Identification of a large cluster of coiled coil-nucleotide binding site--leucine rich repeat-type genes from the Rps1 region containing Phytophthora resistance genes in soybean

Fifteen Rps genes confer resistance against the oomycete pathogen Phytophthora sojae, which causes root and stem rot disease in soybean. We have isolated a disease resistance gene-like sequence from the genomic region containing Rps1-k. Four classes of cDNA of the sequence were isolated from etiolated hypocotyl tissues that express the Rps1-k-encoded Phytophthora resistance. Sequence analyses of a cDNA clone showed that the sequence is a member of the coiled coil-nucleotide binding site–leucine rich repeat (CC-NBS–LRR)-type of disease resistance genes. It showed 36% identity to the recently cloned soybean resistance gene Rpg1-b, which confers resistance against Pseudomonas syringae pv. glycinea, and 56% and 38% sequence identity to putative resistance gene sequences from lotus and Medicago truncatula, respectively. The soybean genome contains about 38 copies of the sequence. Most of these copies are clustered in approximately 600 kb of contiguous DNA of the Rps1-k region. We have identified a recombinant that carries both rps1-k- and Rps1-k-haplotype-specific allelomorphs of two Rps1-k-linked molecular markers. An unequal crossover event presumably led to duplication of alleles for these two physically linked molecular markers. We hypothesize that the unequal crossing over was one of the mechanisms involved in tandem duplication of CC-NBS–LRR sequences in the Rps1-k region.