Abstract
SUMMARY Iron (Fe) is a micronutrient essential for plant growth and development. Iron uptake in alkaline soil is a challenge for most plants. In this study, we investigated the role of the catechol coumarins fraxetin and esculetin in plant Fe acquisition and their Fe chelating properties. Mass spectrometry and capillary electrophoresis were used to characterize Fe-coumarin complexes. To understand the role of these complexes, genetic, molecular and biochemical approaches were deployed. We demonstrated that catechol coumarins are taken up by Arabidopsis thaliana root via an ATP dependent mechanism and that plants defective in IRT1 activity (the main high affinity Fe importer) or bHLH121 (a key regulator of Fe deficiency responses) can be complemented by exogenous supply of fraxetin and to a lesser extent of esculetin. We also showed that Fe and fraxetin can form stable complexes at neutral to alkaline pH that can be taken up by the plant. Overall, these results indicate that at high pH, fraxetin can improve Fe nutrition by directly transporting Fe(III) into the root, circumventing the FRO2/IRT1 system, in a similar way as phytosiderophores do in grasses. This strategy may explain how non-grass species can thrive in alkaline soils.