Stem Rust Resistance Genes Research Articles

Genetics of stem rust resistance in four popular durum wheat cultivars

A study was conducted to understand the mode of inheritance and extent of diversity of stem rust resistance in four popular durum wheat cultivars of central India viz., HI 8498 (Malav Shakti), HI 8663 (Poshan), HI 8713 (Pusa Mangal) and HI 8737 (Pusa Anmol) using Puccinia graminis tritici (Pgt) pathotypes 15-1 (123G15) and 40-3 (127G29). These cultivars were crossed with susceptible parents i.e., Motia and Malvi Local and were also crossed among themselves in half diallel fashion. The F2 and F3 segregation data revealed that a single dominant gene each controlled resistance to the pathotype 40-3 in HI 8713 and HI 8663, while two dominant genes each governed resistance to this pathotype in HI 8737 and HI 8498. A single dominant gene each conditioned resistance to the pathotype 15-1 in all the four cultivars. The F2 segregation data of the intercrosses among the resistant parents showed that three different resistance genes controlled resistance among four cultivars against each Pgt pathotype 40-3 and 15-1. These genes seem to be different from the most commonly postulated stem rust resistance genes in Indian durum wheat germplasm viz., Sr11, Sr12, Sr7b and Sr9e which are ineffective/less effective against the test pathotypes. Hence, the genes identified in the present study can be utilized in broadening the genetic base of stem rust resistance in Indian durum wheat.

Identification of seedling resistance to stem rust in advanced wheat lines and varieties from Pakistan

AbstractStem rust is a major disease of wheat (Triticum aestivum L.) worldwide and new Puccinia graminis f. sp. tritici (Pgt) races including TTKSK (Ug99) pose a serious threat to wheat production. The protection of new varieties against Pgt races can be increased by identifying and combining several types of stem rust resistance genes (Sr). We screened a set of 707 wheat lines and cultivars against 11 Pgt races under glass house conditions. Of the tested lines, groups of 99, 513, 289, and 515 exhibited low infection type (IT < 3) to races TTKSK, TRTTF, TTTTF, and RRTTF, respectively. Screening against Pgt races (QFCSC, QTHJC, MCCFC, RCRSC, RKRQC, TPMKC, and QCCSM) showed that most of the tested lines were resistant. These lines were screened with eight DNA markers for the presence of Sr2, Sr9a, Sr24, Sr25, Sr31, Sr36, Sr38, and Sr57 genes. Sr36 was absent from all the tested lines, whereas Sr9a was detected in four lines. The marker Sr2_ger93p predicted the presence of Sr2 in 40 lines, and marker barc71 suggested the presence of Sr24 in 12 lines. Sr25 and Sr38 were present in 13 and 54 lines, respectively. The highest frequency of Sr genes was observed for Sr57 (199 lines) and Sr31 (177 lines). Except for lines carrying Sr25 and/or Sr24 genes, most lines were susceptible to Pgt race TTKSK. Since Ug99 is overcoming Sr genes worldwide, including Sr24 and Sr36, a strategy to pyramid multiple Sr genes in new cultivars should be pursued to achieve a durable control of stem rust. The effectiveness of lines such as NRL0902, 11050, B‐2(RF)‐11, and CCRI‐6, found in the current study and featuring other Sr genes, warrants further investigation to identify the source of their resistance and use it in Pakistan wheat breeding programs.

Description of Wheat Rusts and Their Virulence Variations Determined through Annual Pathotype Surveys and Controlled Multi-Pathotype Tests

Wheat production started in Australia around 1788 using early maturing varieties adapted to Australian conditions that were able to escape diseases as well as moisture stress conditions. Wheat production is concentrated on mainland Australia in a narrow crescent land considered as the wheat belt occupying an area of about 13.9 million hectares. Rusts are the most important production constraints to wheat production in the world and Australia causing significant yield losses and decreased the qualities of grains. Wheat is affected by three different types of rust diseases: leaf rust, stripe rust or yellow rust, and stem rust. Each species of the rust pathogen has many races or pathotypes that parasitize only on certain varieties of host species, which can only be traced and identified by differential cultivars. Pathotype surveillance is the basis for information on the virulence or pathogenic variations existing in a particular country or wheat growing region of the world. Studies in pathotype variation are conducted in controlled environments using multi-pathotype tests. The currently cultivated commercial wheat varieties of Australia possess leaf rust resistant genes: Lr1, Lr3a, Lr13, Lr13+, Lr14a, Lr17a, Lr17b, Lr20, Lr23, Lr24, Lr26, Lr27, Lr31, Lr34, Lr37, and Lr46; stem rust resistance genes: Sr2, Sr5, Sr8a, Sr8b, Sr9b, Sr9g, Sr11, Sr12, Sr13, Sr15, Sr17, Sr22, Sr24, Sr26, Sr30, Sr36, Sr38, and Sr57; and stripe rust resistance genes: Yr4, Yr9, Yr17, Yr18, Yr27, and Yr33. This paper discusses the historical and current significance of rusts to wheat production in the world with particular reference to Australia viz-a-viz detail description of each of the three rusts and their respective virulence variations through the resistance genes deployed in the commercial cultivars.

