Pm50: a new powdery mildew resistance gene in common wheat derived from cultivated emmer

Fungal diseases of wheat, including powdery mildew, cause significant crop, yield and quality losses throughout the world. Knowledge of the genetic basis of powdery mildew resistance will greatly support future efforts to develop and cultivate resistant cultivars. Studies were conducted on cultivated emmer-derived wheat line K2 to identify genes involved in powdery mildew resistance at the seedling and adult plant growth stages using a BC₁ doubled haploid population derived from a cross between K2 and susceptible cultivar Audace. A single gene was located distal to microsatellite marker Xgwm294 on the long arm of chromosome 2A. Quantitative trait loci (QTL) analysis indicated that the gene was also effective at the adult plant stage, explaining up to 79.0 % of the variation in the progeny. Comparison of genetic maps indicated that the resistance gene in K2 was different from Pm4, the only other formally named resistance gene located on chromosome 2AL, and PmHNK54, a gene derived from Chinese germplasm. The new gene was designated Pm50.     

Identification and mapping of <Emphasis Type="Italic">MLIW30</Emphasis>, a novel powdery mildew resistance gene derived from wild emmer wheat

Powdery mildew, caused by Blumeria graminis f.sp. tritici (Bgt), is a destructive foliar disease of common wheat in areas with cool or maritime climates. Wild emmer wheat, Triticum turgidum ssp. dicoccoides, the progenitor of both domesticated tetraploid durum wheat and hexaploid bread wheat, harbors abundant genetic diversity related to resistance to powdery mildew that can be utilized for wheat improvement. An F₂ segregating population was obtained from a cross between resistant bread wheat line 2L6 and susceptible cultivar Liaochun 10, after which genetic analysis of F₂ and F₂-derived F₃ families was performed by inoculating plants with isolate Bgt E09. The results of this experiment demonstrated that powdery mildew resistance in 2L6, which was derived from wild emmer wheat accession IW30, was controlled by a single dominant gene, temporarily designated MLIW30. Nineteen SSR markers and two STS markers linked with MLIW30 were acquired by applying bulked segregant analysis. Finally, MLIW30 was located to the long arm of chromosome 4A and found to be flanked by simple sequence repeat markers XB1g2000.2 and XB1g2020.2 at 0.1 cM. Because no powdery mildew resistance gene in or derived from wild emmer wheat has been reported in wheat chromosome 4A, MLIW30 might be a novel Pm gene.     

High-density mapping and marker development for the powdery mildew resistance gene PmAS846 derived from wild emmer wheat (Triticum turgidum var. dicoccoides)

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is an important foliar disease of wheat worldwide. The dominant powdery mildew resistance gene PmAS846 was transferred to the hexaploid wheat lines N9134 and N9738 from wild emmer wheat (Triticum dicoccoides) in 1995, and it is still one of the most effective resistance genes in China. A high resolution genetic map for PmAS846 locus was constructed using two F₂ populations and corresponding F_2:3 families developed from the crosses of N9134/Shaanyou 225 and N9738/Huixianhong. Synteny between wheat and Brachypodium distachyon and rice was used to develop closely linked molecular markers to reduce the genetic interval around PmAS846. Twenty-six expressed sequence tag-derived markers were mapped to the PmAS846 locus. Five markers co-segregated with PmAS846 in the F₂ population of N9134/Shaanyou 225. PmAS846 was physically located to wheat chromosome 5BL bin 0.75–0.76 within a gene-rich region. The markers order is conserved between wheat and Brachypodium distachyon, but rearrangements are present in rice. Two markers, BJ261635 and CJ840011 flanked PmAS846 and narrowed PmAS846 to a region that is collinear with 197 and 112 kb genomic regions on Brachypodium chromosome 4 and rice chromosome 9, respectively. The genes located on the corresponding homologous regions in Brachypodium, rice and barley could be considered for further marker saturation and identification of potential candidate genes for PmAS846. The markers co-segregating with PmAS846 provide a potential target site for positional cloning of PmAS846, and can be used for marker-assisted selection of this gene.     

