<Emphasis Type="Italic">Yr32</Emphasis> for resistance to stripe (yellow) rust present in the wheat cultivar Carstens V

Stripe or yellow rust of wheat, caused by Puccinia striiformis f. sp. tritici, is an important disease in many wheat-growing regions of the world. A number of major genes providing resistance to stripe rust have been used in breeding, including one gene that is present in the differential tester Carstens V. The objective of this study was to locate and map a stripe rust resistance gene transferred from Carstens V to Avocet S and to use molecular tools to locate a number of genes segregating in the cross Savannah/Senat. One of the genes present in Senat was predicted to be a gene that is present in Carstens V. For this latter purpose, stripe rust response data from both seedling and field tests on a doubled haploid population consisting of 77 lines were compared to an available molecular map for the same lines using a non-parametric quantitative trait loci (QTL) analysis. Results obtained in Denmark suggested that a strong component of resistance with the specificity of Carstens V was located in chromosome arm 2AL, and this was consistent with chromosome location work undertaken in Australia. Since this gene segregated independently of Yr1, the only other stripe rust resistance gene known to be located in this chromosome arm, it was designated Yr32. Further QTLs originating from Senat were located in chromosomes 1BL, 4D, and 7DS and from Savannah on 5B, but it was not possible to characterize them as unique resistance genes in any definitive way. Yr32 was detected in several wheats, including the North American differential tester Tres.An erratum to this article can be found at Communicated by G. Wenzel     

Wheat stripe rust resistance genes <Emphasis Type="Italic">Yr5</Emphasis> and <Emphasis Type="Italic">Yr7</Emphasis> are allelic

Stripe rust is one of the most destructive diseases of wheat. Breeding for resistance is the most economical and environmentally acceptable means to control stripe rust. Genetic studies on resistance sources are very important. Previous inheritance studies on Triticum aestivum subsp. spelta cv. album and wheat cultivar Lee showed that each possessed a single dominant gene for stripe rust resistance, i.e., Yr5 and Yr7, respectively. Both were located on the long arm of chromosome 2B, but due to the complexities caused by genetic background effects there was no clear evidence on the allelism or linkage status of these genes. Our study, involving an intercross of Avocet S near-isogenic lines possessing the genes, provided clear evidence for allelism or extremely close linkage of Yr5 and Yr7 based on phenotypic and molecular studies.     

Remapping of the stripe rust resistance gene <Emphasis Type="Italic">Yr10</Emphasis> in common wheat

Key message

Yr10 is an important gene to control wheat stripe rust, and the search for Yr10 needs to be continued.

Abstract

Wheat stripe rust or yellow rust is a devastating fungal disease caused by Puccinia striiformis f. sp. tritici (Pst). Host disease resistance offers a primary source for controlling wheat stripe rust. The stripe rust resistance gene Yr10 confers the race-specific resistance to most tested Pst races in China including CYR29. Early studies proposed that Yr10 was a nucleotide-binding site, leucine-rich repeat gene archived as GenBank accession AF149112 (hereafter designated the Yr10 candidate gene or Yr10 _CG ). In this study, we revealed that 15 Chinese wheat cultivars positive for Yr10 _CG are susceptible to CYR29. We then expressed the Yr10 _CG cDNA in the common wheat ‘Bobwhite’. The Yr10 _CG -cDNA positive transgenic plants were also susceptible to CYR29. Thus, it is highly unlikely that Yr10 _CG corresponds to the Yr10 resistance gene. Using the Yr10 donor ‘Moro’ and the Pst-susceptible wheat ‘Huixianhong’, we generated two F₃ populations that displayed a single Mendelian segregation on the Yr10 gene, and used them to remap the Yr10 gene. Six markers were placed in the Yr10 region, with the Yr10 _CG gene now mapping about 1.2-cM proximal to the Yr10 locus and the Xsdauw79 marker is completely linked to the Yr10 locus. Apparently, the Yr10 gene has not yet been identified. Fine mapping and positional cloning of Yr10 is important for gene pyramiding for stripe rust resistance in wheat.

