The powdery mildew resistance gene Pm8 derived from rye is located on a 1BL.1RS chromosome translocation in wheat. However, some wheat lines with this translocation do not show resistance to isolates of the wheat powdery mildew pathogen avirulent to Pm8 due to an unknown genetically dominant suppression mechanism. Here we show that lines with suppressed Pm8 activity contain an intact and expressed Pm8 gene. Therefore, the absence of Pm8 function in certain 1BL.1RS-containing wheat lines is not the result of gene loss or mutation but is based on suppression. The wheat gene Pm3, an ortholog of rye Pm8, suppressed Pm8-mediated powdery mildew resistance in lines containing Pm8 in a transient single-cell expression assay. This result was further confirmed in transgenic lines with combined Pm8 and Pm3 transgenes. Expression analysis revealed that suppression is not the result of gene silencing, either in wheat 1BL.1RS translocation lines carrying Pm8 or in transgenic genotypes with both Pm8 and Pm3 alleles. In addition, a similar abundance of the PM8 and PM3 proteins in single or double homozygous transgenic lines suggested that a post-translational mechanism is involved in suppression of Pm8. Co-expression of Pm8 and Pm3 genes in Nicotiana benthamiana leaves followed by co-immunoprecipitation analysis showed that the two proteins interact. Therefore, the formation of a heteromeric protein complex might result in inefficient or absent signal transmission for the defense reaction. These data provide a molecular explanation for the suppression of resistance genes in certain genetic backgrounds and suggest ways to circumvent it in future plant breeding.

The Powdery Mildew Resistance Gene Pm8 Derived from Rye Is Suppressed By Its Wheat Ortholog Pm3

Durum wheat is one of the most important cultivated cereal crops, providing nutrients to humans and domestic animals Durum breeding programs prioritize the improvement of its main agronomic traits; however, the majority of these traits involve complex characteristics with a quantitative inheritance (quantitative trait loci, QTL) This can be solved with the use of genetic maps, new molecular markers, phenotyping data of segregating populations, and increased accessibility to sequences from next-generation sequencing (NGS) technologies This allows for high-density genetic maps to be developed for localizing candidate loci within a few Kb in a complex genome, such as durum wheat Here, we review the identified QTL, fine mapping, and cloning of QTL or candidate genes involved in the main traits regarding the quality and biotic and abiotic stresses of durum wheat The current knowledge on the used molecular markers, sequence data, and how they changed the development of genetic maps and the characterization of QTL is summarized A deeper understanding of the trait architecture useful in accelerating durum wheat breeding programs is envisioned

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From Genetic Maps to QTL Cloning: An Overview for Durum Wheat

Kernel morphology is one of the major yield traits of wheat, the genetic architecture of which is always important in crop breeding. In this study, we performed a genome-wide association study (GWAS) to appraise the genetic architecture of the kernel traits of 319 wheat accessions using 22,905 single nucleotide polymorphism (SNP) markers from a wheat 90K SNP array. As a result, 111 and 104 significant SNPs for Kernel traits were detected using four multi-locus GWAS models (mrMLM, FASTmrMLM, FASTmrEMMA, and pLARmEB) and three single-locus models (FarmCPU, MLM, and MLMM), respectively. Among the 111 SNPs detected by the multi-locus models, 24 SNPs were simultaneously detected across multiple models, including seven for kernel length, six for kernel width, six for kernels per spike, and five for thousand kernel weight. Interestingly, the five most stable SNPs (RAC875_29540_391, Kukri_07961_503, tplb0034e07_1581, BS00074341_51, and BobWhite_049_3064) were simultaneously detected by at least three multi-locus models. Integrating these newly developed multi-locus GWAS models to unravel the genetic architecture of kernel traits, the mrMLM approach detected the maximum number of SNPs. Furthermore, a total of 41 putative candidate genes were predicted to likely be involved in the genetic architecture underlining kernel traits. These findings can facilitate a better understanding of the complex genetic mechanisms of kernel traits and may lead to the genetic improvement of grain yield in wheat.

