The Barley Mutant emr1 was Identified in a Mutational Screen for Resistance Against Magnaporthe oryzae.

Barley plants can be colonized by the fungus Magnaporthe oryzae, a pathogen initially known from rice plant cultivation. A mutational screen was performed in the barley mlo-genetic background which is, in comparison to wild-type MLO-genotypes, hypersusceptible against this fungus. This led to the identification of a mutant, referred to as emr1 (enhanced Magnaporthe resistance), that showed partially restored resistance. Disease symptoms on leaves of emr1 were significantly less severe than on mlo5-genotypes but still more than on wt MLO-barley plants.Segregation analysis showed that emr1 was inherited as a single recessive trait. Insight into the mode of action of emr1-dependent resistance against M. oryzae was gained by microscopic analysis. The results of these experiments revealed that mutant emr1 blocked penetration by M. oryzae by the formation of effective papillae in approximately half of all incidences. At about 30% of the interaction sites fungal growth was arrested effectively by an HR in the epidermal cell. Only a low frequency of fungal infection sites proceed into the mesophyll where fungal invasion resulted in the onset of a hypersensitive response (HR)-like cell death. Here, we report further evidence that barley shows a mesophyll HR in response to colonisation by M. oryzae. The possibility that the fungus turns this ostensible defence reaction to its own advantage and profits from the dead host tissue by switching to a necrotrophic lifestyle is discussed.