High-temperature Adult-plant Resistance Research Articles

Identification of a Locus for High-Temperature Adult-Plant Resistance to Stripe Rust in the Wheat Yr8 Near-Isogenic Line Through Mutagenesis and Molecular Mapping.

Aegilops species are wheat relatives that harbor valuable disease resistance genes for wheat breeding. The wheat Yr8 near-isogenic line AvSYr8NIL has long been believed to carry only Yr8 for race-specific all-stage resistance to stripe rust, caused by Puccinia striiformis f. sp. tritici, derived from Aegilops comosa. However, AvSYr8NIL has been found to have high-temperature adult-plant (HTAP) resistance in our field and greenhouse tests. To confirm both HTAP and Yr8 resistance, seeds from AvSYr8NIL were treated with ethyl methanesulfonate to generate mutant lines. The mutant lines with only Yr8 (M641) and only HTAP resistance (M488) were crossed with the susceptible recurrent parent Avocet S (AvS). The F1 and F4 lines of AvS/M641 were phenotyped with Yr8-avirulent races in the seedling stage at the low-temperature (4 to 20°C) profile, while the F1, F2, F4, and F5 lines of AvS/M488 were phenotyped with Yr8-virulent races in the adult-plant stage at the high-temperature (10 to 30°C) profile. Both Yr8 and the HTAP resistance gene (YrM488) were recessive. The F4 populations of AvS/M641 and AvS/M488 were genotyped using polymorphic Kompetitive allele-specific PCR markers converted from single-nucleotide polymorphisms. Yr8 was mapped to a 0.66-cM fragment, and YrM488 was mapped to a 1.22-cM interval on chromosome 2D. The physical distance between the two resistance genes was estimated to be more than 500 Mb, indicating their distinct loci. The mutant lines with separated resistance genes would be useful in enhancing our understanding of different types of resistance and in further studying the interactions between wheat and the stripe rust pathogen.

Fine mapping of high-temperature adult-plant resistance to stripe rust in wheat cultivar Louise

Fine mapping of high-temperature adult-plant resistance to stripe rust in wheat cultivar Louise

Characterization and Molecular Mapping of a Gene Conferring High-Temperature Adult-Plant Resistance to Stripe Rust Originally from Aegilops ventricosa.

Wheat near-isogenic line AvSYr17NIL carrying Yr17, originally from Aegilops ventricosa for all-stage resistance to Puccinia striiformis f. sp. tritici, also shows nonrace-specific, high-temperature adult-plant (HTAP) resistance to the stripe rust pathogen. To separate and identify the HTAP resistance gene, seeds of AvSYr17NIL were treated with ethyl methanesulfonate. Mutant lines with only HTAP resistance were obtained, and one of the lines, M1225, was crossed with the susceptible recurrent parent Avocet S (AvS). Field responses of the F2 plants and F3 lines, together with the parents, were recorded at the adult-plant stage in Pullman and Mount Vernon, WA under natural P. striiformis f. sp. tritici infection. The parents and the F4 population were phenotyped with a Yr17-virulent P. striiformis f. sp. tritici race in the adult-plant stage under the high-temperature profile in the greenhouse. The phenotypic results were confirmed by testing the F5 population in the field under natural P. striiformis f. sp. tritici infection. The F2 data indicated a single recessive gene, temporarily named YrM1225, for HTAP resistance. The F4 lines were genotyped with Kompetitive allele-specific PCR markers converted from single-nucleotide polymorphism markers polymorphic between M1225 and AvS. The HTAP resistance gene was mapped on the short arm of chromosome 2A in an interval of 7.5 centimorgans using both linkage and quantitative trait locus mapping approaches. The separation of the HTAP resistance gene from Yr17 should improve the understanding and utilization of the different types of resistance.

