Nodulin 26 is a multifunctional plant aquaporin that facilitates ammonium transport in nitrogen-fixing soybean nodules

At COMBIO 2022 I was awarded an ASPS poster prize for presenting our findings on the soybean aquaporin, Nodulin 26 (NOD26). Legumes have developed a symbiotic relationship with rhizobia, a soil bacterium that fixes atmospheric nitrogen into a form usable by the plant. During the symbiosis, legumes develop highly specialised organs on their roots, called nodules, which are infected by rhizobia. Inside the infected nodule cells, the rhizobia differentiate into nitrogen-fixing bacteroids that are excluded from the plant cytosol in organelle-like structures termed symbiosomes. The symbiosome membrane acts to regulate the metabolite exchange between the symbionts. Interestingly, a non-selective cation channel which facilitates the efflux of fixed nitrogen from symbiosomes remains unidentified (Tyerman et al. 1995). We proposed that NOD26 may be this non-selective cation channel and begin to characterise the channel through two-electrode voltage clamp in Xenopus laevis oocytes, thanks to a collaboration with Stephen Tyerman, Adelaide University.

Ion transport through aquaporins has long been controversial, with Yool and colleagues (1996) first reporting this in human aquaporin 1. More recently, non-selective cation activity has been reported in Arabidopsis PIP2;1, which shares several characteristics with the soybean symbiosome membrane non-selective cation channel. Our work has shown that when NOD26, a major component of the soybean symbiosome membrane, is heterologously expressed in X. laevis oocytes, monovalent cation-induced currents are observed. The channel is permeable to ammonium, methylammonium, potassium and sodium but not choline or anions, and that these currents are inhibited by divalent cations. Through phosphomimetics of S262, we identified a switch in the permeability of NOD26 to water or cations when expressed in X. laevis oocytes (Figure 1). We believe that the soybean aquaporin NOD26 is multifunctional channel that facilitates both ammonia and ammonium transport in nitrogen-fixing nodules and that this transport is regulated by phosphorylation of S262.