Stem Rust Resistance Genes Research Articles

Genome-wide association mapping of seedling and adult plant response to stem rust in a durum wheat panel.

Many of the major stem rust resistance genes deployed in commercial wheat (Triticum spp.) cultivars and breeding lines become ineffective over time because of the continuous emergence of virulent races. A genome-wide association study (GWAS) was conducted using 26,439 single nucleotide polymorphism (SNP) markers and 280 durum wheat [Triticum turgidum L. subsp. Durum (Desf.) Husnot] lines from CIMMYT to identify genomic regions associated with seedling resistance to races TTKSK, TKTTF, JRCQC, and TTRTF and field resistance to TKTTF and JRCQC. The phenotypic data analysis across environments revealed 61-91 and 59-77% of phenotypic variation was explained by the genotypic component for seedling and adult plant response of lines, respectively. For seedling resistance, mixed linear model (MLM) identified eight novel and nine previously reported quantitative trait loci (QTL) while a fixed and random model circulating probability unification (FarmCPU) detected 12 novel and eight previously reported QTL. For field resistance, MLM identified 12 novel and seven previously reported loci while FarmCPU identified seven novel and nine previously reported loci. The regions of Sr7a, Sr8155B1, Sr11, alleles of Sr13, Sr17, Sr22/Sr25, and Sr49 were identified. Novel loci on chromosomes 3B, 4A, 6A, 6B, 7A, and 7B could be used as sources of resistance to the races virulent on durum wheat. Two large-effect markers on chromosome 6A could potentially be used to differentiate resistant haplotypes of Sr13 (R1 and R3). Allelism tests for Sr13, breaking the deleterious effect associated with Sr22/Sr25 and retaining the resistance allele at the Sr49 locus, are needed to protect future varieties from emerging races.

Characterization of synthetic wheat line Largo for resistance to stem rust.

Resistance breeding is an effective approach against wheat stem rust caused by Puccinia graminis f. sp. tritici (Pgt). The synthetic hexaploid wheat line Largo (pedigree: durum wheat “Langdon” × Aegilops tauschii PI 268210) was found to have resistance to a broad spectrum of Pgt races including the Ug99 race group. To identify the stem rust resistance (Sr) genes, we genotyped a population of 188 recombinant inbred lines developed from a cross between the susceptible wheat line ND495 and Largo using the wheat Infinium 90 K SNP iSelect array and evaluated the population for seedling resistance to the Pgt races TTKSK, TRTTF, and TTTTF in the greenhouse conditions. Based on genetic linkage analysis using the marker and rust data, we identified six quantitative trait loci (QTL) with effectiveness against different races. Three QTL on chromosome arms 6AL, 2BL, and 2BS corresponded to Sr genes Sr13c, Sr9e, and a likely new gene from Langdon, respectively. Two other QTL from PI 268210 on 2DS and 1DS were associated with a potentially new allele of Sr46 and a likely new Sr gene, respectively. In addition, Sr7a was identified as the underlying gene for the 4AL QTL from ND495. Knowledge of the Sr genes in Largo will help to design breeding experiments aimed to develop new stem rust-resistant wheat varieties. Largo and its derived lines are particularly useful for introducing two Ug99-effective genes Sr13c and Sr46 into modern bread wheat varieties. The 90 K SNP-based high-density map will be useful for identifying the other important genes in Largo.

Field resistance to wheat stem rust in durum wheat accessions deposited at the USDA National Small Grains Collection.

