Sources Of Genetic Resistance Research Articles

Host sunflower-induced silencing of parasitism-related genes confers resistance to invading Orobanche cumana

Orobanche cumana is a holoparasitic plant that attaches to host-plant roots and seriously reduces the yield of sunflower (Helianthus annuus L.). Effective control methods are lacking with only a few known sources of genetic resistance. In this study, a seed-soak agroinoculation (SSA) method was established, and recombinant tobacco rattle virus vectors were constructed to express RNA interference (RNAi) inducers to cause virus-induced gene silencing (VIGS) in sunflower. A host target gene HaTubulin was systemically silenced in both leaf and root tissues by the SSA-VIGS approach. Trans-species silencing of O. cumana genes were confirmed for 10 out of 11 target genes with silencing efficiency of 23.43%-92.67%. Knockdown of target OcQR1, OcCKX5, and OcWRI1 genes reduced the haustoria number, and silencing of OcEXPA6 caused further phenotypic abnormalities such as shorter tubercles and necrosis. Overexpression of OcEXPA6 caused retarded root growth in alfalfa (Medicago sativa). The results demonstrate that these genes play an important role in the processes of O. cumana parasitism. High-throughput small RNA (sRNA) sequencing and bioinformatics analyses unveiled the distinct features of target gene-derived siRNAs in O. cumana such as siRNA transitivity, strand polarity, hotspot region, and 21/22-nt siRNA predominance, the latter of which was confirmed by Northern blot experiments. The possible RNAi mechanism is also discussed by analyzing RNAi machinery genes in O. cumana. Taken together, we established an efficient host-induced gene silencing technology for both functional genetics studies and potential control of O. cumana. The ease and effectiveness of this strategy could potentially be useful for other species provided they are amenable to SSA.

Identification and validation of Fusarium head blight resistance QTL in the U.S. soft red winter wheat cultivar ‘Jamestown’

AbstractUse of genetic resistance is one of the most important strategies to manage the devastating disease Fusarium head blight (FHB) in wheat. Numerous quantitative trait loci (QTL) having varying effects on reducing FHB and the mycotoxin deoxynivalenol (DON) accumulation have been reported from Asian, European, or distant sources such as wild relatives of wheat (Triticum aestivum L.). However, coming from nonadapted backgrounds, the incorporation of such QTL into regional breeding programs has often resulted in the simultaneous transfer of other undesirable traits. Therefore, it is important to identify, characterize, and deploy sources of genetic resistance that do not suffer from poor adaptability and/or linkage drag. In the present work, QTL associated with FHB resistance in a high‐yielding, moderately resistant soft red winter wheat cultivar ‘Jamestown’ were mapped and validated. The QTL mapping was done using a recombinant inbred line (RIL) population of Pioneer ‘25R47’ × Jamestown having 186 individuals. Phenotyping over 2 yr at three locations, and genotyping using the 90K single nucleotide polymorphism (SNP) platform identified two new QTL, named QFHB.vt‐1B.1 and QFHB.vt‐1B.2, on the chromosome 1B long arm. The QTL contributed to FHB incidence, FHB severity, Fusarium‐damaged kernels, and DON content. Independent mapping of these QTL using two additional RIL populations of FG95195 × Jamestown (170 RILs) and Jamestown × LA97113UC‐124 (77 RILs) validated their stability and effectiveness in different genetic backgrounds. Kompetitive allele specific polymerase chain reaction (KASP) assays were developed using linked SNPs for marker‐assisted selection of the QTL. These QTL are being used in breeding programs to develop FHB‐resistant, high‐yielding varieties.

