Molecular mapping of powdery mildew resistance genes in wheat: A review

Powdery mildew, caused by Blumeria graminis f. sp. tritici (syn. Erysiphe graminis f. sp. tritici), is one of the most important diseases of common wheat (Triticum aestivum L.) worldwide. Molecular mapping and cloning of genes for resistance to powdery mildew in hexaploid wheat will facilitate the study of molecular mechanisms underlying resistance to powdery mildew diseases and help understand the structure and function of powdery mildew resistance genes, and permit marker-assisted selection in breeding programs. So far, 48 genes/alleles for resistance to powdery mildew at 32 loci have been identified and located on 16 different chromosomes, of which 21 resistance genes/alleles have been tagged by restriction fragment length polymorphisms (RFLPs), random-amplified polymorphic DNAs (RAPDs), amplified fragment length polymorphisms (AFLPs), sequence characterized amplified regions (SCARs), sequence-tagged sites (STS) or simple sequence repeats (SSRs). Several quantitative trait loci (QTLs) for adult plant resistance (APR) to powdery mildew have been associated with molecular markers. The detailed information on chromosomal location and molecular mapping of these genes has been reviewed. Isolation of powdery mildew resistance genes and development of valid molecular markers for pyramiding resistance genes in breeding programs is also discussed.     

Molecular mapping of partial resistance to powdery mildew in winter wheat cultivar Folke

The Swedish winter wheat (Triticum aestivum L.) cultivar Folke has a long record of partial and race non-specific resistance to powdery mildew (caused by Blumeria graminis f. sp. tritici) in the field. The aim of the present study was to map the main genetic factors behind the partial resistance in Folke and identify molecular markers for use in marker-assisted selection. A population of 130 recombinant inbred lines was developed from a cross between Folke and the moderately susceptible spring wheat line T2038. The population was tested for powdery mildew resistance over two years at two locations in Norway and genotyped with DArT and SSR markers. Composite interval mapping detected a total of eight quantitative trait loci (QTL) for powdery mildew resistance; six with resistance from Folke on 2BS, 2DL, 5AL, 5BS and 6BS and two with resistance from T2038 on 5BS and 7AL. None of the loci with resistance from Folke mapped to chromosomal regions with known race-specific resistance genes, which confirmed the race non-specific nature of the resistance in this cultivar. The molecular markers linked to the reported QTL will be useful as a tool for selecting partial and potentially durable resistance to powdery mildew based on the resistance in Folke.     

A new powdery mildew resistance gene: Introgression from wild emmer into common wheat and RFLP-based mapping

An Israeli accession (TTD140) of wild emmer, Triticum turgidum var. dicoccoides, was found resistant to several races of powdery mildew. Inoculation of the chromosome-arm substitution lines (CASLs) of TTD140, in the background of the Israeli common wheat cultivar ‘Bethlehem’ (BL), with five isolates of powdery mildew revealed that only the line carrying the short arm of chromosome 2B of wild emmer (CASL 2BS) exhibited complete resistance to four of the five isolates. To map and tag the powdery mildew resistance gene, 41 recombinant substitution lines, derived from a cross between BL and CASL 2BS, were used to construct a linkage map at the gene region. The map, which encompasses 69.5 cM of the distal region of chromosome arm 2BS, contains six RFLP markers, a morphological marker (glaucousness inhibitor, W1 ^I), and the powdery mildew resistance gene. Segregation ratios for resistance in F₂ of BL × CASL 2BS and in the recombinant lines, combined with the susceptability of F₁ progeny to all tested isolates, indicate that resistance is controlled by a single recessive allele. This alleleco-segregated with a polymorphic locus detected by the DNA marker Xwg516, 49.4 cM from the terminal marker Xcdo456. The new powdery mildew resistance gene was designated Pm26. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of a microsatellite marker associated with Pm3 resistance alleles to powdery mildew in wheat

The Pm3 resistance locus, located on chromosome 1A in wheat, confers race‐specific resistance to the obligate biotrophic fungus Blumeria graminis (DC) E.O. Speer f. sp. tritici, the causal agent of powdery mildew. Several Pm3 alleles are still effective in controlling the disease in Europe. A genetic map was constructed to map the Pm3g allele in the recombinant inbred line progeny from the cross ‘RE9001’ (susceptible) בCourtot’ (resistant). Two microsatellite markers were closely mapped to Pm3g. The PSP2999 marker, which cosegregates with this allele, was shown to detect the presence of the Pm3g resistance allele in other cultivars. A collection of 56 wheat cultivars or advanced lines carrying one Pm3 allele was used to assess the allele‐specific amplification of the PSP2999 marker. The same amplification pattern was obtained for lines with Pm3a, Pm3b, Pm3e, Pm3f and Pm3g alleles. Twenty genotypes carrying Pm3d showed a specific amplification pattern. This marker allowed the detection of the Pm3d allele in highly resistant lines whose resistance gene combinations were unknown. It was concluded that PSP2999 is a useful marker to detect Pm3 alleles in parents and to manage them in breeding programmes.     