Wheat stem rust, caused by Puccinia graminis f. sp. tritici, is a re‐emerging disease, posing a significant threat to durum wheat production worldwide. The limited number of stem rust resistance genes in modern cultivars compels us to identify and incorporate new effective genes in durum wheat breeding programs. We evaluated 8,245 spring durum wheat accessions deposited at the USDA National Small Grains Collection (NSGC) for resistance in field stem rust nurseries in Debre Zeit, Ethiopia and St. Paul, MN (USA). A higher level of disease development was observed at the Debre Zeit nursery compared with St. Paul, and the effective alleles of Sr13 in this nursery did not display the level of resistance observed at the St. Paul nursery. Four hundred and ninety‐one (∽6%) accessions exhibited resistant to moderately susceptible responses after three field evaluations at Debre Zeit and two at St. Paul. Nearly 70% of these accessions originated from Ethiopia, Mexico, Egypt, and USA. Eight additional countries, namely Portugal, Turkey, Italy, Canada, Chile, Australia, Syria, and Tunisia contributed to 19% of the resistant to moderately susceptible entries. Among the 491 resistant to moderately susceptible accessions, 53.8% (n = 265) were landraces, and 28.4% (n = 139) and 11.4% (n = 55) were breeding lines and cultivars, respectively. Breeding lines and cultivars displayed a higher level and frequency of resistance than the landraces. We concluded that a large number of durum wheat accessions from diverse origins deposited at the NSGC can be exploited for diversifying and improving stem rust resistance in wheat.

Marker assisted pyramiding of stem rust, leaf rust and powdery mildew resistance genes for durable resistance in wheat (Triticum aestivum L.)

Wheat production is essential for food security. Stem and leaf rust diseases continually pose threat to wheat production. In recent years, climate change and intensive crop cultivation practices are making powdery mildew as a potential threat to wheat production. Deploying resistant cultivars are the most economic, reliable and sustainable way to manage the stem rust, leaf rust and powdery mildew of wheat. Using conventional selection system, it is difficult to select two or more genes in a single genotype. In such a situation, phenotype neutral selection based on marker-trait association along with seedling and adult plant reaction become inevitable. Stem rust, leaf rust and powdery mildew resistance genes, Sr24/Lr24 , Sr26 and Sr36/Pm6 were pyramided in the background of nine Indian wheat cultivars through marker assisted backcross approach. The presence of the rust and powdery mildew resistance genes were confirmed using microsatellite markers such as Sr24#12 ( Sr24/Lr24 ), Sr26#43 ( Sr26 ) and stm773 ( Sr36/Pm6 ) in the pyramided lines. Stable lines were selected at BC 3 F 4 generation. Seedling and adult plant reaction of pyramided lines showed resistance to most of the stem and leaf rust pathotypes prevailing in India. Using gene pyramids ( Sr24/Lr24 , Sr26 and Sr36/Pm6 ) that confer resistance to the predominant pathotypes of stem rust, leaf rust and powdery mildew could impart durability to the cultivars than single gene deployment.

Wheat leaf rust resistance gene Lr13 is a specific Ne2 allele for hybrid necrosis

Wheat leaf rust resistance gene Lr13 is a specific Ne2 allele for hybrid necrosis

Combating Ug99 - Current Scenario

The yield potential of wheat crop is not achieved abundantly because of disease pressure. One of the most destructive of such diseases is stem rust (SR). SR caused by Puccinia graminis f. sp. tritici (Pgt), had been controlled successfully during three decades throughout the world with deployment of semi-dwarf resistant cultivars in the last half of previous century. During 1999 appearance and dispersion of stem rust race Ug-99 in Uganda (a virulent race against Sr31) created an alarming situation worldwide. Widespread germplasm was protected by gene Sr31 found susceptible to this terrible strain as the gene was protecting 80% wheat lines cultivated throughout planet. The emergence of the Ug99 race of stem rust in Africa and the Middle East together with the appearance of new strains in Europe catalyzed a main effort to recognize sources of stem rust resistance genes against new virulent strains and incorporate these genes into wheat lines. Scientific community addressed the dilemma in time and efforts did not go waste. Worldwide concern regarding the danger to global wheat production caused by Ug99 led to breeding wheat for durable resistance against disease and achieved considerably. This success is attributed to team work of experts and serves as an example for research workers in future. However, the continued emergence of stem rust variants that overcome new resistance genes, demands an amplified emphasis on pathogen evolution and virulence mechanisms. A major role for BGRI is to keep ‘the eye on the ball’ with regard to all these aspects. This article enables us to design strategy to tackle a situation which appears without alarm but in this case intellectuals coordinated each other and solution became possible. The same principle does not apply in plant pathology but in Human pathology and Veterinary pathology.

