By STEVE BROWN

Capital Press

Researchers have found a chemical in the potato scab bacterium that could be an effective herbicide for organic growers.

The current issue of the journal Nature Chemical Biology describes a key step toward commercial production of thaxtomin, which occurs naturally in Streptomyces bacteria and could be used as a pre-emergence herbicide.

"I got involved in potato scab early in my career," said Rosemary Loria, a co-author of the article and chair of the plant pathology department at the University of Florida. "As we started discovering more about (its) bacterial structure, a Canadian scientist found the thaxtomin. What we did was find the biosynthetic pathway for thaxtomin and defined it genetically."

In the study, the researchers describe an enzyme in the bacteria that is essential to producing the herbicide. The discovery could allow researchers to increase the amount of thaxtomin the bacterium produces, allowing commercially viable production of the chemical.

"So we could within the next couple of years be able to create a strain of bacteria that would produce much higher levels of thaxtomin than the natural strains produce, and that will assist in getting thaxtomin onto the market," Loria said.

The new strains of bacteria would be selected in the lab and kept in the lab, she said, so they would never be a danger to potato crops.

Because the herbicide is produced from bacteria rather than created in a lab, it can be used on organic farms, she said.

The process would not involve genetic modification, she said. Rather it would be by "mutation and selection, similar to the process used to select strains of antibiotic producing microbes for higher antibiotic production."

Evan Johnson, another co-author of the study, said the advantage of natural herbicides is they target specific organisms and break down more easily in the soil.

Thaxtomin works by hindering the ability of the plant to build a normal cell wall during rapid growth.

"When so much of a plant is involved in growth," Loria said, "without a properly configured cell wall, the plant will fail."

Andy Jensen, director of research at the Washington State Potato Commission, said potato scab is a common disease of only localized concern in most of the Northwest.

"Scab is pretty much everywhere, but some fields have more problem with it than most," he said. "Potatoes are graded on surface blemishes, so scab affects marketability more than yield."

Of greater concern to Northwest growers are late blight, flea beetle and zebra chip, he said.

Finding a solution in the middle of a disease is not unheard of. Loria described other studies in plant pathology where learning the mechanisms by which pathogens can manipulate plant cells can lead to important discoveries regarding the function of those cells.

For example, recently discovered proteins in Xanthomonas -- a bacterium affecting citrus, rice, tomatoes, peppers, soybeans and cotton -- can enable genetic engineering of the plants.