Identification of high-temperature resistance to stripe rust and molecular detection of Yr genes in Chinese core collections of common wheat

Stripe rust is a serious wheat disease caused by Puccinia striiformis Westend f. sp. tritici Erikss. High-temperature resistance, which can be induced when the diurnal cycle changes to 15–28 °C or warmer temperatures, has been demonstrated to be non-race specific and durable. The objectives of this study were to evaluate a core collection of 305 Chinese wheat cultivars and landraces for responses to stripe rust in both seedling and adult-plant stages in the greenhouse and the field, determine the distribution of previously reported Yr genes, and select accessions with effective resistance, especially high-temperature resistance. Fifty-eight (19.0%) entries were found to carry all-stage resistance, 71 (23.3%) entries showed adult-plant resistance, 13 (4.3%) entries showed high-temperature all-stage (HTAS) resistance, and 8 (2.6%) entries showed high-temperature adult-plant (HTAP) resistance. Molecular marker detection identified Yr9, Yr10, Yr17, Yr18, Yr26, Yr30, Yr41, Yr78, and Yr80 at different frequencies in the collection, but did not detect Yr5, Yr15, and Yr36. The results in this study would be useful for the rational distribution of cultivars in different regions and breeding for new cultivars with effective and durable resistance.

Transfer of Durable Stripe Rust Resistance Gene Yr39 into Four Chinese Elite Wheat Cultivars Using Marker-Assisted Selection

Wheat gene Yr39 confers durable high-temperature adult-plant (HTAP) resistance to stripe rust caused by Puccinia striiformis f. sp. tritici, one of the most destructive diseases of wheat worldwide. The objective of this study was to transfer Yr39 into four Chinese elite wheat cultivars. Backcross inbred line populations were developed from four Chinese wheat cultivars Chuanmai 42 (CM42), Bainong Aikang 58 (AK58), Han 6172 (H6172) and Zhengmai 9023 (ZM9023) crossed with a Yr39 single-gene line. The F1, BC1F1 and BC1F6 lines were genotyped using resistance gene analogs polymorphism (RGAP) markers Xwgp36, Xwgp44 and Xwgp43, which are closely linked to Yr39. Progeny lines selected with the markers for Yr39 were evaluated in the field for stripe rust resistance and agronomic traits including plant height, tiller numbers, spike grain numbers and thousand-grain weight. Eleven lines were selected with stripe rust resistance and desirable agronomic traits. These lines with production potential can be used for further testing in various wheat production regions and as germplasm resources for breeding new wheat cultivars with durable stripe rust resistance, high yield, and adaptation to different production environments.

Warm wheat does not rust - how wheat resists stripe rust when temperatures rise.

