The barley mutant emr1 exhibits restored resistance against Magnaporthe oryzae in the hypersusceptible mlo-genetic background

Abstract

Barley plants having wild-type or mutant alleles at the MLO locus show opposite responses to infection with different pathogens, i.e. plants homozygous for mutant alleles (mlo) are resistant to powdery mildew but hypersusceptible to the rice blast fungus Magnaporthe oryzae and vice versa for plants with at least one wild-type MLO-allele. A mutational analysis was performed in the mlo-genetic background aimed at identifying of individuals with restored resistance against M. oryzae. Here, we describe the barley enhanced Magnaporthe resistance (emr1) mutant which showed restored resistance against blast in the absence of wild-type MLO. The emr1 mutant could be classified as a loss of function mutant. It could be excluded that resistance of emr1 is a back-mutation at the mlo-locus, because emr1 retained resistance against Bgh. The mutant did not display generally increased resistance as was evidenced by infection with either brown rust or net blotch pathogens. Additionally, resistance in emr1 was not associated with constitutively activated defence as confirmed by monitoring PR-gene transcript accumulation. Microscopic analysis showed that resistance of the emr1 mutant against M. oryzae was correlated with blocked penetration in epidermal cells and a concomitantly reduced progression into the mesophyll. These findings are reminiscent of the defence phenotypes against M. oryzae previously described for wild-type barley MLO genotypes. Therefore, it is tempting to speculate that resistance in the emr1 mutant was regained by the knockdown of putative suppressor element(s) acting in the defence scenario against M. oryzae, which diminish resistance only in mlo but not in MLO genotypes.