All-stage Resistance Research Articles

Validation and characterization of a QTL for adult plant resistance to stripe rust on wheat chromosome arm 6BS (Yr78)

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

Mapping genes for resistance to stripe rust in spring wheat landrace PI 480035.

Stripe rust caused by Puccinia striiformis Westend. f. sp. tritici Erikks. is an economically important disease of wheat (Triticum aestivum L.). Hexaploid spring wheat landrace PI 480035 was highly resistant to stripe rust in the field in Washington during 2011 and 2012. The objective of this research was to identify quantitative trait loci (QTL) for stripe rust resistance in PI 480035. A spring wheat, “Avocet Susceptible” (AvS), was crossed with PI 480035 to develop a biparental population of 110 recombinant inbred lines (RIL). The population was evaluated in the field in 2013 and 2014 and seedling reactions were examined against three races (PSTv-14, PSTv-37, and PSTv-40) of the pathogen under controlled conditions. The population was genotyped with genotyping-by-sequencing and microsatellite markers across the whole wheat genome. A major QTL, QYr.wrsggl1-1BS was identified on chromosome 1B. The closest flanking markers were Xgwm273, Xgwm11, and Xbarc187 1.01 cM distal to QYr.wrsggl1-1BS, Xcfd59 0.59 cM proximal and XA365 3.19 cM proximal to QYr.wrsggl1-1BS. Another QTL, QYr.wrsggl1-3B, was identified on 3B, which was significant only for PSTv-40 and was not significant in the field, indicating it confers a race-specific resistance. Comparison with markers associated with previously reported Yr genes on 1B (Yr64, Yr65, and YrH52) indicated that QYr.wrsggl1-1BS is potentially a novel stripe rust resistance gene that can be incorporated into modern breeding materials, along with other all-stage and adult-plant resistance genes to develop cultivars that can provide durable resistance.

Reaction of differential wheat and triticale genotypes to natural stripe rust [Puccinia striiformis f. sp. tritici] infection in Saskatchewan, Canada

Stripe rust of wheat (Triticum spp. L.) is an emerging problem in western Canada. The causal pathogen (Puccinia striiformis f. sp. tritici Eriks.) has the potential to overwinter, which is a concern because of green bridging leading to earlier establishment of rust epidemics. Resistance conferred by seedling/all-stage and adult plant genes is the most effective approach to manage this disease. However, many Canadian wheat varieties lack seedling or all-stage resistance. The effectiveness of Yr genes under field conditions was assessed by exposing differentials/varieties carrying Yr genes to natural inoculum in multiple environments. The differentials were planted at multiple Saskatchewan locations in 2013, 2014 and 2016 and significant stripe rust was detected at 12 site-years. Additionally, 33 winter wheat varieties and advanced breeding lines were assessed for stripe rust reaction in 2016 at Swift Current, Saskatchewan. Disease severity was assessed on flag and penultimate leaves between early milk to soft dough growth stages. The genes Yr5, Yr15, YrSP and those in ‘Yamhill’ are effective in Saskatchewan under field conditions. Genes/differentials were classified into two major groups based on cluster analysis, one with defeated genes and the other with partial resistant genes. Genes YrA, Yr6, Yr7, Yr8, Yr9, Yr27, Yr29, Yr31, Yr32 and YrSu were not effective in the field to SK races. At Swift Current, winter wheat varieties/lines carrying Yr17 were resistant in the field, whereas those carrying Yr10 were not.

Mining Vavilov's Treasure Chest of Wheat Diversity for Adult Plant Resistance to Puccinia triticina.

