PLANT PHYSIOLOGY , Vol 105, Issue 1 435-442, Copyright © 1994 by American Society of Plant Biologists
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CELL BIOLOGY AND SIGNAL TRANSDUCTION |
Methylammonium as a Transport Analog for Ammonium in Tomato (Lycopersicon esculentum L.)
K. R. Kosola and A. J. Bloom
Vegetable Crops Department, University of California, Davis, Davis, California 95616-8746
Methylammonium (CH3NH3+) has been widely used as an analog of ammonium
(NH4+) for examining transport in bacteria and fungi. We compared the
kinetics of root CH3NH3+ and NH4+ uptake from solution culture in intact
tomato (Lycopersicon esculentum cv T5) plants. Efflux of NH4+ and CH3NH3+
was negligible. The apparent maximum rate of absorption (apparent Vmax) was
similar for NH4+ and CH3NH3+, but the apparent affinity (apparent Km) was
about 10-fold greater for NH4+ than for CH3NH3+. In characterizing the
interaction between NH4+ and CH3NH3+ transport, we used [15N]NH4+ and
[14C]CH3NH3+ as well as improved methods for analysis of nonisotopic
CH3NH3+ and NH4+. CH3NH3+ acted as an inhibitor of NH4+ influx. Relatively
low concentrations of NH4+ strongly inhibited CH3NH3+ influx. Treatments
with 1 mM methionine sulfoximine that blocked NH4+ assimilation had little
influence on NH4+ inhibition of CH3NH3+ influx. These results suggest that
the two ions share a common transport system in tomato, but because this
transport system has a much greater affinity for NH4+, CH3NH3+ may be used
as a transport analog only when ambient concentrations of NH4+ are very
low.
