Genetic analysis of abiotic and biotic resistance in cowpea [Vigna unguiculata (L.) Walp.]

Cowpea [Vigna unguiculata (L.) Walp.] is a most versatile African crop, it feeds people, their livestock and because of its ability in nitrogen-fixation, it improves soil fertility, and consequently helps to increase the yields of cereal crops when grown in rotation and contributes to the sustainability of cropping systems. Because of its ability to tolerate some level of drought stress, cowpea is a crop of choice where > 10 million hectares are cultivated to cowpea in the semiarid Savanna and Sahelian zones of West and Central Africa. However due to the infrequent drought stress throughout cropping seasons over the last 3 decades, the crop suffers important yield reduction. Moreover, cowpea plants under water stress condition are more vulnerable to diseases, parasites and insect pests attacks. In this thesis we carried out genetic analyses of seedling and terminal drought tolerance and cowpea bacterial blight (CoBB) resistance. Two cowpea genotypes with contrasting reactions to drought and CoBB stresses, Danila (resistant) and TVu7778 (susceptible) were used to develop recombinant inbred lines (RILs) mapping population. The RILs and parents were phenotyped for CoBB resistance and for physiological drought tolerance traits and productivity traits under different water regimes in multiple environments at seedling and adult plant stages. A genetic linkage map of 282 single nucleotide polymorphism (SNP) loci covering a map distance of 633 cM distributed over 11 linkage groups (LG) from the same RILs was used for quantitative traits loci (QTL) analyses. Moisture stress significantly affected RILs performances with number of pods per plant as the yield component most adversely affected by water stress. Correlation and path analyses revealed that grain yield components (mainly number of pods per plant) and plant biomass had the largest direct effects on grain yield under moisture stress and irrigation. Stem greenness was an excellent predictor of seedling survival to drought (r2 = 0.91). Two QTLs were identified for each of the three traits scored under greenhouse, drought-induced trifoliate senescence (DTS), stem greenness (Stg) and survival (Sur) on LG3 and LG7. For all traits measured under field trials, a total of 42 QTLs were detected, 4 for stomatal conductance (Gs), 6 for delayed leaf senescence (DLS), 5 for flowering time and 16 for grain yield components (pod number/plant, seed number/pod, seed weight), 6 for grain yield and 5 for fodder yield. Three QTLs were detected for CoBB resistance, with two major ones (named CoBB-1 and CoBB-2 confirmed over two experiments) on LG3 and LG5 and one minor QTL (CoBB-3 only for experiment 1) on LG9. Although number of QTLs detected seems to be important, association between QTLs of different traits sharing a common genomic region was observed on LG3, LG5, LG7 and LG8 where QTLs for Gs, DLS, and flowering time co-localized with QTLs for yield parameters, DLS and DTS shared common chromosomal regions with CoBB resistance, on LG3 and LG5. These results suggest that common genes might mediate CoBB resistance and DLS. Two QTLs detected for DLS in the field, co-localized with QTLs under greenhouse for seedling drought tolerance traits DTS, Stg and Sur on LG7. This indicates the presence of potential loci controlling senescence in this genomic region. Moreover, this genomic region represents a syntenic genomic region between cowpea, soybean and Medicago. Our findings provide evidence for QTLs mediating seedling and terminal drought tolerance and CoBB resistance in cowpea. QTL and phenotypic analysis revealed that it should be possible to pyramid CoBB resistance with seedling and terminal drought tolerance. The fact that the genetic map of Danila x TVu7778 is integrated in a consensus map of cowpea of 6 RIL populations will permit comparative genomic studies which will enhance the discovery of functional markers for MAS of seedling and terminal drought tolerance and CoBB resistance in cowpea. <br/

Breeding drought tolerant cowpea: constraints, accomplishments, and future prospects

This review presents an overview of accomplishments on different aspects of cowpea breeding for drought tolerance. Furthermore it provides options to enhance the genetic potential of the crop by minimizing yield loss due to drought stress. Recent efforts have focused on the genetic dissection of drought tolerance through identification of markers defining quantitative trait loci (QTL) with effects on specific traits related to drought tolerance. Others have studied the relationship of the drought response and yield components, morphological traits and physiological parameters. To our knowledge, QTLs with effects on drought tolerance have not yet been identified in cowpea. The main reason is that very few researchers are working on drought tolerance in cowpea. Some other reasons might be related to the complex nature of the drought stress response, and partly to the difficulties associated with reliable and reproducible measurements of a single trait linked to specific molecular markers to be used for marker assisted breeding. Despite the fact that extensive research has been conducted on the screening aspects for drought tolerance in cowpea only very few¿like the `wooden box¿ technique¿have been successfully used to select parental genotypes exhibiting different mechanisms of drought tolerance. Field and pot testing of these genotypes demonstrated a close correspondence between drought tolerance at seedling and reproductive stages. Some researchers selected a variety of candidate genes and used differential screening methods to identify cDNAs from genes that may underlie different drought tolerance pathways in cowpea. Reverse genetic analysis still needs to be done to confirm the functions of these genes in cowpea. Understanding the genetics of drought tolerance and identification of DNA markers linked to QTLs, with a clear path towards localizing chromosomal regions or candidate genes involved in drought tolerance will help cowpea breeders to develop improved varieties that combine drought tolerance with other desired traits using marker assisted selection

