Introduction

Powdery mildew (caused by Blumeria graminis f. sp. tritici, Bgt) is a destructive wheat disease all over the world, causing not only significant yield loss but also severe quality deterioration (Bowen et al. 1991; Everts and Leath 1992). Resistant cultivars are the most economical and environmental strategy to reduce the prevalence of powdery mildew, considering that fungicide application can cause environmental problems and likely future acquisition of fungicide tolerance by the pathogen (Huang et al. 1997; Paillard et al. 2000; Huang and Roder 2004).

More than 70 powdery mildew resistance alleles have been identified, a number of which were introduced from wild relatives of common wheat (Friebe et al. 1996; Gill et al. 2011; Mohler et al. 2013; McIntosh et al. 2014; Petersen et al. 2015), such as Triticum monococcum (2n = 2x = 14; genome AA) (Shi et al. 1998; Yao et al. 2007; Schmolke et al. 2012), Aegilops tauschii (2n = 2x = 14; genome DD) (Miranda et al. 2006, 2007; Schneider et al. 2008), Secale cereale L. (2n = 2x = 14; genome RR) (Friebe et al. 1994; Mohler et al. 2001), Haynaldia villosa L. (2n = 2x = 14; genome VV) (Chen et al. 1995, 2013; Xie et al. 2012; Zhang et al. 2012), and Thinopyrum intermedium (2n = 6x = 42; genome JJJsJsSS) (He et al. 2009; Liu et al. 2014; Shen et al. 2015). However, some powdery mildew resistance genes, such as Pm8 from S. cereale L. have become ineffective due to changes in the pathogen population (Hsam and Zeller 2002; McDonald and Linde 2002; Parks et al. 2008). Consequently, researchers are constantly seeking novel germplasms with broad-spectrum resistance to the disease.

Various molecular markers, including restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), simple sequence repeat (SSR), and single nucleotide polymorphism (SNP), have been used to map powdery mildew resistance genes in wheat. SSR markers including genomic SSR or EST-derived SSR markers have most commonly been used (Röder et al. 1998; Paillard et al. 2003; Sourdille et al. 2004; Somers et al. 2004; Yu et al. 2004). Currently, SNP markers are becoming increasingly popular (Akhunov et al. 2009; Berard et al. 2009; Chao et al. 2010; Lai et al. 2012; Allen et al. 2013), followed by the development of the wheat 9 and 90 K SNP chip platforms (Cavanagh et al. 2013; Avni et al. 2014; Wang et al. 2014). Upon SNP identification, various systems have been devised for SNP profiling, such as single strand conformation polymorphism (SSCP), kompetitive allele specific PCR (KASPar) assay, and fluorescent high resolution DNA melting (HRM) analysis. HRM analysis, a powerful tool for discrimination of a single SNP, has been successfully used in wheat (Matsuda et al. 2012; Tan et al. 2013; Terracciano et al. 2013).

Agropyron cristatum (L.) Gaertn (2n = 4x = 28; genomes PPPP), a perennial species of the Triticeae, harbors many favorable traits that can be exploited for wheat genetic improvement (Dewey 1984; Dong et al. 1992), such as enhanced fertile tiller number per plant (Ye et al. 2015), high grain number per spike (Wu et al. 2006; Luan et al. 2010), and high resistance to powdery mildew and other diseases (Han et al. 2014; Lu et al. 2015). Various wheat-A. cristatum derivative lines with elite traits including addition, translocation and introgression lines have been produced (Wu et al. 2006; Han et al. 2014; Ye et al. 2015; Lu et al. 2015; Zhang et al. 2015a). Pubing 74 is a putative wheat-A. cristatum introgression line, which displays a high level of resistance to powdery mildew at both the seedling and adult stages. However, the genetic basis of the resistance was still uncharacterized. We determined the chromosomal location of the resistance gene and also evaluated its effectiveness against a comprehensive set of Bgt isolates from a wide range of wheat-producing regions in China.