     

New slow-rusting leaf rust and stripe rust resistance genes <Emphasis Type="Italic">Lr67</Emphasis> and <Emphasis Type="Italic">Yr46</Emphasis> in wheat are pleiotropic or closely linked

The common wheat genotype ‘RL6077’ was believed to carry the gene Lr34/Yr18 that confers slow-rusting adult plant resistance (APR) to leaf rust and stripe rust but located to a different chromosome through inter-chromosomal reciprocal translocation. However, haplotyping using the cloned Lr34/Yr18 diagnostic marker and the complete sequencing of the gene indicated Lr34/Yr18 is absent in RL6077. We crossed RL6077 with the susceptible parent ‘Avocet’ and developed F₃, F₄ and F₆ populations from photoperiod-insensitive F₃ lines that were segregating for resistance to leaf rust and stripe rust. The populations were characterized for leaf rust resistance at two Mexican sites, Cd. Obregon during the 2008–2009 and 2009–2010 crop seasons, and El Batan during 2009, and for stripe rust resistance at Toluca, a third Mexican site, during 2009. The F₃ population was also evaluated for stripe rust resistance at Cobbitty, Australia, during 2009. Most lines had correlated responses to leaf rust and stripe rust, indicating that either the same gene, or closely linked genes, confers resistance to both diseases. Molecular mapping using microsatellites led to the identification of five markers (Xgwm165, Xgwm192, Xcfd71, Xbarc98 and Xcfd23) on chromosome 4DL that are associated with this gene(s), with the closest markers being located at 0.4 cM. In a parallel study in Canada using a Thatcher × RL6077 F₃ population, the same leaf rust resistance gene was designated as Lr67 and mapped to the same chromosomal region. The pleiotropic, or closely linked, gene derived from RL6077 that conferred stripe rust resistance in this study was designated as Yr46. The slow-rusting gene(s) Lr67/Yr46 can be utilized in combination with other slow-rusting genes to develop high levels of durable APR to leaf rust and stripe rust in wheat.     

Adult plant stripe rust resistance gene <Emphasis Type="Italic">Yr71</Emphasis> maps close to <Emphasis Type="Italic">Lr24</Emphasis> in chromosome 3D of common wheat

Australian cultivar Sunco carries three adult plant stripe rust resistance genes. One of these genes corresponded to Yr18 in chromosome 7DS; the second, YrCK, was mapped on chromosome 2D. Here, we describe the characterization of the third adult plant resistance (APR) gene from Sunco. Sunco/2*Avocet S-derived lines SA65 (resistant) and SA67 (susceptible) were crossed and a recombinant inbred line F₆ population was generated. Monogenic segregation among SA65/SA67-derived RIL population was demonstrated and the resistance locus was designated YrSA3. Selective genotyping using an iSelect 90 K Infinium SNP array and SSR markers located YrSA3 on chromosome 3D. Development of KASP markers for SNP loci showing association with YrSA3 allowed construction of a genetic map harboring the resistance gene. Ten KASP markers (KASP_8306, KASP_9142, KASP_10438, KASP_16434, KASP_17207, KASP_20836, KASP_23518, KASP_23615, KASP_57983 and KASP_63653), one SSR marker (gwm114b) and Lr24/Sr24 were mapped 1.8 cM distal to YrSA3. Comparison of marker data indicated that the previously named seedling stripe rust resistance gene Yr45 was located proximal to YrSA3, and therefore the latter was formally designated Yr71. Two recombinants carrying Lr24/Sr24 and Yr71 in combination were identified for use as donor sources in wheat breeding programs. The robustness of gwm114b, KASP_16434, KASP_17207 and KASP_20836 for marker-assisted selection of these genes was demonstrated through tests on 74 Australian wheat cultivars.     