Identification of markers associated with bacterial blight resistance loci in cowpea (Vigna unguiculata (L.) Walp.)

Cowpea bacterial blight (CoBB), caused by Xanthomonas axonopodis pv. vignicola (Xav), is a worldwide major disease of cowpea [Vigna unguiculata (L.) Walp.]. Among different strategies to control the disease including cultural practices, intercropping, application of chemicals, and sowing pathogen-free seeds, planting of cowpea genotypes with resistance to the pathogen would be the most attractive option to the resource poor cowpea farmers in sub-Saharan Africa. Breeding resistance cultivars would be facilitated by marker-assisted selection (MAS). In order to identify loci with effects on resistance to this pathogen and map QTLs controlling resistance to CoBB, eleven cowpea genotypes were screened for resistance to bacterial blight using 2 virulent Xav18 and Xav19 strains isolated from Kano (Nigeria). Two cowpea genotypes Danila and Tvu7778 were identified to contrast in their responses to foliar disease expression following leaf infection with pathogen. A set of recombinant inbred lines (RILs) comprising 113 individuals derived from Danila (resistant parent) and Tvu7778 (susceptible parent) were infected with CoBB using leaf inoculation method. The experiments were conducted under greenhouse conditions (2007 and 2008) and disease severity was visually assessed using a scale where 0 = no disease and 4 = maximum susceptibility with leaf drop. A single nucleotide polymorphism (SNP) genetic map with 282 SNP markers constructed from the same RIL population was used to perform QTL analysis. Using Kruskall-Wallis and Multiple-QTL model of MapQTL 5, three QTLs, CoBB-1, CoBB-2 and CoBB-3 were identified on linkage group LG3, LG5 and LG9 respectively showing that potential resistance candidate genes cosegregated with CoBB resistance phenotypes. Two of the QTLs CoBB-1, CoBB-2 were consistently confirmed in the two experiments accounting for up to 22.1 and to 17.4% respectively for the first and second experiments. Whereas CoBB-3 was only discovered for the first experiment (2007) with less phenotypic variation explained of about 10%. Our results represent a resource for molecular marker development that can be used for marker assisted selection of bacterial blight resistance in cowpe

Breeding drought tolerant cowpea: constraints, accomplishments, and future prospects

This review presents an overview of accomplishments on different aspects of cowpea breeding for drought tolerance. Furthermore it provides options to enhance the genetic potential of the crop by minimizing yield loss due to drought stress. Recent efforts have focused on the genetic dissection of drought tolerance through identification of markers defining quantitative trait loci (QTL) with effects on specific traits related to drought tolerance. Others have studied the relationship of the drought response and yield components, morphological traits and physiological parameters. To our knowledge, QTLs with effects on drought tolerance have not yet been identified in cowpea. The main reason is that very few researchers are working on drought tolerance in cowpea. Some other reasons might be related to the complex nature of the drought stress response, and partly to the difficulties associated with reliable and reproducible measurements of a single trait linked to specific molecular markers to be used for marker assisted breeding. Despite the fact that extensive research has been conducted on the screening aspects for drought tolerance in cowpea only very few¿like the `wooden box¿ technique¿have been successfully used to select parental genotypes exhibiting different mechanisms of drought tolerance. Field and pot testing of these genotypes demonstrated a close correspondence between drought tolerance at seedling and reproductive stages. Some researchers selected a variety of candidate genes and used differential screening methods to identify cDNAs from genes that may underlie different drought tolerance pathways in cowpea. Reverse genetic analysis still needs to be done to confirm the functions of these genes in cowpea. Understanding the genetics of drought tolerance and identification of DNA markers linked to QTLs, with a clear path towards localizing chromosomal regions or candidate genes involved in drought tolerance will help cowpea breeders to develop improved varieties that combine drought tolerance with other desired traits using marker assisted selection