Effect of Ascochyta blight disease on antioxidant enzymes activities, amount of proline and carbohydrate in some chickpea genotypes

Germplasm evaluation of crop plants as a repository of useful genes to cope with biological and abiotic stresses has always been the focus of plant breeders. The fungus causing Ascochyta blight is one of the most important biological factors limiting chickpea cultivation and production in most parts of the world, including Iran.The present study was conducted to identify the genetic sources of resistance of 20 chickpea genotypes in seedling, flowering, and podding stages in greenhouse conditions. Damages caused by the disease was recorded using a 9-degree scale after observing complete death in the sensitive control genotypes. Analysis of variance of the studied traits of chickpea genotypes was conducted via factorial experiment in a completely randomized design at two levels for factor A (disease-free and disease-contaminated conditions) and 18 levels (genotypes) for factor B (genotypes 13 and 15 were lost due to high susceptibility to the disease in the first stage of growth, samples were taken from 18 genotypes).The results showed that the resistant and susceptible genotypes were more accurately distinguished from each other in the podding stage. At this stage, 9 genotypes with a degree of damage 1, 2, and 3 (less than five) showed high resistance to the causative agent of Ascochyta blight. Physiological and biochemical traits involved in disease resistance were measured. The results showed that all traits except chlorophyll a, chlorophyll b and polyphenol oxidase had significant differences at 1% probability level in terms of disease stress. Chlorophyll a, chlorophyll b contents and polyphenol oxidase activity were significantly different at 5% probability level. In interaction of disease×genotype, only catalase was significantly different among all studied traits. The amount of peroxidase and polyphenol oxidase were affected by the disease and their rates increased. A positive relationship was observed between the level of polyphenol oxidase enzyme and pathogen resistance. Generally speaking, crops’ reactions to harsh environmental conditions seems impossible to predict without the analysis of the relevant mechanisms.

Characterization of exotic germplasm lines for resistance to wheat rusts and spot blotch

Breeding resistant varieties of wheat is a continuous process and empirical disease control depends on deployment of diverse sources of genetic resistance to the deadly rusts. A total of 166 exotic wheat germplasm accessions were evaluated for resistance to wheat rusts having some component of resistance to either any of the three rusts and spot blotch. The data for adult plant resistance (APR) on leaf and stem rusts, was recorded at Wellington while yellow rust epiphytotic was artificially created at Karnal for identification of resistant lines. Based on APR data 85, 96 and 16 accessions were found completely resistant to leaf, stem and stripe rusts respectively. Based on the seedling resistance data against different pathotypes (10 each for leaf and stripe and 5 for stem rust) of each rust, nine, fifteen and five accessions were found highly resistant respectively. Based on the multi-environment adult plant resistance data and race specific seedling data, we were able to identify 28, accessions highly resistant to either one or two rusts. For spot blotch the evaluation was done at hotspot locations Wadura and Coochbehar and 26 accessions were found to be highly resistant against this disease. The lines so identified in this study are being used in the hybridization programs to enhance the levels of resistance.

Genome Sequence Resource for Pathogen Bipolaris sorokiniana Shoemaker GN1 Causing Spot Blotch of Barley (Hordeum vulgare L.).

Spot blotch, caused by fungal pathogen Bipolaris sorokiniana Shoemaker, is one of the most frequent diseases affecting barley-growing regions worldwide. In this study, we reported the genome sequence of the highly virulent B. sorokiniana strain GN1 using the Illumina HiSeq 4000 platform. In total, 57 million 150-nucleotide paired-end clean reads were obtained and assembled into 96 scaffolds with an estimated genome size of 34.33 Mb. Furthermore, we identified genes that may be associated with strain-specific virulence and performed phylogenetic analysis of GN1 with five other Bipolaris spp. These results for GN1 will provide important information in understanding its molecular underpinning of pathogenicity and help identify novel sources of genetic resistance for improving disease resistance in barley.

Constitutive and Induced Expression of Total Phenol and Phenol Oxidases in Wheat Genotypes Ranging in Resistance/Susceptibility to the Root-Lesion Nematode Pratylenchus thornei.