Materials and methods

Materials

Pubing 74 was originally selected from a wide cross between the common wheat cultivar (cv.) Fukuhokomugi (Fukuho) and A. cristatum (accession No. Z559). Common wheat cv. Mingxian 169 highly susceptible to powdery mildew was crossed with Pubing 74 to generate F2 and F2-derived F3 family populations. Common wheat cv. Zhongzuo 9504 was used as the susceptible control in the powdery mildew assessment. Wheat lines with known powdery mildew resistance genes on wheat chromosome arm 5DS, including wheat landrace derivative Ulka/8*Cc (Briggle 1966; Qiu et al. 2006), German wheat cv. Tabasco (Gao et al. 2012), Chinese wheat cv. Liangxing 66 (Huang et al. 2012), wheat-A. cristatum introgression line PB3558 (Lu et al. 2015), common wheat line D57 (Ma et al. 2011), indigenous germplasm X3986-2 (Ma et al. 2014), and Chinese breeding line KM2939 (Ma et al. 2015), were used in this study. Tabasco was provided by Dr. Shibin Cai (Jiangsu Academy of Agricultural Sciences), D57 by Dr. Zhengqiang Ma (Nangjing Agricultural University), X3986-2 and KM2939 by Dr. Diaoguo An (Chinese Academy of Sciences, Shijiazhuang, Hebei province). The other wheat lines are maintained by our laboratory. All 28 single spore-derived Bgt isolates used in this study were kindly provided by Dr. Hongjie Li (Institute of Crop Science, Chinese Academy of Agricultural Sciences).

Genomic in situ hybridization and meiosis observation

Genomic in situ hybridization (GISH) was conducted according to Luan et al. (2010). Agropyron cristatum and Fukuho genomic DNA were used as the probe and blocker, respectively. Agropyron cristatum genomic DNA was labeled with Dig-Nick-Translation Mix (Roche, Mannheim, Germany). Wheat and A. cristatum chromosomes were pseudo-colored as blue and red, respectively. The procedure used for meiotic studies was described by Jauhar and Peterson (2006). Young spikes from Pubing 74 with pollen mother cells (PMCs) at the metaphase I (MI) stage were fixed in Carnoy solution (6-ethanol: 3-chloroform: 1-acetic acid, by vol.) for 24 h and stored at 4 °C until used. All cytological images were taken under a Nikon Eclipse E600 fluorescence microscope and captured with a CCD camera (Diagnostic Instruments, Sterling Heights, MI, USA).

Evaluation of powdery mildew response

The prevailing Bgt isolate E09 was used to test Pubing 74 × Mingxian 169 F1 hybrids, an F2 population, and F2-derived F3 families at the seedling stage. Seedlings at the one-leaf-stage were inoculated by dusting conidiospores of Bgt isolate E09 from susceptible cv. Zhongzuo 9504. Infection types (ITs) were scored on the first leaf using a 0–4 scale around 15 days after inoculation (Liu et al. 2002). Scores of 0–2 were classified as resistant and 3–4 as susceptible. Twenty seedlings of each line in the F2:3 population were tested against Bgt isolate E09 to determine the genotypes of the F2 individuals. We used 20 seedlings for each line, since this was adequate to reduce the probability of erroneously determining a heterozygous plant as homozygous resistant to 0.3 %, and to reduce the probability of determining a heterozygous plant as homozygous susceptible to 9e−13.

Twenty-eight Bgt isolates originating from different wheat-producing regions of Northern China (Sun et al. 2015) were used to compare the reaction patterns of Pubing 74 and lines with known alleles on chromosome arm 5DS. The reactions to 28 Bgt isolates were determined using detached leaf segments as described by Limpert et al. (1988). Three leaf segments from different plants of each genotype were examined and the tests were repeated three times. Besides, a mixture of Bgt isolates mainly composed of E09 was employed to inoculate the populations as well as two parents in the field at the adult stage. All plants were sown in 2.0 m rows, spaced 0.3 m apart. The susceptible control cv. Zhongzuo 9504 was planted in every fifth row to ensure that all plants were evenly infected. Disease reactions were scored using a 0–9 scale at the ear emergence and milky ripe stages.

