Does prior exposure to uranium protect future generations of arabidopsis thaliana plants to metal stress

Background: Uranium is a non-essential radionuclide and heavy metal that occurs naturally in the environment in relatively low concentrations. Anthropogenic practices such as uranium or phosphate mining have increased the disposal of this metal in the environment. It is known that exposure to uranium causes a disturbed growth and development of A. thaliana plants, however little information is known about the underlying mechanisms of its toxicity. We hypothesized that uranium exposure in a previous generation protects A. thaliana plants against future metal-induced stress. Material & methods: A. thaliana plants were grown from control seeds (i.e. with no previous exposure to metal stress) and from seeds that were continuously exposed in their previous generation to 5 μM U (i.e. U-seeds). Both control A. thaliana seedlings and U-seedlings were grown hydroponically for 18 days, after which they were exposed to different concentrations of cadmium (5 or 10 μM Cd) and uranium (25 or 50 μM U) for three days. Subsequently, the effects on several oxidative stress-related parameters, DNA repair and transgenerational stress response, i.e. the degree of DNA methylation, were determined. Results: Higher concentrations of Cd and U were found in the roots than in leaves. In roots, expression of different antioxidative, DNA repair and methylation genes was significantly downregulated in plants grown from control seeds. For U-seeds, however, significant upregulation of some antioxidative, DNA repair and methylation genes was seen for the high Cd- and U-conditions in roots. In contrast, only few significant differences were seen in gene expression in the leaves and significant differences that did occur, were in general similar for both control and U-seeds. In addition, the measured responses of the antioxidative enzyme capacities and the antioxidative metabolite concentrations were different and showed less significant alterations in comparison to the transcriptional responses of the antioxidative genes in both roots and leaves. Discussion & conclusions: In our study, it was shown that exposure to Cd or U induces adverse effects in both roots and leaves of A. thaliana plants after three days. Although, different mechanisms seem to be involved in the effects on the oxidative stress response, DNA repair mechanisms and DNA methylation pathways induced by Cd and U in roots and leaves. The low root-to-shoot transfer of Cd and especially U can possibly explain the more severe effects seen in roots compared to leaves. In addition, our results show for the first time differences between plants from control and U-seeds, especially for the expression of antioxidant, DNA repair and methylation genes in roots. This indicates that changes can occur in the stress response in plants that were exposed to U in the previous generation and that metal exposure in a previous generation can possibly induce a protective abiltiy in the following generation.