The nickel hyperaccumulator Odontarrhena chalcidica (Brassicaceae) preferentially takes up zinc over nickel

Background and aims: Odontarrhena chalcidica is a well-known nickel hyperaccumulator which also take up zinc efficiently, but little is known regarding its zinc uptake mechanisms. Therefore, this study compared the uptake kinetics of zinc and nickel. Methods: Plants were cultivated under various metals (nickel, zinc, cobalt and iron) concentrations of the hydroponic solutions. The plants underwent assessment for a range of parameters, including metal concentrations in the plant, expression level of selected metal transporters, and absorption kinetics models for nickel and zinc in the roots. Results: The plants accumulated high concentrations of zinc in roots (5000 mg kg⁻¹). Zinc uptake was not influenced by nickel, but nickel uptake was severely suppressed by zinc (drop by 87.4%), while significantly stimulated by zinc deficiency (increase by 30.4%). The root uptake of zinc and nickel fit Michaelis-Menten kinetics with K_m for zinc and nickel of 8.99 and 18.3 µM, respectively, while the V_max of nickel was approximately 1.8 times greater than that of zinc. The presence of zinc greatly decreased both the V_max and K_m of nickel. RT-Q-PCR results showed that zinc exposure only stimulated the gene expression levels of zinc transporter (e.g., ZIP10), whilst nickel exposure stimulates gene expression levels of both zinc and iron transporters. Conclusions: O. chalcidica preferentially takes up zinc over nickel, probably via relatively high-affinity transporters. Whilst nickel uptake is via a broader range of transporters, and the zinc transporters appear to be part of the nickel transport systems.