DNA extraction and bulked segregant analysis

Genomic DNA was isolated from leaves of young seedlings following the CTAB method (Allen et al. 2006). To detect A. cristatum chromosomal fragments in Pubing 74, three sequence-tagged-site (STS) markers (Agc2970, Agc6287 and Agc21686) designed according to the expressed sequence tags (EST) of the A. cristatum transcriptome (Zhang et al. 2015b). Bulked segregant analysis (BSA) was applied as described by Michelmore et al. (1991). Briefly, resistant and susceptible DNA bulks were constructed by separately mixing equal amounts of DNA from 20 homozygous resistant (IT = 0) and 20 homozygous susceptible (IT = 4) F2 plants, homozygosity being established on the basis of progeny testing.

Genotyping with publicly available and new developed molecular markers

SSR markers evenly distributed across all the wheat chromosomes were used for a polymorphism survey on the two parents and two DNA bulks, and polymorphic markers were subsequently used for genotyping the segregating population. A series of Xbwm SSR markers, which were developed recently and located on wheat chromosome arm 5DS (Lu et al. 2015), were chosen for genotyping. Three HRM markers were designed based on the flanking sequences of SNPs, taking the sequences and their physical locations on wheat chromosome arm 5DS as reference (Jia et al. 2013). PCR was performed in 10 µL volumes containing 0.1 U Taq DNA polymerase, 2 µM forward and reverse primers, and approximately 30 ng of template DNA. The PCR amplification conditions were: 35 cycles at 95 °C for 30 s, 55–65 °C for 30 s and 72 °C for 30 s with an initial denaturation at 95 °C for 2 min and a final extension step at 72 °C for 5 min.

When PCR was carried out with HRM markers, 1 µL 10 x LC Green (Idaho Technology, Salt Lake City, UT, USA) was included in 10 µL PCR volumes. The resulting PCR products were analyzed using a light scanner (Idaho Technology), by ramping the temperature from 55 to 95 °C at 0.1 °C per second. Data were analyzed using the analytical light scanning software, and the amplification patterns of the HRM markers were shown in two different ways: normalized melting peaks and normalized melting curves.

Statistical analysis and linkage map construction

Chi squared (χ 2) tests were used to compare the observed and theoretically expected ratios. Polymorphic markers were used for constructing a linkage map flanking the powdery mildew resistance gene in Pubing 74, and a linkage map was established with Mapmaker 3.0 and Mapdraw (Lincoln et al. 1993; Liu and Meng 2003). The LOD score threshold was set at 3.0 and a maximum genetic distance at 50 cM. The Kosambi mapping function was used to estimate genetic distances between linked markers and the powdery mildew resistance gene based on recombination values.

Results

Pubing 74 is a novel wheat-A. cristatum introgression line

Pubing 74 was produced from a wide cross between A. cristatum and wheat cv. Fukuho, and then selected over five selfing generations. Pubing 74 displayed full fertility and normal agronomic performance, such as 56–73 grains per spike, and 15–20 fertile tillers per plant (Fig. 1a). GISH was performed to determine whether Pubing 74 is a wheat-A. cristatum introgression line. No visible hybridization signal was detected in Pubing 74 (Fig. 1b), despite strong hybridization signals in the positive control (Fig. 1c). However, evidence for the presence of A. cristatum chromatin was provided by STS markers specific to the A. cristatum P genome. STS markers Agc2970, Agc6287 and Agc21686 were amplified in A. cristatum and Pubing 74, but not in Fukuho or Mingxian 169 (Fig. 1d). These results indicated that A. cristatum chromatin was present in Pubing 74, but was presumably too small to be detected by the standard cytological methods. Meiotic metaphase PMC cells showed normal 21 bivalent chromosome pairing in Pubing 74 (Fig. 1e), indicating that Pubing 74 could be a stable wheat-A. cristatum introgression line.