Leaf rust (LR) caused by Puccinia triticina, is among the most important diseases of wheat (Triticum aestivum L.) crops globally. Deployment of cultivars incorporating genetic resistance, such as adult plant resistance (APR) or all-stage resistance, is considered the most sustainable control method. APR is preferred for durability because it places lower selection pressure on the pathogen and is often polygenic. In the search for new sources of APR, here we explored a diversity panel sourced from the N. I. Vavilov Institute of Plant Genetic Resources. Based on DNA marker screening, 83 of the 300 lines were deemed to carry known APR genes; namely, Lr34, Lr46, and Lr67. Interestingly, lines carrying Lr67 were mostly landraces from India and Pakistan, reconfirming the likely origin of the gene. Rapid phenotypic screening using a method that integrates assessment at both seedling and adult growth stages under accelerated growth conditions (i.e., constant light and controlled temperature) identified 50 lines carrying APR. Levels of APR corresponded well with phenotypes obtained in a field nursery inoculated using the same pathotype (R2 = 0.82). The second year of field testing, using a mixture of pathotypes with additional virulence for race-specific APR genes (Lr13 and Lr37), identified a subset of 13 lines that consistently displayed high levels of APR across years and pathotypes. These lines provide useful sources of resistance for future research. A strategy combining rapid generation advance coupled with phenotyping under controlled conditions could accelerate introgression of these potentially novel alleles into adapted genetic backgrounds.

Stripe rust resistance in wheat breeding lines developed for central Shaanxi, an overwintering region for Puccinia striiformis f. sp. tritici in China

China is one of the largest stripe rust epidemic areas in the world. Central Shaanxi, an important overwintering region, serves as a ‘bridge’ between the western over-summering and eastern epidemic regions. Understanding of resistance levels and Yr-gene distribution in regional wheat breeding lines may provide valuable recommendations for releasing resistant cultivars for managing the disease. A total of 183 wheat breeding lines developed for central Shaanxi were tested for seedling resistance to nine Chinese races of Puccinia striiformis f. sp. tritici (Pst) in the greenhouse. In field tests, entries were evaluated for stripe rust resistance in Yangling, Shaanxi, an overwintering region for the pathogen and artificially inoculated with selected races. In Tianshui, Gansu Province, an over-summering region, entries were evaluated under natural infection of Pst. Molecular markers for Yr5, Yr9, Yr10, Yr15, Yr17, Yr18, Yr26 and Yr61 were used to determine the presence and absence of the genes. Among the 183 entries, 4 (2.2%) entries had effective all-stage resistance; 15 (8.2%) entries had adult-plant resistance; and 164 (89.6%) entries were susceptible to one or more races, especially to the potentially important races V26/CM42 and V26/Gui22. Over 95% of entries showed seedling stage susceptibility in Tianshui across years. Resistance genes Yr9, Yr17 and Yr24/Yr26 were postulated in some of the breeding lines based on the seedling responses and molecular markers. Yr5, Yr10, Yr15, Yr18 and Yr61 were not present in any of the breeding lines. These results suggest that, when multiple races of Pst disperse into Shaanxi from southern Gansu Province, over 90% of wheat breeding lines are susceptible. Based on the results, recommendations are made for releasing or not releasing the individual lines, and a strategy of combining genes for effective all-stage and adult-plant resistance is proposed for developing wheat cultivars with high-level durable resistance to stripe rust.

Molecular Mapping of Stripe Rust Resistance Gene Yr76 in Winter Club Wheat Cultivar Tyee.

Tyee, one of the wheat cultivars used to differentiate races of Puccinia striiformis f. sp. tritici in the United States, was identified to have a single gene for all-stage resistance, tentatively named YrTye. To map the gene, Tyee was crossed with 'Avocet Susceptible' (AvS). Genetic analysis of the F1, F2, F2:3, and BC1 progenies confirmed a single dominant gene for resistance to race PSTv-37 that is avirulent to YrTye. A mapping population of 135 F2 plants was phenotyped with PSTv-37 and the derived F2:3 lines were tested with races PSTv-37, PSTv-40, and PSTv-79. The F2 mapping population was genotyped with simple sequence repeat (SSR) markers. A genetic map comprising 13 SSR markers located YrTye in chromosome 3AS flanked distally by SSR marker wmc11 and proximally by wmc532 at 2.6 and 3.4 cM, respectively. Amplification of Chinese Spring 3A deletion lines placed the gene in the distal bin 3AS4-0.45 to 1.00. Because YrTye is different from all formally named Yr genes in chromosomal location, we permanently name the gene Yr76. A near-isogenic line of spring common wheat was developed and selected by testing F3 lines derived from a AvS*4/Tyee cross with Tyee-avirulent and virulent races and the flanking markers. The specific SSR alleles flanking Yr76 were validated using cultivars and breeding lines with and without the gene, and showed high polymorphisms. The specificity of Yr76 is useful in differentiating P. striiformis f. sp. tritici races, and its tightly linked markers will be useful in developing resistant cultivars when combining the gene with other genes for resistance to stripe rust.

