Article

Detection of Virulence to Resistance Gene Sr24 Within Race TTKS of Puccinia graminis f. sp. tritici

Plant Disease (Impact Factor: 3.02). 06/2008; 92(6). DOI: 10.1094/PDIS-92-6-0923
Source: OAI

ABSTRACT

The stem rust resistance gene Sr24 is effective against most races of Puccinia graminis f. sp. tritici, including race TTKS (syn. Ug99), and is used widely in commercial wheat cultivars worldwide. In 2006, susceptible infection responses were observed on wheat lines and cultivars carrying Sr24 in a field stem rust screening nursery at Njoro, Kenya. We derived 28 single-pustule isolates from stem rust samples collected from the 2006 Njoro nursery. The isolates were evaluated for virulence on 16 North American stem rust differential lines; on wheat lines carrying Sr24, Sr31, Sr38, and SrMcN; and on a wheat cultivar with a combination of Sr24 and Sr31. All isolates were identified as race TTKS with additional virulence on Sr31 and Sr38. These isolates were divided into two groups: group A (seven isolates and the two control isolates), producing a low infection type, and group B (21 isolates), producing a high infection type on Sr24, respectively. Isolates of group B represented a new variant of race TTKS with virulence to Sr24. Eighteen simple sequence repeat (SSR) markers were used to examine the genetic relationship between these two groups of isolates in race TTKS and five North American races (MCCF, QCCQ, RCRS, RTHS, and TPMK) that are representative of distinct lineage groups. All isolates of race TTKS shared an identical SSR genotype and were clearly different from North American races. The virulence and SSR data indicated that the new variant of race TTKS with Sr24 virulence likely has arisen via mutation within the TTKS genetic lineage. We propose to revise the North American stem rust nomenclature system by the addition of four genes (Sr24, Sr31, Sr38, and SrMcN) as the fifth set. This revision recognizes the virulence on Sr31 and differentiates isolates within race TTKS into two separate races: TTKSK and TTKST, with avirulence and virulence on Sr24, respectively. The occurrence of race TTKST with combined virulence on Sr24 and Sr31 has substantially increased the vulnerability of wheat to stem rust worldwide.

