Identification of chelidonic acid as the predominant ligand involved in ni uptake in the hyperaccumulator berkheya coddii

Nickel uptake in Berkheya coddii, a Ni hyperaccumulator (>3 % Ni m/m), was studied using plants from serpentine outcrops in Barberton, Mpumalanga Province, South Africa. Size exclusion chromatography (SEC), high resolution mass spectrometry (HR-MS) and high performance liquid chromatography (HPLC) were used to identify and quantify organic and amino acids typically associated with hyperaccumulation. Calculated molar ratios were used to identify potential Ni-ligand associations with amino and organic acids. The former showed no single acid present at levels sufficient to complex adequately with the high levels of Ni in B. coddii. Only elevated proline concentrations in younger B. coddii leaves were recorded; however, proline may be produced as a stress response to elevated metal concentrations. A combination of chromatography and spectroscopy led to the identification of chelidonic acid as the ligand playing a significant role in Ni uptake in B. coddii. Chelidonic acid and Ni in leaves were quantified in a 3:1 molar ratio. However, during spiking experiments where soluble Ni was added to soil, the chelidonic acid and Ni concentrations increased to a 6:1 molar ratio. The other amino and organic acids present in the plant showed no response to an increase in soluble Ni, thus indicating chelidonic acid has a role in the B. coddii hyperaccumulation process. This finding is the first to link chelidonic acid to hyperaccumulation and will have significant impact on the potential of B. coddii for phytoremediation.