Plant-derived phenolic compounds contribute to the defense against various pathogens, including root-lesion nematodes (Pratylenchus spp.). However, there are no reports on the role of phenolic compounds in wheat (Triticum aestivum) against Pratylenchus thornei. In this study, wheat genotypes ranging from resistant to very susceptible to P. thornei were used to investigate the level of total phenols and phenol oxidases, polyphenol oxidase (PPO), and peroxidase (POD) expressed in root tissues when grown in the presence and absence of P. thornei over time (2–8 weeks). Higher constitutive levels of total phenols were found in resistant synthetic hexaploid wheats CPI133872 (576 µg gallic acid equivalent (GAE)/g root) and CPI133859 (518 µg GAE/g root) at 8 weeks after sowing, compared with moderately resistant and susceptible genotypes (192 to 390 µg GAE/g root). The activity of PPO was induced in resistant (CPI133872) and moderately resistant (GS50a and its derivate QT8343) genotypes, becoming maximal at 4 weeks after P. thornei inoculation. The activity of POD was induced in CPI133872 at 6 weeks after P. thornei inoculation. Different genetic sources of resistance to P. thornei showed diverse defense mechanisms and differences in timing responses. The combined effects of total phenols and oxidative enzymes could be important for defense against P. thornei in some resistant wheat genotypes.

Overview of Biotic Stresses in Pepper (Capsicum spp.): Sources of Genetic Resistance, Molecular Breeding and Genomics.

Pepper (Capsicum spp.) is one of the major vegetable crops grown worldwide largely appreciated for its economic importance and nutritional value. This crop belongs to the large Solanaceae family, which, among more than 90 genera and 2500 species of flowering plants, includes commercially important vegetables such as tomato and eggplant. The genus includes over 30 species, five of which (C. annuum, C. frutescens, C. chinense, C. baccatum, and C. pubescens) are domesticated and mainly grown for consumption as food and for non-food purposes (e.g., cosmetics). The main challenges for vegetable crop improvement are linked to the sustainable development of agriculture, food security, the growing consumers’ demand for food. Furthermore, demographic trends and changes to climate require more efficient use of plant genetic resources in breeding programs. Increases in pepper consumption have been observed in the past 20 years, and for maintaining this trend, the development of new resistant and high yielding varieties is demanded. The range of pathogens afflicting peppers is very broad and includes fungi, viruses, bacteria, and insects. In this context, the large number of accessions of domesticated and wild species stored in the world seed banks represents a valuable resource for breeding in order to transfer traits related to resistance mechanisms to various biotic stresses. In the present review, we report comprehensive information on sources of resistance to a broad range of pathogens in pepper, revisiting the classical genetic studies and showing the contribution of genomics for the understanding of the molecular basis of resistance.

Assessment of the Seedling Resistance of Spring Wheat Lines to Fusarium culmorum

Wheat diseases are one of the constraints limiting wheat yields wherever the crop is grown. Fusarium crown rot, incited by Fusarium culmorum, is one of the most important diseases limiting wheat yields especially in dryland areas. Although there are no wheat varieties which are fully resistant to crown rot, the use of varieties showing some degree of resistance is the most reliable and cost effective method to control this disease. In this study, seedling reactions of 165 spring wheat breeding lines (Triticum aestivum L.) obtained from CIMMYT, Mexico were determined under growth room conditions using an aggressive isolate of Fusarium culmorum. Crown rot severity was assessed using a 1-5 scale. The mean disease severity scores for the lines tested ranged from 1.4 to 4.4. Two out of the 165 lines tested (lines 147 and 158) were resistant (R) in their reaction and had scores of 1.4. Twenty lines showed moderately resistant (MR) reaction and had scores ranging from 1.6 to 2.4. The scores of both the R and MR lines were not significantly different from scores of MR control cultivars. Sixty-three percent of the lines were moderately susceptible (MS). Out of the 165 lines tested, 39 were susceptible (S) in their reaction. The promising wheat lines that showed some degree of resistance to Fusarium culmorum in the present study can serve as useful sources of genetic resistance in breeding for Fusarium crown rot.

