Absorption and efflux of glyphosate by cell suspensions

The absorption and efflux of [14C]-glyphosate (N-[phosphonomethyl]glycine) was studied in maize (Zea mays L. cv. Aussie) and soybean (Glycine max L. Merr. cv. Maple Arrow) cell suspensions. Glyphosate absorption was complex: at low external herbicide concentrations (3–250 (μM) there was evidence for a single active uptake system with an apparent Km of 31 μM and Vmax of 11 nmol g−1 fr. wt. 2 h−1. The system was inhibited by carbonylcyanide m-chlorophenyl hydrazone (CCCP), orthovanadate, diethylstilbestrol (DES), phosphate, and phosphonoformic acid (PFA) suggesting the glyphosate carrier to be a phosphate transporter energized by the plant plasmalemma ATPase. At higher external glyphosate concentrations the operation of this carrier was masked as passive diffusion became the dominant absorption mechanism. Any non-specific binding of glyphosate to the cell surface during absorption was low (0.01–0.02 nmol g−1 fr. wt). Efflux kinetics of [14C]-glyphosate suggests the herbicide to be located in the cells in three kinetically distinct compartments: after 24 h uptake of radiolabelled herbicide, 71% of absorbed glyphosate was found in the slow compartment (t½, 162 h), 19% in the medium (t½, 185 min) and 10% in the fast (t½, 27 min). The implications of these results in relation to the delivery of glyphosate to its subcellular target site and subsequent phytotoxicity are discussed.