Molecular mapping of a new temperature-sensitive gene <Emphasis Type="Italic">LrZH22</Emphasis> for leaf rust resistance in Chinese wheat cultivar Zhoumai 22

Leaf rust, caused by Puccinia triticina, is one of the most widespread diseases in common wheat globally. The Chinese wheat cultivar Zhoumai 22 is highly resistant to leaf rust at the seedling and adult stages. Seedlings of Zhoumai 22 and 36 lines with known leaf rust resistance genes were inoculated with 13 P. triticina races for gene postulation. The leaf rust response of Zhoumai 22 was different from those of the single gene lines. With the objective of identifying and mapping, the new gene(s) for resistance to leaf rust, F₁, F₂ plants and F_2:3 lines from the cross Zhoumai 22/Chinese Spring were inoculated with Chinese P. triticina race FHDQ at the seedling stage. A single dominant gene, tentatively designated LrZH22, conferred resistance. To identify other possible genes in Zhoumai 22, ten P. triticina races avirulent on Zhoumai 22 were used to inoculate 24 F_2:3 lines. The same gene conferred resistance to all ten avirulent races. A total of 1300 simple sequence repeat (SSR) markers and 36 EST markers on 2BS were used to test the parents, and resistant and susceptible bulks. Resistance gene LrZH22 was mapped in the chromosome bin 2BS1-0.53-0.75 and closely linked to six SSR markers (barc183, barc55, gwm148, gwm410, gwm374 and wmc474) and two EST markers (BF202681 and BE499478) on chromosome arm 2BS. The two closest flanking SSR loci were Xbarc55 and Xgwm374 with genetic distances of 2.4 and 4.8 cM from LrZH22, respectively. Six designated genes (Lr13, Lr16, Lr23, Lr35, Lr48 and Lr73) are located on chromosome arm 2BS. In seedling tests, LrZH22 was temperature sensitive, conferring resistance at high temperatures. The reaction pattern of Zhoumai 22 was different from that of RL 4031 (Lr13), RL 6005 (Lr16) and RL 6012 (Lr23), Lr35 and Lr48 are adult-plant resistance genes, and Lr73 is not sensitive to the temperature. Therefore, LrZH22 is likely to be a new leaf rust resistance gene or allele.     

Genotyping-by-sequencing to remap QTL for type II Fusarium head blight and leaf rust resistance in a wheat–tall wheatgrass introgression recombinant inbred population

Fusarium graminearum Schwabe (Fusarium head blight, FHB) and Puccinia triticina Eriks (leaf rust) are two major fungal pathogens posing a continuous threat to the wheat crop; consequently, identifying resistance genes from various sources is always of importance to wheat breeders. We identified tightly linked single nucleotide polymorphism (SNP) markers for the FHB resistance quantitative trait locus (QTL) Qfhs.pur-7EL and the leaf rust resistance locus Lr19 using genotyping-by-sequencing (GBS) in a wheat–tall wheatgrass introgression-derived recombinant inbred line (RIL) population. One thousand and seven hundred high-confidence SNPs were used to conduct the linkage and QTL analysis. Qfhs.pur-7EL was mapped to a 2.9 cM region containing four markers within a 43.6 cM segment of wheatgrass chromosome 7el₂ that was translocated onto wheat chromosome 7DL. Lr19 from 7el₁ was mapped to a 1.21 cM region containing two markers in the same area, in repulsion. Five lines were identified with the resistance-associated SNP alleles for Qfhs.pur-7EL and Lr19 in coupling. Two SNP markers in the Qfhs.pur-7EL region were converted into PCR-based KASP markers. Investigation of the genetic characteristics of the parental lines of this RIL population indicated that they are translocation lines in two different wheat cultivar genetic backgrounds instead of 7E–7D substitution lines in Thatcher wheat background, as previously reported in the literature.     

