Identification of quantitative trait loci associated with resistance to net form net blotch in a collection of Nordic barley germplasm

High resolution mapping of a novel non-transgressive hybrid susceptibility locus in barley exploited by P. teres f. maculata

Hybrid genotypes can provide significant yield gains over conventional inbred varieties due to heterosis or hybrid vigor. However, hybrids can also display unintended negative attributes or phenotypes such as extreme pathogen susceptibility. The necrotrophic pathogen Pyrenophora teres f. maculata (Ptm) causes spot form net blotch, which has caused significant yield losses to barley worldwide. Here, we report on a non-transgressive hybrid susceptibility locus in barley identified between the three parental lines CI5791, Tifang and Golden Promise that are resistant to Ptm isolate 13IM.3. However, F2 progeny from CI5791 × Tifang and CI5791 × Golden Promise crosses exhibited extreme susceptibility. The susceptible phenotype segregated in a ratio of 1 resistant:1 susceptible representing a genetic segregation ratio of 1 parental (res):2 heterozygous (sus):1 parental (res) suggesting a single hybrid susceptibility locus. Genetic mapping using a total of 715 CI5791 × Tifang F2 individuals (1430 recombinant gametes) and 149 targeted SNPs delimited the hybrid susceptibility locus designated Susceptibility to Pyrenophora teres 2 (Spt2) to an ~ 198 kb region on chromosome 5H of the Morex V3 reference assembly. This single locus was independently mapped with 83 CI5791 × Golden Promise F2 individuals (166 recombinant gametes) and 180 genome wide SNPs that colocalized to the same Spt2 locus. The CI5791 genome was sequenced using PacBio Continuous Long Read technology and comparative analysis between CI5791 and the publicly available Golden Promise genome assembly determined that the delimited region contained a single high confidence Spt2 candidate gene predicted to encode a pentatricopeptide repeat-containing protein.

A Moroccan Pyrenophora teres f. teres Population Defeats Rpt5, the Broadly Effective Resistance on Barley Chromosome 6H

Net form net blotch (NFNB), caused by Pyrenophora teres f. teres, is an important barley disease. The centromeric region of barley chromosome 6H has often been associated with resistance or susceptibility to NFNB, including the broadly effective dominant resistance gene Rpt5 derived from barley line CIho 5791. We characterized a population of Moroccan P. teres f. teres isolates that had overcome Rpt5 resistance and identified quantitative trait loci (QTL) that were effective against these isolates. Eight Moroccan P. teres f. teres isolates were phenotyped on barley lines CIho 5791 and Tifang. Six isolates were virulent on CIho 5791, and two were avirulent. A CIho 5791 × Tifang recombinant inbred line (RIL) population was phenotyped with all eight isolates and confirmed the defeat of the 6H resistance locus formerly mapped as Rpt5 in barley line CI9819. A major QTL on chromosome 3H with the resistance allele derived from Tifang, as well as minor QTL, was identified and provided resistance against these isolates. F2 segregation ratios supported dominant inheritance for both the 3H and 6H resistance. Furthermore, inoculation of progeny isolates derived from a cross of P. teres f. teres isolates 0-1 (virulent on Tifang/avirulent on CIho 5791) and MorSM 40-3 (avirulent on Tifang/virulent on CIho 5791) onto the RIL and F2 populations determined that recombination between isolates can generate novel genotypes that overcome both resistance genes. Markers linked to the QTL identified in this study can be used to incorporate both resistance loci into elite barley cultivars for durable resistance.

Genome-wide association mapping of Pyrenophora teres f. maculata and Pyrenophora teres f. teres resistance loci utilizing natural Turkish wild and landrace barley populations

Unimproved landraces and wild relatives of crops are sources of genetic diversity that were lost post domestication in modern breeding programs. To tap into this rich resource, genome-wide association studies in large plant genomes have enabled the rapid genetic characterization of desired traits from natural landrace and wild populations. Wild barley (Hordeum spontaneum), the progenitor of domesticated barley (Hordeum vulgare), is dispersed across Asia and North Africa, and has co-evolved with the ascomycetous fungal pathogens Pyrenophora teres f. teres and P. teres f. maculata, the causal agents of the diseases net form of net blotch and spot form of net blotch, respectively. Thus, these wild and local adapted barley landraces from the region of origin of both the host and pathogen represent a diverse gene pool to identify new sources of resistance, due to millions of years of co-evolution. The barley—P. teres pathosystem is governed by complex genetic interactions with dominant, recessive, and incomplete resistances and susceptibilities, with many isolate-specific interactions. Here, we provide the first genome-wide association study of wild and landrace barley from the Fertile Crescent for resistance to both forms of P. teres. A total of 14 loci, four against P. teres f. maculata and 10 against P. teres f. teres, were identified in both wild and landrace populations, showing that both are genetic reservoirs for novel sources of resistance. We also highlight the importance of using multiple algorithms to both identify and validate additional loci.

QTL mapping of web blotch resistance in peanut by high-throughput genome-wide sequencing

BACKGROUND

Web blotch is one of the most important foliar diseases worldwide in peanut (Arachis hypogaea L.). The identification of quantitative trait loci (QTLs) for peanut web blotch resistance represents the basis for gene mining and the application of molecular breeding technologies.

RESULTS

In this study, a peanut recombinant inbred line (RIL) population was used to map QTLs for web blotch resistance based on high-throughput genome-wide sequencing. Frequency distributions of disease grade and disease index in five environments indicated wide phenotypic variations in response to web blotch among RILs. A high-density genetic map was constructed, containing 3634 bin markers distributed on 20 peanut linkage groups (LGs) with an average genetic distance of 0.5 cM. In total, eight QTLs were detected for peanut web blotch resistance in at least two environments, explaining from 2.8 to 15.1% of phenotypic variance. Two major QTLs qWBRA04 and qWBRA14 were detected in all five environments and were linked to 40 candidate genes encoding nucleotide-binding site leucine-rich repeat (NBS-LRR) or other proteins related to disease resistances.

