Iron-Uptake by Plants and Its Regulatory Mechanism

为了阐明禾本科植物中铁转运系统的立体特异性，我们合成了植物铁载体的对映体（D型），即D-燕麦酸A和D-脱氧二叠子酸A，并将它们与相应的天然植物铁载体（L型）的铁吸收能力进行了比较。Fe（III）与本发明的D-和L-植物铁载体的络合显示几乎相同的ESR谱，表明这些配体形成Fe（III）配位络合物非常相似。然而，与<59>天然L-植物铁载体的高铁吸收能力相反，D-形式的铁吸收显著降低。对缺铁和正常禾本科植物进行的SDS-PAGE比较实验表明，缺铁条件下禾本科植物中存在膜蛋白和少数铁诱导蛋白，其可能与麦红氨酸-Fe（III）复合物和麦红氨酸合成酶的受体蛋白有关，通过Cu^2+的金属螯合亲和层析，发现大麦中存在约30种对Cu^2+具有亲和性的蛋白质。其中一些可能是铁结合蛋白，在植物铁吸收调控机制中起着阻遏物的作用，虽然这些蛋白的功能尚未得到证实，但对其进行详细的分析将有助于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.