Transcriptome-wide association study identifies putative elicitors/suppressor of Puccinia graminis f. sp. tritici that modulate barley rpg4-mediated stem rust resistance

BackgroundStem rust is an economically important disease of wheat and barley. However, studies to gain insight into the molecular basis of these host-pathogen interactions have primarily focused on wheat because of its importance in human sustenance. This is the first extensive study utilizing a transcriptome-wide association mapping approach to identify candidate Puccinia graminis f. sp. tritici (Pgt) effectors/suppressors that elicit or suppress barley stem rust resistance genes. Here we focus on identifying Pgt elicitors that interact with the rpg4-mediated resistance locus (RMRL), the only effective source of Pgt race TTKSK resistance in barley.ResultsThirty-seven Pgt isolates showing differential responses on RMRL were genotyped using Restriction Site Associated DNA-Genotyping by Sequencing (RAD-GBS), identifying 24 diverse isolates that were used for transcript analysis during the infection process. In planta RNAseq was conducted with the 24 diverse isolates on the susceptible barley variety Harrington, 5 days post inoculation. The transcripts were mapped to the Pgt race SCCL reference genome identifying 114 K variants in predicted genes that would result in nonsynonymous amino acid substitutions. Transcriptome wide association analysis identified 33 variants across 28 genes that were associated with dominant RMRL virulence, thus, representing candidate suppressors of resistance. Comparative transcriptomics between the 9 RMRL virulent -vs- the 15 RMRL avirulent Pgt isolates identified 44 differentially expressed genes encoding candidate secreted effector proteins (CSEPs), among which 38 were expressed at lower levels in virulent isolates suggesting that they may represent RMRL avirulence genes. Barley transcript analysis after colonization with 9 RMRL virulent and 15 RMRL avirulent isolates inoculated on the susceptible line Harrington showed significantly lower expression of host biotic stress responses specific to RMRL virulent isolates suggesting virulent isolates harbor effectors that suppress resistance responses.ConclusionsThis transcriptomic study provided novel findings that help fill knowledge gaps in the understanding of stem rust virulence/avirulence and host resistance in barley. The pathogen transcriptome analysis suggested RMRL virulence might depend on the lack of avirulence genes, but evidence from pathogen association mapping analysis and host transcriptional analysis also suggested the alternate hypothesis that RMRL virulence may be due to the presence of suppressors of defense responses.

Identification of Stem Rust Resistance Genes in the Winter Wheat Collection from Southern Russia.

The high yield potential of winter wheats cannot be realized due to disease pressure under field conditions. One of the most harmful of such diseases is stem rust, hence the constant search for sources of resistance and the development of new varieties resistant to stem rust is of great relevance. This study deals with the identification of stem rust resistance genes in a collection of winter wheats grown in Southern Russia. This genepool has not been studied yet. A total of 620 samples of winter soft wheat from various ecological and geographical zones were tested under field conditions. To identify the specific genes or alleles responsible for resistance, all samples were genotyped using PCR. As a result, the groups of resistant samples, carrying the Sr2, Sr31, Sr38 and Sr44 genes in various combinations, were identified. Most of the stem rust resistance was provided by the presence of the effective Sr44 gene. This information can be used in the future breeding work for stem rust resistance.

