Coumarin accumulation and trafficking in Arabidopsis thaliana: a complex and dynamic process.

Iron (Fe) is a major micronutrient required for plant growth and development. Non-grass species have evolved a reduction-based strategy to solubilize and uptake Fe. The secretion of Fe-mobilizing coumarins (e.g. fraxetin, esculetin and sideretin) by plant roots plays an important role in this process. Although the biochemical mechanisms leading to their biosynthesis are well described, very little is known about their cellular and subcellular localization or their mobility within plant tissues. Spectral imaging was used to monitor in Arabidopsis thaliana the in planta localization of Fe-mobilizing coumarins and scopolin. Molecular, genetic and biochemical approaches were also used to investigate the dynamic of coumarins accumulation in roots. These approaches showed that root hairs play a major role in scopoletin secretion whereas fraxetin and esculetin secretion occurs through all epidermis cells. It also highlighted that under Fe deficiency conditions, coumarins transport from the cortex to the rhizosphere relies on the PDR9 transporter. Additional experiments support that coumarins move alongside the plant body via the xylem sap and that several plant species can uptake coumarins present in the surrounding media. Altogether the date presented herein demonstrate that coumarins storage and accumulation in roots is a highly complex and dynamic process.