Mapping of adult plant stripe rust resistance genes in diploid A genome wheat species and their transfer to bread wheat

Stripe rust, caused by Puccinia striiformis West. f.sp. tritici, is one of the most damaging diseases of wheat worldwide. Forty genes for stripe rust resistance have been catalogued so far, but the majority of them are not effective against emerging pathotypes. Triticum monococcum and T. boeoticum have excellent levels of resistance to rusts, but so far, no stripe rust resistance gene has been identified or transferred from these species. A set of 121 RILs generated from a cross involving T. monococcum (acc. pau14087) and T. boeoticum (acc. pau5088) was screened for 3 years against a mixture of pathotypes under field conditions. The parental accessions were susceptible to all the prevalent pathotypes at the seedling stage, but resistant at the adult plant stage. Genetic analysis of the RIL population revealed the presence of two genes for stripe rust resistance, with one gene each being contributed by each of the parental lines. A linkage map with 169 SSR and RFLP loci generated from a set of 93 RILs was used for mapping these resistance genes. Based on phenotypic data for 3 years and the pooled data, two QTLs, one each in T. monococcum acc. pau14087 and T. boeoticum acc. pau5088, were detected for resistance in the RIL population. The QTL in T. monococcum mapped on chromosome 2A in a 3.6 cM interval between Xwmc407 and Xwmc170, whereas the QTL from T. boeoticum mapped on 5A in 8.9 cM interval between Xbarc151 and Xcfd12 and these were designated as QYrtm.pau-2A and QYrtb.pau-5A, respectively. Based on field data for 3 years, their R ² values were 14 and 24%, respectively. T. monococcum acc. pau14087 and three resistant RILs were crossed to hexaploid wheat cvs WL711 and PBW343, using T. durum as a bridging species with the objective of transferring these genes into hexaploid wheat. The B genome of T. durum suppressed resistance in the F₁ plants, but with subsequent backcrossing one resistance gene could be transferred from one of the RILs to the hexaploid wheat background. This gene was derived from T. boeoticum acc. pau5088 as indicated by co-introgression of T. boeoticum sequences linked to stripe rust resistance QTL, QYrtb.pau-5A. Homozygous resistant progenies with 40–42 chromosomes have been identified. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A new powdery mildew resistance allele at the Pm4 wheat locus transferred from einkorn (Triticum monococcum)

Powdery mildew is one of the most destructive foliar diseases of wheat. A set of differential Blumeria graminis f.sp. tritici (Bgt) isolates was used to test the powdery mildew response of a Triticum monococcum-derived resistant hexaploid line, Tm27d2. Segregation analysis of 95 F_2:3 lines from a Chinese Spring/Tm27d2 cross revealed that the resistance of Tm27d2 is controlled by a single dominant gene. Using monosomic analysis and a molecular mapping approach, the resistance gene was localized to the terminal end of chromosome 2AL. The linkage map of chromosome 2AL consisted of nine simple sequence repeat markers and one sequence-tagged site (STS) marker (ResPm4) indicative for the Pm4 locus. According to the differential reactions of 19 wheat cultivars/lines with known powdery mildew resistance genes to 13 Bgt isolates, Tm27d2 carried a new resistance specificity. The complete association of the resistance allele with STS marker ResPm4 indicated that it represented a new allele at the Pm4 locus. This new allele was designated Pm4d. The two flanking markers Xgwm526 and Xbarc122 closely linked to Pm4d at genetic distances of 3.4 and 1.0 cM, respectively, are present in chromosome bin 2AL1-0.85-1.00.     

Characterization of <Emphasis Type="Italic">Pm59</Emphasis>, a novel powdery mildew resistance gene in Afghanistan wheat landrace PI 181356

Key message

A new powdery mildew resistance gene, designated Pm59, was identified in Afghanistan wheat landrace PI 181356, and mapped in the terminal region of the long arm of chromosome 7A.

