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Somatic embryogenesis receptor kinases (SERKs) belong to the leucine-rich repeat receptor-like kinase (LRR-RLK) subfamily, and many LRR-RLKs have been proven to play a key role in plant immune signal transmission. However, the functions of SERKs in resistance to stripe rust caused by Puccinia striiformis f. sp. tritici remains unknown. Here, we identified a gene, TaSERK1 , from Xiaoyan 6, a wheat cultivar possessing high-temperature seedling-plant (HTSP) resistance to the fungal pathogen P. striiformis f. sp. tritici and expresses its resistance at the seedling stage. The expression level of TaSERK1 was upregulated upon P. striiformis f. sp. tritici inoculation under relatively high temperatures. The transcriptional level of TaSERK1 was significantly increased under exogenous salicylic acid and brassinosteroids treatments. The barley stripe mosaic virus-induced gene silencing assay indicated that TaSERK1 positively regulated the HTSP resistance to stripe rust. The transient expression of TaSERK1 in tobacco leaves confirmed its subcellular localization on the plasma membrane. Furthermore, TaSERK1 interacted with and phosphorylated the chaperone protein TaDJA7, which belongs to the heat shock protein 40 subfamily. Silencing TaDJA7 compromised the HTSP resistance to stripe rust. The results indicated that when the membrane immune receptor TaSERK1 perceives the P. striiformis f. sp. tritici infection under relatively high temperatures, it transmits the signal to TaDJA7 to activate HTSP resistance to the pathogen., Competing Interests: The author(s) declare no conflict of interest.

Barley stripe rust is a relatively new disease in the United States. The pathogen, Puccinia striiformis f. sp. hordei ( Psh ), was first observed in Texas in 1991 and has spread north and westwards and mainly caused epidemics in the western United States. A total of 447 isolates collected from 1993 to 2017 were identified as 382 multilocus genotypes (MLGs) using 14 simple sequence repeat markers. The MLGs were clustered into six molecular groups (MGs) using the discriminant analysis of principal components and the hierarchical cluster analysis, and the MGs had significant differences in frequency in different years. MG1 was present in the population prior to the year 2000. MG2, MG3, and MG4 became predominate after 2000. MG5 was detected in all 24 years but more frequent from 2010 to 2017. MG6 was the most recent group detected mainly from 2011 to 2017 and had the highest correlation coefficient with the virulence phenotypes among the MGs. The heterozygosity and genotypic diversity of the Psh populations increased from 2000 to 2017, even more from 2010 to 2017. The results indicate rapid genetic changes from year to year, with major molecular group changes around 2000 and 2010. The possible mechanisms underlying the population changes are discussed.

Stripe rust, caused by Puccinia striiformis f. sp. tritici , is a devastating disease of wheat ( Triticum aestivum ) in the United States. The fungal pathogen can rapidly evolve, producing new virulent races infecting previously resistant cultivars and genotypes adapting to different environments. The objective of this study was to investigate the long-term population dynamics of P. striiformis f. sp. tritici in the United States. Through genotyping 1,083 isolates taken from 1968 to 2009, using 14 simple sequence repeat (SSR) markers and 92 secreted protein single nucleotide polymorphism (SP-SNP) markers, 614 and 945 genotypes were detected, respectively. In general, the two types of markers produced consistent genetic relationships among the P. striiformis f. sp. tritici populations over the 40-year period. The prior-to-2000 and the 2000-to-2009 populations were significantly different, with the latter showing higher genotypic diversity and higher heterozygosity than the earlier populations. Clustering analyses using genotypes of either SSR or SP-SNP markers revealed three molecular groups (MGs), MG1, MG2, and MG3. The prior-to-2000 and the 2000-to-2009 groups both had evidence of MG1 and MG2; however, MG3 was only found in the 2000-to-2009 population. Some of the isolates in the period of 2000 to 2009 formed individual clusters, suggesting exotic incursions. Other isolates of the same period were clustered with prior-to-2000 isolates, indicating that they were developed from the previously established populations. The data suggest the coexistence of newly introduced populations alongside established populations in the United States. Twenty SP-SNP markers were significantly associated to individual avirulence genes. These results are useful for developing more accurate monitoring systems and provide guidance for disease management.

