Genetic dissection of pathotype-specific resistance to ascochyta blight disease in chickpea (Cicer arietinum L.) using microsatellite markers

Comparison of different screening methods for the selection of Ascochyta blight disease on chickpea (Cicer arietinum L.) genotypes

Chickpea (Cicer arietinum L.) is the second most important edible food grain legume, widely grown all over the world. However, the cultivation and production of chickpea are mainly affected by the Ascochyta blight (AB) disease, which causes losses of up to 100% in areas with high humidity and warm temperature conditions. Various screening methods are used in the selection of chickpea genotypes for resistance to AB disease. These methods are natural field condition (NFC), artificial epidemic field condition (AEC), marker-assisted selection (MAS), and real-time PCR (RT-PCR). The study was conducted with 88 chickpea test genotypes between the 2014 and 2016 growing seasons. The results of the screening were used to sort the genotypes into three categories: susceptible (S), moderately resistant (MR), and resistant (R). Using MAS screening, 13, 21, and 54 chickpea genotypes were identified as S, MR, and R, respectively. For RT-PCR screening, 39 genotypes were S, 31 genotypes were MR, and 18 genotypes were R. In the AEC method for NFC screening, 7, 17, and 64 genotypes were S, MR, and R, while 74 and 6 genotypes were S and MR, and 8 genotypes were R-AB disease. As a result of screening chickpea genotypes for AB disease, it was determined that the most effective method was artificial inoculation (AEC) under field conditions. In the study, Azkan, ICC3996, Tüb-19, and Tüb-82 were determined as resistant within all methods for Pathotype 1.

Genomics-assisted genetics of complex region from chickpea chromosome 4 reveals two candidate genes for Ascochyta blight resistance

Ascochyta blight (AB) disease caused by the fungus Ascochyta rabiei is a major threat to global chickpea production. Molecular breeding for improved AB resistance requires the identification of robust fine-mapped QTLs/candidate genes and associated markers. Earlier, we identified three QTLs (qABR4.1, qABR4.2, and qABR4.3) for AB resistance on chickpea chromosome 4 by employing multiple quantitative trait loci sequencing strategy on an intra-specific (FLIP84-92C x PI359075) and an inter-specific (FLIP84-92C x PI599072) crosses derived recombinant inbred lines. Here, we report the identification of AB resistance providing candidate genes under the fine mapped qABR4.2 and qABR4.3 genomic region by combining genetic mapping, haplotype block inheritance, and expression analysis. The qABR4.2 region was narrowed down from 5.94Mb to ~800kb. Among 34 predicted gene models, a secreted class III peroxidase encoding gene showed higher expression in AB resistant parent after A. rabiei conidia inoculation. Under qABR4.3, we identified a frame-shift mutation in a cyclic nucleotide-gated channel CaCNGC1 gene leading to the truncated N-terminal domain in resistant accession of chickpea. This N-terminal domain of CaCNGC1 interacts with chickpea calmodulin. Thus, our analysis has revealed narrowed genomic regions and their associated polymorphic markers, CaNIP43 and CaCNGCPD1. These co-dominant markers significantly associate with AB resistance on qABR4.2 and qABR4.3 regions. Our genetic analysis revealed that the presence of resistant alleles for two major QTLs (qABR4.1 and qABR4.2) together provide AB resistance in the field while minor QTL qABR4.3 determines the degree of resistance. The identified candidate genes and their diagnostic markers will help in biotechnological and AB resistance introgression into farmers adapted local chickpea varieties.

Breeding and Genomic Approaches towards Development of Fusarium Wilt Resistance in Chickpea

Chickpea is an important leguminous crop with potential to provide dietary proteins to both humans and animals. It also ameliorates soil nitrogen through biological nitrogen fixation. The crop is affected by an array of biotic and abiotic factors. Among different biotic stresses, a major fungal disease called Fusarium wilt, caused by Fusarium oxysporum f. sp. ciceris (FOC), is responsible for low productivity in chickpea. To date, eight pathogenic races of FOC (race 0, 1A, and 1B/C, 2-6) have been reported worldwide. The development of resistant cultivars using different conventional breeding methods is very time consuming and depends upon the environment. Modern technologies can improve conventional methods to solve these major constraints. Understanding the molecular response of chickpea to Fusarium wilt can help to provide effective management strategies. The identification of molecular markers closely linked to genes/QTLs has provided great potential for chickpea improvement programs. Moreover, omics approaches, including transcriptomics, metabolomics, and proteomics give scientists a vast viewpoint of functional genomics. In this review, we will discuss the integration of all available strategies and provide comprehensive knowledge about chickpea plant defense against Fusarium wilt.

Chickpea (Cicer arietinum L.) Biology and Biotechnology: From Domestication to Biofortification and Biopharming

Chickpea (Cicer arietinum L.), the world's second most consumed legume crop, is cultivated in more than 50 countries around the world. It is a boon for diabetics and is an excellent source of important nutrients such as vitamins A, C, E, K, B1-B3, B5, B6, B9 and minerals (Fe, Zn, Mg and Ca) which all have beneficial effects on human health. By 2050, the world population can cross 9 billion, and in order to feed the teaming millions, chickpea production should also be increased, as it is a healthy alternative to wheat flour and a boon for diabetics. Moreover, it is an important legume that is crucial for food, nutrition, and health security and the livelihood of the small-scale farmers with poor resources, in developing countries. Although marvelous improvement has been made in the development of biotic and abiotic stress-resistant varieties, still there are many lacunae, and to fulfill that, the incorporation of genomic technologies in chickpea breeding (genomics-assisted breeding, high-throughput and precise-phenotyping and implementation of novel breeding strategies) will facilitate the researchers in developing high yielding, climate resilient, water use efficient, salt-tolerant, insect/pathogen resistant varieties, acceptable to farmers, consumers, and industries. This review focuses on the origin and distribution, nutritional profile, genomic studies, and recent updates on crop improvement strategies for combating abiotic and biotic stresses in chickpea.

Ascochyta rabiei: A threat to global chickpea production

The necrotrophic fungus Ascochyta rabiei causes Ascochyta blight (AB) disease in chickpea. A. rabiei infects all aerial parts of the plant, which results in severe yield loss. At present, AB disease occurs in most chickpea-growing countries. Globally increased incidences of A. rabiei infection and the emergence of new aggressive isolates directed the interest of researchers toward understanding the evolution of pathogenic determinants in this fungus. In this review, we summarize the molecular and genetic studies of the pathogen along with approaches that are helping in combating the disease. Possible areas of future research are also suggested.

TAXONOMY

kingdom Mycota, phylum Ascomycota, class Dothideomycetes, subclass Coelomycetes, order Pleosporales, family Didymellaceae, genus Ascochyta, species rabiei.

PRIMARY HOST

A. rabiei survives primarily on Cicer species.

DISEASE SYMPTOMS

A. rabiei infects aboveground parts of the plant including leaves, petioles, stems, pods, and seeds. The disease symptoms first appear as watersoaked lesions on the leaves and stems, which turn brown or dark brown. Early symptoms include small circular necrotic lesions visible on the leaves and oval brown lesions on the stem. At later stages of infection, the lesions may girdle the stem and the region above the girdle falls off. The disease severity increases at the reproductive stage and rounded lesions with concentric rings, due to asexual structures called pycnidia, appear on leaves, stems, and pods. The infected pod becomes blighted and often results in shrivelled and infected seeds.

DISEASE MANAGEMENT STRATEGIES

Crop failures may be avoided by judicious practices of integrated disease management based on the use of resistant or tolerant cultivars and growing chickpea in areas where conditions are least favourable for AB disease development. Use of healthy seeds free of A. rabiei, seed treatments with fungicides, and proper destruction of diseased stubbles can also reduce the fungal inoculum load. Crop rotation with nonhost crops is critical for controlling the disease. Planting moderately resistant cultivars and prudent application of fungicides is also a way to combat AB disease. However, the scarcity of AB-resistant accessions and the continuous evolution of the pathogen challenges the disease management process.

USEFUL WEBSITES

https://www.ndsu.edu/pubweb/pulse-info/resourcespdf/Ascochyta%20blight%20of%20chickpea.pdf https://saskpulse.com/files/newsletters/180531_ascochyta_in_chickpeas-compressed.pdf http://www.pulseaus.com.au/growing-pulses/bmp/chickpea/ascochyta-blight http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/plant-diseases/grains-pulses-and-cereals/ascochyta-blight-of-chickpea http://www.croppro.com.au/crop_disease_manual/ch05s02.php https://www.northernpulse.com/uploads/resources/722/handout-chickpeaascochyta-nov13-2011.pdf http://oar.icrisat.org/184/1/24_2010_IB_no_82_Host_Plant https://www.crop.bayer.com.au/find-crop-solutions/by-pest/diseases/ascochyta-blight.