Genetic analysis and molecular mapping of a stripe rust resistance gene in wheat–Leymus mollis translocation line M8926-2

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most widespread and destructive diseases of wheat crop worldwide. Growing resistant cultivars is the preferred method of controlling diseases, but only a few effective genes are available. M8926-2, a translocation line developed from interspecific hybridization between Leymus mollis and a common wheat genotype 7182, is highly resistant to the prevalent Chinese Pst races. In order to identify gene(s) for stripe rust resistance, M8926-2 was crossed with susceptible genotype Mingxian 169, and seedlings of parents and F1, F2, and F2:3 progenies were tested with the most prevalent Pst race CYR32 under controlled greenhouse conditions. M8926-2 has a single dominant gene, designated as YrM8926, conferring all-stage resistance to CYR32. Genetic mapping was used to identify molecular markers linked to YrM8926. A linkage group of seven simple sequence repeat (SSR) markers was constructed for YrM8926 using 191 F2-2 population plants and the gene was located on wheat chromosome 2DS. The wheat-L. mollis translocation line background of M8926-2 and disease assessments indicated that YrM8926 is originally derived from L. mollis. The stripe rust resistance gene and closely linked molecular markers should be useful for pyramiding with other genes to develop wheat cultivars with high-level and long-term resistance to stripe rust.

Races of Puccinia striiformis f. sp. tritici in the United States in 2011 and 2012 and Comparison with Races in 2010.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases on wheat in the United States. In 2011, severe wheat stripe rust caused extensive application of fungicides in the western United States, and the disease was more widespread and caused more yield loss in the eastern United States in 2012. In this study, we characterized virulences and identified races of P. striiformis f. sp. tritici by testing the stripe rust samples collected throughout the United States in 2011 and 2012 on a set of 18 Yr single-gene differentials. In 2011, 35 races were identified from 349 viable samples collected from 19 states of the United States and Ontario province of Canada, with PSTv-11 (35.5%), PSTv-37 (12.6%), PSTv-14 (11.8%), PSTv-4 (5.4%), and PSTv-34 (3.4%) as the top five predominant races. In 2012, 23 races were identified from 341 viable samples collected from 24 states of the United States and Ontario of Canada, with PSTv-37 (47.5%), PSTv-11 (11.7%), PSTv-14 (10.0%), PSTv-52 (9.4%), and PSTv-48 (4.4%) as the top five predominant races. Nationally, PSTv-37, PSTv-52, and PSTv-34 were most widely distributed, while PSTv-11, PSTv-14, PSTv-4, and PSTv-48 were mostly detected in the western United States. High frequencies (>80%) were detected for virulences to Yr6, Yr7, Yr8, Yr9, Yr17, Yr27, Yr44, and YrExp2; moderate frequencies (20 to 80%) for virulences to Yr1, Yr43, YrTr1, and YrTye; low frequencies (<10%) for virulences to Yr10, Yr24, Yr32, and YrSP; and virulences to Yr5 and Yr15 were not detected, indicating that these two genes are still effective against the P. striiformis f. sp. tritici population in the United States. Both positive and negative associations were identified between some of the virulences. In total, 55 races identified from 2010 to 2012 in the United States were clustered into two major virulence groups, and dynamics of predominant races and virulence frequencies for the 3 years were presented and discussed. This information is useful for making decisions when screening wheat germplasm for developing stripe-rust-resistant wheat cultivars and managing the disease by growing cultivars with adequate and durable resistance. The severe epidemics and the occurrence of the large number of races in the 3 years indicate that efforts should be made to use diverse resistance genes, especially to combine effective all-stage resistance genes with genes for high-temperature adult-plant resistance.

