In context to a growing world population and loss of arable land it is challenging to make food production more safe, environmentally compatible and healthy for the consumer as well as for the producer. Since plant diseases are still threatening these aims, it is of outstanding importance to understand the molecular and physiological basis of crop diseases and resistance. Therefore, this work focussed on the analysis of these aspects in the interaction of barley and wheat with powdery mildew fungi.
The first part of this work deals the role of reactive oxygen intermediates in the interaction of barley and wheat with inappropriate and appropriate formae speciales of Blumeria graminis. The use of susceptible barley mutants as well as the comparison of host with non-host resistance allowed to link H2O2 accumulation with host and non-host resistance whereas the role of O2 - appears to be rather diverse.
In the second part the isolation and characterisation of candidate genes with a potentially essential role in the interaction of barley with Blumeria graminis f.sp. hordei (Bgh) is described. The candidate approach followed a reverse genetics strategy starting with mRNA sequences in public databases. This led to the isolation of a set of copy DNA sequences. Some of the corresponding genes were strongly expressed in response to attack from Bgh. Functional analysis based on microprojectile mediated single-cell transformation with subsequent challenge of transformed cells by Bgh led to characterisation of some genes as potential resistance or susceptibility factors. The barley family of RAC/ROP-type small GTPases was characterised in detail. The role of the gene products is discussed with respect to penetration resistance and in particular with respect to their role in production of reactive oxygen intermediates and in cytoskeleton rearrangement.
The third part deals with the role of the cell death regulator protein BAX Inhibitor 1 (BAX: BCL-2 associated X protein; BCL-2: B-cell lymphoma protein-2) in barley susceptibility to B. graminis. The corresponding barley mRNA was isolated from leaf tissue and found to accumulate after powdery mildew attack and to transiently disappear after root treatment with a synthetic resistance activator. Importantly, over-expression of barley BAX Inhibitor 1 weakened background resistance of barley to Bgh, broke mlo-mediated penetration resistance and additionally broke non-host resistance to the wheat pathogen B. graminis f.sp. tritici (Bgt). BAX Inhibitor 1 was further analysed based on its amino acid sequence and literature data. This tagged BAX Inhibitor 1 as an ancient cell death suppressor protein conserved in all higher eukaryotes, and traces its origin back to possibly prokaryotic ancestor proteins.
The last part of this work deals with non-host resistance and its mechanistic overlap with host resistance to B. graminis. In this regard, the studies concentrated mainly on the role of the genes mlo, ror1 and ror2, whose role in resistance to Bgh is well established, in response to Bgt. Besides this, a set of different barley accessions was compared for their non-host resistance phenotypes in response to Bgt. Taken together, a similar role for ror1 and ror2 in host and non-host resistance to B. graminis was observed. Additionally, the multiplicity of different non-host defence reactions observed provoked the assumption that non-host resistance to Bgt is based on diverse defence pathways in barley.
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