Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2002
Publication Date: 12/5/2002
Citation: GRAHAM, M.A., MAREK, L.F., SHOEMAKER, R.C. ORGANIZATION, EXPRESSION AND EVOLUTION OF A DISEASE RESISTANCE GENE CLUSTER IN SOYBEAN. GENETICS. 2002. v. 162. p. 1961-1977. Interpretive Summary: Plants are very successful in defending themselves against the myriad of diseases and pests surrounding them. It is not known how the resistance genes evolve to respond to an ever changing array of diseases. Understanding how resistance genes evolve may help us to design more efficient disease resistance genes. In this study the authors decoded the hereditary DNA for a large region of a soybean chromosome that contains several resistance genes. They identified sixteen genes that look like resistance genes and determined that some genes were expressed in some places in the plant while other genes were expressed in other places. By analyzing in detail the linear structure of the genes they also were able to determine that the genes probably arose through a series of duplications of DNA segments and recombining of segments, in addition to the predicted mutations. This study provided valuable information about the origin of new forms of disease resistance and the ability of these genes to be expressed in specialized tissues. This information will be useful for scientists in evaluation of other resistance gene clusters.
Technical Abstract: PCR amplification was previously used to identify a cluster of resistance gene analogs (RGAs) on soybean linkage group J. Resistance to powdery mildew (Rmd-c), Phytophthora stem and root rot (Rps2) and an ineffective nodulation gene (Rj2) map within this cluster. BAC fingerprinting and RGA-specific primers were used to develop a contig of BAC clones spanning this region in cultivar `Williams 82¿ (rps2, Rmd (adult onset), rj2;). Two cDNAs with homology to the TIR/NBD/LRR family of R-genes have also been mapped to opposite ends of a BAC in the contig, BAC 91F11. Sequence analyses of BAC 91F11 identified sixteen different resistance-like gene (RLG) sequences with homology to the TIR/NBD/LRR family of disease resistance genes. Four of these RLGs represent two potentially novel classes of disease resistance genes: TIR/NBD domains fused inframe to a putative secreted protein and TIR domains fused inframe to soybean calmodulin Ca+2 binding domains. RT-PCR analyses using gene-specific primers allowed us to monitor the expression of individual genes in different tissues and developmental stages. Three genes appeared to be constitutively expressed, while three were differentially expressed. Analyses of the R-genes within this BAC suggest that R-gene evolution in soybean is a complex and dynamic process.