Dissection of loci conferring resistance to stripe rust in Chinese wheat landraces from the middle and lower reaches of the Yangtze River via genome-wide association study

Broad-spectrum resistance to fungal foliar diseases in wheat: recent efforts and achievements

Wheat (Triticum spp.) is one of the most important cereal crops in the world. Several diseases affect wheat production and can cause 20-80% yield loss annually. Out of these diseases, stripe rust, also known as yellow rust (Puccinia striiformis f. sp. tritici), stem rust (Puccinia graminis f. sp. tritici), leaf rust (Puccinia recondita), and powdery mildew (Blumeria graminis f. sp. tritici) are the most important fungal diseases that infect the foliar part of the plant. Many efforts were made to improve wheat resistance to these diseases. Due to the continuous advancement in sequencing methods and genomic tools, genome-wide association study has become available worldwide. This analysis enabled wheat breeders to detect genomic regions controlling the resistance in specific countries. In this review, molecular markers significantly associated with the resistance of the mentioned foliar diseases in the last five years were reviewed. Common markers that control broad-spectrum resistance in different countries were identified. Furthermore, common genes controlling the resistance of more than one of these foliar diseases were identified. The importance of these genes, their functional annotation, and the potential for gene enrichment are discussed. This review will be valuable to wheat breeders in producing genotypes with broad-spectrum resistance by applying genomic selection for the target common markers and associated genes.

Favorable Loci Identified for Stripe Rust Resistance in Chinese Winter Wheat Accessions via Genome-Wide Association Study

Stripe rust (or yellow rust), caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most devastating diseases of wheat worldwide. Currently, the utilization of resistant cultivars is the most viable way to reduce yield losses. In this study, a panel of 188 wheat accessions from China was evaluated for stripe rust resistance, and genome-wide association studies were performed using high-quality Diversity Arrays Technology markers. According to the phenotype and genotype data, a total of 26 significant marker-trait associations were identified, representing 18 quantitative trait loci (QTLs) on chromosomes 1B, 2A, 2B, 3A, 3B, 5A, 5B, 6B, 7B, and 7D. Of the 18 QTLs, almost all were associated with adult plant resistance (APR) except QYr.nwsuaf-6B.2, which was associated with all-stage resistance (also known as seedling resistance). Three of the 18 QTLs were mapped far from previously identified Pst resistance genes and QTLs and were considered potentially new loci. The other 15 QTLs were mapped close to known resistance genes and QTLs. Subsequent haplotype analysis for QYr.nwsuaf-2A and QYr.nwsuaf-7B.3 revealed the degrees of resistance of the panel in the APR stage. In summary, the favorable alleles identified in this study may be useful in breeding for disease resistance to stripe rust.

Exome Sequencing from Bulked Segregant Analysis Identifies a Gene for All-Stage Resistance to Stripe Rust on Chromosome 1AL in Chinese Wheat Landrace 'Xiaohemai'

Stripe rust caused by Puccinia striiformis f. sp. tritici is one of the most destructive diseases of wheat. Identifying novel resistance genes applicable for developing disease-resistant cultivars is important for the sustainable control of wheat stripe rust. Chinese wheat landrace 'Xiaohemai' ('XHM') is an elite germplasm line with all-stage resistance (ASR) effective against predominant Chinese P. striiformis f. sp. tritici races. In this study, we performed a bulked segregant analysis coupled with exome capture sequencing (BSE-seq) to identify a candidate genomic region strongly associated with stripe rust resistance on chromosome 1AL in 173 F_2:3 lines derived from the cross 'XHM' × 'Avocet S'. The gene, designated as YrXH-1AL, was validated by a conventional quantitative trait locus analysis using newly developed Kompetitive allele-specific PCR (KASP) markers, explaining up to 48.50% of the phenotypic variance. By testing a secondary mapping population comprising 144 lines from the same cross at the seedling stage with prevalent P. striiformis f. sp. tritici race CYR34, YrXH-1AL was identified as a single Mendelian factor in a 1.5-cM interval flanked by KASP markers KP1A_484.33 and KP1A_490.09. This region corresponded to a 5.76-Mb genomic interval on 'Chinese Spring' chromosome 1AL. Furthermore, two cosegregating KASP markers showed high polymorphisms among 130 Chinese wheat cultivars and could be used for marker-assisted selection. Because no other Yr genes for ASR that originated from common wheat have been detected on chromosome 1AL, YrXH-1AL is likely a novel gene that can be incorporated into modern breeding materials to develop wheat cultivars with enhanced stripe rust resistance.

