Speciation of cadmium and copper in the herbaceous flowering hyperaccumulator Bidens pilosa L. and their detoxification mechanisms

Purpose

We aimed to elucidate the speciation of Cd and Cu in the herbaceous flowering hyperaccumulator Bidens pilosa L. and their detoxification mechanisms.

Methods

Seedlings of B. pilosa were grown in hydroponic solution in a greenhouse. FTIR was used to determine differences in functional groups (including -OH, -CH₂, -CH(CH₃)₂, -CO₂H and -NH₂) in molecular compounds from different tissue cells under different concentrations of Cd and Cu treatments. The subcellular distributions and chemical forms of Cd and Cu were determined by using differential centrifugation and sequential extraction methods.

Results

FTIR revealed that the abundance of -OH, -CH₂ groups, acid amides and lipids increased with increasing Cd concentrations. indicating that corresponding macromolecules, such as alcohols and phenols, proteins and lipids in cells in the stems/roots of B. pilosa reduced Cd stress. The results showed that -CH(CH₃)₂ groups bound Cd on the surface of the root cells. In contrast, the amount of -CH(CH₃)₂ groups was reduced with increasing Cu stress. The amount of -CH(CH₃)₂ groups and protein with acid amides increased in cells in the stems of B. pilosa under Cd treatments. Subcellular distributions and chemical speciation analyses revealed that Cd was mainly sequestered in cytoplasm and cell walls of B. pilosa roots. In contrast, Cu was mostly bound in the cell walls. In the roots, the most Cd and Cu were ethanol extractable, indicating relative lability with a high potential to be translocated to the shoots. In the stems and leaves most Cd was bound in the cell walls. But there were no significant changes for the proportion of ethanol-extracted Cu in stems and leaves.

Conclusions

B. pilosa is a Cd hyperaccumulator, but not a Cu hyperaccumulator. The key compounds associated with Cd hyperaccumulation in B. pilosa were alcohols, phenols, acid amides and lipids.