Validation and Marker-Assisted Selection of Stem Rust Resistance Gene Sr2 in Indian Wheat Using Gel-Based and Gel-Free Methods

Stem rust resistance gene Sr2 is an important slow-rusting gene which has provided resistance against stem rust for many years. The Sr2 gene has durable resistance against all stem rust pathogens including the Ug99 group of races. It shows recessive inheritance and is linked with two phenotypic markers, Pseudo Black Chaff (PBC) and High Temperature-Induced Seedling Chlorosis (HTISC). However, direct screening as well as phenotypic marker-based screening for selection of Sr2 is difficult due to the effect of environmental factors and genetic background. Microsatellite marker Xgwm533 linked to Sr2 gene is useful for rapid screening of large populations. Here we report validation and use of Xgwm533 for screening 21 wheat lines and 24 wheat genotypes for the presence of the Sr2 gene. Furthermore, we also report the presence of a new, smaller allele (97 bp) of Xgwm533 locus in a few genotypes that was also confirmed by sequencing. We also demonstrate a new SYBR green dye, melt-curve/profile-based assay for convenient screening of the Xgwm533 locus, where the presence of different alleles can be differentiated in a gel-free manner.

Mapping of the stem rust resistance gene Pg13 in cultivated oat.

The widely deployed, oat stem rust resistance gene Pg13 was mapped by linkage analysis and association mapping, and KASP markers were developed for marker-assisted selection in breeding programs. Pg13 is one of the most extensively deployed stem rust resistance genes in North American oat cultivars. Identification of markers tightly linked to this gene will be useful for routine marker-assisted selection, identification of gene pyramids, and retention of the gene in backcrosses and three-way crosses. To this end, high-density linkage maps were constructed in four bi-parental mapping populations using SNP markers identified from 6K oat Infinium iSelect and genotyping-by-sequencing platforms. Additionally, genome-wide associations were identified using two sets of association panels consisting of diverse elite oat lines in one set and landrace accessions in the other. The results showed that Pg13 was located at approximately 67.7cM on linkage group Mrg18 of the consensus genetic map. The gene co-segregated with the 7C-17A translocation breakpoint and with crown rust resistance gene Pc91. Co-segregating markers with the best prediction accuracy were identified at 67.7-68.5cM on Mrg18. KASP assays were developed for linked SNP loci for use in oat breeding.

Wheat gene Sr60 encodes a protein with two putative kinase domains that confers resistance to stem rust.

Wheat stem rust, caused by Puccinia graminis Pers. f. sp. tritici (Pgt), is a devastating fungal disease threatening global wheat production. The present paper reports the identification of stem rust resistance gene Sr60, a race-specific gene from diploid wheat Triticum monococcum L. that encodes a protein with two putative kinase domains. This gene, designated asWHEAT TANDEM KINASE 2 (WTK2), confers intermediate levels of resistance to Pgt. WTK2 was identified by map-based cloning and validated by transformation of a c.10-kb genomic sequence including WTK2 into susceptible common wheat variety Fielder (Triticum aestivum L.). Transformation of Fielder with WTK2 was sufficient to confer Pgt resistance. Sr60 transcripts were transiently upregulated 1d post-inoculation with Pgt, but not in mock-inoculated plants. The upregulation of Sr60 was associated with stable upregulation of several pathogenesis-related genes. The Sr60-resistant haplotype found in T.monococcum was not found in polyploid wheat, suggesting an opportunity to introduce a novel resistance gene. Sr60 was successfully introgressed into hexaploid wheat, and wedeveloped a diagnostic molecular marker to accelerate its deployment and pyramiding with other resistance genes. The cloned Sr60 also can be a useful component of transgenic cassettes including other resistance genes with complementary resistance profiles.

Physical Mapping of Stem Rust Resistance Gene Sr52 from Dasypyrum villosum Based on ph1b-Induced Homoeologous Recombination.

Wheat stem rust caused by Puccinia graminis f. sp. tritici (Pgt) had been a devastating foliar disease worldwide during the 20th century. With the emergence of Ug99 races, which are virulent to most stem rust resistance genes deployed in wheat varieties and advanced lines, stem rust has once again become a disease threatening global wheat production. Sr52, derived from Dasypyrum villosum and mapped to the long arm of 6V#3, is one of the few effective genes against Ug99 races. In this study, the wheat–D. villosum Robertsonian translocation T6AS·6V#3L, the only stock carrying Sr52 released to experimental and breeding programs so far, was crossed with a CS ph1b mutant to induce recombinants with shortened 6V#3L chromosome segments locating Sr52. Six independent homozygous recombinants with different segment sizes and breakpoints were developed and characterized using in situ hybridization and molecular markers analyses. Stem rust resistance evaluation showed that only three terminal recombinants (1381, 1380, and 1392) containing 8%, 22%, and 30% of the distal segment of 6V#3L, respectively, were resistant to stem rust. Thus, the gene Sr52 was mapped into 6V#3L bin FL 0.92–1.00. In addition, three molecular markers in the Sr52-located interval of 6V#3L were confirmed to be diagnostic markers for selection of Sr52 introgressed into common wheat. The newly developed small segment translocation lines with Sr52 and the identified molecular markers closely linked to Sr52 will be valuable for wheat disease breeding.

