PLANT PHYSIOLOGY , Vol 103, Issue 2 351-357, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Determination of Auxin-Dependent pH Changes in Coleoptile Cell Walls by a Null-Point Method
P. Schopfer
Biologisches Institut II der Universitat, Schanzlestrasse 1, D-79104 Freiburg, Germany
The present debate on the validity of the "acid-growth theory" of auxin
(indole-3-acetic acid, IAA) action concentrates on the question of whether
IAA-induced proton excretion into the cell wall is quantitatively
sufficient to provide the shift in pH that is required to explain
IAA-induced growth (see D.L. Rayle, R.E. Cleland [1992] Plant Physiol
99:1271-1274 for a recent apologetic review of the acid-growth theory). In
the present paper a null-point method has been employed for determining the
growth-effective cell-wall pH in the presence and absence of IAA after 60
min of treatment. Elongation of abraded maize (Zea mays L.) and oat (Avena
sativa L.) coleoptile segments was measured with the high resolution of a
displacement transducer. The abrasion method employed for rendering the
outer epidermal cell wall permeable for buffer ions was checked with a
dye-uptake method. Evidence is provided demonstrating that externally
applied solutes rapidly and homogeneously penetrate into the epidermal
wall, whereas penetration into the inner tissue walls is strongly retarded.
"Titration" curves of IAA-induced and basal elongation were determined by
measuring the promoting/inhibiting effect of medium pH under iso-osmotic
conditions in the range of pH 4.5 to 6.0. In maize, the null point (no
pH-dependent change in elongation rate after 5-10 min of treatment with 10
mmol L-1 citrate buffer) was pH 5.00 after 60 min of IAA-induced growth,
and the null-point pH determined similarly in IAA-depleted tissue (10 times
smaller elongation rate) was 5.25. Corresponding titration curves with
Avena segments led to slightly lower null-point pH values both in the
presence and absence of IAA-induced growth. After induction of
acid-mediated extension by 1 [mu]mol L-1 fusicoccin (FC) in maize, the
null-point pH shifted to 3.9. At 0.5 [mu]mol L-1, FC induced the same
elongation rate as IAA but a 9-fold larger rate of proton excretion. At
0.033 [mu]mol L-1, FC induced the same rate of proton excretion as IAA but
had no appreciable effect on elongation. The implications of these results
against the background of recent attempts to revitalize the acid-growth
theory of IAA action are discussed.