Characterization and genome-wide association mapping of resistance to leaf rust,stem rust and stripe rust in a geographically diverse collection of spring wheat landraces

The challenge posed by rapidly changing wheat rust pathogens, both in virulence and in environmental adaptation, calls for the development and application of new techniques to accelerate the process of breeding for durable resistance. To expand the resistance gene pool available for germplasm improvement, a panel of 159 landraces plus old cultivars was evaluated for seedling and adult plant resistance (APR) to over 35 Australian pathotypes of Puccinia triticina, Puccinia graminis f. sp. tritici, and Puccinia striiformis f. sp. tritici. Known seedling resistance (SR) genes for leaf rust (Lr2a, Lr3a, Lr13, Lr23, Lr16, and Lr20), stem rust (Sr12, Sr13, Sr23, Sr30, and Sr36), and stripe rust (Yr3, Yr4, Yr5, Yr9, Yr10, Yr17, and Yr27) were postulated. The APR genes identified via field assessments and marker analyses included the pleiotropic genes (Lr34/Yr18/Sr57, Lr46/Yr29/Sr58, Lr67/Yr46/Sr55, and Sr2/Lr27/Yr30), Lr68, Lr74, and uncharacterized APR. A genome-wide association analysis using linear mixed models detected 79 single nucleotide polymorphism (SNP) markers significantly associated with rust resistance, which were mapped on chromosomes 1A, 1B, 1D, 2A, 2B, 3A, 3B, 3D, 4A, 5A, 5B, 6A, 6B, 6D, 7A, 7B and 7D. SNPs associated with multiple rust resistances probably indicate the presence of new pleiotropic or closely linked genes. SNPs were mapped on chromosome positions (1AL, 1DS, 2AL, 4AS, 5BS, 6DL, and 7AL) that have not been known to carry APR genes. This study revealed the presence of a range of possibly unidentified effective seedling and APRs among the landraces, which might represent new sources of rust resistance for the ongoing effort to develop improved wheat cultivars.     

Comparative analysis of <Emphasis Type="Italic">Agropyron intermedium</Emphasis> (Host) Beauv 6Ag<Superscript>i</Superscript> and 6Ag<Superscript>i</Superscript>2 chromosomes in bread wheat cultivars and lines with wheat–wheatgrass substitutions

A comparative study of wheat–wheatgrass substituted cultivars and lines resistant to leaf rust developed by the Agricultural Research Institute for Southeast Regions (Multi 6R, Belyanka, Favorit, Voevoda, Lebedushka) and Samara Agricultural Research Institute (Tulaikovskaya 5, Tulaikovskaya 10, Tulaikovskaya 100, Tulaikovskaya Zolotistaya) breeding was conducted. A complex analysis using molecular cytogenetic (C-differential banding, fluorescent (FISH) and genomic (GISH) in situ hybridization), molecular (PLUG markers), and biochemical (electrophoretic analysis of gliadins) markers demonstrated that they have a substitition of wheat chromosome 6D by the chromosomes 6Agⁱ and 6Agⁱ2 belonging to the J(=E) Agropyron intermedium (Host) Beauv (=Thinopyrum intermedium (Host) Barkworth & D.R. Dewey) subgenome. In spite of the fact that the chromosomes 6Agⁱ and 6Agⁱ2 differ in the C-banding pattern and demonstrated minor differences in the blocks of gliadin components, they had the identical pattern of pSc119.2 and pAs1 probe distribution and conjugated between themselves with insignificant disturbance. Thus, it was demonstrated that 6Agi and 6Agⁱ2 are homologous chromosomes; however, the question about allelism of their leaf rust resistance genes between themselves requires special studies. Nevertheless, using STS and SCAR markers and taking into account the type of reaction to Puccinia triticina, their non-allelism to the Lr9, Lr19, Lr24, Lr29, Lr38, and Lr47 genes was established. It was revealed that the 6Agⁱ and 6Agⁱ2 chromosomes have a different level of transmission in hybrid F₂ populations depending on the hybrid combination gene background.     

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.

     

Mapping and characterization of the new adult plant leaf rust resistance gene <Emphasis Type="Italic">Lr77</Emphasis> derived from Santa Fe winter wheat

Key message

A new gene for adult plant leaf rust resistance in wheat was mapped to chromosome 3BL. This gene was designated as Lr77.