Mapping and characterization of two stem rust resistance genes derived from cultivated emmer wheat accession PI 193883.

Two stem rust resistance genes identified on chromosome arms 2BL and 6AL of the cultivated emmer wheat accession PI 193883 can be used for protecting modern varieties against Ug99 strains. The wheat research community consistently strives to identify new genes that confer resistance to stem rust caused by the fungal pathogen Puccinia graminis f. sp. tritici Eriks & E. Henn (Pgt). In the current study, our objective was to identify and genetically characterize the stem rust resistance derived from the cultivated emmer accession PI 193883. A recombinant inbred line population developed from a cross between the susceptible durum wheat line Rusty and PI 193883 was genotyped and evaluated for reaction to Pgt races TTKSK, TRTTF, and TMLKC. Two QTLs conferring resistance were identified on chromosome arms 2BL (QSr.fcu-2B) and 6AL (QSr.fcu-6A). The stem rust resistance gene (Sr883-2B) underlying QSr.fcu-2B was recessive, and based on its physical location it is located proximal to the Sr9 region. QSr.fcu-6A was located in the Sr13 region, but PI 193883 is known to carry the susceptible haplotype S4 for Sr13, indicating that the gene underlying QSr.fcu-6A (Sr883-6A) is likely a new allele of Sr13 or a gene residing close to Sr13. Three IWGSC scaffold-based simple sequence repeat (SSR) and two SNP-based semi-thermal asymmetric reverse PCR (STARP) markers were developed for the Sr883-2B region, and one STARP marker was developed for Sr883-6A. Sr883-2B was epistatic to Sr883-6A for reaction to TTKSK and TRTTF, and the two genes had additive effects for TMLKC. These two genes and the markers developed in this research provide additional resources and tools for the improvement in stem rust resistance in durum and common wheat breeding programs.

Marker Assisted Transfer of Stripe Rust and Stem Rust Resistance Genes into Four Wheat Cultivars

Three rust diseases namely; stem rust caused by Puccinia graminis f. sp. tritici (Pgt), leaf rust caused by Puccinia triticina (Pt), and stripe rust caused by Puccinia striiformis f. sp. tritici (Pst), are the most common fungal diseases of wheat (Triticum aestivum L.) and cause significant yield losses worldwide including Australia. Recently characterized stripe rust resistance genes Yr51 and Yr57 are effective against pre- and post-2002 Pst pathotypes in Australia. Similarly, stem rust resistance genes Sr22, Sr26, and Sr50 are effective against the Pgt pathotype TTKSK (Ug99) and its derivatives in addition to commercially important Australian pathotypes. Effectiveness of these genes make them good candidates for combining with known pleiotropic adult plant resistance (PAPR) genes to achieve durable resistance against three rust pathogens. This study was planned to transfer rust resistance genes Yr51, Yr57, Sr22, Sr26, and Sr50 into two Australian (Gladius and Livingston) and two Indian (PBW550 and DBW17) wheat cultivars through marker assisted selection (MAS). These cultivars also carry other rust resistance genes: Gladius carries Lr37/Yr17/Sr38 and Sr24/Lr24; Livingston carries Lr34/Yr18/Sr57, Lr37/Yr17/Sr38, and Sr2; PBW550 and DBW17 carry Lr34/Yr18/Sr57 and Lr26/Yr9/Sr31. Donor sources of Yr51 (AUS91456), Yr57 (AUS91463), Sr22 (Sr22/3*K441), Sr26 (Sr26 WA1), and Sr50 (Dra-1/Chinese Spring ph1b/2/3* Gabo) were crossed with each of the recurrent parents to produce backcross progenies. Markers linked to Yr51 (sun104), Yr57 (gwm389 and BS00062676), Sr22 (cssu22), Sr26 (Sr26#43), and Sr50 (Sr50-5p-F3, R2) were used for their MAS and markers csLV34 (Lr34/Yr18/Sr57), VENTRIUP-LN2 (Lr37/Yr17/Sr38), Sr24#12 (Sr24/Lr24), and csSr2 (Sr2) were used to select genes present in recurrent parents. Progenies of selected individuals were grown and selected under field conditions for plant type and adult plant rust responses. Final selections were genotyped with the relevant markers. Backcross derivatives of these genes were distributed to breeding companies for use as resistance donors.