Identification of resistance gene analogue markers closely linked to wheat powdery mildew resistance gene Pm31

Specific oligonucleotide primers, designed for the sequences of known plant disease resistance genes, were used to amplify resistance gene analogues (RGAs) from wheat genomic DNA. This method was applied in a bulked segregant analysis to screen for the RGA markers linked to the powdery mildew resistance gene Pm31, introgressed into common wheat from wild emmer. Two RGA markers (RGA200 and RGA390) were found to be closely linked to Pm31 and completely co‐segregating with the marker allele of Xpsp3029 linked to Pm31, with a genetic distance of 0.6 cM. These two RGA markers were then integrated into the formerly established microsatellite map of Pm31 region. The result showed the effectiveness of the RGA approach for developing molecular markers linked to disease resistance genes and demonstrated the efficiency of denaturing polyacrylamide‐gel electrophoresis for detecting polymerase chain reaction polymorphism.     

Mapping a new source of resistance to powdery mildew in mungbean

Both restriction fragment length polymorphism (RFLP) and amplified fragment length polymorphism (AFLP) analyses were employed to map a new source of resistance to powdery mildew in mungbean. Disease scores of an F₂ population derived from the cross between a moderately resistant breeding line VC1210A and a susceptible wild relative (Vigna radiata var. sublobata, accession TC1966) showed a continuous distribution and was treated as a quantitative trait. Although no significant quantitative trait loci (QTL) that can explain the variation was detected by QTL analysis based on the reconstructed RFLP linkage map, new marker loci associated with resistance were discovered by AFLP analysis. The RFLP loci detected by two of the cloned AFLP bands are associated with resistance and constitute a new linkage group. A major resistance quantitative trait locus was found on this linkage group that accounted for 64.9% of the variation in resistance to powdery mildew. One of the probes developed in this study has the potential to assist in breeding for powdery mildew resistance in mungbean.     

Inheritance and genetic mapping of a gene for seedling resistance to powdery mildew in wheat line X3986-2

Identification and mapping new powdery mildew resistance (Pm) genes is important for resistance breeding in wheat. Common wheat (Triticum aestivum L.) line X3986-2 was tested against 27 isolates of Blumeria graminis f. sp. tritici. To identify the Pm gene(s) in X3986-2, an F₂ population and its derived F_2:3 lines were developed from a cross between X3986-2 and susceptible line Mingxian169. Segregation ratios indicated the presence of a single dominant Pm locus, tentatively designated PmX3986-2. Bulked segregant analysis was applied to screen for molecular markers linked to PmX3986-2. Two sequence characterized amplified region (SCAR) markers SCAR112 and SCAR203, and five simple sequence repeat markers CFD40, CFD78, CFD81, GWM293 and WMC443 on chromosome 5D were linked to PmX3986-2, with CFD81 and SCAR112 flanking PmX3986-2 at 0.6 and 1.5 cM, respectively. This suggests that PmX3986-2 may be a novel allele of loci Pm2, Pm46 and PmLX66 on chromosome arm 5DS. PmX3986-2 with its tightly linked DNA markers should be useful for broadening the genetic basis of Pm and rapidly transferring the resistance gene to susceptible cultivars or for us in gene pyramiding for resistance breeding.     

Assessment of partial resistance to powdery mildew in Chinese wheat varieties

Field trials in two cropping seasons and two locations in central China were conducted on 60 Chinese autumn‐sown wheat varieties to assess their partial resistance to powdery mildew. Mean levels of disease severity ranged from close to 0 to more than 90%. The method of inoculation and the location in which trials were conducted affected the relative performance of the varieties, but these effects were much smaller than the main effect of variety. The area under the disease progress curve was highly correlated with final disease severity, but both were poorly correlated with apparent infection rate. Disease severity was regressed against frequencies of virulence in the Blumeria graminis (syn. Erysiphe graminis) f sp. tritici populations in the trial plots. A vertical distance (D) from the mean mildew severity to the fitted line was calculated for each variety and was used to quantify partial resistance. Five of the 60 varieties, ‘Hx8541’, ‘E28547’, ‘Chuan1066’, ‘Zhe88pin6’ and ‘Lin5064’, consistently expressed relatively low levels of disease despite high frequencies of virulence in the pathogen and had consistently high D‐values. They may therefore have good levels of partial resistance.     

