Fine mapping and comparative genomics integration of two quantitative trait loci controlling resistance to powdery mildew in a Spanish barley landrace

You Had Me at "MAGIC"!: Four Barley MAGIC Populations Reveal Novel Resistance QTL for Powdery Mildew

Blumeria graminis f. sp. hordei (Bgh), the causal agent of barley powdery mildew (PM), is one of the most important barley leaf diseases and is prevalent in most barley growing regions. Infection decreases grain quality and yields on average by 30%. Multi-parent advanced generation inter-cross (MAGIC) populations combine the advantages of bi-parental and association panels and offer the opportunity to incorporate exotic alleles into adapted material. Here, four barley MAGIC populations consisting of six to eight founders were tested for PM resistance in field trials in Denmark. Principle component and STRUCTURE analysis showed the populations were unstructured and genome-wide linkage disequilibrium (LD) decay varied between 14 and 38 Mbp. Genome-wide association studies (GWAS) identified 11 regions associated with PM resistance located on chromosomes 1H, 2H, 3H, 4H, 5H and 7H, of which three regions are putatively novel resistance quantitative trait locus/loci (QTL). For all regions high-confidence candidate genes were identified that are predicted to be involved in pathogen defense. Haplotype analysis of the significant SNPs revealed new allele combinations not present in the founders and associated with high resistance levels.

Specific Resistance of Barley to Powdery Mildew, Its Use and Beyond. A Concise Critical Review

Powdery mildew caused by the airborne ascomycete fungus Blumeria graminis f. sp. hordei (Bgh) is one of most common diseases of barley (Hordeum vulgare). This, as with many other plant pathogens, can be efficiently controlled by inexpensive and environmentally-friendly genetic resistance. General requirements for resistance to the pathogens are effectiveness and durability. Resistance of barley to Bgh has been studied intensively, and this review describes recent research and summarizes the specific resistance genes found in barley varieties since the last conspectus. Bgh is extraordinarily adaptable, and some commonly recommended strategies for using genetic resistance, including pyramiding of specific genes, may not be effective because they can only contribute to a limited extent to obtain sufficient resistance durability of widely-grown cultivars. In spring barley, breeding the nonspecific mlo gene is a valuable source of durable resistance. Pyramiding of nonspecific quantitative resistance genes or using introgressions derived from bulbous barley (Hordeum bulbosum) are promising ways for breeding future winter barley cultivars. The utilization of a wide spectrum of nonhost resistances can also be adopted once practical methods have been developed.

The gene conferring susceptibility to spot blotch caused by Cochliobolus sativus is located at the Mla locus in barley cultivar Bowman

We identified, fine mapped, and physically anchored a dominant spot blotch susceptibility gene Scs6 to a 125 kb genomic region containing the Mla locus on barley chromosome 1H. Spot blotch caused by Cochliobolus sativus is an important disease of barley, but the molecular mechanisms underlying resistance and susceptibility to the disease are not well understood. In this study, we identified and mapped a gene conferring susceptibility to spot blotch caused by the pathotype 2 isolate (ND90Pr) of C. sativus in barley cultivar Bowman. Genetic analysis of F₁ and F₂ progeny as well as F₃ families from a cross between Bowman and ND 5883 indicated that a single dominant gene (designated as Scs6) conferred spot blotch susceptibility in Bowman. Using a doubled haploid (DH) population derived from a cross between Calicuchima-sib (resistant) and Bowman-BC (susceptible), we confirmed that Scs6, contributed by Bowman-BC, was localized at the same locus as the previously identified spot blotch resistance allele Rcs6, which was contributed by Calicuchima-sib and mapped on the short arm of chromosome 1H. Using a genome-wide putative linear gene index of barley (Genome Zipper), 13 cleaved amplified polymorphism markers were developed from 11 flcDNA and two EST sequences and mapped to the Scs6/Rcs6 region on a linkage map constructed with the DH population. Further fine mapping with markers developed from barley genome sequences and F₂ recombinants derived from Bowman × ND 5883 and Bowman × ND B112 crosses delimited Scs6 in a 125 kb genomic interval harboring the Mla locus on the reference genome of barley cv. Morex. This study provides a foundational step for further cloning of Scs6 using a map-based approach.

