For millions of people in developing countries, plant-based foods are the major source of provitamin A carotenoids, vitamin E tocochromanols. More than 125 million preschool-aged children are vitamin A deficient in developing countries, with an estimated 250,000 to 500,000 of them becoming blind every year. Although clinical vitamin E deficiency is rare, suboptimal dietary intake of vitamin E at levels that are associated with an increased risk for cardiovascular disease have been reported in specific population segments of the US and developing countries. Maize is an important staple crop in many of the countries where these nutritional deficiencies are present and has considerable genetic variation for carotenoid and tocochromanol grain levels. However, the varieties of maize grain typically used for human consumption do not provide adequate daily levels of provitamin A and vitamin E. To further our understanding of the biosynthesis of these nutritional compounds, we have undertaken genome-wide association studies of carotenoid and tocochromanol grain traits in two publicly available maize association panels (i.e., the nested association mapping panel and the Goodman-Buckler 281 inbred line association panel). Our research hypotheses address the origin of genetic variation for these metabolic traits, the extent of pleiotropy and epistatic interactions within these pathways, and the efficiency with which genetic mapping results can be used to address biofortification objectives.
In collaboration with Dean DellaPenna (Michigan State University), Robin Buell (Michigan State University), and Jianming Yu (Iowa State University), we have now expanded our efforts in a new project that seeks to leverage the tremendous genetic and genomic tool sets developed in maize the past decade to advance and accelerate our fundamental understanding of the genes, alleles and genetic mechanisms controlling synthesis and accumulation of vitamins that are limiting in maize grain and hence result in vitamin deficiencies in maize-based diets: four B vitamins (B1, thiamine; B2, riboflavin; B3, niacin; B6, pyridoxine) and vitamin E. Specific objectives are to (i) perform genome-wide association studies with the maize Ames inbred line panel (n~2,000) to identify and resolve quantitative trait loci (QTL) controlling accumulation of these micronutrients; (ii) assess the role of rare alleles by constructing and analyzing segregating F2 populations derived from Ames lines that are extreme outliers for traits; (iii) determine the contribution of expression QTL and presence-absence variants (PAVs) to vitamin composition using whole transcriptome sequencing data obtained from grain 24 days after pollination in 500 inbred lines that represents the phenotypic variation of the Ames panel; and, (iv) perform genomic prediction with the Ames panel to accelerate the efficiency of breeding improved grain micronutrient composition in developing countries.