Race-Specific Adult-Plant Resistance in Winter Wheat to Stripe Rust and Characterization of Pathogen Virulence Patterns.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is an important disease of wheat in the Great Plains and southeastern United States. Growing resistant cultivars is the preferred means for managing stripe rust, but new virulence in the pathogen population overcomes some of the resistance. The objectives of this study were to characterize the stripe rust resistance in contemporary soft and hard red winter wheat cultivars, to characterize the virulence of P. striiformis f. sp. tritici isolates based on the resistances found in the cultivars, and to determine wheat breeders' perceptions on the importance and methods for achieving stripe rust resistance. Seedlings of cultivars were susceptible to recent isolates, indicating they lacked effective all-stage resistance. However, adult-plants were resistant or susceptible depending on the isolate, indicating they had race-specific adult-plant resistance. Using isolates collected from 1990 to 2013, six major virulence patterns were identified on adult plants of twelve cultivars that were selected as adult-plant differentials. Race-specific adult-plant resistance appears to be the only effective type of resistance protecting wheat from stripe rust in eastern United States. Among wheat breeders, the importance of incorporating stripe rust resistance into cultivars ranged from high to low depending on the frequency of epidemics in their region, and most sources of stripe rust resistance were either unknown or already overcome by virulence in the pathogen population. Breeders with a high priority for stripe rust resistance made most of their selections based on adult-plant reactions in the field, whereas breeders with a low priority for resistance based selections on molecular markers for major all-stage resistance genes.

Mapping of a new stem rust resistance gene Sr49 in chromosome 5B of wheat.

A new stem rust resistance gene Sr49 was mapped to chromosome 5BL of wheat. Usefulness of the closely linked markers sun209 and sun479 for marker-assisted selection of Sr49 was demonstrated. Landrace AUS28011 (Mahmoudi), collected from Ghardimaou, Tunisia, produced low stem rust response against Australian pathotypes of Puccinia graminis f. sp. tritici (Pgt) carrying virulence for several stem rust resistance genes deployed in modern wheat cultivars. Genetic analysis based on a Mahmoudi/Yitpi F3 population indicated the involvement of a single all-stage stem rust resistance gene and it was temporarily named SrM. Bulked segregant analysis using multiplex-ready SSR technology located SrM on the long arm of chromosome 5B. Since there is no other all-stage stem rust resistance gene located in chromosome 5BL, SrM was permanently designated Sr49. The Mahmoudi/Yitpi F3 population was enhanced to generate F6 recombinant inbred line (RIL) population for detailed mapping of Sr49 using publicly available genomic resources. Markers sun209 and sun479 flanked Sr49 at 1.5 and 0.9cM distally and proximally, respectively. Markers sun209 and sun479 amplified PCR products different than the Sr49-linked alleles in 146 and 145 common wheat cultivars, respectively. Six and seven cultivars, respectively, carried the resistance-linked marker alleles sun209 148bp and sun479 200bp ; however, none of the cultivars carried both resistance-linked alleles. These results demonstrated the usefulness of these markers for marker-assisted selection of Sr49 in breeding programs.

Mapping a Large Number of QTL for Durable Resistance to Stripe Rust in Winter Wheat Druchamp Using SSR and SNP Markers.

