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

, : Identification of a transport mechanism for NH4+ in the symbiosome membrane of pea root nodules. Plant physiology 115(2): 519-526

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 [14C]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 [14C]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 micromolar 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 symbiosome 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.

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