Abstract

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is an important foliar disease of wheat worldwide. In the Great Plains of the USA, Bgt isolates virulent to widely used powdery mildew resistance genes, such as Pm3a, were previously identified. The objectives of this study were to characterize the powdery mildew resistance gene in Afghanistan landrace PI 181356, which exhibited high resistance to Bgt isolates collected in southern Great Plains, and identify molecular markers for marker-assisted selection. An F₂ population and F_2:3 lines derived from a cross between PI 181356 and OK1059060-126135-3 were used in this study. Genetic analysis indicated that PI 181356 carries a single dominant gene, designated Pm59, in the terminal region of the long arm of chromosome 7A. Pm59 was mapped to an interval between sequence tag site (STS) markers Xmag1759 and Xmag1714 with genetic distances of 0.4 cM distal to Xmag1759 and 5.7 cM proximal to Xmag1714. Physical mapping suggested that Pm59 is in the distal bin 7AL 0.99–1.00. Pm59 is a novel powdery mildew resistance gene, and confers resistance to Bgt isolates collected from the Great Plains and the state of Montana. Therefore, Pm59 can be used to breed powdery mildew-resistant cultivars in these regions. Xmag1759 is ideal for marker-assisted selection of Pm59 in wheat breeding.

     

<Emphasis Type="Italic">Pm37</Emphasis>, a new broadly effective powdery mildew resistance gene from <Emphasis Type="Italic">Triticum </Emphasis><Emphasis Type="Italic">timopheevii</Emphasis>

Powdery mildew is an important foliar disease in wheat, especially in areas with a cool or maritime climate. A dominant powdery mildew resistance gene transferred to the hexaploid germplasm line NC99BGTAG11 from T. timopheevii subsp. armeniacum was mapped distally on the long arm of chromosome 7A. Differential reactions were observed between the resistance gene in NC99BGTAG11 and the alleles of the Pm1 locus that is also located on chromosome arm 7AL. Observed segregation in F_2:3 lines from the cross NC99BGTAG11 × Axminster (Pm1a) demonstrate that germplasm line NC99BGTAG11 carries a novel powdery mildew resistance gene, which is now designated as Pm37. This new gene is highly effective against all powdery mildew isolates tested so far. Analyses of the population with molecular markers indicate that Pm37 is located 16 cM proximal to the Pm1 complex. Simple sequence repeat (SSR) markers Xgwm332 and Xwmc790 were located 0.5 cM proximal and distal, respectively, to Pm37. In order to identify new markers in the region, wheat expressed sequence tags (ESTs) located in the distal 10% of 7AL that were orthologous to sequences from chromosome 6 of rice were targeted. The two new EST-derived STS markers were located distal to Pm37 and one marker was closely linked to the Pm1a region. These new markers can be used in marker-assisted selection schemes to develop wheat cultivars with pyramids of powdery mildew resistance genes, including combinations of Pm37 in coupling linkage with alleles of the Pm1 locus.     

Marker Assisted Transfer of Two Powdery Mildew Resistance Genes PmTb7A.1 and PmTb7A.2 from Triticum boeoticum (Boiss.) to Triticum aestivum (L.)

Powdery mildew (PM), caused by Blumeria graminis f. sp. tritici, is one of the important wheat diseases, worldwide. Two PM resistance genes, designated as PmTb7A.1 and PmTb7A.2, were identified in T. boeoticum acc. pau5088 and mapped on chromosome 7AL approximately 48cM apart. Two resistance gene analogue (RGA)-STS markers Ta7AL-4556232 and 7AL-4426363 were identified to be linked to the PmTb7A.1 and PmTb7A.2, at a distance of 0.6cM and 6.0cM, respectively. In the present study, following marker assisted selection (MAS), the two genes were transferred to T. aestivum using T. durum as bridging species. As many as 12,317 florets of F₁ of the cross T. durum /T. boeoticum were pollinated with T. aestivum lines PBW343-IL and PBW621 to produce 61 and 65 seeds, respectively, of three-way F₁. The resulting F₁s of the cross T. durum/T. boeoticum//T. aestivum were screened with marker flanking both the PM resistance genes PmTb7A.1 and PmTb7A.2 (foreground selection) and the selected plants were backcrossed to generate BC₁F₁. Marker assisted selection was carried both in BC₁F₁ and the BC₂F₁ generations. Introgression of alien chromatin in BC₂F₁ plants varied from 15.4 - 62.9 percent. Out of more than 110 BC₂F₁ plants showing introgression for markers linked to the two PM resistance genes, 40 agronomically desirable plants were selected for background selection for the carrier chromosome to identify the plants with minimum of the alien introgression. Cytological analysis showed that most plants have chromosome number ranging from 40-42. The BC₂F₂ plants homozygous for the two genes have been identified. These will be crossed to generate lines combining both the PM resistance genes but with minimal of the alien introgression. The PM resistance gene PmTb7A.1 maps in a region very close to Sr22, a stem rust resistance gene effective against the race Ug99. Analysis of selected plants with markers linked to Sr22 showed introgression of Sr22 from T. boeoticum in several BC₂F₁ plants. Thus, in addition to PM resistance, these progeny might also carry resistance to stem rust race Ug99.     