Conventional and molecular breeding for disease resistance in chickpea: status and strategies

Chickpea (Cicer arietinum L.) is an important grain legume at the global level. Among different biotic stresses, diseases are the most important factor limiting its production, causing yield losses up to 100% in severe condition. The major diseases that adversely affect yield of chickpea include Fusarium wilt, Ascochyta blight and Botrytis gray mold. However, dry root rot, collar rot, Sclerotinia stem rot, rust, stunt disease and phyllody have been noted as emerging biotic threats to chickpea production in many production regions. Identification and incorporation of different morphological and biochemical traits are required through breeding to enhance genetic gain for disease resistance. In recent years, remarkable progress has been made in the development of trait-specific breeding lines, genetic and genomic resources in chickpea. Advances in genomics technologies have opened up new avenues to introgress genes from secondary and tertiary gene pools for improving disease resistance in chickpea. In this review, we have discussed important diseases, constraints and improvement strategies for enhancing disease resistance in chickpea.

Novel Genomic Regions Linked to Ascochyta Blight Resistance in Two Differentially Resistant Cultivars of Chickpea

Ascochyta blight (AB), caused by the fungal pathogen Ascochyta rabiei, is a devastating foliar disease of chickpea (Cicer arietinum L.). The genotyping-by-sequencing (GBS)-based approach was deployed for mapping QTLs associated with AB resistance in chickpea in two recombinant inbred line populations derived from two crosses (AB₃₂₇₉ derived from ILC 1929 × ILC 3279 and AB₄₈₂ derived from ILC 1929 × ILC 482) and tested in six different environments. Twenty-one different genomic regions linked to AB resistance were identified in regions CalG02 and CalG04 in both populations AB₃₂₇₉ and AB₄₈₂. These regions contain 1,118 SNPs significantly associated with AB resistance (p ≤ 0.001), which explained 11.2-39.3% of the phenotypic variation (PVE). Nine of the AB resistance-associated genomic regions were newly detected in this study, while twelve regions were known from previous AB studies. The proposed physical map narrows down AB resistance to consistent genomic regions identified across different environments. Gene ontology (GO) assigned these QTLs to 319 genes, many of which were associated with stress and disease resistance, and with most important genes belonging to resistance gene families such as leucine-rich repeat (LRR) and transcription factor families. Our results indicate that the flowering-associated gene GIGANTEA is a possible key factor in AB resistance in chickpea. The results have identified AB resistance-associated regions on the physical genetic map of chickpea and allowed for the identification of associated markers that will help in breeding of AB-resistant varieties.

Breeding and Genomics Interventions for Developing Ascochyta Blight Resistant Grain Legumes

Grain legumes are a key food source for ensuring global food security and sustaining agriculture. However, grain legume production is challenged by growing disease incidence due to global climate change. Ascochyta blight (AB) is a major disease, causing substantial yield losses in grain legumes worldwide. Harnessing the untapped reserve of global grain legume germplasm, landraces, and crop wild relatives (CWRs) could help minimize yield losses caused by AB infection in grain legumes. Several genetic determinants controlling AB resistance in various grain legumes have been identified following classical genetic and conventional breeding approaches. However, the advent of molecular markers, biparental quantitative trait loci (QTL) mapping, genome-wide association studies, genomic resources developed from various genome sequence assemblies, and whole-genome resequencing of global germplasm has revealed AB-resistant gene(s)/QTL/genomic regions/haplotypes on various linkage groups. These genomics resources allow plant breeders to embrace genomics-assisted selection for developing/transferring AB-resistant genomic regions to elite cultivars with great precision. Likewise, advances in functional genomics, especially transcriptomics and proteomics, have assisted in discovering possible candidate gene(s) and proteins and the underlying molecular mechanisms of AB resistance in various grain legumes. We discuss how emerging cutting-edge next-generation breeding tools, such as rapid generation advancement, field-based high-throughput phenotyping tools, genomic selection, and CRISPR/Cas9, could be used for fast-tracking AB-resistant grain legumes to meet the increasing demand for grain legume-based protein diets and thus ensuring global food security.

A kabuli chickpea ideotype

The concept of 'crop ideotype' is coined as a desirable plant model expected to better perform for seed yield, oils and other useful characteristics when developed as a cultivar, and it consists of two major approaches, namely, (i) 'defect elimination', that is, integration of disease resistance to a susceptible genotype from a resistant genotype and (ii) 'selection for yield' by improving yield after crosses between desirable parents. For consideration of these approaches, here we introduced an ideotype in kabuli chickpea (Cicer arietinum L.) which is high-yielding, extra-large-seeded, and double- or multi-podded, has high plant height and imparipinnate-leafed traits, and is heat tolerant and resistant to ascochyta blight [Ascochyta rabiei (Pass.) Labr.], which causes considerable yield losses, via marker-assisted selection. F3 and F4 lines were evaluated for agro-morphological traits divided into six classes, namely, (i) imparipinnate-leafed and single-podded progeny, (ii) imparipinnate-leafed and double-podded progeny, (iii) imparipinnate-leafed and multi-podded progeny, (iv) unifoliolate-leafed and single-podded progeny, (v) unifoliolate-leafed and double-podded progeny, (vi) unifoliolate-leafed and multi-podded progeny. F3:4 lines having 100-seed weight ≥ 45 g and double- or multi-podded traits were additionally assessed for resistance to ascochyta blight using molecular markers including SCY17590 and CaETR-1. Superior lines having higher values than their best parents were determined for all studied traits indicating that economic and important traits including yield and seed size in chickpea could be improved by crossing suitable parents. Imparipinnate-leafed and multi-podded plants had not only the highest number of pods and seeds per plant but also the highest yield. On the other hand, imparipinnate-leafed and single podded progeny had the largest seed size, followed by imparipinnate-leafed and double-podded progeny. Multi-podded plants produced 23% more seed yield than that of single-podded plants, while multi-podded plants attained 7.6% more seed yield than that of double-podded plants. SCY17590 and CaETR-1 markers located on LG4 related to QTLAR2 and QTLAR1 were found in 14 lines among 152 F3:4 lines. Six superior lines were selected for being double- or multi-podded, imparipinnate-leafed, suitable for combine harvest, heat-tolerant, and resistant to ascochyta blight, and having both of two resistance markers and extra-large seeds as high as 50-60 g per 100-seed weight. Resistance alleles from two different backgrounds for resistance to ascochyta blight were integrated with double- or multi-podded kabuli chickpea lines having high yield, extra-large seeds, high plant height, imparipinnate-leaves and high heat tolerance, playing a crucial role for future demands of population and food security. These approaches seem to be applicable in ideotype breeding for other important crop plants.

Genome-Wide Association Analyses Track Genomic Regions for Resistance to Ascochyta rabiei in Australian Chickpea Breeding Germplasm

Ascochyta blight (AB), caused by a necrotrophic fungus, Ascochyta rabiei (syn. Phoma rabiei) has the potential to destroy the chickpea industry worldwide, due to limited sources of genetic resistance in the cultivated gene pool, high evolutionary potential of the pathogen and challenges with integrated disease management. Therefore, the deployment of stable genetic resistance in new cultivars could provide an effective disease control strategy. To investigate the genetic basis of AB resistance, genotyping-by-sequencing based DArTseq-single nucleotide polymorphism (SNP) marker data along with phenotypic data of 251 advanced breeding lines and chickpea cultivars were used to perform genome-wide association (GWAS) analysis. Host resistance was evaluated seven weeks after sowing using two highly aggressive single spore isolates (F17191-1 and TR9571) of A. rabiei. GWAS analyses based on single-locus and multi-locus mixed models and haplotyping trend regression identified twenty-six genomic regions on Ca1, Ca4, and Ca6 that showed significant association with resistance to AB. Two haplotype blocks (HB) on chromosome Ca1; HB5 (992178-1108145 bp), and HB8 (1886221-1976301 bp) were associated with resistance against both isolates. Nine HB on the chromosome, Ca4, spanning a large genomic region (14.9-56.6 Mbp) were also associated with resistance, confirming the role of this chromosome in providing resistance to AB. Furthermore, trait-marker associations in two F₃ derived populations for resistance to TR9571 isolate at the seedling stage under glasshouse conditions were also validated. Eighty-nine significantly associated SNPs were located within candidate genes, including genes encoding for serine/threonine-protein kinase, Myb protein, quinone oxidoreductase, and calmodulin-binding protein all of which are implicated in disease resistance. Taken together, this study identifies valuable sources of genetic resistance, SNP markers and candidate genes underlying genomic regions associated with AB resistance which may enable chickpea breeding programs to make genetic gains via marker-assisted/genomic selection strategies.

