Monday, November 13, 2006

SSR Mapping of Genes in Soybean Conditioning Resistance to Stem Canker.

Z. P. Shearin1, J. M. Narvel2, Horace Cheney3, and H. R. Boerma1. (1) Univ of Georgia, Ctr for Applied Genetic Tech., 111 Riverbend Rd, Athens, GA 30602-7272, (2) Monsanto, 32545 Galena-Sassafras Rd, Galena, MD 21635, (3) Syngenta Seeds, Inc., 2525 47th Ave SE Apt. 435, Albany, AR 97322-8837

Stem canker, caused by the fungus Diaporthe phaseolorum, is a very devastating disease of soybean worldwide, and particularly in the southeastern United States.  Stem canker symptoms first appear as a small reddish-brown lesion on the main stem of the plant.  As the disease progresses this lesion interferes with the plant’s vascular tissue prohibiting water uptake and eventually causing the plant to die before reaching its full yield potential.  The most successful method for controlling stem canker is the use of resistant soybean cultivars.  The resistance genes Rdc-1, Rdc-2, Rdc-3, Rdc-4, and two previously uncharacterized genes have been identified using traditional phenotypic segregation ratios.  The development of molecular markers that are tightly linked to stem canker resistance genes will allow breeders to use molecular-based screens for stem canker resistance in lieu of the expensive and labor intensive field and greenhouse resistance screens used presently.  Our approach to mapping the Rdc genes involves crossing a source of a single Rdc gene with the highly susceptible soybean breeding line J77-339 to create F2 and F2:3 soybean mapping populations.  These populations will be screened for stem canker reaction (resistant vs. susceptible) using traditional screening procedures.  Bulked segregant analysis using SSR markers will be performed on DNA samples from members of the F2 populations that have been combined based on their stem canker reaction.  Once bulked segregant analysis determines the linkage group on which each Rdc gene resides, the F2 and F2:3 mapping populations will be used to more precisely map the location of each of the Rdc genes.  Two of the Rdc genes have been mapped to LG-B2 using these methods.