PATHWAYS LEADING TO RESISTANCE AND DEATH: INTERACTIONS OF POWDERY MILDEW AND BARLEY
PATHWAYS LEADING TO RESISTANCE AND DEATH: INTERACTIONS OF POWDERY
MILDEW AND BARLEY
Paul Schuize-Lefert 1 , Ken Shirasu 1 , Thomas Lahaye l, Pietro Piffanelli l, Alessandra Devoto l and James Ormel .
l The Sainsbury Laboratory, The John lnnes Centre, Colney, Norwich NR4 7UH,
Background and objectives
Our objective is to identify and to isolate genes required for disease resistance responses in barley to
the fungus Erysiphe graminis f sp hordei the causal agent of powdery mildew.
Results and conclusions
The mio resistance is unusual among characterised powdery mildew resistance genes in that it confer's a broad-spectrum defense to all tested powdery mildew isolates. Mutation-induced mio resistance alleles can be isolated from any badey line carrying the Mio wild type aliele. The deduced 60 kDa Mio protein is predicted to be membrane-anchored by seven membrane-spanning helices. Subcellular fractionation experiments and subsequent immunoblot analysis support the predicted membrane localisation. Analysis of a number of mio resistance alieles has confirmed in each case mutational events in the wild type gene. Barley Mio is the founder of a novel gene family restricted to the plant kingdom. A novel transient tranformation system using detached barley leaves is available for detailed structure/function analysis of Mio and results from these experiments will be reported.
Lack of the Mio vaid type aliele is associated with the appearance of small dead cell lesions in foliar tissue even under pathogen-free conditions. A mutational approach identified two genes, Rorl and Ror2, which are required for mio-mediated resistance. Mutations in these two genes abolish the spontaneous cell death phenotype. Histochemical analysis revealed also a pathogen-triggered and mio-dependent host cell death reaction which is dependent on Rorl and Ror2. The data strongly support a dual function for all three genes (Mio, Rorl, Ror2) in cell death control and pathogen resistance. However, host cell death per se is not essential for arrest of the invading pathogen. It appears that early as yet unknown biochemical events on a road leading to the execution of cell death are critical for fungal arrest.
Another line of research is focused to isolate genes controlling race-specific resistance reactions to the fungus. One of these loci, Mia, is characterized by a large number of dominant alieles enabling the plant to recognize a specific powdery mildew race and mount an efficient defense response upon attack. Mutational analysis has been used to identify Rarl . This gene is required for the function of most tested resistance alieles encoded at Mia, We believe that Rarl represents a point of convergence in the signalling of resistance responses triggered by different resistance specificities. Physical delimitation on barley YAC clones has been completed both for Rarl and Mia.
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