Cucurbitaceae Germplasm Resistance to Verticillium Wilt and Grafting Compatibility with Watermelon

Grafting is used in watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] production as a means to combat soilborne diseases. To support the development of new rootstock cultivars in the United States, we screened cucurbit germplasm accessions for resistance to verticillium wilt (caused by Verticillium dahliae Kleb.) and for compatibility as watermelon rootstocks. Screening was done using a field naturally infested with V. dahliae [5 and 7.5 colony-forming units (cfu)·g−1 soil in 2017 and 2018, respectively], and plants were inoculated at transplanting (1.5 and 104 cfu of V. dahliae per plant in 2017 and 2018, respectively). In 2017, 56 germplasm accessions from three genera commonly used as rootstocks, Cucurbita, Lagenaria, and Benincasa, were sourced from the U.S. Department of Agriculture (USDA) National Plant Germplasm System and area under the verticillium wilt (disease) progress curve (AUDPC) values ranged from 16 to 397. The 14 accessions with the lowest AUDPC values and good germination (>40%) were used as rootstocks along with the commercial rootstock cv. Tetsukabuto (control), and all were grafted with watermelon cv. Secretariat as the scion in 2018. Grafted plant survival rate was greatest for ‘Tetsukabuto’ (90%) and the accession PI 381840 (L. siceraria) (89%), and ranged from 22% to 85% for all other accessions. All grafted treatments that produced mature fruit in 2018 tended to flower at the same time as nongrafted ‘Secretariat’, with first male and female flowers occurring in 45 to 50 days and 44 to 51 days after transplanting, respectively. There were no significant differences in AUDPC values due to grafting or when accessions were compared with ‘Tetsukabuto’. Only six accessions produced mature fruit when grafted with ‘Secretariat’, indicating they were compatible for watermelon grafting. Fruit weight and number as well as total soluble solids, pH, lycopene content, rind firmness and thickness, and dry matter content were similar for all accessions and ‘Tetsukabuto’ grafted on ‘Secretariat’. Only fruit flesh firmness differed and was highest for ‘Secretariat’ grafted on ‘PI 491316’ and lowest for ‘Secretariat’ grafted on ‘PI 49174’. The six verticillium wilt-tolerant accessions that were compatible with watermelon could potentially be used as rootstocks or as sources of genetic resistance in rootstock breeding programs.

Natural resistance to Potato virus Y in Solanum tuberosum Group Phureja

Key MessageNovel major gene resistance against Potato virus Y in diploid populations of Solanum tuberosum Groups Phureja and Tuberosum was biologically and genetically characterised. Named Ry(o)phu, it mapped to chromosome 9.A new source of genetic resistance derived from Solanum tuberosum Group Phureja against Potato virus Y (PVY) was identified and genetically characterised in three diploid biparental potato populations. Segregation data for two populations (05H1 and 08H1) suggested the presence of a single dominant gene for resistance to PVY which, following DaRT analysis of the 08H1 cross, was mapped to chromosome 9. More detailed genetic analysis of resistance utilised a well-characterised SNP-linkage map for the 06H1 population, together with newly generated marker data. In these plants, which have both S. tuberosum Group Phureja and S. tuberosum Group Tuberosum in their pedigree, the resistance was shown to map to chromosome 9 at a locus not previously associated with PVY resistance, although there is evidence for at least one other genetic factor controlling PVY infection. The resistance factor location on chromosome 9 (named as Ry(o)phu) suggests a potential role of NB-LRR genes in this resistance. Phenotypic analysis using a GUS-tagged virus revealed that a small amount of PVY replication occurred in occasional groups of epidermal cells in inoculated leaves of resistant plants, without inducing any visible hypersensitive response. However, the virus did not enter the vascular system and systemic spread was completely prevented.

Understanding the role of phenotypic switching in cancer drug resistance

The emergence of acquired drug resistance in cancer represents a major barrier to treatment success. While research has traditionally focused on genetic sources of resistance, recent findings suggest that cancer cells can acquire transient resistant phenotypes via epigenetic modifications and other non-genetic mechanisms. Although these resistant phenotypes are eventually relinquished by individual cells, they can temporarily ’save’ the tumor from extinction and enable the emergence of more permanent resistance mechanisms. These observations have generated interest in the potential of epigenetic therapies for long-term tumor control or eradication. In this work, we develop a mathematical model to study how phenotypic switching at the single-cell level affects resistance evolution in cancer. We highlight unique features of non-genetic resistance, probe the evolutionary consequences of epigenetic drugs and explore potential therapeutic strategies. We find that even short-term epigenetic modifications and stochastic fluctuations in gene expression can drive long-term drug resistance in the absence of any bona fide resistance mechanisms. We also find that an epigenetic drug that slightly perturbs the average retention of the resistant phenotype can turn guaranteed treatment failure into guaranteed success. Lastly, we find that combining an epigenetic drug with an anti-cancer agent can significantly outperform monotherapy, and that treatment outcome is heavily affected by drug sequencing.