Allocation of the oat powdery mildew resistance gene Pm3 to oat chromosome 1A

Besides the mode of inheritance, the knowledge of the chromosome location and allelic relationships are the essentials towards a successful deployment and stacking of divergent disease resistance genes for a given pathogen in breeding programs. Powdery mildew of oats, to which 11 major resistance genes in the host Avena sativa L. have been characterized so far, is a prevalent fungal disease of the crop in Northwestern Europe. In the present study, the resistance gene Pm3 was mapped by linkage analysis relative to molecular markers from oat consensus linkage group Mrg18 which was recently determined to represent oat chromosome 1A. Pm3 was located at 67.7–72.6 cM on Mrg18 of the oat consensus map, a position at which also stem and crown rust resistance genes Pg13 and Pc91 and a large cluster of resistance gene analogs have been previously mapped. The closely linked marker GMI_ES03_c2277_336 was found to be useful for the prediction of Pm3 in gene postulation studies. Although the major effect of the widespread gene got lost over time, the known genome location with associated markers will assist revealing in future genetic studies whether there is a possible residual effect of the gene contributing to adult plant resistance.

Mapping and validating stem rust resistance genes directly in self-incompatible genetic resources of winter rye

Key messageIndividual stem rust resistance genes could be directly mapped within self-incompatible rye populations.Genetic resources of rye (Secale cereale L.) are cross-pollinating populations that can be highly diverse and are naturally segregating. In this study, we show that this segregation could be used for mapping stem rust resistance. Populations of pre-selected donors from the Russian Federation, the USA and Austria were tested on a single-plant basis for stem rust resistance by a leaf-segment test with three rust isolates. Seventy-four plants per population were genotyped with a 10 K-SNP chip. Using cumulative logit models, significant associations between the ordinal infection score and the marker alleles could be found. Three different loci (Pgs1, Pgs2, Pgs3) in three populations were highly significant, and resistance-linked markers could be validated with field experiments of an independent seed sample from the original population and were used to fix two populations for resistance. We showed that it is possible to map monogenically inherited seedling resistance genes directly in genetic resources, thus providing a competitive alternative to linkage mapping approaches that require a tedious and time-consuming inbreeding over several generations.

Chromosomal composition analysis and molecular marker development for the novel Ug99-resistant wheat-Thinopyrum ponticum translocation line WTT34.

A novel Ug99-resistant wheat-Thinopyrum ponticum translocation line was produced, its chromosomal composition was analyzed and specific markers were developed. Stem rust caused by Puccinia graminis f. sp. tritici Eriks. & E. Henn (Pgt) has seriously threatened global wheat production since Ug99 race TTKSK was first detected in Uganda in 1998. Thinopyrum ponticum is near immune to Ug99 races and may be useful for enhancing wheat disease resistance. Therefore, developing new wheat-Th. ponticum translocation lines that are resistant to Ug99 is crucial. In this study, a novel wheat-Th. ponticum translocation line, WTT34, was produced. Seedling and field evaluation revealed that WTT34 is resistant to Ug99 race PTKST. The resistance was derived from the alien parent Th. ponticum. Screening WTT34 with markers linked to Sr24, Sr25, Sr26, Sr43, and SrB resulted in the amplification of different DNA fragments from Th. ponticum, implying WTT34 carries at least one novel stem rust resistance gene. Genomic in situ hybridization (GISH), multicolor fluorescence in situ hybridization (mc-FISH), and multi-color GISH (mc-GISH) analyses indicated that WTT34 carries a T5DS·5DL-Th translocation, which was consistent with wheat660K single-nucleotide polymorphism (SNP) array results. The SNP array also uncovered a deletion event in the terminal region of chromosome 1D. Additionally, the homeology between alien segments and the wheat chromosomes 2A and 5D was confirmed. Furthermore, 51 PCR-based markers derived from the alien segments of WTT34 were developed based on specific-locus amplified fragment sequencing (SLAF-seq). These markers may enable wheat breeders to rapidly trace Th. ponticum chromosomal segments carrying Ug99 resistance gene(s).