Identification and molecular mapping of a resistance gene to powdery mildew from the synthetic wheat line M53

Powdery mildew disease caused by Blumeria graminis f. sp. tritici (Bgt) is an economically important disease in wheat worldwide. The identification of germplasms resistant to the disease can not only facilitate the breeding of resistant cultivars, but can also broaden the diversity of resistance genes. The Mexican M53 is a synthetic hexaploid wheat line developed at the International Maize and Wheat Improvement Center (CIMMYT) from the cross between Triticum durum and Aegilops tauschii249. Infection of M53 with 15 different pathogen races revealed that the resistance in M53 was race-dependent and effective against the majority of the tested Bgt races, including the race 15 predominant in the Beijing wheat growing area. Inoculation of the parents of M53 with the race 15 demonstrated that M53 and Ae. tauschii249 were resistant, whereas T. durum was susceptible. The inoculation of three segregating F₂ populations developed from the crosses between M53 and three susceptible Chinese wheat cultivars with the race 15 showed that the resistant gene in M53 segregated in a single dominant manner. Amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers were used to map the gene in a segregating F₂ population consisting of 213 lines developed from the cross Wan7107 × M53. Two closely linked AFLP markers, Apm109 and Apm161, were identified to flank the gene with genetic distances of 1.0 cM and 3.0 cM, respectively. The recognized gene was assigned to the long arm of chromosome 5D as determined by three linked SSR markers, Xwmc289b, Xgwm583, and Xgwm292, and by the physical mapping of Apm109 using Chinese Spring nullisomic–tetrasomic and ditelosomic stocks. The resistance gene identified in M53, temporarily designated as Pm-M53, could be used in local wheat-breeding programs to improve powdery mildew resistance.     

Functional characterization of powdery mildew resistance gene MlIW172, a new Pm60 allele and its allelic variation in wild emmer wheat

Wild emmer wheat (Triticum dicoccoides, WEW) is an immediate progenitor of both the cultivated tetraploid and hexaploid wheats and it harbors rich genetic diversity against powdery mildew caused by Blumeria graminis f. sp. tritici (Bgt). A powdery mildew resistance gene MlIW172 originated from WEW accession IW172 (G-797-M) is fine mapped in a 0.048 centimorgan (cM) genetic interval on 7AL, corresponding to a genomic region spanning 233 kb, 1 Mb and 800 kb in Chinese Spring, WEW Zavitan, and T. urartu G1812, respectively. MlIW172 encodes a typical NLR protein NLR^IW172 and physically locates in an NBS-LRR gene cluster. NLR^IW172 is subsequently identified as a new allele of Pm60, and its function is validated by EMS mutagenesis and transgenic complementation. Haplotype analysis of the Pm60 alleles reveals diversifications in sequence variation in the locus and presence and absence variations (PAV) in WEW populations. Four common single nucleotide variations (SNV) are detected between the Pm60 alleles from WEW and T. urartu, indicative of speciation divergence between the two different wheat progenitors. The newly identified Pm60 alleles and haplotypes in WEW are anticipated to be valuable for breeding powdery mildew resistance wheat cultivars via marker-assisted selection.     

Identification and mapping of a new powdery mildew resistance gene on chromosome 6D of common wheat

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is one of the most serious wheat diseases. The rapid evolution of the pathogen's virulence, due to the heavy use of resistance genes, necessitates the expansion of resistance gene diversity. The common wheat line D57 is highly resistant to powdery mildew. A genetic analysis using an F(2) population derived from the cross of D57 with the susceptible cultivar Yangmai 158 and the derived F(2:3) lines indicated that D57 carries two dominant powdery mildew resistance genes. Based on mapping information of polymorphic markers identified by bulk segregant analysis, these two genes were assigned to chromosomes 5DS and 6DS. Using the F(2:3) lines that segregated in a single-gene mode, closely linked PCR-based markers were identified for both genes, and their chromosome assignments were confirmed through linkage mapping. The gene on chromosome 5DS was flanked by Xgwm205 and Xmag6176, with a genetic distance of 8.3 cM and 2.8 cM, respectively. This gene was 3.3 cM from a locus mapped by the STS marker MAG6137, converted from the RFLP marker BCD1871, which was 3.5 cM from Pm2. An evaluation with 15 pathogen isolates indicated that this gene and Pm2 were similar in their resistance spectra. The gene on chromosome 6DS was flanked by co-segregating Xcfd80 and Xmag6139 on one side and Xmag6140 on the other, with a genetic distance of 0.7 cM and 2.7 cM, respectively. This is the first powdery mildew resistance gene identified on chromosome 6DS, and plants that carried this gene were highly resistant to all of the 15 tested pathogen isolates. This gene was designated Pm45. The new resistance gene in D57 could easily be transferred to elite cultivars due to its common wheat origin and the availability of closely linked molecular markers.     