Stripe rust is a destructive wheat disease that is estimated to cause 5.47 million tonnes of wheat loss per year (Beddow et al., 2015). The fungus that causes the disease, Puccinia striiformis f. sp. tritici (Pst), occurs in all wheat-growing regions of the world but causes more problems in cooler climates. One reason for this is that the fungus requires relatively low temperatures for spore germination, growth, and sporulation. But some wheat plants also have a defense mechanism against Pst that only operates under high temperatures. Two types of temperature-sensitive defense mechanisms are known: high-temperature seedling plant resistance and high-temperature adult plant resistance. When grown under high-temperature conditions, wheat varieties with such a defense mechanism are resistant against Pst. Because high-temperature resistance provides immunity to all Pst races, understanding the genetic basis of this defense mechanism might be valuable for breeding. Over the last decades, breeders have developed resistant cultivars by deploying race-specific resistance genes. Race-specific resistant cultivars have limited durability because pathogens evolve quickly. In the case of Pst, new races have overcome most of the known race-specific resistance genes (Fu et al., 2009). Therefore, breeding for non-race-specific resistance such as high-temperature resistance might be a more durable solution in the fight against stripe rust. Xiaoping Hu, Professor of Plant Pathology at Northwest Agricultural and Forestry University in Yangling, China, is trying to understand the molecular mechanisms involved in high-temperature resistance in seedlings. His research group performed transcriptomic analysis on the resistant cultivar Xiaoyan 6 that had been inoculated with Pst and grown under high temperatures. Among the differentially expressed genes, they found a gene encoding a cysteine-rich receptor-like kinase that they designated TaCRK10. In this issue of The Plant Journal, the researchers investigated the role of this gene (Wang et al., 2021). To investigate if TaCRK10 is involved in resistance to Pst, the authors performed virus-induced gene silencing (VIGS) in the Xiaoyan 6 cultivar and inoculated the TaCRK10-silenced plants with Pst. When plants started to show symptoms of stripe rust, they were divided into two groups. One group was placed under high-temperature conditions (20°C) for 24 h, whereas the other group was kept at normal temperatures (16°C). In the TaCRK10-silenced plants grown under high temperatures, there was abundant spore formation on the leaves, whereas non-silenced plants showed only limited sporulation. Under normal temperatures, control plants and silenced plants did not show differences. These results suggest that TaCRK10 is required for high-temperature seedling plant resistance to Pst in the Xiaoyan 6 cultivar. Furthermore, when they overexpressed TaCRK10 from Xiaoyan 6 in the susceptible wheat cultivar Fielder, then inoculated the overexpression lines with Pst, sporulation was much lower than in wild-type plants. Interestingly, the overexpression lines were resistant against Pst when grown at normal temperatures. Further analysis of these lines revealed expression of genes involved in the salicylic acid defense pathway as well as accumulation of reactive oxygen species (ROS), which are common signs of an immunity response. To deduce the molecular action of TaCRK10, the authors performed a yeast-two-hybrid assay using a library constructed from Pst-infected leaves of the Xiaoyan 6 cultivar grown at high temperature. They found that TaCRK10 interacted with the histone variant TaH2A.1. Because TaCRK10 contains a serine/threonine kinase-like domain, the authors wondered if TaCRK10 might phosphorylate histone TaH2A.1. To test this, the researchers produced recombinantly expressed proteins and mixed them in the presence of ATP. Using an electrophoresis technique that separates phosphorylated and non-phosphorylated proteins followed by Western blot analysis, they found that TaH2A.1 was indeed phosphorylated in the presence of TaCRK10. To find out if H2A.1 is involved in high-temperature seedling plant resistance to Pst, the authors used VIGS to silence the histone variant. Subsequent inoculation experiments showed that sporulation was higher on TaH2A.1-silenced plants at both normal and high temperatures than on non-silenced plants. Hence, TaH2A.1 appears to be involved in wheat resistance to Pst (Figure). Xiaoping Hu wants to better understand the TaCRK10-TaH2A.1 pathway by identifying the phosphorylation sites on H2A.1 on which CRK10 acts and to determine if phosphorylation of TaH2A.1 plays a role in high-temperature resistance to stripe rust in seedlings. He explains that the findings might be used to develop wheat varieties with a durable resistance against Pst. Overexpression of CRK10 from Xiaoyan 6 in the susceptible variety Fielder resulted in plants that were resistant to the pathogen grown at normal temperatures, showing that susceptible cultivars can become resistant in this way, but also that constitutive overexpression of the allele leads to resistance that is no longer dependent on temperature.

Mapping Quantitative Trait Loci for High-Temperature Adult-Plant Resistance to Stripe Rust in Spring Wheat PI 197734 Using a Doubled Haploid Population and Genotyping by Multiplexed Sequencing.

Stripe rust caused by Puccinia striiformis f. sp. tritici (Pst) is a global concern for wheat production. Spring wheat cultivar PI 197734, of Sweden origin, has shown high-temperature adult-plant resistance (APR) to stripe rust for many years. To map resistance quantitative trait loci (QTL), 178 doubled haploid lines were developed from a cross of PI 197734 with susceptible AvS. The DH lines and parents were tested in fields in 2017 and 2018 under natural infection of Pst and genotyped with genotyping by multiplexed sequencing (GMS). Kompetitive allele specific PCR (KASP) and simple sequence repeat (SSR) markers from specific chromosomal regions were also used to genotype the population to validate and saturate resistance QTL regions. Two major QTL on chromosomes 1AL and 3BL and one minor QTL on 2AL were identified. The two major QTL, QYrPI197734.wgp-1A and QYrPI197734.wgp-3B, were detected in all tested environments explaining up to 20.7 and 46.8% phenotypic variation, respectively. An awnletted gene mapped to the expected distal end of chromosome 5AL indicated the accuracy of linkage mapping. The KASP markers converted from the GMS-SNPs in the 1A and 3B QTL regions were used to genotype 95 US spring wheat cultivars and breeding lines, and they individually showed different percentages of polymorphisms. The haplotypes of the three markers for the 1A QTL and four markers for the 3B QTL identified 37.9 and 21.1% of the wheat cultivar/breeding lines possibly carrying these two QTL, indicating their usefulness in marker-assisted selection (MAS) for incorporating the two major QTL into new wheat cultivars.

