Isolation and characterisation of a NBS-LRR resistance gene analogue from pearl millet

Plant resistance (R) proteins belonging to nucleotide-binding site–leucine-rich repeat (NBS–LRR) family are mainly involved in recognition of effectors secreted by pathogens. Pearl millet [Pennisetum glaucum (L.) R.Br] is one of the most drought tolerant cereals, staple food crop of the semi-arid tropics but is highly susceptible to the downy mildew disease caused by oomycetous Sclerospora graminicola (Sacc) schroet. Earlier studies have identified several resistance gene analogues (RGAs) in pearl millet which may be involved in resistance against downy mildew. Of these, a clone RGPM213 was shown to have more than 60% identity with R-proteins coding for NBS–LRR-like protein kinase. The exact nature and function of the R-protein encoded by this gene was not known. In the present study, the cDNA of RGPM213 encompassing NBS–LRR region was inserted into an expression vector pRSET-A and transformed into BL21 E.coli cells. The expressed recombinant fusion protein with a His tag was purified using nickel affinity purification and it had a molecular weight of 35 kDa on SDS-PAGE. Immunoaffinity purification using antibodies raised against this recombinant R-protein identified two proteins of molecular weights 55 kDa and 66 kDa from pearl millet seedling extracts. Peptide mass fingerprinting of these proteins followed by homology search in database revealed similarity of the 55 kDa protein with a protein kinase from Brassica oleracia containing serine/ threonine kinase domain.     

Construction of a sequence-based bin map and mapping of QTLs for downy mildew resistance at four developmental stages in Chinese cabbage (<Emphasis Type="Italic">Brassica rapa</Emphasis> L. ssp. <Emphasis Type="Italic">pekinensis</Emphasis>)

Specific-locus amplified fragment sequencing is a high-resolution method for genetic mapping, genotyping, and single nucleotide polymorphism (SNP) marker discovery. Previously, a major QTL for downy mildew resistance, BraDM, was mapped to linkage group A08 in a doubled-haploid population derived from Chinese cabbage lines 91–112 and T12–19. The aim of the present study was to improve the linkage map and identify the genetic factors involved in downy mildew resistance. We detected 53,692 high quality SLAFs, of which 7230 were polymorphic, and 3482 of the polymorphic markers were used in genetic map construction. The final map included 1064 bins on ten linkage groups and was 858.98 cM in length, with an average inter-locus distance of 0.81 cM. We identified six QTLs that are involved in downy mildew resistance. The four major QTLs, sBrDM8, yBrDM8, rBrDM8, and hBrDM8, for resistance at the seedling, young plant, rosette, and heading stages were mapped to A08, and are identical to BraDM. The two minor resistance QTLs, rBrDM6 (A06) and hBrDM4 (A04), were active at the rosette and heading stages. The major QTL sBrDM8 defined a physical interval of ~228 Kb on A08, and a serine/threonine kinase family gene, Bra016457, was identified as the possible candidate gene. We report here the first high-density bin map for Chinese cabbage, which will facilitate mapping QTLs for economically important traits and SNP marker development. Our results also expand knowledge of downy mildew resistance in Chinese cabbage and provide three SNP markers (A08-709, A08-028, and A08-018) that we showed to be effective when used in MAS to breed for downy mildew resistance in B. rapa.     

Pearl millet downy mildew as affected by integrating seed dressing,sowing date and cultivar

The effects of seed dressing, sowing date and cultivar on incidence and severity of downy mildew of pearl millet induced by Sclerospora graminicola and yield were studied in a two-year field trial conducted at the Research farm of University of Maiduguri. The millet cultivars, Ex-Borno, SOSAT-C88, GB 8735 and Gwagwa were each dressed with metalaxyl at 0.75 and 1.50 g a.i./kg seed; and a batch of undressed seeds of each cultivar served as control. Both dressed and undressed seeds were used for dry-planting and wet-planting in early and late seasons. The results showed that seed dressing with the fungicides significantly (p ≤ 0.05) reduced the incidence and severity of downy mildew and increased grain yield. Dry-planting also significantly (p ≤ 0.01) increased grain yield irrespective of disease incidence. Delay in sowing led to a significant reduction in incidence and severity of downy mildew. Differences between the cultivars in relation to incidence and severity of downy mildew and grain yield were significant. SOSAT-C88 developed low or no downy mildew in both seasons. Sowing of dressed SOSAT-C88 as soon as rainfall established appeared most beneficial in the control of downy mildew. Dry- or wet-planting Ex-Borno dressed with any of the metalaxyl formulations proved to be effective for downy mildew management and for high yield.     