Molecular marker dissection of stem rust resistance in Nebraska bread wheat germplasm

Stem rust (caused by Puccinia graminis f. sp. tritici) is a major disease of wheat. To understand the genetic basis of stem rust resistance in Nebraska winter wheat, a set of 330 genotypes representing two nurseries (DUP2015 and TRP2015) were evaluated for resistance to a Nebraska stem rust race (QFCSC) in two replications. The TRP2015 nursery was also evaluated for its resistance to an additional 13 stem rust races. The analysis of variance revealed significant variation among genotypes in both populations for stem rust resistance. Nine stem rust genes, Sr6, Sr31, Sr1RSAmigo, Sr24, Sr36, SrTmp, Sr7b, Sr9b, and Sr38, were expected and genotyped using gene-specific markers. The results of genetic analysis confirmed the presence of seven stem rust resistance genes. One genotype (NE15680) contained target alleles for five stem rust resistance genes and had a high level of stem rust resistance against different races. Single marker analysis indicated that Sr24 and Sr38 were highly significantly associated with stem rust resistance in the DUP2015 and TRP2015 nurseries, respectively. Linkage disequilibrium analysis identified the presence of 17 SNPs in high linkage with the Sr38-specific marker. These SNPs potentially tagging the Sr38 gene could be used in marker-assisted selection after validating them in additional genetic backgrounds.

Mapping of Novel Leaf Rust and Stem Rust Resistance Genes in the Portuguese Durum Wheat Landrace PI 192051

Leaf rust caused by Puccinia triticina Erikss. (Pt) and stem rust caused by Puccinia graminis f. sp. tritici Erikss. & E. Henn (Pgt) are serious constraints to production of durum wheat (Triticum turgidum L). The objective of this study was to identify leaf rust resistance (Lr) and stem rust resistance (Sr) genes/QTL in Portuguese durum landrace PI 192051. Four Pt-isolates, representing three virulence phenotypes (BBBQJ, BBBSJ & EEEEE) and six Pgt-races TTKSK, JRCQC, TKTTF, QFCFC, TPMKC and TMLKC were used to evaluate 180 recombinant inbred lines (RILs) derived from the cross Rusty (rust susceptible) × PI 192051-1 (rust resistant) at the seedling stage. The RILs were also phenotyped at the adult-plant stage in a stem rust nursery in Ethiopia in 2017. The RILs were genotyped using the Illumina iSelect 9K wheat SNP array. PI 192051-1 carries a previously unidentified major Sr gene designated as QSr.ace-7A on chromosome arm 7AS and Lr gene Lr.ace-4A in the pericentromeric region of chromosome 4A. In addition, three minor Sr QTL QSr.ace-1A, QSr.ace-2B and QSr.ace-4A were mapped in PI 192051-1 on chromosomes 1AL, 2BL, and 4A, respectively Lr.ace-4A could be co-located or tightly linked to QSr.ace-4A. Markers linked to the identified QTL/genes can be used for marker assisted selection. These findings enrich the genetic basis of rust resistance in both durum and common wheat.

Characterization of Ethiopian Wheat Germplasm for Resistance to Four Puccinia graminis f. sp. tritici Races Facilitated by Single-Race Nurseries.

In Ethiopia, breeding rust resistant wheat cultivars is a priority for wheat production. A stem rust epidemic during 2013 to 2014 on previously resistant cultivar Digalu highlighted the need to determine the effectiveness of wheat lines to multiple races of Puccinia graminis f. sp. tritici in Ethiopia. During 2014 and 2015, we evaluated a total of 97 bread wheat and 14 durum wheat genotypes against four P. graminis f. sp. tritici races at the seedling stage and in single-race field nurseries. Resistance genes were postulated using molecular marker assays. Bread wheat lines were resistant to race JRCQC, the race most virulent to durum wheat. Lines with stem rust resistance gene Sr24 possessed the most effective resistance to the four races. Only three lines with adult plant resistance possessed resistance effective to the four races comparable with cultivars with Sr24. Although responses of the wheat lines across races were positively correlated, wheat lines were identified that possessed adult plant resistance to race TTKSK but were relatively susceptible to race TKTTF. This study demonstrated the importance of testing wheat lines for response to multiple races of the stem rust pathogen to determine if lines possessed non-race-specific resistance.

