The ambivalence of the barley Mlo locus:: Mutations conferring resistance against powdery mildew (Blumeria graminis f. sp, hordei) enhance susceptibility to the rice blast fungus Magnaporthe grisea

Recessive alleles of the barley Mlo locus confer non-race-specific resistance against the powdery mildew fungus Blumeria graminis f, sp, hordei (Bgh), Recently the Mlo gene has been isolated and it was suggested that the Mlo product is a negative regulator of cell death. Thus, loss of function can precondition cells to a higher responsiveness for the onset of multiple defense functions, Here, we document an enhanced susceptibility of barley mlo mutants to the rice blast fungus Magnaporthe grisea, The disease phenotype is independent of the barley cultivar in which the mlo allele has been introgressed and occurs in equal amounts in barley backcross lines of cv, Ingrid carrying the mlo-1, mlo-3, or mlo-5 allele, Ror genes, which are required for the full expression of mlo resistance in barley against Bgh, do not affect the specific mlo-mediated phenotype observed after M. grisea infection. Formation of an effective papilla restricts blast development in epidermal cells of Mlo plants. In contrast, papillae are mostly penetrated in mlo mutants and, as a consequence, the fungus spreads into adjacent mesophyll cells, Both wild-type plants and mlo mutants did not differ in perception of a purified elicitor derived from M, grisea, Thus, we hypothesize that a functional Mlo protein is a prerequisite for penetration resistance of barley to fungal pathogens like M, grisea, The benefit of mlo alleles for durable resistance in barley and a proposed role of mlo-type-mutations in rice are discussed.