West Lafayette, Indiana
November 26, 2003
A western
American pasture grass crossed with wheat can improve resistance
to a fungus that can be toxic to plants, animals and people,
according to Purdue University
researchers.
Resistance
genes in the grass that replaced genes in wheat
increased protection against Fusarium head blight, or wheat
scab, the scientists said. In the December issue of the journal
Theoretical and Applied Genetics the researchers also report
that they located and mapped the small bits of DNA, or markers,
associated with the resistance gene in the grass, called tall
wheatgrass.
"In the
past 10 or 15 years, the fungus Fusarium graminearum has emerged
as one of the diseases of primary concern in wheat," said
Herb Ohm, Purdue agronomy professor. "This is because the
widespread practice of reduced tillage in fields provides a
perfect environment for growth of the fungus."
Reduced
tillage, meaning the soil is not plowed for planting,
cuts farmers' costs and helps prevent erosion, he said. In the
eastern United States, the upper Midwest and other places where
large amounts of corn and wheat are both grown, Fusarium is a
major problem, especially when the weather is warm and humid or
rainy. Corn stalks left as natural mulch after harvest also
foster fungus growth.
The fungus
causes head blight that leads to major wheat crop losses.
In 1996, crop losses due to Fusarium totaled at least $38
million just in Indiana, according to the U.S. Department of
Agriculture.
"The
disease has occurred most years since the early 1990s," Ohm
said. "Its increase in frequency and severity coincide with
reduced soil tillage, along with favorable weather - warm, humid
conditions - for several weeks prior to and during wheat
flowering in mid- to late-May." |
 |
|
Both wheat
plants displayed by Purdue agronomy professor Herb Ohm have been
infected with Fusarium head blight, or wheat scab. The plant on
the left was crossed with a pasture grass to create a high level
of resistance to the fungus, which is one of the primary
diseases affecting wheat production. (Agricultural
Communications photo/Tom Campbell) |
The fungus
also produces a mycotoxin that sickens animals and people. Pigs,
cattle, horses, poultry and people can develop vomiting, loss
of appetite, diarrhea, staggering, skin irritation and
immunosuppression when they eat grain or hay infected by
Fusarium. The most severe cases can be fatal.
Research
has found evidence that these toxins may be
cancer-causing. People usually ingest the fungus when they eat
contaminated grains and cereals. According to the United
Nations' Food and Agriculture Organization, people in developing
countries face the greatest risk from Fusarium mycotoxins.
"Fusarium
production of mycotoxins is a more serious problem than
wheat production loss," Ohm said. "The toxin results in complete
loss because you can't use the grain to make food for people or
livestock."
The fungus
can infect most cereal grains, including corn, wheat, barley and
some oats.
Replacement
of the wheat gene was done with conventional crossbreeding and
selection and didn't involve any genetic engineering. Because
the two plants are closely related, the wheat is not altered,
except for the added protection against Fusarium.
The newly
identified resistance gene in the wheat grass is on a different
chromosome in the genome than other known resistance genes used
in wheat. This will enable researchers to combine the
newly discovered effective resistance gene from wheatgrass with
other genes that protect wheat against Fusarium. This breeding
of a plant with more than one resistance gene is called gene
pyramiding.
"For some
diseases, such as Fusarium, a single resistance gene will
not give you complete resistance," Ohm said. "So we try to
identify genes from different resistant varieties or sources
that will give some resistance.
"Then we
use genetics to determine whether resistance genes from
two different sources are on different locations in the genome.
If they are, then we can pyramid them."
Now that
Ohm and his team of researchers know they can combine the
tall wheatgrass resistance gene with other resistance genes,
they will try to produce a line of wheat with several genes
resistant to Fusarium. The seed will then be available through
the U.S. Department of Agriculture-Agricultural Research Service
laboratory in Aberdeen, Idaho, that is a seed repository for
wheat lines from around the world.
"The whole
basis of plant breeding is to put the favorable genetic traits
of different parent lines into one progeny line," Ohm said.
"Prior to
the DNA era, we had to rely on characterizing plants just
on phenotype (observable traits). For certain traits that's
fairly easy to do. For many traits it's difficult because of
environmental effects.
"Some
phenotypes have good crop yield in one environment but not
in other environments. This makes it difficult to determine
which genes affect yield in one wheat variety compared to
another."
For
instance, under perfect conditions, one wheat line might have
a very high yield but may not in an arid climate. By using DNA
to compare traits, environmental impact is not a factor because
if a gene is present and activated, then scientists can
ascertain if a characteristic is genetic or environmental, Ohm
said. The bits of DNA known as markers allow scientists to
determine more quickly whether the gene carrying the desired
trait is present in a plant.
Other
researchers on this study were Xiaorong Shen and Lingrang
Kong, both postdoctoral fellows in Ohm's research group.
The Ag
Alumni Seed Improvement Association and Purdue
Agricultural Research Programs provided funding for this
research.
Writer:
Susan A. Steeves, (765) 496-7481,
ssteeves@purdue.edu
Source: Herbert Ohm, (765) 494-8072, hohm@purdue.edu
Related Web
sites:
Herbert Ohm:
http://www.agry.purdue.edu/staffbio/hwobio.htm
Department
of Agronomy:
http://www.agry.purdue.edu/
Purdue
Genetics Program:
http://www.biochem.purdue.edu/~genetics/
Purdue/USDA-ARS Wheat Genomics: http://www.btny.purdue.edu/usda-ars/wheatgen/
Theoretical and Applied Genetics
ABSTRACT
Fusarium head blight resistance in hexaploid wheat-Lophopyrum
genetic lines and tagging of the alien chromatin by PCR markers
Xiaorong Shen, Lingrang Kong, Herbert Ohm
The
objective of this research was to identify Fusarium head
blight (FHB) resistance in wheat (Triticum aestivum)-Lophopyrum
genetic lines that might complement FHB resistance in common
wheat; and identify DNA markers that can be used to tag the
resistance in the alien (E genome) chromatin in the development
of improved wheat cultivars. FHB resistance was evaluated in 19
Chinese Spring-Lophopyrum elongatum (EE) substitution lines, two
Thatcher-L. ponticum (el1 and el2) substitution lines, and four
Thatcher-L. ponticum translocation lines. Significant resistance
was identified in the 7E(7A), 7E(7B), and 7E(7D) substitution
lines. The homoeologous chromosome, 7el2, also showed resistance
in the Thatcher genetic background. Both the
Thatcher-7el1 substitution and translocation lines were
susceptible like Thatcher, indicating no resistance on the 7el1
chromosome. Simple sequence repeat (SSR) and cleaved amplified
polymorphic sequences (CAPS) in homoeologous group 7 chromosomes
were used to identify DNA markers that located on 7E and 7el2.
As expected, the transferability of wheat SSR markers to
Lophopyrum is low. Of 52 SSR markers that we tested, only five
were found to be co-dominant on 7E of L. elongatum versus 7A,
7B, and 7D, one of which is also positive on 7el2. A CAPS
marker, derived from the RFLP probe Xpsr129, can serve as a
dominant marker for 7el2 chromatin. |
