Interactions between absorption of paraquat and the polyamines putrescine, cadaverine, and spermine in roots of intact maize (Zea mays L. cv 3377 Pioneer) seedlings were examined. Concentrationdependent kinetics for paraquat and putrescine influx were similar and both kinetic curves could be resolved into a linear and a saturable component. The linear component was previously shown to represent cell wall/membrane binding. The saturable components for paraquat and putrescine uptake, which represent influx across the plasmalemma, had Km values of 98 and 120 micromolar, respectively, and Vmax values of 445 and 456 nanomoles per gram fresh weight per hour, respectively. Lineweaver-Burk transformation of the saturable component of paraquat influx in the presence of varying concentrations of putrescine indicated that the diamine competitively inhibited the saturable component of paraquat uptake. Reciprocal experiments similarly demonstrated that paraquat competitively inhibited the saturable component of putrescine uptake. Competitive inhibition of both paraquat and putrescine influx could also be demonstrated with the diamine cadaverine, which has a charge distribution similar to that of paraquat and putrescine. In contrast, the larger, tetravalent polyamine spermine appeared to noncompetitively inhibit the influx of paraquat and putrescine. These results strongly suggest that paraquat enters maize root cells via a carrier system that normally functions in the transport of diamines with a charge distribution similar to that of paraquat.Paraquat is a widely used postmergence contact herbicide that acts by accepting electrons from PSI of chloroplast membranes (3). To reach its site of action following foliar application, paraquat must be capable of crossing the plasmalemma (as well as the chloroplast envelope). In aqueous solution, however, paraquat exists as a divalent cation and is therefore unlikely to passively diffuse across the lipid bilayer of plant cell membranes. Until recently (11), however, little evidence has been reported for a paraquat transport mechanism in plant cell membranes.With the exception of 2,4-D (6, 18), herbicides have seldom been reported to require a carrier-mediated transport system for uptake across the plasmalemma. Recent evidence from our laboratories suggested that paraquat is taken up by maize (Zea mays L. cv 3377 Pioneer) root epidermal and cortical cells via a protein-mediated system. This is based primarily on concentration-dependent kinetic studies of paraquat absorption. In these studies, we demonstrated that paraquat influx across the plasmalemma is via a saturable system with a Km of 90 tiM and a Vmax of 458 nmol g-1 fresh weight h-1 (11).Paraquat uptake in animal tissues has also been reported to be mediated by a saturable system with a Km of 40 to 80 AM (17,21) and Vmax of 300 to 600 nmol g-' fresh weight h-1 (15,17). In contrast with our data from maize roots, influx into animal cells was reported to be sensitive to respiratory inhibitors such as cyanide and rotenone ...

No abstract

No abstract