Genome-Wide Mapping of Quantitative Trait Loci Conferring All-Stage and High-Temperature Adult-Plant Resistance to Stripe Rust in Spring Wheat Landrace PI 181410.

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases of wheat in the world. Genetic resistance is the best strategy for control of the disease. Spring wheat landrace PI 181410 has shown high level resistance to stripe rust. The present study characterized the landrace to have both race-specific all-stage resistance and nonrace-specific high-temperature adult-plant (HTAP) resistance. To map quantitative trait loci (QTL) for the resistance in PI 181410, it was crossed with Avocet S (AvS), from which a recombinant inbred line population was developed. The F5–F8 populations were consecutively phenotyped for stripe rust response in multiple field environments under natural Pst infection, and the F7 population was phenotyped in seedlings at low temperature and in adult-plant stage with selected Pst races in the greenhouse. The F7 population was genotyped using the 90K wheat SNP chip. Three QTL, QYrPI181410.wgp-4AS, QYrPI181410.wgp-4BL, and QYrPI181410.wgp-5BL.1, from PI 181410 for all-stage resistance, were mapped on chromosome arms 4AS, 4BL, and 5BL, respectively. Four QTL, QYrPI181410.wgp-1BL, QYrPI181410.wgp-4BL, QYrPI181410.wgp-5AS, and QYrPI181410.wgp-5BL.2, were identified from PI 181410 for HTAP resistance and mapped to 1BL, 4BL, 5AS, and 5BL, respectively. Two QTL with minor effects on stripe rust response were identified from AvS and mapped to 2BS and 2BL. Four of the QTL from PI 181410 and one from AvS were potentially new. As the 4BL QTL was most effective and likely a new gene for stripe rust resistance, three kompetitive allele specific PCR (KASP) markers were developed for incorporating this gene into new wheat cultivars.

Characterization and diagnostic marker development for Yr28-rga1 conferring stripe rust resistance in wheat

Stripe rust is one of the most devastating wheat diseases and developing resistant cultivars is the most economical way to manage this disease. More than 80 stripe rust resistance genes (Yr genes) have formally been mapped. However, a lack of diagnostic markers has partially restricted the deployment of some resistance genes in molecular breeding. Among them, Yr28 showed high-temperature adult plant resistance (HT-APR) to stripe rust in the field, but it is rarely employed in wheat breeding. Here, Yr28-rga1 was found and isolated through the co-segregated marker mwg634 of Yr28. The gene sequence of Yr28-rga1-4A and Yr28-rga1-4B were almost the same between PI648511 and other wheat germplasm, including Chinese Spring (CS), Avocet, Yanzhan1, and Chuanyu12, but Yr28-rga1-4D showed only 88% identity between PI648511 and the others. Further, the high similarity in conserved domains to other wheat R proteins indicated that Yr28-RGA1 was a typical NBS-LRR protein. But the low identity in amino sequences also suggested that it was a novel gene. The specific 3-D protein structure in LRR regions also indicated that Yr28-rga1-4D from PI648511 might have different functional responses to disease. Furthermore, two gene-specific markers of cib-Yr28M1 and cib-Yr28M2 were developed for Yr28-rga1. Only 24 accessions showed identity to PI648511 on Yr28-rga1 locus in a panel of 672 wheat germplasms. The two diagnostic markers also revealed the positive correlation of Yr28-rga1 and stripe rust resistance (R2 > 0.8; p < 0.01) in field testing. These results indicated that the newly characterized Yr28-rga1 was a novel NBS-LRR gene conferring stripe rust resistance. The two diagnostic markers would be useful for marker assisted selection in wheat breeding against stripe rust.

