A Novel Pear Scab (Venturia nashicola) Resistance Gene, Rvn3, from Interspecific Hybrid Pear (Pyrus pyrifolia × P. communis)

Asian pear scab is a fungal disease caused by Venturia nashicola. The identification of genes conferring scab resistance could facilitate the breeding of disease-resistant cultivars. Therefore, the present study aimed to identify a scab-resistance gene using an interspecific hybrid population ((Pyrus pyrifolia × P. communis) × P. pyrifolia). Artificial inoculation of V. nashicola was carried out for two years. The segregation ratio (1:1) of resistant to susceptible individuals indicated that resistance to V. nashicola was inherited from P. communis and controlled by a single dominant gene. Based on two years phenotypic data with the Kruskal–Wallis test and interval mapping, 12 common markers were significantly associated with scab resistance. A novel scab resistance gene, Rvn3, was mapped in linkage group 6 of the interspecific hybrid pear, and co-linearity between Rvn3 and one of the apple scab resistance genes, Rvi14, was confirmed. Notably, an insertion in pseudo-chromosome 6 of the interspecific hybrid cultivar showed homology with apple scab resistance genes. Hence, the newly discovered Rvn3 was considered an ortholog of the apple scab resistance gene. Since the mapping population used in the present study is a pseudo-BC1 population, pyramiding of multiple resistance genes to pseudo-BC1 could facilitate the breeding of pear cultivars with durable resistance.

Successful intergeneric transfer of a major apple scab resistance gene (Rvi6) from apple to pear and precise comparison of the downstream molecular mechanisms of this resistance in both species

Background Scab is the most important fungal disease of apple and pear. Apple (Malus x domestica Borkh.) and European pear (Pyrus communis L.) are genetically related but they are hosts of two different fungal species: Venturia inaequalis for apple and V. pyrina for European pear. The apple/V. inaequalis pathosystem is quite well known, whereas knowledge about the pear/V. pyrina pathosystem is still limited. The aim of our study was to analyse the mode of action of a major resistance gene of apple (Rvi6) in transgenic apple and pear plants interacting with the two scab species (V. inaequalis and V. pyrina), in order to determine the degree of functional transferability between the two pathosystems. Results Transgenic pear clones constitutively expressing the Rvi6 gene from apple were compared to a scab transgenic apple clone carrying the same construct. After inoculation in greenhouse with V. pyrina, strong defense reactions and very limited sporulation were observed on all transgenic pear clones tested. Microscopic observations revealed frequent aborted conidiophores in the Rvi6 transgenic pear / V. pyrina interaction. The macro- and microscopic observations were very comparable to the Rvi6 apple / V. inaequalis interaction. However, this resistance in pear proved variable according to the strain of V. pyrina, and one of the strains tested overcame the resistance of most of the transgenic pear clones. Comparative transcriptomic analyses of apple and pear resistant interactions with V. inaequalis and V. pyrina, respectively, revealed different cascades of molecular mechanisms downstream of the pathogen recognition by Rvi6 in the two species. Signal transduction was triggered in both species with calcium (and G-proteins in pear) and interconnected hormonal signaling (jasmonic acid in pear, auxins in apple and brassinosteroids in both species), without involvement of salicylic acid. This led to the induction of defense responses such as a remodeling of primary and secondary cell wall, lipids biosynthesis (galactolipids in apple and cutin and cuticular waxes in pear), systemic acquired resistance signal generation (in apple) or perception in distal tissues (in pear), and the biosynthesis of phenylpropanoids (flavonoids in apple but also lignin in pear). Conclusion This study is the first example of a successful intergeneric transfer of a resistance gene among Rosaceae, with a resistance gene functioning towards another species of pathogen.

