The powdery mildew disease of barley, caused by the obligate fungal pathogen Erysiphe graminis f. sp. hordei, offers several advantages for the study of host genes mediating resistance. This disease has been studied intensively for more than 50 years; thus, there is an extensive base of physiological data and genetic materials available that is unmatched by most other host/pathogen systems. The barley M1-a locus is of particular interest in that more than 30 resistance alleles have been identified at this locus. By screening 118,000 M2 seedlings, I have isolated 41 partially or fully susceptible mutants. Complementation analysis of these new alleles will be used to determine the genetic structure of the M1-a locus, which may represent either a complex locus or cluster of tightly linked genes. Also, the mutants will be exploited in subtractive hybridization experiments to isolate M1-a cDNA clones. No such disease resistance gene has been cloned from any plant. Analysis of the M1-a clones will provide information about the nature of the allelic variation at the M1-a locus and about the biochemical role of the M1-a gene product in mediating disease resistance.***//