Identification and characterization of a novel powdery mildew resistance gene <Emphasis Type="Italic">PmG3M</Emphasis> derived from wild emmer wheat, <Emphasis Type="Italic">Triticum dicoccoides</Emphasis>

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt) is one of the most important wheat diseases worldwide. Wild emmer wheat, Triticum turgidum ssp. dicoccoides, the tetraploid ancestor (AABB) of domesticated bread and durum wheat, harbors many important alleles for resistance to various diseases, including powdery mildew. In the current study, two tetraploid wheat mapping populations, derived from a cross between durum wheat (cv. Langdon) and wild emmer wheat (accession G-305-3M), were used to identify and map a novel powdery mildew resistance gene. Wild emmer accession G-305-3M was resistant to all 47 Bgt isolates tested, from Israel and Switzerland. Segregation ratios of F₂ progenies and F₆ recombinant inbred line (RIL) mapping populations, in their reactions to inoculation with Bgt, revealed a Mendelian pattern (3:1 and 1:1, respectively), indicating the role of a single dominant gene derived from T. dicoccoides accession G-305-3M. This gene, temporarily designated PmG3M, was mapped on chromosome 6BL and physically assigned to chromosome deletion bin 6BL-0.70-1.00. The F₂ mapping population was used to construct a genetic map of the PmG3M gene region consisted of six simple sequence repeats (SSR), 11 resistance gene analog (RGA), and two target region amplification polymorphism (TRAP) markers. A second map, constructed based on the F₆ RIL population, using a set of skeleton SSR markers, confirmed the order of loci and distances obtained for the F₂ population. The discovery and mapping of this novel powdery mildew resistance gene emphasize the importance of the wild emmer wheat gene pool as a source for crop improvement.     

Molecular mapping of a recessive powdery mildew resistance gene in spelt wheat cultivar Hubel

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is one of the most important wheat diseases worldwide. The basis for wheat powdery mildew resistance breeding consists of screening diversified host genetic resources with a range of races of the powdery mildew pathogen. Spelt wheat (Triticum aestivum ssp. spelta 2n = 6x = 42, AABBDD) is a close relative of common wheat (T. aestivum ssp. aestivum) and contains several known disease resistance genes, including Pm1d, Yr5, and Lr65. Here, we report the identification and mapping of a powdery mildew resistance gene in spelt wheat cultivar Hubel, which was introduced to China from Europe and is resistant to Chinese Bgt isolate E09 at the seedling stage. Genetic analysis of a recombinant inbred line population derived from a cross of Hubel and a susceptible early maturing mutant line indicated that Hubel possessed a recessive powdery mildew resistance gene (temporarily designated MlHubel). Markers linked to MlHubel were identified using bulked segregant analysis, simple sequence repeat, and expressed sequence tag-derived sequence tagged site methods. The linked markers were physically located on wheat chromosome 2D. Comparative genomic analysis indicated that the genetic interval covering MlHubel in wheat is highly colinear with the corresponding regions on Brachypodium distachyon chromosome 5 and Oryza sativa chromosome 4. Accordingly, the genetic map of MlHubel was established in comparison with B. distachyon 5L and O. sativa 4L, with the closest marker Xgwm265 being 0.4 cM from MlHubel. The identification of the recessive powdery mildew gene in spelt wheat suggests the potential of this accession along with its closely linked markers in breeding for resistance to powdery mildew.     