<Emphasis Type="Italic">Basidiophora delawarensis</Emphasis>, a new downy mildew species infecting cultivated goldenrod (<Emphasis Type="Italic">Solidago sphacelata</Emphasis>) in the USA

Species in the genus Basidiophora (Oomycota, Peronosporales) are pathogens causing downy mildew disease on several closely related plant hosts in the family Asteraceae, including the genera Conyza, Erigeron, Solidago, and Symphyotrichum. Despite their widespread occurrence, little is known about the diversity, biology, and economic impact of downy mildew pathogens of ornamental or wild plants in the Asteraceae. In June 2017, Solidago sphacelata plants of the cultivar ‘Golden Fleece’ were found in the state of Delaware, USA, showing typical symptoms of downy mildew disease. Initial morphological observations indicated the specimen belonged to the genus Basidiophora. Phylogenetic analysis of a sequence matrix of the partial cox2 mitochondrial gene using Bayesian and maximum likelihood methods showed that the Basidiophora isolate from S. sphacelata represents an undescribed species that is here described as B. delawarensis. This new species constitutes the third accepted species in the genus Basidiophora and can be distinguished from other Basidiophora species by differences in size of sporangia and sporangiophores and nucleotide sequence divergence. This new species poses a potential threat to its host, warranting close monitoring and the implementation of disease management programs.     

Sclerospora graminicola- and arachidonic acid-induced autofluorescence in downy mildew resistant and susceptible genotypes of pearl millet

Autofluorescence of downy mildew resistant and susceptible cells of pearl millet seedlings undergoing hypersensitive reaction (HR) upon Sclerospora graminicola-inoculation and arachidonic acid (AA)-treatment was studied. Two-day-old seedlings of a highly resistant (IP 18296) and a highly susceptible (23D₂B) genotype of pearl millet were either inoculated with zoospore suspension of S. graminicola or treated with AA for 24 h. The coleoptiles with hypersensitive necrotic spots were processed by the standard procedure, and the tissues were subjected to fluorescence microscopy. A differential accumulation of autofluor-escent compounds in resistant and susceptible pearl millet genotypes was observed with most accumulation occurring in resistant cells treated with AA. The variation in the degree of fluorescence and the spatial accumulation of autofluorescent compounds among the two inoculated/treated genotypes is discussed.     

Biochemical events involved in downy mildew disease resistance in pearl millet in relation to H+-ATPase

Pearl millet (Pennisetum glaucum L. Br.) is the most important crop in India and Africa. Downy mildew disease of pearl millet caused by the oomycetous fungus Sclerospora graminicola (Sacc.) Schroet., is the major biological constraint in the production of pearl millet. Plasma membrane H⁺-ATPase is induced in resistant pearl millet against downy mildew pathogen. Sodium orthovanadate, an inhibitor of H⁺-ATPase, was used in this study to understand its effect on other known defence responses in pearl millet including H⁺-ATPase. Results suggest that vanadate down-regulates all defence responses tested, such as H⁺-ATPase (53 ± 5.0%), peroxidase (36 ± 5.6%), phenylalanine ammonia lyase (43 ± 4.5%), β-1,3 glucanase (25 ± 4.2%), lytic activity (32 ± 3.0%), hypersensitive response (57 ± 4.3%) and pathogen colonisation. These data indicate that the plasma membrane H⁺-ATPase may be a key step in the signaling pathway leading to defence activation in pearl millet against downy mildew disease.     

