Iron uptake, trafficking and homeostasis in plants.

Iron is an essential micronutrient with numerous cellular functions, and its deficiency represents one of the most serious problems in human nutrition worldwide. Plants have two major problems with iron as a free ion: its insolubility and its toxicity. To ensure iron acquisition from soil and to avoid iron excess in the cells, uptake and homeostasis are tightly controlled. Plants meet the extreme insolubility of oxidized iron at neutral pH values by deficiency-inducible chelation and reduction systems at the root surface that facilitate uptake. Inside the cells the generation of highly toxic hydroxyl radicals by iron redox changes is avoided by intricate chelation mechanisms. Organic acids, most notably nicotianamine, and specialized proteins bind iron before it can be inserted into target molecules for biological function. Uptake and trafficking of iron throughout the plant is therefore a highly integrated process of membrane transport and reduction, trafficking between chelator species, whole-plant allocation and genetic regulation. The improvement of crop plants with respect to iron efficiency on iron-limiting soils and to iron fortification for human nutrition has been initiated by breeding and biotechnology. These efforts have to consider molecular and physiological evidence to overcome the inherent barriers and problems of iron metabolism.