Transfer and mapping of a gene conferring later-growth-stage powdery mildew resistance in a tetraploid wheat accession

Wheat powdery mildew is a severe foliar disease and causes significant yield losses in epidemic years. Breeding and using resistant cultivars is the most widely employed strategy to curb this disease. To identify and transfer powdery mildew resistance genes in wild emmer wheat accession TA1410 into common wheat, a resistant F3 line derived from the cross of TA1410 × durum wheat line Zhongyin1320 was crossed with common wheat cultivar Yangmai158. The homozygous resistant BC5F2 lines derived from the backcross with Yangmai158 exhibited susceptibility at seedling stage and conferred increasing resistance when the plants were closer to heading stage. In two segregating BC5F3 families investigated at heading stage, the segregation of the resistance fit a 3:1 ratio, suggesting that a single dominant gene controls the resistance. This resistance gene, designated HSM1, was mapped to the 0.6-cM Xmag5825.1–Xgwm344 interval on chromosome 7AL and co-segregated with Xrga-C3 and Xrga-C6. A mapping position comparison with other powdery mildew resistance genes on this chromosome suggested that HSM1 belongs to the Pm1 resistance gene cluster. HSM1 is a useful candidate gene for resistance breeding, particularly in winter-wheat growing areas.     

Identification and mapping of a new powdery mildew resistance allele in the Chinese wheat landrace Hongyoumai

Powdery mildew (Pm) caused by Blumeria graminis f. sp. tritici (Bgt) is one of the world’s major wheat diseases and results in large grain yield losses. Discovery and utilization of Pm resistance genes constitute the most common strategy for wheat Pm control. Hongyoumai, a wheat landrace from Henan Province in China, has excellent resistance to infection by Bgt. In order to identify the basis of such Pm resistance, a segregating population was submitted to genetic analysis, which showed that Pm resistance in Hongyoumai was conferred by a single recessive resistance gene. This gene was temporarily named pmHYM. Molecular marker analysis, chromosomal location, resistance spectrum analysis, and an allelism test showed that pmHYM was located on the long arm of chromosome 7B (7BL), most likely representing a new recessive resistance gene allelic with Pm5e and mlXBD. By using 90-kb single-nucleotide polymorphism sequences (SNP) in the BLASTn analysis against the wheat 7BL genome sequence, 12 new simple sequence repeat (SSR) markers linked with pmHYM were developed to map pmHYM co-segregating with the marker Xmp1207 and between markers Xmp925 and Xmp1158, at genetic distances of 2.8 and 2.7 cM, respectively. In addition, physical mapping of the markers linked with pmHYM using Chinese Spring deletion lines indicated a location in the 0.86–1.00 bin of 7BL.     

Genetic and comparative genomics mapping reveals that a powdery mildew resistance gene Ml3D232 originating from wild emmer co-segregates with an NBS-LRR analog in common wheat (Triticum aestivum L.)

Powdery mildew caused by Blumeria graminis f. sp. tritici is one of the most important wheat diseases worldwide and breeding for resistance using diversified disease resistance genes is the most promising approach to prevent outbreaks of powdery mildew. A powdery mildew resistance gene, originating from wild emmer wheat (Triticum turgidum var. dicoccoides) accessions collected from Israel, has been transferred into the hexaploid wheat line 3D232 through crossing and backcrossing. Inoculation results with 21 B. graminis f. sp. tritici races indicated that 3D232 is resistant to all of the powdery mildew isolates tested. Genetic analyses of 3D232 using an F₂ segregating population and F₃ families indicated that a single dominant gene, Ml3D232, confers resistance in the host seedling stage. By applying molecular markers and bulked segregant analysis (BSA), we have identified polymorphic simple sequence repeats (SSR), expressed sequence tags (EST) and derived sequence tagged site (STS) markers to determine that the Ml3D232 is located on chromosome 5BL bin 0.59–0.76. Comparative genetic analyses using mapped EST markers and genome sequences of rice and Brachypodium established co-linearity of the Ml3D232 genomic region with a 1.4 Mb genomic region on Brachypodium distachyon chromosome 4, and a 1.2 Mb contig located on the Oryza sativa chromosome 9. Our comparative approach enabled us to develop new EST–STS markers and to delimit the genomic region carrying Ml3D232 to a 0.8 cM segment that is collinear with a 558 kb region on B. distachyon. Eight EST markers, including an NBS-LRR analog, co-segregated with Ml3D232 to provide a target site for fine genetic mapping, chromosome landing and map-based cloning of the powdery mildew resistance gene. This newly developed common wheat germplasm provides broad-spectrum resistance to powdery mildew and a valuable resource for wheat breeding programs.     

