INTERACTION OF GROWTH-STIMULATING PHENOXYAMINOBUTYRIC ACIDS WITH THE TRANSPORT-SYSTEM FOR NEUTRAL AMINO-ACIDS IN EGERIA-DENSA

Agrostemma hypocotyl sections exhibited dose-dependent elongation response to chloro-substituted alpha-aminophenoxybutyric acids. In order to investigate aminophenoxybutyric acid transport, measurements of C-14-AIB uptake in the presence of aminophenoxybutyric acids as well as of the membrane potential of leaf cells were performed using the fresh-water plant Egeria densa. alpha-aminophenoxybutyric acids transiently depolarize the plasmalemma of leaf cells thus suggesting a proton cotransport-like mechanism. Furthermore, C-14-AIB uptake is competitively inhibited by aminophenoxybutyric acids, the (R)-form being more effective (K(i) = 16.6-mu-M) than the (S)-form (K(i) = 28-mu-M). Under the same conditions the dipeptide glycyl-2,4-dichloraminophenoxybutyric acid (where the 2 functional groups alpha-NH3+ and alpha-COO- are far apart) caused only a moderate inhibition, whereas the phenoxybutyric acid could not inhibit AIB uptake. Correspondingly, in the presence of aminophenoxybutyric acids the electric response of the plasmalemma to AIB is strongly reduced and of similar magnitude to the response to the aminophenoxybutyric acids alone. It is concluded that at least two sites for recognition (alpha-amino and carboxyl groups) are involved in the interference of aminophenoxybutyric acids with the uptake system for neutral amino acids in Egeria densa. The results suggest that the binding affinity of aminophenoxybutyric acids to the cotransporter is much higher than that of neutral amino acids, whereas the transport step through the membrane is hampered.