FIGURE 1.

Overview of iron (Fe) transport in non-graminaceous plants based on studies in Arabidopsis thaliana. A proton pump, AHA2, contributes to the acidification of rhizosphere to facilitate solubilization of Fe(III) under Fe-limited conditions. Ferric-chelate reductase FRO2 reduces Fe(III) to Fe(II) at the root surface and Fe²⁺ is then transported into root epidermal cells by IRT1. Phenolic and flavin compounds are also exuded into the rhizosphere to facilitate Fe(III) solubilization by ABCG37/PDR9. Fe–NA chelates prevail in the cytosol, where nicotianamine (NA) in roots is produced by NASes. Fe–NA then moves radially via plasmodesmata toward the vasculature. FPN1 is implicated in loading Fe into the xylem vessels, while FRD3 provides citrate. In the xylem, Fe forms a (tri)Fe³⁺-(tri)citrate complex, which moves toward the shoot via the transpiration stream. In shoots Fe(III) is reduced likely by the action of the FRO family members. YSL2 is proposed to transport Fe–NA to photosynthetic cells (mesophyll cells). Whether the ionic form of Fe is reabsorbed from the xylem vessels into xylem parenchyma cells in the shoot, is not known. In the shoot symplast, Fe predominantly exists as Fe(II)–NA complex. Fe is then loaded into the phloem for subsequent partitioning to sink tissues such as young leaves, seeds as well as is recirculated to the root. OPT3 and YSL1 and YSL3 are implicated in Fe²⁺ and Fe–NA loading into the phloem, respectively. While Fe–NA is a predominant form of Fe in the phloem sap, Fe transporting protein (ITP), metallothionein-like protein type 2B, and a low-molecular weight protein with the similarity to a phloem-specific protein from M. truncatula, have been identified associated with Fe in the phloem sap of R. communis and L. texensis. CC, companion cells; TC, phloem companion cells that de-differentiated into transfer cells.