Premier Bacteria Suppress Wheat Take-All
Inside a growth chamber, technician Kurtis Schroeder examines
plants grown in soil samples collected from long-term wheat-cultivated fields
in nine states.
It was the fifth consecutive time that plant pathologists Jos M. Raaijmakers
and David M. Weller grew wheat seedlings in soil they'd collected from Lind,
Washington. The soil was special, but not uniquesimilar soils had been
found worldwide. These soils, on which only wheat had grown year after year,
naturally suppressed the fungus that causes one of the world's worst wheat
The scientists, working for USDA's Agricultural Research Service, were trying
to identify why wheat thrived in these soils when, in most places, it succumbed
to the ravages of take-all. The Gaeumannomyces graminis fungus blackens
plant roots and can reduce yields by 50 percent or more, costing U.S. wheat
growers millions of dollars annually.
Weller is a plant pathologist in the ARS Root Disease and Biological Control
Research Unit at Pullman, Washington. Raaijmakers, formerly a plant pathologist
with ARS, is now an assistant professor at Wageningen University in the
The researchers succeededand more. They discovered that specific
strains of bacteria possess extraordinary abilities to reproduce on roots and
fight the disease. Their discoveries not only give new hope for take-all
control, they may have broad applications for biological control of many
diseases in a variety of crops.
Natural suppression of take-all was first observed more than 60 years ago.
If a wheat crop was grown every year and no other crops were grown in the same
soil, the disease would eventually disappearor exist only at very low
levels. This phenomenon was labeled "take-all decline."
By the 1970s, several scientists, including ARS researchers, knew that
fluorescent Pseudomonas bacteria were somehow involved.
A decade ago, retired ARS plant pathologist R. James Cook (now with
Washington State University), Weller, and ARS geneticist Linda S. Thomashow
discovered a key to the role of the pseudomonads. They proved that some of
these bacteria produce antibiotics and showed that the antibiotics naturally
suppress the G. graminis fungus. And they isolated the genes responsible
for producing the antibiotics 2,4-diacetylphloroglucinol (Phl) and
phenazine-1-carboxylic acid (PCA).
The scientists also successfully transferred the antibiotic-producing
ability to other Pseudomonas strains. [For a brief history of important
research about take-all and its control, see Tackling Wheat Take-All,
Agricultural Research, August 1995, pp. 4-7.]
These findings opened the door to using the bacteria as a biological control
for take-all. But there were still questions. Although take-all decline
occurred worldwide, there were enough differences that scientists believed many
mechanisms were involved.
And using the Pseudomonas bacteria to suppress take-all presented a
challenge common to all biological control efforts: how to achieve
"Usually, you start with very large numbers of a biological control
organism, such as a million on each plant seed," says Weller. "But as
the control organisms compete with the hundreds of other soil microbes for
nutrients, their population declines sharply. That gives the fungus a chance to
regain a foothold."
Three Candidates Are Standouts
Near Pullman, Washington, (left to right) Kurtis Schroeder,
Linda Thomashow, Jim Cook, and David Weller examine healthy wheat thriving in a
filed infected by the fungus that causes wheat take-all.
Weller's team discovered that nature provided the solution. In three
Washington soils that naturally suppress take-all, they've identified three
different bacteria that outperform all the other microbes. "At least in
the U.S. Pacific Northwest, there seems to be just one mechanism for take-all
control," says Weller.
These bacteria have a specific genetic fingerprint that indicates superior
"These premier strains have two unique qualities that have never been
seen in any biological control agent. They colonize the wheat's roots very
aggressively, and they control disease when applied in relatively low
Unlike other bacteria, the populations of these premier strains decline much
less from competition with other organisms. Even starting with only 10,000
bacteria on a seed, the premier strains reproduce rapidly and soon reach the
level of millions necessary to suppress take-all. And they stay at that level
throughout the growing season.
The bacteria's ability to reproduce so rapidly has an enormous cost-savings
potential. "Every factor of 10 by which you decrease the dose, or number
of organisms appliedsay from 1,000,000 to 100,000
organismssignificantly reduces the cost of the biological control
treatment," says Weller. ARS has applied for a patent on the premier
bacterial strains, the genetic fingerprint, and the methods for finding similar
strains in other soils. Two companies plan to license the technology to produce
seed coatings or other control methods. Farmers should start to reap the
benefits in just a few years.
"With these strains, it is possible to control disease on as many as
1,000 acres with only a quart of bacterial culture," says Weller.
The scientists have demonstrated that in 2 to 6 years of cropping only
wheat, the bacteria's presence can change the soil from an environment that
favors take-all to one that naturally suppresses it.
Each soil may have a different bacterium that works best, but all these
premier bacteria possess the same qualities. The next step is to find the
actual genes responsible for their root-colonizing abilities. In the meantime,
the researchers can enhance the bacteria by using genetic technology they
The premier strains already produce the Phl antibiotic. "By adding a
gene, we can transform the bacteria so they also produce the antibiotic PCA,
while retaining their strong reproductive and competitive abilities," says
Strains that produce both antibiotics fight two other important root
diseases, Rhizoctonia root rot and Pythium root rot. These diseases cost
millions of dollars annually in lost yields and control chemicals.
The transgenic bacteria are also more potent. Only 100 to 1,000 of them are
needed per seed, compared with the 10,000 of the premier strain or the
1,000,000 typically used now for other biological control bacteria. Thomashow
and Weller are already testing the transgenic organisms in the field, with
approval from the U.S. Environmental Protection Agency.
Visiting Russian scientist Olga Mavrodi withdraws DNA from
bacteria taken from soil infected with wheat take-all. Computer-assisted
pattern analysis aids comparision of the DNA from different samples.
Even though they're powerful, they're safe. "There's no worry that the
bacteria will take over," says Thomashow. "They only thrive as long
as the wheat is growing, and they are part of the natural ecosystem
Once these bacteria become commercially available, cereal and turfgrass
growers will definitely benefit. But the researchers believe the findings have
a much broader application.
"We've illustrated that there is a biochemical and molecular basis for
biological control, and this should apply to a wide variety of diseases and
crops," says Thomashow.
The technology, she says, is generic. So far, each suppressive soil they've
examined contains a premier strain. "Take-all decline has been studied
extensively, but other instances of diseases naturally declining in the
environment have been recorded."
Weller is especially hopeful that the technique may be a boon to strawberry
growers as a replacement for methyl bromide. This fumigant has been essential
for strawberry growers to combat soilborne diseases, but it is scheduled for
elimination in the year 2005. Tomatoes, watermelon, and peas are other likely
crops to investigate.
"In each crop we've looked at so far, there appears to be a premier
strain. This may be a universal phenomenon," Weller says.By
Kathryn Barry Stelljes,
Agricultural Research Service Information Staff.
This research is part of the ARS National Program on Plant Sciences
described on the World Wide Web at
David M. Weller
Linda S. Thomashow
are in the
USDA-ARS Root Disease and
Biological Control Research Unit
, 365 Johnson Hall, Washington State
University, Pullman, WA 99164-6430; phone (509) 335-1116, fax (509) 335-7674.
"Premier Bacteria Suppress Wheat Take-All" was published in
the March 1999 issue of
Agricultural Research magazine.