Thesis presented October 10, 2018
Abstract: Plant response and adaptation to metal stress involve numerous mechanisms with the aim of limiting the deleterious effects of toxic elements. Although a large number of these mechanisms are well characterized, many processes still need to be identified in order to have a better understanding of the strategies involved in plant response to toxic metals.
First, we investigated the role of two post-translational modifications, methylation and phosphorylation of non-histone proteins, in response to stresses induced by two toxic metals, cadmium (Cd) and uranium (U). We analyzed the dynamic of these modifications in three species of Arabidopsis:
A. thaliana and
A. lyrata, two species that are sensitive to Cd, and
A. halleri, a species which naturally tolerates and hyperaccumulates Cd in leaves. By using Western blot analyses and protein mass spectrometry, we showed that the pattern of protein lysine methylation was modified during metal stress. Next, we analyzed the expression of genes coding for enzymes involved in lysine methylation and phosphorylation processes in plants exposed to Cd or U. These analyses showed that the expression of numerous kinase genes was differentially regulated in response to metal stress. For protein lysine methyltransferases, only a few genes were differentially regulated by metals. Finally, we set up a genetic screening and identified two genes coding for protein lysine methyltransferases involved in the tolerance of
A. thaliana to a stress induced by Cd.
Second, we studied the physiological and cellular processes involved in the response of the
A. thaliana root system to U. Through the utilization of different staining procedures and reporter genes, we showed that the root architecture was strongly modulated in response to a stress induced by U. We showed that, at a sub-toxic dose, U stimulated the apex mitotic activity, resulting in improved primary root growth and reduced secondary root formation. At a toxic level, U arrested primary root growth and increased the formation of secondary and higher-order lateral roots. These effects were linked to the inhibition of the cell cycle and the synthesis and accumulation of reactive oxygen species and nitric oxide, which contribute to cell death in the root apex. These changes were associated with perturbations in auxin transport and distribution at the root apex, and were correlated to the deposition of defense polymers (callose and lignin) involved in cell wall stiffening. U stress was linked with a depletion of Pi and a redistribution of Fe in root tissues. These results showed that part of the radionuclide effects are linked with the signaling cascade of Pi sensing in the root apex. Other mechanisms involved in U toxicity are likely related to perturbations of Fe homeostasis and direct deleterious effects of U on root components. Together, these results revealed new mechanisms involved in plant response and adaptation to metal trace elements through the fine-tuned regulation of protein function by post-translational modifications and the identification of cellular processes involved in the response to U toxicity in roots.
Keywords: Plant, Arabidopsis, stress, cadmium, uranium, post-translational modifications, methylation, phosphorylation, root development
On-line thesis.