Mapping of Quantitative Trait Loci for Scab Resistance in Apple (Malus × Domestica)

Scab caused by Venturia inaequalis (Cke.) Wint. is the most important fungal disease of apple. Fungicide application is a widely practiced method of disease control. The use of chemicals is however, cost intensive, tedious and ecologically unsafe. Development of genetic resistance and breeding of resistant cultivars is most reliable and a safest option. One such source of scab resistance happens to be the variety ‘Shireen’, released from SKUASTKashmir. However, till date the nature of resistance and its genetic control has not been characterized. The aim of this research was to elucidate the genetic basis of scab resistance in Shireen. The present study helped us to identify two quantitative trait loci (QTLs) on chromosome 2 and 8 and six potential CDGs for the polygenic resistance in ‘Shireen’. The genomic region corresponding to the mapped QTLs in LG 2 and LG 8 of ‘Shireen’ was examined for candidate genes possibly related to scab resistance using in silico analysis. The QTLs mapped in the genetic background of Shireen are the novel QTLs and may be transferred to desirable genetic backgrounds and provide opportunities for isolation and cloning of genes apart from their utility in order to achieve durable resistance to scab.

Phenotypic and transcriptomic analyses reveal major differences between apple and pear scab nonhost resistance

Background: Nonhost resistance is the outcome of most plant/pathogen interactions, but it has rarely been described in Rosaceous fruit species. Apple (Malus x domestica Borkh.) is a nonhost for Venturia pyrina, the scab species attacking European pear (Pyrus communis L.). Reciprocally, P. communis is a nonhost for Venturia inaequalis, the scab species attacking apple. The major objective of our study was to compare the scab nonhost resistance in apple and in European pear, at the phenotypic and transcriptomic levels. Results: Macro- and microscopic observations after reciprocal scab inoculations indicated that, after a similar germination step, nonhost apple/V. pyrina interaction remained nearly symptomless, whereas hypersensitive reactions were observed during nonhost pear/V. inaequalis interaction. Comparative transcriptomic analyses of apple and pear nonhost interactions with V. pyrina and V. inaequalis, respectively, revealed considerable differences. Very few differentially expressed genes were detected during apple/V. pyrina interaction, which is consistent with a symptomless type I nonhost resistance. On the contrary, numerous genes were differentially expressed during pear/V. inaequalis interaction, as expected in a type II nonhost resistance involving visible hypersensitive reaction. Pre-invasive defense, such as stomatal closure, was detected, as well as several post-invasive defense mechanisms (apoplastic reactive oxygen species accumulation, phytoalexin production and alterations of the epidermis composition). In addition, a comparative analysis between pear scab host and nonhost interactions indicated that, although specificities were observed, two major defense lines were shared in these resistances: cell wall and cuticle modifications and phenylpropanoid pathway induction. Conclusion: This first deciphering of the molecular mechanisms underlying a nonhost scab resistance in pear offers new possibilities for the genetic engineering of sustainable scab resistance in this species.

Successful intergeneric transfer of a major apple scab resistance gene (Rvi6) from apple to pear and precise comparison of the downstream molecular mechanisms of this resistance in both species