Characterization and mapping of cryptic alien introgression from <Emphasis Type="Italic">Aegilops geniculata</Emphasis> with new leaf rust and stripe rust resistance genes <Emphasis Type="Italic">Lr57</Emphasis> and <Emphasis Type="Italic">Yr40</Emphasis> in wheat

Leaf rust and stripe rust are important foliar diseases of wheat worldwide. Leaf rust and stripe rust resistant introgression lines were developed by induced homoeologous chromosome pairing between wheat chromosome 5D and 5M^g of Aegilops geniculata (U^gM^g). Characterization of rust resistant BC₂F₅ and BC₃F₆ homozygous progenies using genomic in situ hybridization with Aegilops comosa (M) DNA as probe identified three different types of introgressions; two cytologically visible and one invisible (termed cryptic alien introgression). All three types of introgression lines showed similar and complete resistance to the most prevalent pathotypes of leaf rust and stripe rust in Kansas (USA) and Punjab (India). Diagnostic polymorphisms between the alien segment and recipient parent were identified using physically mapped RFLP probes. Molecular mapping revealed that cryptic alien introgression conferring resistance to leaf rust and stripe rust comprised less than 5% of the 5DS arm and was designated T5DL·5DS-5M^gS(0.95). Genetic mapping with an F₂ population of Wichita × T5DL·5DS-5M^gS(0.95) demonstrated the monogenic and dominant inheritance of resistance to both diseases. Two diagnostic RFLP markers, previously mapped on chromosome arm 5DS, co-segregated with the rust resistance in the F₂ population. The unique map location of the resistant introgression on chromosome T5DL·5DS-5M^gS(0.95) suggested that the leaf rust and stripe rust resistance genes were new and were designated Lr57 and Yr40. This is the first documentation of a successful transfer and characterization of cryptic alien introgression from Ae. geniculata conferring resistance to both leaf rust and stripe rust in wheat.     

Characterisation of a new stripe rust resistance gene <Emphasis Type="Italic">Yr47</Emphasis> and its genetic association with the leaf rust resistance gene <Emphasis Type="Italic">Lr52</Emphasis>

Two Iranian common wheat landraces AUS28183 and AUS28187 from the Watkins collection showed high levels of seedling resistance against Australian pathotypes of leaf rust and stripe rust pathogens. Chi-squared analyses of rust response segregation among F₃ populations derived from crosses of AUS28183 and AUS28187 with a susceptible genotype AUS27229 revealed monogenic inheritance of leaf rust and stripe rust resistance. As both genotypes produced similar leaf rust and stripe rust infection types, they were assumed to carry the same genes. The genes were temporarily named as LrW1 and YrW1. Molecular mapping placed LrW1 and YrW1 in the short arm of chromosome 5B, about 10 and 15 cM proximal to the SSR marker gwm234, respectively, and the marker cfb309 mapped 8–12 cM proximal to YrW1. LrW1 mapped 3–6 cM distal to YrW1 in two F₃ populations. AUS28183 corresponded to the accession V336 of the Watkins collection which was the original source of Lr52. Based on the genomic location and accession records, LrW1 was concluded to be Lr52. Because no other seedling stripe rust resistance gene has previously been mapped in chromosome 5BS, YrW1 was permanently named as Yr47. A combination of flanking markers gwm234 and cfb309 with phenotypic assays could be used to ascertain the presence of Lr52 and Yr47 in segregating populations. This investigation characterised a valuable source of dual leaf rust and stripe rust resistance for deployment in new wheat cultivars. Transfer of Lr52 and Yr47 into current Australian wheat backgrounds is in progress.     

