Genetics of powdery mildew resistance in pea

The genetics of powdery mildew (Erysiphe pisi) resistance in pea (Pisum sativum) was studied using five isolates of E. pisi in a controlled environment. Resistance was controlled by a single recessive gene in all the pea cvs. included in the study. The same recessive gene controlled resistance to all the five isolates of E. pisi. Tests for allelism showed that resistance in cvs. HPPC-63, HPPC-95, DPP-26, DPP-54, Mexique-4, SVP-950, Wisconsin-7104 and P-6588 is conferred by the same recessive gene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sequence tagged site markers linked to the sbm1 gene for resistance to pea seedborne mosaic virus in pea

Resistance to pea seed‐borne mosaic virus (PSbMV) pathotype P‐1 in peas is conferred by sbm1 with recessive inheritance. PSbMV is an economically important pathogen with world‐wide distribution that causes significant losses in pea yield and reduces seed and produce quality. The sbm1 gene was previously mapped to linkage group VI on molecular linkage maps of the pea genome. To improve plant breeders’ ability to develop varieties resistant to PSbMV, two random amplified polymorphic DNA markers (G05_2537 and L01_910) and one restriction fragment length polymorphism (P446) linked to sbm1 have been identified. The genomic sequences for these markers have been characterized and the information used to develop three simple polymerase chain reaction‐based STS (sequence tagged site) assays. Linkage analysis in two F₂ populations showed that the most tightly linked of these three STS loci (sG05_2537) is approximately 4 cM from sbm1. Characterization of a collection of resistant and susceptible germplasm demonstrated a strong correlation between STS alleles and sbm1 alleles, indicating the utility of these markers for marker‐assisted selection in breeding programmes using a range of germplasm sources.     

Development of SCAR markers linked to the Pm21 gene conferring resistance to powdery mildew in common wheat

Powdery mildew is an important disease in most of the wheat production areas of the world. The resistance gene Pm21 (6AL/6VS trans-location) derived from Haynaldia villosa confers resistance to all available isolates of Erysiphe (Blumeria) graminis f. sp. tritici in China and Europe. The objective of this study was to develop fast and reliable sequence characterized amplified region (SCAR) markers linked to the Pm21 gene. A random amplified polymorphic DNA (RAPD) marker for Pm21, OPH17₁₄₀₀, was converted to SCAR markers after sequencing the two ends of the polymorphic DNA fragment. Two SCAR markers, SCAR₁₂₆₅ and SCAR₁₄₀₀, were developed to detect the Pm21 gene in different genetic backgrounds. The specific SCAR₁₂₆₅ marker enable large-scale accurate screening for the presence/absence of Pm21 allele.     

Identification and characterization of sources of resistance to Erysiphe pisi Syd. in Pisum spp.

A collection of 67 accessions of Pisum species originating from different countries was screened in a glasshouse test for resistance to Erysiphe pisi. All Pisum fulvum accessions were completely resistant. Incomplete resistance was identified in some accessions of P. sativum subsp. sativum var. arvense and P. sativum subsp. elatius and abyssinicum. Microscopy revealed several distinct cellular mechanisms governing resistance. In P. fulvum, it was mainly due to a high frequency of cell death that occurred both as a rapid response to attempted infection and a delayed response that followed colony establishment. Cell death following colony establishment was also key to the incomplete resistance in some accessions of P. sativum subsp. sativum var. arvense. In addition, impaired spore germination, and to a lesser extent appressorium formation, contributed to pre‐penetration resistance in some accessions. In some cases, resistance also retarded colony growth, possibly through effects on haustorial development or function in cells that survived the attack. Thus, these wild pea accessions offer diverse resistances that could be introduced to cultivated peas to increase the efficacy of powdery mildew resistance.     

