Functional characterization of a new ammonium transporter in soybean

Uddin MK, Wen Z and Kaiser BN

Centre for Carbon, Water and Food, School of Life and Environmental Sciences, The University of Sydney, Brownlow Hill, NSW, 2570.

Biological nitrogen fixation (BNF) is an alternative nitrogen source to commercial synthetic nitrogen fertilizers. Soybean (Glycine max) is a major legume crop used globally in rotation with non-nitrogen fixing plants and has a calculated annual N2 fixation capacity of 16.4 Teragram (Tg) N, representing 77% of the total nitrogen fixed by all grain legumes. Soybean like other legumes form a symbiotic partnership with nitrogen fixing soil rhizobia bacteria. The symbiosis results in the formation of the root nodule where invaded bacteria (bacteroids) express the enzyme nitrogenase, which reduces atmospheric N2 to NH3. Fixed NH3 is rapidly protonated to NH4+ and assumed transported across cellular membranes and then assimilated into amino acids for export to the shoot. Ammonium transport proteins (AMTs and AMFs) are considered the primary mechanisms responsible for the transport of NH4+ in plants; however their role in legume nodule remains poorly understood. To address this, we have begun the process of characterising select members of the AMT and AMF families shown to be expressed in soybean nodules. We have cloned the uncharacterised GmAMT2;1 transport protein from nodule cDNA. GmAMT2;1 was found to functionally complement a yeast strain deficient in ammonium transport while chemical flux analysis indicated GmAMT2;1 behaves as a high-affinity ammonium transport protein. Using real time qPCR, GmAMT2;1 was found broadly expressed across the plant but showed elevated expression in the nodule as plants matured and began to fix atmospheric nitrogen. The functional role of GmAMT2;1 will be tested against other AMT and AMF genes found expressed in nitrogen fixing nodules.