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Journal of Experimental Botany, Vol 49, 527-533, Copyright © 1998 by Oxford University Press

ARTICLES

Absorption and efflux of glyphosate by cell suspensions

P Hetherington, G Marshall, R Kirkwood and J Warner
Plant Science Department, SAC, Auchincruive, Ayr KA6 5HW, UK; Department of Bioscience and Biotechnology, University of Strathclyde, Glasgow G4 0NR, UK; Monsanto Company, 800 N. Lindbergh Blvd, Mail Zone, Q2E, St Louis, MO 63167, USA; Corresponding author; Fax: +44 1292 525314; E-mail: R.Hetherington@au.sac.ac.uk

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 M) there was evidence for a single active uptake system with an apparent Km of 31 M 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 (0.02-0.02 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 (t1/2 162 h), 19% in the medium (t1/2 185 min) and 10% in the fast (t1/2 27 min). The implications of these results in relation to the delivery of glyphosate to its subcellular target site and subsequent phytotoxicity are discussed.Keywords: Zea mays, Glycine max, glyphosate (N-[phosphonomethyl]glycine), absorption, compartmentation.
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