The chemical speciation of uranium (U), and hence its toxicity, is strongly dependent on pH. However, oxidative stress responses after U exposure have mainly been investigated in Arabidopsis thaliana plants at pH 5.5, the ideal pH for growing plants in a hydroponic setup. As the pH of pore water can vary strongly, the aim of this study is to investigate oxidative stress responses induced in roots of A. thaliana plants exposed to different U concentrations at pH 4.5 and hence at a high free uraynl (UO22+) concentration. In addition to analyzing growth reduction, effects were analysed at enzyme, metabolite and genetic level. Results indicate that U is highly toxic at low pH resulting in a significant decrease in fresh weight (EC50 value for root growth reduction: 28.14 ± 1.59 μM U). In addition, no intact RNA could be extracted from the roots exposed to 75 and 100 μM U and the ascorbate concentrations could not be determined in roots exposed to 50, 75 or 100 μM U indicating that the roots are seriously damaged. Concerning the antioxidative defence system, the involvement of miRNA398b/c in the regulation of the superoxide dismutase (SOD) response was observed after U exposure. As such, a significant increase in MIR398b/c expression was observed, accompanied by a decreased copper/zinc SOD (CSD1/2) expression. The latter was compensated by an increased expression of iron SOD (FSD1) to maintain the SOD capacity in the plastids. While the involvement of miRNA398b/c was already reported before under copper or cadmium stress, this is the first time that it is reported for U.