A Genome Resource for Several North American Venturia inaequalis Isolates with Multiple Fungicide Resistance Phenotypes.

The apple scab pathogen, Venturia inaequalis, is among the most economically important fungal pathogens that affects apples. Fungicide applications are an essential part of disease management. Implementation of cultural practices and genetic sources of resistance in the host are vital components of scab management. This is the first presentation of multiple, high quality, well-annotated genomes of four North American V. inaequalis isolates having both sensitive and multiple fungicide resistance phenotypes. We envision that these isolates will enable investigations into fungicide resistance mechanisms, exploring fungal virulence factors and delineating phylogenomic relationships among apple scab isolates from around the world.

Screening of cowpea accessions for cowpea aphid resistance

ABSTRACT Cowpea (Vigna unguiculata) genotypes may preserve important characteristics of potential use for the genetic breeding of this species. This study aimed to characterize sources of genetic resistance to the cowpea aphid (Aphis craccivora), among accessions maintained at the cowpea germplasm bank of the Universidade Federal do Ceará (Brazil), in order to suggest crossing options for obtaining superior cultivars. Twenty-one local accessions were assessed together with two genotypes as resistant checks and two others as susceptible checks. A randomized complete block design, with four replications, was used. Counts of live adults and nymphs per plant were used to evaluate the genotypic resistance and, based on the ranking of these data, the effective resistance was estimated. The applied approach showed to be effective to discriminate the genetic variability for cowpea aphid resistance, identifying antibiosis as the probable resistance mechanism. Four genotypic groups were identified: resistant, intermediate, susceptible and highly susceptible. Thus, for a short-time cultivar release strategy, backcrosses are suggested between the BRS Guariba cultivar (recurrent parental, with good agronomic traits) and aphid-resistant accessions, such as Enrica Pobre (CE-36), Das Almas (CE-07), CE-51 (selected within CE-13) and Ritinha (CE-08). However, for a long-term breeding strategy, crossings between the most dissimilar accessions are suggested, using resistant genitors (e.g. BRS Guariba, Enrica Pobre, Das Almas, CE-51 and Ritinha) and others with good commercial and agronomic attributes.

QTL mapping of resistance to Fusarium oxysporum f. sp. niveum race 2 and Papaya ringspot virus in Citrullus amarus.

A Citrullus amarus mapping population segregating for resistance to Fusarium oxysporum f. sp. niveum race 2 and Papaya ringspot virus was used to identify novel QTL, important for the improvement in watermelon disease resistance. Multiple disease screens of the USDA Citrullus spp. germplasm collection have highlighted the value of Citrullus amarus (citron melon or wild watermelon) as a resource for enhancing modern watermelon cultivars (Citrullus lanatus) with resistance to a broad range of fungal, bacterial and viral diseases of watermelon. We have generated a genetic population of C. amarus segregating for resistance to two important watermelon diseases: Fusarium wilt (caused by the fungus Fusarium oxysporum f. sp. niveum; Fon race 2) and Papaya ringspot virus-watermelon strain (PRSV-W). QTL mapping of Fon race 2 resistance identified seven significant QTLs, with the major QTL representing a novel genetic source of resistance and an opportunity for gene pyramiding. A single QTL was associated with resistance to PRSV-W, which adhered to expectations of a prior study indicating a single-gene recessive inheritance in watermelon. The resistance loci identified here provide valuable genetic resources for introgression into cultivated watermelon for the improvement in disease resistance.

