ABSTRACT
Even in Iron (Fe)-deficient conditions, some plants can acquire Fe from the soil. The Fe-acquisition mechanisms are summarized by two strategies: Strategy I for dicotyledonous plants, and strategy II for graminaceous plants. In strategy II, mugineic acid (MA) and its derivatives, which are low-molecular-weight (M.W., 310 to 340) natural amino acids, are secreted from roots of Fe-deficient plants, and they can dissolve Fe in the soil by forming MA-Fem complex with high efficiency and high dissolution rate, and their Fe-dissolution ability are maximized under Fe-deficient and problematic pH soil conditions. The first step of the Fe-dissolution reaction is the adsorption reaction of MA on the surfaces of Fe-containing minerals by ligand exchange reaction. The second step is the desorption of the adsorbed MA from the minerals holding an Fe atom, resulting in the formation of soluble MA-Fem complex in solution. The MA-Fem complex is also re-adsorbed on soil minerals, although its affinity is lower than that of MA. Free MA has six vacant functional groups to bind with one Fe atom (three carboxylic groups, two imino groups, and one hydroxyl group), whereas in the MA-Fem complex, these functional groups are occupied by an Fe atom. Crystallinity of Fe minerals is the important factor in MA-promoted Fe dissolution: poorly crystalline ferrihydrite can be an Fe source of MA-Fem complex, but not the more crystalline Fe minerals (e.g., goethite, hematite, lepidocrocite and maghemite) because they cannot be dissolved by MA. This information is useful in understanding the behavior of MA and its derivatives in the soil and for predicting Fe-deficiency in graminaceous plants.
