![]() ![]() However, some of the so-called allelic relationships might result from tight linkage of different R genes. For instance, at Pm3, 17 functional alleles have been identified at the sequence level ( Yahiaoui et al., 2004, 2006, 2009 Srichumpa et al., 2005 Bhullar et al., 2009, 2010), which form a true allelic series. Of the 50 loci identified for wheat powdery mildew resistance, six, including Pm1, Pm2, Pm3, Pm4, Pm5, and Pm24, were known to have multiple alleles according to allelic tests or mapping results. The Rp1 encompassed up to >50 copies of R genes was probably the largest R-gene cluster known so far in plants. Barley ( Hordeum vulgare L.) Mla ( Wei et al., 1999), maize ( Zea mays L.) Rp1 ( Ramakrishna et al., 2002 Smith et al., 2004), and lettuce ( Lactuca sativa L.) RGC2 ( Meyers et al., 1998) are all known R-gene clusters. In Arabidopsis thaliana L., two-thirds of the NBS-LRR R genes were in clusters ( Meyers et al., 2003) 74.3% R genes in rice ( Oryza sativa L.) were in clusters ( Monosi et al., 2004 Luo et al., 2012). Resistance-gene clusters widely exist in plants. However, in consideration of the vast amount of wheat R genes, lack of knowledge about their evolution, products, and functions has limited our understanding of the strategies employed by wheat to combat diseases. An RGA gene isolated from chromosome 6V of Haynaldia villosa (L.) Schur, the carrier of Pm21, encodes a serine–threonine kinase and provided powdery mildew resistance ( Cao et al., 2011). The previously popular Pm8, originated from rye ( Secale cereale L.), is the ortholog of Pm3 ( Hurni et al., 2013). (2004) isolated Pm3b on chromosome 1A of common wheat through positional cloning, which is an allele of the R locus encoding coiled-coil–nucleotide-binding-site–leucine-rich repeat (CC-NBS-LRR) proteins. ![]() Most of these genes confer race-specific resistance to wheat powdery mildew. To overcome powdery mildew in wheat, over 70 R genes, mapping to 50 distinct loci of the genome, have been identified in common wheat and its relatives ( McIntosh et al., 2013, 2014 Hao et al., 2015 Wang et al., 2015 Ma et al., 2015). Thus, it is necessary to constantly discover and use new powdery mildew R genes in the breeding program. However, R genes with large effect are often defeated, when widely employed over a large geographical area, by rapidly evolved pathogen populations within a short period ( McDonald and Linde 2002). Deployment of host R genes is the most effective and environmentally safe approach to control this disease in agricultural production. Severe epidemic of this disease could cause significant yield reduction especially in the areas with cool or maritime climates ( Everts and Leath 1992). P owdery mildew is an important foliar wheat disease caused by biotrophic fungus Bgt. New markers tightly linked to Mlm2033 were developed.Mlm2033, Mlm80, and MlIW172 are different powdery mildew resistance genes in the Pm1 cluster.High-resolution maps were constructed for both Mlm2033 and Mlm80.These results provided useful information for map-based cloning of the R genes in the Pm1 cluster and interpretation of their evolution. Severe recombination suppression and disruption of synteny were noted in the region encompassing Mlm2033. Based on the closely linked common markers, Mlm2033, Mlm80, and MlIW172 (another powdery mildew resistance gene in the Pm1 cluster) were not allelic to one another. Eventually, Mlm2033 and Mlm80 were restricted to a 0.12- and 0.86-cM interval, respectively. Moreover, a physical map of Mlm2033 was constructed through chromosome walking with bacterial artificial chromosome (BAC) clones and comparative mapping. With these and other newly developed and published markers, high-resolution maps were constructed for both Mlm2033 and Mlm80 using large F 2 populations. ![]() chromosome 1 (Bd1) and the Mlm2033– Mlm80–Pm1a region, which allowed us to develop markers based on the wheat sequences orthologous to genes contained in the Bd1 region. Comparative genomics study showed that macrocolinearity exists between Brachypodium distachyon L. This study was initiated to fine map these two genes toward map-based cloning. monococcum L.) were two of the R genes belonging to this cluster. Mlm2033 and Mlm80 from einkorn wheat ( T. More than a dozen genes against this disease, identified from wheat germplasms of different ploidy levels, have been mapped to the region surrounding the Pm1 locus on the long arm of chromosome 7A, which forms a resistance ( R)-gene cluster. tritici ( Bgt) is a globally devastating foliar disease of wheat ( Triticum aestivum L.). Powdery mildew caused by Blumeria graminis (DC.) f. ![]()
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