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.

Evolutionary Conserved Function of Barley and Arabidopsis 3-KETOACYL-CoA SYNTHASES in Providing Wax Signals for Germination of Powdery Mildew Fungi

“…The barley mutant enhanced Magnaporthe resistance gene1 (emr1), generated in our laboratory, was identified in a suppressor screen for restoration of resistance against M. oryzae in the hypersusceptible genetic background mildew resistance locus O allele5 (Ingridmlo5; Jansen et al, 2007 Jansen and Schaffrath, 2009). For powdery mildew, however, the presence of the strong resistance allele mlo5 could have masked potential effects.…”

The Barley Mutantemr1was Identified in a Mutational Screen for Resistance AgainstMagnaporthe oryzae

“…In this context a mutational screen was set up in the barley mlo-genetic background and resulted in the identification of mutant emr1. 5 Resistance in this mutant did not result from a back-mutation to wild-type MLO-alleles as evidenced by retained resistance against Bgh. Infection experiments with the obligate-biotropic rust fungus Puccinia hordei or the necrotrophic fungus Drechslera teres revealed no altered resistance phenotype with these pathogens.…”

“…More precisely, plants homozygous for mutant alleles (mlo) are resistant to powdery mildew but hypersusceptible to M. oryzae and vice versa for plants with at least one wild-type MLO-allele. 4,5 This clearly showed that the transfer of the mlo-scenario to rice would probably result in more devastating blast epidemics.…”

“…At the cellular level resistance against M. oryzae was At this stage it must be stated clearly that all barley cultivars under investigation were hosts for M. oryzae, however they differed in their degree of susceptibility. 4,5,9 On question aroused from this observation targeted the mechanisms by which the pathogen establish compatibility on each genotype. On the microscopic level enhanced susceptibility correlated strictly with cell collapse and the accumulation of autofluorescent material in the mesophyll.…”

“…On the microscopic level enhanced susceptibility correlated strictly with cell collapse and the accumulation of autofluorescent material in the mesophyll. 4,5 These histological criteria were reminiscent of a typical HR as described for resistant barley cultivars attacked by Bgh. 10 However, since the term HR is normally used to describe a rapid form of programmed cell death asssociated with resistance, 11 we refer to the observed phenomenon of cell death in M. oryzae attacked mesophyll of barley as "HR-like cell death".…”