Winter wheat Druchamp has both high-temperature adult-plant (HTAP) resistance and all-stage resistance to stripe rust caused by Puccinia striiformis f. sp. tritici (Pst). The HTAP resistance in Druchamp is durable as the variety has been resistant in adult-plant stage since it was introduced from France to the United States in late 1940s. To map the quantitative trait loci (QTL) for stripe rust resistance, an F8 recombinant inbred line (RIL) population from cross Druchamp × Michigan Amber was phenotyped for stripe rust response in multiple years in fields under natural infection and with selected Pst races under controlled greenhouse conditions, and genotyped with simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers. Composite interval mapping (CIM) identified eight HTAP resistance QTL and three all-stage resistance QTL. Among the eight HTAP resistance QTL, QYrdr.wgp-1BL.2 (explaining 2.36-31.04% variation), QYrdr.wgp-2BL (2.81–15.65%), QYrdr.wgp-5AL (2.27–17.22%) and QYrdr.wgp-5BL.2 (2.42–15.13%) were significant in all tests; and QYrdr.wgp-1BL.1 (1.94–10.19%), QYrdr.wgp-1DS (2.04–27.24%), QYrdr.wgp-3AL (1.78–13.85%) and QYrdr.wgp-6BL.2 (1.69–33.71%) were significant in some of the tests. The three all-stage resistance QTL, QYrdr.wgp-5BL.1 (5.47–36.04%), QYrdr.wgp-5DL (9.27–11.94%) and QYrdr.wgp-6BL.1 (13.07-20.36%), were detected based on reactions in the seedlings tested with certain Pst races. Among the eleven QTL detected in Druchamp, at least three (QYrdr.wgp-5DL for race-specific all-stage resistance and QYrdr.wgp-3AL and QYrdr.wgp-6BL.2 for race non-specific HTAP resistance) are new. All these QTL, especially those for durable HTAP resistance, and their closely linked molecular markers could be useful for developing wheat cultivars with durable resistance to stripe rust.

Genome-wide association mapping for stripe rust (Puccinia striiformis F. sp. tritici) in US Pacific Northwest winter wheat (Triticum aestivum L.).

Potential novel and known QTL for race-specific all-stage and adult plant resistance to stripe rust were identified by genome-wide association mapping in the US PNW winter wheat accessions. Stripe rust (Puccinia striiformis F. sp. tritici; also known as yellow rust) is a globally devastating disease of wheat (Triticum aestivum L.) and a major threat to wheat production in the US Pacific Northwest (PNW), therefore both adult plant and all-stage resistance have been introduced into the winter wheat breeding programs in the PNW. The goal of this study was to identify quantitative trait loci (QTL) and molecular markers for these resistances through genome-wide association (GWAS) mapping in winter wheat accessions adapted to the PNW. Stripe rust response for adult plants was evaluated in naturally occurring epidemics in a total of nine environments in Washington State, USA. Seedling response was evaluated with three races under artificial inoculation in the greenhouse. The panel was genotyped with the 9K Illumina Wheat single nucleotide polymorphism (SNP) array and additional markers linked to previously reported genes and QTL for stripe rust resistance. The population was grouped into three sub-populations. Markers linked to Yr17 and previously reported QTL for stripe rust resistance were identified on chromosomes 1B, 2A, and 2B. Potentially novel QTL associated with race-specific seedling response were identified on chromosomes 1B and 1D. Potentially novel QTL associated with adult plant response were located on chromosomes 2A, 2B, 3B, 4A, and 4B. Stripe rust was reduced when multiple alleles for resistance were present. The resistant allele frequencies were different among sub-populations in the panel. This information provides breeders with germplasm and closely linked markers for stripe rust resistance to facilitate the transfer of multiple loci for durable stripe rust resistance into wheat breeding lines and cultivars.

Genetic mapping of a putative Agropyron cristatum-derived powdery mildew resistance gene by a combination of bulked segregant analysis and single nucleotide polymorphism array

