Almost every year, a disease called head scab wreaks havoc somewhere in
Scab now ranks as the worst plant disease to hit the United States since the
stem rust of the 1950s. In the past decade, outbreaks in Illinois, Indiana,
Michigan, Minnesota, North Dakota, Ohio, and South Dakota have led to more than
$1 billion in crop losses.
Geneticist Ann Blechl looks at root growth on genetically engineered wheat
plants that may carry new genes for resistance to Fusarium.
Caused by a fungus known as Fusarium graminearum, scab gets its name
from the whitish-gray lesions that form on the kernel-bearing portion, or head,
of infected wheat or barley plants. The Fusarium microbe can also cause
root, stalk, and ear rots in corn.
Infected wheat kernels often shrivel and become discolored, and they may
harbor compounds called mycotoxins that are manufactured by the fungus. Most
notable of these natural compounds is deoxynivalenol, which can make wheat
unsuitable for flour or cereals and too toxic for sale as animal feed.
Today, Agricultural Research Service
scientists at a half-dozen laboratories around the United States are exploring
an array of tactics to thwart the fungus.
Deactivating the Toxin
One defensive strategy relies on exploiting the mechanism apparently used by
F. sporotrichioides, a relative of F. graminearum, to protect
itself from its own toxin, known as T-2.
"Because both of these Fusarium species produce toxins the same
way," explains ARS microbiologist Nancy J. Alexander, "the process
they use to deactivate those toxins may also be similar."
Alexander and colleagues with ARS at Peoria, Illinois, foundin F.
sporotrichioidesa T-2 resistance gene that they named TRI-R.
"This gene," says Alexander, "cues the fungus to make an
enzyme that alters the structure of the toxin. The enzyme does this by placing
a protective chemical group on the toxin. The altered toxin apparently can't
harm the fungus."
"Equipping wheat and barley plants with a gene to deactivate the toxin
made by the scab fungus, F. graminearum, might give the plants a new and
powerful form of protection."
Expelling the Toxin
Another organism in naturethe Saccharomyces cerevisiae yeast
used in breadmakinghas a gene called PDR5 that can pump toxins out
of its cells. Alexander and co-researchers borrowed PDR5 from scientists
at The Catholic University of America in Washington, D.C. They rebuilt it to
work in plants.
Now, ARS colleagues in Fargo, North Dakota, are moving PDR5 and
TRI-R into barley, while co-workers in Albany, California, are
transferring the genes into wheat.
Plant pathologist James D. Miller (right) inoculates progeny of a
durum-emmer cross with Fusarium fungus as geneticists Leonard R. Joppa
(left) and Norman D. Williams assist in selcting flowering heads for testing.
ARS geneticist Lynn S. Dahleen at Fargo has some healthy plants with the
TRI-R gene working inside and others that boast the PDR5 gene.
"Now our goal," says Dahleen, "is to get both genes up and
running in the same barley plantpreferably one that's either a commercial
variety or at least similar to a commercial barley."
One target of Dahleen's experiments: MNBrite, a commercial barley from the
University of Minnesota. The North Dakota Barley Council is financing some of
A handful of wheat plants nurtured in an Albany lab and greenhouse are also
now outfitted with either the PDR5 or TRI-R gene. Ann E. Blechl
and Patricia A. Okubara at Albany used a gene gun, or bioblaster, to propel
gold particles coated with these genes into wheat cells.
Fusarium-resistance testing will follow. The North American Millers
Association is funding part of this research. Proteins To Knock
Blechl and Okubara are also examining wheat genes that may knock out the
In tests elsewhere, proteins produced on command by these genes hindered
still another F. graminearum relative, F. oxysporum. A scientist
at the University of Zurich provided one of these experimental genes. It tells
plants to make what's known as a thaumatin-like protein. A second gene,
isolated by Kent F. McCue at the Albany center, directs plants to produce a
protein called purothionin.
Another protein in the thionin familyhordothioninis a target of
research by ARS scientists and their colleagues in Madison, Wisconsin.
ARS molecular biologist Ronald W. Skadsen says, "We think hordothionin
and another protein, permatin, may play a role in keeping Fusarium from
gaining a foothold in barley kernels." Skadsen and colleagues cloned two
genes that barley plants use to make the proteins.
"In barley," Skadsen says, "these proteins are found only
inside the kernel. We're hoping to rebuild the genes so that plants will
produce the proteins on leaflike structures that surround the kernel, where
Fusarium can begin its attack."
The American Malting Barley Association and the North American Barley Genome
Mapping Project help fund the research. Help From a Wild Relative
Lighter, discolored barley heads are infected with Fusarium head scab
A wheat cousin called wild emmer has resistance genes that might be moved
into domesticated wheat. Emmer is a relative of durum wheat, which is used to
make flour for pasta and for pizza doughs.
North Dakota State University researchers, along with ARS plant pathologist
James D. Miller and ARS geneticists Leonard R. Joppa and Norman D. Williams at
Fargo, are crossing emmer with domesticated durum wheats. The work gets some
support from grants from the Agricultural Production and Utilization Committee
of North Dakota, North Dakota Wheat Commission, and U.S. Durum Growers
In other experiments, they are crossing wild emmer with special wheats
"Aneuploids are invaluable to us," says Williams, "because
they take up individual emmer chromosomes intact. That makes it easier to find
which emmer chromosome contains a resistance gene. Once we locate that
chromosome, we may be able to pinpoint the location of the resistance gene on
itor at least that of an associated marker gene."