Genetic mapping of the powdery mildew resistance gene Pm6 in wheat by RFLP analysis

Pm6 in bread wheat (Triticum aestivum L.), which was transferred from Triticum. timopheevii L., is a gene conferring resistance to the powdery mildew disease caused by Erysiphe graminis f. sp. tritici. Six near-isogenic lines ( NILs ) of Pm6 in a cultivar ’Prins’ background were analyzed to map this gene using restriction fragment length polymorphism (RFLP). Each of the six NILs possessed a T. timopheevii-derived segment, varying in length, and associated with powdery mildew resistance. Lines IGV1–465 (FAO163b/ 7*Prins) and IGV1–467 (Idaed 59B/7*Prins) had the shortest introgressed segments, which were detected only by DNA probes BCD135 and PSR934, respectively. The polymorphic loci detected by both probes were mapped to the long arm of chromosome 2B. Lines IGV1–458 (CI13250/7*Prins) and IGV1–456 (CI12559/8*Prins) contained the longest T. timopheevii segments involving both arms of donor chromosome 2G across the centromere. All these introgressed segments had an overlapping region flanked by the loci xpsr934 and xbcd135 on 2BL. Thus, Pm6 was located in this region since the powdery mildew resistance in all the NILs resulted from the introgressed fragments. Using the F₂ mapping population from a cross of IGV1–463 (PI170914/7*Prins)×Prins, Pm6 was shown to be closely linked to the loci xbcd135 and xbcd266 at a genetic distance of 1.6 cM and 4.8 cM, respectively. BCD135 was successfully used in detecting the presence of Pm6 in different genetic backgrounds. Received: 29 June 1999 / Accepted: 6 July 1999     

小麦抗白粉病基因Pm23的RAPD标记

小麦抗白粉病基因Pm23对世界上很多麦区流行的白粉病表现高抗或免疫.本研究以Pm23和Chancellor为抗感亲本,用集群分离分析法对抗性基因Pm23进行了RAPD分析,从320个十碱基随机引物中筛选到一个与Pm23紧密连锁的相引相标记OPE051100. 对F2分离群体进行RAPD分析表明,该标记与Pm23基因之间的连锁距离为10.65±3.25 cM.该标记可以有效用于小麦育种分子标记辅助选择中.     

Microsatellite mapping of a Triticum urartu Tum. derived powdery mildew resistance gene transferred to common wheat (Triticum aestivum L.)

A powdery mildew resistance gene from Triticum urartu Tum. accession UR206 was successfully transferred into hexaploid wheat (Triticum aestivum L.) through crossing and backcrossing. The F₁ plants, which had 28 chromosomes and an average of 5.32 bivalents and 17.36 univalents in meiotic pollen mother cells (PMC), were obtained through embryos rescued owing to shriveling of endosperm in hybrid seed of cross Chinese Spring (CS) × UR206. Hybrid seeds were produced through backcrossing F₁ with common wheat parents. The derivative lines had normal chromosome numbers and powdery mildew resistance similar to the donor UR206, indicating that the powdery mildew resistance gene originating from T. urartu accession UR206 was successfully transferred and expressed in a hexaploid wheat background. Genetic analysis indicated that a single dominant gene controlled the powdery mildew resistance at the seedling stage. To map and tag the powdery mildew resistance gene, 143 F₂ individuals derived from a cross UR206 × UR203 were used to construct a linkage map. The resistant gene was mapped on the chromosome 7AL based on the mapped microsatellite makers. The map spanned 52.1 cM and the order of these microsatellite loci agreed well with the established microsatellite map of chromosome arm 7AL. The resistance gene was flanked by the microsatellite loci Xwmc273 and Xpsp3003, with the genetic distances of 2.2 cM and 3.8 cM, respectively. On the basis of the origin and chromosomal location of the gene, it was temporarily designated PmU.     