Effects of Host Plant Resistance and Fungicide Applications on Ascochyta Blight Symptomology and Yield of Chickpea

Ascochyta blight (AB), caused by the pathogen Ascochyta rabiei, is a major threat to chickpea production worldwide, causing major yield losses and decreasing quality. Control of AB requires integrating pest management options including resistant cultivars and fungicide applications. To address this, fungicides with different modes of action were evaluated on three chickpea cultivars with differing levels of susceptibility to AB under irrigated and dryland conditions in 2015 to 2017. The fungicides were applied once or twice and compared with a no-fungicide application control on AB score and yield. The mean grain yields across locations and years were 1,753, 1,283, and 981 kg/ha, with a corresponding AB mean score of 2.6, 3.2, and 3.3 on 0 to 7 scale (where 0 is no disease and 7 is completely dead) for the moderately resistant, moderately susceptible, and susceptible chickpea cultivars, respectively. Fungicide application was not enough to control disease throughout the season. The use of AB-resistant cultivars had the most significant impact on minimizing the disease and maximizing yield, irrespective of year and location. This study supports previous research indicating that planting AB-resistant chickpea cultivars is essential for disease control, regardless of the fungicides applied.

Chickpea tolerance to temperature stress: Status and opportunity for improvement

Chickpea is a globally important commercial crop and a key source of protein for vegetarian populations. Though chickpea was domesticated at least 3000 years ago, research into abiotic stress tolerance has been limited compared to cereal crops such as wheat. This review investigates the impacts of heat stress on chickpea, focusing on reproductive development. The fertilisation process is particularly sensitive to environmental stress, such as drought and heat that can reduce yields by up to 70%. Current research has largely focused on breeding cultivars that reach maturity faster to avoid stress rather than true thermotolerance and little is known of the impact of heat on cellular processes. This review suggests that there is ample variation within the chickpea gene pool for selective breeding to achieve improved abiotic stress tolerance. Rates of genetic progress will improve once key QTL are identified and the link between thermotolerance and pollen viability confirmed. Other benefits may arise from better understanding of heat shock proteins and molecular chaperones and their role in the protection of reproductive processes.

Molecular mapping of quantitative trait loci for ascochyta blight and botrytis grey mould resistance in an inter-specific cross in chickpea (Cicer arietinum L.) using genotyping by sequencing

Ascochyta blight (AB) and botrytis grey mould (BGM) are the most devastating fungal diseases of chickpea worldwide. The wild relative of chickpea, C. reticulatum acc. ILWC 292 was found resistant to BGM whereas, GPF2 (Cicer arietinum L.) is resistant to AB. A total of 187 F₈ Recombinant Inbred Lines (RILs) developed from an inter-specific cross of GPF2 × C. reticulatum acc. ILWC 292 were used to identify quantitative trait loci (QTLs) responsible for resistance to AB and BGM. RILs along with parents were evaluated under artificial epiphytotic field/laboratory conditions for two years. Highly significant differences (P < 0.001) were observed for reaction to both pathogens in both years. Parents and RILs were genotyped-by-sequencing to identify genome wide single nucleotide polymorphism (SNPs). A total of 1365 filtered and parental polymorphic SNPs were used for linkage map construction, of which, 673 SNPs were arranged on eight linkage groups. Composite interval mapping revealed three QTLs for AB and four QTLs for BGM resistance. Out of which, two QTLs for AB and three QTLs for BGM were consistent in both years. These QTLs can be targeted for further fine mapping for deployment of resistance to AB and BGM in elite chickpea cultivars using marker-assisted-selection.

Identification of Novel Sources of Resistance to Ascochyta Blight in a Collection of Wild Cicer Accessions

Chickpea production is constrained worldwide by the necrotrophic fungal pathogen Ascochyta rabiei, the causal agent of Ascochyta blight (AB). To reduce the impact of this disease, novel sources of resistance are required in chickpea cultivars. Here, we screened a new collection of wild Cicer accessions for AB resistance and identified accessions resistant to multiple, highly pathogenic isolates. In addition to this, analyses demonstrated that some collection sites of C. echinospermum harbor predominantly resistant accessions, knowledge that can inform future collection missions. Furthermore, a genome-wide association study identified regions of the C. reticulatum genome associated with AB resistance and investigation of these regions identified candidate resistance genes. Taken together, these results can be utilized to enhance the resistance of chickpea cultivars to this globally yield-limiting disease.

Genome-wide SNP discovery for development of high-density genetic map and QTL mapping of ascochyta blight resistance in chickpea (Cicer arietinum L.)

A high-density linkage map of chickpea using 3430 SNPs was constructed and used to identify QTLs and candidate genes for ascochyta blight resistance in chickpea. Chickpea cultivation in temperate conditions is highly vulnerable to ascochyta blight infection. Cultivation of resistant cultivars in combination with fungicide application within an informed disease management package is the most effective method to control ascochyta blight in chickpeas. Identifying new sources of resistance is critical for continued improvement in ascochyta blight resistance in chickpea. The objective of this study was to identify genetic loci and candidate genes controlling the resistance to ascochyta blight in recombinant inbred lines derived from crossing cultivars Amit and ICCV 96029. The RILs were genotyped using the genotyping-by-sequencing procedure and Illumina^® GoldenGate array. The RILs were evaluated in the field over three site-years and in three independent greenhouse experiments. A genetic map with eight linkage groups was constructed using 3430 SNPs. Eight QTLs for resistance were identified on chromosomes 2, 3, 4, 5 and 6. The QTLs individually explained 7-40% of the phenotypic variations. The QTLs on chromosomes 2 and 6 were associated with the resistance at vegetative stage only. The QTLs on chromosomes 2 and 4 that were previously reported to be conserved across diverse genetic backgrounds and against different isolates of Ascochyta rabiei were confirmed in this study. Candidate genes were identified within the QTL regions. Their co-localization with the underlying QTLs was confirmed by genetic mapping. The candidate gene-based SNP markers would lead to more efficient marker-assisted selection for ascochyta blight resistance and would provide a framework for fine mapping and subsequent cloning of the genes associated with the resistance.

QTL sequencing strategy to map genomic regions associated with resistance to ascochyta blight in chickpea

Whole-genome sequencing-based bulked segregant analysis (BSA) for mapping quantitative trait loci (QTL) provides an efficient alternative approach to conventional QTL analysis as it significantly reduces the scale and cost of analysis with comparable power to QTL detection using full mapping population. We tested the application of next-generation sequencing (NGS)-based BSA approach for mapping QTLs for ascochyta blight resistance in chickpea using two recombinant inbred line populations CPR-01 and CPR-02. Eleven QTLs in CPR-01 and six QTLs in CPR-02 populations were mapped on chromosomes Ca1, Ca2, Ca4, Ca6 and Ca7. The QTLs identified in CPR-01 using conventional biparental mapping approach were used to compare the efficiency of NGS-based BSA in detecting QTLs for ascochyta blight resistance. The QTLs on chromosomes Ca1, Ca4, Ca6 and Ca7 overlapped with the QTLs previously detected in CPR-01 using conventional QTL mapping method. The QTLs on chromosome Ca4 were detected in both populations and overlapped with the previously reported QTLs indicating conserved region for ascochyta blight resistance across different chickpea genotypes. Six candidate genes in the QTL regions identified using NGS-based BSA on chromosomes Ca2 and Ca4 were validated for their association with ascochyta blight resistance in the CPR-02 population. This study demonstrated the efficiency of NGS-based BSA as a rapid and cost-effective method to identify QTLs associated with ascochyta blight in chickpea.

mQTL-seq and classical mapping implicates the role of an AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED (AHL) family gene in Ascochyta blight resistance of chickpea

Ascochyta blight (AB) caused by the fungal pathogen Ascochyta rabiei is a serious foliar disease of chickpea (Cicer arietinum L.). Despite many genetic studies on chickpea-Ascochyta interaction, genome-wide scan of chickpea for the identification of AB-associated quantitative trait loci (QTLs) and their gene(s) has not been accomplished. To elucidate narrow QTLs for AB resistance, here, we report the use of multiple QTL-sequencing approach on 2 sets of extreme AB phenotype bulks derived from Cicer intraspecific and interspecific crosses. Two major QTLs, qABR4.1 and qABR4.2, and a minor QTL, qABR4.3, were identified on assembled chickpea pseudomolecule 4. We narrowed qABR4.1 to a "robust region" at 4.568-4.618 Mb through mapping on a larger intraspecific cross-derived population and comparative analysis. Among 4 genes, the CaAHL18 gene showed higher expression under Ascochyta stress in AB resistant parent suggesting that it is the candidate gene under "robust qABR4.1." Dual-luciferase assay with CaAHL18 polymorphic cis-regulatory sequences showed that allelic variation is associated with higher expression. Thus, our findings on chickpea-Ascochyta interaction have narrowed down AB resistance associated QTLs on chickpea physical map. The narrowed QTLs and gene-associated markers will help in biotechnological and breeding programs for chickpea improvement.

