Fe Chelation and Zinc Solubilization: A Promising Approach for Cereals Biofortification

Micronutrient deficiency, micronutrient malnutrition or hidden hunger is an increasingly severe global challenge for humankind. As wheat, rice and maize are the major staple food crops and are frequently consumed, it has become necessary to biofortify cereal crops with micronutrients, especially iron and zinc, in order to fulfill the requirements for better human health. Cellular and molecular mechanisms should be understood to maintain the homeostasis of micronutrients for increasing iron and zinc in plants. For metal ions uptake under deficiency conditions, plants have established strongly regulated two different strategies. Reduction-based strategy-I is followed by non-graminaceous monocots and dicots, whereas chelation-based strategy-II is followed by grasses. Strategy-II plants (graminaceous species) facing Zn, Fe and other micronutrient deficiency have mechanism of Fe chelation which depends on methionine derivative synthesis known as mugineic acid family phytosiderophores. Graminaceous plants also show quantitative and qualitative differences in MAs production. Wheat, rice and maize release only 2-deoxymugineic acid (DMA) in a very small fraction, thus due to low Fe availability these are reported as susceptible. Whereas, in low Fe availability, barley is reported more tolerant due to production of large amounts of different types of MAs, including MA, 3-epi-hydroxymugineic acid and 3-hydroxymugineic acid. In graminaceous plants, chelation and uptake of non-Fe metals are also facilitated by MAs, like Zn in the form of Zn (II)-MAs. Therefore, to solve the above problem, there is a requirement for alternative and eco-friendly technology such as plant growth-promoting rhizobacteria (PGPR) and organic farming practices to enhance zinc solubilization and its availability to plants.