Degree Type


Date of Award


Degree Name

Doctor of Philosophy




Plant Physiology

First Advisor

Randy C. Shoemaker


In soybean, PCR amplification has been used to identify a cluster of RGAs (Resistance Gene Analogs) on soybean linkage group J (Kanazin et al. 1996). Resistance to powdery mildew (Rmd-c), Phytophthora stem and root rot (Rps2) and an ineffective nodulation gene (Rj2) map within this cluster. Using RGA-specific primers and BAC fingerprinting, a contig of BACs was developed for this region in cultivar 'Williams 82' [rps2, Rmd (adult onset), rj2; Marek and Shoemaker, 1997]. Two cDNAs showing homology to RGAs have also been placed in the contig (Graham et al. 2000). Since the two cDNAs were derived from different tissues, we became interested in determining if differential expression of the R-genes occurs within this cluster. A PCR-based sequencing approach and shotgun sequencing of two overlapping BACs were used to identify 15 R-gene sequences within this cluster. The R-genes show homology to the TIR/NBD/LRR family of disease resistance genes. Two of these R-genes represent a novel class of disease resistance genes; TIR/NBD domains fused inframe to a putative secretory protein. RT-PCR analyses using gene-specific primers allowed us to monitor the expression of the individual genes in different tissues and at different stages of development. We identified six expressed genes, three of which where differentially expressed. Sequence analyses of the BAC 91F11 R-genes suggest that these genes evolve by the accumulation of point mutations in the LRR, not by unequal intragenic recombination. In addition, the cluster of genes has most likely expanded through unequal intergenic recombination. Using a variety of techniques we have been able to examine the organization, evolution and expression of a disease resistance gene cluster in soybean.



Digital Repository @ Iowa State University,

Copyright Owner

Michelle Anne Graham



Proquest ID


File Format


File Size

125 pages