Abstract

‘Santa Fe’ is a hard red winter cultivar that has had long-lasting resistance to the leaf rust fungus, Puccinia triticina. The objective of this study was to determine the chromosome location of the adult plant leaf rust resistance in Santa Fe wheat. A partial backcross line of ‘Thatcher’ (Tc) wheat with adult plant leaf rust resistance derived from Santa Fe was crossed with Thatcher to develop a Thatcher//Tc*2/Santa Fe F₆ recombinant inbred line (RIL) population. The RIL population and parental lines were evaluated for segregation of leaf rust resistance in three field plot tests and in an adult plant greenhouse test. A genetic map of the RIL population was constructed using 90,000 single-nucleotide polymorphism (SNP) markers with the Illumina Infinium iSelect 90K wheat bead array. A significant quantitative trait locus for reduction of leaf rust severity in all four tests was found on chromosome 3BL that segregated as a single adult plant resistance gene. The RILs with the allele from the resistant parent for SNP marker IWB10344 had lower leaf rust severity and a moderately resistant to moderately susceptible response compared to the susceptible RILs and Thatcher. The gene derived from Santa Fe on chromosome 3BL was designated as Lr77. Kompetitive allele-specific polymerase chain reaction assay markers linked to Lr77 on 3BL should be useful for selection of wheat germplasm with this gene.

     

Marker-assisted pyramiding of <Emphasis Type="BoldItalic">Thinopyrum-</Emphasis>derived leaf rust resistance genes <Emphasis Type="BoldItalic">Lr19</Emphasis> and <Emphasis Type="BoldItalic">Lr24</Emphasis> in bread wheat variety HD2733

This study was undertaken to pyramid two effective leaf rust resistance genes (Lr19 and Lr24) derived from Thinopyrum (syn. Agropyron), in the susceptible, but agronomically superior wheat cultivar HD2733 using marker-assisted selection. In the year 2001, HD2733 was released for irrigated timely sown conditions of the north eastern plains zone (NEPZ) of India became susceptible to leaf rust, a major disease of the region. Background selection helped in developing near-isogenic lines (NILs) of HD2733 with Lr19 and Lr24 with 97.27 and \(98.94\%\), respectively, of genomic similarity with the parent cultivar, after two backcrossing and one generation of selfing. NILs were intercrossed to combine the genes Lr19 and Lr24. The combination of these two genes in the cultivar HD2733 is expected to provide durable leaf rust resistance in farmers’ fields.     

Phytopathological and molecular genetic identification of leaf rust resistance genes in common wheat accessions with alien genetic material

Leaf rust resistance genes were sought in 23 resistant common wheat accessions with alien genetic material of Aegilops speltoides, Ae. triuncialis, and Triticum kiharae from the Arsenal collection. The genes were identified by common phytopathological tests and PCR analysis with STS markers linked with the known Lr genes. None of the methods identified the resistance genes in two accessions. In the other accessions, the combination of the two methods broadened the spectrum of detectable genes and, in some cases, allowed double verification of the presence of a resistance gene. Most accessions proved to contain several leaf rust resistance genes, combining juvenile and adult plant ones. The accessions were found to contain gene combinations that ensured field resistance (Lr13 + Lr10 and Lr12 + Lr34) and immunity under the conditions of the Non-Chernozem region. Accessions with alien genetic material contained a unique combination of five or six resistance genes. Since the accessions were rich in leaf rust resistance genes, including effective ones, and carried rare combinations of these genes, they were proposed as donors to be universally employed in breeding for immunity in all regions of Russia.     