Collinearity Analysis and High-Density Genetic Mapping of the Wheat Powdery Mildew Resistance Gene Pm40 in PI 672538

The wheat powdery mildew resistance gene Pm40, which is located on chromosomal arm 7BS, is effective against nearly all prevalent races of Blumeria graminis f. sp tritici (Bgt) in China and is carried by the common wheat germplasm PI 672538. A set of the F1, F2 and F2:3 populations from the cross of the resistant PI 672538 with the susceptible line L1034 were used to conduct genetic analysis of powdery mildew resistance and construct a high-density linkage map of the Pm40 gene. We constructed a high-density linkage genetic map with a total length of 6.18 cM and average spacing between markers of 0.48 cM.Pm40 is flanked by Xwmc335 and BF291338 at genetic distances of 0.58 cM and 0.26 cM, respectively, in deletion bin C-7BS-1-0.27. Comparative genomic analysis based on EST-STS markers established a high level of collinearity of the Pm40 genomic region with a 1.09-Mbp genomic region on Brachypodium chromosome 3, a 1.16-Mbp genomic region on rice chromosome 8, and a 1.62-Mbp genomic region on sorghum chromosome 7. We further anchored the Pm40 target intervals to the wheat genome sequence. A putative linear index of 85 wheat contigs containing 97 genes on 7BS was constructed. In total, 9 genes could be considered as candidates for the resistances to powdery mildew in the target genomic regions, which encoded proteins that were involved in the plant defense and response to pathogen attack. These results will facilitate the development of new markers for map-based cloning and marker-assisted selection of Pm40 in wheat breeding programs.

Isolation and fine mapping of Rps6: an intermediate host resistance gene in barley to wheat stripe rust

We uncouple host and nonhost resistance in barley to Puccinia striiformis ff. spp. hordei and tritici . We isolate, fine map, and physically anchor Rps6 to chromosome 7H in barley. A plant may be considered a nonhost of a pathogen if all known genotypes of a plant species are resistant to all known isolates of a pathogen species. However, if a small number of genotypes are susceptible to some known isolates of a pathogen species this plant may be considered an intermediate host. Barley (Hordeum vulgare) is an intermediate host for Puccinia striiformis f. sp. tritici (Pst), the causal agent of wheat stripe rust. We wanted to understand the genetic architecture underlying resistance to Pst and to determine whether any overlap exists with resistance to the host pathogen, Puccinia striiformis f. sp. hordei (Psh). We mapped Pst resistance to chromosome 7H and show that host and intermediate host resistance is genetically uncoupled. Therefore, we designate this resistance locus Rps6. We used phenotypic and genotypic selection on F2:3 families to isolate Rps6 and fine mapped the locus to a 0.1 cM region. Anchoring of the Rps6 locus to the barley physical map placed the region on a single fingerprinted contig spanning a physical region of 267 kb. Efforts are now underway to sequence the minimal tiling path and to delimit the physical region harboring Rps6. This will facilitate additional marker development and permit identification of candidate genes in the region.

Fine mapping and identification of a candidate gene for the barley Un8 true loose smut resistance gene

The candidate gene for the barley Un8 true loose smut resistance gene encodes a deduced protein containing two tandem protein kinase domains. In North America, durable resistance against all known isolates of barley true loose smut, caused by the basidiomycete pathogen Ustilago nuda (Jens.) Rostr. (U. nuda), is under the control of the Un8 resistance gene. Previous genetic studies mapped Un8 to the long arm of chromosome 5 (1HL). Here, a population of 4625 lines segregating for Un8 was used to delimit the Un8 gene to a 0.108 cM interval on chromosome arm 1HL, and assign it to fingerprinted contig 546 of the barley physical map. The minimal tilling path was identified for the Un8 locus using two flanking markers and consisted of two overlapping bacterial artificial chromosomes. One gene located close to a marker co-segregating with Un8 showed high sequence identity to a disease resistance gene containing two kinase domains. Sequence of the candidate gene from the parents of the segregating population, and in an additional 19 barley lines representing a broader spectrum of diversity, showed there was no intron in alleles present in either resistant or susceptible lines, and fifteen amino acid variations unique to the deduced protein sequence in resistant lines differentiated it from the deduced protein sequences in susceptible lines. Some of these variations were present within putative functional domains which may cause a loss of function in the deduced protein sequences within susceptible lines.

Brachypodium as an emerging model for cereal-pathogen interactions

BACKGROUND

Cereal diseases cause tens of billions of dollars of losses annually and have devastating humanitarian consequences in the developing world. Increased understanding of the molecular basis of cereal host-pathogen interactions should facilitate development of novel resistance strategies. However, achieving this in most cereals can be challenging due to large and complex genomes, long generation times and large plant size, as well as quarantine and intellectual property issues that may constrain the development and use of community resources. Brachypodium distachyon (brachypodium) with its small, diploid and sequenced genome, short generation time, high transformability and rapidly expanding community resources is emerging as a tractable cereal model.