Molecular mapping of stripe rust resistance gene <Emphasis Type="Italic">YrJ22</Emphasis> in Chinese wheat cultivar Jimai 22

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is an important disease of wheat worldwide. Host resistance is the best way to control the disease. Genetic analysis of F₂ and F_2:3 populations from an Avocet S/Jimai 22 cross indicated that stripe rust resistance in Jimai 22 was conferred by a single dominant gene, tentatively designated YrJ22. A total of 377 F₂ plants and 127 F_2:3 lines were tested with Chinese Pst race CYR32 and genotyped with simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers. A linkage map was constructed with five SSR and two SNP markers. Xwmc658 and IWA1348 flanked YrJ22 at genetic distances of 1.0 and 7.3 cM, proximally and distally, respectively. The chromosomal location was confirmed using Chinese Spring nulli-tetrasomic, ditelosomics and deletion lines. Seedling reactions to 21 Pst races demonstrated differences in specificity between YrJ22 and other resistance genes on chromosome 2AL, indicating that YrJ22 is likely to be a new wheat stripe rust resistance gene.     

Validation and characterization of a QTL for adult plant resistance to stripe rust on wheat chromosome arm 6BS (<Emphasis Type="Italic">Yr78</Emphasis>)

Key message

This study validated one QTL for adult plant resistance to stripe rust, identified donor lines of the resistance allele, and demonstrated that it is different from previously named Yr genes.

Abstract

The spread of more virulent and aggressive races of Puccinia striiformis f. sp. tritici (Pst, causal pathogen of stripe rust) after the year 2000 has caused substantial yield losses worldwide. To find new sources of resistance, we previously performed a genome-wide association study and identified a strong QTL for adult plant resistance on the short arm of chromosome 6B (QYr.ucw-6B). In this study, we validated QYr.ucw-6B in ten biparental populations, and mapped it 0.6 cM proximal to IWA7257 and 3.9 cM distal to IWA4408. We showed that QYr.ucw-6B is located approximately 15 cM proximal to the all-stage resistance gene Yr35 and that none of the resistant lines carries the previously cloned Yr36 gene. Based on these results, QYr.ucw-6B was assigned the name Yr78. This gene was not effective against Pst at the seedling stage, suggesting that it is an adult plant resistance gene. Yr78 has been effective against Pst races present in field experiments performed in the Western USA between 2011 and 2016. Since this gene is predicted to be present at low frequency in wheat germplasm from this region, it can provide a useful tool to diversify the sources of resistance against this devastating pathogen.

     

Molecular mapping of stripe rust resistance gene <Emphasis Type="Italic">YrHu</Emphasis> derived from <Emphasis Type="Italic">Psathyrostachys huashanica</Emphasis>

Wheat stripe rust is a destructive disease that affects most wheat-growing areas worldwide. Resistance genes from related species and genera add to the genetic diversity available to wheat breeding programs. The stripe rust-resistant introgression line H9020-17-25-6-4 was developed from a cross of resistant Psathyrostachys huashanica with the susceptible wheat cultivar 7182. H9020-17-25-6-4 is resistant to all existing Chinese stripe rust races, including the three most widely virulent races, CYR32, CYR33, and V26. We attempted to characterize this new line by genomic in situ hybridization (GISH) and genetic analysis. GISH using P. huashanica genomic DNA as a probe indicated that the translocated segment was too small to be detected. Genetic analysis involving F₁, F₂, and F_2:3 materials derived from a cross of Mingxian 169 and H9020-17-25-6-4 indicated that a single dominant gene from H9020-17-25-6-4, temporarily designated YrHu, conferred resistance to CYR29 and CYR33. A genetic map consisting of four simple sequence repeat, two sequence-tagged site (STS), and two sequence-related amplified polymorphism markers was constructed. YrHu was located on the short arm of chromosome 3A and was about 0.7 and 1.5 cM proximal to EST-STS markers BG604577 and BE489244, respectively. Both the gene and the closely linked markers could be used in marker-assisted selection.     