Xiaoyan6 (XY6) is a wheat ( Triticum aestivum ) cultivar possessing nonrace-specific high-temperature seedling plant (HTSP) resistance against stripe rust, caused by Puccinia striiformis f. sp. tritici . Previously, we identified one particular gene, TaRPS2 , for its involvement in the HTSP resistance. To elucidate the role of TaRPS2 in the HTSP resistance, we cloned the full length of TaRPS2 from XY6. The transcriptional expression of TaRPS2 was rapidly upregulated (19.11-fold) under the normal-high-normal temperature treatment that induces the HTSP resistance. The expression level of TaRPS2 in leaves was higher than that in the stems and roots. Quantification of the endogenous hormones in wheat leaves after P. striiformis f. sp. tritici inoculation showed that 1-aminocyclopropane-1-carboxylic acid, salicylic acid (SA), and jasmonic acid were involved in the HTSP resistance. In addition, detection of hydrogen peroxide (H 2 O 2 ) accumulation indicated that reactive oxygen species burst was also associated with the HTSP resistance. Two hours after exogenous H 2 O 2 treatment or 0.5 h after SA treatment, the expression level of TaRPS2 was increased by 2.66 and 2.35 times, respectively. The subcellular localization of enhanced green fluorescent protein-TaRPS2 fusion protein was in the nuclei and plasma membranes. Virus-induced gene silencing of TaRPS2 reduced the level of HTSP resistance in XY6. Compared with the nonsilenced leaves, the TaRPS2 -silenced leaves had the reduction of necrotic cells but a greater number of uredinia. These results indicated that TaRPS2 positively regulates the HTSP resistance of XY6 against P. striiformis f. sp. tritici and is related to the SA and H 2 O 2 signaling pathways.

Puccinia striiformis f. sp. tritici causes stripe rust (yellow rust), one of the most important wheat diseases worldwide. To understand the genetic variation of the pathogen in a global scale, 283 P. striiformis f. sp. tritici isolates collected from 16 countries in eight geographic regions were genotyped using 24 codominant simple sequence repeat markers. The overall collection had a high level of genetic diversity, and the diversity levels in the Asian populations were generally higher than those of the other regions. Heterozygosity of isolates ranged from 0 to 75%, with an average of 46%. Mean heterozygosity in individual countries ranged from 34 to 59%. A total of 265 multilocus genotypes (MLGs) were detected, which were classified into eight molecular groups. Some of the molecular groups were present in all geographic regions. Moreover, many isolates from different regions were found to be identical or very closely related MLGs. Analysis of molecular variance revealed high variation within countries and intermediate variation between countries, but it revealed low and insignificant variation among geographic regions. Pairwise comparisons of regional populations detected considerable effective migrants and only low to moderate levels of differentiation. The molecular genotypes had a moderate level of correlation with the virulence phenotypes, and some of the molecular/virulence groups contained isolates from different continents. The results indicate tremendous migrations of P. striiformis f. sp. tritici and warrant the development of management strategies considering the global pathogen population.

A previous genome-wide association study (GWAS) for leaf rust (caused by Puccinia triticina ) resistance identified 46 resistance quantitative trait loci (QTL) in an elite spring wheat leaf rust resistance diversity panel. With the aim of characterizing the pleiotropic resistance sources to both leaf rust and stripe rust (caused by P. striiformis f. sp. tritici ), stripe rust responses were tested in five U.S. environments at the adult-plant stage and to five U.S. races at the seedling stage. The data revealed balanced phenotypic distributions in this population except for the seedling response to P. striiformis f. sp. tritici race PSTv-37. GWAS for stripe rust resistance discovered a total of 21 QTL significantly associated with all-stage or field resistance on chromosomes 1B, 1D, 2B, 3B, 4A, 5A, 5B, 5D, 6A, 6B, 7A, and 7B. Previously documented pleiotropic resistance genes Yr18/Lr34 and Yr46 / Lr67 and tightly linked genes Yr17 - Lr37 and Yr30-Sr2 - Lr27 were also detected in this population. In addition, stripe rust resistance QTL Yrswp-2B.1 , Yrswp-3B , and Yrswp-7B colocated with leaf rust resistance loci 2B_3 , 3B_t2 , and 7B_4 , respectively. Haplotype analysis uncovered that Yrswp-3B and 3B_t2 were either tightly linked genes or the same gene for resistance to both stripe and leaf rusts. Single nucleotide polymorphism markers IWB35950 , IWB74350 , and IWB72134 for the 3B QTL conferring resistance to both rusts should be useful in incorporating the resistance allele(s) in new cultivars.

