ICOM1 Abstract

talk session 1 MICHEL CHALOT1, ANNICK BRUN1, BERNARD BOTTON1 & BENGT SODERSTROM2. 1University Henri Poincare-Nancy I, Faculty of Sciences, Laboratory of Forest Biology, U.A. INRA 977, BP 239, F-54500 VandÏuvre Cedex, France. 2Department of Microbial Ecology, University of Lund, S-223 62 Lund, Sweden. - Kinetics, energetics and specificity of a general amino acid transporter from the ectomycorrhizal fungus Paxillus involutus. The kinetics, energetics and specificity of a general amino acid transporter were studied in the ectomycorrhizal fungus Paxillus involutus. The uptake of amino acids showed characteristic features of active transport. After correction for a non-mediated component, the kinetics of glutamate, glutamine, alanine and aspartate uptake measured over a wide concentration range followed the simple Michaelis-Menten saturation curves. The apparent Km derived from the Eadie-Hofstee plots ranged from 7 µM for alanine to 27 µM for glutamate. Dinitrophenol, carbonyl-cyanide-m-chlorophenylhydrazone and NaN3 strongly inhibited amino acid uptake whereas dicyclohexyl-carbodiimide, vanadate and the ionophores monensin and nonactin were without effect on the uptake. Both pH dependence and inhibition by protonophores are consistent with a proton symport mechanism for amino acid uptake by P. involutus. Competition studies indicate a broad substrate recognition by the uptake system, which resembles general amino acid permease of yeast. Dixon plots of the inhibition of glutamate uptake by alanine, lysine and methionine sulfoximine showed that inhibitions were competitive. The physiological importance of this transporter for the exchange of nitrogenous compounds between fungal and host plant cells in ectomycorrhizal associations is discussed. Further experiments using modern molecular techniques will be directed to elucidate nitrogen transport mechanisms at the biotrophic interface during the early stages of mycorrhiza formation. They will lead to a better understanding of many aspects of ammonium, amino acid and peptide transport that are not well defined at present.