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New Method to Speed Breeders' Search for
Disease-Resistant Beans By
Breeding disease-resistant bean varieties could get easier,
thanks to a new method of identifying homozygous plants. Homozygous describes
plants that are "true breeding," meaning their offspring will consistently show
the same traits.
Bean breeders rely on such plants as a genetic base from which
to develop cultivars that resist diseases caused by the bean common mosaic
virus, and bean common mosaic necrosis. Both can cause losses of up to 60
percent in snap and dry bean crops.
At the forefront in the war on these viruses is marker-assisted
selection (MAS). That's a biotechnology tool that rapidly identifies resistant
plants by confirming the presence of the virus-resistance gene
bc-12 (referred to as the "recessive bc one two gene").
Otherwise, bean breeders must observe disease symptoms induced in a greenhouse.
MAS, however, can't distinguish homozygous plants, which have two identical
copies of bc-12, from heterozygous plants. The latter, which
are not true breeding, have one copy of bc-12 and a slightly
different version that actually gives the plant susceptibility to diseases.
Now, Agricultural Research
Service scientists George Vandemark and Phil Miklas have overcome the
problem by modifying the use of a key technology on which MAS is
based--polymerase chain reaction, or PCR. Their modified PCR method works by
revealing which of the two kinds of plants have more genetic marker material
for bc-12, based on the degree to which the marker material
glows when exposed to a flourescent compound. Since homozygous plants have two
copies of bc-12, their markers glow twice as brightly as
those for heterozygous plants, which only have one copy of the gene.
Bean breeders normally discard heterozygous plants, but only
after using an elimination process that takes six to 12 months. The new PCR
method takes about two hours, according to Vandemark, with ARS'
Vegetable and Forage Crops
Production Research Unit in Prosser, Wash.
A longer article on the research is in the
May issue of
Agricultural Research magazine.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.