Abstract
Previously isolated susceptible host mutants were used in a genetic and functional study of the resistance response of barley specified by resistance gene Mla12 to the fungal pathogen Erysiphe graminis f sp hordei. Mutant M66 represents a defective allele of Mla12, whereas M22, M82, and M100 represent mutations in loci unlinked to Mla12. Intermutant crosses of the latter three show that susceptibility in M82 and M100 is caused by allelic, recessive mutations that define the Nar-1 gene (necessary for Mla12 resistance gene 1), whereas the semidominant mutation in M22 defines a second unlinked locus, Nar-2. We show that both genes are required for resistance specified by Mlal2 in different genetic backgrounds of barley. Nar-1 maps on barley chromosome 2 within an ~6-centimorgan restriction fragment length polymorphism interval: this is 0.5 centimorgans from the anthocyanin pigmentation gene Ant2. Quantitative cytological analysis showed that functional alleles of Mla12, Nar-1, and Nar-2 are required for triggering a cell death reaction of attacked host cells at early stages during infection. Functional alleles of all three genes were also required for high-level transcript accumulation of barley defense-related genes that encode chitinase, peroxidase, and pathogenesis-related protein-1. The data support the hypothesis that host cell death and high-level accumulation of defense-related gene transcripts, which are under common control of Mla12, Nar-1, and Nar-2, are crucial events of race-specific resistance to powdery mildew.