Mechanisms of cobalt uptake in plants: Co-60 uptake and distribution in Chara

The mechanism of cobalt uptake was investigated using cells of the giant al ga Chara corallina in which it is possible to resolve separately uptake by the cell wall and actual influx across the cell membrane. The absorption of Co-60 by Chara cells appeared to saturate within 2 h, but this was mainly due to rapid uptake into the cell wall which accounted for 87-92% of the to tal activity. Even after prolonged desorption most of the cell-associated C o-60 was found on the cell wall. The intracellular distribution of absorbed Co-60 was investigated by fractionating the cell into cytoplasm and vacuol e. It was shown that Co-60 influx to the vacuole occurs simultaneously with influx to the cytoplasm. The transported species appears to be Co2+ rather than the less charged Co(OH)(+) or Co(OH)(2). Co-60 influx is pH dependent (optimum pH 7-9), and is sensitive to some other divalent metals. Influx f rom solutions containing 1 mu M Co-60 was inhibited by 5 mu M Cd2+, Cu2+, a nd Zn2+, but Mn2+ and Ni2+ had no significant effect. The sensitivity of Co uptake to N-ethyl maleimide (NEM) and cysteine suggests that the transport system involves direct binding of Co2+ to -SH groups.