Iron and Zinc in Maize in the Developing World: Deficiency, Availability, and Breeding

Maize (Zea mays L.) is the third most important cereal in the world and the most important food security crop in sub-Saharan Africa. Maize provides energy and micronutrients. Deficiencies of the essential micronutrients Zn and Fe are fifth and sixth ranked among the top 10 most important risk factors for conditions such as anemia, low cognitive functioning, and impaired immune system (Fe deficiency) and diarrhea, skin inflammation, and recurrent infections (Zn deficiency) in humans, affecting more than two billion people worldwide. Poverty, lack of access to balanced diets and awareness, and low phytoavailability and bioavailability of these nutrients are major reasons for deficiencies. Breeding for mineral-rich maize is a sustainable and cost-effective approach to reduce micronutrient deficiencies. Since 2004, there has been significant progress in improving maize for Zn content. The aim of this review was to capture recent developments, trends, and progress in maize Fe and Zn biofortification and to identify challenges and ways to overcome them. HarvestPlus has set target levels for Fe (60 mu g g(-1)) and Zn (38 mu g g(-1)) in maize. Zinc target levels have been reached, but conventional breeding alone cannot enhance Fe to the recommended levels. Techniques such as oligo-directed mutagenesis, reverse breeding, RNA-directed DNA methylation, and gene editing could be used in future to speed up maize Fe biofortification. Additional research is required on Fe and Zn bioavailability in maize products, and on interactions of Fe and Zn with Ca and phytate and their influence on absorption, to better understand the underlying mechanisms.