YR36/WKS1-Mediated Phosphorylation of PsbO, an Extrinsic Member of Photosystem II, Inhibits Photosynthesis and Confers Stripe Rust Resistance in Wheat

Wheat stripe rust, due to infection by Puccinia striiformis f. sp. tritici (Pst), is a devastating disease that causes significant global grain yield losses. Yr36, which encodes Wheat Kinase START1 (WKS1), is an effective high-temperature adult-plant resistance gene and confers resistance to a broad spectrum of Pst races. We previously showed that WKS1 phosphorylates the thylakoid ascorbate peroxidase protein and reduces its ability to detoxify peroxides, which may contribute to the accumulation of reactive oxygen species (ROS). WKS1-mediated Pst resistance is accompanied by leaf chlorosis in Pst-infected regions, but the underlying mechanisms remain elusive. Here, we show that WKS1 interacts with and phosphorylates PsbO, an extrinsic member of photosystem II (PSII), to reduce photosynthesis, regulate leaf chlorosis, and confer Pst resistance. A point mutation in PsbO-A1 or reduction in its transcript levels by RNA interference resulted in chlorosis and reduced Pst sporulation. Biochemical analyses revealed that WKS1 phosphorylates PsbO at two conserved amino acids involved in physical interactions with PSII and reduces the binding affinity of PsbO with PSII. Presumably, phosphorylated PsbO proteins dissociate from the PSII complex and then undergo rapid degradation by cysteine and aspartic proteases. Taken together, these results demonstrate that perturbations of wheat PsbO by point mutation or phosphorylation by WKS1 reduce the rate of photosynthesis and delay the growth of Pst pathogen before the induction of ROS.

QTL analysis of durable stripe rust resistance in the North American winter wheat cultivar Skiles.

This study determined the effects of growth stage and temperature on expression of high-temperature adult-plant resistance to stripe rust, mapped six QTL for durable resistance in winter wheat Skiles using a doubled haploid population, and selected breeding lines with different combinations of the QTL using marker-assisted selection. The winter wheat cultivar Skiles has a high level of high-temperature adult-plant (HTAP) resistance to stripe rust caused by Puccinia striiformis f. sp. tritici (Pst). The Skiles HTAP resistance was highly effective at the adult-plant stage even under low temperatures, but high temperatures induced earlier expression and increased levels of resistance. To map resistance genes, Skiles was crossed with the susceptible cultivar Avocet S and a doubled haploid (DH) population was developed. The DH population was tested in fields at Pullman, WA, in 2016, 2017 and 2018, Mount Vernon, WA, in 2017 and 2018 under natural infection, and an environmentally controlled greenhouse at the adult-plant stage with the currently predominant race PSTv-37. The population was genotyped using the 90K Illumina iSelect wheat SNP chip and selected SSR markers on specific chromosomes. In total, 2526 polymorphic markers were used for QTL mapping and six QTL were detected. Two of the six QTL had major effects across all environments, with one mapped on chromosome 3BS, explaining up to 28.2% of the phenotypic variation and the other on chromosome 4BL, explaining up to 41.8%. Minor QTL were mapped on chromosomes 1BL, 5AL, 6B and 7DL. Genotyping 140 wheat cultivars from the US Pacific Northwest revealed high polymorphism of markers for five of the QTL, and five highly resistant lines with the five QTL were selected from Skiles-derived breeding lines using the markers. This study demonstrated that multiple QTL with mostly additive effects contributed to the high-level HTAP resistance in Skiles.

Identification of a major QTL on chromosome arm 2AL for reducing yellow rust severity from a Chinese wheat landrace with evidence for durable resistance.

