High-Tech Detectives Seek Natural Control for Take-All

PULLMAN, Wash.–Roll down your window next time you drive past a wheat field. Listen carefully. Or better yet, get out and grab a handful of soil. Hidden in that soil is a conversation between the wheat’s roots and a variety of soil bacteria. It takes a special ear, though, to hear this “cross talk,” says USDA-Agricultural Research Service plant pathologist David Weller. For the conversation takes place via chemical signals.
Identifying these signals and other chemicals produced during interactions between bacteria and wheat is one part of a cooperative effort between scientists in Washington State University’s Department of Plant Pathology and those of the USDA-ARS. Because the scientists are from different fields – molecular genetics, microbiology and bioanalytical chemistry – it’s also a cooperation that offers a unique all-around approach to plant disease.
The scientists expect to use what they learn to develop biocontrol agents that will be useful for combating a variety of wheat and barley diseases. Biocontrol involves using one organism, such as bacteria, to control another, such as a fungus.
“Biocontrol measures are going to become more necessary,” says Weller, “for it’s clear that ecological concerns are coming to have equal status with agricultural concerns. There is tremendous pressure from society on agriculture to reduce the use of pesticides.”
Research technologist Bob Bonsall is responsible for identifying the soil chemicals, and he does it in a lab that looks far removed from plants and farm fields. It’s all machines, in various shades of beige, with nothing green in sight except for a line or two on a computer screen.
Bonsall, Weller and their colleagues do most of their research on “take-all,” a root disease of wheat caused by the fungus Gaeumannomyces graminis var. tritici. Take-all occurs everywhere that wheat is grown and results in millions of dollars in losses each year. Currently there’s no cure. A farmer can either change to a less profitable crop or keep planting wheat and wait through several years of reduced crops for a natural resistance to build up in his soil, a process that results in “take-all decline.”
“We’re using the natural phenomenon of take-all decline as the basis for a novel approach for the biological control of the disease,” says Weller. “We’re looking to nature for solutions.”
What they know now is that wheat plants in soil that develops take-all decline are not bystanders in the process. The plants attract bacteria that cause the decline by signaling to them. The bacteria then colonize the area around the plants’ roots and produce small, localized amounts of the antibiotics that ultimately result in the decline.
For the scientists, the key to using nature is understanding how the bacteria work, how they “do their thing,” says Weller. Then the bacteria can be manipulated. They might be made to produce more or better antibiotics, or to respond more quickly to the signals from wheat. Once improved strains of bacteria are produced, wheat seeds can be inoculated with them prior to planting. Instant take-all decline.
When Bonsall joined the collaboration several years ago, no one was using sophisticated analytical equipment to look at the antibiotics, and no one was trying to find them in the natural soil environment. Bonsall has not only developed a method of isolating the minute quantities of antibiotics and other chemicals found in the natural soil environment, but he’s also developed techniques for separating and identifying them using sophisticated analytical equipment – those beige machines in his lab. One important early result was the resolution of 50 years of speculation about the role of antibiotics in soil: his work has shown that antibiotics are present in the natural soil environment and result in the control of take-all.
This year a mass spectrometer was added to the lab’s equipment inventory. It allows Bonsall to identify previously unknown substances in the soil – such as other antibiotics, the signals that pass between the bacteria and the plant, or other unknown compounds made by bacteria that may be critical in biocontrol or growth-promoting activities.
In just the first two months the spectrometer was operating, Bonsall found and identified two new antibiotics. If they exhibit high anti-fungal activity, the molecular geneticists will take over and determine if either can be incorporated into better bacteria for controlling take-all.
“We’re the only ones taking a team approach, combining tools of molecular biology and bioanalytical chemistry to do this work,” says Bonsall. They’re also the only ones in the world using these analytical equipment to determine the specific components of interactions that take place in soil between biocontrol agents and plant hosts, which is why this WSU lab gets requests from around the world to analyze soils.


by Mary Aegerter
for WSU News and Information