Molecular Breeding for Ascochyta Blight Resistance in Lentil: Current Progress and Future Directions

Lentil (Lens culinaris Medik.) is a diploid (2n = 2x = 14), self-pollinating, cool-season, grain legume that is cultivated worldwide and is highly valuable due to its high protein content. However, lentil production is constrained by many factors including biotic stresses, majority of which are fungal diseases such as ascochyta blight (AB), fusarium wilt, rust, stemphylium blight, anthracnose, and botrytis gray mold. Among various diseases, AB is a major -problem in many lentil-producing countries and can significantly reduce crop production. Breeding for AB resistance has been a priority for breeding programs across the globe and consequently, a number of resistance sources have been identified and extensively exploited. In order to increase the efficiency of combining genes from different genetic backgrounds, molecular genetic tools can be integrated with conventional breeding methods. A range of genetic linkage maps have been generated based on DNA-based markers, and quantitative trait loci (QTLs) for AB resistance have been identified. Molecular markers linked to these QTLs may potentially be used for efficient pyramiding of the AB disease resistance genes. Significant genomic resources have been established to identify and characterize resistance genes, including an integrated genetic map, expressed sequence tag libraries, gene based markers, and draft genome sequences. These resources are already being utilized for lentil crop improvement, to more effectively select for disease resistance, as a case study of the Australian breeding program will show. The combination of genomic resources, effective molecular genetic tools and high resolution phenotyping tools will improve the efficiency of selection for ascochyta blight resistance and accelerate varietal development of global lentil breeding programs.

Fine Mapping of QTLs for Ascochyta Blight Resistance in Pea Using Heterogeneous Inbred Families

Ascochyta blight (AB) is an important disease of pea which can cause severe grain yield loss under wet conditions. In our previous study, we identified two quantitative trait loci (QTLs) abIII-1 and abI-IV-2 for AB resistance and these QTLs were consistent across locations and/or years in an inter-specific pea population (PR-19) developed from a cross between Alfetta (Pisum sativum) and P651 (P. fulvum). The objectives of this study were to fine map the abIII-1 and abI-IV-2 QTLs using a high density single nucleotide polymorphism (SNP)-based genetic linkage map and analyze identified markers in heterogeneous inbred family (HIF) populations. Selective genotyping of 51 PR-19 recombinant inbred lines was performed using genotyping-by-sequencing (GBS) and the resulting high density genetic linkage map was used to identify eight new SNP markers within the abI-IV-2 QTL, whereas no additional SNPs were identified within the abIII-1 QTL. Two HIF populations HIF-224 (143 lines) and HIF-173 (126 lines) were developed from F6 RILs PR-19-224 and PR-19-173, respectively. The HIF populations evaluated under field conditions in 2015 and 2016 showed a wide range of variation for reaction to AB resistance. Lodging score had significant positive (P < 0.001) correlation with AB scores. HIFs were genotyped using SNP markers within targeted QTLs. The genotypic and phenotypic data of the HIFs were used to identify two new QTLs, abI-IV-2.1 and abI-IV-2.2 for AB resistance within the abI-IV-2 QTL. These QTLs individually explained 5.5 to 14% of the total phenotypic variation. Resistance to lodging was also associated with these two QTLs. Identified SNP markers will be useful in marker assisted selection for development of pea cultivars with improved AB resistance.

QTL mapping of early flowering and resistance to ascochyta blight in chickpea

In western Canada, chickpea (Cicer arietinum L.) production is challenged by short growing seasons and infestations with ascochyta blight. Research was conducted to determine the genetic basis of the association between flowering time and reaction to ascochyta blight in chickpea. Ninety-two chickpea recombinant inbred lines (RILs) developed from a cross between ICCV 96029 and CDC Frontier were evaluated for flowering responses and ascochyta blight reactions in growth chambers and fields at multiple locations and during several years. A wide range of variation was exhibited by the RILs for days to flower, days to maturity, node of first flowering, plant height, and ascochyta blight resistance. Moderate to high broad sense heritability was estimated for ascochyta blight reaction (H(2) = 0.14-0.34) and for days to flowering (H(2) = 0.45-0.87) depending on the environments. Negative correlations were observed among the RILs for days to flowering and ascochyta blight resistance, ranging from r = -0.21 (P < 0.05) to -0.58 (P < 0.0001). A genetic linkage map consisting of eight linkage groups was developed using 349 SNP markers. Seven QTLs for days to flowering were identified that individually explained 9%-44% of the phenotypic variation. Eight QTLs were identified for ascochyta blight resistance that explained phenotypic variation ranging from 10% to 19%. Clusters of QTLs for days to flowering and ascochyta blight resistances were found on chromosome 3 at the interval of 8.6-23.11 cM and on chromosome 8 at the interval of 53.88-62.33 cM.

Genome Analysis Identified Novel Candidate Genes for Ascochyta Blight Resistance in Chickpea Using Whole Genome Re-sequencing Data

Ascochyta blight (AB) is a fungal disease that can significantly reduce chickpea production in Australia and other regions of the world. In this study, 69 chickpea genotypes were sequenced using whole genome re-sequencing (WGRS) methods. They included 48 Australian varieties differing in their resistance ranking to AB, 16 advanced breeding lines from the Australian chickpea breeding program, four landraces, and one accession representing the wild chickpea species Cicer reticulatum. More than 800,000 single nucleotide polymorphisms (SNPs) were identified. Population structure analysis revealed relatively narrow genetic diversity amongst recently released Australian varieties and two groups of varieties separated by the level of AB resistance. Several regions of the chickpea genome were under positive selection based on Tajima's D test. Both Fst genome- scan and genome-wide association studies (GWAS) identified a 100 kb region (AB4.1) on chromosome 4 that was significantly associated with AB resistance. The AB4.1 region co-located to a large QTL interval of 7 Mb∼30 Mb identified previously in three different mapping populations which were genotyped at relatively low density with SSR or SNP markers. The AB4.1 region was validated by GWAS in an additional collection of 132 advanced breeding lines from the Australian chickpea breeding program, genotyped with approximately 144,000 SNPs. The reduced level of nucleotide diversity and long extent of linkage disequilibrium also suggested the AB4.1 region may have gone through selective sweeps probably caused by selection of the AB resistance trait in breeding. In total, 12 predicted genes were located in the AB4.1 QTL region, including those annotated as: NBS-LRR receptor-like kinase, wall-associated kinase, zinc finger protein, and serine/threonine protein kinases. One significant SNP located in the conserved catalytic domain of a NBS-LRR receptor-like kinase led to amino acid substitution. Transcriptional analysis using qPCR showed that some predicted genes were significantly induced in resistant lines after inoculation compared to non-inoculated plants. This study demonstrates the power of combining WGRS data with relatively simple traits to rapidly develop "functional makers" for marker-assisted selection and genomic selection.

QTL-seq for rapid identification of candidate genes for 100-seed weight and root/total plant dry weight ratio under rainfed conditions in chickpea

Terminal drought is a major constraint to chickpea productivity. Two component traits responsible for reduction in yield under drought stress include reduction in seeds size and root length/root density. QTL-seq approach, therefore, was used to identify candidate genomic regions for 100-seed weight (100SDW) and total dry root weight to total plant dry weight ratio (RTR) under rainfed conditions. Genomewide SNP profiling of extreme phenotypic bulks from the ICC 4958 × ICC 1882 population identified two significant genomic regions, one on CaLG01 (1.08 Mb) and another on CaLG04 (2.7 Mb) linkage groups for 100SDW. Similarly, one significant genomic region on CaLG04 (1.10 Mb) was identified for RTR. Comprehensive analysis revealed four and five putative candidate genes associated with 100SDW and RTR, respectively. Subsequently, two genes (Ca_04364 and Ca_04607) for 100SDW and one gene (Ca_04586) for RTR were validated using CAPS/dCAPS markers. Identified candidate genomic regions and genes may be useful for molecular breeding for chickpea improvement.