A QTL on 2AL for reducing yellow rust severity was identified from a Chinese wheat landrace, being more effective than Yr18, with evidence for durable resistance from field observations. Utilization of wheat resistance is an important strategy to control yellow rust. The Chinese wheat landrace Hong Qimai (HQM) and the advanced breeding line AQ24788-83 (AQ; a progeny of HQM) can significantly reduce disease severity at the adult-plant growth stage. HQM has maintained adult-plant resistance for a prolonged period of time. To study the inheritance of the resistance, 126 recombinant inbred lines (RILs) derived from the cross Thatcher (TC) × HQM and 138 RILs from Luke × AQ were assessed for disease severity in six field trials. A genetic map of TC × HQM was constructed by genotyping these RILs using the 90K wheat single-nucleotide polymorphism chip. Luke × AQ map was previously constructed for another disease study and also utilized here. Based on these maps and disease data, a quantitative trait locus (QTL) was detected on the chromosome arm 2AL from both TC × HQM and Luke × AQ and designated as QYr.cau-2AL. The resistance allele at QYr.cau-2AL came from HQM and AQ. QYr.cau-2AL was significantly effective across all the test environments and different genetic backgrounds, with its effect magnitude being higher than that of Yr18. QYr.cau-2AL synergistically acted with Yr18 and a QTL for high-temperature adult-plant resistance on 2BS, resulting in an elevated resistance from the juvenile plant growth stage onward, although QYr.cau-2AL alone displayed no substantial resistance at juvenile stage. Evidence indicates that QYr.cau-2AL is novel and confers durable resistance, and thus, should have high potential value for practical breeding.

Pathogenesis-related protein genes involved in race-specific all-stage resistance and non-race specific high-temperature adult-plant resistance to Puccinia striiformis f. sp. tritici in wheat

Interactions of the stripe rust pathogen (Puccinia striiformis f. sp. tritici) with wheat plants activate a wide range of host responses. Among various genes involved in the plant-pathogen interactions, the expressions of particular pathogenesis-related (PR) protein genes determine different defense responses. Different types of resistance have been recognized and utilized for developing wheat cultivars for resistance to stripe rust. All-stage resistance can be detected in seedling stage and remains at high levels throughout the plant growth stages. This type of resistance is race-specific and not durable. In contrast, plants with only high-temperature adult-plant (HTAP) resistance are susceptible in seedling stage, but become resistant when plants grow older and the weather becomes warmer. HTAP resistance controlled by a single gene is partial, but usually non-race specific and durable. The objective of this study was to analyze the expression of PR protein genes involved in different types of wheat resistance to stripe rust. The expression levels of 8 PR protein genes (PR1, PR1.2, PR2, PR3, PR4, PR5, PR9 and PR10) were quantitatively evaluated at 0, 1, 2, 7 and 14 days after inoculation in single resistance gene lines of wheat with all-stage resistance genes YrTr1, Yr76, YrSP and YrExp2 and lines carrying HTAP resistance genes Yr52, Yr59, Yr62 and Yr7B. Races PSTv-4 and PSTv-37 for compatible and incompatible interactions were used in evaluation of PR protein gene expression in wheat lines carrying all-stage resistance genes in the seedling-stage experiment while PSTv-37 was used in the HTAP experiment. Analysis of quantitative real-time polymerase chain reaction (qRT-PCR) revealed that all of the PR protein genes were involved in the different types of resistance controlled by different Yr genes. However, these genes were upregulated at different time points and at different levels during the infection process among the wheat lines with different Yr genes for either all-stage resistance or HTAP resistance. Some of the genes were also induced in compatible interactions, but the levels were almost always higher in the incompatible interaction than in the compatible interaction at the same time point for each Yr gene. These results indicate that both allsalicylic acid and jasmonate signaling pathways are involved in both race-specific all-stage resistance and non-race specific HTAP resistance. Although expressing at different stages of infection and at different levels, these PR protein genes work in concert for contribution to different types of resistance controlled by different Yr genes.

