Genotype Heterogeneity in Accessions of a Winter Barley Core Collection Assessed on Postulated Specific Powdery Mildew Resistance Genes

Gene bank accessions are necessary for implementing many research and breeding projects. However, a great number of accessions are contaminated or confused. If such accessions are used, the results obtained from these projects are inaccurate and non-reproducible. There are methods that allow almost perfect genotype identification; nevertheless, they are relatively recent and results cannot be compared with the characteristics of the original accessions. Growing resistant cultivars is an environmentally safe and cheap way of disease management and knowledge of diverse resistance genes and their combinations can be used to identify varieties and verify their authenticity and homogeneity. For this purpose, all 172 accessions of the core collection (CC) of the Czech winter barley (Hordeum vulgare) gene bank, originating from 35 countries, were studied. For resistance tests, 51 reference isolates of Blumeria graminis f. sp. Hordei, collected in all nonpolar continents over a period of 63 years and representing the global virulence/avirulence diversity of the pathogen, were used. Only 25 barley accessions were homogeneous (genetically uniform), whereas 147 accessions were heterogeneous due to presence of different genotypes. In total, 17 resistance genes were found singly or in combinations; 76.3% of accessions with identified resistance genes carried alleles at the Mla locus. To purify the CC, progenies of individual plants must be multiplied and authenticity and homogeneity of the seed should be confirmed with resistance tests, and subsequently can be studied with more advanced methods.

Newly discovered genes for resistance to powdery mildew in the subtelomeric region of the short arm of barley chromosome 7H

Two dominant genes for resistance to powdery mildew (caused by Blumeria graminis f.sp. hordei) from the PI296825 and PI466461 accessions of wild barley (Hordeum vulgare subsp. spontaneum) were identified close to the subtelomeric region of the short arm of chromosome 7H. Genetic analyses predicted two resistance loci in F₂ populations established from crosses between each of the two accessions and the winter barley (H. vulgare) variety Tiffany. Genetic mapping revealed a highly effective (52% of phenotypic variation) resistance gene from PI296825 located between the markers GBMS192 and GBM1060. In F₂ plants exhibiting resistance reaction types (RT) 0 to RT1–2, specific DNA fragments for co-segregating markers were amplified. In plants with RT2 and RT2–3, the resistance was conferred by another unidentified resistance gene. In PI466461, the resistance gene found on the short arm of chromosome 7H was flanked by the markers GBM1126 and GBM1060. Another resistance gene coincided with the Mla locus. Resistance in RT0 plants was conferred by both resistance genes, which accounted for 58% of the total phenotypic variation. The two resistance genes with the same location on chromosome 7H have different phenotypic effects on the resistance in RT0 plants; therefore, the resistance alleles could be at different loci.

Powdery Mildew Resistance of Czech and Slovak Spring Barley Breeding Lines in Variety Trials

In 2001–2005, resistance to powdery mildew was studied in 227 Czech and Slovak breeding lines of spring barley included in the breeding station trials or official trials. Seventeen known resistances were identified (Al, Ar, At, HH, Kr, La, Ly, Mlo, N81, Ri, Ru, Sp, St, Tu, We, Mla21, and Mlp1). Unknown resistances were found in 11 breeding lines, in five of which resistance was effective against all used pathotypes of the pathogen. Besides the identified resistances, unknown resistances were detected in another three breeding lines. Sixty-five breeding lines (= 29%) exhibited heterogeneity in the examined trait, i.e. they are composed of components with different resistances to powdery mildew. Comparison of current results with the previous ones shows a considerable increase in the proportion of breeding lines carrying the resistance Mlo (72%), on the account of the resistances located at the Mla locus, particularly Ru. The examined set is characterised by a high proportion of breeding lines resistant to all used Czech pathotypes of the given pathogen (78%), however of low diversity in the resistance.  

