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Trace elements are vital for the operation of metabolic pathways that promote growth and structural integrity. Trace elements are essential for plant growth, i.e., iron, zinc, copper, manganese, cobalt, nickel, boron, molybdenum, and chlorine. Unfortunately, the first two i.e. iron and zinc deficiency in humans has increased on a global scale. The main reasons for these two micronutrients deficiency are dietary intake of food with low levels of Fe and Zn. Biofortification approaches would result in the enrichment of these two elements on target tissue to a considerable extent. Absorption of trace elements is a complex physiological trait which is mainly governed by element transporters and metal chelators of the plant system. Different factors like crop growth stage, edaphic factors, season, etc. also influence the element efficiency of a particular crop. The absorption and transport of trace elements in crop plants are based on the thermodynamics of adsorption on charged solid surfaces embedded in a solution phase of charged ions and metal-binding ligands together with redox systems in the case of iron, zinc, and some other elements. During the normal condition, constitutive absorption systems function in nutrient uptake while in deficiency inducible turbo systems function which increases the supply of a particular nutrient. Crops are divided into two groups, i.e., dicotyledonous crops and monocotyledonous crops. Fe and Zn uptakes in dicotyledonous crops have a turbo system that is an upregulated version of the constitutive system which consists of a membrane-bound reductase and an ATP-driven hydrogen ion extrusion pump while monocotyledonous crops have a constitutive system with an inducible system remarkably different from dicot that uses the mugenic acid class of phytosiderophores and chelators. Biofortification of these metals involves through enhancing uptake and translocation of Fe and Zn by introducing genes responsible for the biosynthesis of mugineic acid family phytosiderophores (MAs). MAs played an important role in iron transport from an iron-rich soil into the iron-starved crops. Iron is stored in crops after absorption as phytoferritin and transported to active sites by transport-specific ligands. In the grains, Fe and Zn are transported by the phloem sap system through chelation of heavy metals which is governed by three or four genes each that control chelation, membrane transport, and deposition as phytate. |