Comprehensive evaluation of mapping complex traits in wheat using genome-wide association studies

Genome-wide association studies (GWAS) are effectively applied to detect the marker trait associations (MTAs) using whole genome-wide variants for complex quantitative traits in different crop species. GWAS has been applied in wheat for different quality, biotic and abiotic stresses, and agronomic and yield-related traits. Predictions for marker-trait associations are controlled with the development of better statistical models taking population structure and familial relatedness into account. In this review, we have provided a detailed overview of the importance of association mapping, population design, high-throughput genotyping and phenotyping platforms, advancements in statistical models and multiple threshold comparisons, and recent GWA studies conducted in wheat. The information about MTAs utilized for gene characterization and adopted in breeding programs is also provided. In the literature that we surveyed, as many as 86,122 wheat lines have been studied under various GWA studies reporting 46,940 loci. However, further utilization of these is largely limited. The future breakthroughs in area of genomic selection, multi-omics-based approaches, machine, and deep learning models in wheat breeding after exploring the complex genetic structure with the GWAS are also discussed. This is a most comprehensive study of a large number of reports on wheat GWAS and gives a comparison and timeline of technological developments in this area. This will be useful to new researchers or groups who wish to invest in GWAS.

Molecular Mapping and Analysis of an Excellent Quantitative Trait Loci Conferring Adult-Plant Resistance to Stripe Rust in Chinese Wheat Landrace Gaoxianguangtoumai

The Chinese wheat landrace "Gaoxianguangtoumai" (GX) has exhibited a high level of adult-plant resistance (APR) to stripe rust in the field for more than a decade. To reveal the genetic background for APR to stripe rust in GX, a set of 249 F_6:8 (F₆, F₇, and F₈) recombinant inbred lines (RILs) was developed from a cross between GX and the susceptible cultivar "Taichung 29." The parents and RILs were evaluated for disease severity at the adult-plant stage in the field by artificial inoculation with the currently predominant Chinese Puccinia striiformis f. sp. tritici races during three cropping seasons and genotyped using the Wheat 55K single-nucleotide polymorphism (SNP) array to construct a genetic map with 1,871 SNP markers finally. Two stable APR quantitative trait loci (QTL), QYr.GX-2AS and QYr.GX-7DS in GX, were detected on chromosomes 2AS and 7DS, which explained 15.5-27.0% and 11.5-13.5% of the total phenotypic variation, respectively. Compared with published Yr genes and QTL, QYr.GX-7DS and Yr18 may be the same, whereas QYr.GX-2AS is likely to be novel. Haplotype analysis revealed that QYr.GX-2AS is likely to be rare which presents in 5.3% of the 325 surveyed Chinese wheat landraces. By analyzing a heterogeneous inbred family (HIF) population from a residual heterozygous plant in an F₈ generation of RIL, QYr.GX-2AS was further flanked by KP2A_36.85 and KP2A_38.22 with a physical distance of about 1.37Mb and co-segregated with the KP2A_37.09. Furthermore, three tightly linked Kompetitive allele-specific PCR (KASP) markers were highly polymorphic among 109 Chinese wheat cultivars. The results of this study can be used in wheat breeding for improving resistance to stripe rust.

A Stable Quantitative Trait Locus on Chromosome 5BL Combined with Yr18 Conferring High-Level Adult Plant Resistance to Stripe Rust in Chinese Wheat Landrace Anyuehong

Chinese wheat landrace Anyuehong (AYH) has displayed high levels of stable adult plant resistance (APR) to stripe rust for >15 years. To identify quantitative trait loci (QTLs) for stripe rust resistance in AYH, a set of 110 recombinant inbred lines (RILs) was developed from a cross between AYH and susceptible cultivar Taichung 29. The parents and RILs were evaluated for final disease severity (FDS) in six field tests with a mixture of predominant Puccinia striiformis f. sp. tritici races at the adult plant stage and genotyped via the wheat 55K single-nucleotide polymorphism (SNP) array to construct a genetic map with 1,143 SNP markers. Three QTLs, designated as QYr.AYH-1AS, QYr.AYH-5BL, and QYr.AYH-7DS, were mapped on chromosome 1AS, 5BL, and 7DS, respectively. RILs combining three QTLs showed significantly lower FDS compared with the lines in other combinations. Of them, QYr.AYH-5BL and QYr.AYH-7DS were stably detected in all environments, explaining 13.6 to 21.4% and 17.6 to 33.6% of phenotypic variation, respectively. Compared with previous studies, QYr.AYH-5BL may be a new QTL, whereas QYr.AYH-7DS may be Yr18. Haplotype analysis revealed that QYr.AYH-5BL is probably present in 6.2% of the 323 surveyed Chinese wheat landraces. The kompetitive allele specific PCR (KASP) markers for QYr.AYH-5BL were developed by the linked SNP markers to successfully confirm the effects of the QTL in a validation population derived from a residual heterozygous line and were further assessed in 38 Chinese wheat landraces and 92 cultivars. Our results indicated that QYr.AYH-5BL with linked KASP markers has potential value for marker-assisted selection to improve stripe rust resistance in breeding programs.