Identification of AFLP and RAPD markers linked to anthracnose resistance in grapes and their conversion to SCAR markers

Resistance to anthracnose or black spot ( Elsinoe ampelina ), a serious fungal pathogen in viticulture and table grape production, was investigated on 25 grape cultivars. Bioassays performed with culture filtrates produced by the pathogen revealed 14 resistant genotypes. In most plants resistance originated from Vitis labrucsa but also genotypes with V. rupestris and V. riparia  ×  V. rupestris background showed resistance. Genetic analysis was conducted in F₁, S₁ and BC₁ plants developed from various cultivars. In total, 326 F₁ plants were evaluated, 172 genotypes proofed to be resistant, whereas 154 were susceptible to anthracnose. A Mendelian segregation ratio of 1 : 1 (χ² = 0.30–0.65) indicating that anthracnose resistance is controlled by a single dominant gene. To facilitate the use of marker-assisted selection in grape-breeding PCR-based markers were developed by random amplified polymorphic DNA and amplified fragment length polymorphism in bulk segregant analysis. Finally, OPB 15₁₂₄₇ was developed as a sequence characterized amplified region marker being diagnostic for the locus of resistance to anthracnose in all resistant genotypes tested. Within the 25 grape cultivars OPB 15₁₂₄₇ is diagnostic in the genetic background of both V. labrucsa and V. rupestris and V. riparia  ×  V. rupestris .     

Characterization of resistance to powdery mildew (Erysiphe pisi) in a germplasm collection of Lathyrus sativus

Lathyrus sativus has a considerable potential for dry land‐farming systems but can be severely damaged by powdery mildew infection. Little is known about the availability of resistance against powdery mildew and the underlying mechanisms in the Lathyrus genus. A range of resistance reactions was identified in a collection of Iberian‐cultivated L. sativus germplasm. In most cases a compatible reaction to powdery mildew with no macroscopically visible necrosis was observed. Nevertheless, accessions with reduced disease severity despite being of a high infection type have been identified. The genes responsible for this partial resistance will be potentially useful for the development of durable resistant cultivars of Lathyrus spp., and may also be of value for the improvement of other related cultivated leguminous plants.     

RAPD markers linked to resistance to downy mildew disease in soybean

To determine and utilize RAPD markers linked to resistance to downymildew incited by Peronospora manshurica in soybean, a resistantcultivar `AGS129' was crossed to a susceptible cultivar `Nakhon Sawan 1'(NS1). F₂ and BC₁ populations were advanced from the F₁ and evaluatedfor resistance to the disease. ²-test demonstrated that the resistancewas controlled by a single dominant gene (Rpmx). Near-isogenic lines(NILs) and bulked segregant analysis (BSA) were used to identify RAPDmarkers linked to the gene. Six DNA bulks namely F₅(R), F₅(S),BC₆F₃(R), BC₆F₃(S), F₂(R) and F₂(S) were set up by pooling equalamount of DNA from 8 randomly selected plants of each disease responsetype. A total of 180 random sequence decamer oligonucleotide primerswere used for RAPD analysis. Primer OPH-02 (5 TCGGACGTGA 3 andOPP-10 (5 TCCCGCCTAC 3) generated OPH-02₁₂₅₀ and OPP-10₈₃₁fragments in donor parent and resistant bulks, but not in the recurrentparent and susceptible ones. Co-segregation analysis using 102 segregatingF₂ progenies confirmed that both markers were linked to the Rpmxgene controlling downy mildew disease resistance with a genetic distance of4.9 cm and 23.1 cm, respectively. Marker OPH-02₁₂₅₀ was presentin 13 of 16 resistant soybean cultivars and absent in susceptible cultivars,thus confirming a potential for MAS outside the mapping population.     

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.     

Development of molecular markers linked to the wheat powdery mildew resistance gene Pm4b and marker validation for molecular breeding

Powdery mildew, caused by Blumeria graminis (DC.) E.O. Speer f. sp. tritici, is an important disease in wheat (Triticum aestivum L.). Bulk segregant analysis (BSA) was employed to identify SRAP (sequence‐related amplified polymorphism), sequence tagged site (STS) and simple sequence repeat (SSR) markers linked to the Pm4b gene, which confers good resistance to powdery mildew in wheat. Out of 240 SRAP primer combinations tested, primer combinations Me8/Em7 and Me12/Em7 yielded 220‐bp and 205‐bp band, respectively, each of them associated with Pm4b. STS‐₂₄₁ also linked to Pm4b with a genetic distance of 4.9 cM. Among the eight SSR markers located on wheat chromosome 2AL, Xgwm382 was found to be polymorphic and linked to Pm4b with a genetic distance of 11.8 cM. Further analysis was carried out using the four markers to investigate marker validation for marker‐assisted selection (MAS). The results showed that a combination of the linked markers STS_?241, Me8/Em7_?220 and Xgwm382 could be used for marker‐assisted selection of the resistance gene Pm4b in wheat breeding programmes.     