Inheritance of the Translocation in Chromosome 2D of Common Wheat from <Emphasis Type="Italic">Aegilops speltoides</Emphasis> Tausch with Leaf Rust Resistance Gene

The variety of common spring wheat Chelyaba 75 carries a translocation from Aegilops speltoides Tausch in the chromosome 2D, which contains the leaf rust resistance gene and gametocidal genes. The length of this translocation was determined by molecular-genetic analysis. It is shown that the long arm of chromosome 2D is completely replaced by the long arm of chromosome 2S; it is possible that translocation involves the near-centromere region of the short arm. According to molecular analysis data, the translocation from Ae. speltoides in the Chelyaba 75 variety differs from the 2S chromosome region carrying the Lr35/Sr39 genes. This makes it possible to designate the leaf rust resistance gene of the Chelyaba 75 as LrSp2. The inheritance of LrSp2 in four populations from crossing Chelyaba 75 with different varieties of common wheat was studied. Estimation of leaf rust resistance of F₂ and F₃ hybrids in field conditions (2015–2016) revealed the absence of susceptible plants. The presence of 2DS.2SL translocation in hybrid plants was confirmed by molecular analysis. The results indicate the action of the gametocidal gene localized in the 2DS.2SL translocation and the fact that its tight linkage to the LrSp2 gene is inherited in a series of generations.     

Molecular markers for adult plant leaf rust resistance gene <Emphasis Type="Italic">Lr48</Emphasis> in wheat

Leaf rust of wheat, caused by Puccinia triticina, is an important disease throughout the world. The adult plant leaf rust resistance gene Lr48 reported in CSP44 was previously mapped in chromosome 2B, but the marker–gene association was weak. In this study, we confirmed the location of Lr48 to be in the short arm of chromosome 2B and identified closely linked markers suitable for use in breeding. The CSP44/WL711 recombinant inbred line (RIL) population (90 lines) showed monogenic segregation for Lr48. Twelve resistant and 12 susceptible RILs were used for selective genotyping using an iSelect 90K Infinium SNP assay. Closely linked SNPs were converted into Kompetitive allele-specific primers (KASP) and tested on the parental lines. KASP markers giving clear clusters for alternate genotypes were assayed on the entire RIL population. SNP markers IWB31002, IWB39832, IWB34324, IWB72894 and IWB36920 co-segregated with Lr48 and the marker IWB70147 was mapped 0.3 cM proximal to this gene. Closely linked KASP markers were tested on a set of Australian and Nordic wheat genotypes. The amplification of SNP alleles alternate to those linked with Lr48 in the majority of the Australian and Nordic wheat genotypes demonstrated the usefulness of these markers for marker-assisted pyramiding of Lr48 with other rust resistance genes.     

Characterization of <Emphasis Type="Italic">Lr75</Emphasis>: a partial,broad-spectrum leaf rust resistance gene in wheat

Key message

Here, we describe a strategy to improve broad-spectrum leaf rust resistance by marker-assisted combination of two partial resistance genes. One of them represents a novel partial adult plant resistance gene, named Lr75.

Abstract

Leaf rust caused by the fungal pathogen Puccinia triticina is a damaging disease of wheat (Triticum aestivum L.). The combination of several, additively-acting partial disease resistance genes has been proposed as a suitable strategy to breed wheat cultivars with high levels of durable field resistance. The Swiss winter wheat cultivar ‘Forno’ continues to show near-immunity to leaf rust since its release in the 1980s. This resistance is conferred by the presence of at least six quantitative trait loci (QTL), one of which is associated with the morphological trait leaf tip necrosis. Here, we used a marker-informed strategy to introgress two ‘Forno’ QTLs into the leaf rust-susceptible Swiss winter wheat cultivar ‘Arina’. The resulting backcross line ‘ArinaLrFor’ showed markedly increased leaf rust resistance in multiple locations over several years. One of the introgressed QTLs, QLr.sfr-1BS, is located on chromosome 1BS. We developed chromosome 1B-specific microsatellite markers by exploiting the Illumina survey sequences of wheat cv. ‘Chinese Spring’ and mapped QLr.sfr-1BS to a 4.3 cM interval flanked by the SSR markers gwm604 and swm271. QLr.sfr-1BS does not share a genetic location with any of the described leaf rust resistance genes present on chromosome 1B. Therefore, QLr.sfr-1BS is novel and was designated as Lr75. We conclude that marker-assisted combination of partial resistance genes is a feasible strategy to increase broad-spectrum leaf rust resistance. The identification of Lr75 adds a novel and highly useful gene to the small set of known partial, adult plant leaf rust resistance genes.