Construction of a genome-anchored,high-density genetic map for melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.) and identification of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melonis</Emphasis> race 1 resistance QTL

Key message

Four QTLs and an epistatic interaction were associated with disease severity in response to inoculation with Fusarium oxysporum f. sp. melonis race 1 in a recombinant inbred line population of melon.

Abstract

The USDA Cucumis melo inbred line, MR-1, harbors a wealth of alleles associated with resistance to several major diseases of melon, including powdery mildew, downy mildew, Alternaria leaf blight, and Fusarium wilt. MR-1 was crossed to an Israeli cultivar, Ananas Yok’neam, which is susceptible to all of these diseases, to generate a recombinant inbred line (RIL) population of 172 lines. In this study, the RIL population was genotyped to construct an ultra-dense genetic linkage map with 5663 binned SNPs anchored to the C. melo genome and exhibits the overall high quality of the assembly. The utility of the densely genotyped population was demonstrated through QTL mapping of a well-studied trait, resistance to Fusarium wilt caused by Fusarium oxysporum f. sp. melonis (Fom) race 1. A major QTL co-located with the previously validated resistance gene Fom-2. In addition, three minor QTLs and an epistatic interaction contributing to Fom race 1 resistance were identified. The MR-1 × AY RIL population provides a valuable resource for future QTL mapping studies and marker-assisted selection of disease resistance in melon.

     

Genetic analysis of a novel broad-spectrum powdery mildew resistance gene from the wheat-<Emphasis Type="Italic">Agropyron cristatum</Emphasis> introgression line Pubing 74

Main conclusion

A novel broad-spectrum powdery mildew resistance gene PmPB74 was identified in wheat- Agropyron cristatum introgression line Pubing 74. Development of wheat cultivars with broad-spectrum, durable resistance to powdery mildew has been restricted by lack of superior genetic resources. In this study, a wheat-A. cristatum introgression line Pubing 74, originally selected from a wide cross between the common wheat cultivar Fukuhokomugi (Fukuho) and Agropyron cristatum (L.) Gaertn (2n = 4x = 28; genome PPPP), displayed resistance to powdery mildew at both the seedling and adult stages. The putative alien chromosomal fragment in Pubing 74 was below the detection limit of genomic in situ hybridization (GISH), but evidence for other non-GISH-detectable introgressions was provided by the presence of three STS markers specific to A. cristatum. Genetic analysis indicated that Pubing 74 carried a single dominant gene for powdery mildew resistance, temporarily designated PmPB74. Molecular mapping showed that PmPB74 was located on wheat chromosome arm 5DS, and flanked by markers Xcfd81 and HRM02 at genetic distances of 2.5 and 1.7 cM, respectively. Compared with other lines with powdery mildew resistance gene(s) on wheat chromosome arm 5DS, Pubing 74 was resistant to all 28 Blumeria graminis f. sp tritici (Bgt) isolates from different wheat-producing regions of northern China. Allelism tests indicated that PmPB74 was not allelic to PmPB3558 or Pm2. Our work showed that PmPB74 is a novel gene with broad resistance to powdery mildew, and hence will be helpful in broadening the genetic basis of powdery mildew resistance in wheat.

     

10,000 year history of plant use at Bosumpra Cave,Ghana

Investigations of hunter-gatherer subsistence, early food production, and the development of agroforestry systems during the Later Stone Age (LSA) of West Africa have proven challenging because of limited recovery and analysis of archaeological evidence relating directly to subsistence. This paper examines changes in the use of plant resources over a 10,000 year period at Bosumpra Cave, southern Ghana. Large quantities of recovered Canarium schweinfurthii (incense tree) and Elaeis guineensis (oil palm) endocarp preserved at the site allow for the assessment of previous observations about changes in the relative importance of tree fruit resources over time. Results point to the possibility that C. schweinfurthii was a managed resource and may be useful as a marker of forager subsistence in tropical forest regions. The exploitation of C. schweinfurthii persisted in the early and middle Holocene, but was eventually overshadowed in the late Holocene by Kintampo food-producing economies based on Pennisetum glaucum (pearl millet), Vigna unguiculata (cowpea) and E. guineensis. The Bosumpra deposits also yielded domesticated pearl millet and cowpea, allowing for the comparison of LSA hunter-gatherer and early food producer subsistence practices and cultural interactions in southern Ghana.     