<Emphasis Type="Italic">Yr45</Emphasis>, a new wheat gene for stripe rust resistance on the long arm of chromosome 3D

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases of wheat worldwide. Growing resistant cultivars is the most effective approach to control the disease, but only a few genes confer effective all-stage resistance against the current populations of the pathogen worldwide. It is urgent to identify new genes for diversifying sources of resistance genes and for pyramiding genes for different types of resistance in order to achieve high levels of durable resistance for sustainable control of stripe rust. The common spring wheat genotype ‘PI 181434’, originally from Afghanistan, was resistant in all greenhouse and field tests in our previous studies. To identify the resistance gene(s) PI 181434 was crossed with susceptible genotype ‘Avocet Susceptible’. Adult plants of 103 F₂ progeny were tested in the field under the natural infection of P. striiformis f. sp. tritici. Seedlings of the parents, F₂ and F₃ were tested with races PST-100 and PST-127 of the pathogen under controlled greenhouse conditions. The genetic study showed that PI 181434 has a single dominant gene conferring all-stage resistance. Resistance gene analog polymorphism (RGAP) and simple sequence repeat (SSR) techniques were used to identify molecular markers linked to the gene. A linkage map of 8 RGAP and 2 SSR markers was constructed for the gene using data from the 103 F₂ plants and their derived F₃ lines tested in the greenhouse. Amplification of the complete set of nulli-tetrasomic lines and selected ditelosomic lines of Chinese Spring with an RGAP marker and the two SSR markers mapped the gene on the long arm of chromosome 3D. Because it is the first gene for stripe rust resistance mapped on chromosome 3DL and different from all previously named Yr genes, the gene in PI 181434 was designated Yr45. Polymorphism rates of the two closest flanking markers, Xwgp115 and Xwgp118, in 45 wheat genotypes were 73.3 and 82.2%, respectively. Single nucleotide polymorphisms (SNPs) were identified in the eight wheat genotypes sharing both flanking markers. The RGAP markers and potential SNP markers should be useful in incorporating the gene into wheat cultivars and in pyramiding it with other genes for durable resistance.     

Development of an STS marker tightly linked to <Emphasis Type="Italic">Yr26</Emphasis> against wheat stripe rust using the resistance gene-analog polymorphism (RGAP) technique

The gene Yr26 confers resistance to all races of Puccinia striiformis f. sp. tritici (PST), the casual pathogen of wheat stripe rust in China. Here, we report development of a molecular marker closely linked to Yr26 using a resistance gene-analog polymorphism (RGAP) technique. A total of 787 F₂ plants and 165 F₃ lines derived from the cross Chuanmai 42/Taichung 29 were used for linkage analysis. Eighteen near-isogenic lines (NILs) and 18 Chinese wheat cultivars and advanced lines with different genes for stripe rust resistance were employed for the validation of STS markers. A total of 1,711 RGAP primer combinations were used to test the parents and resistant and susceptible bulks. Five polymorphic RGAP markers were used for genotyping all F₂ plants. Linkage analysis showed that the five RGAP markers were closely linked to Yr26 with genetic distances ranging from 0.5 to 2.9 cM. These markers were then converted into STS markers, one, CYS-5, of which was located 0.5 cM to Yr26 and was closely associated with the resistance gene when validated over 18 NILs and 18 Chinese wheat cultivars and lines. The results indicated that CYS-5 can be used in marker-assisted selection targeted at pyramiding Yr26 and other genes for stripe rust resistance.     

Genome-wide identification of QTL conferring high-temperature adult-plant (HTAP) resistance to stripe rust (<Emphasis Type="Italic">Puccinia striiformis</Emphasis> f. sp<Emphasis Type="Italic">. tritici</Emphasis>) in wheat

