Plant Physiol, April 2001, Vol. 125, pp. 1743-1753

Real Time Visualization of ¹³N-Translocation in Rice under Different Environmental Conditions Using Positron Emitting Tracer Imaging System¹

Department of Applied Biological Chemistry, The University of Tokyo, 1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan (S.K., H.N., Sa.M.); Central Research Laboratory, Hamamatsu Photonics K. K., Shizuoka 434-8601, Japan (H.U., A.T., S.N., M.F., H.T.); Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Gunma 370-1292, Japan (N.S.I., S.W., T.I., C.M., A.O., Sh.M., S.H., T.S.); and Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, 2-1-6 Sengen, Tsukuba 305-0047, Japan (Sa.M.)

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 ¹³N-labeled ammonium (¹³NH₄⁺) and ¹⁵O-water to monitor the movement. In plants cultured under normal conditions, ¹³NH₄⁺ supplied to roots was taken up, and a ¹³N signal was detected at the discrimination center, the basal part of the shoots, within 2 minutes. This rapid translocation of ¹³N was almost completely inhibited by a glutamine synthetase inhibitor, methionine sulfoximine. In general, nitrogen deficiency enhanced ¹³N translocation to the discrimination center. In the dark, ¹³N translocation to the discrimination center was suppressed to 40% of control levels, whereas ¹⁵O-water flow from the root to the discrimination center stopped completely in the dark. In abscisic acid-treated rice, ¹³N translocation to the discrimination center was doubled, whereas translocation to leaves decreased to 40% of control levels. Pretreatment with NO₃ for 36 hours increased ¹³N 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 NH₄⁺-transporter(s) or glutamine synthetase(s).