PROGRESS TOWARDS MAP-BASED CLONING OF THE DISEASE RESISTANCE GENE RPS-K IN SOYBEAN

Plant Biology Division, The Noble Foundation, P.O. Box 2180, Ardmore, OK 73402 USA

Soybean is an agronomically important oil seed crop in many countries. In Northern America, a serious root and stem rot disease caused by Phytophthora sojae, leads to a heavy yield loss specifically in susceptible soybean cultivars. The resistance of soybean against this fungal pathogen is governed by a series of single dominant genes Rps We have applied a map-based cloning strategy in isolating one of these six genes Rps1-k, mapped in this locus. Cloning of this gene will assist us in understanding the recognition and signal transduction events that are involved in the expression race-specific resistance of soybean cultivars. As a first step, a high-density genetic map around the gene region was completed. This gene, thereafter, was physically located in an about 145 kb region flanked by two AFLP markers TC1 and CG1. In search of this gene, three large-insert BAC genomic libraries representing 14 soybean genome equivalents were constructed, and screened. As a result, six BAC clones of the Rps1 region were identified, and further characterized by the restriction analysis, Southern hybridization, and sequencing. Two contigs representing CG1 and TC1 loci were developed from these BAC clones. It appears that there may be a gap of approximately 70 kb between these two contigs. Apparently, DNA fragments of the Rps1 region are highly under-represented among these three different BAC libraries. Furthermore, BAC clones isolated for this region are also very small in size. Current experiments include characterization of a BAC clone which is closest to the Rps1-k gene, and isolation of DNA fragments to complete constructing the contig of the Rps1 region.