Stripe rust caused by Puccinia striiformis is a disastrous disease of cereal crops and various grasses. To date, 14 stripe rust genomes are publicly available, including 13 P . striiformis f. sp. tritici and 1 P . striiformis f. sp. hordei . In this study, one isolate (11-281) of P . striiformis collected from wheatgrass ( Agropyron cristatum ), which is avirulent to most of standard differential genotypes of wheat and barley, was sequenced, assembled, and annotated. The sequences were assembled to a draft genome of 84.75 Mb, which is comparable with previously sequenced P. striiformis f. sp. tritici and P. striiformis f. sp. hordei isolates. The draft genome comprised 381 scaffolds and contained 1,829 predicted secreted proteins. The high-quality draft genome of the isolate is a valuable resource in shedding light on the evolution and pathogenicity of P . striiformis .

Winter wheat cultivar Eltan has been one of the most widely grown cultivars in the U.S. Pacific Northwest. It has shown variable levels of resistance to stripe rust in different years since it was released in 1990. To map all currently effective and defeated resistance genes in Eltan and understand the factors causing the resistance changes, 112 F 2:5 recombinant inbred lines (RILs) were developed from a cross of Eltan with cultivar Avocet S. The RILs were evaluated in fields of Pullman, Washington in 2015, 2016, 2017, and 2018 and Mount Vernon, Washington in 2016 and 2017 under natural infections; they were also evaluated in the greenhouse with races PSTv-4 and PSTv-40 of Puccinia striiformis f. sp. tritici . The RILs were genotyped with the 90K Illumina iSelect wheat single-nucleotide polymorphism chip. A total of five quantitative trait loci (QTLs) were identified in Eltan. Two major QTLs on chromosome arms 2BS and 4AL were detected in the greenhouse tests, explaining up to 28.0 and 42.0% of phenotypic variation, respectively. The two race-specific QTLs were also detected in some field experiments but with reduced effects. A minor QTL on 5BS was detected in the greenhouse and field tests, explaining 10.0 to 14.8% of the phenotypic variation. The other two minor QTLs were mapped on 6AS and 7BL and detected only in field experiments, explaining up to 20.5 and 13.5% of phenotypic variation, respectively. All stripe rust samples collected in the experimental fields in 2015 and 2016 were identified as P. striiformis f. sp. tritici races virulent on seedlings of Eltan. The resistance reduction of Eltan was caused by changes of the P. striiformis f. sp. tritici population from avirulent to virulent, overcoming the race-specific all-stage resistance in Eltan.