Combination of all-stage and high-temperature adult-plant resistance QTL confers high-level, durable resistance to stripe rust in winter wheat cultivar Madsen.

Wheat cultivar Madsen has a new gene on the short arm of chromosome 1A and two QTL for all-stage resistance and three QTL for high-temperature adult-plant resistance that in combination confer high-level, durable resistance to stripe rust. Wheat cultivar Madsen has maintained a high-level resistance to stripe rust over 30years. To map quantitative trait loci (QTL) underlying the high-level, durable resistance, 156 recombinant inbred lines (RILs) developed from cross Avocet S × Madsen were phenotyped with selected races of Puccinia striiformis f. sp. tritici in the greenhouse seedling tests, and in naturally infected fields during 2015-2017. The RILs were genotyped by SSR and SNP markers from genotyping by sequencing and the 90K wheat SNP chip. Three QTL for all-stage resistance were mapped on chromosomes 1AS, 1BS and 2AS, and two QTL for high-temperature adult-plant (HTAP) resistance were mapped on 3BS and 6BS. The most effective QTL on 2AS, explaining 8.97-23.10% of the phenotypic variation in seedling tests and 8.60-71.23% in field tests, contained Yr17 for all-stage resistance and an additional gene for HTAP resistance. The 6BS QTL, detected in all field tests, was identified as Yr78. The 1AS QTL, conferring all-stage resistance, was identified as a new gene, which explained 20.45 and 30.23% of variation in resistance to races PSTv-37 and PSTv-40, respectively, and contributed significantly to field resistance at Pullman in 2015-2017, but was not detected at Mount Vernon. The interactions among QTL were mostly additive, and RILs with all five QTL had the highest level of resistance in the field, similar to Madsen. Genotyping 148 US Pacific Northwest wheat cultivars with markers for the 1AS, 2AS and 6BS QTL validated the genes and markers, and indicated their usefulness for marker-assisted selection.

Characterization of Novel Gene Yr79 and Four Additional Quantitative Trait Loci for All-Stage and High-Temperature Adult-Plant Resistance to Stripe Rust in Spring Wheat PI 182103.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is an important disease of wheat worldwide. Exploring new resistance genes is essential for breeding resistant wheat cultivars. PI 182103, a spring wheat landrace originally from Pakistan, has shown a high level of resistance to stripe rust in fields for many years, but genes for resistance to stripe rust in the variety have not been studied. To map the resistance gene(s) in PI 182103, 185 recombinant inbred lines (RILs) were developed from a cross with Avocet Susceptible (AvS). The RIL population was genotyped with simple sequence repeat (SSR) and single nucleotide polymorphism markers and tested with races PST-100 and PST-114 at the seedling stage under controlled greenhouse conditions and at the adult-plant stage in fields at Pullman and Mt. Vernon, Washington under natural infection by the stripe rust pathogen in 2011, 2012, and 2013. A total of five quantitative trait loci (QTL) were detected. QyrPI182103.wgp-2AS and QyrPI182103.wgp-3AL were detected at the seedling stage, QyrPI182103.wgp-4DL was detected only in Mt. Vernon field tests, and QyrPI182103.wgp-5BS was detected in both seedling and field tests. QyrPI182103.wgp-7BL was identified as a high-temperature adult-plant resistance gene and detected in all field tests. Interactions among the QTL were mostly additive, but some negative interactions were detected. The 7BL QTL was mapped in chromosomal bin 7BL 0.40 to 0.45 and identified as a new gene, permanently designated as Yr79. SSR markers Xbarc72 and Xwmc335 flanking the Yr79 locus were highly polymorphic in various wheat genotypes, indicating that the molecular markers are useful for incorporating the new gene for potentially durable stripe rust resistance into new wheat cultivars.

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.