Mapping of the apple powdery mildew resistance gene Pl1 and its genetic association with an NBS-LRR candidate resistance gene

Molecular markers for the major apple powdery mildew resistance gene Pl1 were identified and are presently used in marker-assisted selection in apple breeding. However, the precise map position of the Pl1 gene in the apple genome was not known. The objectives of this investigation were the identification of the Malus linkage group (LG) carrying the Pl1 locus, mapping of the resistance gene by simple sequence repeat (SSR) markers, and the analysis of genetic associations between the Pl1 gene and the numerous NBS-LRR resistance gene candidates already mapped in the apple genome. A two-step linkage mapping was used, based on two different apple families. The identification of LG 12 carrying Pl1 was performed indirectly by mapping the SCAR marker AT20 in an apple progeny for which there was a core genetic map but no mildew data available. Then, the position of Pl1 on LG 12 was determined by SSR markers in a second population which has been scored for mildew over 6 years in a greenhouse and in the field. The SSR Hi07f01, previously mapped on LG 12 [Tree Genet. Genomes, 2 (2006), 202] cosegregated with AT20 and was closely linked (∼1 cM) to the Pl1 gene. The TIR-NBS-LRR resistance gene analogue 15G11 mapped by the SSCP technique was also closely linked to the Pl1 resistance locus and might be a candidate for Pl1 itself, a second powdery mildew major resistance gene ( Pld , [Theor. Appl. Genet., 110 (2004), 175]), or two scab resistance genes ( Vg , [IOBC/WPRS Bull., 23 (2000), 245]; Vb , [Genome, 49 (2006), 1238]) which all seem to be located in a common R gene cluster at the distal end of apple LG 12.     

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.     

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.     

Genetic analysis of adult plant resistance to powdery mildew in pea (Pisum sativum L.)

Summary An analysis of adult plant resistance of powdery mildew in 15 F₁, F₂ and F₃ populations of pea derived from crossing 15 diverse and susceptible lines with one resistant line revealed that resistance to powdery mildew is controlled by duplicate recessive genes. The genes were designated as er₁ and er₂.Disease reaction showed independent segregation with three known markers in the resistant parent, namely, af (afila, chromosome 1), st (stipule reduced, chromosome 3) and tl (clavicula, chromosome 7).Contribution form the Department of Genetics and Plant Breeding Banaras Hindu University, Varanasi-221005, India.     

Linkage of random amplified polymorphic DNA markers to downy mildew resistance in cucumber (Cucumis sativus L.)

Marker assisted selection (MAS) may improve the efficiency of breeding downy mildew resistant cucumber cultivars. A study was conducted to identify random amplified polymorphic DNA (RAPD) markers linked to the downy mildew resistance gene (dm) which would be suitable for MAS. A total of 145 F₃ families from two populations (55 from the WI 1983G × Straight 8 population and 90 from the Zudm1 × Straight 8 population) were evaluated over five locations in North America and Europe. Resistant and susceptible F₃ families were identified and mean family resistance ratings were used to type individual F₂ plants. No evidence for race differences in the pathogen (Psuedoperonospora cubensis (Berk. & Curt.) Rostow) between North America and Europe was found. Phenotypic correlations between locations ranged from 0.3 to 0.7. Of the 135 polymorphic RAPD markers identified from 960 primers, five were linked to dm - G14₈₀₀, X15₁₁₀₀, AS5₈₀₀, BC519₁₁₀₀, and BC526₁₀₀₀. In the WI 1983G × Straight 8 population, G14₈₀₀ was linked to dm at 16.5 cm, AS5₈₀₀ at 32.8 cm, BC519₁₁₀₀ at 9.9 cm, and BC526₁₀₀₀ at 19.2 cm. In the Zudm1 ×Straight 8 population, G14₈₀₀ was linked at 20.9 cm, X15₁₁₀₀at 14.8 cm, AS5₈₀₀ at 24.8 cm, and BC526_{_1000} at 32.9 cm. MarkersG14₈₀₀ and BC519₁₁₀₀ were linked in repulsion to the dm allele, and X15₁₁₀₀, AS5₈₀₀, and BC526₁₀₀₀ were linked in coupling phase. These genetic markers may be exploited to develop an efficient MAS strategy for breeding resistant cucumber cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Cell selection as a possible reason for the specificity of somaclonal variation in pea

