Association and QTL Mapping of Carotenoid and Color Loci in Maize Grains
Food security and sustainable development are interdependent. Development can be hindered by food insecurity as undernourished people have a reduced working capacity, greater vulnerability to illness, and lower school performance. Having sufficient caloric intake is necessary for survival, but having nutritious food is also necessary for health. Micronutrient deficiencies contribute to the global burden of disease by increased rates of illness, death and disability. Vitamin A deficiency (VAD) is a serious problem in the developing world affecting an estimated 250 million pre-school age children. Between 250,000 and 500,000 of the affected children become blind every year, and half of these then die within one year. Diets that lack diversity, in which people are mainly dependent on staple crops such as rice, wheat, maize and white flesh sweet potato, can lead to VAD. HarvestPlus is developing and releasing orange high provitamin A maize germplasm in Zambia, and this program will be expanded to include other sub-Saharan African countries where maize is a staple food. This maize is bred through the process known as biofortification, the nutritional enhancement of staple crops to provide micronutrients. While white maize is traditionally preferred for human consumption in many areas. The provitamin A compounds, β-carotene, β-cryptoxanthin and α-carotene are produced in the carotenoid biosynthetic pathway, and can be metabolized by humans to form Vitamin A, or retinol. While the enzymes in the carotenoid biosynthetic pathway are well known, the allelic variation required to produce high provitamin A maize grain is not fully understood. Recent research has produced informative results. Natural polymorphisms in the lcyE gene in maize affect flux through the α-branch versus β-branch of the carotenoid pathway. Flux towards the β-branch is desirable to produce higher levels of β-carotene and β-cryptoxanthin. A weak allele of crtRB1 has been identified that results in lower expression of the enzyme. Reduced expression of crtRB1 results in decreased conversion of β-carotene to β-cryptoxanthin and zeaxanthin and thus a greater accumulation of β-carotene. Different phenotypic and statistical methods were successfully used to further characterize the nature of allelic variation in a number of maize genes encoding enzymes in the carotenoid pathway, upstream to the pathway, and in the degradation of carotenoids. QTL mapping for carotenoid related traits in biparental populations using visual color-score, colorimeter, and NIR-VIS data identified a number of regions containing genes involved in carotenoid biosynthesis and degradation including psy1, crtRB1, and z-iso. A genome-wide association study of carotenoid levels in a panel of 201 diverse maize inbreds identified four major loci (zep1, lut1, lcyE and crtRB1) contributing to variation in the abundance of various carotenoid compounds and carotenoid ratios and the dxs2 and lut5 loci were also identified at the pathway level. A genome-wide association study of colorimeter data in a panel of 1471 diverse maize inbreds identified three major loci ( dxs2, psy1 and lcyE) affecting color and the dmes2 and zep1 loci were identified in pathway level analysis. These collective results provide further support for the concept that genes responsible for orange color and carotenoid concentration overlap. The genes reported here provide a logical focus for breeding efforts to enhance provitamin A and orange color in maize.
Rocheford, Purdue University.
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