Characterization of Paraquat Transport in Protoplasts from Maize (Zea mays L.) Suspension Cells

Protoplasts isolated from maize (Zea mays L.) suspension cells were used to study transport of paraquat. [14C]Paraquat uptake was measured in 400-[mu]L centrifuge tubes using silicon oil centrifugation techniques. Approximately 50% of accumulation from a 100 [mu]M paraquat solution occurred in the first 10 s, and net accumulation reached a maximum after about 10 min. Membrane binding accounted for about 30% of apparent accumulation. Concentration-dependent uptake kinetics were characterized by a non-saturating curve, which was resolved into a linear and a saturable component. The Km of the saturable component was 132 [mu]M, and the Vmax was 0.512 nmol [mu]L of protoplasts-1 min-1. In the absence of sucrose, the Vmax of the saturable component was reduced by 52%, suggesting that paraquat uptake across the plasmalemma is energy dependent. Measurement of concentration-dependent binding of paraquat to burst protoplasts showed a linear response. This suggests that the linear component from intact protoplast concentration kinetics represented paraquat binding to the plasmalemma surface. Calcium inhibited the saturable component, and this inhibition was shown by Lineweaver-Burk analysis to be noncompetitive. Putrescine, a divalent cationic polyamine with a charge distribution similar to that of paraquat, competitively inhibited paraquat uptake. These results show that paraquat transport characteristics at the plasmalemma of maize protoplasts are similar to those reported earlier for paraquat transport in roots of intact maize seedlings.