Glyphosate uptake in Catharanthus roseus cells: Involvement of a plasma membrane redox system?

In Catharanthus roseus cells, recent data have indicated that glyphosate up take is mediated by a phosphate carrier for concentrations lower than 100 m u M (F. Morin et nl., Pestic. Biochem. Physiol. 158, 13, 1997). Evidence is presented in this study which indicates that two mechanisms interplay in t he energization of cellular glyphosate uptake: (i) the classical proton/ani on symport, as demonstrated by the increased glyphosate absorption when tra nsport experiments were carried out with an imposed pH gradient, by bufferi ng the medium at acidic pH values; and (ii) another mechanism independent o f the formation of a pH gradient but involving the combined action of calci um and iron, which may be energized by the functioning of a redox pump at t he plasmalemma level. Ca/Fe-dependent glyphosate uptake is not strictly rel ated to the presence of calcium in the medium, since other bi- and trivalen t cations such as lanthanum, in combination with Fe2+, also increase glypho sate absorption. In contrast, Fe2+, which plays a specific role in the incr eased glyphosate uptake, appears to be one of the major elements of the ene rgization process. Experiments carried out with various chemical agents whi ch decrease the availability of intracellular NAD(P)H as the electron donor of the plasmalemma redox pump, or using a competitive inhibitor of molybda te in electron transfer (Na2WO4), and an artificial electron acceptor (FeCN ), suggest that transplasmalemma electron transport is involved both in Fe2 + and in glyphosate uptake. Using the above data, we propose a model which explains the combined action of calcium and iron and the coupling reactions between the redox pump, iron uptake, and the cellular absorption of glypho sate. (C) 1998 Academic Press.