Characterization of aspartate transport across the symbiosome membrane in pea root nodules

Aspartate transport was studied in symbiosome membrane (SM) vesicles, facing bacteroid-side-out, purified from pea (Pisum sativum L.) root nodules. An intravesicular alkaline pH gradient (ΔpH) and an intravesicular negative membrane potential (Δψ) were imposed to energize the vesicles. Energized vesicles were capable of accumulating aspartate from the extravesicular medium against a concentration gradient.

The uptake of aspartate in SM vesicles was electrogenic and the transport capacity was influenced by both ΔpH and Δψ. Furthermore, vesicle uptake of aspartate had an extravesicular pH optimum at 6. The uptake of aspartate driven by the energization of SM vesicles was biphasic showing saturation kinetics in the range of 0.01 to 1 mmol/L aspartate.

Two protein modifying reagents, p-chloromercuribenzenesulphonic acid and phenylglyoxal, inhibited the uptake of aspartate in energized vesicles. The data are consistent with H+/aspartate symport(s) transporting aspartate from the bacteroid side of the SM to the plant cytosol.