Vitamin A Breakthrough

Maize is one of six key staple food crops consumed in developing countries that the HarvestPlus Challenge Program aims to biofortify with micronutrients. Maize is the preferred staple in Africa, in many parts of which consumption is higher even than in Mexico, where the crop originated. Meanwhile, vitamin A deficiency is widely prevalent in Africa, afflicting millions of children with morbidity, blindness and even death. To help combat this, HarvestPlus scientists are breeding varieties of maize biofortified with higher levels of beta-carotene, which the body converts into vitamin A. Now, a major scientific breakthrough has brought HarvestsPlus one step closer to this goal.

Maize is one of the most genetically diverse crops in the world. More than 20,000 accessions are held in the genebanks of Centers supported by the CGIAR. Thanks to this tremendous diversity, lines of maize that have naturally high levels of beta-carotene have been found. Some lines, in fact, exceed the intermediate target levels set by HarvestPlus, and a hybrid of two lines meets the final target level. But additional lines high in beta-carotene must be identified.

Maize lines that are rich in carotenoids tend to be more yellow and orange in color. Photo: Robin Allscheid Stevens.

Maize kernels that have higher total carotenoid content tend to be distinctively dark yellow or orange in color, but not all carotenoids are converted to vitamin A in the body. Therefore, varieties with higher levels of the desired beta-carotene cannot be identified by color alone; parent plants and their progeny must be screened specifically for beta-carotene. This process has, until now, been time consuming and expensive, greatly limiting the number of plants that can be processed.

In research partly funded by HarvestPlus and recently published in Science, a team led by Ed Buckler, of the United States Department of Agriculture Agricultural Research Service and Cornell University, and Torbert Rocheford, of the University of Illinois, analyzed a genetically diverse panel of almost 300 maize lines. These lines showed a range in values for the concentration of total carotenoids and beta-carotene. For the first time, they were able to identify a gene and develop genetic markers that are associated with higher levels of beta-carotene in the grain. This method of identifying and selecting breeding lines that are rich in beta-carotene is much simpler and faster than running conventional chemical tests - and up to 1,000 times cheaper.

Buckler expects it to significantly accelerate research in maize biofortification, especially by scientists working in basic labs in developing countries. To this end, Buckler and Rocheford are working with CGIAR Centers, including the International Maize and Wheat Improvement Center (CIMMYT) and the International Institute of Tropical Agriculture (IITA), to help train plant breeders in developing countries to use their techniques, which are being made freely available. CIMMYT and IITA are partner organizations leading HarvestPlus efforts to breed biofortified maize.

Breeding for maize with enhanced provitamin A is just one step in the process. Sufficient micronutrients have to be retained during processing and cooking, and subsequently converted in the body into vitamin A. Studies to date have shown that nutrient retention in orange maize after processing and cooking exceeds the initial assumption of 50%. Furthermore, the conversion factor of provitamin A in the grain to vitamin A in the body has also been found to be approximately 7:1, far better than the expected 12:1. Based on these positive results, the next step is to conduct, in a clinical setting, a study of how well maize that is high in provitamin A improves people's vitamin A status.

Finally, consumers, especially those in regions where vitamin A deficiency is endemic, have to be convinced that eating biofortified maize will improve their health. One significant hurdle is that white varieties of maize are generally preferred in many parts of Africa. However, in a study conducted in Mozambique, Rocheford and his colleagues found that orange maize is an acceptable product to many consumers, particularly when offered at a modest price discount, and that those who are most likely to suffer from vitamin A deficiency were the most likely to accept orange maize meal.

These positive findings, in tandem with the recent breakthrough in genetic marker screening that will make it far easier to identify desirable maize varieties, should help HarvestPlus and its national agricultural research partners remain on target to release naturally biofortified provitamin A maize in Africa as early as 2012.