Evaluation of resistant genotypes and their characterization using molecular markers linked for powdery mildew resistance in cucumber (Cucumis sativus L.)

Powdery mildew (PM) is one of the most severe fungal diseases of cucumber that limits its production worldwide. In this study, 140 genotypes of cucumber were screened for disease resistance under field and artificial conditions, and then validated with eight known SSR markers linked to PM resistance. Among these genotypes, genotype GS140 was found resistant (R), whereas GS148, GS16 and GS70 were moderately resistant, and GS169 was found to be tolerant. Of all the eight markers, only C31, C80, C162, SSR16472 and SSR16881 amplified the reported linked allele. The 127 bp allele of SSR16881 was found to be associated with the lowest disease severity of 37.65%. The associated markers could further be verified for their usability using linkage studies and the contrast genotypes in the present study could serve as a tool for selection in future marker-assisted selection breeding strategies for PM resistance.

Introgression of the Powdery Mildew Resistance Genes Pm60 and Pm60b from Triticum urartu to Common Wheat Using Durum as a ‘Bridge’

Powdery mildew, caused by the fungus Blumeria graminis f. sp. tritici (Bgt), has limited wheat yields in many major wheat-production areas across the world. Introducing resistance genes from wild relatives into cultivated wheat can enrich the genetic resources for disease resistance breeding. The powdery mildew resistance gene Pm60 was first identified in diploid wild wheat Triticum urartu (T. urartu). In this study, we used durum as a ‘bridge’ approach to transfer Pm60 and Pm60b into hexaploid common wheat. Synthetic hexaploid wheat (SHW, AABBAuAu), developed by crossing T. urartu (AuAu) with durum (AABB), was used for crossing and backcrossing with common wheat. The Pm60 alleles were tracked by molecular markers and the resistance to powdery mildew. From BC1F1 backcross populations, eight recombinant types were identified based on five Pm60-flanking markers, which indicated different sizes of the introgressed chromosome segments from T. urartu. Moreover, we have selected two resistance-harboring introgression lines with high self-fertility, which could be easily used in wheat breeding system. Our results showed that the durum was an excellent ‘bridge’ for introducing the target gene from diploid T. urartu into the hexaploid cultivated wheat. Moreover, these introgression lines could be deployed in wheat resistance breeding programs, together with the assistance of the molecular markers for Pm60 alleles.

Genetic control of juvenile resistance to powdery mildew in spring bread wheat cultivars from the VIR collection

Background. Bread wheat (Triticum aestivum L.) is one of the major food crops of humankind. Powdery mildew, caused by Blumeria graminis f. sp. tritici, is the most destructive foliar disease capable of causing great yield losses in epidemic years. Breeding for resistance to powdery mildew is the most economical and effective way to control this disease. By now, 68 loci were identified to contain more than 90 alleles of resistance to powdery mildew in wheat. However, there is a permanent necessity in finding new sources of resistance.The objective of the present study was to characterize the seedling powdery mildew resistance in some spring bread wheat varieties from the VIR collection and determine the inheritance of powdery mildew resistance in these accessions.Materials and methods. The powdery mildew resistant varieties ‘SW Kungsjet’ (k-66036), ‘SW Kronjet’ (k-66097), ‘Boett’ (k-66353), ‘Batalj’ (k-67116), ‘Stilett’ (k-67119) ‘Pasteur’ (k-66093) were crossed with a resistant line ‘Wembley 14.31’ (k-62557) containing the Pm12 gene, and with ‘SW Milljet’ (k-64434); the variety ‘Sibirka Yartsevskaya’ (k-38587) was used as a susceptible parent and control. The hybrid populations F2 were inoculated with the fungus population from local field and evaluated. The powdery mildew population manifested virulence to Pm1a, Pm2, Pm3a-f, Pm4a-b, Pm5a, Pm6, Pm7, Pm8, Pm9, Pm10, Pm11, Pm16, Pm19, Pm28, and avirulence to Pm12. The degree of resistance was assessed on days 8 and 10 after the inoculation using the Mains and Dietz scale (Mains, Dietz, 1930). The castrated flowers in the spikes were pollinated using the twell-method (Merezhko et al., 1973). Chi-squared for goodness of fit test was used to determine deviation of the observed data from the theoretically expected segregation.Results. According phytopathological and genetic tests, juvenile resistance in the varieties ‘SW Kungsjet’, ‘SW Kronjet’, ‘Boett’, ‘Batalj’, ‘Stilett’ and ‘Pasteur’ is controlled by dominant genes, which differ from Pm1a, Pm2, Pm3a-f, Pm4a-b, Pm5a, Pm6, Pm7, Pm8, Pm9, Pm10, Pm11, Pm12, Pm16, Pm19, and Pm28. The varieties ‘SW Milljet’, ‘SW Kronjet’ and ‘Pasteur’ had identical resistance genes. Genetic control of juvenile resistance to powdery mildew in ‘Batalj’, ‘Boett’, ‘Stilett’, ‘SW Milljet’, ‘SW Kungsjet’, ‘Pasteur’ was governed by different genes.Conclusions. The varieties ‘SW Kungsjet’, ‘SW Kronjet’, ‘Boett’ have been maintaining adult and seedling resistance since 2005, and ‘Batalj’, ‘Stilett’ and ‘Pasteur’ since 2017. Seedling resistance of these varieties to local powdery mildew population is controlled by dominant genes. A high degree of resistance was displayed by ‘SW Kungsjet’ and ‘SW Kronjet’ in the Novosibirsk Province, while ‘SW Kungsjet’ was resistant to mildew populations of Tatarstan. The variety ‘Pasteur’ manifested seedling resistance to leaf rust, and ‘SW Kungsjet’ was resistant to loose smut. By summing all the results, it may be suggested that the varieties ‘SW Kungsjet’, ‘SW Kronjet’, ‘Boett’, ‘Batalj’, ‘Stilett’ and ‘Pasteur can serve as good donors of powdery mildew resistance in wheat breeding.

