Genetic analysis of carotenoid accumulation in tropical maize inbred lines using molecular tools and field tests for accelerating the biofortification of maize with provitamin A
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Improving the nutritional content of staple crops through breeding (biofortification) is promoted as cheap and sustainable approach for alleviating vitamin A and other micronutrient deficiencies across the world. Maize is one of the target crops for provitamin A biofortification, since it is a major staple for millions of people in developing countries, and has a great genetic potential for accumulating provitamin A in its grain. This PhD study addressed three research objectives aimed at accelerating the biofortification of maize with provitamin A. To achieve the objectives, a diverse set of inbred lines developed from different crosses having mixed genetic background of temperate and tropical germplasm were employed. In the first research objective, the efficiency of PCR based functional markers proposed for high provitamin A maize breeding was investigated. Out of eight polymorphisms for three key carotenoid biosynthesis genes tested, the 3'TE and 5'TE polymorphisms of the gene crtRB1 showed the strongest association. About 18 percent of the inbred lines studied had the favourable alleles for these two polymorphisms. The second research objective assessed the association of genome-wide SNP markers with provitamin A and non-provitamin A carotenoids using GWAS and pathway level association analysis. This study detected known major and minor effect carotenoid genes (lcyE, crtRB1, ZEP1 and others), plus novel loci, such as ARF20, that may be involved in regulation of carotenoid biosynthesis in maize endsosperm. In the last research objective 24 yellow maize inbred lines were inter-crossed using factorial mating scheme for studying their combining abilities and heterotic effects for carotenoids and key agronomic traits. The result demonstrated the predominance of additive genetic effects in the inheritance of endosperm carotenoid content. Small percentage of non-additive genetic effect was also detected for provitamin A which is important for expression of heterosis in the trait. Hybrids combining good agronomic performance and high provitamin A content were identified. This study will serve as a basis for establishing heterotic pattern in IITA's yellow maize germplasm which is crucial for development of hybrid and synthetic varieties that can meet existing and emerging demands, while incorporating high level of provitamin A in the grain.