Bidirectional backcrosses between wild and cultivated lettuce identify loci involved in nonhost resistance to downy mildew

Key message

The nonhost resistance of wild lettuce to lettuce downy mildew seems explained by four components of a putative set of epistatic genes.

Abstract

The commonplace observation that plants are immune to most potential pathogens is known as nonhost resistance (NHR). The genetic basis of NHR is poorly understood. Inheritance studies of NHR require crosses of nonhost species with a host, but these crosses are usually unsuccessful. The plant-pathosystem of lettuce and downy mildew, Bremia lactucae, provides a rare opportunity to study the inheritance of NHR, because the nonhost wild lettuce species Lactuca saligna is sufficiently cross-compatible with the cultivated host Lactuca sativa. Our previous studies on NHR in one L. saligna accession led to the hypothesis that multi-locus epistatic interactions might explain NHR. Here, we studied NHR at the species level in nine accessions. Besides the commonly used approach of studying a target trait from a wild donor species in a cultivar genetic background, we also explored the opposite, complementary approach of cultivar introgression in a wild species background. This bidirectional approach encompassed (1) nonhost into host introgression: identification of L. saligna derived chromosome regions that were overrepresented in highly resistant BC1 plants (F1?×?L. sativa), (2) host into nonhost introgression: identification of L. sativa derived chromosome regions that were overrepresented in BC1 inbred lines (F1?×?L. saligna) with relatively high infection levels. We demonstrated that NHR is based on resistance factors from L. saligna and the genetic dose for NHR differs between accessions. NHR seemed explained by combinations of epistatic genes on three or four chromosome segments, of which one chromosome segment was validated by the host into nonhost approach.

     

Discovery and introgression of the wild sunflower-derived novel downy mildew resistance gene <Emphasis Type="Italic">Pl</Emphasis><Subscript><Emphasis Type="Italic">19</Emphasis></Subscript> in confection sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis> L.)

Key message

A new downy mildew resistance gene, Pl ₁₉ , was identified from wild Helianthus annuus accession PI 435414, introduced to confection sunflower, and genetically mapped to linkage group 4 of the sunflower genome.

Abstract

Wild Helianthus annuus accession PI 435414 exhibited resistance to downy mildew, which is one of the most destructive diseases to sunflower production globally. Evaluation of the 140 BC₁F_2:3 families derived from the cross of CMS CONFSCLB1 and PI 435414 against Plasmopara halstedii race 734 revealed that a single dominant gene controls downy mildew resistance in the population. Bulked segregant analysis conducted in the BC₁F₂ population with 860 simple sequence repeat (SSR) markers indicated that the resistance derived from wild H. annuus was associated with SSR markers located on linkage group (LG) 4 of the sunflower genome. To map and tag this resistance locus, designated Pl ₁₉ , 140 BC₁F₂ individuals were used to construct a linkage map of the gene region. Two SSR markers, ORS963 and HT298, were linked to Pl ₁₉ within a distance of 4.7 cM. After screening 27 additional single nucleotide polymorphism (SNP) markers previously mapped to this region, two flanking SNP markers, NSA_003564 and NSA_006089, were identified as surrounding the Pl ₁₉ gene at a distance of 0.6 cM from each side. Genetic analysis indicated that Pl ₁₉ is different from Pl ₁₇ , which had previously been mapped to LG4, but is closely linked to Pl ₁₇ . This new gene is highly effective against the most predominant and virulent races of P. halstedii currently identified in North America and is the first downy mildew resistance gene that has been transferred to confection sunflower. The selected resistant germplasm derived from homozygous BC₂F₃ progeny provides a novel gene for use in confection sunflower breeding programs.

