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Photo: Snap beans that possess the gene bc-1*2* resist most strains of bean common mosaic virus and bean common mosaic necrosis virus. Link to photo information
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New Method to Speed Breeders' Search for Disease-Resistant Beans

By Jan Suszkiw
May 5, 2003

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.

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