A novel barley scald resistance gene: genetic mapping of the Rrs15 scald resistance gene derived from wild barley, Hordeum vulgare ssp. spontaneum

Most genes for resistance to barley leaf scald map either to the Rrs1 locus on the long arm of chromosome 3H, or the Rrs2 locus on the short arm of chromosome 7H. Other loci containing scald resistance genes have previously been identified using lines derived from wild barley, Hordeum vulgare ssp. spontaneum. A single dominant gene conditioning resistance to scald was identified in a third backcross (BC3F3) line derived from an Israeli accession of wild barley. The resistance gene is linked to three microsatellite markers that map to the long arm of chromosome 7H; the closest of these loci, HVM49, maps 11.5 cM from the resistance gene. As no other scald resistance genes have been mapped to this chromosome arm, it is considered to be a novel scald resistance locus. As the Acp2 isozyme locus is linked to this scald resistance locus, at 17.7 cM, Acp2 is assigned to chromosome 7H. Molecular markers linked to the novel scald resistance gene, designated Rrs15, can be used in breeding for scald resistance.     

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 of isolate specific sources of scald resistance in Turkish barley (Hordeum vulgare) accessions

Disease reactions to specific Rhynchosporium secalis isolates from western Canada were characterized on forty Turkish Hordeum vulgare accessions. One accession, MEH151-1, exhibited resistance against isolate WRS1860 and was also resistant to isolates WRS1493 and WRS1824 which were avirulent to the Rh gene. A related line, MEH151-2 exhibited resistance to WRS1493 but not to WRS1860. Crosses between both MEH151-1 and MEH151-2, and the susceptible cultivar, Harrington, were used to demonstrate a linkage between resistance to WRS1493 and an allele specific amplicon, Falc666. Resistance to WRS1860 imparted by MEH151-1 was mapped to the other side of the Falc666 locus. Falc666 was previously shown to be located near the centromere on the long arm of chromosome 3. Characterization of the genetic basis for the scald resistance phenotype exhibited by these Turkish accessions, coupled with the identification of marker linkages, provides evidence for genetic variation in scald resistance in this chromosomal region. This revised version was published online in July 2006 with corrections to the Cover Date.     

The transfer of a gene conferring resistance to scald (Rhynchosporium secalis) from Hordeum bulbosum into H. vulgare chromosome 4HS

Scald is a serious foliar disease that infects barley (Hordeum vulgare L.) causing reduced yields and adversely affecting quality. A means to combat the disease is to breed cultivars that possess genetic resistance. However, all known resistance alleles have so far originated from within the primary genepool of barley. This reliance on H. vulgare and H. vulgare ssp. spontaneum as resistance sources may encourage virulent forms of the pathogen to become established. To broaden the genetic base of cultivated barley and provide novel resistances to many diseases we have used a species from the secondary genepool of barley, H. bulbosum, in a resistance‐breeding programme. In this study we describe the development and trialling of a scald‐resistant recombinant line derived from a hybrid between H. vulgare and H. bulbosum. The scald resistance is simply inherited and located on the short arm of barley chromosome 4 (4HS).     

Inheritance of resistance to Rhynchosporium secalis in spring barley (Hordeum vulgare L.)

The inheritance of durable resistance of selected spring barley varieties to Rhynchosporium secalis was investigated. Data from the F₂ generation of a 4 × 4 diallel, without reciprocals and the F₄ generation of three crosses selected out of this diallel, suggest that resistance in this sample of varieties tested is complex in inheritance. Significant additive effects were detected indicating that the resistance level of barley cultivars may be improved by the hybridisation of suitable varieties. However, the genes conferring resistance seem to be concealed by the expression of one completely dominant resistance gene in our set of varieties. These results are partly in conflict with previous results on the inheritance of resistance to R. secalis in the breeder's line ‘11258/2286₁₃A’ indicating that the effectiveness of this resistance gene may be greatly influenced by the genetic background of the current population of R. secalis.     