A study of regenerants obtained from long-term callus cultures of different pea (Pisum sativum L.) genotypes revealed specificity of in vitro mutagenesis. The specificity was displayed in the preferential generation of the somaclonal variations affecting quantitative and developmental characters. About 60% of regenerated lines obtained from the cultivar Ranny Zeleny carried mutations in the Lf and Sn loci, which control initiation of flowering. An in vitro study of isogenic lines differing at the Lf and Sn loci illustrates the growth advantages of mutant genotypes in tissue culture. These experiments suggest that mutations affecting developmental characters (e.g. mutations in loci which control flowering behaviour) may be expressed in tissue culture and may cause increased adaptation in mutant cells to the in vitro conditions. Rapid propagation of mutant cells during in vitro culture may lead to a higher proportion of quantitative mutations among the regenerated plants.     

Validation of a major QTL for scab resistance with SSR markers and use of marker-assisted selection in wheat

The objectives of this study were to validate the major quantitative trait locus (QTL) for scab resistance on the short arm of chromosome 3B in bread wheat and to isolate near‐isogenic lines for this QTL using marker‐assisted selection (MAS). Two resistant by susceptible populations, both using ‘Ning7840’ as the source of resistance, were developed to examine the effect of the 3BS QTL in different genetic backgrounds. Data for scab resistance and simple sequence repeat (SSR) markers linked to the resistance QTL were analyzed in the F_2:3 lines of one population and in the F_3:4 lines of the other. Markers linked to the major QTL on chromosome 3BS in the original mapping population (‘Ning7840’/‘Clark’) were closely associated with scab resistance in both validation populations. Marker‐assisted selection for the QTL with the SSR markers combined with phenotypic selection was more effective than selection based solely on phenotypic evaluation in early generations. Marker‐assisted selection of the major QTL during the seedling stage plus phenotypic selection after flowering effectively identified scab resistant lines in this experiment. Near‐isogenic lines for this 3BS QTL were isolated from the F₆ generation of the cross ‘Ning7840’/‘IL89‐7978’ based on two flanking SSR markers, Xgwm389 and Xbarc147. Based on these results, MAS for the major scab resistance QTL can improve selection efficiency and may facilitate stacking of scab resistance genes from different sources.     

Mapping of a gene for leaf scald resistance in barley line 'B87/14' and validation of microsatellite and RFLP markers for marker-assisted selection

Seedlings of the barley line ‘B87/14’ were resistant to 22 out of 23 Australian isolates of Rhynchosporium secalis, the causal agent of leaf scald.‘B87/14’‐based populations were developed to determine the location of the resistance locus. Scald resistance segregated as a single dominant trait in BC₁F₂ and BC₁F₃ populations. Bulked segregant analysis identified amplified fragment length polymorphisms (AFLPs) with close linkage to the resistance locus. Fully mapped populations not segregating for scald resistance located these AFLP markers on chromosome 3H, possibly within the complex Rrs1 scald locus. Microsatellite and restriction fragment length polymorphism markers adjacent to the AFLP markers were identified and validated for their linkage to scald resistance in a second segregating population, with the closest marker 2.2 cM from the resistance locus. These markers can be used for selection of the Rrs.B87 scald‐resistance locus, and other genes at the chromosome 3H Rrs1 locus.     

Identification and inheritance of a new gene for powdery mildew resistance in mungbean (Vigna radiata L. Wilczek)

Powdery mildew (PM) is one of the important foliar diseases of mungbean. Resistance sources have been identified in India and the inheritance studies showed that complete resistance (RO) was controlled by two dominant genes, Pm1 , Pm2 . The breakdown of complete resistance (RO) into moderate resistance (R2) by race-2 (Akola) has been reported. It is assumed that the change in resistance reaction is due to a mutation in the pathogen. The present investigation was carried out with a view to screen germplasm, cultivars and mutants for identification of complete resistance (RO) sources against race-2 and to study their inheritance. 'Mulmarada', a local mungbean cultivar from Maharashtra state of India was identified as a complete resistance (RO) source for race-2. The inheritance of Mulmarada's resistance (RO) was studied. The F₁ and the segregation in F₂ and F₃ showed that the complete resistance (RO) in 'Mulmarada' is controlled by a single dominant gene, which is different from the earlier identified Pm1 and Pm2 resistance genes. Mulmarada's resistance gene is designated as Pm3 for PM resistance.