Background: Scab is the most important fungal disease of apple and pear. Apple (Malus x domestica Borkh.) and European pear (Pyrus communis L.) are genetically related but they are hosts of two different fungal species: Venturia inaequalis for apple and V. pyrina for European pear. The apple/V. inaequalis pathosystem is quite well known, whereas knowledge about the pear/V. pyrina pathosystem is still limited. The aim of our study was to analyse the mode of action of a major resistance gene of apple (Rvi6) in transgenic apple and pear plants interacting with the two scab species (V. inaequalis and V. pyrina), in order to determine the degree of functional transferability between the two pathosystems. Results: Transgenic pear clones constitutively expressing the Rvi6 gene from apple were compared to a scab transgenic apple clone carrying the same construct. After inoculation in greenhouse with V. pyrina, strong defense reactions and very limited sporulation were observed on all transgenic pear clones tested. Microscopic observations revealed frequent aborted conidiophores in the Rvi6 transgenic pear / V. pyrina interaction. The macro- and microscopic observations were very comparable to the Rvi6 apple / V. inaequalis interaction. However, this resistance in pear proved variable according to the strain of V. pyrina, and one of the strains tested overcame the resistance of most of the transgenic pear clones. Comparative transcriptomic analyses of apple and pear resistant interactions with V. inaequalis and V. pyrina, respectively, revealed different cascades of molecular mechanisms downstream of the pathogen recognition by Rvi6 in the two species. Signal transduction was triggered in both species with calcium (and G-proteins in pear) and interconnected hormonal signaling (jasmonic acid in pear, auxins in apple and brassinosteroids in both species), without involvement of salicylic acid. This led to the induction of defense responses such as a remodeling of primary and secondary cell wall, lipids biosynthesis (galactolipids in apple and cutin and cuticular waxes in pear), systemic acquired resistance signal generation (in apple) or perception in distal tissues (in pear), and the biosynthesis of phenylpropanoids (flavonoids in apple but also lignin in pear). Conclusion: This study is the first example of a successful intergeneric transfer of a resistance gene among Rosaceae, with a resistance gene functioning towards another species of pathogen.

Multilocus Sequence Analysis of Selected Housekeeping- and Pathogenicity-Related Genes in Venturia inaequalis

The relationship between housekeeping and pathogenicity-related genes and virulence or avirulence towards the primary Malus resistance genes (R) has not been previously studied for Venturia inaequalis fungus, the causal agent of apple scab. In this study, the sequences of two housekeeping genes encoding elongation factor alpha (EF-1α) and β-tubulin and two previously unstudied effector genes of V. inaequalis from mannosidase and glucosidase families of 100 strains collected from apple cultivars with Rvi6, Rvi1, and Rvi17 and without known scab resistance genes were submitted to the analyses. Based on the phylogenetic and diversity data, as well as recombination analyses of the sequenced regions, we assessed the phylogenetic relationships and genetic structure of the pathogen within the species and the evolutionary forces that are currently acting upon this microorganism. The topology of the obtained phylograms demonstrates the lack of a relationship between the phylogenetic position of the strain and the host cultivar and the geographical origin or race of the strain. The isolates from different hosts were differentiated but did not form diagnosable, distinct phylogenetic groups. These results suggest that the analyzed genes may be too conserved to reflect the adaptation of pathogens to apple genotypes with different R genes; thus, they do not adequately reflect race discrimination. In contrast, based on variation and gene flow estimation, genetic divergence was observed among strains virulent to apple trees containing Rvi6. The results of this study confirmed a lack of free recombination between strains and demonstrated that the analyzed regions are in linkage disequilibrium and contain non-random polymorphisms associated with the strain.

Quantitative trait locus mapping for common scab resistance in a tetraploid potato full-sib population

Despite the negative impact of common scab (Streptomyces spp.) on the potato industry, little is known about the genetic architecture of resistance to this bacterial disease in the crop. We evaluated a mapping population (~150 full-sibs) derived from a cross between two tetraploid potatoes (‘Atlantic’ × B1829-5) in three environments (MN11, PA11, ME12) under natural common scab pressure. Three measures to common scab reaction, namely percentage of scabby tubers, and disease area and lesion indices, were found to be highly correlated (>0.76). Due to the large environmental effect, heritability values were zero for all three traits in MN11, but moderate to high in PA11 and ME12 (0.44~0.79). We identified a single quantitative trait locus (QTL) for lesion index in PA11, ME12 and joint analyses on linkage group 3, explaining 22~30% of the total variation. The identification of QTL haplotypes and candidate genes contributing to disease resistance can support genomics-assisted breeding approaches in the crop.

