The symbiosome-localised aquaporin GmNOD26 functions as a non-selective monovalent cation channel capable of transporting NH4+ in heterologous systems.

Abstract

Legumes have developed a symbiotic relationship with soil bacteria known collectively as rhizobia, which when present in root nodules fix atmospheric nitrogen into a form usable by the plant. The functioning unit of symbiotic nitrogen fixation is referred to as the symbiosome and the membrane separating symbionts acts to regulate the metabolite exchange within the root nodule cells. The efflux of fixed nitrogen from the symbiosome is through an unidentified non-selective cation channel, which is inwardly rectified by cytosolic Mg2+ and has a preference for ammonium. Expression of Nodulin 26 (NOD26), a major component of the soybean symbiosome membrane, in Xenopus laevis oocytes reveals monovalent cation-induced currents. These currents display time- and voltage dependent characteristic similar to those previously observed in patch-clamp studies of the symbiosome membrane and exhibited an apparent monovalent cation preference of NH4+ > methylammonium (MeA+) ≥ K+ > Na+ ≥ C5H15NO+. GmNOD26-mediated currents in X. laevis oocytes were inhibited by divalent cations and inhibited by phosphorylation at serine 262. Together, these findings demonstrate that GmNOD26 functions as a non-selective cation channel capable of facilitating ammonium transport in nitrogen-fixing soybean nodules. Furthermore, pronounced inhibition of GmNOD26 by divalent cations provides as likely explanation for why monovalent cation-induced currents have not previously been characterised in GmNOD26-expressing X. laevis oocytes.