Identification of a transport mechanism for NH₄⁺ in the symbiosome membrane of pea root nodules

Symbiosome membrane vesicles, facing bacteroid-side-out, were purified from pea (Pisum sativum L.) root nodules and used to study NH4+ transport across the membrane by recording vesicle uptake of the NH4+ analog [C-14]methylamine (MA). Membrane potentials (Delta psi) were imposed on the vesicles using K+ concentration gradients and valinomycin, and the size of the imposed Delta psi was determined by measuring vesicle uptake of [C-14]tetraphenylphosphonium. Vesicle uptake of MA was driven by a negative Delta psi and was stimulated by a low extravesicular pH. Protonophore-induced collapse of the pH gradient indicated that uptake of MA was not related to the presence of a pH gradient. The MA-uptake mechanism appeared to have a large capacity for transport, and saturation was not observed at MA concentrations in the range of 25 mu M to 150 mM. MA uptake could be inhibited by NH4+, which indicates that NH4+ and MA compete for the same uptake mechanism. The observed fluxes suggest that voltage-driven channels are operating in the wsymbiosome membrane and that these are capable of transporting NH4+ at high rates from the bacteroid side of the membrane to the plant cytosol. The pH of the symbiosome space is likely to be involved in regulation of the flux.