Calcium in Carbonate Water Facilitates the Transport of U(VI) in Brassica juncea Roots and Enables Root-to-Shoot Translocation

Abstract

The role of calcium (Ca) on the cellular distribution of U(VI) in Brassica juncea roots and root-toshoot translocation was investigated using hydroponic experiments, microscopy, and spectroscopy. Uranium accumulated mainly in the roots (727-9376 mg kg −1) after 30 days of exposure to 80 μM dissolved U in water containing 1 mM HCO 3 − at different Ca concentrations (0-6 mM) at pH 7.5.

Calcium-permeable cation channels are involved in uranium uptake in Arabidopsis thaliana

“…The accumulation of U in the apoplast, which is a temperature-insensitive adsorption and diffusion process (Barberon and Geldner, 2014), accounts for 60-70% bioaccumulated U. This is in agreement with previous experiments showing that plant roots or plant cells exposed to U accumulate the radionuclide principally in the cell wall and intracellular spaces (Laurette et al, 2012a;Laurette et al, 2012b;El Hayek et al, 2018;El Hayek et al, 2019;Lai et al, 2020;Sarthou et al, 2020;Wu et al, 2020). The absorption of U by the symplastic pathway, which is mainly temperature-sensitive (reduction of energy metabolism, enzymatic activities, membrane fluidity, plasmodesmatal permeability) (Barberon and Geldner, 2014;Sager and Lee, 2014), accounts for 30-40% bioaccumulated U, and is virtually fully inhibited by external Ca at high concentration (Figure 3) or by inhibitors of NSCCs and Ca 2+ -permeable cation channels (Figure 4).…”

“…For example, in carbonated water at nearly neutral pH, uranyl carbonate anionic forms are predominant and possibly incorporated into cells through anionic channels. Interestingly, in these conditions, Ca inhibits the radial transfer of U through the apoplastic pathway but facilitates the symplastic route and results in an increased translocation of U to shoot organs (El Hayek et al, 2018;El Hayek et al, 2019). Our study paves the way for the identification of U transport machinery in environmental conditions with changing U bioavailability as well as in other organisms, in which the molecular basis of U uptake is not known.…”

“…The link between U and the homeostasis of Ca has been previously reported in several plant species (Vanhoudt et al, 2011b;Tawussi et al, 2017;El Hayek et al, 2018;El Hayek et al, 2019;Rajabi et al, 2021) but no evidence was provided to support the uptake of the radionuclide by the Ca assimilation machinery. In this work, we provide several lines of evidence that U is taken up by the roots of Arabidopsis plants through Ca 2+ -permeable cation channels.…”

“…Complexation with phosphate considerably reduces U bioavailability (by precipitation) and limits accumulation in all plant organs. In carbonate water, calcium (Ca) has been shown to modify U bioavailability and to facilitate U(VI) symplastic transport and translocation to shoots (El Hayek et al, 2018;El Hayek et al, 2019). Thus, depending on U speciation in soils, several assimilation pathways are anticipated for free or complexed anionic and cationic U species.…”

Development of a metalloproteomic approach to analyse the response of Arabidopsis cells to uranium stress

“…1) is consistent with previous data showing that U-treated plants display important U precipitates in the cell wall of root cells. 9,10,53 Cell walls are primarily composed of polysaccharides (cellulose, pectins, hemicelluloses) and (glyco)proteins that provide negatively charged groups on which the uranyl ion may bind. For example, carboxyl groups from low methyl-esterified pectins, 51 together with phosphate, 10 confer to the cell wall a high cation exchange capacity.…”