Characterization of a Segregation Distortion Locus with Powdery Mildew Resistance in a Wheat-Thinopyrum intermedium Introgression Line WE99

Fighting wheat powdery mildew: from genes to fields

KEY MESSAGE

Host resistance conferred by Pm genes provides an effective strategy to control powdery mildew. The study of Pm genes helps modern breeding develop toward more intelligent and customized. Powdery mildew of wheat is one of the most destructive diseases seriously threatening the crop yield and quality worldwide. The genetic research on powdery mildew (Pm) resistance has entered a new era. Many Pm genes from wheat and its wild and domesticated relatives have been mined and cloned. Meanwhile, modern breeding strategies based on high-throughput sequencing and genome editing are emerging and developing toward more intelligent and customized. This review highlights mining and cloning of Pm genes, molecular mechanism studies on the resistance and avirulence genes, and prospects for genomic-assisted breeding for powdery mildew resistance in wheat.

GWAS Reveals a Novel Candidate Gene CmoAP2/ERF in Pumpkin (Cucurbita moschata) Involved in Resistance to Powdery Mildew

Pumpkin (Cucurbita moschata Duchesne ex Poir.) is a multipurpose cash crop rich in antioxidants, minerals, and vitamins; the seeds are also a good source of quality oils. However, pumpkin is susceptible to the fungus Podosphaera xanthii, an obligate biotrophic pathogen, which usually causes powdery mildew (PM) on both sides of the leaves and reduces photosynthesis. The fruits of infected plants are often smaller than usual and unpalatable. This study identified a novel gene that involves PM resistance in pumpkins through a genome-wide association study (GWAS). The allelic variation identified in the CmoCh3G009850 gene encoding for AP2-like ethylene-responsive transcription factor (CmoAP2/ERF) was proven to be involved in PM resistance. Validation of the GWAS data revealed six single nucleotide polymorphism (SNP) variations in the CmoAP2/ERF coding sequence between the resistant (IT 274039 [PMR]) and the susceptible (IT 278592 [PMS]). A polymorphic marker (dCAPS) was developed based on the allelic diversity to differentiate these two haplotypes. Genetic analysis in the segregating population derived from PMS and PMR parents provided evidence for an incomplete dominant gene-mediated PM resistance. Further, the qRT-PCR assay validated the elevated expression of CmoAP2/ERF during PM infection in the PMR compared with PMS. These results highlighted the pivotal role of CmoAP2/ERF in conferring resistance to PM and identifies it as a valuable molecular entity for breeding resistant pumpkin cultivars.

Fine mapping of powdery mildew resistance gene MlWE74 derived from wild emmer wheat (Triticum turgidum ssp. dicoccoides) in an NBS-LRR gene cluster

Powdery mildew resistance gene MlWE74, originated from wild emmer wheat accession G-748-M, was mapped in an NBS-LRR gene cluster of chromosome 2BS. Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is a globally devastating disease. Wild emmer wheat (Triticum turgidum var. dicoccoides) is a valuable genetic resource for improving disease resistance in common wheat. A powdery mildew resistance gene was transferred to hexaploid wheat line WE74 from wild emmer accession G-748-M. Genetic analysis revealed that the powdery mildew resistance in WE74 is controlled by a single dominant gene, herein temporarily designated MlWE74. Bulked segregant analysis (BSA) and molecular mapping delimited MlWE74 to the terminal region of chromosome 2BS flanking by markers WGGBD412 and WGGBH346 within a genetic interval of 0.25 cM and corresponding to 799.9 kb genomic region in the Zavitan reference sequence. Sequence annotation revealed two phosphoglycerate mutase-like genes, an alpha/beta-hydrolases gene, and five NBS-LRR disease resistance genes that could serve as candidates for map-based cloning of MlWE74. The geographical location analysis indicated that MlWE74 is mainly distributed in Rosh Pinna and Amirim regions, in the northern part of Israel, where environmental conditions are favorable to the occurrence of powdery mildew. Moreover, the co-segregated marker WGGBD425 is helpful in marker-assisted transfer of MlWE74 into elite cultivars.