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.     

Identification and mapping of PmG16, a powdery mildew resistance gene derived from wild emmer wheat

The gene-pool of wild emmer wheat, Triticum turgidum ssp. dicoccoides, harbors a rich allelic repertoire for disease resistance. In the current study, we made use of tetraploid wheat mapping populations derived from a cross between durum wheat (cv. Langdon) and wild emmer (accession G18-16) to identify and map a new powdery mildew resistance gene derived from wild emmer wheat. Initially, the two parental lines were screened with a collection of 42 isolates of Blumeria graminis f. sp. tritici (Bgt) from Israel and 5 isolates from Switzerland. While G18-16 was resistant to 34 isolates, Langdon was resistant only to 5 isolates and susceptible to 42 isolates. Isolate Bgt#15 was selected to differentiate between the disease reactions of the two genotypes. Segregation ratio of F_2-3 and recombinant inbreed line (F₇) populations to inoculation with isolate Bgt#15 indicated the role of a single dominant gene in conferring resistance to Bgt#15. This gene, temporarily designated PmG16, was located on the distal region of chromosome arm 7AL. Genetic map of PmG16 region was assembled with 32 simple sequence repeat (SSR), sequence tag site (STS), Diversity array technology (DArT) and cleaved amplified polymorphic sequence (CAPS) markers and assigned to the 7AL physical bin map (7AL-16). Using four DNA markers we established colinearity between the genomic region spanning the PmG16 locus within the distal region of chromosome arm 7AL and the genomic regions on rice chromosome 6 and Brachypodium Bd1. A comparative analysis was carried out between PmG16 and other known Pm genes located on chromosome arm 7AL. The identified PmG16 may facilitate the use of wild alleles for improvement of powdery mildew resistance in elite wheat cultivars via marker-assisted selection.     

Molecular mapping of powdery mildew resistance gene <Emphasis Type="Italic">PmHNK</Emphasis> in winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) cultivar Zhoumai 22

The Chinese winter wheat cultivar Zhoumai 22 is highly resistant to powdery mildew. The objectives of this study were to map a powdery mildew resistance gene in Zhoumai 22 using molecular markers and investigate its allelism with Pm13. A total of 278 F₂ and 30 BC₁ plants, and 143 F₃ lines derived from the cross between resistant cultivar Zhoumai 22 and susceptible cultivar Chinese Spring were used for resistance gene tagging. The 137 F₂ plants from the cross Zhoumai 22/2761-5 (Pm13) were employed for the allelic test of the resistance genes. Two hundred and ten simple sequence repeat (SSR) markers were used to test the two parents, and resistant and susceptible bulks. Subsequently, seven polymorphic markers were used for genotyping the F₂ and F₃ populations. The results indicated that the powdery mildew resistance in Zhoumai 22 was conferred by a single dominant gene, designated PmHNK tentatively, flanked by seven SSR markers Xgwm299, Xgwm108, Xbarc77, Xbarc84, Xwmc326, Xwmc291 and Xwmc687 on chromosome 3BL. The resistance gene was closely linked to Xwmc291 and Xgwm108, with genetic distances of 3.8 and 10.3 cM, respectively, and located on the chromosome bin 3BL-7-0.63-1.0 in the test with a set of deletion lines. Seedling tests with seven isolates of Blumeria graminis f. sp. tritici (Bgt) and allellic test indicated that PmHNK is different from Pm13, and Pm41 seems also to be different from PmHNK due to its origin from T. dicoccoides and molecular evidence. These results indicate that PmHNK is likely to be a novel powdery mildew resistance gene in wheat.     

E3 ubiquitin ligase gene CMPG1–V from Haynaldia villosa L. contributes to powdery mildew resistance in common wheat (Triticum aestivum L.)