High-temperature adult-plant (HTAP) resistance to stripe rust (caused by Puccinia striiformis f. sp. tritici) is a durable type of resistance in wheat (Triticum aestivum L.). This study identified quantitative trait loci (QTL) conferring HTAP resistance to stripe rust in a population consisting of 169 F_8:10 recombinant inbred lines (RILs) derived from a cross between a susceptible cultivar Rio Blanco and a resistant germplasm IDO444. HTAP resistance was evaluated for both disease severity and infection type under natural infection over two years at two locations. The genetic linkage maps had an average density of 6.7 cM per marker across the genome and were constructed using 484 markers including 96 wheat microsatellite (SSR), 632 Diversity Arrays Technology (DArT) polymorphisms, two sequence-tagged-site (STS) from semi-dwarf genes Rht1 and Rht2, and two markers for low molecular-weight glutenin gene subunits. QTL analysis detected a total of eight QTL significantly associated with HTAP resistance to stripe rust with two on chromosome 2B, two on 3B and one on each of 1A, 4A, 4B and 5B. QTL on chromosomes 2B and 4A were the major loci derived from IDO444 and explained up to 47 and 42% of the phenotypic variation for disease severity and infection type, respectively. The remaining five QTL accounted for 7–10% of the trait variation. Of these minor QTL, the resistant alleles at the two QTL QYrrb.ui-3B.1 and QYrrb.ui-4B derived from Rio Blanco and reduced infection type only, while the resistant alleles at the other three QTL, QYrid.ui-1A, QYrid.ui-3B.2 and QYrid.ui-5B, all derived from IDO444 and reduced either infection type or disease severity. Markers linked to 2B and 4A QTL should be useful for selection of HTAP resistance to stripe rust.     

Fine mapping of the chromosome 5B region carrying closely linked rust resistance genes <Emphasis Type="Italic">Yr47</Emphasis> and <Emphasis Type="Italic">Lr52</Emphasis> in wheat

Key message

Fine mapping of Yr47 and Lr52 in chromosome arm 5BS of wheat identified close linkage of the marker sun180 to both genes and its robustness for marker-assisted selection was demonstrated.

Abstract

The widely effective and genetically linked rust resistance genes Yr47 and Lr52 have previously been mapped in the short arm of chromosome 5B in two F₃ populations (Aus28183/Aus27229 and Aus28187/Aus27229). The Aus28183/Aus27229 F₃ population was advanced to generate an F₆ recombinant inbred line (RIL) population to identify markers closely linked with Yr47 and Lr52. Diverse genomic resources including flow-sorted chromosome survey sequence contigs representing the orthologous region in Brachypodium distachyon, the physical map of chromosome arm 5BS, expressed sequence tags (ESTs) located in the 5BS6-0.81-1.00 deletion bin and resistance gene analog contigs of chromosome arm 5BS were used to develop markers to saturate the target region. Selective genotyping was also performed using the iSelect 90 K Infinium wheat SNP assay. A set of SSR, STS, gene-based and SNP markers were developed and genotyped on the Aus28183/Aus27229 RIL population. Yr47 and Lr52 are genetically distinct genes that mapped 0.4 cM apart in the RIL population. The SSR marker sun180 co-segregated with Lr52 and mapped 0.4 cM distal to Yr47. In a high resolution mapping population of 600 F₂ genotypes Yr47 and Lr52 mapped 0.2 cM apart and marker sun180 was placed 0.4 cM distal to Lr52. The amplification of a different sun180 amplicon (195 bp) than that linked with Yr47 and Lr52 (200 bp) in 204 diverse wheat genotypes demonstrated its robustness for marker-assisted selection of these genes.

     

Genome-wide association mapping reveals a rich genetic architecture of stripe rust resistance loci in emmer wheat (<Emphasis Type="Italic">Triticum turgidum</Emphasis> ssp. <Emphasis Type="Italic">dicoccum</Emphasis>)

Key message

SNP-based genome scanning in worldwide domesticated emmer germplasm showed high genetic diversity, rapid linkage disequilibrium decay and 51 loci for stripe rust resistance, a large proportion of which were novel.

