9402315 Somerville 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 that 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 Ml-a locus is of particular interest in that more that 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 Ml-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 isolated Ml-a cDNA clones. No such disease resistance gene has been cloned form any plant. %%% Analysis of the Ml-a clones will provide information about the nature of the allelic variation at the Ml-a locus and about the biochemical role of the Ml-a gene product in mediating disease resistance. ***
