Elevated Esterases Exhibiting Arylesterase-like Characteristics in an Organophosphate-Resistant Clone of the Greenbug, Schizaphis graminum (Homoptera: Aphididae)

In Catharanthus roseus cells, recent data have indicated that glyphosate is taken up in cell suspensions by an Fe/glyphosate cotransport, stimulated by the action of Ca. In this work, experiments were designed to determine the nature of the coupling mechanisms involved. Two biochemical mechanisms were shown to interplay in the cellular absorption of Fe, the transported form of iron: a fast diffusional process after the transfer of cells into the medium and a slow component mediated by an Fe transporter and stimulated by the action of Ca. There was evidence to suggest that glyphosate absorption is associated with the slow component of Ca-dependent iron uptake: (i) glyphosate and iron uptakes exhibit the same kinetics with respect to medium FeS04 concentrations, with the presence of a saturable phase and the existence of a similar K for Fe and glyphosate absorption; (ii) in presence of the ferrous form of Fe in the medium, glyphosate uptake is greatly inhibited by bathophenanthrolinedisulfonic acid (BPDS), which specifically forms a complex with Fe; and (iii) BPDS also inhibits the cellular absorption of glyphosate with the ferric form of iron, suggesting that the effectiveness of Fe in promoting the transport of glyphosate is due, in part, to its ability to be reduced to Fe. In support of this observation, cadmium, an inhibitor of the Fe reductase, also inhibited the uptake of the herbicide in the presence of FeCl Ca-dependent Fe/glyphosate cotransport, which was demonstrated in C. roseus, was found to occur with different degrees of efficiency in various plant cell suspensions, indicating that this glyphosate uptake process can be considered to be a general mechanism in plant cells.