The wheat relative Agropyron cristatum (L.) Gaertn. (2n = 4x = 28; genomes PPPP) has often been used as a donor of useful genetic variation for wheat improvement, including enhanced disease resistance to powdery mildew caused by Blumeria graminis f. sp. tritici (Bgt). In this report, resistance to powdery mildew was transferred from A. cristatum to common wheat, and the resulting introgression line PB3558 exhibited all-stage resistance. To identify the resistance gene, genetic analysis was conducted using F2, F2:3 and recombinant inbreed line populations derived from the cross of PB3558 and the susceptible cultivar Jing 4841. Segregation ratios from inoculation with Bgt isolate E09 indicated that the resistance was conferred by a single dominant gene, temporarily designated PmPB3558. Bulked segregant analysis (BSA) was applied to screen for molecular markers linked to PmPB3558, and five published markers were found. In order to increase the density of the genetic map, we developed ten novel single sequence repeat markers based on the single nucleotide polymorphism (SNP) loci with polymorphisms produced from a combination wheat 90 k SNP array and BSA. PmPB3558 was located on wheat chromosome arm 5DS and flanked by markers Xcfd81 and Xbwm25. Because there are other powdery mildew resistance genes located on 5DS, 21 Bgt isolates were used to compare the reaction differences. PmPB3558 showed unique reactions, suggesting that it was most likely a novel allele. This is the first documentation on transferring an alien powdery mildew resistance gene from A. cristatum, and the germplasm acquired in this study will be useful for broadening the genetic basis for wheat breeding.

Identification of QTL for adult plant resistance to stripe rust in Chinese wheat landrace Caoxuan 5

Yellow (or stripe) rust, caused by Puccinia striiformis f. sp. tritici (Pst), is an important wheat disease worldwide. The development of wheat cultivars with adult plant resistance (APR) has been given increasing emphasis in recent years because of the reputed durability of APR compared to all-stage resistance. Wheat landrace Caoxuan 5 showed a high level of APR in field nurseries from 2008 to 2014. Bulked segregant analysis using resistance gene analog polymorphism, simple sequence repeat and sequence-tagged site markers identified molecular markers linked to resistance genes. A total of 176 F3 lines were evaluated for stripe rust response in field nurseries at two sites for 2 years. Broad sense heritability of APR in the population was estimated as 0.81. Inclusive composite interval mapping identified quantitative trait loci (QTL) QYrcx.nafu-3BS and QYrcx.nafu-6AL conferring adult plant resistance and they explained 17.3–38.1 % and 9.7–19.8 % of the phenotypic variations, respectively. Based on molecular markers, both QTL appear to be different from previously identified stripe rust resistance genes. The resistance QTL and their molecular markers identified in Caoxuan 5 should be useful in breeding for stripe rust resistance.

The Stripe Rust Resistance Gene Yr10 Encodes an Evolutionary-Conserved and Unique CC–NBS–LRR Sequence in Wheat

The first seedling or all-stage resistance (R) R gene against stripe rust isolated from Moro wheat (Triticum aestivum L.) using a map-based cloning approach was identified as Yr10. Clone 4B of this gene encodes a highly evolutionary-conserved and unique CC-NBS-LRR sequence. Clone 4E, a homolog of Yr10, but lacking transcription start site (TSS) and putative TATA-box and CAAT-box, is likely a non-expressed pseudogene. Clones 4B and 4E are 84% identical and divergent in the intron and the LRR domain. Gene silencing and transgenesis were used in conjunction with inoculation with differentially avirulent and virulent stripe rust strains to demonstrate Yr10 functionality. The Yr10 CC-NBS-LRR sequence is unique among known CC-NBS-LRR R genes in wheat but highly conserved homologs (E = 0.0) were identified in Aegilops tauschii and other monocots including Hordeum vulgare and Brachypodium distachyon. Related sequences were also identified in genomic databases of maize, rice, and in sorghum. This is the first report of a CC-NBS-LRR resistance gene in plants with limited homologies in its native host, but with numerous homologous R genes in related monocots that are either host or non-hosts for stripe rust. These results represent a unique example of gene evolution and dispersion across species.

Mapping stripe rust resistance in a BrundageXCoda winter wheat recombinant inbred line population.