Quantitative trait loci for scald resistance in barley localized by a non-interval mapping procedure

Three quantitative trait loci (QTL) for scald resistance in barley were identified and mapped in relation to molecular markers using a population of chromosome doubled‐haploid lines produced from the F₁ generation of a cross between the spring barley varieties ‘Alexis’ and ‘Regatta’. Two field experiments were conducted in Denmark and two in Norway to assess disease resistance. The percentage leaf area covered with scald (Rhynchosporium secalis) ranged from 0 to 40% in the 189 doubled‐haploid (DH) lines analysed. One quantitative trait locus was localized in the centromeric region of chromosome 3H, Qryn3, using the MAPQTL program. MAPQTL was unable to provide proper localization of the other two resistance genes and so a non‐interval QTL mapping method was used. One was found to be located distally to markers on chromosome 4H (Qryn4) and the other, Qryn6, was located distally to markers on chromosome 6H. The effects of differences between the Qryn3, Qryn4 and Qryn6 alleles in two barley genotypes for the QTL were estimated to be 8.8%, 7.3% and 7.0%, respectively, of leaf covered by scald. No interactions between the QTLs were found.     

Variation in Ethiopian barley landrace populations for resistance to barley leaf scald and netblotch

One-hundred and eighty landrace populations and six-hundred single-head plants selected from 60 promising populations were evaluated for resistance to scald and netblotch at three locations in Ethiopia. Each accession was tested with and without the application of 50% of the recommended rate of fertilizer at planting. Plants were rated for disease attack two to four times during the season. Both diseases were enhanced by the application of fertilizer and were more severe at the testing sites of Holetta and Bekoji than at Sheno. The difference in disease resistance among and within populations was considerable. Moreover, populations from Arsi and Bale tend to be more susceptible to scald but more resistant to netblotch than populations from other regions. Populations collected from higher altitudes were more resistant to scald, but susceptible to netblotch, than were populations from lower altitudes. The paper illustrates approaches to the identification of valuable genotypes from landrace populations that can be incorporated into a breeding programme for the development of improved varieties with resistance to the principal diseases of barley in Ethiopia.     

Molecular mapping of greenbug (Schizaphis graminum) resistance gene Rsg1 in barley

The greenbug, Schizaphis graminum (Rondani) is an extremely damaging aphid pest of barley (Hordeum vulgare L.) particularly in the southern Great Plains of the USA. The simply inherited, dominant resistance gene Rsg1 is in all greenbug‐resistant US barley cultivars. In this study, we conducted molecular mapping of Rsg1 using an F_2:3 population derived from a cross between the greenbug‐resistant Post 90*4/R015 and susceptible CI2260 inbred lines. Segregation of host responses to greenbug biotype E infestation confirmed that a single dominant gene is responsible for greenbug resistance in Post 90*4/R015. Simple sequence repeat (SSR) markers evenly distributed along the seven barley chromosomes were employed for the construction of a framework genetic map. Linkage analysis placed the Rsg1 locus in the long arm of chromosome 3H (3HL) flanked by SSR markers Bmag0877 and GBM1420 that were 35 cM apart. Polymorphic single‐nucleotide polymorphism (SNP) markers in 3HL were identified from an Illumina GoldenGate SNP assay and used for targeted mapping to locate Rsg1 to an 8.4‐cM interval. Comparative analysis identified syntenic genomic regions in Brachypodium distachyon chromosome 2, in which 37 putative genes were annotated including a NB‐LRR‐type resistance gene homologue that may be a potential candidate gene for the Rsg1 locus of barley. Results from this study offer a starting point for fine mapping and cloning of this aphid resistance gene in barley.     

Molecular mapping of the leaf rust resistance gene Rph7 in barley

Leaf rust of barley, caused by Puccinia hordei Otth, is an important foliar disease in most temperate regions of the world. Sixteen major leaf rust resistance (Rph) genes have been described from barley, but only a few have been mapped. The leaf rust resistance gene Rph7 was first described from the cultivar ‘Cebada Capa’ and has proven effective in Europe. Previously mapped restriction fragment length polymorphism (RFLP) markers have been used to determine the precise location of this gene in the barley genome. From the genetic analysis of a ‘Bow‐man’/‘Cebada Capa’ cross, Rph7 was mapped to the end of chromosome 3HS, 1.3 recombination units distal to the RFLP marker cMWG691. A codominant cleaved amplified polymorphic site (CAPS) marker was developed by exploiting allele‐specific sequence information of the cMWG691 site and adjacent fragments of genomic DNA. Based on the large amount of polymorphism present in this region, the CAPS marker may be useful for the marker‐assisted selection of Rph7 in most diverse genetic backgrounds.     