Abstract

Cultivated emmer wheat (Triticum turgidum ssp. dicoccum), one of the oldest domesticated crops in the world, is a potentially rich reservoir of variation for improvement of resistance/tolerance to biotic and abiotic stresses in wheat. Resistance to stripe rust (Puccinia striiformis f. sp. tritici) in emmer wheat has been under-investigated. Here, we employed genome-wide association (GWAS) mapping with a mixed linear model to dissect effective stripe rust resistance loci in a worldwide collection of 176 cultivated emmer wheat accessions. Adult plants were tested in six environments and seedlings were evaluated with five races from the United States and one from Italy under greenhouse conditions. Five accessions were resistant across all experiments. The panel was genotyped with the wheat 90,000 Illumina iSelect single nucleotide polymorphism (SNP) array and 5106 polymorphic SNP markers with mapped positions were obtained. A high level of genetic diversity and fast linkage disequilibrium decay were observed. In total, we identified 14 loci associated with field resistance in multiple environments. Thirty-seven loci were significantly associated with all-stage (seedling) resistance and six of them were effective against multiple races. Of the 51 total loci, 29 were mapped distantly from previously reported stripe rust resistance genes or quantitative trait loci and represent newly discovered resistance loci. Our results suggest that GWAS is an effective method for characterizing genes in cultivated emmer wheat and confirm that emmer wheat is a rich source of stripe rust resistance loci that can be used for wheat improvement.

     

SNP-based pool genotyping and haplotype analysis accelerate fine-mapping of the wheat genomic region containing stripe rust resistance gene <Emphasis Type="Italic">Yr26</Emphasis>

Key message

NGS-assisted super pooling emerging as powerful tool to accelerate gene mapping and haplotype association analysis within target region uncovering specific linkage SNPs or alleles for marker-assisted gene pyramiding.

Abstract

Conventional gene mapping methods to identify genes associated with important agronomic traits require significant amounts of financial support and time. Here, a single nucleotide polymorphism (SNP)-based mapping approach, RNA-Seq and SNP array assisted super pooling analysis, was used for rapid mining of a candidate genomic region for stripe rust resistance gene Yr26 that has been widely used in wheat breeding programs in China. Large DNA and RNA super-pools were genotyped by Wheat SNP Array and sequenced by Illumina HiSeq, respectively. Hundreds of thousands of SNPs were identified and then filtered by multiple filtering criteria. Among selected SNPs, over 900 were found within an overlapping interval of less than 30 Mb as the Yr26 candidate genomic region in the centromeric region of chromosome arm 1BL. The 235 chromosome-specific SNPs were converted into KASP assays to validate the Yr26 interval in different genetic populations. Using a high-resolution mapping population (>?30,000 gametes), we confined Yr26 to a 0.003-cM interval. The Yr26 target region was anchored to the common wheat IWGSC RefSeq v1.0 and wild emmer WEWSeq v.1.0 sequences, from which 488 and 454 kb fragments were obtained. Several candidate genes were identified in the target genomic region, but there was no typical resistance gene in either genome region. Haplotype analysis identified specific SNPs linked to Yr26 and developed robust and breeder-friendly KASP markers. This integration strategy can be applied to accelerate generating many markers closely linked to target genes/QTL for a trait of interest in wheat and other polyploid species.

     

Genetic mapping of stem rust resistance gene <Emphasis Type="Italic">Sr13</Emphasis> in tetraploid wheat (<Emphasis Type="Italic">Triticum turgidum</Emphasis> ssp. <Emphasis Type="Italic">durum</Emphasis> L.)

Wheat stem rust caused by Puccinia graminis f. sp. tritici, can cause significant yield losses. To combat the disease, breeders have deployed resistance genes both individually and in combinations to increase resistance durability. A new race, TTKSK (Ug99), identified in Uganda in 1999 is virulent on most of the resistance genes currently deployed, and is rapidly spreading to other regions of the world. It is therefore important to identify, map, and deploy resistance genes that are still effective against TTKSK. One of these resistance genes, Sr13, was previously assigned to the long arm of chromosome 6A, but its precise map location was not known. In this study, the genome location of Sr13 was determined in four tetraploid wheat (T. turgidum ssp. durum) mapping populations involving the TTKSK resistant varieties Kronos, Kofa, Medora and Sceptre. Our results showed that resistance was linked to common molecular markers in all four populations, suggesting that these durum lines carry the same resistance gene. Based on its chromosome location and infection types against different races of stem rust, this gene is postulated to be Sr13. Sr13 was mapped within a 1.2–2.8 cM interval (depending on the mapping population) between EST markers CD926040 and BE471213, which corresponds to a 285-kb region in rice chromosome 2, and a 3.1-Mb region in Brachypodium chromosome 3. These maps will be the foundation for developing high-density maps, identifying diagnostic markers, and positional cloning of Sr13.     