A recombinant inbred line (RIL) mapping population developed from a cross between winter wheat (Triticum aestivum L.) cultivars Coda and Brundage was evaluated for reaction to stripe rust (caused by Puccinia striiformis f. sp. tritici). Two hundred and sixty eight RIL from the population were evaluated in replicated field trials in a total of nine site-year locations in the U.S. Pacific Northwest. Seedling reaction to stripe rust races PST-100, PST-114 and PST-127 was also examined. A linkage map consisting of 2,391 polymorphic DNA markers was developed covering all chromosomes of wheat with the exception of 1D. Two QTL on chromosome 1B were associated with adult plant and seedling reaction and were the most significant QTL detected. Together these QTL reduced adult plant infection type from a score of seven to a score of two reduced disease severity by an average of 25% and provided protection against race PST-100, PST-114 and PST-127 in the seedling stage. The location of these QTL and the race specificity provided by them suggest that observed effects at this locus are due to a complementation of the previously known but defeated resistances of the cultivar Tres combining with that of Madsen (the two parent cultivars of Coda). Two additional QTL on chromosome 3B and one on 5B were associated with adult plant reaction only, and a single QTL on chromosome 5D was associated with seedling reaction to PST-114. Coda has been resistant to stripe rust since its release in 2000, indicating that combining multiple resistance genes for stripe rust provides durable resistance, especially when all-stage resistance genes are combined in a fashion to maximize the number of races they protect against. Identified molecular markers will allow for an efficient transfer of these genes into other cultivars, thereby continuing to provide excellent resistance to stripe rust.

Genetic mapping of a putative Thinopyrum intermedium-derived stripe rust resistance gene on wheat chromosome 1B

Stripe rust resistance transferred from Thinopyrum intermedium into common wheat was controlled by a single dominant gene, which mapped to chromosome 1B near Yr26 and was designated YrL693. Stripe rust caused by Puccinia striiformis f. sp. tritici (Pst) is a highly destructive disease of wheat (Triticum aestivum). Stripe rust resistance was transferred from Thinopyrum intermedium to common wheat, and the resulting introgression line (L693) exhibited all-stage resistance to the widely virulent and predominant Chinese pathotypes CYR32 and CYR33 and to the new virulent pathotype V26. There was no cytological evidence that L693 had alien chromosomal segments from Th. intermedium. Genetic analysis of stripe rust resistance was performed by crossing L693 with the susceptible line L661. F(1), F(2), and F(2:3) populations from reciprocal crosses showed that resistance was controlled by a single dominant gene. A total 479 F(2:3) lines and 781 pairs of genomic simple sequence repeat (SSR) primers were employed to determine the chromosomal location of the resistance gene. The gene was linked to six publicly available and three recently developed wheat genomic SSR markers. The linked markers were localized to wheat chromosome 1B using Chinese Spring nulli-tetrasomic lines, and the resistance gene was localized to chromosome 1B based on SSR and wheat genomic information. A high-density genetic map was also produced. The pedigree, molecular marker data, and resistance response indicated that the stripe rust resistance gene in L693 is a novel gene, which was temporarily designated YrL693. The SSR markers that co-segregate with this gene (Xbarc187-1B, Xbarc187-1B-1, Xgwm18-1B, and Xgwm11-1B) have potential application in marker-assisted breeding of wheat, and YrL693 will be useful for broadening the genetic basis of stripe rust resistance in wheat.

Genetic Analysis and Molecular Mapping of an All-Stage Stripe Rust Resistance Gene in Triticum aestivum-Haynaldia villosa Translocation Line V3

