The Regulation of Proton/Amino Acid Symport inRiccia fluitansL. by Cytosolic pH and Proton Pump Activity

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Abstract
In the aquatic liverwort Riccia fluitans the regulation of the plasma membrane H+/amino acid symport has been investigated. Cytosolic pH (pHc), membrane potential (Em) and membrane conductance have been measured and related to transport data, (i) The release of [14C]amino acids is strongly stimulated by cytosolic acidification, induced by the external addition of acetic acid, a decrease in external K+, and in the change from light to dark. On average, a decrease in pHc of 0.5 to 0.6 units corresponded with a 4-fold stimulation in amino acid efflux. (ii) External pH changes have far less effect on substrate transport than the cytosolic pH shifts of the same order. (iii) The inwardly directed positive current, induced by amino acids, is severely inhibited by cytosolic acidification. (iv) Fusicoccin (FC) stimulates amino acid uptake without considerable change in proton motive force. (v) When the proton motive force is kept constant, the uptake of amino acids into Riccia thalli is much lower than when the pump is deactivated. It is suggested that both the proton pump activity and cytosolic pH are the dominant factors in the regulation of the H+/amino acid symport across the plasma membrane of Riccia fluitans, and it is concluded that the proton motive force is not a reliable quantity to predict and interpret transport kinetics.