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

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.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a major yield-limiting foliar disease of wheat (Triticum aestivum) worldwide. In this study, the genetic variability of elite spring wheat germplasm from North America was investigated to characterize the genetic basis of effective all-stage and adult plant resistance (APR) to stripe rust. A genome-wide association study was conducted using 237 elite spring wheat lines genotyped with an Illumina Infinium 90K single-nucleotide polymorphism array. All-stage resistance was evaluated at seedling stage in controlled conditions and field evaluations were conducted under natural disease pressure in eight environments across Washington State. High heritability estimates and correlations between infection type and severity were observed. Ten loci for race-specific all-stage resistance were confirmed from previous mapping studies. Three potentially new loci associated with race-specific all-stage resistance were identified on chromosomes 1D, 2A, and 5A. For APR, 11 highly significant quantitative trait loci (QTL) (false discovery rate < 0.01) were identified, of which 3 QTL on chromosomes 3A, 5D, and 7A are reported for the first time. The QTL identified in this study can be used to enrich the current gene pool and improve the diversity of resistance to stripe rust disease.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most devastating diseases of wheat worldwide. Growing resistant cultivars is considered the best approach to manage this disease. In order to identify the resistance gene(s) in wheat line 03031-1-5 H62, which displayed high resistance to stripe rust at adult plant stage, a cross was made between 03031-1-5 H62 and susceptible cultivar Avocet S. The mapping population was tested with Chinese P. striiformis f. sp. tritici race CYR32 through artificial inoculation in a field in Yangling, Shaanxi Province and under natural infection in Tianshui, Gansu Province. The segregation ratios indicated that the resistance was conferred by a single dominant gene, temporarily designated as YrH62. A combination of bulked segregant analysis (BSA) with wheat 90K single nucleotide polymorphism (SNP) array was used to identify molecular markers linked to YrH62. A total of 376 polymorphic SNP loci identified from the BSA analysis were located on chromosome 1B, from which 35 kompetitive allele-specific PCR (KASP) markers selected together with 84 simple sequence repeat (SSR) markers on 1B were used to screen polymorphism and a chromosome region associated with rust resistance was identified. To saturate the chromosomal region covering the YrH62 locus, a 660K SNP array was used to identify more SNP markers. To develop tightly linked markers for marker-assisted selection of YrH62 in wheat breeding, 18 SNPs were converted into KASP markers. A final linkage map consisting of 15 KASP and 3 SSR markers was constructed with KASP markers AX-109352427 and AX-109862469 flanking the YrH62 locus in a 1.0 cM interval. YrH62 explained 63.8 and 69.3% of the phenotypic variation for disease severity and infection type, respectively. YrH62 was located near the centromeric region of chromosome 1BS based on the positions of the SSR markers in 1B deletion bins. Based on the origin, responses to P. striiformis f. sp. tritici races, and marker distances, YrH62 is likely different from the other reported stripe rust resistance genes/quantitative trait loci on 1B. The gene and tightly linked KASP markers will be useful for breeding wheat cultivars with resistance to stripe rust.

Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen, is a dikaryotic, biotrophic, and macrocyclic fungus. Genetic study of P. striiformis f. sp. tritici virulence was not possible until the recent discovery of Berberis spp. and Mahonia spp. as alternate hosts. To determine inheritance of virulence and map virulence genes, a segregating population of 119 isolates was developed by self-fertilizing P. striiformis f. sp. tritici isolate 08-220 (race PSTv-11) on barberry leaves under controlled greenhouse conditions. The progeny isolates were phenotyped on a set of 29 wheat lines with single genes for race-specific resistance and genotyped with simple sequence repeat (SSR) markers, single nucleotide polymorphism (SNP) markers derived from secreted protein genes, and SNP markers from genotyping-by-sequencing (GBS). Using the GBS technique, 10,163 polymorphic GBS-SNP markers were identified. Clustering and principal component analysis grouped these markers into six genetic groups, and a genetic map, consisting of six linkage groups, was constructed with 805 markers. The six clusters or linkage groups resulting from these analyses indicated a haploid chromosome number of six in P. striiformis f. sp. tritici. Through virulence testing of the progeny isolates, the parental isolate was found to be homozygous for the avirulence loci corresponding to resistance genes Yr5, Yr10, Yr15, Yr24, Yr32, YrSP, YrTr1, Yr45, and Yr53 and homozygous for the virulence locus corresponding to resistance gene Yr41. Segregation was observed for virulence phenotypes in response to the remaining 19 single-gene lines. A single dominant gene or two dominant genes with different nonallelic gene interactions were identified for each of the segregating virulence phenotypes. Of 27 dominant virulence genes identified, 17 were mapped to two chromosomes. Markers tightly linked to some of the virulence loci may facilitate further studies to clone these genes. The virulence genes and their inheritance information are useful for understanding the host-pathogen interactions and for selecting effective resistance genes or gene combinations for developing stripe rust resistant wheat cultivars.

Puccinia striiformis causes stripe rust on wheat, barley, and grasses. Natural population studies have indicated that somatic recombination plays a possible role in P. striiformis variation. To determine whether somatic recombination can occur, susceptible wheat or barley plants were inoculated with mixed urediniospores of paired isolates of P. striiformis. Progeny isolates were selected by passing through a series of inoculations of wheat or barley genotypes. Potential recombinant isolates were compared with the parental isolates on the set of 18 wheat or 12 barley genotypes that are used to differentiate races of P. striiformis f. sp. tritici (the wheat stripe rust pathogen) and P. striiformis f. sp. hordei (the barley stripe rust pathogen), respectively, for virulence changes. They were also tested with 51 simple-sequence repeat and 90 single-nucleotide polymorphism markers for genotype changes. From 68 possible recombinant isolates obtained from nine combinations of isolates based on virulence tests, 66 were proven to be recombinant isolates by molecular markers. Various types of recombinants were determined, including lost virulence from both virulent parental isolates, gained virulence from both avirulent isolates, combined virulences from both parents, and inherited virulence from one parent and avirulence from another. Marker data indicate that most of the recombinants were produced through chromosome reassortment and crossover after the hybridization of two parental isolates. The results demonstrate that somatic recombination is a mechanism by which new variants can be generated in P. striiformis.

Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust, frequently produces new races overcoming resistance in wheat cultivars. A recently identified race, V26 with virulence to Yr26 and many other stripe rust resistance genes, has a high potential to cause epidemics in China. In this study, teliospores from a single-urediniospore isolate of V26 (Pinglan 17-7) produced on the wheat line 92R137 (Yr26) were used to produce a sexual population through selfing by infecting Berberis shensiana plants under controlled conditions. One hundred and eighteen progeny isolates and the parental isolate were phenotyped for virulence/avirulence on 24 Yr gene lines of wheat. These progeny isolates were all avirulent to Yr5, Yr8, Yr15, and YrTr1 and virulent to Yr1, Yr2, Yr7, Yr9, Yr10, Yr17, Yr24, Yr25, Yr26, YrA, YrExp2, and YrV23, indicating that the parental isolate is homozygous avirulent or homozygous virulent at these loci. The progeny population segregated for avirulence to Yr6, Yr43, and YrSP at one locus (3 avirulent:1 virulent ratio); for virulence to Yr27 and Yr28 at one locus (3 virulent:1 avirulent); and for Yr4, Yr32, and Yr44 at two loci (15 virulent:1 avirulent). Among the eight segregating avirulence/virulence loci, association was found between virulence to Yr4 and Yr32, as well as between virulence to Yr6 and Yr43 based on χ(2) tests. From 82 genotypically different progeny isolates, 24 pathotypes and 82 multilocus genotypes were identified. The results show that a highly diverse population can be produced from a single isolate by selfing on a barberry plant and sexually produced population can be used to genetically characterize virulence of the stripe rust pathogen.

The spring wheat 'Canthatch' has been shown to suppress stem rust resistance genes in the background due to the presence of a suppressor gene located on the long arm of chromosome 7D. However, it is unclear whether the suppressor also suppresses resistance genes against leaf rust and stripe rust. In this study, we investigated the specificity of the resistance suppression. To determine whether the suppression is genome origin specific, chromosome location specific, or rust species or race specific, we introduced 11 known rust resistance genes into the Canthatch background, including resistance to leaf, stripe, or stem rusts, originating from A, B, or D genomes and located on different chromosome homologous groups. F1 plants of each cross were tested with the corresponding rust race, and the infection types were scored and compared with the parents. Our results show that the Canthatch 7DL suppressor only suppressed stem rust resistance genes derived from either the A or B genome, and the pattern of the suppression is gene specific and independent of chromosomal location.

ABSTRACT The wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici) population in China has been reported to be a distinct genetic group with higher diversity than those in many other countries. Genetic recombination in the P. striiformis f. sp. tritici population has been identified with molecular markers but whether sexual reproduction occurs in China is unknown. In this study, we surveyed barberry plants for infection by rust fungi in the stripe rust "hotspot" regions in Gansu, Sichuan, and Shaanxi provinces; collected barberry plants and inoculated plants of 20 Berberis spp. with germinated teliospores under controlled greenhouse conditions for susceptibility to P. striiformis f. sp. tritici; and tested P. striiformis f. sp. tritici isolates obtained from aecia on naturally infected barberry plants on the wheat genotypes used to differentiate Chinese P. striiformis f. sp. tritici races to determine virulence variations. Different Berberis spp. were widely distributed and most surveyed plants had pycnia and aecia of rust fungi throughout the surveyed regions. In total, 28 Berberis spp. were identified during our study. From 20 Berberis spp. tested with teliospores of P. striiformis f. sp. tritici from wheat plants, 18 species were susceptible under greenhouse conditions. Among 3,703 aecia sampled from barberry plants of three species (Berberis shensiana, B. brachypoda, and B. soulieana) under natural infections in Gansu and Shaanxi provinces, four produced P. striiformis f. sp. tritici uredinia on susceptible wheat 'Mingxian 169'. Sequence of the internal transcribed spacer (ITS) regions of the four isolates from barberry shared 99% identity with the P. striiformis f. sp. tritici sequences in the National Center for Biotechnology Information database. The four isolates had virulence patterns different from all previously reported races collected from wheat plants. Furthermore, 82 single-uredinium isolates obtained from the four barberry isolates had high virulence diversity rates of 9.0 to 28.1%, indicating that the diverse isolates were produced through sexual reproduction on barberry plants under natural conditions. In addition to P. striiformis f. sp. tritici, sequence analysis of polymerase chain reaction products of the ITS regions and inoculation tests on wheat identified P. graminis (the stem rust pathogen). Our results indicated that P. striiformis f. sp. tritici can infect some Berberis spp. under natural conditions, and the sexual cycle of the fungus may contribute to the diversity of P. striiformis f. sp. tritici in China.