Powdery mildew is one of the most devastating wheat fungal diseases. A diploid wheat relative, Haynaldia villosa L., is highly resistant to powdery mildew, and its genetic resource of resistances, such as the Pm21 locus, is now widely used in wheat breeding. Here we report the cloning of a resistance gene from H. villosa, designated CMPG1–V, that encodes a U–box E3 ubiquitin ligase. Expression of the CMPG1–V gene was induced in the leaf and stem of H. villosa upon inoculation with Blumeria graminis f. sp. tritici (Bgt) fungus, and the presence of Pm21 is essential for its rapid induction of expression. CMPG1–V has conserved key residues for E3 ligase, and possesses E3 ligase activity in vitro and in vivo. CMPG1–V is localized in the nucleus, endoplasmic reticulum, plasma membrane and partially in trans‐Golgi network/early endosome vesicles. Transgenic wheat over‐expressing CMPG1–V showed improved broad‐spectrum powdery mildew resistance at seedling and adult stages, associated with an increase in expression of salicylic acid‐responsive genes, H₂O₂ accumulation, and cell‐wall protein cross‐linking at the Bgt infection sites, and the expression of CMPG1–V in H. villosa was increased when treated with salicylic acid, abscisic acid and H₂O₂. These results indicate the involvement of E3 ligase in defense responses to Bgt fungus in wheat, particularly in broad‐spectrum disease resistance, and suggest association of reactive oxidative species and the phytohormone pathway with CMPG1–V‐mediated powdery mildew resistance.     

Chromosomal Location and Comparative Genomics Analysis of Powdery Mildew Resistance Gene Pm51 in a Putative Wheat-Thinopyrum ponticum Introgression Line

Powdery mildew (PM) is a very destructive disease of wheat (Triticum aestivum L.). Wheat-Thinopyrum ponticum introgression line CH7086 was shown to possess powdery mildew resistance possibly originating from Th. ponticum. Genomic in situ hybridization and molecular characterization of the alien introgression failed to identify alien chromatin. To study the genetics of resistance, CH7086 was crossed with susceptible genotypes. Segregation in F₂ populations and F_2:3 lines tested with Chinese Bgt race E09 under controlled conditions indicated that CH7086 carries a single dominant gene for powdery mildew resistance. Fourteen SSR and EST-PCR markers linked with the locus were identified. The genetic distances between the locus and the two flanking markers were 1.5 and 3.2 cM, respectively. Based on the locations of the markers by nullisomic-tetrasomic and deletion lines of ‘Chinese Spring’, the resistance gene was located in deletion bin 2BL-0.89-1.00. Conserved orthologous marker analysis indicated that the genomic region flanking the resistance gene has a high level of collinearity to that of rice chromosome 4 and Brachypodium chromosome 5. Both resistance specificities and tests of allelism suggested the resistance gene in CH7086 was different from previously reported powdery mildew resistance genes on 2BL, and the gene was provisionally designated PmCH86. Molecular analysis of PmCH86 compared with other genes for resistance to Bgt in the 2BL-0.89-1.00 region suggested that PmCH86 may be a new PM resistance gene, and it was therefore designated as Pm51. The closely linked flanking markers could be useful in exploiting this putative wheat-Thinopyrum translocation line for rapid transfer of Pm51 to wheat breeding programs.     

Pyramiding of transgenic <Emphasis Type="Italic">Pm3</Emphasis> alleles in wheat results in improved powdery mildew resistance in the field

Key message

The combined effects of enhanced total transgene expression level and allele-specificity combination in transgenic allele-pyramided Pm3 wheat lines result in improved powdery mildew field resistance without negative pleiotropic effects.

Abstract

Allelic Pm3 resistance genes of wheat confer race-specific resistance to powdery mildew (Blumeria graminis f. sp. tritici, Bgt) and encode nucleotide-binding domain, leucine-rich repeat (NLR) receptors. Transgenic wheat lines overexpressing alleles Pm3a, b, c, d, f, and g have previously been generated by transformation of cultivar Bobwhite and tested in field trials, revealing varying degrees of powdery mildew resistance conferred by the transgenes. Here, we tested four transgenic lines each carrying two pyramided Pm3 alleles, which were generated by crossbreeding of lines transformed with single Pm3 alleles. All four allele-pyramided lines showed strongly improved powdery mildew resistance in the field compared to their parental lines. The improved resistance results from the two effects of enhanced total transgene expression levels and allele-specificity combinations. In contrast to leaf segment tests on greenhouse-grown seedlings, no allelic suppression was observed in the field. Plant development and yield scores of the pyramided lines were similar to the mean scores of the corresponding parental lines, and thus, the allele pyramiding did not cause any negative effects. On the contrary, in pyramided line, Pm3b × Pm3f normal plant development was restored compared to the delayed development and reduced seed set of parental line Pm3f. Allele-specific RT qPCR revealed additive transgene expression levels of the two Pm3 alleles in the pyramided lines. A positive correlation between total transgene expression level and powdery mildew field resistance was observed. In summary, allele pyramiding of Pm3 transgenes proved to be successful in enhancing powdery mildew field resistance.