Identification of QTLs for powdery mildew and scald resistance in barley

A population of 103 recombinant inbred lines (RILs, F₉-derived lines) developed from the two-row spring barley cross L94 × ‘Vada’ was evaluated under field conditions for resistance against powdery mildew (Blumeria graminis f.sp. hordei) and scald (Rhynchosporium secalis). Apart from the major resistance gene mlo on chromosome 4 (4H), three QTLs (Rbgq1, Rbgq2 and Rbgq3) for resistance against powdery mildew were detected on chromosomes 2 (2H), 3 (3H), and 7 (5H), respectively. Rbgq1 and Rbgq2 have not been reported before, and did not map to a chromosome region where a major gene for powdery mildew had been reported. Four QTLs (Rrsq1, Rrsq2, Rrsq3 and Rrsq4) for resistance against scald were detected on chromosomes 3 (3H), 4 (4H) and 6 (6H). All four mapped to places where QTLs for scald resistance had been reported before in different populations.     

STS markers for powdery mildew resistance gene Pm6 in wheat

The powdery mildew resistance gene Pm6, transferred to common wheat from the tetraploid Triticum timopheevii, is effective in most epidemic areas for powdery mildew in China. RFLP probe BCD135 was previously associated with Pm6. In the present research, four STS primers (NAU/STS_BCD135-1, NAU/STS_BCD135-2, STS003 and STS004) were designed from the sequence data of BCD135. These primers were used for PCR amplification using the genomic DNA of resistant near-isogenic lines with Pm6 and their recurrent parent, cv. Prins. No polymorphic product was observed using primers STS003 and STS004; however, primers NAU/STS_BCD135-1 and NAU/STS_BCD135-2 amplified two and one bands, respectively, polymorphic between the resistant near-isogenic-lines and Prins. The two primers were then used to amplify the F₂ population from the cross IGV1-465 (FAO163b/7*Prins) × Prins. The amplification and the powdery mildew resistance identification data were analyzed using the software Mapmaker 3.0. The results indicated that both NAU/STS_BCD135-1 and NAU/STS_BCD135-2 were closely linked to Pm6 with a genetic distance of 0.8 cM. A total of 175 commercial varieties without Pm6 from different ecological areas of China were tested using marker NAU/STS_BCD135-2 and none of them amplified the 230 bp-specific band. This marker thus has high practicability and can be used in MAS of Pm6 in wheat breeding programs for powdery mildew resistance. Jianhui Ji and Bi Qin contributed equally to this work.     

Cytogenetical studies in wheat iv. Chromosome location and linkage studies involving thePM2 locus for powdery mildew resistance

Summary The genePm2 conditioning resistance to powdery mildew in the cultivar Ulka was located on chromosome 5D by monosomic analysis. It showed genetic segregation independent of geneLr3 conditioning resistance to leaf rust on the same chromosome. Results of telocentric mapping demonstrated thatLr1 was on the long arm of 5D whereasPm2 was very close to the centromere on this arm or, more likely, on the opposite arm. Evidence from chimaeric sectoring favoured the latter alternative.     

Transfer of powdery mildew resistance from Aegilops variabilis into bread wheat

Winter hexaploid wheat (Triticum aestivum L.) was crossed with Aegilops variabilis to transfer resistance to powdery mildew into wheat. Following two backcrosses to wheat and from 5 to 9 generations of selfing, several disomic addition and substitution lines of hexaploid wheat resistant to the mildew pathogen were isolated. A pair of short satellited chromosomes was always observed in the resistant lines. Further evidence utilizing as markers for homoeologous group 1 HMW glutenin subunits and DNA hybridization with probe pGBX 3076 showed that an alien substitution involved this homoeologous group.     