     

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.     

Genetic analysis and mapping of adult plant resistance loci to leaf rust in durum wheat cultivar Bairds

Key message

New leaf rust adult plant resistance (APR) QTL QLr.cim - 6BL was mapped and confirmed the known pleotropic APR gene Lr46 effect on leaf rust in durum wheat line Bairds.

Abstract

CIMMYT-derived durum wheat line Bairds displays an adequate level of adult plant resistance (APR) to leaf rust in Mexican field environments. A recombinant inbred line (RIL) population developed from a cross of Bairds with susceptible parent Atred#1 was phenotyped for leaf rust response at Ciudad Obregon, Mexico, during 2013, 2014, 2015 and 2016 under artificially created epidemics of Puccinia triticina (Pt) race BBG/BP. The RIL population and its parents were genotyped with the 50 K diversity arrays technology (DArT) sequence system and simple sequence repeat (SSR) markers. A genetic map comprising 1150 markers was used to map the resistance loci. Four significant quantitative trait loci (QTLs) were detected on chromosomes 1BL, 2BC (centromere region), 5BL and 6BL. These QTLs, named Lr46, QLr.cim-2BC, QLr.cim-5BL and QLr.cim-6BL, respectively, explained 13.5–60.8%, 9.0–14.3%, 2.8–13.9%, and 11.6–29.4%, respectively, of leaf rust severity variation by the inclusive composite interval mapping method. All of these resistance loci were contributed by the resistant parent Bairds, except for QLr.cim-2BC, which came from susceptible parent Atred#1. Among these, the QTL on chromosome 1BL was the known pleiotropic APR gene Lr46, whereas QLr.cim-6BL, a consistently detected locus, should be a new leaf rust resistance locus in durum wheat. The mean leaf rust severity of RILs carrying all four QTLs ranged from 8.0 to 17.5%, whereas it ranged from 10.9 to 38.5% for three QTLs (Lr46 + 5BL + 6BL) derived from the resistant parent Bairds. Two RILs with four QTLs combinations can be used as sources of complex APR in durum wheat breeding.

     

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.     

Effect of <Emphasis Type="Italic">Lr34/Yr18</Emphasis> on agronomic and quality traits in a spring wheat mapping population and implications for breeding

The locus Lr34/Yr18 plays an important role in conferring resistance to a number of fungal diseases and is thus an important component of global wheat breeding efforts. We investigated the differences in disease response and agronomic traits of the ‘CDC Teal’ × ‘CDC Go’ spring wheat population of 187 recombinant inbred lines (RILs) in relation to the presence/absence of the rust resistance gene Lr34/Yr18. Lines carrying the resistant allele of Lr34/Yr18 were taller, matured earlier, and yielded less grain with lower test weights than lines without Lr34/Yr18. Lines with or without the resistant allele of Lr34/Yr18 did not differ for grain protein content, SDS sedimentation volume, and for resistance to leaf spotting and common bunt. Lines with Lr34/Yr18 exhibited lower leaf and stripe rust infection than lines without it. We selected superior lines from the population based on high yield, protein content, SDS sedimentation, and the presence of the resistant allele of Lr34/Yr18 and grew them with continued selection in replicated yield trials over nine site-years. We attempted to combine Lr34/Yr18 with high yield, protein content, and SDS sedimentation suitable for the Canadian western red spring wheat class. Our results suggested that the population size we used was not large enough to obtain recombinants with high yield potential, high grain protein, and acceptable quality attributes. Moreover, selection for Lr34/Yr18 resulted in the elimination of lines with high yield potential. We therefore suggest using a population size of at least 310 to increase the potential of pooling Lr34/Yr18 with high grain yield and desirable agronomic and end-use quality attributes.     