Identification of a Novel Genomic Region Associated With Resistance to Fusarium Head Blight in Chinese Winter Wheat

Background: Fusarium head blight (FHB) is a disease affecting wheat spikes caused by Fusarium species, which leads to cases of severe yield reduction and seed contamination. Therefore, identifying resistance genes from various sources is always of importance to wheat breeders. In this study, a genome-wide association study (GWAS) focusing on FHB using a high-density genetic map constructed with 90K single nucleotide polymorphism (SNP) arrays in a panel of 205 elite winter wheat accessions, was conducted in 3 environments. Results: Sixty-six significant marker–trait associations (MTAs) were identified (P<0.001) on fifteen chromosomes explaining 5.4–11.2% of the phenotypic variation therein. Some important new genomic regions involving FHB resistance were found on chromosomes 2A, 3B, 5B, 6A, and 7B. On chromosome 7B, 6 MTAs at 92 genetic positions were found in 2 environments. Moreover, there were 11 MTAs consistently associated with diseased spikelet rate and diseased rachis rate as pleiotropic effect loci. Eight new candidate genes of FHB resistance were predicated in wheat. Of which, three genes: TraesCS5D01G006700, TraesCS6A02G013600, and TraesCS7B02G370700 on chromosome 5DS, 6AS, and 7BL, respectively, were important in defending against FHB by regulating chitinase activity, calcium ion binding, intramolecular transferase activity, and UDP-glycosyltransferase activity in wheat. In addition, a total of six excellent alleles associated with wheat scab resistance were discovered. Conclusion: These results provide important genes/loci for enhancing FHB resistance in wheat breeding populations by marker-assisted selection.

New apple-tree (Malus domestica Mill.) varieties as sourses of breeding valuable traits

Aim. To identify sources of several valuable economic and biological traits among new Ukrainian and foreign cultivars. Results and Discussion. The weather in the spring of 2019 and 2020 contributed to the epiphytotic development of scab, which allowed for assessment of susceptibility of new cultivars to this disease and for identification of sources of high resistance to it. There were no pathogen signs in immune cvs. Harant, Goldrush, Topaz, and William's Pride, which are carriers of the scab resistance oligogen (Vf). No cultivars that would be consistently highly-resistant to this disease in the field have been found. The creation of cultivars with a columnar crown remains a promising trend in breeding. Cv. Ruslan is a columnarity source. As to fruit yield, the leaders were Elise, Geneva Early, Goldrush, Honeycrisp, Pilot, and Topaz. Cvs. Harant, Ruslan, Elise, William's Pride gave stable fruit yields. Cvs. Kakhovske, Alionushkino, Julia, Honeycrisp, and Topaz give yields every year. Cvs. Harant, Malakhit and Rubin are sources of large fruits. Cv. Harant is noticeable for stable expression of this trait, which makes it a reference of large fruits. Mid-sized apples are considered the most optimal for selling. The sources of this trait are cvs. Ruslan, Elise, Honeycrisp, Szampion Arno, and Topaz. Most of the studied cultivars have round or flat-round fruits. Kakhovske apples are distinguished by their unordinary oval (sinapoid) shape. Sources of several features that determine highly-attractive appearance of their fruits have been distinguished. These are cvs. Harant, Malakhit, Ruslan, Elise, Geneva Early, Honeycrisp, Rubin, and William's Pride. Apples of cvs. Kakhovske, Elise, Honeycrisp, Rubin, and William's Pride are characterized by excellent palatability. Cultivars that are sources of several valuable economic and biological traits have been selected. Conclusions. Modern apple cultivars are starting material for creating new cultivars that will meet the future requirements. Cvs. Harant, Ruslan, Elise, Honeycrisp, Topaz, and William's Pride are sources of the maximum number of selectively valuable traits. By crossing them with cvs. Kakhovske, Malakhit, Alionushkino, Geneva Early, Goldrush, Julia, Pilot, and Rubin, it is possible to obtain genotypes with various ripening periods, which would combine immunity to scab, compact crown, high and stable yield, and high quality fruits.