Employing genome-wide SNP discovery and genotyping strategy to extrapolate the natural allelic diversity and domestication patterns in chickpea

The genome-wide discovery and high-throughput genotyping of SNPs in chickpea natural germplasm lines is indispensable to extrapolate their natural allelic diversity, domestication, and linkage disequilibrium (LD) patterns leading to the genetic enhancement of this vital legume crop. We discovered 44,844 high-quality SNPs by sequencing of 93 diverse cultivated desi, kabuli, and wild chickpea accessions using reference genome- and de novo-based GBS (genotyping-by-sequencing) assays that were physically mapped across eight chromosomes of desi and kabuli. Of these, 22,542 SNPs were structurally annotated in different coding and non-coding sequence components of genes. Genes with 3296 non-synonymous and 269 regulatory SNPs could functionally differentiate accessions based on their contrasting agronomic traits. A high experimental validation success rate (92%) and reproducibility (100%) along with strong sensitivity (93-96%) and specificity (99%) of GBS-based SNPs was observed. This infers the robustness of GBS as a high-throughput assay for rapid large-scale mining and genotyping of genome-wide SNPs in chickpea with sub-optimal use of resources. With 23,798 genome-wide SNPs, a relatively high intra-specific polymorphic potential (49.5%) and broader molecular diversity (13-89%)/functional allelic diversity (18-77%) was apparent among 93 chickpea accessions, suggesting their tremendous applicability in rapid selection of desirable diverse accessions/inter-specific hybrids in chickpea crossbred varietal improvement program. The genome-wide SNPs revealed complex admixed domestication pattern, extensive LD estimates (0.54-0.68) and extended LD decay (400-500 kb) in a structured population inclusive of 93 accessions. These findings reflect the utility of our identified SNPs for subsequent genome-wide association study (GWAS) and selective sweep-based domestication trait dissection analysis to identify potential genomic loci (gene-associated targets) specifically regulating important complex quantitative agronomic traits in chickpea. The numerous informative genome-wide SNPs, natural allelic diversity-led domestication pattern, and LD-based information generated in our study have got multidimensional applicability with respect to chickpea genomics-assisted breeding.

Genomics-assisted breeding for drought tolerance in chickpea

Terminal drought is one of the major constraints in chickpea (Cicer arietinum L.), causing more than 50% production losses. With the objective of accelerating genetic understanding and crop improvement through genomics-assisted breeding, a draft genome sequence has been assembled for the CDC Frontier variety. In this context, 544.73Mb of sequence data were assembled, capturing of 73.8% of the genome in scaffolds. In addition, large-scale genomic resources including several thousand simple sequence repeats and several million single nucleotide polymorphisms, high-density diversity array technology (15360 clones) and Illumina GoldenGate assay genotyping platforms, high-density genetic maps and transcriptome assemblies have been developed. In parallel, by using linkage mapping approach, one genomic region harbouring quantitative trait loci for several drought tolerance traits has been identified and successfully introgressed in three leading chickpea varieties (e.g. JG 11, Chefe, KAK 2) by using a marker-assisted backcrossing approach. A multilocation evaluation of these marker-assisted backcrossing lines provided several lines with 10-24% higher yield than the respective recurrent parents.Modern breeding approaches like marker-assisted recurrent selection and genomic selection are being deployed for enhancing drought tolerance in chickpea. Some novel mapping populations such as multiparent advanced generation intercross and nested association mapping populations are also being developed for trait mapping at higher resolution, as well as for enhancing the genetic base of chickpea. Such advances in genomics and genomics-assisted breeding will accelerate precision and efficiency in breeding for stress tolerance in chickpea.

Integrated physical, genetic and genome map of chickpea (Cicer arietinum L.)

Physical map of chickpea was developed for the reference chickpea genotype (ICC 4958) using bacterial artificial chromosome (BAC) libraries targeting 71,094 clones (~12× coverage). High information content fingerprinting (HICF) of these clones gave high-quality fingerprinting data for 67,483 clones, and 1,174 contigs comprising 46,112 clones and 3,256 singletons were defined. In brief, 574 Mb genome size was assembled in 1,174 contigs with an average of 0.49 Mb per contig and 3,256 singletons represent 407 Mb genome. The physical map was linked with two genetic maps with the help of 245 BAC-end sequence (BES)-derived simple sequence repeat (SSR) markers. This allowed locating some of the BACs in the vicinity of some important quantitative trait loci (QTLs) for drought tolerance and reistance to Fusarium wilt and Ascochyta blight. In addition, fingerprinted contig (FPC) assembly was also integrated with the draft genome sequence of chickpea. As a result, ~965 BACs including 163 minimum tilling path (MTP) clones could be mapped on eight pseudo-molecules of chickpea forming 491 hypothetical contigs representing 54,013,992 bp (~54 Mb) of the draft genome. Comprehensive analysis of markers in abiotic and biotic stress tolerance QTL regions led to identification of 654, 306 and 23 genes in drought tolerance "QTL-hotspot" region, Ascochyta blight resistance QTL region and Fusarium wilt resistance QTL region, respectively. Integrated physical, genetic and genome map should provide a foundation for cloning and isolation of QTLs/genes for molecular dissection of traits as well as markers for molecular breeding for chickpea improvement.

Fast track genetic improvement of ascochyta blight resistance and double podding in chickpea by marker-assisted backcrossing

Ascochyta blight (AB) caused by the fungus Ascochyta rabiei Pass. Lab. is one of the major diseases of chickpea worldwide and a constraint to production in western Canada. The use of varieties with high levels of resistance is considered the most economical solution for long-term ascochyta blight management in chickpea. QTL for resistance to ascochyta blight have been identified in chickpea. The availability of molecular markers associated with QTL for ascochyta blight resistant and double podding provides an opportunity to apply marker-assisted backcrossing to introgress the traits into adapted chickpea cultivars. In the present study, molecular markers that were linked to the QTL for ascochyta blight resistance and the double podding trait, and those unlinked to the resistance were used in foreground and background selection, respectively, in backcrosses between moderately resistant donors (CDC Frontier and CDC 425-14) and the adapted varieties (CDC Xena, CDC Leader and FLIP98-135C). The strategy included two backcrosses and selection for two QTL for ascochyta blight resistance and a locus associated with double podding. The fixation of the elite genetic background was monitored with 16-22 SSR markers to accelerate restoration of the genetic background at each backcross. By the BC2F1 generation, plants with improved ascochyta blight resistance and double podding were identified. The selected plants possessed the majority of elite parental type SSR alleles on all fragments analyzed except the segment of LG 4, LG 6 and LG 8 that possessed the target QTL. The results showed that the adapted variety could be efficiently converted into a variety with improved resistance in two backcross generations.

Achievements and prospects of genomics-assisted breeding in three legume crops of the semi-arid tropics

Advances in next-generation sequencing and genotyping technologies have enabled generation of large-scale genomic resources such as molecular markers, transcript reads and BAC-end sequences (BESs) in chickpea, pigeonpea and groundnut, three major legume crops of the semi-arid tropics. Comprehensive transcriptome assemblies and genome sequences have either been developed or underway in these crops. Based on these resources, dense genetic maps, QTL maps as well as physical maps for these legume species have also been developed. As a result, these crops have graduated from 'orphan' or 'less-studied' crops to 'genomic resources rich' crops. This article summarizes the above-mentioned advances in genomics and genomics-assisted breeding applications in the form of marker-assisted selection (MAS) for hybrid purity assessment in pigeonpea; marker-assisted backcrossing (MABC) for introgressing QTL region for drought-tolerance related traits, Fusarium wilt (FW) resistance and Ascochyta blight (AB) resistance in chickpea; late leaf spot (LLS), leaf rust and nematode resistance in groundnut. We critically present the case of use of other modern breeding approaches like marker-assisted recurrent selection (MARS) and genomic selection (GS) to utilize the full potential of genomics-assisted breeding for developing superior cultivars with enhanced tolerance to various environmental stresses. In addition, this article recommends the use of advanced-backcross (AB-backcross) breeding and development of specialized populations such as multi-parents advanced generation intercross (MAGIC) for creating new variations that will help in developing superior lines with broadened genetic base. In summary, we propose the use of integrated genomics and breeding approach in these legume crops to enhance crop productivity in marginal environments ensuring food security in developing countries.

Characterization and genetic analysis of an EIN4-like sequence (CaETR-1) located in QTL(AR1) implicated in ascochyta blight resistance in chickpea

Two alleles of a chickpea (Cicer arietinum L.) ethylene receptor-like sequence (CaETR-1) were sequence-characterized using synteny analysis with genome sequences of Medicago truncatula L. The full length of the sequence obtained in the accession FLIP84-92C resistant to ascochyta blight (CaETR-1a) span 4,428 bp, including the polyadenylation signal in the 3'-untranslated region (UTR), whereas it has a 730 bp deletion in the 3'-UTR region in the susceptible accession PI359075 (CaETR-1b). The deduced protein belongs to subfamily II of the ethylene receptors and contains all the domains that define EIN4 homologs in Arabidopsis. The EIN4-like sequence (CaETR-1) has been mapped using a recombinant inbred line (RIL) population derived from an intraspecific cross between ILC3279 and WR315, resistant and susceptible to blight, respectively. The locus was located in LGIVa of the genetic map, flanked by markers NCPGR91 and GAA47 (at distances of 11.3 and 17.9 cM, respectively). This is the first potentially functional sequence identified under a QTL peak for ascochyta blight resistance in chickpea (QTL(AR1)). This EIN4-like (CaETR-1) sequence explained up to 33.8% of the total phenotypic variation. This sequence could be directly related to blight resistance, together with other QTLs that have been found to be involved in resistance to this major chickpea disease.