Identification of barley accessions from the VIR collection carrying the mlo11(cnv2) powdery mildew resistance allele

Background. The search for barley (Hordeum vulgare L.) genotypes that carry effective genes for resistance to powdery mildew agent Blumeria graminis f. sp. hordei is a present-day issue for Russian plant breeding. The mlo11 allele that confers long-term protection of barley against the pathogen is rarely found among the varieties, approved for cultivation in the territory of Russia. There is no information on the occurrence among Russian varieties of another effective allele, mlo11 (cnv2), therefore, the search for its source is a current necessity. Materials and methods. Seven barley accessions from Ethiopia and 7 accessions from Japan have been tested for resistance to the northwestern population of the powdery mildew agent in the field and in laboratory conditions. To identify of the Mlo gene alleles, nucleotide sequences of the Stowaway-MITE (Miniature Inverted-repeat Transposable Elements) and the adjacent promoter fragments were determined. Results. Phytopathological tests in the field and greenhouse conditions, as well as molecular markers were used to study 14 barley accessions from Ethiopia and Japan. According to the preliminary tests, plants were resistant to powdery mildew. The highly effective allele of powdery mildew resistance mlo11 (cnv2) was for the first time identified in four barley accessions from Ethiopia, k-20087, k-20523, k-20524 and k-28126. Under field conditions, adult plants were resistant, and in the greenhouse they were moderately damaged by powdery mildew (1-2 points). The disease symptoms were similar to those described for the sample Eth295, a carrier of the mlo11(cnv2) allele variant: single pustules and the absence of necrotic spots on the leaves. The fragments of Stowaway-MITE and adjacent Mlo 5' promoter sequences were amplified in all 14 accessions. The amplicons were cloned and sequenced. The unique marker SNPs within the MITE and Mlo 5’ promoter sequences, i.e. the substitutions of cytosine by thymine in positions 262 and 452, were found only in k-20087, k-20523, k-20524 and k-28126. These accessions belong to different botanical varieties and differ from each other in a number of morphological features, i.e. they are not duplicates. Conclusions. The genotypes selected as a result of the study can serve as a source of the mlo11(cnv2) allele in breeding powdery mildew-resistant barley varieties.

Discovery of a novel powdery mildew (Blumeria graminis) resistance locus in rye (Secale cereale L.)

