Coumarin-facilitated iron transport: an IRT1 independent strategy for iron acquisition in <i>Arabidopsis thaliana</i>

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 <i>Arabidopsis thaliana</i>, reduce Fe(III) to Fe(II) at the root surface and take up Fe(II) into the root via the high-affinity transporter IRT1 (Strategy I). In addition, they also secrete catechol coumarins like fraxetin into the rhizosphere to improve Fe acquisition. Although the importance of catechol coumarins in Fe reduction has been clearly demonstrated in acidic soils, their functions in alkaline condition remains enigmatic. In the present work, we first showed that at circumneutral pH, the catechol coumarin fraxetin forms stable complexes with Fe(III). We further demonstrated that fraxetin can significantly improve Fe nutrition, even in mutant plants lacking IRT1 and in the presence of the strong Fe(II) chelator ferrozine, suggesting that the plant can bypass the conventional Fe(II)-dependent uptake pathway. These findings support that Fe-coumarin complexes are taken up by plant roots in a manner similar to that of Fe(III)-PS complexes in grass species, and therefore challenge the actual paradigm for plant Fe uptake pointing toward a more unified and flexible model in which Strategy I plants can also employ Fe(III)-chelating mechanisms, similar to that of Strategy II.