Markers Tell Us Where It's At
A marker gene is a sequence of DNA that acts somewhat like a signpost to
indicate the presence of a gene of interest. "When we find wheat-emmer
hybrids that have a resistance gene or marker," Williams says, "we
can select the best of these plants for further breeding."
"The more scab-resistance markers scientists can identify," says
geneticist James A. Anderson, "the closer we should all come to locating
the actual resistance genes that we're seeking." Anderson, formerly with
ARS at Pullman, Washington, and now with the University of Minnesota at St.
Paul, is among the first to find a scab-resistance marker in wheat.
He did the work with researchers at North Dakota State University. The
research was funded by a USDA National Research Initiative Grant, the North
American Millers Association, and the North Dakota State University Research
In all, the team discovered five markers. They crossed a scab-resistant
wheat from China, called Sumai 3, with a moderately susceptible variety, called
Stoa, from North Dakota. Then they examined the offspring.
Geneticist Robert Busch examines a newly developed, high-yielding,
scab-tolerant spring wheat variety.
"We chose a Chinese variety as one of the parents," says Anderson,
"because some Chinese wheats appear to have the best natural
The researchers located markers for two genes that, Anderson says,
"seem to have significant impact on scab resistance. The three other
markers indicate genes that make minor contributions."
Other promising markers have been discovered by scientists with the
University of Illinois at Urbana, working with ARS molecular biologists Guihua
Bainow at Peoriaand Leslie L. Domier at Urbana. They crossed Ning
7840, a scab-resistant Chinese wheat, with Clark, a susceptible variety
developed in Indiana.
"One marker," says Bai, "is linked to a gene that may account
for up to 50 percent of the scab resistance. Several other markers are for two
minor genes that may account for another 20 percent of the resistance."
Further research may reveal other new markers linked to different resistance
genes. "Resistance from different genetic sources," says Bai,
"is the best option, because it will give breeders a larger pool from
which to develop new varieties for the future."
Screening for Resistance
The best-performing plants from these and other experiments may make their
way to the Uniform Regional Scab Nursery, a rigorous program of scab-resistance
screening of new wheats for the upper Midwest. ARS plant geneticist Robert H.
Busch, who is at St. Paul, coordinates the nursery, helping breeders field-test
Previously, Busch helped oversee distribution of USDA funds for
scab-resistance research at ARS labs and universities nationwide. This year
those funds totaled $3.5 million.
In his own breeding research, Busch develops scab-tolerant wheats for the
upper Midwest, including one he is releasing this year. "Through both
conventional breeding and biotechnology," Busch says, "we expect to
be able to offer high-yielding wheats with even more scab resistance in the
Some of the other new wheats and barleys for the upper Midwest might contain
genes discovered or rebuilt by Busch's ARS colleagues at St. Paul. William R.
Bushnell, for example, is developing a faster, less expensive test to determine
if experimental scab-resistance genes can help cells foil Fusarium.
Plant physiologist William Bushnell and technician Tessa Goff examine a
computerized image of red barley cells expressing antifungal genes shot into
He links a gene he is testing for scab resistance to another gene that cues
plants to make anthocyanin, a natural pigment that gives fall leaves their red
colors. After the genes are shot into plant tissue, cells with the anthocyanin
gene turn red. This means they have likely taken up the gene for possible scab
resistance, as well.
When the tissue is inoculated with Fusarium and monitored under a
microscope, the distinctive color makes it faster and easier to pinpoint these
In addition to giving plants new, more powerful genes to boost scab
resistance, scientists are scrutinizing about 700 microbes for their potential
to serve as biological controls of the fungus.
ARS plant pathologist David A. Schisler at Peoria and colleagues at Ohio
State University are testing the microbes for their ability to gobble up two
compounds naturally present on wheat heads when Fusarium strikes. The
compounds are choline and betaine. They nourish the fungus as it grows from the
male organs of wheat flowerswhere it sometimes landsto where it can
infect the developing kernel deeper inside the flower.
Says Schisler, "We are also perfecting a liquid fermentation medium to
economically produce the most effective microbes." The medium could be
sprayed on wheat during flowering.
Other new tactics for outflanking the Fusarium fungi might come from
sleuthing the microbe's development. To stalk the fungus, former Peoria
microbiologist Thomas M. Hohn borrowed a gene from a bioluminescent jellyfish,
Aequorea victoria, and hooked it to the fungus.
The jellyfish gene causes the fungus to glow a telltale green when
fungal-infected tissues are exposed to fluorescent light. Though the idea of
using the jellyfish gene in lab experiments isn't new, Hohn was apparently the
first to use it for spying on Fusarium.
"This innovative pairing of jellyfish and Fusarium," says
molecular biologist Skadsen, "should shed new light on the biology of this
destructive fungus."By Marcia Wood,
Linda Cooke McGraw, and
Kathryn Barry Stelljes,
Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program
described on the World Wide Web at
Contact scientists mentioned in this story through:
Marcia Wood, phone (510) 559-6070,
fax (510) 559-5882.
Don Comis, phone (301)
504-1625, fax (301) 504-1641.
"Fighting Fusarium" was published in the
June 1999 issue of Agricultural