Molecular characterization of major gene resistance in a Populus-leaf rust pathosystem

The genetic basis underlying disease resistance in plants is frequently controlled by single dominant resistance (R) genes in the host plant that recognize pathogens expressing specific avirulence genes. Plant R genes have been genetically mapped in numerous plant-pathosystems involving a wide variety of pathogens and pests, including fungi, bacteria, viruses, nematodes and insects. For decades, classical breeding for disease resistance in plants has focused on resistance conferred by R genes due the ease and efficacy of introducing such resistance into agronomically important cultivars. Within the past eight years, the isolation and characterization of over thirty R genes from different plant species has provided important insights into the structure, function, and evolution of plant R genes. The knowledge that is gained from studies of characterized plant R genes should eventually lead to the design of new and effective methods for preventing disease.; In this study, genetic and physical mapping approaches were used to characterize a single locus (MXC3) governing resistance to poplar leaf rust caused by pathotype 3 of the fungus Melampsora x columbiana. The long-term goal was to positionally clone MXC3 to assist in understanding its role in both natural and plantation forests, and to genetically engineer susceptible but commercially valuable hybrid poplars for resistance to this important pathogen. AFLP markers were used to saturate the chromosomal region around the MXC3 locus in a large (N = 1902) Populus trichocarpa x P. deltoides (TxD) mapping pedigree segregating 1:1 for rust resistance and susceptibility. The high-resolution genetic map developed around the MXC3 locus contains 19 AFLP markers and spans a genetic distance of 2.73cM. Of the 19 AFLP markers, seven were found to cosegregate with the locus. One co-segregating AFLP marker, CCG.GCT_01, was converted to an STS marker (BVSI) and used to identify a physical contig of overlapping BAC clones from the MXC3 region. Genetic and physical mapping of markers isolated from the BAC contig failed to delimit the MXC3 locus within a ca. 300kb interval defined by the overlapping BAC clones. This result indicates a >25-fold reduction in recombination frequency in the MXC3 region compared to the average rate of recombination for the Populus genome.