Thesis presented March 03, 2022

Abstract:
Uranium is a naturally occurring radionuclide in the environment. Anthropogenic activities related to its exploitation are sources of environmental pollution. Exposure to this toxic element is detrimental to health because its accumulation in water and soil leads to contamination of the food chain, which is potentially hazardous for the population. It is therefore necessary to remediate these environments and to this end, the decontamination of contaminated areas can be achieved by using plants. Phytoremediation is based on the ability of plants to absorb pollutants, including metals. Thus, in order to use plants in phytoremediation and to optimise the technique, it is essential to understand the molecular processes involved in the accumulation of uranium. Uranium-stress interferes with plant biology at different levels: growth is inhibited, homeostasis of essential elements is modified, oxidative stress is generated, but the intracellular proteins that bind uranium or the proteins responsible for its transport are still unknown.
In a first part of my PhD thesis, we developed a metalloproteomic approach to identify uranium-binding proteins in Arabidopsis thaliana cells. This approach is based on the chromatographic fractionation of the proteome and quantification of metals associated with the different protein fractions. This work led to the identification of several protein fractions containing uranium, demonstrating for the first time the existence of uranium-binding proteins in Arabidopsis. The continuation of this work allowed the biochemical and structural characterisation of two uranium-binding proteins.
In a second step, we focused on the accumulation of uranium in A. thaliana roots, and more particularly on the import mechanisms of the radionuclide. We studied uranium accumulation in roots using a physiological approach based on the induction of essential element transporters during nutritional deficiency. This study allowed us to identify calcium channels as the main pathway for uranium uptake in roots. We then showed that the three calcium channels MCA1, MCA2 and ANN1 are involved in uranium uptake in roots.
Finally, in a third part, we performed a genome-wide association study (GWAS) on uranium accumulation in roots and leaves of A. thaliana. By analysing these traits in 142 ecotypes, we were able to show that uranium accumulation in roots is probably not determined by a single strong gene, but by several genes and/or environmental factors. We also identified 34 genes that could be involved in the translocation of the metal from roots to leaves and/or its accumulation in leaves. Further characterisation of these genes should provide new insights into these processes, which are still poorly understood.

Keywords:
Remediation, Uranium, Plants, Homeostasis, Mineral nutrition