Virulence Diversity and Physiological Race Composition of Wheat Stem Rust (Puccinia graminis f.sp. tritici) in Tigray Region, Northern Ethiopia

Wheat stem rust (black rust) is one of the most important airborne diseases of wheat (Triticum aestivum) caused by Puccinia graminis f. sp. tritici remains a constraint to the world’s wheat production. Because of the sudden changes in stem rust race patterns, commercial varieties tend to become vulnerable globally at large and particularly in Ethiopia. It was responsible to cause 6.2 million metric tonnes per year or higher losses under severe epidemics at global level. Therefore, this study was initiated to identify the physiological races and virulence diversity of Puccinia graminis f. sp. tritici in Tigray region. Race identification through inoculation of stem rust isolates, multiplication of single-pustule of the pathogen and race designation by inoculating on a set of wheat differential lines were done in the greenhouse. Forty-seven stem rust isolates were analyzed on the twenty stem rust differentials and resulted in the identification of six races namely; TTTTF, TKTTF, TRTTF, TTRTF, RRTTF and TKPTF. In this study, race TTRTF was detected for the first time in Ethiopia during 2017 cropping season. Out of the six races identified, TTTTF was detected from 25 (53.19%) isolates and TKTTF from 15 (31.91%) isolates. The most virulent race that made 18 stem rust resistant genes non-effective was TTTTF, which virulent on 90% of stem rust resistance genes. Differential hosts carrying Sr24 and Sr31 were effective genes which confers resistance to all of the races identified. Hence, the stem rust resistance gene Sr24 and Sr31 can be used as sources of resistance in the wheat breeding program.

Registration of the wheat D‐genome nested association mapping (DNAM) population

AbstractModern wheat is lacking diversity in the D genome due to the genetic bottleneck from the hybridization between tetraploid Triticum turgidum L. and diploid Aegilops tauschii Coss. The D‐genome nested association mapping (DNAM) population (Reg. no. MP‐14, NSL 536301 MAP) was developed to expand D‐genome variation in hexaploid wheat (Triticum aestivum L.). The DNAM population is a wheat nested association mapping population developed with direct crosses between the hard‐white winter Kansas State University breeding line KS05HW14‐3 and Ae. tauschii accessions TA10187, TA1693, TA10171, TA1662, TA1617, TA1615, TA1642, and TA1718. In total, there are 1,164 BC₂F₄ recombinant inbred lines (RILs) in 19 families. The DNAM was originally created for introgression of novel stem rust resistance genes but has since been used to identify resistance to other fungal pathogens. A subset of 420 lines were selected for important agronomic traits, including height and threshability, and named the DNAM Core RILs. Research with the DNAM has potential to provide novel genes that can be introgressed into elite cultivars, as well as knowledge and understanding of the D genome in wheat.

Extensive Genetic Variation at the Sr22 Wheat Stem Rust Resistance Gene Locus in the Grasses Revealed Through Evolutionary Genomics and Functional Analyses.

In the last 20 years, severe wheat stem rust outbreaks have been recorded in Africa, Europe, and Central Asia. This previously well controlled disease, caused by the fungus Puccinia graminis f. sp. tritici, has reemerged as a major threat to wheat cultivation. The stem rust (Sr) resistance gene Sr22 encodes a nucleotide-binding and leucine-rich repeat receptor which confers resistance to the highly virulent African stem rust isolate Ug99. Here, we show that the Sr22 gene is conserved among grasses in the Triticeae and Poeae lineages. Triticeae species contain syntenic loci with single-copy orthologs of Sr22 on chromosome 7, except Hordeum vulgare, which has experienced major expansions and rearrangements at the locus. We also describe 14 Sr22 sequence variants obtained from both Triticum boeoticum and the domesticated form of this species, T. monococcum, which have been postulated to encode both functional and nonfunctional Sr22 alleles. The nucleotide sequence analysis of these alleles identified historical sequence exchange resulting from recombination or gene conversion, including breakpoints within codons, which expanded the coding potential at these positions by introduction of nonsynonymous substitutions. Three Sr22 alleles were transformed into wheat cultivar Fielder and two postulated resistant alleles from Schomburgk (hexaploid wheat introgressed with T. boeoticum segment carrying Sr22) and T. monococcum accession PI190945, respectively, conferred resistance to P. graminis f. sp. tritici race TTKSK, thereby unequivocally confirming Sr22 effectiveness against Ug99. The third allele from accession PI573523, previously believed to confer susceptibility, was confirmed as nonfunctional against Australian P. graminis f. sp. tritici race 98-1,2,3,5,6.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Marker assisted stacking/pyramiding of stem rust, leaf rust and powdery mildew disease resistance genes (Sr2/Lr27/Yr30, Sr24/Lr24 and Sr36/Pm6) for durable resistance in wheat (Triticum aestivum L.)

