Speciation of uranium in plants upon root accumulation and root-to-shoot translocation: A XAS and TEM study

► Phosphate complexation leads to precipitation and reduces U mobility in plants. ► Citrate complexation promotes U translocation to shoot but reduces root uptake. ► U chemical species mobile in plants is a U-carboxylate complex. Uranium mobilization in surface waters and soils is highly dependent on its speciation. Links between U speciation and in planta mobility remain unclear, although understanding this relationship is essential in a view to properly develop efficient phytoremediation strategies. To address this question, we used X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) to determine U speciation and distribution in plant roots and leaves when exposed to U in the form of different chemical species. Our results indicate that U complexation with endogenous phosphate residues leads to its precipitation and fixation in plant organs, avoiding translocation from roots to leaves. We also show that complexation with a strong ligand such as citrate in exposure solution circumvents this precipitation, and enhances root-to-shoot translocation, in a U-carboxylate complex form. These results highlight correlations between U speciation in the environment and its mobility pattern in plants, which would help for phytoremediation purposes.