     

Cultured Pearl Surface Quality Profiling by the Shell Matrix Protein Gene Expression in the Biomineralised Pearl Sac Tissue of <Emphasis Type="Italic">Pinctada margaritifera</Emphasis>

Nucleated pearls are produced by molluscs of the Pinctada genus through the biomineralisation activity of the pearl sac tissue within the recipient oyster. The pearl sac originates from graft tissue taken from the donor oyster mantle and its functioning is crucial in determining key factors that impact pearl quality surface characteristics. The specific role of related gene regulation during gem biogenesis was unknown, so we analysed the expression profiles of eight genes encoding nacreous (PIF, MSI60, PERL1) or prismatic (SHEM5, PRISM, ASP, SHEM9) shell matrix proteins or both (CALC1) in the pearl sac (N?=?211) of Pinctada margaritifera during pearl biogenesis. The pearls and pearl sacs analysed were from a uniform experimental graft with sequential harvests at 3, 6 and 9 months post-grafting. Quality traits of the corresponding pearls were recorded: surface defects, surface deposits and overall quality grade. Results showed that (1) the first 3 months of culture seem crucial for pearl quality surface determination and (2) all the genes (SHEM5, PRISM, ASP, SHEM9) encoding proteins related to calcite layer formation were over-expressed in the pearl sacs that produced low pearl surface quality. Multivariate regression tree building clearly identified three genes implicated in pearl surface quality, SHEM9, ASP and PIF. SHEM9 and ASP were clearly implicated in low pearl quality, whereas PIF was implicated in high quality. Results could be used as biomarkers for genetic improvement of P. margaritifera pearl quality and constitute a novel perspective to understanding the molecular mechanism of pearl formation.     

QTL mapping for downy mildew resistance in cucumber via bulked segregant analysis using next-generation sequencing and conventional methods

Key message

QTL mapping using NGS-assisted BSA was successfully applied to an F ₂ population for downy mildew resistance in cucumber. QTLs detected by NGS-assisted BSA were confirmed by conventional QTL analysis.

Abstract

Downy mildew (DM), caused by Pseudoperonospora cubensis, is one of the most destructive foliar diseases in cucumber. QTL mapping is a fundamental approach for understanding the genetic inheritance of DM resistance in cucumber. Recently, many studies have reported that a combination of bulked segregant analysis (BSA) and next-generation sequencing (NGS) can be a rapid and cost-effective way of mapping QTLs. In this study, we applied NGS-assisted BSA to QTL mapping of DM resistance in cucumber and confirmed the results by conventional QTL analysis. By sequencing two DNA pools each consisting of ten individuals showing high resistance and susceptibility to DM from a F₂ population, we identified single nucleotide polymorphisms (SNPs) between the two pools. We employed a statistical method for QTL mapping based on these SNPs. Five QTLs, dm2.2, dm4.1, dm5.1, dm5.2, and dm6.1, were detected and dm2.2 showed the largest effect on DM resistance. Conventional QTL analysis using the F₂ confirmed dm2.2 (R ² = 10.8–24 %) and dm5.2 (R ² = 14–27.2 %) as major QTLs and dm4.1 (R ² = 8 %) as two minor QTLs, but could not detect dm5.1 and dm6.1. A new QTL on chromosome 2, dm2.1 (R ² = 28.2 %) was detected by the conventional QTL method using an F₃ population. This study demonstrated the effectiveness of NGS-assisted BSA for mapping QTLs conferring DM resistance in cucumber and revealed the unique genetic inheritance of DM resistance in this population through two distinct major QTLs on chromosome 2 that mainly harbor DM resistance.