EST-derived genic molecular markers: development and utilization for generating an advanced transcript map of chickpea

Well-saturated linkage maps especially those based on expressed sequence tag (EST)-derived genic molecular markers (GMMs) are a pre-requisite for molecular breeding. This is especially true in important legumes such as chickpea where few simple sequence repeats (SSR) and even fewer GMM-based maps have been developed. Therefore, in this study, 2,496 ESTs were generated from chickpea seeds and utilized for the development of 487 novel EST-derived functional markers which included 125 EST-SSRs, 151 intron targeted primers (ITPs), 109 expressed sequence tag polymorphisms (ESTPs), and 102 single nucleotide polymorphisms (SNPs). Whereas ESTSSRs, ITPs, and ESTPs were developed by in silico analysis of the developed EST sequences, SNPs were identified by allele resequencing and their genotyping was performedusing the Illumina GoldenGate Assay. Parental polymorphism was analyzed between C. arietinum ICC4958 and C. reticulatum PI489777, parents of the reference chickpea mapping population, using a total of 872 markers: 487 new gene-based markers developed in this study along with 385 previously published markers, of which 318 (36.5%) were found to be polymorphic and were used for genotyping. The genotypic data were integrated with the previously published data of 108 markers and an advanced linkage map was generated that contained 406 loci distributed on eight linkage groups that spanned 1,497.7 cM. The average marker density was 3.68 cM and the average number of markers per LG was 50.8. Among the mapped markers, 303 new genomic locations were defined that included 177 gene-based and 126 gSSRs (genomic SSRs) thereby producing the most advanced gene-rich map of chickpea solely based on co-dominant markers.

Bacterial artificial chromosome libraries of pulse crops: characteristics and applications

Pulse crops are considered minor on a global scale despite their nutritional value for human consumption. Therefore, they are relatively less extensively studied in comparison with the major crops. The need to improve pulse crop production and quality will increase with the increasing global demand for food security and people's awareness of nutritious food. The improvement of pulse crops will require fully utilizing all their genetic resources. Bacterial artificial chromosome (BAC) libraries of pulse crops are essential genomic resources that have the potential to accelerate gene discovery and enhance molecular breeding in these crops. Here, we review the availability, characteristics, applications, and potential applications of the BAC libraries of pulse crops.

Advancing the STMS genomic resources for defining new locations on the intraspecific genetic linkage map of chickpea (Cicer arietinum L.)

Background

Chickpea (Cicer arietinum L.) is an economically important cool season grain legume crop that is valued for its nutritive seeds having high protein content. However, several biotic and abiotic stresses and the low genetic variability in the chickpea genome have continuously hindered the chickpea molecular breeding programs. STMS (Sequence Tagged Microsatellite Sites) markers which are preferred for the construction of saturated linkage maps in several crop species, have also emerged as the most efficient and reliable source for detecting allelic diversity in chickpea. However, the number of STMS markers reported in chickpea is still limited and moreover exhibit low rates of both inter and intraspecific polymorphism, thereby limiting the positions of the SSR markers especially on the intraspecific linkage maps of chickpea. Hence, this study was undertaken with the aim of developing additional STMS markers and utilizing them for advancing the genetic linkage map of chickpea which would have applications in QTL identification, MAS and for de novo assembly of high throughput whole genome sequence data.

Results

A microsatellite enriched library of chickpea (enriched for (GT/CA)_n and (GA/CT)_n repeats) was constructed from which 387 putative microsatellite containing clones were identified. From these, 254 STMS primers were designed of which 181 were developed as functional markers. An intraspecific mapping population of chickpea, [ICCV-2 (single podded) × JG-62 (double podded)] and comprising of 126 RILs, was genotyped for mapping. Of the 522 chickpea STMS markers (including the double-podding trait, screened for parental polymorphism, 226 (43.3%) were polymorphic in the parents and were used to genotype the RILs. At a LOD score of 3.5, eight linkage groups defining the position of 138 markers were obtained that spanned 630.9 cM with an average marker density of 4.57 cM. Further, based on the common loci present between the current map and the previously published chickpea intraspecific map, integration of maps was performed which revealed improvement of marker density and saturation of the region in the vicinity of sfl (double-podding) gene thereby bringing about an advancement of the current map.

Conclusion

An arsenal of 181 new chickpea STMS markers was reported. The developed intraspecific linkage map defined map positions of 138 markers which included 101 new locations.Map integration with a previously published map was carried out which revealed an advanced map with improved density. This study is a major contribution towards providing advanced genomic resources which will facilitate chickpea geneticists and molecular breeders in developing superior genotypes with improved traits.

A BAC/BIBAC-based physical map of chickpea, Cicer arietinum L

Background

Chickpea (Cicer arietinum L.) is the third most important pulse crop worldwide. Despite its importance, relatively little is known about its genome. The availability of a genome-wide physical map allows rapid fine mapping of QTL, development of high-density genome maps, and sequencing of the entire genome. However, no such a physical map has been developed in chickpea.

Results

We present a genome-wide, BAC/BIBAC-based physical map of chickpea developed by fingerprint analysis. Four chickpea BAC and BIBAC libraries, two of which were constructed in this study, were used. A total of 67,584 clones were fingerprinted, and 64,211 (~11.7 ×) of the fingerprints validated and used in the physical map assembly. The physical map consists of 1,945 BAC/BIBAC contigs, with each containing an average of 28.3 clones and having an average physical length of 559 kb. The contigs collectively span approximately 1,088 Mb. By using the physical map, we identified the BAC/BIBAC contigs containing or closely linked to QTL4.1 for resistance to Didymella rabiei (RDR) and QTL8 for days to first flower (DTF), thus further verifying the physical map and confirming its utility in fine mapping and cloning of QTL.

Conclusion

The physical map represents the first genome-wide, BAC/BIBAC-based physical map of chickpea. This map, along with other genomic resources previously developed in the species and the genome sequences of related species (soybean, Medicago and Lotus), will provide a foundation necessary for many areas of advanced genomics research in chickpea and other legume species. The inclusion of transformation-ready BIBACs in the map greatly facilitates its utility in functional analysis of the legume genomes.

Genetic analyses and conservation of QTL for ascochyta blight resistance in chickpea (Cicer arietinum L.)

Ascochyta blight (AB) caused by Ascochyta rabiei (teleomorph, Didymella rabiei) Pass. Lab. is an important fungal disease of chickpea worldwide. Only moderate sources of resistance are available within the cultivated species and we hypothesized that the available sources may carry different genes for resistance, which could be pyramided to improve field resistance to AB. Four divergent moderately resistant cultivars CDC Frontier, CDC Luna, CDC Corinne, and Amit were each crossed to a highly susceptible germplasm ICCV 96029. Parents, F(1) and F(2) generations were evaluated under controlled conditions for their reactions to AB. A total of 144 simple sequence repeat (SSR) markers were first mapped to eight linkage groups (LG) for the CDC Frontier x ICCV 96029 population. Then based on the evidence from this population, 76, 61, and 42 SSR markers were systematically chosen and mapped in CDC Luna, CDC Corinne, and Amit populations, respectively. Frequency distributions of the AB rating in the F(2) generation varied among the four populations. Composite interval mapping revealed five QTLs (QTL1-5), one on each of LG 2, 3, 4, 6, and 8, respectively, distributed across different sources, controlling resistance to AB. CDC Frontier contained QTL2, 3, and 4 that simultaneously accounted for 56% of phenotypic variations. CDC Luna contained QTL 1 and 3. CDC Corinne contained QTL 3 and 5, while only QTL 2 was identified in Amit. Altogether these QTL explained 48, 38, and 14% of the estimated phenotypic variations in CDC Luna, CDC Corinne, and Amit populations, respectively. The results suggested that these QTLs could be combined into a single genotype to enhance field resistance to AB.

