Allele-mining and natural diversity in wheat powdery mildew resistance genes

Using map-based cloning, we have isolated the Pm3b powdery mildew resistance gene from hexaploid bread wheat (Triticum aestivum L.). Based on haplotype studies, we have developed molecular tools to isolate all the 10 known Pm3 genes conferring resistance. We found that the Pm3 genes form a true allelic series and that they are highly conserved at the molecular level. The molecular work on Pm3 resistance genes has lead to very diagnostic tools for these genes which support the cloning of new functional alleles from this locus by allele-mining. We have used these tools to screen for new Pm3 alleles in the gene pools of (i) wild and domesticated tetraploid accessions and (ii) hexaploid wheat landraces. The Pm3 locus is conserved in tetraploid wheat, allowing a comparative evolutionary study of the same resistance locus in a domesticated species and one of its wild ancestors. We have identified 61 Pm3 allelic sequences from wild and domesticated tetraploid wheat subspecies. These alleles showed low sequence diversity, differing by few polymorphic sequence blocks that were further reshuffled between alleles by gene conversion and recombination. A new functional gene was identified in a wild wheat accession from Syria. This gene, Pm3k, conferred intermediate resistance to powdery mildew and consists of a mosaic of gene segments derived from non-functional alleles. From the hexaploid wheat gene pool, a set of 1320 landraces, mostly from Asia, was screened for powdery mildew resistance and the presence of a Pm3 haplotype. Most of these lines were found to contain a susceptible Pm3 allele which is closely related to the functional Pm3 resistance genes. We have also identified resistant lines with new types of Pm3 allelic sequences, resulting from point mutations, gene conversion and illegitimate recombination events. These new alleles are currently tested for resistance activity in a transient expression assay. (author)