     

Production of thermotolerant entomopathogenic fungal conidia on millet grain

Thermotolerance of entomopathogenic (insect-killing) fungi should be seriously considered before industrialization. This work describes the feasibility of millet grain as a substrate for production of thermotolerant Beauveria bassiana (Bb) GHA and ERL1170 and Metarhizium anisopliae (Ma) ERL1171 and ERL1540 conidia. First, conidial suspensions of the Bb isolates, produced on millet grain in polyethylene bags, were exposed to five temperatures (43–47°C) at 15-min intervals for up to 120 min (experiment I). Agar-based quarter-strength (¼) Sabouraud dextrose agar supplemented with yeast extract (SDAY) and whey permeate media served as controls. Millet-grain-based culture was superior in producing the most thermotolerant Bb conidia, followed by whey permeate agar and ¼SDAY-based cultures. Secondly, to compare the thermotolerance of conidia produced at the same conditions, the Bb isolates were then produced on agar-based millet powder medium, with ¼SDAY and whey permeate agar media as controls, and the two Ma isolates were added (experiment II). They were then exposed to the same temperatures as above. More thermotolerant Bb and Ma conidia were produced on millet powder agar than on whey permeate agar and ¼SDAY overall. These results suggest that millet grain can be used as a substrate to produce thermotolerant conidia in a mass production system.     

Phosphorus supply,arbuscular mycorrhizal fungal species,and plant genotype impact on the protective efficacy of mycorrhizal inoculation against wheat powdery mildew

A potential alternative strategy to chemical control of plant diseases could be the stimulation of plant defense by arbuscular mycorrhizal fungi (AMF). In the present study, the influence of three parameters (phosphorus supply, mycorrhizal inoculation, and wheat cultivar) on AMF protective efficiency against Blumeria graminis f. sp. tritici, responsible for powdery mildew, was investigated under controlled conditions. A 5-fold reduction (P/5) in the level of phosphorus supply commonly recommended for wheat in France improved Funneliformis mosseae colonization and promoted protection against B. graminis f. sp. tritici in a more susceptible wheat cultivar. However, a further decrease in P affected plant growth, even under mycorrhizal conditions. Two commercially available AMF inocula (F. mosseae, Solrize®) and one laboratory inoculum (Rhizophagus irregularis) were tested for mycorrhizal development and protection against B. graminis f. sp. tritici of two moderately susceptible and resistant wheat cultivars at P/5. Mycorrhizal levels were the highest with F. mosseae (38 %), followed by R. irregularis (19 %) and Solrize® (SZE, 8 %). On the other hand, the highest protection level against B. graminis f. sp. tritici was obtained with F. mosseae (74 %), followed by SZE (58 %) and R. irregularis (34 %), suggesting that inoculum type rather than mycorrhizal levels determines the protection level of wheat against B. graminis f. sp. tritici. The mycorrhizal protective effect was associated with a reduction in the number of conidia with haustorium and with an accumulation of polyphenolic compounds at B. graminis f. sp. tritici infection sites. Both the moderately susceptible and the most resistant wheat cultivar were protected against B. graminis f. sp. tritici infection by F. mosseae inoculation at P/5, although the underlying mechanisms appear rather different between the two cultivars. This study emphasizes the importance of taking into account the considered parameters when considering the use of AMF as biocontrol agents.     

Fine mapping and candidate gene screening of the downy mildew resistance gene <Emphasis Type="Italic">RPF1</Emphasis> in Spinach

Key message

A SLAF-BSA approach was used to locate the RPF1 locus. The three most likely candidate genes were identified which provide a basic for cloning the resistance gene at the RPF1 locus.

Abstract

Spinach downy mildew is a globally devastating oomycete disease. The use of downy mildew resistance genes constitutes the most effective approach for disease management. Hence, the objective of the present study was to fine map the first-reported resistance locus RPF1. The resistance allele at this resistance locus was effective against races 1–7, 9, 11, 13, and 15 of Peronospora farinosa f. sp. spinaciae (P. effusa). The approach fine mapped RPF1 using specific-locus amplified fragment sequencing (SLAF-Seq) technology combined with bulked segregant analysis. A 1.72 Mb region localized on chromosome 3 was found to contain RPF1 based on association analysis. After screening recombinants with the SLAF markers within the region, the region was narrowed down to 0.89 Mb. Within this region, 14 R genes were identified based on the annotation information. To identify the genes involved in resistance, resequencing of two resistant inbred lines (12S2 and 12S3) and three susceptible inbred lines (12S1, 12S4, and 10S2) was performed. The three most likely candidate genes were identified via amino acid sequence analysis and conserved domain analysis between resistant and susceptible inbred lines. These included Spo12729, encoding a receptor-like protein, and Spo12784 and Spo12903, encoding a nucleotide-binding site and leucine-rich repeat domains. Additionally, based on the sequence variation in the three genes between the resistant and susceptible lines, molecular markers were developed for marker-assisted selection. The results could be valuable in cloning the RPF1 alleles and improving our understanding of the interaction between the host and pathogen.

