| dc.description.abstract | Cowpea (Vigna unguiculata L. Walp.) is an important legume crop in African countries where it is used as a source of proteins and a substitute for animal proteins in low-income communities. Its production is however faced with biotic and abiotic challenges. Flower bud thrips (Megalurothrips sjostedti Trybom) is among the biotic factors which constrain cowpea production. Although pesticides have been employed to control flower bud thrips, the technicalities in application and the negative effect to the environment are sources of concern. Consequently, use of host plant resistance could be low cost and efficient means of reducing flower bud thrips infestation. The objectives of this study were to a) identify sources of resistance to flower bud thrips in cowpea minicore population, b) determine the genomic regions and candidate genes associated with resistance of cowpea to flower bud thrips, c) determine the effect of environment on secondary metabolites associated with resistance of cowpea to flower bud thrips, and d) determine the mode of inheritance of secondary metabolites associated with resistance of cowpea to flower bud thrips. A total of 220 minicore genotypes were evaluated for resistance to flower bud thrips in three locations for one season and one location for three seasons. Genotypes TVU-3804, TVU-7647 and Ecute were identified as resistant while genotypes Sanzi and Lori Niebe were moderately resistant. These genotypes could be used as parents in a breeding program for improvement of adapted but susceptible genotypes for resistance to flower bud thrips. The 220 minicore genotypes were genotyped using 51,128 single nucleotide polymorphism (SNP) and together with the phenotypic data, were used to determine the genomic regions and candidate genes associated with cowpea resistance to flower bud thrips. Twenty-two SNP markers were significantly associated with resistance to flower bud thrips on chromosomes Vu01, Vu02, Vu03, Vu04, Vu07, Vu08, Vu10 and Vu11. Positional candidate genes including Vigun08g215700, Vigun08g215600 and Vigun08g218500, which encode for nucleotide binding factor (NB-ARC), leucine rich repeat (LRR), and WD40 respectively, were identified on chromosome Vu08. These genes were involved in signaling pathway of plant innate immunity. The SNPs and candidate genes could be incorporated in future marker assisted selection and genomic selection programs, for improvement of cowpea resistance to flower bud thrips.
Six cowpea genotypes, among them two resistant, two moderately resistant and two susceptible to flower bud thrips, were evaluated for biochemicals including flavonoids, antioxidants, phenolics, proteins, lignin, tannins, and reducing sugars in field and screenhouse environments with varying temperature and rainfall. The data collected was subjected to analysis of variance and Genotype and Genotype × Environment (GGE) analysis. The flower bud thrips damage, flavonoids, antioxidants, lignin, tannins and reducing sugars varied significantly (p<0.001) among genotypes owing to their genetic differences, with genotype Sanzi leading in the antioxidants content while TVU-201 led in flavonoid content. Metabolites were significantly (p<0.001) different among environments with long rain field experiment leading in the content of flavonoids, proteins, lignin and tannins. Resistant genotype TVU-3804 was stable for contents of flavonoids, antioxidants, phenolics, proteins and reducing sugars. The environment influenced the content of the secondary metabolites which in turn affected cowpea resistance to flower bud thrips. Flavonoids and antioxidants played key roles in reducing flower bud thrips on cowpea genotypes.
To study the inheritance of secondary metabolites associated with cowpea resistance to flower bud thrips, five cowpea genotypes with varying level of resistance to flower bud thrips were crossed in full diallel method one. The F2 were evaluated for secondary metabolites and flower bud thrips damage in field and screenhouse. The results confirmed genotype TVU-3804 as resistant, and TVU-9820 and TVU-201 as susceptible to flower bud thrips. Flavonoids, antioxidants, proteins and reducing sugars had significant (p<0.001) general combining ability (GCA) and specific combining ability (SCA) effects, indicating the importance of additive and dominance effect in controlling resistance of flower bud thrips. Furthermore, significant reciprocal observed for crosses such as Lori Niebe × Sanzi and Sanzi × TVU-3804 for flavonoids and antioxidants was an indication that maternal effect was key in governing resistance of cowpea to flower bud thrips. The narrow sense heritability was low (less than 42%) for secondary metabolites, an indication that delaying selection to a later generation would yield better results while breeding for resistance to flower bud thrips based on secondary metabolites. The results of this experiment showed that it was possible to use secondary metabolites to breed for improved cowpea resistance to flower bud thrips. | en_US |