     

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.     

几个四倍体小麦-山羊草双二倍体及其部分亲本的抗小麦白粉病基因分析

用离体叶段接种方法鉴定了11个四倍体小麦一山羊草双二倍体、波斯小麦PS5、硬粒小麦DR147、5份山羊草、杂交高代材料Am9／莱州953*^2F5和(DR147／Ael4)／／莱州953*^2F4对20个具有不同毒力白粉菌株的抗谱。通过与含有已知抗病基因品种或品系的反应模式比较,推测Am9／莱州953*^2F5含有Pm4b,波斯小麦PS5含有Pm4b与一个未知抗病基因组合;(DR147／Ael4)／／莱州953*^2F4和硬粒小麦DR147含有Pm4a和一个未知抗病基因组合;尾状山羊草Ael4和小伞山羊草Y39抗所有白粉菌株,由于迄今还没有在尾状山羊草和小伞山羊草中鉴定出抗白粉病基因,推测这2份山羊草含有新的抗白粉病基因。除Am9外,在其它双二倍体中波斯小麦或硬粒小麦的抗性部分受到抑制。山羊草的抗性部分或完全量到抑制。     

The adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29 are important determinants of partial resistance to powdery mildew in bread wheat line Saar

Powdery mildew, caused by Blumeria graminis f. sp. tritici is a major disease of wheat (Triticum aestivum L.) that can be controlled by resistance breeding. The CIMMYT bread wheat line Saar is known for its good level of partial and race non-specific resistance, and the aim of this study was to map QTLs for resistance to powdery mildew in a population of 113 recombinant inbred lines from a cross between Saar and the susceptible line Avocet. The population was tested over 2 years in field trials at two locations in southeastern Norway and once in Beijing, China. SSR markers were screened for association with powdery mildew resistance in a bulked segregant analysis, and linkage maps were created based on selected SSR markers and supplemented with DArT genotyping. The most important QTLs for powdery mildew resistance derived from Saar were located on chromosomes 7DS and 1BL and corresponded to the adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29. A major QTL was also located on 4BL with resistance contributed by Avocet. Additional QTLs were detected at 3AS and 5AL in the Norwegian testing environments and at 5BS in Beijing. The population was also tested for leaf rust (caused by Puccinia triticina) and stripe rust (caused by P. striiformis f. sp. tritici) resistance and leaf tip necrosis in Mexico. QTLs for these traits were detected on 7DS and 1BL at the same positions as the QTLs for powdery mildew resistance, and confirmed the presence of Lr34/Yr18 and Lr46/Yr29 in Saar. The powdery mildew resistance gene at the Lr34/Yr18 locus has recently been named Pm38. The powdery mildew resistance gene at the Lr46/Yr29 locus is designated as Pm39.     

Isoprenylation modification is required for HIPP1-mediated powdery mildew resistance in wheat

Powdery mildew (Pm), caused by Blumeria graminis f.sp. tritici (Bgt), is one of the most important wheat diseases. Heavy-metal-associated isoprenylated plant protein (HIPP1) has been proved playing important roles in response to biotic and a biotic stress. In present study, we proved HIPP1-V from Haynalidia villosa is a positive regulator in Pm resistance. HIPP1-V was rapidly induced by Bgt. Transiently or stably heterologous overexpressing HIPP1-V in wheat suppressed the haustorium formation and enhanced Pm resistance. HIPP1-V isoprenylation was critical for plasma membrane (PM) localization, interaction with E3-ligase CMPG1-V and function in Pm resistance. Bgt infection recruited the isoprenylated HIPP1-V and CMPG1s on PM; blocking the HIPP1 isoprenylation reduced such recruitment and compromised the resistance of OE-CMPG1-V and OE-HIPP1-V. Overexpressing HIPP1-VC^148G could not enhance Pm resistance. These indicated the Pm resistance was dependent on HIPP1-V's isoprenylation. DGEs related to the ROS and SA pathways were remarkably enriched in OE-HIPP1-V, revealing their involvement in Pm resistance. Our results provide evidence on the important role of protein isoprenylation in plant defense.     