Triticum aestivum-Hayaldia villosa translocation line V3 has shown effective all-stage resistance to the seven dominant pathotypes of Puccinia striiforms f. sp. tritici prevalent in China. To elucidate the genetic basis of the resistance, the segregating populations were developed from the cross between V3 and susceptible genotype Mingxian 169, seedlings of the parents and F2 progeny were tested with six prevalent pathotypes, including CYR29, CYR31, CYR32-6, CYR33, Sun11-4, and Sun11-11, F1 plants and F3 lines were also inoculated with Sun11-11 to confirm the result further. The genetic studied results showed that the resistance of V3 against CYR29 was conferred by two dominant genes, independently, one dominant gene and one recessive gene conferring independently or a single dominant gene to confer resistance to CYR31, two complementary dominant genes conferring resistance to both CYR32-6 and Sun11-4, two independently dominant genes or three dominant genes (two of the genes show cumulative effect) conferring resistance to CYR33, a single dominant gene for resistance to Sun11-11. Resistance gene analog polymorphism (RGAP) and simple-sequence repeat (SSR) techniques were used to identify molecular markers linked to the single dominant gene (temporarily designated as YrV3) for resistance to Sun11-11. A linkage map of 2 RGAP and 7 SSR markers was constructed for the dominant gene using data from 221 F2 plants and their derived F2:3 lines tested with Sun11-11 in the greenhouse. Amplification of the complete set of nulli-tetrasomic lines of Chinese Spring with a RGAP marker RG1 mapped the gene on the chromosome 1B, and then the linked 7 SSR markers located this gene on the long arm of chromosome 1B. The linkage map spanned a genetic distance of 25.0 cM, the SSR markers Xgwm124 and Xcfa2147 closely linked to YrV3 with genetic distances of 3.0 and 3.8 cM, respectively. Based on the linkage map, it concluded that the resistance gene YrV3 was located on chromosome arm 1BL. Given chromosomal location, the reaction patterns and pedigree analysis, YrV3 should be a novel gene for resistance to stripe rust in wheat. These closely linked markers should be useful in stacking genes from different sources for wheat breeding and diversification of resistance genes against stripe rust.

Stripe rust resistance and genes in Chinese wheat cultivars and breeding lines

AbstractStripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases on wheat in China. To assess resistance in wheat cultivars and breeding lines in China, 330 leading cultivars and 164 advanced breeding lines were evaluated with stripe rust. In the greenhouse tests, seedlings of the entries were inoculated separately with several Pst pathotypes. In the field tests, the entries were evaluated for stripe rust resistance in Yangling, Shaanxi Province artificially inoculated and in Tianshui, Gansu Province under natural infection of Pst. The oversummering/wintering and spring epidemic zones of resistance genes were postulated using molecular markers for Yr5, Yr9, Yr10, Yr15, Yr17, Yr18, and Yr26, in combination with resistance spectra. Out of the 494 wheat entries, 16 (3.24 %) entries had all-stage resistance (ASR) in all race tests, 99 (20.04 %) had adult-plant resistance (APR), 28 (5.67 %) were considered to have slow-rusting (SR), and 351 (71.05 %) were susceptible to one or more races in both seedling and adult-plant stages. Advanced breeding lines had a higher percentage (37.2 %) of resistant entries (The sum of ASR, APR and SR) than leading cultivars (24.85 %). Among the epidemic regions, southern Gansu had a higher percentage of resistant entries than any other regions. Based on stripe rust reactions and molecular markers, two cultivars were found to possibly have Yr5 while no entries have Yr10 or Yr15. Resistance genes Yr9, Yr17, Yr18, and Yr26 were found in 134 (29.4 %), 45 (9.1 %), 10 (2 %), and 15 (3 %) entries, respectively.

Understanding Molecular Mechanisms of Durable and Non-durable Resistance to Stripe Rust in Wheat Using a Transcriptomics Approach

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici, continues to cause severe damage worldwide. Durable resistance is necessary for sustainable control of the disease. High-temperature adult-plant (HTAP) resistance, which expresses when the weather becomes warm and plants grow older, has been demonstrated to be durable. We conducted numerous studies to understand the molecular mechanisms of different types of stripe rust resistance using a transcriptomics approach. Through comparing gene expression patterns with race-specific, all-stage resistance controlled by various genes, we found that a greater diversity of genes is involved in HTAP resistance than in all-stage resistance. The genes involved in HTAP resistance are induced more slowly and their expression induction is less dramatic than genes involved in all-stage resistance. The high diversity of genes and less dramatic induction may explain durability and the incomplete expression level of HTAP resistance. Identification of transcripts may be helpful in identifying resistance controlled by different genes and in selecting better combinations of genes to combine for achieving adequate and durable resistance.