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Simultaneous fine mapping and locus characterization of the Andean Ur-4 and Mesoamerican Ur-5 rust resistance genes in Common bean
Oscar Hurtado-Gonzales: Soybean Genomics & Improvement Lab, BARC-West, ARS-USDA; Giseli Valentini: Universidade Estadual de Maringá; Thiago Gilio: Universidade Estadual de Maringá; Qijian Song: Soybean Genomics & Improvement Lab, BARC-West, ARS-USDA; Marcial Pastor-Corrales: Soybean Genomics & Improvement Lab, BARC-West, ARS-USDA
<div>Bean rust, caused by the hyper variable <i>Uromyces appendiculatus</i> fungus, is a destructive disease of common bean in the Americas and Africa. Effective management of bean rust is accomplished by pyramiding complementary disease resistance genes into single cultivars. Combining <i>Ur-4</i> with <i>Ur-5</i> confers broad-spectrum resistance to 95% of the races of <i>U. appendiculatus</i> maintained in ARS-Beltsville. However, pyramiding these genes is difficult due to the epistatic interaction between <i>Ur-4</i> and <i>Ur-5. </i>Conversely, co-dominant molecular markers facilitate the combination of these genes. We report here the simultaneous fine mapping of the <i>Ur-4</i> and <i>Ur-5</i> genes to obtain tightly-linked and highly accurate markers for marker-assisted selection of both loci. A total of 393 F<sub>2</sub> plants from the Early Gallatin (<i>Ur-4</i>) x Mexico 309 (<i>Ur-5</i>) cross was phenotyped with various races of <i>U. appendiculatus</i>. Bulk segregant analysis combined with SNP genotyping using the BARCBEAN6K_3 SNP chip with 5,398 SNPs, identified the genomic region containing the <i>Ur-4 </i>and <i>Ur-5</i> genes in chromosome Pv06 and Pv04 of the common bean, respectively. Targeted SSR and KASP markers were developed to accurately map the location of both genes using recombinant F<sub>2:3</sub> families. Our analysis established that while <i>Ur-4</i> was located in a region of high recombination on the long arm of Pv06, <i>Ur-5</i> was positioned in a cold-spot for recombination on the short arm of Pv04. Markers tightly linked to these genes useful for marker-assisted selection are being developed and validated. Moreover, candidate disease resistance genes were also identified in the small genomic region containing both loci. <p> </div>

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