Evaluation of genomic selection and marker-assisted selection in Miscanthus and energycane

Although energycane (Saccharum spp. hybrids) is widely used as a source of lignocellulosic biomass for bioethanol, breeding this crop for disease resistance is challenging due to its narrow genetic base. Therefore, efforts are underway to introgress novel sources of genetic resistance from Miscanthus into energycane. Given that disease resistance in energycane could be either qualitative or quantitative in nature, careful examination of a wide variety of genomic-enabled breeding approaches will be crucial to the success of such an undertaking. Here we examined the efficiency of both genomic selection (GS) and marker-assisted selection (MAS) for traits simulated under different genetic architectures in F1 and BC1 populations of Miscanthus × Miscanthus and sugarcane × sugarcane crosses. We observed that the performance of MAS was comparable and sometimes superior to GS for traits simulated with four quantitative trait nucleotides (QTNs). In contrast, as the number of simulated QTN increased, all four GS models that were evaluated tended to outperform MAS, select more phenotypically optimal F1 individuals, and accurately predict simulated trait values in subsequent BC1 generations. We therefore conclude that GS is preferable to MAS for introgressing genetic sources of horizontal disease resistance from Miscanthus to energycane, while MAS remains a suitable option for introgressing vertical disease resistance.

Genome wide association study reveals novel QTL for barley yellow dwarf virus resistance in wheat

BackgroundBarley yellow dwarf (BYD) is an important virus disease that causes significant reductions in wheat yield. For effective control of Barley yellow dwarf virus through breeding, the identification of genetic sources of resistance is key to success. In this study, 335 geographically diverse wheat accessions genotyped using an Illumina iSelect 90 K single nucleotide polymorphisms (SNPs) bead chip array were used to identify new sources of resistance to BYD in different environments.ResultsA genome-wide association study (GWAS) performed using all the generalised and mixed linkage models (GLM and MLM, respectively) identified a total of 36 significant marker-trait associations, four of which were consistently detected in the K model. These four novel quantitative trait loci (QTL) were identified on chromosomes 2A, 2B, 6A and 7A and associated with markers IWA3520, IWB24938, WB69770 and IWB57703, respectively. These four QTL showed an additive effect with the average visual symptom score of the lines containing resistance alleles of all four QTL being much lower than those with less favorable alleles. Several Chinese landraces, such as H-205 (Baimazha) and H-014 (Dahongmai) which have all four favorable alleles, showed consistently higher resistance in different field trials. None of them contained the previously described Bdv2, Bdv3 or Bdv4 genes for BYD resistance.ConclusionsThis study identified multiple novel QTL for BYD resistance and some resistant wheat genotypes. These will be useful for breeders to generate combinations with and/or without Bdv2 to achieve higher levels and more stable BYD resistance.

In vivo and in vitro screening for resistance against Penicillium allii in garlic accessions

Garlic blue mold disease, caused by Penicillium allii, is responsible for important economic losses produced yearly in garlic field crops and in postharvest storage. The identification of genetic resistance sources may help in the management of this disease. The present study investigated in vivo and in vitro antifungal effects of a genetically-diverse garlic collection against P. allii in two growing seasons. Cloves of garlic accessions were inoculated with P. allii conidia, and lesion area and sporulation of the fungus in the clove were estimated as a means to characterize the accessions response to the infection. Significant and continuous variation was found among the accessions (p < 0.001), suggesting a quantitative mode of resistance against P. allii in this garlic collection. ‘Castano’ and ‘Peteco’ were consistently the most resistant and susceptible genotypes, respectively. Culture media of the fungus supplemented with crude garlic extracts (CGEs) revealed significant inhibition of P. allii growth in vitro by all the garlic accessions, with fungal-growth inhibition being dose-dependent up to a concentration of 20–26% (v/v), beyond which the CGEs became fungicidal. These data indicate that garlic bulbs have antifungal compounds, which at high concentration can be fungicidal for P. allii. CGEs of ‘Castano’ and a few other accessions were consistently the more inhibitory extracts. Total phenolics and pyruvate (an estimator of the total thiosulfinates content) concentration in the garlic bulbs was not associated with P. allii resistance in vivo, but their content in the CGEs correlated significantly and negatively with the fungal growth in vitro (r = −0.38 to −0.72), suggesting that these compounds have antifungal effect, but their content is not the main factor conditioning P. allii resistance in vivo.

