Iron Biofortification in Maize: A Review of Recent Progress and Future Prospects`
Iron Deficiency (ID) continues to be one of the main hidden hunger issues in the world today, impacting billions of people and becoming a major hurdle for human food security and health. Maize (Zea mays L.) is one of the widely cultivated and consumed cereal crops and thus has the potential to be an important target for iron biofortification to improve dietary iron intake, especially in cereal-centric diets. During the last decade efforts have been undertaken to understand the physiological, agronomic, genetic and molecular aspects associated to Fe uptake, transport, accumulation and bioavailability in maize. The recent progress in biofortification of maize with iron is reviewed, with a focus on soil Fe, strategy II Fe acquisition, Fe transport and Fe homeostasis and their effects on the accumulation of Fe in grain. Modern breeding technologies including conventional breeding, quantitative trait locus (QTL) mapping, genome wide association studies (GWAS), genomic selection, transgenic technologies and CRISPR/Cas mediated genome editing are discussed critically along with the recent advances of agronomic biofortification including soil and foliar fertilization, seed priming, nano-fertilizers and organic amendments. The review also focusses on the new functions of plant growth-promoting rhizobacteria (PGPR), microorganisms producing siderophores and arbuscular mycorrhizal fungi in the improvement of the iron availability and uptake in a sustainable biological way. Moreover, recent advances in multi-nutrient biofortification, precision agriculture, omics, artificial intelligence and genome-assisted breeding are explored as tools to facilitate the acceleration of the development of biofortified maize cultivars rich in iron. However, a number of challenges still exist, such as the bioavailability of iron, genotype × environment interaction, soil limitations, climate change, and regulation and farmer adaptation. It is concluded that the combination of molecular breeding, microbial-assisted technologies, precision agriculture and agronomic management are most likely to lead to the development of nutritionally improved maize varieties which will reduce the prevalence of ID and help to achieve sustainable FSNSS. Finally, future research priorities are seen which will assist in the efficient development and scale-up of iron-biofortified maize.