DNA Markers... With a Twist for
Improved Bean Breeding
Plant geneticists Phil Miklas
(left) and George Vandemark
inspect damaged snap bean
plants at Prosser, Washington.
Americans eat nearly 8 pounds of dry beans each year in
soups, salads, and other dishes and more than 7 pounds of frozen, canned,
or fresh snap beans. But only the best bean cultivars make the cut when
it comes to traits that will help growers profit and satisfy consumer
One cultivar trait farmers prize most is disease resistance,
especially against bean common mosaic virus and bean common mosaic necrosis
virus. Severe outbreaks of these aphid-borne viruses can cause yield
losses of up to 60 percent and threaten the $512 million dry bean crop
of California, Colorado, Idaho, Michigan, Nebraska, North Dakota, and
Washington. Also at risk is the $190 million snap bean crop of Florida,
Illinois, New York, Oregon, Wisconsin, and other states.
Geneticist Phil Miklas
inoculates selected bean
plants with a virus.
Forefront in the war on such viruses is marker-assisted
selection (MAS). It's a biotech approach some breeders are using to
rapidly identify resistant plants by confirming the presence of certain
genes rather than by observing actual disease symptoms induced in a
greenhouse. Now, an advance by ARS
scientists in Prosser, Washington, could make MAS even more useful by
enabling it to do what it often couldn't before: distinguish homozygous
from heterozygous plants. Homozygous plants carry the desirable gene
on each of the two chromosomes. Heterozygous plants carry the beneficial
gene on one chromosome, while the other chromosome contains a slightly
different version of the gene that does not confer the desirable trait.
The advance, called co-dominant interpretation, should
save breeders substantial time, labor, and money spent ensuring that
promising cultivars are homozygous, or " true breeding," meaning
their offspring will consistently show the same traits over many generations.
Geneticists Phil Miklas (left)
and George Vandemark analyze
results of quantitative
polymerase chain reaction
assays used to rapidly
genotype bean plants for
a virus-resistance gene.
In beans, the gene for resistance to the two viruses is dominant, say
George Vandemark and Phillip Miklas, plant geneticists at ARS' Vegetable
and Forage Crops Production Research Unit. When a gene is dominant,
a single copy in a heterozygous plant will confer the desirable trait,
making the plant visually indistinguishable from the true-breeding,
homozygous plant. This presents an obvious problem in excluding heterozygous
individuals from the pool of plants used for seed generation, say the
X Marks the Spot
In some quarters, classical plant breeding techniques have given way
to MAS because it detects certain traits more easily. The markers themselves
are specific sequences of DNA inside the plant's chromosomes. A marker
serves as a road sign that says a desired gene is close by.
Geneticist George Vandemark
isolates DNA from a bean
plant that will be examined
with a polymerase chain
reaction assay to determine
whether it has the virus-resistant
gene. The assay can be completed
in 1 day, whereas greenhouse
methods require up to 4 months
to determine the same thing.
Key to finding markers is the polymerase chain reaction (PCR). It's
the same technology that forensics experts use to identify DNA fingerprints
in blood, hair, or other biological samples. But bean breeders use it
to search for plants that possess a specific virus-resistance gene called
bc-12. This gene, which confers resistance to most
bean mosaic virus strains, is often bred into pinto, navy, and other
dry beans as well as snap beans.
A breeder prefers plants with two identical copies (homozygous) of
the virus-resistance gene because the plants will be true breeding,
meaning all their offspring will be virus resistant. But if the plant
is heterozygous, it will have one copy of the bc-12
resistance gene and one copy of an alternate gene called bc-1,
which confers susceptibility to most bean mosaic virus strains. Heterozygous
plants won't breed true, and some of their offspring will be susceptible
to virus attack.
Geneticist Phil Miklas compares
susceptible (left) and resistant
(right) bean lines exposed to
Breeders want to exclude the bc-1 gene from their breeding programs,
but this is difficult with existing PCR detection methods, Miklas says.
Breeders now rely on a process called progeny testing, where the plants'
offspring (progeny) are tested for resistance to the virus in a greenhouse.
Such testing allows breeders to detect heterozygous plants and eliminate
them from the breeding program. This process can take 6 months to 1
year and adds considerably to the cost of breeding programs.
Making a Good Biotech Tool Better
Use of DNA markers typically begins with PCR to identify plants having
the bc-12 resistance gene. Until recently, the PCR
technique was able to detect the bc-12 gene but not
the number of copies present.
Vandemark and Miklas overcame this problem by using a newer PCR method
that can accurately tell how much marker DNA is present by labeling
it with a fluorescent compound and then measuring the fluorescence.
" Since our quantitative PCR assay is specific for the bc-12
gene, we can expect that homozygous plants will fluoresce twice as much
as heterozygous plants," says Vandemark.
Snap beans that possess
the gene bc-12 are
resistant to most strains
of bean common mosaic virus
and bean common mosaic
In trials, the scientists used a small reference group of known heterozygous
plants to create a confidence interval (CI). The CI establishes the
amount of fluorescence expected from heterozygous plants. " We
then said that any plant that shows marker fluorescence greater than
the CI is homozygous," Vandemark says. They validated the technique
by performing progeny testing on all the plants, showing that it was
99 percent accurate in determining which ones had one copy or two of
the bc-12 gene.
Vandemark and Miklas note that their new approach to using quantitative
PCR can make such determinations in about 2 hours. This should make
it easier for breeders to develop new resistant bean cultivars that
will help growers protect their crops from viral diseases.
Geneticists Phil Miklas (left)
and George Vandemark examine
healthy, disease-resistant snap
(left) and pinto bean breeding
lines developed at Prosser,
| The scientists reported their
advance in a recent issue of the journal Molecular Breeding.By
Agricultural Research Service Information Staff.
This research is part of Rangeland, Pasture, and Forages
(#205) and Genomic Characterization and Genetic Improvement (#301),
two ARS National Programs described on the World Wide Web at www.nps.ars.usda.gov.
George J. Vandemark
and Phillip N. Miklas
are in the USDA-ARS Vegetable
and Forage Crops Production Research Unit, 24106 North Bunn Rd.,
Prosser, WA 99350; phone (509) 786-9218, fax (509) 786-9277.
"DNA Markers... With a Twist for Improved Bean Breeding"
was published in the May
2003 issue of Agricultural Research magazine.