Wheat being a staple food crop is essential for food security. Rust diseases like stem and leaf rust continues to be a serious threat to wheat production at national and international level. In recent years, powdery mildew disease is emerging as a threat to wheat production due to the climate change and intensive crop cultivation practices. Development of resistant cultivar is the most economical, reliable and sustainable way to control rusts and powdery mildew diseases. Stem rust, leaf rust and powdery mildew resistance genes viz., Sr2/Lr27/Yr30, Sr24/Lr24 and Sr36/ Pm6 were pyramided using marker assisted backcross breeding approach. Based on phenotypic observations, five populations were selected at BC4 F6 with respect to yield traits, leaf and stem rust and powdery mildew reactions. These populations were subjected to molecular marker analyses. Out of the five populations, Sr2+ were present in all the populations, while Sr24/Lr24 and Sr36/Pm6 was present in the fourth and fifth population. The identified population carrying Sr24/Lr24, Sr36/Pm6 and Sr2+ genes were stabilized at BC4 F7 generation. The use of gene stacks/pyramids of major (Sr24/Lr24 and Sr36/Pm6) and minor gene (Sr2+) that confer resistance to most of the pathotypes of stem rust, leaf rust and powdery mildew could impart durability and sustainability to the cultivars than single gene deployment.

Assessments and perspectives on stem rust resistance in South African malting barley

The production of malting barley under dryland and irrigated environments in South Africa (SA) has increased recently, making it the second most important winter crop after wheat. To assess the potential vulnerability of adapted barleys from SA to wheat stem rust (caused by Puccinia graminis f. sp. tritici, Pgt, race PTKST) and rye stem rust (Puccinia graminis f. sp. secalis, Pgs, isolate UVPgs1), 50 accessions were evaluated as seedlings and as adult plants. The same barley germplasm was tested against Pgt and Pgs cultures from the USA that have proven useful for detecting the stem rust resistance genes Rpg1 and the rpg4/Rpg5 complex. The possible presence of these resistance genes was assayed using molecular markers and sequence analysis. Barley accessions exhibited mostly resistant infection types (ITs) to the South African cultures, whilst field data revealed that several of the barley varieties displayed high severities in combination with more compatible host response types to race PTKST. The barley accessions exhibited mostly high ITs to cultures from the USA. None of the seedling IT data was sufficiently distinct to reliably postulate the presence of possible resistance genes. This result was corroborated from the molecular assays for Rpg1 and rpg4/Rpg5. Except for one line that tested heterozygous for the Rpg1 gene, none of the test entries contained this gene or a functional rpg4/Rpg5 complex due to the prevalence of mutations in the rpg4/Rpg5 + carriers. Susceptibility among barley varieties to wheat stem rust race PTKST under field conditions in the present study is of concern.

Virulence and race structure of Puccinia graminis f. sp. tritici in Kazakhstan

Severe epidemics of wheat stem rust caused by Puccinia graminis f. sp. tritici (Pgt) have been observed in recent years in major spring wheat producing regions in Kazakhstan. However, information on the virulence structure and race composition of Pgt is currently not available. Stem rust samples were collected in 2015–2018 in three regions of Kazakhstan to determine the virulence diversity and race distribution in the Pgt populations. A total of 203 single-pustule isolates were derived and evaluated on the stem rust differential and supplemental lines and 38 races were identified. Among them, the races QHHSF and THMTF were found in all the regions and in all the years. The races RFRTF, RHMRF, TKRPF and MHCTC were the most common races in the Akmola and Kostanay regions, and the races LHCSF, QKCSF and LKCSF were only widely distributed in East Kazakhstan. The virulence complexity (i.e., number of Sr genes on which the races were virulent) ranged from 5 to 16, with about 40% of the races having 14 or more virulence. The stem rust resistance genes Sr11, Sr13, Sr22, Sr26, Sr31, Sr33 and Sr35 were found to confer resistance to all the races identified during the study period. Hence, these genes can be used as sources of resistance in wheat breeding programmes in Kazakhstan.