Appraising the Genetic Architecture of Kernel Traits in Hexaploid Wheat Using GWAS.

Wheat is the most widely grown crop and represents the staple food for one third of the world’s population. Wheat is attacked by a large variety of pathogens and the use of resistant cultivars is an effective and environmentally safe strategy for controlling diseases and eliminating the use of fungicides. In this study, a collection of wild and cultivated tetraploid wheats (Triticum turgidum) were evaluated for seedling resistance (SR) and adult plant resistance (APR) to powdery mildew (Blumeria graminis) and genotyped with a 90K single nucleotide polymorphism (SNP) array to identify new sources of resistance genes. The genome-wide association mapping detected 18 quantitative trait loci (QTL) for APR and 8 QTL for SR, four of which were identical or at least closely linked to four QTL for APR. Thirteen candidate genes, containing nucleotide binding sites and leucine-rich repeats, were localized in the confidence intervals of the QTL-tagging SNPs. The marker IWB6155, associated to QPm.mgb-1AS, was located within the gene TRITD1Av1G004560 coding for a disease resistance protein. While most of the identified QTL were described previously, five QTL for APR (QPm.mgb-1AS, QPm.mgb-2BS, QPm.mgb-3BL.1, QPm.mgb-4BL, QPm.mgb-7BS.1) and three QTL for SR (QPm.mgb-3BL.3, QPm.mgb-5AL.2, QPm.mgb-7BS.2) were mapped on chromosome regions where no resistance gene was reported before. The novel QTL/genes can contribute to enriching the resistance sources available to breeders.

https://www.mdpi.com/1422-0067/21/21/7910/pdf

Mapping Powdery Mildew (Blumeria graminis f. sp. tritici) Resistance in Wild and Cultivated Tetraploid Wheats.

Genome-wide characterization of NBS-LRR family genes and expression analysis under powdery mildew stress in Lagenaria siceraria

Powdery mildew (Blumeria graminis f. sp. tritici) results in serious economic loss in wheat production. Exploration of plant resistance to wheat powdery mildew over several decades has led to the discovery of a wealth of resistance genes and quantitative trait loci (QTLs). We have provided a comprehensive summary of over 200 powdery mildew genes (permanently and temporarily designated genes) and QTLs reported in common bread wheat. This highlights the diverse and rich resistance sources that exist across all 21 chromosomes. To manage different data for breeders, here we also present a bridged mapping result from previously reported powdery mildew resistance genes and QTLs with the application of a published integrated wheat map. This will provide important insights to empower further breeding of powdery mildew resistant wheat via marker‐assisted selection (MAS).

Mechanisms of powdery mildew resistance of wheat – a review of molecular breeding

Blumeria graminis f. sp. tritici, the causal agent of wheat powdery mildew disease, can occur at all stages of the crop and constantly threatens wheat production. To identify candidate resistance genes for powdery mildew, we performed GWAS (genome-wide association studies) on a total set of 329 wheat varieties obtained from different origins. These wheat materials were genotyped using wheat 90K SNP array and evaluated for their resistance in either field or glasshouse condition from 2016 to 2018. Using a mixed linear model, 33 SNP markers of which 14 QTL (quantitative trait loci) were later defined were observed to associate with powdery mildew resistance. Among these, QTL on chromosome 3A, 3B, 6D and 7D were concluded as potentially new QTL. Exploration of candidate genes for new QTL suggested roles of these genes involved in encoding disease resistance and defence-related proteins, and regulating early immune response to the pathogen. Overall, the results reveal that GWAS can be an effective means of identifying marker-trait associations, though further functional validation and fine-mapping of gene candidates are required before creating opportunities for developing new resistant genotypes.

Yichen Kang

Papers

Mechanisms of powdery mildew resistance of wheat – a review of molecular breeding

Genome-wide association mapping for adult resistance to powdery mildew in common wheat.