Iron-Uptake by Plants and Its Regulatory Mechanism

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In order to clarify the existence of stereospecific iron transport system in gramineous plants, we newly synthesized the enantiomers (D-form) of phytosiderophores, namely D-avenic acid A and D-deoxydistichonic acid A, and then their iron-uptake ability was compared with that of the corresponding natural phytosiderophores (L-forms). The Fe(III) complexation with the present D- and L-phytosiderophores showed almost same ESR spectra, suggesting that these ligands form Fe(III) coordination complex very similar to each other. In contrast to high ^<59>Fe-uptake ability of the natural L-phytosiderophores, nevertheless, the iron-uptake of the D-forms dramatically decreased. The present significant distinction of the biological activity between the optical isomers indicates the existence of stereospecific iron-uptake mediated by phytosiderophores in gramineous plants.Comparative experiment using SDS-PAGE between iron-deficient and normal barely indicated the existences of membrane proteins and a few proteins induced by iron deficient, which may be related to the receptor protein for mugineic acid-Fe(III) complex and mugineic acid - synthesis enzymes, respectively.The presence of about 30 proteins with affinity for Cu^<2+> in barely was revealed by metal chelate affinity chromatography using Cu^<2+>. Some of them may be iron-binding proteins playing a role as the repressor in iron-uptake regulatory mechanism in plants.Although we have not yet confirmed the functions of these proteins, the detailed analysis for them could make it possible to understand the regulatory mechanism for plants iron-uptake at molecular levels.