SCOPE

Recent research reviewed here has demonstrated that brachypodium is either susceptible or partially susceptible to many of the major cereal pathogens. Thus, the study of brachypodium-pathogen interactions appears to hold great potential to improve understanding of cereal disease resistance, and to guide approaches to enhance this resistance. This paper reviews brachypodium experimental pathosystems for the study of fungal, bacterial and viral cereal pathogens; the current status of the use of brachypodium for functional analysis of cereal disease resistance; and comparative genomic approaches undertaken using brachypodium to assist characterization of cereal resistance genes. Additionally, it explores future prospects for brachypodium as a model to study cereal-pathogen interactions.

CONCLUSIONS

The study of brachypodium-pathogen interactions appears to be a productive strategy for understanding mechanisms of disease resistance in cereal species. Knowledge obtained from this model interaction has strong potential to be exploited for crop improvement.

Genotyping-by-sequencing based intra-specific genetic map refines a ''QTL-hotspot" region for drought tolerance in chickpea

To enhance the marker density in the “QTL-hotspot” region, harboring several QTLs for drought tolerance-related traits identified on linkage group 04 (CaLG04) in chickpea recombinant inbred line (RIL) mapping population ICC 4958 × ICC 1882, a genotyping-by-sequencing approach was adopted. In total, 6.24 Gb data from ICC 4958, 5.65 Gb data from ICC 1882 and 59.03 Gb data from RILs were generated, which identified 828 novel single-nucleotide polymorphisms (SNPs) for genetic mapping. Together with these new markers, a high-density intra-specific genetic map was developed that comprised 1,007 marker loci spanning a distance of 727.29 cM. QTL analysis using the extended genetic map along with precise phenotyping data for 20 traits collected over one to seven seasons identified 49 SNP markers in the “QTL-hotspot” region. These efforts have refined the “QTL-hotspot” region to 14 cM. In total, 164 main-effect QTLs including 24 novel QTLs were identified. In addition, 49 SNPs integrated in the “QTL-hotspot” region were converted into cleaved amplified polymorphic sequence (CAPS) and derived CAPS (dCAPS) markers which can be used in marker-assisted breeding.

Towards positional isolation of three quantitative trait loci conferring resistance to powdery mildew in two Spanish barley landraces

Three quantitative trait loci (QTL) conferring broad spectrum resistance to powdery mildew, caused by the fungus Blumeria graminis f. sp. hordei, were previously identified on chromosomes 7HS, 7HL and 6HL in the Spanish barley landrace-derived lines SBCC097 and SBCC145. In the present work, a genome-wide putative linear gene index of barley (Genome Zipper) and the first draft of the physical, genetic and functional sequence of the barley genome were used to go one step further in the shortening and explicit demarcation on the barley genome of these regions conferring resistance to powdery mildew as well as in the identification of candidate genes. First, a comparative analysis of the target regions to the barley Genome Zippers of chromosomes 7H and 6H allowed the development of 25 new gene-based molecular markers, which slightly better delimit the QTL intervals. These new markers provided the framework for anchoring of genetic and physical maps, figuring out the outline of the barley genome at the target regions in SBCC097 and SBCC145. The outermost flanking markers of QTLs on 7HS, 7HL and 6HL defined a physical area of 4 Mb, 3.7 Mb and 3.2 Mb, respectively. In total, 21, 10 and 16 genes on 7HS, 7HL and 6HL, respectively, could be interpreted as potential candidates to explain the resistance to powdery mildew, as they encode proteins of related functions with respect to the known pathogen defense-related processes. The majority of these were annotated as belonging to the NBS-LRR class or protein kinase family.

Genomics-based high-resolution mapping of the BaMMV/BaYMV resistance gene rym11 in barley (Hordeum vulgare L.)

Soil-borne barley yellow mosaic virus disease, caused by different strains of Barley yellow mosaic virus (BaYMV) and Barley mild mosaic virus (BaMMV), is one of the most important diseases of winter barley (Hordeum vulgare L.) in Europe and East Asia. The recessive resistance gene rym11 located in the centromeric region of chromosome 4HL is effective against all so far known strains of BaMMV and BaYMV in Germany. In order to isolate this gene, a high-resolution mapping population (10,204 meiotic events) has been constructed. F2 plants were screened with co-dominant flanking markers and segmental recombinant inbred lines (RILs) were tested for resistance to BaMMV under growth chamber and field conditions. Tightly linked markers were developed by exploiting (1) publicly available barley EST sequences, (2) employing barley synteny to rice, Brachypodium distachyon and sorghum and (3) using next-generation sequencing data of barley. Using this approach, the genetic interval was efficiently narrowed down from the initial 10.72 % recombination to 0.074 % recombination. A marker co-segregating with rym11 was developed providing the basis for gene isolation and efficient marker-assisted selection.