IDENTIFICATION OF GENES REQUIRED FOR THE FUNCTION OF NON-RACE-SPECIFIC MLO RESISTANCE TO POWDERY MILDEW IN BARLEY

Recessive alleles (mlo) of the Mlo locus in barley mediate a broad, no n-race-specific resistance reaction to the powdery mildew fungus Erysi phe graminis f sp hordei. A mutational approach was used to identify g enes that are required for the function of mlo. Six susceptible M(2) i ndividuals were isolated after inoculation with the fungal isolate K1 from chemically mutagenized seed carrying the mlo-5 allele. Susceptibi lity in each of these individuals is due to monogenic, recessively inh erited mutations in loci unlinked to mlo. The mutants identify two unl inked complementation groups, designated Ror1 and Ror2 (required for m lo-specified resistance), Both Ror genes are required for the function of different tested mlo alleles and for mlo function after challenge with different isolates of E. g. f sp hordei. A quantitative cytologic al time course analysis revealed that the host cell penetration effici ency in the mutants is intermediate compared with mlo-resistant and Ml o-susceptible genotypes. Ror1 and Ror2 mutants could be differentiated from each other by the same criterion. The spontaneous formation of c ell wall appositions in mlo plants, a subcellular structure believed t o represent part of the mlo defense, is suppressed in mlo/ror genotype s. In contrast, accumulation of major structural components in the app ositions is seemingly unaltered. We conclude that there is a regulator y function for the Ror genes in mlo-specified resistance and propose a model in which the Mlo wild-type allele functions as a negative regul ator and the Ror genes act as positive regulators of a non-race-specif ic resistance response.