Plant Physiol, April 2001, Vol. 125, pp. 1620-1632

Carrier-Mediated Uptake and Phloem Systemy of a 350-Dalton Chlorinated Xenobiotic with an -Amino Acid Function¹

Laboratoire de Physiologie et Biochimie Végétales (Unité Mixte de Recherche 6161, Centre National de la Recherche Scientifique), Université de Poitiers, 25 rue du Faubourg, St. Cyprien, 86000 Poitiers, France (C.D.-G.,M.F., F.R., J.-L.B.); Laboratoire de Synthèse Organique (Unité Mixte de Recherche 6514, Centre National de la Recherche Scientifique), Université de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers, France (J.-F.C.); and Pflanzenphysiologie, Universität Bayreuth, D-95440 Bayreuth, Germany (E.K.)

In a previous paper we have shown that -(phenoxyalkanecarboxylyl)-L-Lys conjugates are potent inhibitors of amino acid transport systems and that it is possible to modulate the uptake inhibition by hydrophobic or hydrophilic additions in the 4-position of the aromatic ring (J.F. Chollet, C. Delétage, M. Faucher, L. Miginiac, J.L. Bonnemain [1997] Biochem Biophys Acta 1336: 331-341). In this report we demonstrate that -(2,4-dichlorophenoxyacetyl)-L-Lys (2,4D-Lys), one of the largest molecules of the series and one of the most potent inhibitors, is a highly permeant conjugate. Uptake of 2,4D-Lys by broad bean (Vicia faba) leaf discs is mediated by an active carrier system (K_m1 = 0.2 mM; V_max1 = 2.4 nmol cm² h¹ at pH 5.0) complemented by an important diffusive component. Among the compounds tested (neutral, basic, and acidic amino acids, auxin, glutathione, and sugars), only the aromatic amino acids clearly compete with 2,4D-Lys. The conjugate accumulates in the vein network, is exported toward the growing organs, and exhibits a distribution pattern different from that of the herbicide moiety. However, over time 2,4D-Lys progressively splits into 2,4D and lysine. Analyses by high-performance liquid chromatography and liquid scintillation spectrometry of the phloem sap collected from the castor bean system, used as a systemy test, indicate decreasing capacities of 2,4D, 2,4D-Lys, and glyphosate, respectively, to move from the epidermis cell wall to the sieve element. Our results show that it is possible to design synthesis of large-size xenobiotics (approximately 350 D) with a lipophilic pole, exhibiting high mobility within the vascular system.