Molecular cytogenetics and development of St-chromosome-specific molecular markers of novel stripe rust resistant wheat-Thinopyrum intermedium and wheat-Thinopyrum ponticum substitution lines

BACKGROUND

Owing to their excellent resistance to abiotic and biotic stress, Thinopyrum intermedium (2n = 6x = 42, JJJ^sJ^sStSt) and Th. ponticum (2n = 10x = 70) are both widely utilized in wheat germplasm innovation programs. Disomic substitution lines (DSLs) carrying one pair of alien chromosomes are valuable bridge materials for transmission of novel genes, fluorescence in situ hybridization (FISH) karyotype construction and specific molecular marker development.

RESULTS

Six wheat-Thinopyrum DSLs derived from crosses between Abbondanza nullisomic lines (2n = 40) and two octoploid Trititrigia lines (2n = 8x = 56), were characterized by sequential FISH-genome in situ hybridization (GISH), multicolor GISH (mc-GISH), and an analysis of the wheat 15 K SNP array combined with molecular marker selection. ES-9 (DS2St (2A)) and ES-10 (DS3St (3D)) are wheat-Th. ponticum DSLs, while ES-23 (DS2St (2A)), ES-24 (DS3St (3D)), ES-25(DS2St (2B)), and ES-26 (DS2St (2D)) are wheat-Th. intermedium DSLs. ES-9, ES-23, ES-25 and ES-26 conferred high thousand-kernel weight and stripe rust resistance at adult stages, while ES-10 and ES-24 were highly resistant to stripe rust at all stages. Furthermore, cytological analysis showed that the alien chromosomes belonging to the same homoeologous group (2 or 3) derived from different donors carried the same FISH karyotype and could form a bivalent. Based on specific-locus amplified fragment sequencing (SLAF-seq), two 2St-chromosome-specific markers (PTH-005 and PTH-013) and two 3St-chromosome-specific markers (PTH-113 and PTH-135) were developed.

CONCLUSIONS

The six wheat-Thinopyrum DSLs conferring stripe rust resistance can be used as bridging parents for transmission of valuable resistance genes. The utility of PTH-113 and PTH-135 in a BC1F2 population showed that the newly developed markers could be useful tools for efficient identification of St chromosomes in a common wheat background.

Fine mapping of a powdery mildew resistance gene MlIW39 derived from wild emmer wheat (Triticum turgidum ssp. dicoccoides)

Powdery mildew resistance gene MlIW39, originated from wild emmer wheat accession IW39, was mapped to a 460.3 kb genomic interval on wheat chromosome arm 2BS. Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is destructive disease and a significant threat to wheat production globally. The most effective way to control this disease is genetic resistance. However, when resistance genes become widely deployed in agriculture, their effectiveness is compromised by virulent variants that were previously minor components of the pathogen population or that arise from mutation. This necessitates continual search for new sources of resistance in both wheat and its near relatives. In this study, we produced a common wheat line 8D49 (87-1/IW39//2*87-1), which has all-stage immunity to Bgt isolate E09 and many other Chinese Bgt isolates, by transferring powdery mildew resistance from Israeli wild emmer wheat (WEW) accession IW39 to the susceptible common wheat line 87-1. Genetic analysis indicated that the powdery mildew resistance in 8D49 was controlled by a single dominant gene, temporarily designated MlIW39. Genetic linkage analyses with molecular markers showed that MlIW39 was located in a 0.7 cm genetic region between markers QB-3-16 and 7Seq546 on the short arm of chromosome 2B. Fine mapping using three large F₂ populations delimited MlIW39 to a physical interval of approximately 460.3 kb region in the WEW reference genome (Zavitan v1.0) that contained six annotated protein-coding genes, four of which had gene structures similar to known disease resistance genes. This provides a foundation for map-based cloning of MlIW39. Markers 7Seq622 and 7Seq727 co-segregating with MlIW39 can be utilized for marker-assisted selection in further genetic studies and wheat breeding.