     

Exploring almond genetic variability useful for peach improvement: mapping major genes and QTLs in two interspecific almond × peach populations

Genetic analysis of a diverse set of 42 traits for flower (5), phenology (9), fruit quality (19), leaf (8) and disease resistance (1) was carried out in two interspecific almond × peach populations, an F₂ (T × E) and a BC₁ (T1E), from the cross between ‘Texas’ almond and ‘Earlygold’ peach. Traits related to flower, phenology, fruit quality, leaf morphology and resistance to powdery mildew were phenotyped over 3 years in two locations and studied for co-segregation with a large set of SNP and SSR markers. Three maps were used, one for the T × E and two for the T1E (T1E and E) population. Nine major genes were identified and mapped: anther color (Ag/ag and Ag2/ag2), flower color (Fc2/fc2), maturity date (MD/md), almond fruit type (almond vs. peach; Alf/alf), juiciness (Jui/jui), blood flesh (DBF2/dbf2), powdery mildew resistance (Vr3) and flower type (showy/non-showy; Sh/sh). These genes were often located in genome positions different from those for major genes for similar traits mapped before. Two of them explain fundamental aspects that define the fruit of peach with respect to that of almond: Alf and Jui, for its thick and juicy mesocarp, respectively. The genetics of quantitative traits was studied, and 32 QTLs were detected, with consistent behavior over the years. New alleles identified from almond for important traits such as red skin color, blood flesh, fruit weight and powdery mildew resistance may prove useful for the introduction of new variability into the peach gene pool used in commercial breeding programs.     

QTL mapping of downy and powdery mildew resistances in PI 197088 cucumber with genotyping-by-sequencing in RIL population

Key message

Host resistances in PI 197088 cucumber to downy and powdery mildew pathogens are conferred by 11 (3 with major effect) and 4 (1 major effect) QTL, respectively, and three of which are co-localized.

Abstract

The downy mildew (DM) and powdery mildew (PM) are the two most important foliar diseases of cucurbit crops worldwide. The cucumber accession PI 197088 exhibits high-level resistances to both pathogens. Here, we reported QTL mapping results for DM and PM resistances with 148 recombinant inbred lines from a cross between PI 197088 and the susceptible line ‘Coolgreen’. Phenotypic data on responses to natural DM and PM infection were collected in multi-year and multi-location replicated field trials. A high-density genetic map with 2780 single nucleotide polymorphisms (SNPs) from genotyping-by-sequencing and 55 microsatellite markers was developed, which revealed genomic regions with segregation distortion and mis-assemblies in the ‘9930’ cucumber draft genome. QTL analysis identified 11 and 4 QTL for DM and PM resistances accounting for more than 73.5 and 63.0% total phenotypic variance, respectively. Among the 11 DM resistance QTL, dm5.1, dm5.2, and dm5.3 were major-effect contributing QTL, whereas dm1.1, dm2.1, and dm6.2 conferred susceptibility. Of the 4 QTL for PM resistance, pm5.1 was the major-effect QTL explaining 32.4% phenotypic variance and the minor-effect QTL pm6.1 contributed to disease susceptibility. Three PM QTL, pm2.1, pm5.1, and pm6.1, were co-localized with DM QTL dm2.1, dm5.2, and dm6.1, respectively, which was consistent with the observed linkage of PM and DM resistances in PI 197088. The genetic architecture of DM resistance in PI 197088 and another resistant line WI7120 (PI 330628) was compared, and the potential of using PI 197088 in cucumber breeding for downy and powdery mildew resistances is discussed.