Light-stimulated Electrogenic Uptake of Neutral Amino Acids by Leaf Cells of Egeria densa and Vicia faba

Abstract

Summary The kinetics and energetics of amino acid uptake into leaf tissues of Egeria and Vicia by means of 14C-amino acids (alanine, aminoisobutyric acid, leucine) and the accompanying electric response at the plasmalemma were compared. Amino acid uptake in the concentration range of 0.1 to 5 mmol/l at pH 6 exhibits biphasic isotherms (Egeria: saturable phase, [S]: 0.1-1.5 mmol/l, linear phase, [S]:, 1.5 mmol/l; Vicia: two Michaelis-Menten terms, without any linear component). Uptake of Ala, AIB and Leu is strongly pH-dependent and is enhanced in the light. Effects of light and various inhibitors on amino acid influx, medium-acidifying activity of the leaf tissues and on the membrane potential prove the existence of a light-stimulated H+ transport system at the plasmalemma, creating a protonmotive force capable to provide transport energy. The transient depolarization of the membrane potential during amino acid influx and the sensitivity of this influx to the membrane potential point to coupling of amino acid transport with electrochemical proton gradient. It is suggested that in Egeria as well as in Vicia neutral amino acids were taken up by proton symport in the range of pH 4 to 8.