Wooster, Ohio
March 5, 2007
A new way of characterizing
partial resistance to one of the most devastating soybean
diseases may enable germplasm companies to incorporate effective
genes more quickly into plant lines that are the most beneficial
to growers.
Ohio State University plant
pathologists and soybean breeders are teaming up with
researchers from the Virginia Bioinformatics Institute and the
Department of Crop and Soil Environmental Sciences at Virginia
Polytechnic Institute to study the mechanisms of partial
resistance to Phytophthora sojae – the pathogen that causes
Phytophthora root and stem rot. With a five-year, $6.74 million
National Science Foundation grant, the team is evaluating 289
genetic lines from a Virginia plant population to identify those
with partial resistance. The researchers are using microarray
chips – technology that produces an instant readout of which
genes might be most useful in producing germplasm with high
levels of partial resistance.
Anne Dorrance, an Ohio State plant pathologist with the Ohio
Agricultural Research and Development Center, said that using
microarray chips produces a more informative result than the
standard marker technology.
“A standard marker amplifies a region in a genome where
resistance is expressed, and then maps that area. At most, we
know where the region is but not what the mechanisms are that
control this trait,” said Dorrance. “With microarray chips, we
can map over 30,000 genes at the same time and know instantly
which genes are involved in partial resistance and which are
not. It allows us to more quickly target which genes are
important for companies to incorporate in their germplasm and
get it into the hands of growers faster.”
The technology also enables researchers to better understand the
mechanisms of gene expression and how partial resistance to
Phytophthora works. Dorrance, who also holds a partial Ohio
State University Extension appointment, said that partial
resistance is more durable, more consistent and more effective
in controlling Phytophthora than single resistance genes alone.
A combination of the two will give growers the best protection.
“The 'R' genes only have a certain life span. Using these
R-genes wisely will get the longest length of time out of genes,
but eventually plants with just R-gene resistance will no longer
be effective against the disease,” said Dorrance. “High levels
of partial resistance helps maintain yields across disease
pressures and disease locations. Of course we continue to
identify R-genes because if you have both partial and R-gene
resistance in a plant, you aren’t going to see losses to
Phytophthora.”
Single-resistance genes, like the newly discovered Rps8, work by
killing the pathogen before it ever has a chance to establish in
the plant. However, if the pathogen is not detected by the
resistant gene, then that gene becomes ineffective and the plant
succumbs to disease.
“It’s the reason why so many single-resistant gene packages,
specifically Rps1a, Rps1b, Rps1c, Rps1k, Rps3a and Rps6, are no
longer able to control Phytophthora in many Ohio fields,” said
Dorrance.
Partial resistance genes allow Phytophthora to colonize a
soybean plant, but only to a certain extent, keeping the disease
at bay and preventing it from killing the plant as long as
resistance is high enough.
“Partial resistance basically means that the pathogen has little
effect on the plant once it has grown up and out of the ground,”
said Dorrance. “Partial resistance varieties can be very
effective, sometimes having a 30 percent difference in yields
compared to soybean plants that have no resistance to
Phytophthora at all, depending on the disease pressure.”
One advantage of partial resistance genes is that, unlike
single-resistance genes, they are not race specific, meaning
that partial resistance works against any Phytophthora isolate
that exists. The result is partially resistant soybean cultivars
that yield consistently, no matter what race of Phytophthora may
be present in a particular field.
Phytophthora is a major problem in Midwest states that have
heavy clay soils, such as Ohio. Heavy rains saturate the soil
producing areas with standing water, which provides an outlet
for the pathogen to infect plant roots. This water mold grows in
the roots and into the plant stem, eventually killing the plant.
Economic losses to Phytophthora can be as high as $120 million
in any given year, with yield reductions ranging from five to 30
bushels per acre depending on variety. |
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