Development of SCAR markers linked to a scald resistance gene derived from wild barley

The F₂ progeny of a third backcross(BC₃) line, BC line 240, derived from a Turkish accession of wild barley (Hordeum vulgare ssp. spontaneum),segregated for resistance to scald (Rhynchosporium secalis) in a manner indicating the presence of a single dominant resistance gene. Two SCAR marker slinked to this resistance were developed from AFLP markers. Screens of disomic and ditelosomic wheat-barley addition lines with the SCAR markers demonstrated that the scald resistance gene is located in the centromeric region of barley chromosome 3H,a region previously reported to contain a major scald resistance locus, Rrs1. Markers that flank the Rrs1 locus were used to screen the wild barley-derivedBC₃F₂ population. These markers also flank the wild barley-derived scald resistance, indicating that it maps to the same locus as Rrs1; it may beallelic, or a separate gene within a complex locus. However, BC line 240 does not respond to treatment with the Rhynchosporium secalis avirulence factorNIP1 in the same way as the Rrs1-carrying cultivar Atlas46. This suggests that the scald resistance gene derived from wild barley confers a different specificity of response to theRrs1 allele in Atlas46.In order to increase the durability of scald resistance in the field, we suggest that at least two scald resistances should be combined into barley cultivars before release. The scald resistance gene described here will be of value in the Australian environment, and the several markers linked to it will facilitate pyramiding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genes for scald resistance from wild barley (Hordeum vulgare ssp spontaneum) and their linkage to isozyme markers

Summary Accessions of Hordeum vulgare ssp. spontaneum, the wild progenitor of barley, collected in Israel (70), Iran (15) and Turkey (6) were screened for seedling response to four isolates of Rhynchosporium secalis, the pathogen causing leaf scald in barley. Resistance was very common in the collection (77%) particularly among accessions from the more mesic sites (90%). The genetics of this resistance were investigated in fifteen backcross (BC₃) lines that contained an isozyme variant from H.v. ssp. spontaneum in a H.v. ssp. vulgare (cv. Clipper) background and were resistant to scald. Segregation in the BC₃F₂ families conformed with a single dominant resistance gene in 9 of the 15 lines. Scald resistance and the isozyme marker were closely linked in three of the BC₃-lines, loosely linked in four and unlinked in the remaining eight. Scald resistance genes were identified on barley chromosomes 1, 3, 4 and 6. Crosses between several of the scald resistant BC-lines together with the linkage data indicated that at least five genetically independent resistances are available for combining together for deployment in barley. The linkage of scald resistance in several BC₃-lines to the isozyme locus Acp2 is of special interest as this locus is highly polymorphic in wild barley.     

Genetic analysis of resistance in barley to barley yellow dwarf virus

The inheritance of resistance to barley yellow dwarf virus (BYDV) was studied in the selected 24 spring and winter barley cultivars that showed a high or intermediate resistance level in 1994‐97 field infection tests. The polymerase chain reaction diagnostic markers YLM and Ylp were used to identify the resistance gene Yd2. The presence of the Yd2 gene was detected with both markers in all the resistant spring barley cultivars and lines from the CIMMYT/ICARDA BYDV nurseries. The results of field tests and genetic analyses in winter barley corresponded with marker analyses only when the Ylp marker was used. Genes non‐allelic with Yd2 were detected by genetic analyses and the Ylp marker in moderately resistant spring barley cultivars ‘Malvaz’, ‘Atribut’ and ‘Madras’, and in the winter barley cultivars ‘Perry’ and ‘Sigra’. Significant levels of resistance to BYDV were obtained by combining the resistance gene Yd2 with genes detected in moderately resistant cultivars. The utilization of analysed resistance sources in barley breeding is discussed.     

