Real Time Visualization of (13)N-Translocation in Rice under Different Environmental Conditions Using Positron Emitting Tracer Imaging System

The ammonium ion is an indispensable nitrogen source for crops, especially paddy rice (Oryza sativa L. cv Nipponbare). Until now, it has been impossible to measure ammonium uptake and nitrogen movement in plants in real time. Using the new technologies of PETIS (positron emitting tracer imaging system) and PMPS (positron multi-probe system), we were able to visualize the real time translocation of nitrogen and water in rice plants. We used positron-emitting (13)N-labeled ammonium ((13)NH(4)(+)) and (15)O-water to monitor the movement. In plants cultured under normal conditions, (13)NH(4)(+) supplied to roots was taken up, and a (13)N signal was detected at the discrimination center, the basal part of the shoots, within 2 minutes. This rapid translocation of (13)N was almost completely inhibited by a glutamine synthetase inhibitor, methionine sulfoximine. In general, nitrogen deficiency enhanced (13)N translocation to the discrimination center. In the dark, (13)N translocation to the discrimination center was suppressed to 40% of control levels, whereas (15)O-water flow from the root to the discrimination center stopped completely in the dark. In abscisic acid-treated rice, (13)N translocation to the discrimination center was doubled, whereas translocation to leaves decreased to 40% of control levels. Pretreatment with NO(3)(−) for 36 hours increased (13)N translocation from the roots to the discrimination center to 5 times of control levels. These results suggest that ammonium assimilation (from the roots to the discrimination center) depends passively on water flow, but actively on NH(4)(+)-transporter(s) or glutamine synthetase(s).