Kinetics and Specificity of Amino Acid Uptake by the Duckweed Spirodela polyrhiza (L.) Schleiden

Borstlap, A. G, Meenks, J. L. D., van Eck, W. F. and Bicker,J. T. E. 1986. Kinetics and specificity of amino acid uptakeby the duckweed Spirodela polyrhiza (L.) Schleiden.—J.exp. Bot. 37: 1020–1035. Uptake of ¹⁴C-labelled amino acids by intact, axenically grownplants of Spirodela polyrhiza (L.) Schleiden was investigated.Experiments in which uptake was measured from the decrease inthe amino acid concentration in the medium, indicated that saturableuptake conforms to the sum of two Michaelis-Menten terms, possiblycorresponding with a high-affinity and a low-affinity system.Further experiments with L-leucine, L-glutamic acid, and L-lysine,in which uptake was measured by assaying the amount of ¹⁴ inthe plants, showed the presence of a non-saturable componentin addition to the dual saturable uptake. Uptake of L-glutamic acid precipitously declined between pH4?0 and 6? and that of L-leucine between pH 4?0 and 8? whereasL-lysine uptake was optimal at pH 6?0. No evidence was foundthat the apparent high-affinity and low-affinity systems respondeddifferently to changes in external pH or to the addition ofCCCP. The non-saturable uptake component was not affected bychanges in external pH or by adding CCCP, and might have beendue to free space uptake. Mutual inhibition of uptake was found between acidic and neutralamino acids (L-leucine, L-methionine, L-glutamic acid) and betweenbasic amino acids (L-lysine, L-ornithine). The basic amino acidshad no effect on the uptake of L-leucine, L-methionine and L-glutamicacid, although the uptake of basic amino acids was inhibitedby glutaminc acid and several neutral amino acids. It is suggested that the duckweed has a high-affinity transportsystem for neutral and acidic amino acids, and a distinct high-affinitysystem for basic amino acids. It is argued that the first systemtransports zwitterionic amino acids (z-system), and that thesecond system transports cationic amino acids(y⁺-system). Thespecificity of the low-affinity system is less certain, butthere is some evidence that it is similar to that of their high-affinitycounterparts. Key words: Kinetics, membrane transport, pH-dependency, transport systems, uptake isotherms