Molecular Mapping of a Novel Quantitative Trait Locus Conferring Adult Plant Resistance to Stripe Rust in Chinese Wheat Landrace Guangtoumai

Stripe rust (yellow rust), caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases of wheat worldwide. Chinese wheat landrace Guangtoumai (GTM) exhibited a high level of resistance against predominant P. striiformis f. sp. tritici races in China at the adult plant stage. The objective of this research was to identify and map the major locus/loci for stripe rust resistance in GTM. A set of 212 recombinant inbred lines (RILs) was developed from a cross between GTM and Avocet S. The parents and RILs were evaluated in three field tests (2018, 2019, and 2020 at Chongzhou, Sichuan) with the currently predominant P. striiformis f. sp. tritici races for final disease severity and genotyped with the Wheat 55K single nucleotide polymorphism (SNP) array to construct a genetic map with 1,031 SNP markers. A major locus, named QYr.GTM-5DL, was detected on chromosome 5DL in GTM. The locus was mapped in a 2.75-cM interval flanked by SNP markers AX-109855976 and AX-109453419, explaining up to 44.4% of the total phenotypic variation. Since no known Yr genes have been reported on chromosome 5DL, QYr.GTM-5DL is very likely a novel adult plant resistance locus. Haplotype analysis revealed that the resistance allele displayed enhanced levels of stripe rust resistance and is likely present in 5.3% of the 247 surveyed Chinese wheat landraces. The derived cleaved amplified polymorphic sequence (dCAPS) marker dCAPS-5722, converted from a SNP marker tightly linked to QYr.GTM-5DL with 0.3 cM, was validated on a subset of RILs and 48 commercial wheat cultivars developed in Sichuan. The results indicated that QYr.GTM-5DL with its linked dCAPS marker could be used in marker-assisted selection to improve stripe rust resistance in breeding programs, and this quantitative trait locus will provide new and possibly durable resistance to stripe rust.

Genome-wide association mapping reveals potential novel loci controlling stripe rust resistance in a Chinese wheat landrace diversity panel from the southern autumn-sown spring wheat zone

BACKGROUND

Stripe rust, caused by the fungal pathogen Puccinia striiformis f. sp. tritici (Pst), is a serious foliar disease of wheat. Identification of novel stripe rust resistance genes and cultivation of resistant cultivars are considered to be the most effective approaches to control this disease. In this study, we evaluated the infection type (IT), disease severity (DS) and area under the disease progress curve (AUDPC) of 143 Chinese wheat landrace accessions for stripe rust resistance. Assessments were undertaken in five environments at the adult-plant stage with Pst mixture races under field conditions. In addition, IT was assessed at the seedling stage with two prevalent Pst races (CYR32 and CYR34) under a controlled greenhouse environment.

RESULTS

Seventeen accessions showed stable high-level resistance to stripe rust across all environments in the field tests. Four accessions showed resistance to the Pst races CYR32 and CYR34 at the seedling stage. Combining phenotypic data from the field and greenhouse trials with 6404 markers that covered the entire genome, we detected 17 quantitative trait loci (QTL) on 11 chromosomes for IT associated with seedling resistance and 15 QTL on seven chromosomes for IT, final disease severity (FDS) or AUDPC associated with adult-plant resistance. Four stable QTL detected on four chromosomes, which explained 9.99-23.30% of the phenotypic variation, were simultaneously associated with seedling and adult-plant resistance. Integrating a linkage map of stripe rust resistance in wheat, 27 QTL overlapped with previously reported genes or QTL, whereas four and one QTL conferring seedling and adult-plant resistance, respectively, were mapped distantly from previously reported stripe rust resistance genes or QTL and thus may be novel resistance loci.

CONCLUSIONS

Our results provided an integrated overview of stripe rust resistance resources in a wheat landrace diversity panel from the southern autumn-sown spring wheat zone of China. The identified resistant accessions and resistance loci will be useful in the ongoing effort to develop new wheat cultivars with strong resistance to stripe rust.

Genome-wide association analysis of stripe rust resistance in modern Chinese wheat

BACKGROUND

Stripe rust (yellow rust) is a significant disease for bread wheat (Triticum aestivum L.) worldwide. A genome-wide association study was conducted on 240 Chinese wheat cultivars and elite lines genotyped with the wheat 90 K single nucleotide polymorphism (SNP) arrays to decipher the genetic architecture of stripe rust resistance in Chinese germplasm.

