Regulation of NH4 + transport pathways in Pisum arvense roots

Abstract

The effects of metabolic and protein synthesis inhibitors on NH4 + uptake by Pisum arvense plants at low (0.05 mM) and high (1 mM) external ammonium concentration were studied. In short-time experiments cycloheximide decreased the ammonium uptake rate at low level of NH4 + and increased the absorption of NH4 + from uptake medium containing high ammonium concentration. Arsenate and azide supplied into uptake solutions at low ammonium concentration strongly decreased or completely suppressed the NH4 + uptake rate, respectively. When the experiments were carried out at high level of ammonium only azide decreased the uptake rate of NH4 + and arsenate stimulated this process. Dinitrophenol very strongly repressed the uptake rate of NH4 + at both ammonium concentrations. After removing dinitrophenol from both solutions, neither at low nor high external ammonium level the recovery of NH4 + uptake rate was achieved within 150 min or 3 h, respectively. The recovery of NH4 + uptake rate after removing azide was observed within 90 min and 3 h at low and high ammonium concentrations, respectively. The regulation of NH4 + uptake by some inhibitors at low external ammonium level was investigated using plasma membrane vesicles isolated from roots by two-phase partitioning. Orthovanadate completely suppressed the uptake of NH4 + by vesicles and quinacrine decreased the NH4 + uptake which 55 suggests that ammonium uptake depends on activities of plasma membrane-bound enzymes. On the other hand, it was found that dinitrophenol completely reduced the NH4 + uptake by vesicles. The various effects of inhibitors on ammonium uptake dependent on external ammonium concentration suggest the action of different ammonium transport systems in Pisum arvense roots. The ammonium transport into root cells at low NH4 + level requires energy and synthesis of protein in the cytoplasm.