Powdery mildew is one of the most destructive diseases in the world, causing substantial grain yield losses and quality reduction in cereal crops. At present 23 powdery mildew resistance genes have been identified in rye, of which the majority are in wheat-rye translocation lines developed for wheat improvement. Here, we investigated the genetics underlying powdery mildew resistance in the Gülzow-type elite hybrid rye (Secale cereale L.) breeding germplasm. In total, 180 inbred breeding lines were genotyped using the state-of-the-art 600 K SNP array and phenotyped for infection type against three distinct field populations of B. graminis f. sp. secalis from Northern Germany (2013 and 2018) and Denmark (2020). We observed a moderate level of powdery mildew resistance in the non-restorer germplasm population, and by performing a genome-wide association study using 261,406 informative SNP markers, we identified a powdery mildew resistance locus, provisionally denoted PmNOS1, on the distal tip of chromosome arm 7RL. Using recent advances in rye genomic resources, we investigated whether nucleotide-binding leucine-rich repeat genes residing in the identified 17 Mbp block associated with PmNOS1 on recent reference genomes resembled known Pm genes.

Genetic Mapping of Powdery Mildew Resistance Genes in Wheat Landrace ‘Guizi 1’ Using Genotyping-By-Sequencing

Wheat powdery mildew (Pm), caused by Blumeria graminis f. sp. tritici (Bgt), is a destructive disease of wheat (Triticum aestivum L.) worldwide that causes severe yield losses. Resistant wheat cultivars easily lose effective resistance against newly emerged Bgt strains; therefore, identifying new resistance genes is necessary for breeding resistant cultivars. ‘Guizi 1’ is a Chinese wheat cultivar with effective moderate and stable resistance against powdery mildew. A genetic analysis indicated that powdery mildew resistance in ‘Guizi 1’ was controlled by a single dominant gene, designated PmGZ1. In total, 110 F2 individual plants and the 2 parents were used for genotyping-by-sequencing, which produced 23,134 high-quality single-nucleotide polymorphisms (SNPs). The SNP distributions on the 21 chromosomes ranged from 134 on chromosome 6D to 6,288 on chromosome 3B. Chromosome 6A has 1,866 SNPs, among which 16 are located in a physical region between positions 307,802,221 and 309,885,836 in an approximate 2.3-cM region, which possessed the greatest SNP density. The average map distance between SNP markers was 0.1 cM. A quantitative trait locus with a significant epistatic effect on powdery mildew resistance was mapped to Chromosome 6A. The LOD value of PmGZ1 reached 34.8, and PmGZ1 was located within the confidence interval marked by chr6a-307802221 and chr6a-309885836. The phenotypic variance explained by PmGZ1 was 74.7%. Four candidate genes (two each encoding TaAP2-A and actin proteins) were annotated as resistance genes. The present results provide valuable information for wheat genetic improvement, quantitative trait loci fine mapping, and candidate gene validation.

Marker-Assisted Evaluation of Two Powdery Mildew Resistance Candidate Genes in Korean Cucumber Inbred Lines

Two genes, CsLRR-RPK2 (CsGy5G015660) and CsaMLO8 (Csa5G623470), have been considered as powdery mildew (PM) resistance genes in cucumbers. In this study, we evaluated the involvement of the alleles of these two genes in PM resistance in 100 commercial Korean cucumber inbred lines. To achieve this, we developed cleaved amplified polymorphic sequences (CAPS) and InDel markers from CsLRR-RPK2 and CsaMLO8. Genotyping analysis indicated that the CsLRR-RPK2-CAPS marker showed a stronger correlation with the PM-resistant phenotype, with an 84% consistency compared to the CsaMLO8-InDel marker. The use of the CsaMLO8-InDel marker showed a 70% consistency between phenotype and genotype results. It was proposed that the CsLRR-RPK2-CAPS marker successfully eliminated PM-susceptible inbred lines, since both genotype and phenotype results were 100% identical. Furthermore, the present study revealed that the introduction of one of these alleles is probably enough to confer PM resistance in cucumbers. However, seven PM-resistant inbred lines harbored either CsaMLO8 or CsLRR-RPK2 alleles, indicating that there is another PM-resistant resource(s) besides CsaMLO8- and CsLRR-RPK2–originated resistance in the commercial Korean inbred lines. Our results provide reliable evidence confirming two PM-resistant candidate genes for the detection of PM resistance resources in cucumber inbred lines.