CONCLUSIONS

The results of this study provide a basis for breeding peanut cultivars with web blotch resistance.

Identification of quantitative trait loci for net form net blotch resistance in contemporary barley breeding germplasm from the USA using genome-wide association mapping

Association mapping study conducted in a population of 3490 elite barley breeding lines from ten barley breeding programs of the USA identified 12 QTLs for resistance/susceptibility to net form of net blotch. Breeding resistant varieties is the best management strategy for net form of net blotch (NFNB) in barley (Hordeum vulgare L.) caused by Pyrenophora teres f. teres (Ptt). Several resistance QTL have been previously identified in barley via linkage mapping and genome-wide association studies (GWAS). A GWAS conducted in a collection of advanced breeding lines (n = 3490) representing elite germplasm from ten barley breeding programs of the USA identified 42 unique marker-trait associations (MTA) for NFNB resistance. The lines were genotyped with 3072 SNP markers and phenotyped with four Ptt isolates in controlled environment. The lines were used to construct 13 different GWAS panels. Efficient mixed model association method with principal components and kinship was used for GWAS. Significance threshold for MTA was set at a false discovery rate of 0.05. Two, eight, six, one and 25 MTA were identified in chromosomes 1H, 3H, 4H, 5H and 6H, respectively. Based on genetic positions and linkage disequilibrium, these MTA's correspond to two, three, two, one and four QTLs in chromosome 1H, 3H, 4H, 5H and 6H, respectively. A comparison with previous linkage and GWAS studies revealed several previously identified and novel QTLs. Moreover, different genomic regions were found to be responsible for NFNB resistance in two-row versus six-row germplasm. The germplasm-specific SNP markers with additive effects and allelic distribution is reported to facilitate breeders in selection of markers for MAS to introgress novel net blotch resistance.

Research advances in the Pyrenophora teres-barley interaction

Pyrenophora teres f. teres and P. teres f. maculata are significant pathogens that cause net blotch of barley. An increased number of loci involved in P. teres resistance or susceptibility responses of barley as well as interacting P. teres virulence effector loci have recently been identified through biparental and association mapping studies of both the pathogen and host. Characterization of the resistance/susceptibility loci in the host and the interacting effector loci in the pathogen will provide a path for targeted gene validation for better-informed release of resistant barley cultivars. This review assembles concise consensus maps for all loci published for both the host and pathogen, providing a useful resource for the community to be used in pathogen characterization and barley breeding for resistance to both forms of P. teres.

Genetic analysis of a worldwide barley collection for resistance to net form of net blotch disease (Pyrenophora teres f. teres)

A total of 449 barley accessions were phenotyped for Pyrenophora teres f. teres resistance at three locations and in greenhouse trials. Genome-wide association studies identified 254 marker-trait associations corresponding to 15 QTLs. Net form of net blotch is one of the most important diseases of barley and is present in all barley growing regions. Under optimal conditions, it causes high yield losses of 10-40% and reduces grain quality. The most cost-effective and environmentally friendly way to prevent losses is growing resistant cultivars, and markers linked to effective resistance factors can accelerate the breeding process. Here, 449 barley accessions expressing different levels of resistance comprising landraces and commercial cultivars from the centres of diversity were selected. The set was phenotyped for seedling resistance to three isolates in controlled-environment tests and for adult plant resistance at three field locations (Belarus, Germany and Australia) and genotyped with the 50 k iSelect chip. Genome-wide association studies using 33,818 markers and a compressed mixed linear model to account for population structure and kinship revealed 254 significant marker-trait associations corresponding to 15 distinct QTL regions. Four of these regions were new QTL that were not described in previous studies, while a total of seven regions influenced resistance in both seedlings and adult plants.

Harnessing Novel Diversity From Landraces to Improve an Elite Barley Variety

The Spanish Barley Core Collection (SBCC) is a source of genetic variability of potential interest for breeding, particularly for adaptation to Mediterranean environments. Two backcross populations (BC2F5) were developed using the elite cultivar Cierzo as the recurrent parent. The donor parents, namely SBCC042 and SBCC073, were selected from the SBCC lines due to their outstanding yield in drought environments. Flowering time, yield and drought-related traits were evaluated in two field trials in Zaragoza (Spain) during the 2014-15 and 2015-16 seasons and validated in the 2017-18 season. Two hundred sixty-four lines of each population were genotyped with the Barley Illumina iSelect 50k SNP chip. Genetic maps for each population were generated. The map for SBCC042 × Cierzo contains 12,893 SNPs distributed in 9 linkage groups. The map for SBCC073 × Cierzo includes 12,026 SNPs in 7 linkage groups. Both populations shared two QTL hotspots. There are QTLs for flowering time, thousand-kernel weight (TKW), and hectoliter weight on a segment of 23 Mb at ~515 Mb on chromosome 1H, which encompasses the HvFT3 gene. In both populations, flowering was accelerated by the landrace allele, which also increased the TKW. In the same region, better soil coverage was contributed by SBCC042 but coincident with a lower hectoliter weight. The second large hotspot was on chromosome 6H and contained QTLs with wide intervals for grain yield, plant height and TKW. Landrace alleles contributed to increased plant height and TKW and reduced grain yield. Only SBCC042 contributed favorable alleles for "green area," with three significant QTLs that increased ground coverage after winter, which might be exploited as an adaptive trait of this landrace. Some genes of interest found in or very close to the peaks of the QTLs are highlighted. Strategies to deploy the QTLs found for breeding and pre-breeding are proposed.