way of making soybean plants with resistance or tolerance to SCN is to identify genes in populations of soybeans and to breed these genes into favorable varieties," says Stephen Farrand, professor of molecular biology in the U of I's Department of Crop Sciences. "What we are trying to do is to use the new tricks of biotechnology to look for genes from sources such as bacteria or other species of plants that can be put into the soybean to confer unique types of resistance to SCN."

Stephen Farrand (left), professor of molecular biology in the Department of Crop Sciences, and Hyeon-Je Cho, post doctoral research associate, use a fluorescence dissecting microscope to examine soybean "fuzzy roots" that have been genetically modified. The target genes are inserted with a marker that allows them to be easily screened for resistance to soybean cyst nematode.

This project is funded by the Illinois Soybean Checkoff Board in collaboration with the United Soybean Board. Along with Farrand, the U of I research team includes plant physiologist Jack Widholm, USDA nematologist Greg Noel, and post-doctoral research associate Hyeon-Je Cho.

Farrand notes that the first step is to identify genes of interest that might have an effect on infection by SCN. The principle tool in this process is the bacterium known as Agrobacterium rhizogenes.

"This is one of a group of bacteria that have become the plant genetic engineer's favorite tools because they know how to send any DNA to plants," he says. "The DNA gets integrated directly into the plant and, if the DNA contains a properly constructed gene, the plant will express the trait in which we are interested. What's unique about Agrobacterium rhizogenes is that it causes plants to produce transformed roots—called hairy roots—that contain a piece of bacterial DNA."

These so-called hairy roots can be cut from the plant and grown virtually forever in an artificial medium in the laboratory.

"We know that we can use this technique to get nice hairy roots on soybeans and that we can grow the nematodes on these cultured roots," Farrand says. "This allows us to grow nematodes on the roots in the laboratory instead of on plants in the greenhouse."

The use of Agrobacterium rhizogenes provides an additional advantage because the bacterium can transfer a second gene along with the gene that causes the hairy roots.

"Suddenly, we have an efficient method for introducing foreign genes into soybean plants and testing them for their ability to confer nematode resistance." Farrand says.

As part of the project, Widholm is working to identify and clone foreign genes of interest. The researchers are looking at a number of different types of genes in that process.

"For example, much of the outer layer of a nematode is made up of complex chemicals, including collagen," Farrand says. "Many bacteria make enzymes that digest these products. Putting genes for these enzymes into the soybean could confer a potent form of resistance."

He explains that the new system allows researchers to screen through these genes to identify those that work best without the worry of having to develop a transgenic soybean plant at that stage of the project. It also lets the researchers focus on the root, which is the site of SCN infection.

"Another major advantage is that the types of genes we are looking at are not race specific," Farrand says. "If you put a collagenase gene in the root and it affects the outer layer of a nematode, then it would not make any difference which race the nematode came from."

"Because SCN infects roots, it is perfectly conceivable to place the resistance gene behind an on-off switch that is on only in the root," Farrand says. "The product of this gene would not show up in the seed. So the foreign gene product would be absent in that part of the soybean that is used for commercial purposes."

The biggest advantage for this technique, however, is that researchers are no longer restricted to soybean germplasm as the source of new genes. Nevertheless, once a candidate gene has been identified as useful, then researchers still face the hurdle of getting it into a transgenic soybean plant.

"Once that has been done, it will still be up to breeders to move that gene into cultivars with high yield and other disease or herbicide resistant traits," Farrand says. "Although these molecular techniques cannot supplant classical breeding, they can identify and provide the breeder with genes from a much wider range of sources."

The NSRL Bulletin is published three times a year by the National Soybean Research Laboratory at the University of Illinois, 170 Environmental and Agricultural Sciences Building, 1101 W. Peabody Drive, Urbana, IL 61801; telephone (217)244-1706; e-mail nsrl@uiuc.edu; FAX (217)244-1707. Steven T. Sonka, director; Robert J. Wynstra, editor; David Riecks, photographer; Lynn Hawkinson Smith, graphic designer.

Unless otherwise stated, articles may be reproduced or quoted if credit is given to the NSRL Bulletin. The National Soybean Research Laboratory at the University of Illinois is an affirmative action and equal opportunity institution.