Fig. 1
figure 1

Identification of wheat-A. cristatum introgression line Pubing 74. a Normal appearance of a Pubing 74 plant. b, c GISH detection of Pubing 74 (b) and one wheat-A. cristatum translocation line WAT12-9 as the positive control (c). d PCR patterns of three STS markers specific to A. cristatum (Agc2970, Agc6287 and Agc21686). Lanes M DNA ladder; 1 A. cristatum; 2 Fukuho; 3 Pubing 74; 4 Mingxian 169. e PMC from Pubing 74 with 21 bivalents at meiotic metaphase I (MI)

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Inheritance of powdery mildew resistance in Pubing 74

Pubing 74 was highly resistant under natural field epidemic conditions over several years. It was also resistant at the seeding and adult plant stages under controlled conditions (Fig. 2). The inheritance data presented in Table 1 showed that resistance to Bgt race E09 was conferred by a single dominant gene, which was tentatively designated PmPB74. Results from seedling and adult plant tests were identical.

Fig. 2
figure 2

Responses to powdery mildew of Pubing 74 at the adult (a) and seedling (b) stages. 1 A. cristatum; 2 Fukuho; 3 Pubing 74; 4 Mingxian 169; 5 Zhongzuo 9504

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Table 1 Genetic analysis of powdery mildew resistance to the Bgt isolate E09 in Pubing 74 × Mingxian 169 F1, F2 and F2:3 populations
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Chromosomal location of PmPB74 in Pubing 74

Three hundred and seventy-eight wheat SSR markers distributed randomly throughout the wheat genome were screened for polymorphisms between the parents and DNA bulks, and nine markers showed polymorphisms. Among these nine markers, only Xcfd81 showed evidence of linkage with PmPB74. Since Xcfd81 was known to be on chromosome arm 5DS, additional SSR markers on that chromosome arm were surveyed for polymorphisms. Four SSR markers (Xgpw5201, Xwmc805, Xgpw302 and Xcfd40) were polymorphic and linked to PmPB74 (Fig. 3). Four of 25 Xbwm SSR markers, and three of 15 HRM markers were also polymorphic and linked to PmPB74 (Fig. 3; Table 2). The linkage map based on the powdery mildew response and marker data indicated that PmPB74 was flanked by markers Xcfd81 and HRM02 at genetic distances of 2.5 and 1.7 cM, respectively (Fig. 3). Xcfd81 was earlier reported to be located on the bin C-5DS1-0-0.63, hence PmPB74 is also likely to be located on this bin. However, the three STS markers specific to A. cristatum were not linked with PmPB74, nor were they linked to each other, suggesting that there were multiple small chromosome segments from A. cristatum distributed at different chromosomal locations in Pubing 74. Amplification patterns of two SSR markers (Xcfd81 and Xbwm25) and two HRM markers (HRM01 and HRM02) are shown as examples in Figs. 4, 5, respectively.

Fig. 3
figure 3

Genetic mapping of PmPB74 and comparison with the documented powdery mildew genes on wheat chromosome 5DS. Genetic distances in cM are shown on the left, and black arrows point to the centromeres

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Table 2 Primer sequences of all the markers used in this study
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Fig. 4
figure 4

Amplification profiles of SSR markers Xcfd81 (a) and Xbwm25 (b). Lanes M DNA ladder; PR resistant parent Pubing 74; PS susceptible parent Mingxian 169; BR resistant DNA bulk, BS susceptible DNA bulk; HR homozygous resistant F2 individual; HZ heterozygous resistant F2 individual; HS homozygous susceptible F2 individual. Polymorphic PCR bands are indicated by arrows

