The rice OsAMT1;1 is a proton-independent feedback regulated ammonium transporter.

KEY MESSAGE: Functional identification of a relatively lower affinity ammonium transporter, OsAMT1;1, which is a proton-independent feedback regulated ammonium transporter in rice. Rice genome contains at least 12 ammonium transporters, though their functionality has not been clearly resolved. Here, we demonstrate the functional properties of OsAMT1;1 applying functional complementation and (15)NH4 (+) uptake determination in yeast cells in combination with electrophysiological measurements in Xenopus oocytes. Our results show that OsAMT1;1 is a NH4 (+) transporter with relatively lower affinity to NH4 (+) (110-129 μM in oocytes and yeast cells, respectively). Under our experimental conditions, OsAMT1;1-mediated NH4 (+) uptake or current is not significantly modulated by extra- or intracellular pH gradient, suggesting that this transporter probably functions as a NH4 (+) uniporter. Inhibition of yeast growth or currents elicited from oocytes by ammonium assimilation inhibitor L-methionine sulfoximine indicates that NH4 (+) transport by OsAMT1;1 is likely feedback regulated by accumulation of the substrate. In addition, effects of phosphorylation inhibitors imply that NH4 (+) uptake by OsAMT1;1 is also modulated by tyrosine-specific protein kinase or calcium-regulated serine/threonine-specific protein phosphatase involved phosphorylation processes.