Genetic characterization of powdery mildew resistance in U.S. hard winter wheat

Powdery mildew significantly affects grain yield and end-use quality of winter wheat in the southern Great Plains. Employing resistance resources in locally adapted cultivars is the most effective means to control powdery mildew. Two types of powdery mildew resistance exist in wheat cultivars, i.e., qualitative and quantitative. Qualitative resistance is controlled by major genes, is race-specific, is not durable, and is effective in seedlings and in adult plants. Quantitative resistance is controlled by minor genes, is non-race-specific, is durable, and is predominantly effective in adult plants. In this study, we found that the segregation of powdery mildew resistance in a population of recombinant inbred lines developed from a cross between the susceptible cultivar Jagger and the resistant cultivar 2174 was controlled by a major QTL on the short arm of chromosome 1A and modified by four minor QTLs on chromosomes 1B, 3B, 4A, and 6D. The major QTL was mapped to the genomic region where the Pm3 gene resides. Using specific PCR markers for seven Pm3 alleles, 2174 was found to carry the Pm3a allele. Pm3a explained 61% of the total phenotypic variation in disease reaction observed among seedlings inoculated in the greenhouse and adult plants grown in the field and subjected to natural disease pressure. The resistant Pm3a allele was present among 4 of 31 cultivars currently being produced in the southern Great Plains. The genetic effects of several minor loci varied with different developmental stages and environments. Molecular markers associated with these genetic loci would facilitate incorporating genetic resistance to powdery mildew into improved winter wheat cultivars.     

<Emphasis Type="Italic">Pm61</Emphasis>: a recessive gene for resistance to powdery mildew in wheat landrace Xuxusanyuehuang identified by comparative genomics analysis

Key message

A single recessive powdery mildew resistance gene Pm61 from wheat landrace Xuxusanyuehuang was mapped within a 0.46-cM genetic interval spanning a 1.3-Mb interval of the genomic region of chromosome arm 4AL.

Abstract

Epidemics of powdery mildew incited by the biotrophic fungus Blumeria graminis f. sp. tritici (Bgt) have caused significant yield reductions in many wheat (Triticum aestivum)-producing regions. Identification of powdery mildew resistance genes is required for sustainable improvement of wheat for disease resistance. Chinese wheat landrace Xuxusanyuehuang was resistant to several Bgt isolates at the seedling stage. Genetic analysis based on the inoculation of Bgt isolate E09 on the F₁, F₂, and F_2:3 populations produced by crossing Xuxusanyuehuang to susceptible cultivar Mingxian 169 revealed that the resistance of Xuxusanyuehuang was controlled by a single recessive gene. Bulked segregant analysis and simple sequence repeat (SSR) mapping placed the gene on chromosome bin 4AL-4-0.80-1.00. Comparative genomics analysis was performed to detect the collinear genomic regions of Brachypodium distachyon, rice, sorghum, Aegilops tauschii, T. urartu, and T. turgidum ssp. dicoccoides. Based on the use of 454 contig sequences and the International Wheat Genome Sequence Consortium survey sequence of Chinese Spring wheat, four EST-SSR and seven SSR markers were linked to the gene. An F₅ recombinant inbred line population derived from Xuxusanyuehuang?×?Mingxian 169 cross was used to develop the genetic linkage map. The gene was localized in a 0.46-cM genetic interval between Xgwm160 and Xicsx79 corresponding to 1.3-Mb interval of the genomic region in wheat genome. This is a new locus for powdery mildew resistance on chromosome arm 4AL and is designated Pm61.

     

Chromosomal location of a gene suppressing powdery mildew resistance genes Pm8 and Pm17 in common wheat (Triticum aestivum L. em. Thell.)

The chromosomal location of a suppressor for the powdery mildew resistance genes Pm8 and Pm17 was determined by a monosomic set of the wheat cultivar Caribo. This cultivar carries a suppressor gene inhibiting the expression of Pm8 in cv Disponent and of Pm17 in line Helami-105. In disease resistance assessments, monosomic F₁ hybrids (2n=41) of Caribo x Disponent and Caribo x Helami-105 lacking chromosome 7D were resistant, whereas monosomic F₁ hybrids involving the other 20 chromosomes, as well as disomic F₁ hybrids (2n=42) of all cross combinations, were susceptible revealing that the suppressor gene for Pm8 and Pm17 is localized on chromosome 7D. It is suggested that genotypes without the suppressor gene be used for the exploitation of genes Pm8 and Pm17 in enhancing powdery mildew resistance in common wheat.