Genetic mapping of a barley leaf rust resistance gene <Emphasis Type="Italic">Rph26</Emphasis> introgressed from <Emphasis Type="Italic">Hordeum bulbosum</Emphasis>

Key message

The quantitative barley leaf rust resistance gene, Rph26, was fine mapped within a H. bulbosum introgression on barley chromosome 1HL. This provides the tools for pyramiding with other resistance genes.

Abstract

A novel quantitative resistance gene, Rph26, effective against barley leaf rust (Puccinia hordei) was introgressed from Hordeum bulbosum into the barley (Hordeum vulgare) cultivar ‘Emir’. The effect of Rph26 was to reduce the observed symptoms of leaf rust infection (uredinium number and infection type). In addition, this resistance also increased the fungal latency period and reduced the fungal biomass within infected leaves. The resulting introgression line 200A12, containing Rph26, was backcrossed to its barley parental cultivar ‘Emir’ to create an F₂ population focused on detecting interspecific recombination within the introgressed segment. A total of 1368 individuals from this F₂ population were genotyped with flanking markers at either end of the 1HL introgression, resulting in the identification of 19 genotypes, which had undergone interspecific recombination within the original introgression. F₃ seeds that were homozygous for the introgressions of reduced size were selected from each F₂ recombinant and were used for subsequent genotyping and phenotyping. Rph26 was genetically mapped to the proximal end of the introgressed segment located at the distal end of chromosome 1HL. Molecular markers closely linked to Rph26 were identified and will enable this disease resistance gene to be combined with other sources of quantitative resistance to maximize the effectiveness and durability of leaf rust resistance in barley breeding. Heterozygous genotypes containing a single copy of Rph26 had an intermediate phenotype when compared with the homozygous resistant and susceptible genotypes, indicating an incompletely dominant inheritance.

     

Genetic analysis and localization of loci controlling leaf rust resistance of <Emphasis Type="Italic">Triticum aestivum</Emphasis> × <Emphasis Type="Italic">Triticum timopheevii</Emphasis> introgression lines

Introgressive lines resulting from crossing common wheat Triticum aestivum with the tetraploid T. timopheevii are characterized by effective resistance to leaf rust caused by Puccinia triticina Eriks. Molecular analysis using 350 specific simple sequence repeat (SSR) markers determined localization of the T. timopheevii genome in chromosomes 1A, 2A, 2B, 5A, 5B, and 6B. A population of F₂ offspring of crossing hybrid line 842-2 with common wheat cultivar Skala was obtained for mapping the loci controlling leaf rust resistance. Analysis of association of phenotypic and genotypic data by means of simple interval mapping (SIM) and composite interval mapping (CIM) has shown that the resistance of adult plants is determined by two loci in chromosomes 5B and 2A. The major locus QLr.icg-5B, transferred from T. timopheevii chromosome 5G mapped to the interval of microsatellite loci Xgwm408-Xgwm1257 controls 72% of the phenotypic variance of the trait. The other, minor locus QLr.icg-2A located to chromosome 2A at a distance of 10 cM from Xgwm312 accounts for 7% of the trait expression. Microsatellite markers located near these loci may be used for controlling the transfer of agronomically valuable loci when new lines and cultivars are created.     

Discovery and characterization of two new stem rust resistance genes in <Emphasis Type="Italic">Aegilops sharonensis</Emphasis>

Key message

We identified two novel wheat stem rust resistance genes, Sr-1644-1Sh and Sr-1644-5Sh in Aegilops sharonensis that are effective against widely virulent African races of the wheat stem rust pathogen.

