Coumarin-facilitated iron transport: An IRT1-independent strategy for iron acquisition in Arabidopsis thaliana.

Iron (Fe) is an essential micronutrient for plant growth and development. Despite its importance, Fe uptake in alkaline soils is challenging for most plants because of its poor bioavailability. Plants have evolved two main strategies to acquire Fe. Grass species release phytosiderophores (PS) into the rhizosphere and take up Fe as Fe(III)-PS complexes via specific transporters (strategy II). Non-grass species, such as Arabidopsis thaliana, reduce Fe(III) to Fe(II) at the root surface and transport Fe(II) into the root via the high-affinity transporter IRT1 (strategy I). Additionally, these species secrete catechol coumarins, such as fraxetin, into the rhizosphere to enhance Fe acquisition. Although the role of catechol coumarins in Fe reduction has been clearly demonstrated in acidic soils, their functions under alkaline conditions remain unclear. In this study, we demonstrate that, at circumneutral pH, the catechol coumarin fraxetin forms stable complexes with Fe(III). We also demonstrate that fraxetin significantly improves Fe nutrition, even in mutant plants lacking IRT1 and in the presence of the strong Fe(II) chelator ferrozine, suggesting that plants can bypass the conventional Fe(II)-dependent uptake pathway. These findings support the hypothesis that Fe-coumarin complexes are taken up by plant roots in a manner analogous to Fe(III)-PS complexes in grass species, thereby challenging the current paradigm for plant Fe uptake and suggesting a more unified and flexible model in which strategy I plants can utilize Fe(III)-chelating mechanisms similar to strategy II.