RESULTS

Stripe rust resistance was evaluated at the adult plant stage in Pixian and Xindu in Sichuan province in the 2015-2016 cropping season, and in Wuhan in Hubei province in the 2013-2014, 2016-2017 and 2018-2019 cropping seasons. Twelve stable loci for stripe rust resistance were identified by GWAS using TASSEL and GAPIT software. These loci were distributed on chromosomes 1B, 1D, 2A, 2B, 3A, 3B, 4B (3), 4D, 6D, and 7B and explained 3.6 to 10.3% of the phenotypic variation. Six of the loci corresponded with previously reported genes/QTLs, including Sr2/Yr30/Lr27, while the other six (QYr.hbaas-1BS, QYr.hbaas-2BL, QYr.hbaas-3AL, QYr.hbaas-4BL.3, QYr.hbaas-4DL, and QYr.hbaas-6DS) are probably novel. The results suggest high genetic diversity for stripe rust resistance in this population. The resistance alleles of QYr.hbaas-2AS, QYr.hbaas-3BS, QYr.hbaas-4DL, and QYr.hbaas-7BL were rare in the present panel, indicating their potential use in breeding for stripe rust resistance in China. Eleven penta-primer amplification refractory mutation system (PARMS) markers were developed from SNPs significantly associated with seven mapped QTLs. Twenty-seven genes were predicted for mapped QTLs. Six of them were considered as candidates for their high relative expression levels post-inoculation.

CONCLUSION

The resistant germplasm, mapped QTLs, and PARMS markers developed in this study are resources for enhancing stripe rust resistance in wheat breeding.

Identification of Stripe Rust Resistance Loci in U.S. Spring Wheat Cultivars and Breeding Lines Using Genome-Wide Association Mapping and Yr Gene Markers

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), poses a major threat to wheat production worldwide, especially in the United States. To identify loci for effective stripe rust resistance in U.S. wheat, a genome-wide association study (GWAS) was conducted using a panel of 616 spring wheat cultivars and breeding lines. The accessions in this panel were phenotyped for stripe rust response in the greenhouse at seedling stage with five predominant and highly virulent races of Pst and in different field environments at adult-plant stage in 2017 and 2018. In total, 2,029 single-nucleotide polymorphism markers that cover the whole genome were generated with genotyping by multiplexed sequencing and used in GWAS. In addition, 23 markers of previously reported resistance genes or quantitative trait loci (QTLs) were used to genotype the population. This spring panel was grouped into three subpopulations based on principal component analysis. A total of 37 genes or QTLs including 10 potentially new QTLs for resistance to stripe rust were detected by GWAS and linked marker tests. The frequencies of the resistance genes or QTLs in various nurseries were determined, indicating different intensities of these genes or QTLs used in breeding programs of different regions. These resistance loci and the information on their markers, effectiveness, and distributions should be useful for improving stripe rust resistance in wheat cultivars.

Genome-Wide Association Study and Gene Specific Markers Identified 51 Genes or QTL for Resistance to Stripe Rust in U.S. Winter Wheat Cultivars and Breeding Lines

Stripe (yellow) rust, caused by fungal pathogen Puccinia striiformis f. sp. tritici (Pst), is a serious disease of wheat in the United States and many other countries. Growing resistant cultivars has been approved to be the best approach for control of stripe rust. To determine stripe rust resistance genes in U.S. winter wheat cultivars and breeding lines, we analyzed a winter wheat panel of 857 cultivars and breeding lines in a genome-wide association study (GWAS) using genotyping by multiplexed sequencing (GMS) and by genotyping with molecular markers of 18 important stripe rust resistance genes or quantitative trait loci (QTL). The accessions were phenotyped for stripe rust response at adult-plant stage under natural infection in Pullman and Mount Vernon, Washington in 2018 and 2019, and in the seedling stage with six predominant or most virulent races of Pst. A total of 51 loci were identified to be related to stripe rust resistance, and at least 10 of them (QYrww.wgp.1D-3, QYrww.wgp.2B-2, QYrww.wgp.2B-3, QYrww.wgp.2B-4, QYrww.wgp.3A, QYrww.wgp.5A, QYrww.wgp.5B, QYrww.wgp.5D, QYrww.wgp.6A-2 and QYrww.wgp.7B-3) were previously reported. These genes or QTL were found to be present at different frequencies in breeding lines and cultivars developed by breeding programs in various winter wheat growing regions. Both Yr5 and Yr15, which are highly resistant to all races identified thus far in the U.S., as well as Yr46 providing resistance to many races, were found absent in the breeding lines and commercially grown cultivars. The identified genes or QTL and their markers are useful in breeding programs to improve the level and durability of resistance to stripe rust.