Diverse transporters for neutral amino acids in Ricinus communis L. seedlings

Abstract

Uptake mechanisms for neutral amino acids were investigated by expression of mRNA isolated from seedlings of Ricinus communis L. in Xenopus laevis oocytes. After injection of mRNA from root, hypocotyl and cotyledon currents elicited by saccharose and neutral amino acids ranged from 0.3 nA up to 2 nA depending on the respective substrate and the source of mRNA. These currents were due to expression of low affinity uptake mechanisms and the KM values found for amino acid induced charge flow range from 1 to 2 mM. The abundance and/or the specificity of the expressed mechanisms differ in the various tissues. Currents of similar magnitude were recorded for alanine and glutamine with mRNA isolated from root, hypocotyl and cotyledons. Serine and proline induced currents after injection of mRNA from hypocotyl and roots, in case of α-aminoisobutyric acid (AIB) induced currents were generally small with mRNA from all tissues tested. In addition, differential sensitivity of glutamine and AIB uptake in the high affinity range was evident towards the amino acid analogue 2-chloro-aminophenoxybutyric acid which indicated an additional set of carriers operating in the micromolar concentration range. The results suggest that multiple transporters for neutral amino acids exist in various tissues of the plant differing in specificity of charge flow and in sensitivity towards the inhibitor 2-chloro-aminophenoxybutyric acid.