Iron is an essential element for plant growth. Gramineous plants have generally developed a distinct strategy to efficiently acquire insoluble iron, which is characterized by the synthesis and secretion of an iron-chelating substance, phytosiderophore (PS) such as mugineic acid (MA), and by a specific uptake system for iron(III)-MAs complexes. In this study, we have identified a gene specifically encoding an iron(III)-MAS transporter (HvYS1) in barley. This gene as well as the encoded protein is specifically expressed in ghe epidermal cells of the roots and gene expression is greatly enhanced under iron-deficient conditions. The localization and substrate specificity of HvYS1 indicate that it is a specific transporter in barley roots. In contrast, ZmYS1, reported as iron-MAs transporter from maize, possesses broad substrate specificity despite a high homology with HvYS1. By assessing the transport activity of a series of HvYS1-ZmYS1 chimeras, we revealed that the outer membrane loop between the sixth and seenth transmembrane regions is essential for the substrate specificity. Circular dichroism spectra revealed that a synthetic peptide corresponding to the loop of HvYS1 forms an α-helix in solution, whereas that of ZmYS1 is flexible. We disclosed that the structural difference at this particular loop determines the substrate specificity of the HvYS1 transporter. We also achieved an efficient short-step synthesis of MA and 2'-deoxymugineic acid (DMA). Our new synthetic method enabled a sufficient supply of these compounds that are essential for further biological studies.