Molecular identification of the stem rust resistance genes in the introgression lines of spring bread wheat

A total of 57 introgression lines and 11 cultivars of spring bread wheat developed by All-Russian Institute of Plant Protection and cultivated in the Volga Region were analyzed. The lines were obtained with the participation of CIMMYT synthetics, durum wheat cultivars, direct crossing with Agropyron elongatum (CI-7-57) and have introgressions from related species of bread wheat, namely translocations from Ag. elongatum (7DS-7DL-7Ae#1L), Aegilops speltoides (2D-2S), Ae. ventricosum (2AL-2AS-2MV#1), Secale cereale (1BL-1R#1S), 6Agi (6D) substitution from Ag. intermedium and triticale Satu. Cultivars and lines were assessed for resistance to Saratov, Lysogorsk, Derbent and Omsk stem rust pathogen populations (Puccinia graminis f. sp. tritici), and analyzed for the presence of the known Sr resistance genes using molecular markers. The analysis of the cultivars’ and lines’ resistance to the Saratov pathogen population in the field, as well as to Omsk, Derbent and Lysogorsk populations at the seedling stage, showed the loss of efficiency of the Sr25 and Sr6Agi genes. The Sr31 gene remained effective. Thirty one wheat lines out of 57 (54.4 % of samples) were resistant to all pathogen populations taken into analysis. The Sr31/Lr26, Sr25/Lr19, Sr28, Sr57/Lr34 and Sr38/Lr37 genes were identified in the introgression lines. The Sr31/Lr26 gene was identified in 19 lines (33.3 % of samples). All lines carrying the 1RS.1BL translocation (Sr31/Lr26) were resistant to all pathogen populations taken into analysis. The Sr25/Lr19 gene was identified in 49 lines (86 %). The gene combination Sr31/Lr26+ Sr25/Lr19 was identified in 15 lines (26.3 %). The gene combinations Sr38/Lr37+Sr25/Lr19, Sr57/Lr34+Sr25/Lr19 and Sr31/Lr26+Sr25/Lr19+Sr28 were identified in 3 introgression lines. These three lines were characterized by resistance to the pathogen populations studied in this work. The Sr2, Sr24, Sr26, Sr32, Sr36 and Sr39 genes were not detected in the analyzed wheat lines.

Introgression of rpg4/Rpg5 Into Barley Germplasm Provides Insights Into the Genetics of Resistance to Puccinia graminis f. sp. tritici Race TTKSK and Resources for Developing Resistant Cultivars.

Stem rust (incited by Puccinia graminis f. sp. tritici) is a devastating disease of wheat and barley in many production areas. The widely virulent African P. graminis f. sp. tritici race TTKSK is of particular concern, because most cultivars are susceptible. To prepare for the possible arrival of race TTKSK in North America, we crossed a range of barley germplasm-representing different growth habits and end uses-with donors of stem rust resistance genes Rpg1 and rpg4/Rpg5. The former confers resistance to prevalent races of P. graminis f. sp. tritici in North America, and the latter confers resistance to TTKSK and other closely related races from Africa. We produced doubled haploids from these crosses and determined their allele type at the Rpg loci and haplotype at 7,864 single-nucleotide polymorphism loci. The doubled haploids were phenotyped for TTKSK resistance at the seedling stage. Integration of genotype and phenotype data revealed that (i) Rpg1 was not associated with TTKSK resistance, (ii) rpg4/Rpg5 was necessary but was not sufficient for resistance, and (iii) specific haplotypes at two quantitative trait loci were required for rpg4/Rpg5 to confer resistance to TTKSK. To confirm whether lines found resistant to TTKSK at the seedling resistance were also resistant at the adult plant stage, a subset of doubled haploids was evaluated in Kenya. Additionally, adult plant resistance to leaf rust and stripe rust (incited by Puccinia hordei and Puccinia striiformis f. sp. hordei, respectively) was also assessed on the doubled haploids in field trials at three locations in the United States over a 2-year period. Doubled haploids were identified with adult plant resistance to all three rusts, and this germplasm is available to the research and breeding communities.