Sustainable control of plant diseases can be achieved by developing cultivars with durable resistance. 'Bancroft' barley has durable high-temperature, adult-plant (HTAP) resistance to stripe rust caused by Puccinia striiformis f. sp. hordei. The objectives of this study were to determine the inheritance of the HTAP resistance in Bancroft, develop molecular markers for the HTAP resistance using the resistance gene analog polymorphism (RGAP) technique, map the HTAP resistance quantitative trait locus or loci (QTL) on barley chromosomes, and determine the usefulness of the RGAP markers in other barley cultivars for marker-assisted selection. The parents and F(4) recombinant inbred lines (RIL) and the parents and F(5) RIL were evaluated in 2004 and 2005 in one and three field sites, respectively, in Washington State. Infection type (IT) and disease severity (DS) were recorded three times at each location during each growing season. Area under the disease progress curve (AUDPC) was calculated for each parent and RIL based on the DS data. Genetic analyses of IT data of the parents, F(1), and F(2) tested in the adult-plant stage under controlled high-temperature cycle in the greenhouse and the parents, F(4), and F(5) RIL in the field indicated that one dominant gene controlled the HTAP resistance in Bancroft. Using 119 F(5:6) RIL and IT data, a linkage map on chromosome arm 3HL was constructed with eight RGAP markers and three simple sequence repeat (SSR) markers. Using the QTL analysis, a QTL for HTAP resistance was mapped with the DS and AUDPC data on the same chromosome location as with the IT data. The QTL explained >70% of the total phenotypic variation for the DS and AUDPC. The heritability of the HTAP resistance based on the AUDPC data was 76%. The two markers most close to the QTL peak detected polymorphisms in 84 and 88% of 25 barley genotypes that do not have the Bancroft HTAP resistance when used individually, and detected polymorphism in 100% of the genotypes when used in combination, indicating that the markers could be used in incorporating the HTAP resistance into these barley genotypes to improve the level and durability of resistance to stripe rust.

ABSTRACT Stripe rust, caused by Puccinia striiformis f. sp. hordei, is one of the most important diseases of barley in the south-central and western United States. Growing resistant cultivars is the best approach for controlling the disease. The barley genotype BBA 2890 has all-stage resistance against all races of P. striiformis f. sp. hordei (PSH) identified thus far in the United States. The resistance in BBA 2890 is controlled by a single recessive gene, rps1.a. The objectives of this study were to identify resistance gene analog polymorphism (RGAP) markers for the all-stage resistance gene rps1.a, to map the gene on a barley chromosome using chromosome-specific simple sequence repeat (SSR) markers, and to determine the presence or absence of the flanking RGAP markers for the gene in 24 barley genotypes. Seedlings of the parents and 200 F(8) recombinant inbred lines (RILs) were tested for resistance to pathogen races PSH-14, PSH-48, and PSH-54 in the greenhouse in 2005. Genomic DNA was extracted from the parents and 150 F(8) RILs. The RGAP technique was used to identify molecular markers for the rps1.a gene. Twelve primer pairs generating repeatable polymorphic bands were selected for genotyping the 150 F(8) RILs. A genetic linkage group was constructed for the resistance gene with 13 RGAP markers and four chromosome-specific SSR markers. The four SSR markers mapped the gene on the long arm of barley chromosome 3H. The closest RGAP marker for the resistant allele was within a genetic distance of 2.1 centimorgans (cM). The closest marker for the susceptible allele was 6.8 cM away from the locus. The two closest RGAP markers for the resistant allele detected polymorphisms in 67 and 71% of the 24 barley genotypes when used individually, and detected polymorphism in 88% of the genotypes when used in combination. This information should be useful in incorporating the resistance gene into barley cultivars and in pyramiding the gene with other resistance genes for superior stripe rust resistance.

