X-ray absorption spectroscopy evidence of sulfur-bound Cadmium in the Cd-hyperaccumulator Solanum nigrum and the non-accumulator Solanum melongena

International audienceThe environmental pollution by ecotoxic heavy metals is a problem of increasing significance for ecological and human health reasons. Among these elements, cadmium (Cd) is of special concern due to its high mobility in the soil/plant system and its acute toxicity. To limit Cd contamination in plants, and to enhance food safety, it is critical to understand Cd biogeochemical cycle, in particular what controls the bioavailability of this element. Previous studies have demonstrated that Cd bioavailability in soils depends ultimately on its chemical speciation. It has been proposed that non-protein thiols and organic acids play a major role in cadmium phytoavailability and distribution in plants. In the Cd-hyperaccumulator Solanum nigrum and non-accumulator Solanum melongena, the role of these organic ligands in the accumulation and detoxification mechanisms of Cd are debated. Here, we used X-ray absorption spectroscopy to investigate Cd speciation in these plants (roots, stem, leaves) and in the soils used for their culture to unravel the plants responses to Cd exposure. The results show that Cd in the 100 mg.kg-1 Cd-doped clayey loam soil is sorbed onto iron oxyhydroxides. In both S. nigrum and S. melongena, Cd in roots and fresh leaves is mainly bound to thiol ligands, with a small contribution of inorganic S ligands in S. nigrum leaves. We interpret the Cd binding to sulfur ligands as detoxification mechanisms, possibly involving the sequestration of Cd complexed with glutathione or phytochelatins in the plant vacuoles. In the stems, results show an increase binding of Cd to -O ligands (>50% for S. nigrum). We suggest that Cd is partly complexed by organic acids for transportation in the sap. To further constrain the fate of Cd in S. nigrum and S. melongena, we performed Cd stable isotope analysis of our samples. These new results will be presented at Goldschmidt 2021, and put in perspective with the XAS measurements