Characterization of Pm68, a new powdery mildew resistance gene on chromosome 2BS of Greek durum wheat TRI 1796

New powdery mildew resistance gene Pm68 was found in the terminal region of chromosome 2BS of Greek durum wheat TRI 1796. The co-segregated molecular markers could be used for MAS. Durum wheat (Triticum turgidum L. var. durum Desf.) is not only an important cereal crop for pasta making, but also a genetic resource for common wheat improvement. In the present study, a Greek durum wheat TRI 1796 was found to confer high resistance to all 22 tested isolates of Blumeria graminis f. sp. tritici (Bgt). Inheritance study on the F₁ plants and the F₂ population derived from the cross TRI 1796/PI 584832 revealed that the resistance in TRI 1796 was controlled by a single dominant gene, herein designated Pm68. Using the bulked segregant RNA-Seq (BSR-Seq) analysis combined with molecular analysis, Pm68 was mapped to the terminal part of the short arm of chromosome 2B and flanked by markers Xdw04 and Xdw12/Xdw13 with genetic distances of 0.22 cM each. According to the reference genome of durum wheat cv. Svevo, the corresponding physical region spanned the Pm68 locus was about 1.78-Mb, in which a number of disease resistance-related genes were annotated. This study reports the new powdery mildew resistance gene Pm68 that would be a valuable resource for improvement of both common wheat and durum wheat. The co-segregated markers (Xdw05-Xdw11) developed here would be useful tools for marker-assisted selection (MAS) in breeding.

Characterization of a Powdery Mildew Resistance Gene in Wheat Breeding Line 10V-2 and Its Application in Marker-Assisted Selection

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a serious disease of wheat (Triticum aestivum L.) throughout the world. Host resistance is the most effective and preferred means for managing this disease. Line 10V-2, a wheat breeding line with superior agronomic performance, shows broad-spectrum seedling resistance to powdery mildew. Genetic analysis demonstrated that its resistance was controlled by a single dominant gene, tentatively designated Pm10V-2. This gene was localized near the documented Pm2 locus on chromosome 5DS using the simple sequence repeat (SSR) marker Cfd81. To saturate the marker map of Pm10V-2, more markers were developed using bulked segregant RNA-Seq. Two single-nucleotide polymorphism (SNP) markers (Swgi047 and Swgi064), three expressed sequence tag markers (Swgi007, Swgi029, and Swgi038), and one SSR marker (Swgi066) were polymorphic between the resistant and susceptible bulks and showed tightly linked to the Pm10V-2 gene. Pm10V-2 was flanked by the new developed markers Swgi064 and Swgi066 at genetic distances of 0.4 and 1.2 centimorgans (cM), respectively, and cosegregated with Swgi007 and Swgi038. The homologous sequence of Pm2a was cloned from 10V-2 based on a recent study. Although the sequence cloned from 10V-2 was completely identical to that of the reported Pm2a-related gene, they did not cosegregate but were separated at a genetic distance of 1.6 cM, indicating that Pm10V-2 was different from the reported of Pm2a-related gene. When inoculated with multiple B. graminis f. sp. tritici isolates, Pm10V-2 had a significantly different resistance spectrum from Pm2a and other powdery mildew (Pm) resistance genes at or near the Pm2 locus. Therefore, Pm10V-2 may be a new Pm2 allele or Pm2-linked gene. To use Pm10V-2 in marker-assisted selection (MAS) breeding, seven markers applicable for MAS were confirmed, including three newly developed markers (Swgi029, Swgi038, and Swgi064) in the present work. Using these markers, a great number of resistant lines with desirable agronomic performance were selected from crosses involving 10V-2, including the breeding line KM5016, which has been entered in the Regional trials in Hebei Province, China.