Integration of new CAPS and dCAPS-RGA markers into a composite chickpea genetic map and their association with disease resistance

A composite linkage map was constructed based on two interspecific recombinant inbred line populations derived from crosses between Cicer arietinum (ILC72 and ICCL81001) and Cicer reticulatum (Cr5-10 or Cr5-9). These mapping populations segregate for resistance to ascochyta blight (caused by Ascochyta rabiei), fusarium wilt (caused by Fusarium oxysporum f. sp. ciceris) and rust (caused by Uromyces ciceris-arietini). The presence of single nucleotide polymorphisms in ten resistance gene analogs (RGAs) previously isolated and characterized was exploited. Six out of the ten RGAs were novel sequences. In addition, classes RGA05, RGA06, RGA07, RGA08, RGA09 and RGA10 were considerate putatively functional since they matched with several legume expressed sequences tags (ESTs) obtained under infection conditions. Seven RGA PCR-based markers (5 CAPS and 2 dCAPS) were developed and successfully genotyped in the two progenies. Six of them have been mapped in different linkage groups where major quantitative trait loci conferring resistance to ascochyta blight and fusarium wilt have been reported. Genomic locations of RGAs were compared with those of known Cicer R-genes and previously mapped RGAs. Association was detected between RGA05 and genes controlling resistance to fusarium wilt caused by races 0 and 5.

The marker SCK13(603) associated with resistance to ascochyta blight in chickpea is located in a region of a putative retrotransposon

The sequence characterized amplified region (SCAR) marker SCK13(603), associated with ascochyta blight resistance in a chickpea recombinant inbred line (RIL) population, was used as anchored sequence for genome walking. The PCRs performed in the walking steps to walk in the same direction produced eight bands in 5' direction and five bands in 3' direction with a length ranking from 530 to 2,871 bp. The assembly of the bands sequences along with the sequence of SCK13(603) resulted in 7,815 bp contig. Blastn analyses showed stretches of DNA sequence mainly distributed from the nucleotides 1,500 to 4,500 significantly similar to Medicago truncatula genomic DNA. Three open reading frames (ORFs) were identified and blastp analysis of predicted amino acids sequences revealed that ORF1, ORF2 and ORF3 had significant similarity to a CCHC zinc finger protein, to an integrase, and to a precursor of the glucoamylase s1/s2, respectively, from M. truncatula. The high homology of the putative proteins derived from ORF1 and ORF2 with retrotransposon proteins and the prediction of the existence of conserved domains usually present in retrotransposon proteins indicate that the marker SCK13(603) is located in a region of a putative retrotransposon. The information generated in this study has contributed to increase the knowledge of this important region for blight resistance in chickpea.

Molecular genetic diversity and linked resistance to ascochyta blight in Australian chickpea breeding materials and their wild relatives

Simple sequence-repeat (SSR) and sequence characterised amplified regions (SCARs) have been used to characterise the genetic diversity of chickpea germplasm. A set of 48 genotypes comprising cultigen, landraces, and wild relatives important for breeding purposes was used to determine the genetic similarity between genotypes and to assess the association between ascochyta blight (AB) and SCAR phenotypes. The 21 SSR markers amplified a total of 370 alleles, with an average of ~17 alleles per SSR locus among the 48 genotypes. Polymorphic information content (PIC) values ranged from 0.37 for the XGA13 locus to 0.93 for the XGA106. Principal coordinate analysis (PCO) of genetic similarity (GS) estimates revealed a clear differentiation of the chickpea genotypes into 5 groups, which were generally consistent with available pedigree information. Comparison of SCAR and AB phenotypes enabled us to tag the common source(s) of AB resistance in the breeding collection. Based on the SCAR phenotypes, it was evident that the studied chickpea genotypes, including worldwide-known AB-resistant lines (ICC12004, ILC72, ILC3279), carry at least one common source of resistance to AB. Since SSR markers are polymerase chain reaction (PCR)-based markers, highly polymorphic, and amenable to high-throughput technologies, they are therefore well suited for studies of genetic diversity and cultivar identification in chickpea. The broad level of genetic diversity detected in the chickpea germplasm should be useful for selective breeding for specific traits such as AB, backcrossing, and in enhancing the genetic base of breeding programs.

Transcriptional profiling of chickpea genes differentially regulated in response to high-salinity, cold and drought

Background

Cultivated chickpea (Cicer arietinum) has a narrow genetic base making it difficult for breeders to produce new elite cultivars with durable resistance to major biotic and abiotic stresses. As an alternative to genome mapping, microarrays have recently been applied in crop species to identify and assess the function of putative genes thought to be involved in plant abiotic stress and defence responses. In the present study, a cDNA microarray approach was taken in order to determine if the transcription of genes, from a set of previously identified putative stress-responsive genes from chickpea and its close relative Lathyrus sativus, were altered in chickpea by the three abiotic stresses; drought, cold and high-salinity. For this, chickpea genotypes known to be tolerant and susceptible to each abiotic stress were challenged and gene expression in the leaf, root and/or flower tissues was studied. The transcripts that were differentially expressed among stressed and unstressed plants in response to the particular stress were analysed in the context of tolerant/susceptible genotypes.

Results

The transcriptional change of more than two fold was observed for 109, 210 and 386 genes after drought, cold and high-salinity treatments, respectively. Among these, two, 15 and 30 genes were consensually differentially expressed (DE) between tolerant and susceptible genotypes studied for drought, cold and high-salinity, respectively. The genes that were DE in tolerant and susceptible genotypes under abiotic stresses code for various functional and regulatory proteins. Significant differences in stress responses were observed within and between tolerant and susceptible genotypes highlighting the multiple gene control and complexity of abiotic stress response mechanism in chickpea.

Conclusion

The annotation of these genes suggests that they may have a role in abiotic stress response and are potential candidates for tolerance/susceptibility.

Genetic mapping of ascochyta blight resistance in chickpea (Cicer arietinum L.) using a simple sequence repeat linkage map

Ascochyta blight, caused by the fungus Ascochyta rabiei (Pass.) Lab., is one of the most devastating diseases of chickpea (Cicer arietinum L.) worldwide. Research was conducted to map genetic factors for resistance to ascochyta blight using a linkage map constructed with 144 simple sequence repeat markers and 1 morphological marker (fc, flower colour). Stem cutting was used to vegetatively propagate 186 F2 plants derived from a cross between Cicer arietinum L. 'ICCV96029' and 'CDC Frontier'. A total of 556 cutting-derived plants were evaluated for their reaction to ascochyta blight under controlled conditions. Disease reaction of the F1 and F2 plants demonstrated that the resistance was dominantly inherited. A Fain's test based on the means and variances of the ascochyta blight reaction of the F3 families showed that a few genes were segregating in the population. Composite interval mapping identified 3 genomic regions that were associated with the reaction to ascochyta blight. One quantitative trait locus (QTL) on each of LG3, LG4, and LG6 accounted for 13%, 29%, and 12%, respectively, of the total estimated phenotypic variation for the reaction to ascochyta blight. Together, these loci controlled 56% of the total estimated phenotypic variation. The QTL on LG4 and LG6 were in common with the previously reported QTL for ascochyta blight resistance, whereas the QTL on LG3 was unique to the current population.

Development of an integrated intraspecific map of chickpea (Cicer arietinum L.) using two recombinant inbred line populations

A composite intraspecific linkage map of chickpea was developed by integrating individual maps developed from two F(8:9) RIL populations with one common parent. Different molecular markers viz. RAPD, ISSR, RGA, SSR and ASAP were analyzed along with three yield related traits: double podding, seeds per pod and seed weight. A total of 273 markers and 186 RILs were used to generate the map with eight linkage groups at a LOD score of >/=3.0 and maximum recombination fraction of 0.4. The map spanned 739.6 cM with 230 markers at an average distance of 3.2 cM between markers. The predominantly used SSR markers facilitated identification of homologous linkage groups from the previously published interspecific linkage map of chickpea and confirmed conservation of the SSR markers across the two maps as well as the variation in terms of marker distance and order. The double podding gene was tagged by the markers NCPGR33 and UBC249z at 2.0 and 1.1 cM, respectively. Whereas, seeds per pod, was tagged by the markers TA2x and UBC465 at 0.1 and 1.8 cM, respectively. Eight QTLs were identified that influence seed weight. The joint map approach allowed mapping a large number of markers with a moderate coverage of the chickpea genome and few linkage gaps.

Expression profiling of chickpea genes differentially regulated during a resistance response to Ascochyta rabiei

Using microarray technology and a set of chickpea (Cicer arietinum L.) unigenes, grasspea (Lathyrus sativus L.) expressed sequence tags (ESTs) and lentil (Lens culinaris Med.) resistance gene analogues, the ascochyta blight (Ascochyta rabiei (Pass.) L.) resistance response was studied in four chickpea genotypes, including resistant, moderately resistant, susceptible and wild relative (Cicer echinospermum L.) genotypes. The experimental system minimized environmental effects and was conducted in reference design, in which samples from mock-inoculated controls acted as reference against post-inoculation samples. Robust data quality was achieved through the use of three biological replicates (including a dye swap), the inclusion of negative controls and strict selection criteria for differentially expressed genes, including a fold change cut-off determined by self-self hybridizations, Student's t-test and multiple testing correction (P < 0.05). Microarray observations were also validated by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). The time course expression patterns of 756 microarray features resulted in the differential expression of 97 genes in at least one genotype at one time point. k-means clustering grouped the genes into clusters of similar observations for each genotype, and comparisons between A. rabiei-resistant and A. rabiei-susceptible genotypes revealed potential gene 'signatures' predictive of effective A. rabiei resistance. These genes included several pathogenesis-related proteins, SNAKIN2 antimicrobial peptide, proline-rich protein, disease resistance response protein DRRG49-C, environmental stress-inducible protein, leucine-zipper protein, polymorphic antigen membrane protein, Ca-binding protein and several unknown proteins. The potential involvement of these genes and their pathways of induction are discussed. This study represents the first large-scale gene expression profiling in chickpea, and future work will focus on the functional validation of the genes of interest.

