Résumé
Iron (Fe) is an essential micronutrient for plant development. Fe is present under two forms in the cells: Fe3+ and Fe2+. Thanks to its capacity to gain or lose an electron, Fe is a precious cofactor for enzymes involved in oxido-reduction reactions, many of which are required for key plant metabolisms such as photosynthesis or antioxidant responses. Fe uptake in plants is well documented and requires Fe reduction or Fe3+ chelation with secreted ligands to be transported by dedicated transporters into the root epidermal cell. A number of Fe transporters have been identified and functionally characterized in plants. However, there is no reported method allowing to monitor Fe fluxes and its transport in time and space in living plant cells with a subcellular resolution. We have implemented a robust staining technique based on the Fe-specific Perls stain, which enables us to visualize Fe distribution in plant tissues at the cellular and subcellular levels (Roschzttardtz et al., 2009 Plant Physiol 151:1329-38). This technique however is neither quantitative nor able to distinguish between Fe3+ and Fe2+ forms since the tissue sample must be fixed, which causes potential loss of labile Fe localization. Using specific fluorescent sensor dyes, we are developing a method to visualize both Fe2+ and Fe3+ in the root of living seedlings of wild-type and Fe homeostasis mutants of Arabidopsis. This method allows detection of labile Fe as well as different Fe-chelate complexes in plant cells. First results show that the distribution of Fe2+ and Fe3+ is heterogeneous along the primary root and that distinct gradients are observed for the two Fe forms. Using confocal microscopy, Fe distribution in the different cell layers of the root can also be visualized. We found that in root epidermal cells, Fe is polarized, which is consistent with the presence of IRT1/FRO2 in this cell layer. Moreover, the change in Fe redox status in some mutants or upon external Fe application could be monitored and the dynamics of Fe fluxes could be visualized in the primary root vascular tissue. In conclusion, the development of Fe sensors are powerful tools for Fe localization in living cells and optimization of these optical sensors should provide a valuable tool for future studies of Fe homeostasis of different genotypes under several culture conditions.