Screening for resistance in the primary and secondary gene pool of barley against the root-lesion nematode Pratylenchus neglectus

Root-lesion nematodes of the genus Pratylenchus are significant pests in crop cultivation throughout many parts of the world. A study was initiated to determine the resistance of Hordeum vulgare and H. vulgare ssp. spontaneum (wild barley) against one major representative of the genus Pratylenchus , P. neglectus . A glasshouse test was first established. Barley seedlings were grown in 20 cm³ tubes filled with sand. Each plant was inoculated with 400 P. neglectus juveniles. After 12 weeks of cultivation nematodes were isolated from roots and sand using a misting chamber. The nematodes were counted under a microscope. A representative collection of 565 barley and wild barley accessions was tested in this way. The average number of nematodes per accession ranged from 350 to 12 000. In a verification experiment, 35 accessions with low and high infection rates were tested. This experiment identified a number of accessions with low infection rates. The perspectives for future breeding of barley cultivars resistant to root-lesion nematodes are discussed.     

Combination of resistances to barley yellow mosaic virus and Rhynchosporium secalis by recurrent selection with repeated haploid steps

The aim of this investigation was the incorporation of resistance to BaYMV and Rhynchosporium secalis into German winter barley varieties together with the maintenance of their high agronomic characters. The agronomic value of DH-lines after two backcrosses and three haploid steps were compared with the original parents. Lines with similar or superior results could be selected for all the agronomic features evaluated. This method of recurrent selection with repeated haploid steps can therefore lead to the development of lines useful for practical purposes.     

Evaluating resistance of barley breeding lines to scald in field plots

Summary A 0–4 scoring system to quantify scald (Rhynchosporium secalis) infection is suggested. Scores 1, 2, 3 and 4 allocated to represent 1/4, 1/2, 3/4 and 4/4 of the crop canopy scalded are easy to comprehend and intermediate scores e.g. 0.5, 1.5, 2.5 and 3.5 give it the breadth of a quantitative scale. Scores on a large number of lines showed a high degree of repeatability and were found to be highly correlated with the log transformed values of the actual leaf area damage. Although it was suggested that predictions of leaf area damage at scores 3–4 should be applied with caution, broad generalization of the scores in discriminating the amount of disease were shown to be soundly based and offered plant breeders a tool to standardize the evaluation of scald resistance in field plots on a large scale with this quick and reliable scoring system.     

Approaches for field assessment of resistance to leaf pathogens in spring barley varieties

The resistance of spring barley varieties to powdery mildew, leaf rust, leaf scald and net blotch was characterized by using results from inoculated small‐plot nurseries and larger survey plots subject to natural infection. The experiments were conducted in different environments. Both trial types often yielded complementary results with respect to the ranking of varieties suggesting that a recommended variety characterization should include both naturally infected survey‐type trials and nursery trials in which the most relevant pathogen isolates and/or isolate mixtures or populations are used for inoculation. Average and median values of the diseased leaf area of a variety were highly correlated with each other and with the ‘genotype main effect’ determined by joint regression analysis, whereas maximum diseased leaf area was poorly correlated with them. Statistics based on absolute disease severity values were highly correlated with the corresponding statistics derived from relative values. It is suggested that one should use at least two parameters to characterize the disease resistance of a variety, a parameter indicating the overall resistance level and a parameter indicating the potential susceptibility and/or resistance instability of a variety. For practical purposes, the genotype median and maximum, respectively, may represent these, although statistically more appropriate parameters do exist.     