Molecular marker-facilitated pyramiding of different genes for powdery mildew resistance in wheat

Breeding durable resistance to pathogens and pests is a major task for modern plant breeders and pyramiding different resistance genes into a genotype is one way of achieving this. Three powdery mildew resistance gene combinations, Pm2+Pm4a, Pm2+Pm21, Pm4a+Pm21 were successfully integrated into an elite wheat cultivar ‘Yang047′. Double homozygotes were selected from a small F₂ population with the help of molecular markers. As the parents were near‐isogenic lines (NILs) of ‘Yang158′, the progenies showed good uniformity in morphological and other non‐resistance agronomic traits. The present work illustrates the bright prospects for the utilization of molecular markers in breeding for host resistance.     

Fine mapping of two major QTLs conferring resistance to powdery mildew in tomato

Tomato (Solanum lycopersicum) is the most cultivated crop in the Solanaceae family and is a host for Oidium neolycopersici, the cause agent of powdery mildew disease. In wild species of tomato, genes (Ol-1–Ol-6) for monogenic resistance have been identified. Moreover, three quantitative resistance loci (QRLs), namely Ol-qtl1, Ol-qtl2 and Ol-qtl3, have been mapped in Solanum neorickii G1.1601. In this work, we developed several advanced backcross populations in order to fine-map these Ol-qtls. Resistant lines harboring individual Ol-qtl were produced and used in recombinant screening. Ten recombinants were identified in chromosomal regions carrying Ol-qtl1s. The recombinant individuals were used to produce recombinant families (RFs). By screening these RFs with molecular markers and testing them with O. neolycopersici, we could localize Ol-qtl1 in a region of about 2.3 Mbp on the long arm of chromosome 6 and Ol-qtl2 in a region of 2.5 Mbp on the short arm of chromosome 12. On the other hand, the presence of Ol-qtl3 locus was not confirmed in this study. The fine-mapping results further demonstrated the co-localization between Ol-qtls and genes for monogenic resistance; the Ol-qtl1 interval contains the Ol-1 gene and the Ol-qtl2 interval harbors the Lv gene that confers monogenic resistance to Leveillula taurica, another species of tomato powdery mildew.     

Two unnecessary powdery mildew resistance genes in a synthetic rye population are neutral on fitness

A synthetic winter rye population was produced with two race-specific powdery mildew resistance genes, one dominant (Rm1) and the other (rm2) recessive, each at a frequency of about 0.50. The population was advanced by open-pollination in an isolated plot under mildew-free conditions for eight years. Samples of generations Syn-0 through Syn-7 were inoculated in the laboratory with two mildew isolates, one avirulent to either resistance gene, the other virulent to Rm1 and avirulent to rm2, to discriminate resistant and susceptible phenotypes. From the proportions of resistant plants, frequencies of Rm1 and rm2 were calculated and the fitness of carriers of resistance alleles was estimated in relation to carriers of susceptibility alleles at the two loci using continuous models and linear regression analyses. Frequencies of the two resistance genes oscillated only weakly over the eight generations. Coefficients of selection against Rm1-and rm2rm2 genotypes were –0.04 and –0.02, respectively, and not significantly different from zero. Thus the two resistance genes were selectively neutral. It is concluded that pyramiding of major powdery mildew resistance genes in rye varieties should not reduce their yield potential in the absence of mildew.     

Characterisation and origin of rust and powdery mildew resistance genes in VPM1 wheat

Summary The expression of rust resistances conferred by closely linked genes derived from VPM1 varied with environmental conditions and with genetic backgrounds. Under low light and low temperature conditions seedlings carrying Yr17 showed susceptible responses. Stem rust and leaf rust resistance genes Sr38 and Lr37 tended to confer more resistance at 17±2° C than at normal temperatures above > 20° C. These studies supported the hypothesis that Yr17, Lr37 and Sr38 were derived from Aegilops ventricosa, whereas Pm4b was probably derived from T. persicum. Studies on certain addition lines and parental stocks indicated that wheat cytoplasm may enhance the expression of Sr38.     

Characterisation of new oat germplasm for resistance to powdery mildew

Summary Wild oat accessions from the Institute of Plant Genetics and Crop Plant Research (IPK) Gatersleben were selected as potential sources of resistance to powdery mildew (Erysiphe graminis f.sp. avenae). The Avena strigosa accessions AVE 128, AVE 488 and AVE 264, along with A. occidentalis accession CAV 3889 showed high levels of mildew resistance in tests at early and late growth stages over two years; the latter is a particularly useful source because it is a hexaploid species. Moderate levels of resistance were also found in A. fatua accession AVE 1981, AVE 2032 and A.sterilis accession AVE 1373 but this could be due to the later maturity of these genotypes.