Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 104

induced generation of local, not spreading over long distances impulses (Prudnikov
et al. 2010 ). The pharmacological analysis showed the involvement of ionic chan-
nels in pulse generation. The first depolarizing stage of the pulse activity was sen-
sitive to inhibition of voltage gated and mechanosensitive calcium channels of
plasmalemma. Lantan, a non-specific Ca 2+ competitor for binding sites, suppressed
initial depolarization efficiently. Gadolinium, the inhibitor of mechanosensitive
Ca 2+ channels, and ruthenium red, inhibitor of Ca 2+ exit from intracellular depots,
and also verapamil inhibited this process. The action of these inhibitors indicates the
involvement of calcium penetration into the cytoplasm during initial steps of
depolarization. Calcium can entry from the apoplast (effects of lantan, gadolinium,
verapamil), as well as from intracellular depots (action of ruthenium red). These data
demonstrate clearly the role of calcium in the formation of local responses, primarily
at the stage of initial depolarization. The initial slow depolarization was followed by
a fast depolarizing shift, which was sensitive to the anion channel inhibitor
4-acetamido-4 0 -isothiocyano-stilbene-2,2 0 -dilsulfonic acid. At the next stage repo-
larization developed. The blockers of potassium channels, tetraethylammonium and
quinine sulfate, suppressed the second stage of the impulse, that means the partic-
ipation K + channels at stage of repolarization. Both stages of electric pulse gener-
ation were retarded by the H + -ATPase inhibitors, sodium orthovanadate and
dicyclohexylcarbodiimide, which implies the involvement of the proton pump in the
origin of electric pulses examined.
Pretreatment of intact leaf on plant with SA for 1 h changed parameters of
electric response to cooling of the leaf region (Fig. 4 ). At the concentration of
0.1 mM SA increased markedly the amplitude of response and lowered the speed
of initial depolarization possibly through the effect on the calcium entry. The
higher SA concentrations resulted in the stress response and a decrease in the rate
and amplitude of depolarization.
SA also affected the generation of another form of electric response to irritation,
e.g., the action potential (AP). As distinct from the local response, AP spreads over
the plant and can induce changes in distant plant parts. After Chara cell treatment
with SA, the shape of generated impulse changed (Fig. 5 ). The response of
Characea algae to excitation is much more rapid than the local response recorded
for the whole tissue. In our experiments Chara cells were not detected the initial
slow phase of depolarization. However, it was well notable that in the presence of
0.1 mM SA the shape of impulse changed: the phase of repolarization determined
by potassium outflow started later and its rate was lower. The inhibitor of potas-
sium channels TEA induced similar changes in repolarization phase, which con-
firms dependence of K + exit from the cell on SA. SA action is not explained by its
properties as an acid because another weak acid, acetate, did not change the shape
of the impulse. The inefficient, in defensive responses, SA analogue, 4-oxybenzoic
acid, was also inactive in the action on the shape of impulse. This suggests a
specificity of SA action.
In these experiments, tissue pretreatment with SA was used. In the suspension of
tobacco cells, during SA treatment the content of free Ca in the cytosol increased
rapidly, as early as in 10 s, whereas the highest increase was observed in 90 s.