Uranium (U) is a naturally occurring radionuclide and heavy metal present in both aquatic and terrestrial environments at relatively low concentrations (4 mg/kg in the earth’s crust). Due to anthropogenic activities such as metal mining milling, nuclear energy production and the phosphate industry, U concentrations have locally risen causing ecological problems (Vandenhove, 2002).
The U toxicity and availability to plants has been intensively studied in recent years. However, to conduct a proper risk assessment, it is necessary to predict the uptake by the roots and the translocation to other parts of the plant. Since the U uptake and translocation mechanisms are largely unknown, there is still a lack of information.
The general consensus is that toxic heavy metals enter plant cells through transport systems involved in micronutrient uptake. For example, Cd2+ uptake occurs through transporters involved in the uptake of Ca2+, Fe2+, Mg2+, Cu2+ and Zn2+ (Clemens 2006, Roth et al. 2006). This could be confirmed since some of these metals can inhibit Cd uptake and prevent its accumulation in plant roots. The same experimental approach can be used to unravel U uptake mechanisms.
It is already demonstrated that exposure to high, toxic U concentrations will lead to disturbed nutrient profiles in plants (Vanhoudt et al. 2011). Further transcriptomic studies on U-exposed Arabidopsis plants suggest that U exposure triggers a phosphate starvation stress response. In addition, the main components of the iron uptake and signalling pathway were highly affected by U-exposure (Doustaly et al. 2014). Based on recent data obtained within the group Biosphere Impact Studies (SCK•CEN), we found that also the calcium homeostasis was disturbed in Arabidopsis thaliana plants after U exposure. This was confirmed in the framework of setting up a biotic ligand model for plants: preliminary data from studying the influence of different cations (Ca, K, Mg and Na) and pH on the toxicity of U in Lemna minor showed a clear influence of Ca and pH-levels (Horemans, personal communication) on U toxicity that could indicate among others the possible uptake of U through Ca-channels