Genetic Characterization of Apulian Olive Germplasm as Potential Source in New Breeding Programs.

The olive is a fruit tree species with a century-old history of cultivation in the Mediterranean basin. In Apulia (Southern Italy), the olive is of main social, cultural and economic importance, and represents a hallmark of the rural landscape. However, olive cultivation in this region is threatened by the recent spread of the olive quick decline syndrome (OQDS) disease, thus there is an urgent need to explore biodiversity and search for genetic sources of resistance. Herein, a genetic variation in Apulian olive germplasm was explored, as a first step to identify genotypes with enhanced bio-agronomic traits, including resistance to OQDS. A preselected set of nuclear microsatellite markers allowed the acquisition of genotypic profiles, and to define genetic relationships between Apulian germplasm and widespread cultivars. The analysis highlighted the broad genetic variation in Apulian accessions and the presence of different unique genetic profiles. The results of this study lay a foundation for the organization of new breeding programs for olive genetic improvement.

Durable powdery mildew resistance in grapevines: myth or reality

Grape powdery mildew (PM) is an economically important disease worldwide, and necessitates high input of fungicides for successful grape production. Development of cultivars that carry resistance from multiple backgrounds is an important strategy to obtain durable resistance in the field. This strategy requires a) the identification of sources of resistance and marker development to track those resistances, b) characterization of the mechanisms of resistance, c) marker-assisted breeding to introduce the selected loci into elite cultivars, and, finally, d) understanding of pathogen biology and responses to different resistance sources. We have identified and tagged novel sources of genetic resistance from eight genetic backgrounds, including Muscadinia rotundifolia, two different Chinese species (Vitis piasezkii and Vitis romanetii), a wide array of Vitis vinifera cultivars and wild forms of V. vinifera subsp. sylvestris from Asia. Molecular markers that are tightly linked to different loci are used at the early stage to select seedlings that carry resistance loci. PM resistance from two or more genetic backgrounds is combined in multiple steps, while maintaining high fruit-quality attributes and other important horticultural traits. Resistance is confirmed by careful evaluation for disease development in the field, the greenhouse or by in-vitro assay. Multipronged testing of seedling plants for resistance allows better assessment of pathogen biology and interaction with different resistance loci and allows us to make decisions about combining resistance loci. Results will be presented with elucidation of the mechanism of resistance from different backgrounds, specificity to different isolates and the effectiveness of molecular markers in the breeding program.

Prospection of genetic resistance resources to root-knot nematodes in cucurbit genotypes

ABSTRACT The aim of this research was to prospect genetic resistant sources to root-knot-nematode in Cucurbitaceae accessions. Two assays were conducted, the first to evaluate the resistance to root-knot nematode, M. incognita race 1 in accessions of Lagenaria vulgaris; Sicana odorifera; Cucurbita facifolia; Luffa sp.; Citrullus lanatus cv. Citroides; Lagenaria vulgaris; Cucurbita moschata; Cucurbita moschata x Cucurbita maxima; Cucumis melo and Cucumis metuliferus. The second to evaluate the reaction to nematode M. incognita race 1, M. javanica and M. enterolobii of the three accessions of Cucumis metuliferus (‘Kino’) considered resistant to M. incognita race 1 in the first experiment. The assays were carried out in a greenhouse at Embrapa Hortaliças in a completely randomized design with six replications. The seedlings in pots were inoculated with 5000 eggs and juveniles of 2nd stage (J2) of each Meloidogyne species alone. Fifty-three and eighty-four days after inoculation, respectively in both experiments, the plants were evaluated for egg mass index (IMO), gall index (IG), number of eggs per gram of root (NEGR) and reproduction factor (RF). The melon Cucumis metuliferus known as ‘Kino’ was identified as genetic source of resistance to root-knot nematodes and can be explored in breeding program as resistant rootstock.