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Fig. 5
figure 5

Amplification profiles of HRM markers: HRM01 (a) and HRM02 (b). Left and right panels display normalized melting peaks and normalized melting curves for the same amplicons, respectively. Lines with different colours represent different genotypes: red lines represent Mingxian 169 and the homozygous susceptible F2 plants; blue lines represent Pubing 74 and the homozygous resistant F2 plants; gray lines represent heterozygous F2 plants

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Comparative reactions of Pubing 74 and other lines with known powdery mildew resistance genes on chromosome arm 5DS to 28 Bgt isolates

To determine the relationships of PmPB74 with other genes located on chromosome 5DS, 28 Bgt isolates including E09 from different wheat-producing regions of Northern China were used in inoculations. As shown in Table 3, Pubing 74 and A. cristatum were resistant to all the 28 isolates tested. However, Ulka/8*Cc (Pm2) was susceptible to 12 isolates, Tabasco (Pm48) was susceptible to eight isolates, Liangxing 66 (PmLX66) was susceptible to 12 isolates, and PB3558 (PmPB3558) was susceptible to seven isolates. The disease reactions of seven lines to six Bgt isolates as examples are shown in Fig. 6. Besides, seven isolates were used to compare the reactions of Pubing 74 and three lines (D57, X3986-2 and KM2939). As shown in Table 4, both D57 and X3986-2 were each susceptible to two Bgt isolates, and KM2939 was susceptible to E21. Therefore, Pubing 74 displayed a broader spectrum of disease resistance than any other wheat line tested above. Finally, when 20 F2:3 lines homozygous resistant and 20 F2:3 lines homozygous susceptible to isolate E09 were tested with the other 27 isolates at the seedling stage, all these lines showed disease reactions identical to those inoculated with E09, thus showing that PmPB74 conferred resistance to all the isolates.

Table 3 Comparative reactions to 28 Bgt isolates on Pubing 74 and other lines with known powdery mildew resistance genes on 5DS
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Fig. 6
figure 6

Powdery mildew reactions of Pubing 74, Ulka/8*Cc, Tabasco, Liangxing 66, PB3558, Mingxian 169 and Zhongzuo 9504 to six Bgt isolates

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Table 4 Disease reactions to seven Bgt isolates on Pubing 74, D57-5D, X3986 and KM2939
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PmPB74 and PmPB3558 were both derived from A. cristatum accession Z559. To determine whether PmPB74 was PmPB3558, the Bgt isolate Bg44-6, which was virulent to PmPB3558 (and Mingxian 169) but avirulent to PmPB74, was used to inoculate the Pubing 74 × Mingxian 169 F2:3 population at the seedling stage. A segregation pattern identical with that of E09 was obtained and PmPB74 was assigned to the same chromosomal location, indicating that PmPB74 was indeed not identical to PmPB3558. To further determine whether PmPB74 and PmPB3558, and also PmPB74 and Pm2 were different alleles in a single locus, crosses between Pubing 74 and Ulka/8*Cc, and between Pubing 74 and PB3558, respectively, were made. When large F2 seedling populations were tested with isolate E09, which was avirulent to all three host lines, two of 6986 F2 plants from Pubing 74 × Ulka/8*Cc were susceptible to powdery mildew (IT = 4), and just one of 2260 F2 plants from Pubing 74 × PB3558 displayed susceptibility to powdery mildew (IT = 4). Thus, PmPB74 was not allelic to Pm2 nor to PmPB3558. To confirm the conclusion that PmPB74 represents a novel locus, we inoculated Pubing 74  × PB3558 F2:3 lines with E09, and two lines showing 3 resistant:1 susceptible segregation were identified from 168 F2:3 lines. Similarly, we tested the relationship between PmPB74 and Pm2 in Pubing 74 × Ulka/8*Cc F2:3 lines, and two segregating lines were identified from 392 F2:3 lines.