Detection of two quantitative trait loci for resistance to ascochyta blight in an intra-specific cross of chickpea (Cicer arietinum L.): development of SCAR markers associated with resistance

Two quantitative trait loci (QTLs), (QTL(AR1) and QTL(AR2)) associated with resistance to ascochyta blight, caused by Ascochyta rabiei, have been identified in a recombinant inbred line population derived from a cross of kabulixdesi chickpea. The population was evaluated in two cropping seasons under field conditions and the QTLs were found to be located in two different linkage groups (LG4a and LG4b). LG4b was saturated with RAPD markers and four of them associated with resistance were sequenced to give sequence characterized amplified regions (SCARs) that segregated with QTL(AR2). This QTL explained 21% of the total phenotypic variation. However, QTL(AR1), located in LG4a, explained around 34% of the total phenotypic variation in reaction to ascochyta blight when scored in the second cropping season. This LG4a region only includes a few markers, the flower colour locus (B/b), STMS GAA47, a RAPD marker and an inter-simple-sequence-repeat and corresponds with a previously reported QTL. From the four SCARs tagging QTL(AR2), SCAR (SCY17(590)) was co-dominant, and the other three were dominant. All SCARs segregated in a 1:1 (presence:absence) ratio and the scoring co-segregated with their respective RAPD markers. QTL(AR2) on LG4b was mapped in a highly saturated genomic region covering a genetic distance of 0.8 cM with a cluster of nine markers (three SCARs, two sequence-tagged microsatellite sites (STMS) and four RAPDs). Two of the four SCARs showed significant alignment with genes or proteins related to disease resistance in other species and one of them (SCK13(603)) was sited in the highly saturated region linked to QTL(AR2). STMS TA72 and TA146 located in LG4b were described in previous maps where QTL for blight resistance were also localized in both inter and intraspecific crosses. These findings may improve the precision of molecular breeding for QTL(AR2) as they will allow the choice of as much polymorphism as possible in any population and could be the starting point for finding a candidate resistant gene for ascochyta blight resistance in chickpea.

A linkage map of chickpea (Cicer arietinum L.) based on populations from Kabuli x Desi crosses: location of genes for resistance to fusarium wilt race 0

Two recombinant inbred line (RIL) populations derived from intraspecific crosses with a common parental line (JG62) were employed to develop a chickpea genetic map. Molecular markers, flower colour, double podding, seed coat thickness and resistance to fusarium wilt race 0 (FOC-0) were included in the study. Joint segregation analysis involved a total of 160 markers and 159 RILs. Ten linkage groups (LGs) were obtained that included morphological markers and 134 molecular markers (3 ISSRs, 13 STMSs and 118 RAPDs). Flower colour (B/b) and seed coat thickness (Tt/tt) appeared to be linked to STMS (GAA47). The single-/double-podding locus was located on LG9 jointly with two RAPD markers and STMS TA80. LG3 included a gene for resistance to FOC-0 (Foc0(1)/foc0(1)) flanked by RAPD marker OPJ20(600) and STMS marker TR59. The association of this LG with FOC-0 resistance was confirmed by QTL analysis in the CA2139 x JG62 RIL population where two genes were involved in the resistance reaction. The STMS markers enabled comparison of LGs with preceding maps.

Construction of BAC and BIBAC libraries and their applications for generation of SSR markers for genome analysis of chickpea, Cicer arietinum L

Large-insert bacterial artificial chromosome (BAC) libraries, plant-transformation-competent binary BAC (BIBAC) libraries, and simple sequence repeat (SSR) markers are essential for many aspects of genomics research. We constructed a BAC library and a BIBAC library from the nuclear DNA of chickpea, Cicer arietinum L., cv. Hadas, partially digested with HindIII and BamHI, respectively. The BAC library has 14,976 clones, with an average insert size of 121 kb, and the BIBAC library consists of 23,040 clones, with an average insert size of 145 kb. The combined libraries collectively cover ca. 7.0 x genomes of chickpea. We screened the BAC library with eight synthetic SSR oligos, (GA)10, (GAA)7, (AT)10, (TAA)7, (TGA)7, (CA)10, (CAA)7, and (CCA)7. Positive BACs were selected, subcloned, and sequenced for SSR marker development. Two hundred and thirty-three new chickpea SSR markers were developed and characterized by PCR, using chickpea DNA as template. These results have demonstrated that BACs are an excellent source for SSR marker development in chickpea. We also estimated the distribution of the SSR loci in the chickpea genome. The SSR motifs (TAA)n and (GA)n were much more abundant than the others, and the distribution of the SSR loci appeared non-random. The BAC and BIBAC libraries and new SSR markers will provide valuable resources for chickpea genomics research and breeding (the libraries and their filters are available to the public at http://hbz.tamu.edu).

Pathotype-specific genetic factors in chickpea (Cicer arietinum L.) for quantitative resistance to ascochyta blight

Ascochyta blight in chickpea ( Cicer arietinum L.) is a devastating fungal disease caused by the necrotrophic pathogen, Ascochyta rabiei (Pass.) Lab. To elucidate the genetic mechanism of pathotype-dependent blight resistance in chickpea, F(7)-derived recombinant inbred lines (RILs) from the intraspecific cross of PI 359075(1) (blight susceptible) x FLIP84-92C(2) (blight resistant) were inoculated with pathotypes I and II of A. rabiei. The pattern of blight resistance in the RIL population varied depending on the pathotype of A. rabiei. Using the same RIL population, an intraspecific genetic linkage map comprising 53 sequence-tagged microsatellite site markers was constructed. A quantitative trait locus (QTL) for resistance to pathotype II of A. rabiei and two QTLs for resistance to pathotype I were identified on linkage group (LG)4A and LG2+6, respectively. A putative single gene designated as Ar19 (or Ar21d) could explain the majority of quantitative resistance to pathotype I. Ar19 (or Ar21d) appeared to be required for resistance to both pathotypes of A. rabiei, and the additional QTL on LG4A conferred resistance to pathotype II of A. rabiei. Further molecular genetic approach is needed to identify individual qualitative blight resistance genes and their interaction for pathotype-dependent blight resistance in chickpea.

Quantitative trait loci for broomrape (Orobanche cumana Wallr.) resistance in sunflower

Broomrape (Orobanche cumana Wallr.) is a root parasite of sunflower that is regarded as one of the most important constraints of sunflower production in the Mediterranean region. Breeding for resistance is the most effective method of control. P-96 is a sunflower line which shows dominant resistance to broomrape race E and recessive resistance to the very new race F. The objective of this study was to map and characterize quantitative trait loci (QTL) for resistance to race E and to race F of broomrape in P-96. A population from a cross between P-96 and the susceptible line P-21 was phenotyped for broomrape resistance in four experiments, two for race E and two for race F, by measuring different resistance parameters (resistance or susceptibility, number of broomrape per plant, and proportion of resistant plants per F(3) family). This population was also genotyped with microsatellite and RFLP markers. A linkage map comprising 103 marker loci distributed on 17 linkage groups was developed, and composite interval mapping analyses were performed. In total, five QTL ( or1.1, or3.1, or7.1 or13.1 and or13.2) for resistance to race E and six QTL ( or1.1, or4.1, or5.1, or13.1, or13.2 and or16.1) for resistance to race F of broomrape were detected on 7 of the 17 linkage groups. Phenotypic variance for race E resistance was mainly explained by the major QTL or3.1 associated to the resistance or susceptibility character ( R(2)=59%), while race F resistance was explained by QTL with a small to moderate effect ( R(2) from 15.0% to 38.7%), mainly associated with the number of broomrape per plant. Or3.1 was race E-specific, while or1.1, or13.1 and or13.2 of were non-race specific. Or13.1, and or13.2 were stable across the four experiments. Or3.1, and or7.1 were stable over the two race E experiments and or1.1 and or5.1 over the two race F experiments. The results from this study suggest that resistance to broomrape in sunflower is controlled by a combination of qualitative, race-specific resistance affecting the presence or absence of broomrape and a quantitative non-race specific resistance affecting their number.