PLANT PHYSIOLOGY , Vol 113, Issue 2 451-461, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Differential Ammonia-Elicited Changes of Cytosolic pH in Root Hair Cells of Rice and Maize as Monitored by 2[prime],7[prime]-bis-(2-Carboxyethyl)-5 (and -6)-Carboxyfluorescein-Fluorescence Ratio
H. Kosegarten, F. Grolig, J. Wieneke, G. Wilson and B. Hoffmann
Institut fur Pflanzenernahrung, Sudanlage 6, Justus-Liebig-Universitat, D-35390 Giessen, Germany (H.K., G.W.)
Intact hair cells of young rice (Oryza sativa L.) and maize roots (Zea mays
L.), grown without external nitrogen, were specifically loaded with
2[prime],7[prime]-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein
acetoxymethyl ester to monitor fluorescence ratio cytosolic pH changes in
response to external ammonia (NH4+/NH3) application. In neutral media,
cytosolic pH of root hairs was 7.15 [plus or minus] 0.13 (O. sativa) and
7.08 [plus or minus] 0.11 (Z. mays). Application of 2 mM ammonia at
external pH 7.0 caused a transient cytosolic alkalization (7.5 [plus or
minus] 0.15 in rice; 7.23 [plus or minus] 0.13 in maize). Alkalization
increased with an increase of external pH; no pH changes occurred at
external pH 5.0. The influx of 13N-labeled ammonia in both plant species
did not differ between external pH 5.0 and 7.0 but increased significantly
with higher pH. Pretreatment with 1 mM 1-methionine sulfoximine
significantly reduced the ammonia-elicited pH increase in rice but not in
maize. Application of 2 mM methylammonia only caused a cytosolic pH
increase at high external pH; the increase in both species compared with
the ammonia-elicited alkalization in 1-methionine sulfoximine-treated
roots. The differential effects indicate that cytosolic alkalization
derived from (a) NH3 protonation after passive permeation of the plasma
membrane and, particularly in rice, (b) additional proton consumption via
the glutamine synthetase/glutamate synthase cycle.