Over-expressing a UDP-glucosyltransferase gene (<Emphasis Type="Italic">Ta-UGT</Emphasis><Subscript><Emphasis Type="Italic">3</Emphasis></Subscript>) enhances <Emphasis Type="Italic">Fusarium</Emphasis> Head Blight resistance of wheat

Wheat Fusarium Head Blight (FHB), mainly caused by Fusarium graminearum (F.g), is a destructive fungal disease worldwide. FHB can not only cause considerable reduction in yield, but more seriously, can contaminate grain by trichothecene toxins released by the fungus. Here, we report new insights into the function and underlying mechanisms of a UDP-glycosyltransferase gene, Ta-UGT ₃ , that is involved in FHB resistance in wheat. In our previous study, Ta-UGT ₃ was found to enhance host tolerance against deoxynivalenol (DON) in Arabidopsis. In this study, four transgenic lines over-expressing Ta-UGT ₃ in a FHB highly susceptible wheat variety, Alondra’s, were obtained and characterized. 3 years of assays using single floret inoculation with F.g indicated that all four transgenic lines exhibited significantly enhanced type II resistance to FHB and less DON accumulation in the grains compared to the untransformed control. Histological observation using GFP labelled F.g was in agreement with the above test results since over-expression of Ta-UGT ₃ dramatically inhibited expansion of F.g. To explore the putative mechanism of resistance mediated by Ta-UGT ₃ , microarray analysis, qRT-PCR and hormone measurements were performed. Microarray analysis showed that DON up-regulated genes, such as TaNPR1, in the susceptible control, and down-regulated genes in F.g inoculated transgenic lines, while qRT-PCR showed that some defence related genes were up-regulated in F.g inoculated transgenic lines. Ta-UGT ₃ over-expression also changed the contents of the endogenous hormones SA and JA in the spikes. These data suggest that Ta-UGT ₃ positively regulates the defence responses to F.g, perhaps by regulating defence-related and DON-induced downstream genes.     

Molecular mapping of leaf rust resistance gene <Emphasis Type="Italic">LrBi16</Emphasis> in Chinese wheat cultivar Bimai 16

Leaf rust, caused by Puccinia triticina, is one of the most widespread diseases in common wheat (Triticum aestivum L.) globally. With the objective of identifying and mapping new genes for resistance to leaf rust, F₁, F₂ plants and F₃ lines from a cross between resistant cultivar Bimai 16 and susceptible cultivar Thatcher were inoculated with Chinese Puccinia triticina pathotypes FHTT and PHTS in the greenhouse. In the first seedling test, Bimai 16, Thatcher, 20 F₁ plants, 359 F₂ plants and 298 F₃ lines were inoculated with pathotype FHTT. A set of 1,255 simple sequence repeat (SSR) primer pairs were used to test the parents, and resistant and susceptible bulks. Seven polymorphic markers on chromosome 7BL were used for genotyping the F₂ and F₃ populations. The results indicated that Bimai 16 carried a single dominant resistance gene, temporarily designated LrBi16, closely linked to SSR markers Xcfa2257 and Xgwm344, with genetic distances of 2.8 and 2.9 cM, respectively. In the second seedling test, two dominant resistance genes were identified in Bimai 16 based on seedling reactions of 254 F₂ plants inoculated with pathotype PHTS. One of the genes was LrBi16, and the other was likely to be LrZH84, which is located in chromosome 1BL. The seedling reaction pattern of plants with LrBi16 was different from that of the Thatcher lines, with Lr14a and Lr14b located on chromosome 7BL. It was concluded that LrBi16 is likely to be a new leaf rust resistance gene.