The powdery mildew resistance gene Pm8 derived from rye is suppressed by its wheat ortholog Pm3

The powdery mildew resistance gene Pm8 derived from rye is located on a 1BL.1RS chromosome translocation in wheat. However, some wheat lines with this translocation do not show resistance to isolates of the wheat powdery mildew pathogen avirulent to Pm8 due to an unknown genetically dominant suppression mechanism. Here we show that lines with suppressed Pm8 activity contain an intact and expressed Pm8 gene. Therefore, the absence of Pm8 function in certain 1BL.1RS‐containing wheat lines is not the result of gene loss or mutation but is based on suppression. The wheat gene Pm3, an ortholog of rye Pm8, suppressed Pm8‐mediated powdery mildew resistance in lines containing Pm8 in a transient single‐cell expression assay. This result was further confirmed in transgenic lines with combined Pm8 and Pm3 transgenes. Expression analysis revealed that suppression is not the result of gene silencing, either in wheat 1BL.1RS translocation lines carrying Pm8 or in transgenic genotypes with both Pm8 and Pm3 alleles. In addition, a similar abundance of the PM8 and PM3 proteins in single or double homozygous transgenic lines suggested that a post‐translational mechanism is involved in suppression of Pm8. Co‐expression of Pm8 and Pm3 genes in Nicotiana benthamiana leaves followed by co‐immunoprecipitation analysis showed that the two proteins interact. Therefore, the formation of a heteromeric protein complex might result in inefficient or absent signal transmission for the defense reaction. These data provide a molecular explanation for the suppression of resistance genes in certain genetic backgrounds and suggest ways to circumvent it in future plant breeding.     

Inheritance and mapping of a powdery mildew resistance gene introgressed from Avena macrostachya in cultivated oat

The powdery mildew resistance from Avena macrostachya was successfully introgressed into hexaploid oat (A. sativa). Genetic analysis of F₁, F₂, F₃ and BC₁ populations from two powdery-mildew resistant introgression lines revealed that the resistance is controlled by a dominant gene, tentatively designated Eg-5. Molecular marker analysis was conducted using bulked-segregant analysis in two segregating F₃ populations. One codominant simple sequence repeats (SSR) marker AM102 and four AFLP-derived PCR-based markers were successfully developed. The SSR marker AM102 and the STS marker ASE41M56 were linked to the gene Eg-5, with genetic distances of 2 and 0.4 cM, respectively, in both mapping populations. Three STS markers (ASE45M56, ASE41M61, ASE36M55) co-segregated with Eg-5 in one population while two (ASE45M56, ASE36M55) of them linked to Eg-5 with a genetic distance of 1 cM in another population. The gene was further mapped to be in a region corresponding to linkage group 22_44+18 in the Kanota × Ogle (KO) hexaploid oat map by comparative mapping. To our knowledge, this is the first report of mapping powdery-mildew resistance in hexaploid oat. The new resistance source of A. macrostachya, together with the tightly linked markers identified here, could be beneficial in oat breeding programmes.     

Molecular mapping of the wheat powdery mildew resistance gene Pm24 and marker validation for molecular breeding

Molecular markers were identified in common wheat for the Pm24 locus conferring resistance to different isolates of the powdery mildew pathogen, Erysiphe graminis DM f. sp. tritici (Em. Marchal). Bulked segregant analysis was used to identify amplified fragment length polymorphism (AFLP) markers and microsatellite markers linked to the gene Pm24 in an F₂ progeny from the cross Chinese Spring (susceptible)× Chiyacao (resistant). Two AFLP markers XACA/CTA-407 and XACA/CCG-420, and three microsatellite markers Xgwm106, Xgwm337 and Xgwm458, were mapped in coupling phase to the Pm24 locus. The AFLP marker locus XACA/CTA-407 co-segregated with the Pm24 gene, and XACA/CCG-420 mapped 4.5 cM from this gene. Another AFLP marker locus XAAT/CCA-346 co- segregated in repulsion phase with the Pm24 locus. Pm24 was mapped close to the centromere on the short arm of chromosome 1D, contrary to the previously reported location on chromosome 6D. Pm24 segregated independently of gene Pm22, also located on chromosome 1D. An allele of microsatellite locus Xgwm337 located 2.4±1.2 cM from Pm24 was shown to be diagnostic and therefore potentially useful for pyramiding two or more genes for powdery mildew resistance in a single genotype. Received: 25 August 1999 / Accepted: 16 December 1999