ABSTRACT Most barley cultivars are resistant to stripe rust of wheat that is caused by Puccinia striiformis f. sp. tritici. The barley cv. Steptoe is susceptible to all identified races of P. striiformis f. sp. hordei (PSH), the barley stripe rust pathogen, but is resistant to most P. striiformis f. sp. tritici races. To determine inheritance of the Steptoe resistance to P. striiformis f. sp. tritici, a cross was made between Steptoe and Russell, a barley cultivar susceptible to some P. striiformis f. sp. tritici races and all tested P. striiformis f. sp. hordei races. Seedlings of parents and F(1), BC(1), F(2), and F(3) progeny from the barley cross were tested with P. striiformis f. sp. tritici races PST-41 and PST-45 under controlled greenhouse conditions. Genetic analyses of infection type data showed that Steptoe had one dominant gene and one recessive gene (provisionally designated as RpstS1 and rpstS2, respectively) for resistance to races PST-41 and PST-45. Genomic DNA was extracted from the parents and 150 F(2) plants that were tested for rust reaction and grown for seed of F(3) lines. The infection type data and polymorphic markers identified using the resistance gene analog polymorphism (RGAP) technique were analyzed with the Mapmaker computer program to map the resistance genes. The dominant resistance gene in Steptoe for resistance to P. striiformis f. sp. tritici races was mapped on barley chromosome 4H using a linked microsatellite marker, HVM68. A linkage group for the dominant gene was constructed with 12 RGAP markers and the microsatellite marker. The results show that resistance in barley to the wheat stripe rust pathogen is qualitatively inherited. These genes might provide useful resistance against wheat stripe rust when introgressed into wheat from barley.

ABSTRACT Stripe rust is one of the most important diseases of wheat and barley worldwide. On wheat it is caused by Puccinia striiformis f. sp. tritici and on barley by P. striiformis f. sp. hordei Most wheat genotypes are resistant to P. striiformis f. sp. hordei and most barley genotypes are resistant to P. striiformis f. sp. tritici. To determine the genetics of resistance in wheat to P. striiformis f. sp. hordei, crosses were made between wheat genotypes Lemhi (resistant to P. striiformis f. sp. hordei) and PI 478214 (susceptible to P. striiformis f. sp. hordei). The greenhouse seedling test of 150 F(2) progeny from the Lemhi x PI 478214 cross, inoculated with race PSH-14 of P. striiformis f. sp. hordei, indicated that Lemhi has a dominant resistance gene. The single dominant gene was confirmed by testing seedlings of the F(1), BC(1) to the two parents, and 150 F(3) lines from the F(2) plants with the same race. The tests of the F(1), BC(1), and F(3) progeny with race PSH-48 of P. striiformis f. sp. hordei and PST-21 of P. striiformis f. sp. tritici also showed a dominant gene for resistance to these races. Cosegregation analyses of the F(3) data from the tests with the two races of P. striiformis f. sp. hordei and one race of P. striiformis f. sp. tritici suggested that the same gene conferred the resistance to both races of P. striiformis f. sp. hordei, and this gene was different but closely linked to Yr21, a previously reported gene in Lemhi conferring resistance to race PST-21 of P. striiformis f. sp. tritici. A linkage group consisting of 11 resistance gene analog polymorphism (RGAP) markers was established for the genes. The gene was confirmed to be on chromosome 1B by amplification of a set of nullitetrasomic Chinese Spring lines with an RGAP marker linked in repulsion with the resistance allele. The genetic information obtained from this study is useful in understanding interactions between inappropriate hosts and pathogens. The gene identified in Lemhi for resistance to P. striiformis f. sp. hordei should provide resistance to barley stripe rust when introgressed into barley cultivars.