H. Frohnmeyer et al., A ROLE FOR THE VACUOLE IN AUXIN-MEDIATED CONTROL OF CYTOSOLIC PH BY VICIA MESOPHYLL AND GUARD-CELLS, Plant journal, 13(1), 1998, pp. 109-116
A role for cytosolic pH (pH(i)) in hormonal signalling and transport c
ontrol in plants has long been mooted. Yet, while changes in pH(i) are
a common consequence of hormonal stimuli in plant cells and contribut
e to hormonally evoked ion channel control, the origins of these chang
es remain unknown. To examine a possible role for the tonoplast and va
cuolar compartment in these events, pH(i) was measured in the presence
of auxins and during cytosolic H+ loading with weak acid in vacuolate
and evacuolate protoplasts, both from mesophyll and guard cells of Vi
cia faba L. Evacuolate protoplasts were obtained following ultracentri
fugation on Percoll gradients, and pH(i) of single protoplasts was rec
orded in both vacuolate and evacuolate! preparations using fluorescenc
e ratio microphotometry and the pH-sensitive dye BCECF. External pH me
asurements indicated a roughly twofold increase in the rate of net Hsecretion in evacuolate compared with vacuolate protoplasts, and showe
d that evacuolate protoplasts retained the characteristic stimulation
of H+ secretion in the presence of auxin. BCECF fluorescence recording
gave resting pH(i) values near 7.5, and evacuolation had no significa
nt effect on this parameter. Reversible decreases of 0.1-0.2 units in
pH(i) were evoked in vacuolate protoplasts by 10 mu M concentrations o
f the auxins 1-naphthalene acetic acid and 3-indoyl-acetic acid, and n
ot by the inactive (anti-auxin) analogue 2-naphthalene-acetic acid. Ho
wever, auxin treatments failed to evoke a change in pH(i) in all but o
ne of 12 experiments with evacuolate protoplasts. Evacuolation also ap
peared to reduce the transient, dynamic H+ buffering capacity of the p
rotoplasts in the face of acid pH(i) loads imposed by adding Na+-butyr
ate to the bath. These results implicate the tonoplast or vacuolar com
partment in short-term pH(i) homeostasis and generation of hormonally
evoked H+ signalling in plant cells; they also conform with the view t
hat the decrease in pH(i) per se is not a primary determinant in the s
timulation of H+ secretion by auxin.