Mutagenesis ofPuccinia graminisf. sp.tritici and Selection of Gain-of-Virulence Mutants.

Wheat stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt), is regaining prominence due to the recent emergence of virulent isolates and epidemics in Africa, Europe and Central Asia. The development and deployment of wheat cultivars with multiple stem rust resistance (Sr) genes stacked together will provide durable resistance. However, certain disease resistance genes can suppress each other or fail in particular genetic backgrounds. Therefore, the function of each Sr gene must be confirmed after incorporation into an Sr-gene stack. This is difficult when using pathogen disease assays due to epistasis from recognition of multiple avirulence (Avr) effectors. Heterologous delivery of single Avr effectors can circumvent this limitation, but this strategy is currently limited by the paucity of cloned Pgt Avrs. To accelerate Avr gene cloning, we outline a procedure to develop a mutant population of Pgt spores and select for gain-of-virulence mutants. We used ethyl methanesulphonate (EMS) to mutagenize urediniospores and create a library of > 10,000 independent mutant isolates that were combined into 16 bulks of ~658 pustules each. We sequenced random mutants and determined the average mutation density to be 1 single nucleotide variant (SNV) per 258 kb. From this, we calculated that a minimum of three independently derived gain-of-virulence mutants is required to identify a given Avr gene. We inoculated the mutant library onto plants containing Sr43, Sr44, or Sr45 and obtained 9, 4, and 14 mutants with virulence toward Sr43, Sr44, or Sr45, respectively. However, only mutants identified on Sr43 and Sr45 maintained their virulence when reinolculated onto the lines from which they were identified. We further characterized 8 mutants with virulence toward Sr43. These also maintained their virulence profile on the stem rust international differential set containing 20 Sr genes, indicating that they were most likely not accidental contaminants. In conclusion, our method allows selecting for virulent mutants toward targeted resistance (R) genes. The development of a mutant library from as little as 320 mg spores creates a resource that enables screening against several R genes without the need for multiple rounds of spore multiplication and mutagenesis.

Localization of the Stem Rust Resistance Gene Pg2 to Linkage Group Mrg20 in Cultivated Oat (Avena sativa).

Stem rust is an important disease of cultivated oat (Avena sativa) caused by Puccinia graminis f. sp. avenae. In North America, host resistance is the primary strategy to control this disease and is conferred by a relatively small number of resistance genes. Pg2 is a widely deployed stem rust resistance gene that originates from cultivated oat. Oat breeders wish to develop cultivars with multiple Pg genes to slow the breakdown of single gene resistance, and often require DNA markers suited for marker-assisted selection. Our objectives were to (i) construct high density linkage maps for a major oat stem rust resistance gene using three biparental mapping populations, (ii) develop Kompetitive allele-specific PCR (KASP) assays for Pg2-linked single-nucleotide polymorphisms (SNPs), and (iii) test the prediction accuracy of those markers with a diverse panel of spring oat lines and cultivars. Genotyping-by-sequencing SNP markers linked to Pg2 were identified in an AC Morgan/CDC Morrison recombinant inbred line (RIL) population. Pg2-linked SNPs were then analyzed in an AC Morgan/RL815 F2 population and an AC Morgan/CDC Dancer RIL population. Linkage analysis identified a common location for Pg2 in all three populations on linkage group Mrg20 of the oat consensus genetic map. The most predictive markers were identified and converted to KASP assays for use in oat breeding programs. When used in combination, the KASP assays for the SNP loci avgbs2_126549.1.46 and avgbs_cluster_23819.1.27 were highly predictive of Pg2 status in panel of 54 oat breeding lines and cultivars.

The wheat Sr22, Sr33, Sr35 and Sr45 genes confer resistance against stem rust in barley.

