Feedback Regulation of the Ammonium Transporter Gene Family AMT1 by Glutamine in Rice

The three members of the rice OsAMT1 gene family of ammonium transporters show distinct expression patterns; constitutive and ammonium-promoted expression in shoots and roots for OsAMT1;1; root-specific and ammonium-inducible expression for OsAMT1;2; root-specific and nitrogen-repressible expression for OsAMT1;3 [Sonoda et al. (2003), Plant Cell Physiol. 44: 726]. To clarify the feedback mechanisms, and to identify regulatory factors of the OsAMT1 genes, the accumulation of the three mRNAs and its dependence on endogenous nitrogen compounds (as quantified by capillary electrophoresis) was studied. Ammonium application to roots following a period of nitrogen starvation induced accumulation of OsAMT1;1 and OsAMT1;2 mRNA, but a decrease of OsAMT1;3 mRNA levels. The expression patterns of the three genes showed good correlation (positive in OsAMT1;1 and OsAMT1;2, negative in OsAMT1;3) with the root tissue contents of glutamine but not of ammonium. The ammonium effects on OsAMT1 expression were prevented by methionine sulfoximine, an inhibitor of glutamine synthetase. Moreover, glutamine had the same effect on transcriptional regulation of OsAMT1 genes as ammonium, indicating that glutamine rather than ammonium controls the expression of ammonium transporter genes in rice. These results imply that rice possesses unique mechanisms of adaptation to variable nitrogen sources in the soil.