| Summary: | Powdery mildew caused by <i>Blumeria graminis</i> f. sp. <i>tritici</i> (<i>Bgt</i>) is one of many severe diseases that threaten bread wheat (<i>Triticum aestivum</i> L.) yield and quality worldwide. The discovery and deployment of powdery mildew resistance genes (<i>Pm</i>) can prevent this disease epidemic in wheat. In a previous study, we transferred the powdery mildew resistance gene <i>Pm57</i> from <i>Aegilops searsii</i> into common wheat and cytogenetically mapped the gene in a chromosome region with the fraction length (FL) 0.75−0.87, which represents 12% segment of the long arm of chromosome 2S<sup>s</sup>#1. In this study, we performed RNA-seq using RNA extracted from leaf samples of three infected and mock-infected wheat-<i>Ae</i>. <i>searsii</i> 2S<sup>s</sup>#1 introgression lines at 0, 12, 24, and 48 h after inoculation with <i>Bgt</i> isolates. Then we designed 79 molecular markers based on transcriptome sequences and physically mapped them to <i>Ae</i>. <i>searsii</i> chromosome 2S<sup>s</sup>#1- in seven intervals. We used these markers to identify 46 wheat-<i>Ae</i>. <i>searsii</i> 2S<sup>s</sup>#1 recombinants induced by <i>ph1b</i>, a deletion mutant of pairing homologous (<i>Ph</i>) genes. After analyzing the 46 <i>ph1b</i>-induced 2S<sup>s</sup>#1L recombinants in the region where <i>Pm57</i> is located with different <i>Bgt</i>-responses, we physically mapped <i>Pm57</i> gene on the long arm of 2S<sup>s</sup>#1 in a 5.13 Mb genomic region, which was flanked by markers <i>X67593</i> (773.72 Mb) and <i>X62492</i> (778.85 Mb). By comparative synteny analysis of the corresponding region on chromosome 2B in Chinese Spring (<i>T.</i> <i>aestivum</i> L.) with other model species, we identified ten genes that are putative plant defense-related (R) genes which includes six coiled-coil nucleotide-binding site-leucine-rich repeat (CNL), three nucleotide-binding site-leucine-rich repeat (NL) and a leucine-rich receptor-like repeat (RLP) encoding proteins. This study will lay a foundation for cloning of <i>Pm57</i>, and benefit the understanding of interactions between resistance genes of wheat and powdery mildew pathogens.
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