Molecular and Functional Analysis of Resistance Expansion and Suppression for Alleles of the Powdery Mildew Resistance Gene Pm3 in Wheat

| 5 Abstract Developing high yielding and high quality varieties with broad-spectrum and durable disease resistance is the ultimate goal of crop breeding. Resistance (R) genes encoding proteins with nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domains provide high levels of resistance, but this type of R genes usually only confers resistance to a subset of pathogen races. In addition, major R-gene resistance is often rapidly overcome by newly adapted pathogen races when employed in agriculture. In this thesis the allelic series of the NBS-LRR-encoding powdery-mildew resistance gene Pm3 of wheat was analyzed in two projects.Developing high yielding and high quality varieties with broad-spectrum and durable disease resistance is the ultimate goal of crop breeding. Resistance (R) genes encoding proteins with nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domains provide high levels of resistance, but this type of R genes usually only confers resistance to a subset of pathogen races. In addition, major R-gene resistance is often rapidly overcome by newly adapted pathogen races when employed in agriculture. In this thesis the allelic series of the NBS-LRR-encoding powdery-mildew resistance gene Pm3 of wheat was analyzed in two projects. In the first project, Pm3 alleles with a broad and a narrow resistance spectrum were investigated. It was found that Pm3 alleles with a broad resistance spectrum can induce a fast and intense hypersensitive response (HR) in a Nicotiana transient-expression system and this activity could be attributed to two particular amino acids in the ARC2 subdomain of the NBS. The combined substitution of these amino acids in narrow-spectrum PM3 proteins enhanced their capacity to induce an HR in Nicotiana benthamiana, and it was found that these substitutions also enlarge the resistance spectrum of the Pm3f allele in wheat. Using the rice gene Bph14, it was confirmed that the region carrying the two relevant amino acids also plays a role in the HR regulation of another coiled-coil NBS-LRR resistance protein. These results improve the mechanistic understanding of NBS-LRR-protein function and suggest an approach to extend the effectiveness of resistance genes via minimal targeted modifications in