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    • "The broad virulence of these races, together with a continuing evolution for virulence (Jin et al. 2008; 2009; Pretorius et al. 2010; 2012) and steadily broadening distribution, threaten wheat production in many regions, with serious implications for food security (Vurro et al. 2010; Singh et al. 2011; 2015; Shiferaw et al. 2013). The Durable Rust Resistance in Wheat (DRRW) project, coordinated by Cornell University (http:// wheatrust.cornell.edu), "
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    ABSTRACT: Bread wheat is an allopolyploid species with a large, highly repetitive genome. To investigate the impact of selection on variants distributed among homoeologous wheat genomes and to build a foundation for understanding genotype-phenotype relationships, we performed population-scale re-sequencing of a diverse panel of wheat lines. A sample of 62 diverse lines was re-sequenced using the whole exome capture and genotyping-by-sequencing approaches. We describe the allele frequency, functional significance, and chromosomal distribution of 1.57 million single nucleotide polymorphisms and 161,719 small indels. Our results suggest that duplicated homoeologous genes are under purifying selection. We find contrasting patterns of variation and inter-variant associations among wheat genomes; this, in addition to demographic factors, could be explained by differences in the effect of directional selection on duplicated homoeologs. Only a small fraction of the homoeologous regions harboring selected variants overlapped among the wheat genomes in any given wheat line. These selected regions are enriched for loci associated with agronomic traits detected in genome-wide association studies. Evidence suggests that directional selection in allopolyploids rarely acted on multiple parallel advantageous mutations across homoeologous regions, likely indicating that a fitness benefit could be obtained by a mutation at any one of the homoeologs. Additional advantageous variants in other homoelogs probably either contributed little benefit, or were unavailable in populations subjected to directional selection. We hypothesize that allopolyploidy may have increased the likelihood of beneficial allele recovery by broadening the set of possible selection targets.
    No preview · Article · Dec 2015 · Genome Biology
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    • "A recent example is the emergence of the Ug99 (TTKSK) race group of the stem rust fungus in East Africa (Jin et al. 2008; Pretorius et al. 2010; Singh et al. 2011). The TTKSK race group has defeated the resistance provided by stem rust resistance genes (Sr) Sr24, Sr31, Sr36, and Sr9h (Jin et al. 2008; Pretorius et al. 2010) and can infect more than 90 % of the wheat crop grown worldwide (Singh et al. 2011). This race group is quickly evolving and migrating throughout Africa and Southwest Asia, posing a serious threat to world wheat production (Singh et al. 2011). "
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    ABSTRACT: The stem rust fungus, particularly race TTKSK (Ug99), poses a serious threat to world wheat production. Gene Sr42 or SrCad (which could be the same gene or an allele of Sr42) is effective against race TTKSK. However, known genetic markers for Sr42 are mostly SSR markers which are generally labor intensive to use. In this study, we mapped a race TTKSK resistance gene derived from PI 595667 at the same locus as Sr42 on chromosome 6DS. Based on position, pedigree and infection-type information, we propose that this gene is SrCad (Sr42). We enriched the genetic map for the Sr42 region using genotyping by sequencing (GBS) and array-derived SNP markers. In total, 21 SNP markers were discovered, spanning a genetic distance of 27.2 cM. Nine of them are derived from GBS and twelve from the Illumina iSelect 90K SNP assay. Ten of the twenty-one SNP markers are closely linked (<2.2 cM, or co-segregating) with Sr42. We converted five of the closely linked SNP markers into uniplex KASP assays which will better facilitate marker-assisted selection. We validated the KASP assay in a doubled haploid wheat population derived from a three-way cross between accessions PI 410954, RB07, and Faller that shared an uncharacterized resistance gene mapped at approximately the same locus as PI 595667. The development of closely linked (co-segregating), codominant, sequence-based SNP assays will aid marker-assisted selection and map-based cloning of Sr42.
    Full-text · Article · Nov 2015 · Molecular Breeding
    • "The broad virulence of these races, together with a continuing evolution for virulence (Jin et al. 2008; 2009; Pretorius et al. 2010; 2012) and steadily broadening distribution, threaten wheat production in many regions, with serious implications for food security (Vurro et al. 2010; Singh et al. 2011; 2015; Shiferaw et al. 2013). The Durable Rust Resistance in Wheat (DRRW) project, coordinated by Cornell University (http:// wheatrust.cornell.edu), "
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    ABSTRACT: Bread wheat (Triticum aestivum) is an important crop in South Asia and epidemics of stem rust, caused by Puccinia graminis f. sp. tritici, can impact negatively on food security. In a pro-active assessment of the risk posed by this disease, germplasm collections received from Bangladesh and Nepal in 2011 were screened for their response to Sr31-virulent stem rust races belonging to the Ug99 group. According to a molecular marker assay, 44% of Bangladesh lines and 48% of Nepal lines contained Sr31. In seedling tests, most entries were susceptible to P. graminis f. sp. tritici race PTKST, confirming the ineffectiveness of Sr31. No lines contained Sr24 or Sr36, two major genes that have been overcome by races within the Ug99 group. Despite seedling susceptibility, many lines exhibited high levels of adult plant resistance in the field. The presence of the Sr2 marker allele of csSr2 in some of these lines indicated that this gene is likely to play a role in the adult plant resistance observed. This study showed that sufficient protection to Ug99 exists in Bangladesh and Nepal wheat germplasm, and that the appropriate release of resistant cultivars will insure against a possible future incursion of this dangerous pathogen.
    No preview · Article · Sep 2015 · Phytoparasitica
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