Iron-Uptake by Plants and Its Regulatory Mechanism

为了阐明禾本科植物中铁的立体专一性运输系统的存在，我们合成了植物铁载体对映体(D-型)，即D-燕麦草酸A和D-脱氧二雄酸A，并比较了它们与相应的天然植物铁载体(L-型)的铁吸收能力。Fe(III)与D-和L-植物铁载体形成的配位络合物的ESR谱基本相同，表明这些配体形成的Fe(III)配位络合物非常相似。与天然L铁载体的高吸铁能力相比，D型铁载体的吸铁能力显著下降。目前光学异构体之间的生物学活性的显著差异表明，禾本科植物中存在植物铁载体介导的立体特异性铁吸收。通过对缺铁植物和正常植物进行的SDS-PAGE比较实验表明，缺铁植物存在膜蛋白和少数受缺铁诱导的蛋白质，这可能分别与麦芽酸-Fe(III)络合物受体蛋白和麦芽酸合成酶有关。其中一些可能是在植物铁吸收调控机制中起抑制作用的铁结合蛋白，虽然我们还没有确定这些蛋白的功能，但对它们的详细分析可以从分子水平上了解植物铁吸收的调控机制。

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.