High-temperature adult-plant resistance to stripe rust in facultative winter wheat

Stripe (yellow) rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss., is one of the most damaging diseases in wheat and is especially damaging for winter and facultative wheat. The objective of this study was to understand stripe rust resistance in 100 wheat and facultative wheat entries from the International Winter Wheat Improvement Program by conducting experiments in a greenhouse and in four field environments in Washington State, USA, and by genotyping molecular markers linked to Yr genes. Percentages of entries resistant to the rust races at the seedling stage were: PST-17, 44%; PST-37, 32%; PST-43, 45%; PST-45, 49%; PST-116, 18%; PST-100, 17%; and PST-127, 8%. Molecular markers were positive for genes Yr9, Yr17, and Yr18 and negative for Yr5, Yr10, and Yr15. Yr18 was present in 44 entries (44%). By using the highly virulent races PST-127 and PST-100 under controlled conditions, 16 entries were shown to have high-temperature adult-plant (HTAP) resistance and resistant–moderately resistant field reactions at all four field sites. Resistant entries, especially those with HTAP resistance, were also identified in the field experiments.

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.

Mapping of Yr62 and a small-effect QTL for high-temperature adult-plant resistance to stripe rust in spring wheat PI 192252.

This manuscript reports a new gene (Yr62) and a small-effect QTL for potentially durable resistance to stripe rust and usefulness of Yr62 markers for marker-assisted selection. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a devastating disease of wheat worldwide. Spring wheat germplasm PI 192252 showed a high level of high-temperature adult-plant (HTAP) resistance to stripe rust in germplasm evaluation over 8years in the State of Washington. To elucidate the genetic basis of resistance, PI 192252 was crossed with 'Avocet susceptible'. A mapping population of 150 F5 recombinant inbred lines was developed using single-seed descent. Stripe rust tests were conducted with selected Pst races in a greenhouse and in field conditions under natural infections. The relative area under the disease progress curve (rAUDPC) data showed continuous distributions, indicating that HTAP resistance of PI 192252 was controlled by quantitative trait loci (QTL). Two QTL were identified in PI 192252, explaining 74.2% of the total phenotypic variation for rAUDPC. These two QTL were mapped to chromosomes 4BL (QYrPI192252.wgp-4BL) and 5BS (QYrPI192252.wgp-5BS) with SSR and SNP markers and explained 40-60 and 22-27%, respectively, of the phenotypic variation across the four environments. Because the major-effect QTL on 4BL is different from previously named Yr genes and inherited as a single gene, it is named Yr62. The SSR marker alleles Xgwm192 222 and Xgwm251 133 flanking Yr62 were different from the alleles in various wheat varieties, suggesting that these markers could be useful in marker-assisted selection for incorporating Yr62 into commercial cultivars.

Identification of Yr59 conferring high-temperature adult-plant resistance to stripe rust in wheat germplasm PI 178759

This manuscript reports a new gene for non-race-specific resistance to stripe rust and molecular markers for incorporating it into wheat cultivars for control of the disease with durable resistance. Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive wheat diseases worldwide. The spring wheat germplasm 'PI 178759' originating from Iraq showed effective resistance to stripe rust in field evaluations over 8 years in Washington state, USA. To map the resistance gene(s), PI 178759 was crossed with 'Avocet Susceptible', and the parents and 176 F2:3 lines were phenotyped in the fields under natural infection and in a greenhouse with selected races of P. striiformis f. sp. tritici. PI 178759 was identified to have high-temperature adult-plant (HTAP) resistance. Resistance gene analog polymorphism and simple sequence repeat techniques were used to identify molecular markers linked to the resistance gene and a chromosome region was mapped using a quantitative trait locus approach. One major gene was mapped to the long arm of chromosome 7B. Flanked by Xwgp5175 and Xbarc32 in a 2.1 cM region, the gene explained 31.8 and 54.7 % of the phenotypic variation in rAUDPC and IT, respectively. Based on genetic distances among markers and allelism tests, the HTAP resistance gene in PI 178759 is different from the previously reported Yr39, Yr52, YrZH84, and YrC591, also located on chromosome 7BL, and is therefore designated as Yr59. The gene and its flanking markers should be useful for developing wheat cultivars with durable resistance.