Most plants are resistant to the majority pathogenic organisms. In specific cases, however, pathogens have evolved to overcome these broadly active defense mechanisms by means of host specific virulence genes, which results in disease. Many fungal and bacterial pathogens were found to produce strain specific complements of virulence factors that are secreted or injected into the apoplastic space and/or the plant cell cytoplasm.These effector proteins are thought to actively perturb host cell metabolism and defense in favor of the pathogen. Some plant cultivars have evolved to recognize specific effector molecules from their pathogens both directly or indirectly by means of resistance (R-) genes. In such cases, virulence genes become avirulence determinants (Avr genes) for the pathogen, resulting in race / cultivar specific resistance. Race specific resistance is among the most powerful defense plants have developed. To better understand race specific resistance on a molecular level, a number of matching R- and Avr-gene pairs have been cloned from plants and different pathogen species, respectively. Among them are the barley (Hordeum vulgare) Mla gene, which mediates resistance against barley powdery mildew (Blumeria graminis f. sp. hordei), and AvrMla from barley powdery mildew. The Mla gene encodes for a NBS-LRR type of receptor, with a central nucleotide binding domain (NBS) followed by a variable number of lucine-rich repeats (LRRs). NBS-LRR types of proteins are common in plants, and cloned R-genes from diverse plant species often encode this type of protein. The LRR domain is the structure that recognizes cognate AVR molecules by direct interaction with the protein or the product of its enzymatic activity, and therefore provides the specificity of the receptor. A full understanding of this recognition process is among the most relevant but unresolved questions related to this type of receptor. A full understanding would allow the targeted manipulation of specificity, and thus the engineering of R-genes that recognize effector proteins that currently evade recognition by naturally occurring R-gene variants.We propose to make use of the allelic series of barley Mla genes that recognize a probably equally large and variable number of powdery mildew Avrs to study the molecular basis for the recognition of AvrMla gene products by individual Mla alleles. Barley land races and wild barley relatives accessible through large germplasm collections will be mined for novel and functional Mla alleles. We will also try to identify functional orthologous sequences from plant accessions in the related genera Triticum and Brachypodium that can be expected to contribute even more variable sequences. All together, a large number of active gene sequences will be generated, grouped by recognition specificity and used to identify the sequence requirement for the detection of specific AVR proteins.