Pyramiding stem rust resistance genes to race TTKSK (Ug99) in wheat

There has not been a major wheat stem rust epidemic in Canada since 1954. The detection of race TTKSK, with virulence to Sr31 and most other stem rust resistance genes in wheat, presents a threat to the wheat crop worldwide. Single disease resistance genes are usually effective for only several years before the pathogen changes genetically to overcome the resistance. To produce more durable resistance to race TTKSK, as well as to leaf rust and Fusarium head blight (FHB), we produced pyramided lines with several resistance genes. A population of 68 doubled haploids was produced from the hybrid ‘AC Cadillac’ (SrCad, Lr34)/‘Carberry’ (Lr34, Fhb1) × ‘RL5405' (Sr33)/‘Carberry’ (Lr34, Fhb1). A total of 16 combinations of variable numbers of the four genes were recovered by marker assisted selection. The most notable combination with four of the genes was Sr33, SrCad, Lr34 and Fhb1. Two such lines were recovered. All pyramided lines with Lr34 and SrCad or Sr33 were resistant to race TTKSK, while two control wheat varieties 'Shaw’ and ‘Little Club’ were susceptible. The two pyramided lines (SrCad-Sr33-Lr34-Fhb1) were relatively more resistant than the lines containing only one gene for stem rust resistance. The pyramided lines with Lr34 showed good resistance to leaf rust races 12–3 MBDS and TDBG. The pyramids with Fhb1 did not show a lowered expression of Fusarium head blight.

Resistance gene cloning from a wild crop relative by sequence capture and association genetics.

Disease resistance (R) genes from wild relatives could be used to engineer broad-spectrum resistance in domesticated crops. We combined association genetics with R gene enrichment sequencing (AgRenSeq) to exploit pan-genome variation in wild diploid wheat and rapidly clone four stem rust resistance genes. AgRenSeq enables R gene cloning in any crop that has a diverse germplasm panel.

Mapping of genes for leaf and stem rust resistance in bread wheat genotype Selection 212

Resistance genes for leaf and stem rusts in bread wheat line Selection212 are recessive in nature. Both leaf and stem rust resistance genes, named tentatively as LrSel212 and SrSel212, have been mapped to the short arm of chromosome 2B separated by genetic distance of 16.4 cM. Xwmc474 was the closest marker located between two genes, 5.6 cM proximal to LrSel212 and 10.8 cM distal to SrSel212. Leaf rust pathotype 77-5 is virulent to leaf rust resistance genes located on chromosome 2B viz., Lr13, Lr16, Lr23, Lr35 and Lr73, but avirulent to Selection212, suggesting that LrSel212 is distinct from these genes. Six stem rust resistance genes have been assigned to chromosome 2B viz., Sr19, Sr20, Sr23, Sr36, Sr39 and Sr40. Stem rust pathotype 40A used in genetic analysis was virulent to Sr19 and Sr20, but avirulent to Selection212; and the latter showed a significantly lower infection type in comparison to Sr39. Sr23 and Sr36 showed susceptibility to few other stem rust pathotypes to which Selection212 was resistant. While the response of Sr40 to Indian pathotypes of Pgt is not known, differences in the genetic distance and nature of inheritance between Selection212 and Sr40 indicate their distinct identity. However, test of allelism with Sr40 is required to confirm whether SrSel212 represents a different locus. Selection212 may be useful in broadening the genetic base of rust resistance in wheat.

Identification of recombinants carrying stripe rust resistance gene Yr57 and adult plant stem rust resistance gene Sr2 through marker‐assisted selection

AbstractMany stem rust resistance genes have been formally named in wheat. Adult plant stem rust resistance gene Sr2 was mapped in the short‐arm of chromosome 3B. Stripe rust resistance gene Yr57, identified in Aus91463, was mapped about 5 cM away from Sr2 based on its linkage with Sr2‐linked marker gwm533. The objective of this study was to combine Sr2 and Yr57 in a single genotype. A mapping population containing 107 recombinant inbred lines was developed from a cross between Aus91463‐Yr57 and Hartog‐Sr2. This population was tested at the seedling stage in the glasshouse for variation in stripe rust response, and high temperature induced Sr2‐linked seedling chlorosis. The RIL population was screened for Sr2‐linked pseudo black chaff phenotype at the adult plant stage in field. Five recombinants carrying Sr2 and Yr57 in coupling were detected using phenotypic and marker data. Four recombinants also carried leaf rust resistance gene Lr23 from Aus91463. These recombinants are being used as triple rust resistance source in the Australian Cereal Rust Control Program.