Diversity at the Mla Powdery Mildew Resistance Locus from Cultivated Barley Reveals Sites of Positive Selection

The Mla locus in barley (Hordeum vulgare) conditions isolate-specific immunity to the powdery mildew fungus (Blumeria graminis f. sp. hordei) and encodes intracellular coiled-coil (CC) domain, nucleotide-binding (NB) site, and leucine-rich repeat (LRR)-containing receptor proteins. Over the last decades, genetic studies in breeding material have identified a large number of functional resistance genes at the Mla locus. To study the structural and functional diversity of this locus at the molecular level, we isolated 23 candidate MLA cDNAs from barley accessions that were previously shown by genetic studies to harbor different Mla resistance specificities. Resistance activity was detected for 13 candidate MLA cDNAs in a transient gene-expression assay. Sequence alignment of the deduced MLA proteins improved secondary structure predictions, revealing four additional, previously overlooked LRR. Analysis of nucleotide diversity of the candidate and validated MLA cDNAs revealed 34 sites of positive selection. Recombination or gene conversion events were frequent in the first half of the gene but positive selection was also found when this region was excluded. The positively selected sites are all, except two, located in the LRR domain and cluster in predicted solvent-exposed residues of the repeats 7 to 15 and adjacent turns on the concave side of the predicted solenoid protein structure. This domain-restricted pattern of positively selected sites, together with the length conservation of individual LRR, suggests direct binding of effectors to MLA receptors.

Genome-Wide and Mla Locus-Specific Characterisation of Latvian Barley Varieties

Genome-Wide and Mla Locus-Specific Characterisation of Latvian Barley Varieties Genetic diversity in locally adapted germplasm forms the basis for crop improvement through breeding. While single loci have been routinely used for studies of genetic diversity, the highthroughput genotyping platforms that have recently become available for large genome crop plants offer an unbiased view on genetic diversity on a genome-wide scale. We assessed genetic diversity in Latvian barley varieties and some progenitors using DArT markers and studied the extent of linkage disequilibrium in Latvian germplasm. Further, genetic diversity at three loci flanking the barley powdery mildew Mla locus conferring race-specific resistance was studied in Latvian barley germplasm. The Mla locus encompasses several closely related resistance gene homologues with a complex evolutionary history, which complicates the design of molecular markers for different Mla genes. We observed significant linkage disequilibrium between the single nucleotide polymorphisms (SNPs) at the three loci, 206i20_T7, ABC15612, and 538P8, flanking the Mla locus. SNP haplotypes were largely in agreement with known phenotypic data and, thus, may be potentially diagnostic for Mla resistance genes in hybrids.

Resistance genes for rye stem rust (SrR) and barley powdery mildew (Mla) are located in syntenic regions on short arm of chromosome

Genetic stocks were developed for the localization and eventual cloning of the stem rust resistance gene SrR that occurs in wheat lines carrying the 1RS translocation from Secale cereale 'Imperial' rye. We have used a mutation-based approach for molecular analysis of the SrR region in rye. Forty-one independent mutants resulting in loss of SrR resistance were isolated: many of these were deletions of various sizes that were used to locate SrR with respect to chromosome group 1S markers. The analysis of the mutants showed that markers about 1 Mb apart flanking the barley Mla locus also flank SrR. Additionally, three of the approximately 20 closely related sequences of Mla in rye are deleted in each of six interstitial deletion mutants of SrR. The results indicate that the SrR region in rye is syntenic to the Mla region in barley or that SrR is possibly orthologous to the Mla locus.Key words: DRA-1, Gabo 1BL.1RS, imperial rye, interstitial deletion, resistance gene analog (RGA).

Recombination of alleles conferring specific resistance to powdery mildew at the Mla locus in barley

In barley (Hordeum vulgare L.), the Mla locus conditions reaction to the powdery mildew fungus Erysiphe graminis f.sp. hordei. Enrichment for genetic recombinants in the Mla region is possible by screening for recombination events between the flanking endosperm storage proteins hordeins C and B. Reciprocal crosses were made between the Franger (C.I. 16151) and Rupee (C.I. 16155) lines carrying the (Mla6 + Mla14) and Mla13 alleles, respectively. Recombinants were identified from F2 segregants by analyzing the extracted hordein polypeptides by sodium dodecyl sulphate – polyacrylamide gel electrophoresis. Two hundred and seventy-six recombinant gametes were identified from the 1800 seeds that were screened. Recombination of Mla alleles was analyzed by inoculating F4 recombinant lines with three isolates of E. graminis (A27, 5874, and CR3), which recognize specific Mla alleles. The linkage order established is Hor1–Mla6–Mla13–Mla14–Hor2. The genetic distances between Hor1–Mla6, Mla6–Mla13, and Mla13–Hor2, obtained using Mapmaker 3.0b F3 intercross analysis, are 3.9, 0.2, and 5.2 cM, respectively.Key words: recombinant, barley, powdery mildew, Mla, hordein.