Allelic Series of Four Powdery Mildew Resistance Genes at the Pm3 Locus in Hexaploid Bread Wheat

At the Pm3 locus in hexaploid wheat (Triticum aestivum), 10 alleles conferring race-specific resistance to powdery mildew (Blumeria graminis f. sp. tritici) are known. A cluster of genes encoding coiled-coil-nucleotide-binding site-leucine-rich repeat proteins spans the Pm3 locus on wheat chromosome 1A, and one member of this gene family has recently been identified as the Pm3b resistance gene. Using molecular markers closely linked to Pm3b, we performed haplotype analysis of 10 lines carrying different Pm3 alleles. All these lines have a conserved genomic region delimited by markers cosegregating with Pm3b and including a structurally conserved Pm3b-like gene. A polymerase chain reaction-based strategy allowed the amplification of one Pm3b-like sequence from lines carrying Pm3a, Pm3d, and Pm3f alleles. These candidate genes for Pm3a, Pm3d, and Pm3f conferred AvrPm3a-, AvrPm3d-, and AvrPm3f-dependent resistance, respectively, to wheat powdery mildew in a single cell transient transformation assay. A high level of amino acid similarity (97.8%) was found between the PM3A, PM3B, PM3D, and PM3F proteins. The coiled-coil domain was 100% conserved, whereas, in the nucleotide binding site region, sequence exchange was detected, indicating intragenic recombination or gene conversion between alleles. All these results indicate that Pm3a, Pm3b, Pm3d, and Pm3f form a true allelic series. The low level of sequence divergence between the four characterized alleles as well as the finding of a conserved Pm3 haplotype are in agreement with the hypothesis of a recent evolution of Pm3-based resistance, suggesting that some or most of the diversity found at the Pm3 locus in modern wheat has evolved after wheat domestication.