Discussion

Introgression lines are valuable sources of resistance genes in common wheat

The wild relatives of wheat have been used effectively as donors of desirable genes conferring superior agronomic traits. However, large alien chromosomal fragments may carry additional genes that confer undesirable traits in wheat, a phenomenon known as ‘linkage drag’. Therefore, smaller alien chromosomal fragments are generally preferred. Nevertheless, small alien chromosomal fragments with desirable genes are sometimes too small to be detected by standard cytological methods. Examples include the alien introgression of small chromosomal fragments from Dasypyrum villosum and Thinopyrum ponticum to wheat (Caceres et al. 2012; Chen et al. 2012), rust resistance genes introgressed from Ae. geniculata and Ae. triuncialis to wheat (Kuraparthy et al. 2007a, b), and powdery mildew and stripe rust resistance genes putatively derived from T. intermedium (He et al. 2009; Liu et al. 2013; Huang et al. 2014). Pubing 74, was obtained from distant hybridization between the common wheat cv. Fukuho and A. cristatum, displayed high resistance to powdery mildew at both the seedling and adult stages. The putative chromosomal fragment from A. cristatum was below the detection limit of GISH, but evidence for other non-GISH-detectable introgressions was provided by the presence of three STS markers specific to A. cristatum. However, these markers were genetically independent of PmPB74 and also independent of each other, suggesting that there were multiple small chromosome segments from A. cristatum distributed at different chromosomal locations in Pubing 74. Therefore, further studies are needed to develop more markers specific to A. cristatum and also linked to PmPB74. Nevertheless, the resistance in Pubing 74 was presumably derived from A. cristatum, considering that A. cristatum was the only parent highly resistant to powdery mildew.

PmPB74 is a novel resistance gene

PmPB74 occupied a different chromosomal position and conferred a different array of powdery mildew response compared with other genes at or near the Pm2 locus. PmPB74 and PmPB3558 are present in wheat-A. cristatum introgression lines, both of which display high levels of resistance to powdery mildew at both the seedling and adult stages (Lu et al. 2015). However, PmPB74 and PmPB3558 conferred different response spectra, and an allelism test indicated that they were not allelic. Similar results were also obtained for PmPB74 and Pm2. We also compared the response spectra of lines carrying PmPB74 and other resistance genes located on wheat chromosome 5DS. When inoculated with 28 Bgt isolates, the lines with Pm48 and PmLX66 was susceptible to 8 and 12 Bgt isolates, respectively (Table 3). When inoculated with seven out of 28 Bgt isolates, the lines with PmD57-5D and PmX3986-2 were susceptible to two isolates, and KM2939 was susceptible to one isolate (Table 4). By contrast, PmPB74 was highly resistant to all the 28 Bgt isolates, showing that the resistance spectrum of PmPB74 was broader than all of the other genes mentioned above. We concluded that PmPB74 is a novel gene.

PmPB74 is potentially valuable for resistance breeding

Most powdery mildew resistance sources derived from wild relatives of wheat are usually not directly applicable in wheat breeding because of linkage drag. Pubing 74 not only displayed a broad-spectrum of resistance against Bgt isolates from northern China, but also exhibited superior agronomic performance without linkage drag. Therefore, Pubing 74 was an ideal germplasm potentially useful to enhance the powdery mildew resistance at different wheat genetic backgrounds. In this study, no virulence to PmPB74 was found; therefore, identification of PmPB74 and its closely linked markers will be useful for breeders to combine it with other powdery mildew resistance genes. Actually, transferring PmPB74 into different commercial wheat varieties is currently being conducted in our group, and a range of powdery mildew resistant introgression lines have been obtained. In conclusion, the identification of PmPB74 reported here and its corresponding closely linked molecular markers will be beneficial to increasing the diversity of the genetic sources of powdery mildew resistance.

Author contribution statement

L. H. Li and Y. Q. Lu designed the research; M. M. Yao, Y. Q. Lu, J. P. Zhang, L. Q. Song, W. H. Liu, X. M. Yang and X. Q. Li performed technical work; Y. Q. Lu and M. M. Yao analyzed the data. Y. Q. Lu wrote the manuscript.