Quantitative characterization of nitrogen regulation of OsAMT1;1, OsAMT1;2, and OsAMT2;2 expression in rice seedlings

The effects of nitrogen forms and supply levels on rice ammonium transporter expression have previously been studied by Northern blot or semiquantitative PCR in a model rice Oryza sativa L., cv. Nipponbare. However, most ammonium transporters (AMT) have low abundance in rice, and it is difficult to accurately analyze their transcript levels. In this study, an analysis of the transcript levels of the OsAMT1;1, OsAMT1;2, and OsAM7-2;2 in rice seedlings, cv. Guidan 4 has been performed under various nitrogen conditions, using the technique of real-time quantitative reverse transcription polymerase chain reaction (RT-PCR). The OsAMT1;1 and OsAMT2;2 mRNA levels in nitrogen prestarved plants were induced by supply of NH4+, NH4NO3, and NO3-, while OsAMT1;2 mRNA in roots was repressed by the same treatment. Expression of OsAMT1;2 in nitrogen-starved rice shoots was induced steeply when rice plants were exposed to 0.5 mM NH4NO3 or 2 MM NH4+-N whereas both OsAMT1;1 and OsAMT2;2 exhibited only modest changes under same conditions. OsAMT1;1 and OsAMT1;2 were expressed preferentially in roots while OsAMT2;2 was expressed evenly in roots and shoots, suggesting that OsAMT2;2 may play a different physiological role in ammonium uptake. The upregulation effect of NH4NO3 on OsAMT1;1 mRNA in roots is between that of NO3- and NH+, and its induction effect on OsAMT2;2 mRNA level is higher than that of NO3- or NH+ in both roots and shoots. These three OsAMTs genes in the Guidan 4 rice cultivar show dissimilar transcriptional regulation by various levels of different nitrogen species, and the nitrogen-regulated expression patterns are different from those of the model rice cv. Nipponbare.