Increase of free cysteine and citric acid in plant cells exposed to cobalt ions

Cobalt complexation was investigated in a suspension cell culture of the cobalt hyperaccumulator Crotalaria cobalticola. C. cobalticola cells were more tolerant towards cobalt ions than the suspension cells of the non-accumulators Rauvolfia serpentina and Silene cucubalus. While the concentration of various compounds increased in cells of C. cobalticola challenged with cobalt ions, phytochelatin biosynthesis was not induced. Instead, the exposure to cobalt ions resulted in the increase of citrate and cysteine in cells. Size exclusion chromatography demonstrated the co-elution of cobalt and cysteine in C. cobalticola cell extracts. A significant increase in cysteine was observed also in cells of R. serpentina and S. cucubalus when they were exposed to cobalt ions. These results suggest that free cysteine is involved in cobalt ion complexation in plant cells.