THE BARLEY MLO GENE - A NOVEL CONTROL ELEMENT OF PLANT PATHOGEN RESISTANCE

Mutation-induced recessive alleles (mlo) of the barley Mlo locus confe r a leaf lesion phenotype and broad spectrum resistance to the fungal pathogen, Erysiphe graminis f. sp. hordei. The gene has been isolated using a positional cloning approach. Analysis of 11 mutagen-induced ml o alleles revealed mutations leading in each case to alterations of th e deduced Mlo wild-type amino acid sequence. Susceptible intragenic re combinants, isolated from mlo heteroallelic crosses, show restored Mlo wild-type sequences. The deduced 60 kDa protein is predicted to be me mbrane-anchored by at least six membrane-spanning helices. The finding s are compatible with a dual negative control function of the Mlo prot ein in leaf cell death and in the onset of pathogen defense; absence o f Mlo primes the responsiveness for the onset of multiple defense func tions.