In the last 20 years, stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt), has re‐emerged as a major threat to wheat and barley production in Africa and Europe. In contrast to wheat with 60 designated stem rust (Sr) resistance genes, barley’s genetic variation for stem rust resistance is very narrow with only ten resistance genes genetically identified. Of these, only one complex locus consisting of three genes is effective against TTKSK, a widely virulent Pgt race of the Ug99 tribe which emerged in Uganda in 1999 and has since spread to much of East Africa and parts of the Middle East. The objective of this study was to assess the functionality, in barley, of cloned wheat Sr genes effective against race TTKSK. Sr22, Sr33, Sr35 and Sr45 were transformed into barley cv. Golden Promise using Agrobacterium‐mediated transformation. All four genes were found to confer effective stem rust resistance. The barley transgenics remained susceptible to the barley leaf rust pathogen Puccinia hordei, indicating that the resistance conferred by these wheat Sr genes was specific for Pgt. Furthermore, these transgenic plants did not display significant adverse agronomic effects in the absence of disease. Cloned Sr genes from wheat are therefore a potential source of resistance against wheat stem rust in barley.

Molecular marker aided selection for developing rust resistant genotypes by pyramiding Lr19/Sr25 and Yr15 in wheat (Triticum aestivum L.)

The study was aimed to develop rust resistant genotypes with genome of prominent wheat variety HS240 by pyramiding of linked leaf rust and stem rust resistance genes Lr19 and Sr25 with stripe rust resistance gene Yr15. The genotypes FLW13 and FLW20 were used as gene donors for transfer of Yr15 and Lr19/Sr25, respectively. The molecular markers scs265, wmc221, PSY1-E1, and Gb linked to Lr19/Sr25 and barc8 and gwm11 linked to Yr15 were used for foreground selection. Background selection, involving 58 simple sequence repeat (SSR) markers polymorphic between HS240 and FLW20; 72 between HS240 and FLW13 was carried out to recover the genome of HS240 in advanced bulks derived from a cross HS240*2/FLW20//HS240*2/FLW13. The SSR based genome recovery in selected advanced breeding lines, WBM3682 and WBM3684 was 90.8% and 93.3%, respectively. The 35 K SNP array based analysis of genomic regions of HS240 also substantiated the results of genomic recovery estimated through SSR markers. The selected line WBM3682 recorded average grain yield of 25.7q/ha and showed its superiority over parental check HS240 under rainfed situations. This study has led to development of rust resistant wheat carrying genome of HS240.

Molecular Marker Based Design for Breeding Wheat Lines with Multiple Resistance and Superior Quality.

There has not been a major wheat stem rust epidemic worldwide since the 1970s, but the emergence of race TTKSK of Puccinia graminis f. sp. tritici in 1998 presented a great threat to the world wheat production. Single disease-resistance genes are usually effective for only several years before the pathogen changes genetically to overcome the resistance. Stripe rust caused by Puccinia striiformis f. sp. tritici (Pst) is one of the most common and persistent wheat diseases worldwide. The development of varieties with multiple resistance is the most economical and effective strategy for preventing stripe rust and stem rust, the two main rust diseases constraining wheat production. Plateau 448 has been widely used in the spring wheat growing region in northwest China, but it has become susceptible to stripe rust and is susceptible to TTKSK. To produce more durable resistance to race TTKSK as well as to stripe rust, four stem rust resistance genes (Sr33, Sr36, Sr-Cad, and Sr43) and three stripe rust resistance genes (Yr5, Yr18, and Yr26) were simultaneously introgressed into Plateau 448 to improve its stem rust (Ug99) and stripe rust resistance using a marker-assisted backcrossing strategy combined with phenotypic selection. We obtained 131 BC1F5 lines that pyramided two to four Ug99 resistance genes and one to two Pst resistance genes simultaneously. Thirteen of these lines were selected for their TTKSK resistance, and all of them exhibited near immunity or high resistance to TTKSK. Among the 131 pyramided lines, 95 showed high resistance to mixed Pst races. Nine lines exhibited not only high resistance to TTKSK and Pst but also better agronomic traits and high-molecular-weight glutenin subunit compositions than Plateau 448.