Development of an STS marker and SSRs suitable for marker-assisted selection for the BaMMV resistance gene rym9 in barley

Based on the RAPD marker OP‐C04H910 which is closely linked to the barley mild mosaic virus (BaMMV) resistance gene rym9 derived from the variety ‘Bulgarian 347’ the marker STS‐C04H910 cosegregating with OP‐C04H910 and generating a single additional band on plants carrying the recessive resistance encoding allele has been developed. Furthermore, the simple sequence repeats (SSRs) WMS6 and HVM67 have been integrated into the genetic map of the rym9 region on chromosome 4HL. Because of their close linkage to rym9 and distinct banding pattern STS‐C04H910 and HVM67 are well‐suited for marker‐ assisted selection, enhanced backcrossing procedures and pyramiding of resistance genes.     

Genetic diversity of barley cultivars grown in Spain, estimated by RFLP, similarity and coancestry coefficients

This study was conducted with the objective of characterizing the genetic variation among a representative set of 37 barley cultivars currently grown in Spain, using restriction fragment length polymorphism (RFLP) markers. Thirty-two RFLP probes, in combination with three restriction enzymes, were used to analyse polymorphism at the molecular level. Genetic distances (GD), based on RFLP band patterns, and coancestry coefficients (f), based on pedigree records, were calculated. Of the 95 clone-enzyme combinations analysed, 71 (74.7%) were polymorphic, representing 246 RFLP patterns. A cluster analysis of GD split the sample into five distinct germplasm groups that were consistent with the history of the cultivars (winter European, spring European, CIMMYT-ICARDA materials, the single cultivar ‘Dobla’ and Spanish local materials). The Spanish group was the most distinct one and had unique alleles at markers close to major loci determining phonological adaptation. The probes which best distinguished among groups were also identified. Genetic similarity estimates were moderately consistent with f (for cultivars with complete pedigrees). The implications for integration of diversity studies into breeding programmes are discussed.     

Cloning, genetic and physical mapping of resistance gene analogs in barley (Hordeum vulgare L.)

The majority of verified plant disease resistance genes isolated to date belong to the NBS‐LRR class, encoding proteins with a predicted nucleotide binding site (NBS) and a leucine‐rich repeat (LRR) region. Using degenerate primers, designed from the conserved motifs of the NBS region in tobacco N and Arabidopsis RPS2 genes, we isolated 190 resistance gene analogs (RGA) clones from barley genomic DNA. A total of 13 single‐ and low‐copy RGAs were genetically mapped onto chromosomes 1H–7H (except 5H) using three barley double haploid (DH) mapping populations: Steptoe × Morex, Harrington × TR306 and LUGC × Bowman. Sequence analysis of the RGAs showed that they are members of a diverse group. As a result of BLAST searches, one RGA proved unique as it did not detect any significant hit. Another RGA is putatively functional, because it detected several barley expressed sequence tag (EST) matches. To physically map the RGAs, 13 sequences were used to screen a 6.3 × cv. ‘Morex’ bacterial artificial chromosome (BAC) library. After fingerprint analysis, eight contigs were constructed incorporating 62 BAC clones. These BAC contigs are of great value for positional cloning of disease resistance genes, because they span the regions where various barley R genes have been genetically mapped.     

Characterization and mapping of gene Rph19 conferring resistance to Puccinia hordei in the cultivar 'Reka 1' and several Australian barleys

Previous studies established that the Australian barley cultivar ‘Prior’ possessed resistance to Puccinia hordei (RphP), displaying the same specificity as an uncharacterized resistance in the differential cultivar ‘Reka 1’ (also possessing Rph2). Multipathotype tests confirmed the presence RphP in nine additional barley cultivars and indicated that RphP differed in specificity to the genes Rph1 to Rph15 and Rph18, plus the gene RphX present in the barley cultivar ‘Shyri’. RphP was inherited as a single dominant gene. Mapping studies using a doubled haploid population derived from ‘Chebec’/‘Harrington’ located RphP to the long arm of chromosome 7H, and demonstrated linkage with an restriction fragment length polymorphism marker (pTAG732), a resistance gene analogue marker (RLch4(Nc)), and two microsatellite markers (HVM11 and HVM49) at genetic distances of about 4‐10 cM. RphP showed linkage of 28 ± 4.3 cM with Rph3. RphP was designated Rph19, with the allele designation Rph19.ah. Previous studies have established that virulence for Rph19 occurs in many barley growing regions of the world.