Abstract

Stem rust is one of the most important diseases of wheat in the world. When single stem rust resistance (Sr) genes are deployed in wheat, they are often rapidly overcome by the pathogen. To this end, we initiated a search for novel sources of resistance in diverse wheat relatives and identified the wild goatgrass species Aegilops sharonesis (Sharon goatgrass) as a rich reservoir of resistance to wheat stem rust. The objectives of this study were to discover and map novel Sr genes in Ae. sharonensis and to explore the possibility of identifying new Sr genes by genome-wide association study (GWAS). We developed two biparental populations between resistant and susceptible accessions of Ae. sharonensis and performed QTL and linkage analysis. In an F₆ recombinant inbred line and an F₂ population, two genes were identified that mapped to the short arm of chromosome 1S^sh, designated as Sr-1644-1Sh, and the long arm of chromosome 5S^sh, designated as Sr-1644-5Sh. The gene Sr-1644-1Sh confers a high level of resistance to race TTKSK (a member of the Ug99 race group), while the gene Sr-1644-5Sh conditions strong resistance to TRTTF, another widely virulent race found in Yemen. Additionally, GWAS was conducted on 125 diverse Ae. sharonensis accessions for stem rust resistance. The gene Sr-1644-1Sh was detected by GWAS, while Sr-1644-5Sh was not detected, indicating that the effectiveness of GWAS might be affected by marker density, population structure, low allele frequency and other factors.

     

Association mapping of leaf rust resistance loci in a spring wheat core collection

Key message

We identified 15 potentially novel loci in addition to previously characterized leaf rust resistance genes from 1032 spring wheat accessions. Targeted AM subset panels were instrumental in revealing interesting loci.

Abstract

Leaf rust is a common disease of wheat, consistently reducing yields in many wheat-growing regions of the world. Although fungicides are commonly applied to wheat in the United States (US), genetic resistance can provide less expensive, yet effective control of the disease. Our objectives were to map leaf rust resistance genes in a large core collection of spring wheat accessions selected from the United States Department of Agriculture-Agricultural Research Service National Small Grains Collection (NSGC), determine whether previously characterized race-nonspecific resistance genes could be identified with our panel, and evaluate the use of targeted panels to identify seedling and adult plant resistance (APR) genes. Association mapping (AM) detected five potentially novel leaf rust resistance loci on chromosomes 2BL, 4AS, and 5DL at the seedling stage, and 2DL and 7AS that conditioned both seedling and adult plant resistance. In addition, ten potentially novel race-nonspecific resistance loci conditioned field resistance and lacked seedling resistance. Analyses of targeted subsets of the accessions identified additional loci not associated with resistance in the complete core panel. Using molecular markers, we also confirmed the presence and effectiveness of the race-nonspecific genes Lr34, Lr46, and Lr67 in our panel. Although most of the accessions in this study were susceptible to leaf rust in field and seedling tests, many resistance loci were identified with AM. Through the use of targeted subset panels, more loci were identified than in the larger core panels alone.

     

Lr70, a new gene for leaf rust resistance mapped in common wheat accession KU3198

Key message

KU3198 is a common wheat accession that carries one novel leaf rust resistance (Lr) gene, Lr70 , and another Lr gene which is either novel, Lr52 or an allele of Lr52.

Abstract

Leaf rust, caused by Puccinia triticina Eriks. (Pt), is a broadly distributed and economically important disease of wheat. Deploying cultivars carrying effective leaf rust resistance (Lr) genes is a desirable method of disease control. KU3198 is a common wheat (Triticum aestivum L.) accession from the Kyoto collection that was highly resistant to Pt in Canada. An F₂ population from the cross HY644/KU3198 showed segregation for two dominant Lr genes when tested with Pt race MBDS which was virulent on HY644. Multiple bulk segregant analysis (MBSA) was employed to find putative chromosome locations of these Lr genes using SSR markers that provided coverage of the genome. MBSA predicted that the Lr genes were located on chromosomes 5B and 5D. A doubled haploid population was generated from the cross of JBT05-714 (HY644*3/KU3198), a line carrying one of the Lr genes from KU3198, to Thatcher. This population segregated for a single Lr gene conferring resistance to Pt race MBDS, which was mapped to the terminal region of the short arm of chromosome 5B with SSR markers and given the temporary designation LrK1. One F₃ family derived from the HY644/KU3198 F₂ population that segregated only for the second Lr gene from KU3198 was identified. This family was treated as an F₂-equivalent population and used for mapping the Lr gene, which was located to the terminal region of chromosome 5DS. As no other Lr gene has been mapped to 5DS, this gene is novel and has been designated as Lr70.