Search

The Yr5 gene confers resistance to all races of the stripe rust pathogen (Puccinia striiformis f. sp. tritici) of wheat in the United States. To develop molecular markers for Yr5, a BC7:F3 population was developed by backcrossing the Yr5 donor 'Triticum spelta album' (TSA) with the recurrent parent 'Avocet Susceptible' (AVS). Seedlings of the Yr5 near-isogenic lines (AVS/6*Yr5), AVS, TSA, and the BC7:F3 lines were tested with North American races of P. striiformis f. sp. tritici under controlled greenhouse conditions. The single gene was confirmed by a 1:2:1 segregation ratio for homozygous-resistant, heterozygous and homozygous-susceptible BC7:F3 lines. Genomic DNA was extracted from the parents (the Yr5 near-isogenic line and AVS) and 202 BC7:F3 lines. The resistance gene-analog polymorphism (RGAP) technique was used to identify molecular markers. The parents and the homozygous-resistant and homozygous-susceptible BC7:F3 bulks were used to identify putative RGAP markers for Yr5. Association of the markers with Yr5 was determined using segregation analysis with DNA from the individual BC7:F3 lines. Of 16 RGAP markers confirmed by segregation analysis with 109 BC7:F3 lines, and nine of the markers confirmed with an additional 93 BC7:F3 lines, three markers co-segregated with the resistance allele and three markers co-segregated with the susceptibility allele at the Yr5 locus. The other four markers were tightly linked to the locus. Analysis of a set of Chinese Spring nulli-tetrasomic lines with three markers that co-segregated with, or were linked to, the susceptibility allele confirmed that the Yr5 locus is on chromosome 2B. Of five RGAP markers that were cloned and sequenced, markers Xwgp-17 and Xwgp-18 that co-segregated with the Yr5 locus were co-dominant and had 98% homology with each other in both DNA and translated amino-acid sequences. The two markers had 97% homology with a resistance gene-like sequence from Aegilops ventricosa and had significant homology with many known plant resistance genes, resistance gene analogs and expressed sequence tags (ESTs) from wheat and other plant species. The markers Xwgp-17 and Xwgp-18 also had significant homology with the NB-ARC domain that is in several genes for plant resistance to diseases, nematode cell death and human apoptotic signaling. These markers should be useful to clone Yr5 and combine Yr5 with other genes for durable and superior resistance for the control of stripe rust. [ABSTRACT FROM AUTHOR]

A National Wheat Rust Control Program, that has operated in Australiasince 1975, provides national pathotype surveys for the three rustpathogens, undertakes ongoing searches for new sources of resistance andcarries out genetic analyses of these sources, and provides rust screeningand germplasm enhancement support for breeders. The program wasexpanded in the 1980s to address problems arising from the introductionof wheat stripe rust. A new form of stripe rust, described as barley grassstripe rust, was first detected in 1998. Although virulent on some wheatseedlings this new rust appears to be a greater threat to barley. Molecularmarkers are expected to make an increasing impact on our ability to selectthe gene combinations needed to enhance the durability of resistance. [ABSTRACT FROM AUTHOR]