References

398 J.D.Rhodes,J.F.Thain,D.C.Wildon
We obtained intracellular electrical recordings from the epidermal cells,
cortical cells, phloem parenchyma cells, vascular parenchyma cells, and the
STE/CC complex. The identification of the various cell types was achieved
by iontophoretic injection of LY at the end of electrical recording. Figure
26.3 shows three of the cell types, and electrical recordings from these
cell types: an epidermal cell (Fig. 26.3a,b), a cortical cell (Fig. 26.3c,d) and
a STE/CC (Fig. 26.3e,f). Results from the other two cell types (phloem
parenchyma cells, vascular parenchyma cells) were similar to those obtained
from the cortical cells.
Only the phloem STE/CC complexes showed large electrical responses to
wounding. These were large ‘spike’ depolarisations, which were very similar
in shape and duration to known plant action potentials. The other four
cell types showed only small depolarisations of longer duration, usually
(except in the case of epidermal cells) with a small initial spike as is seen in
thecorticalcellrecordofFig.26.3d.Itispossiblethatthesmallinitialspikes
seen with these other cell types are due to current from the large STE/CC
spikes dissipating through surrounding cells via limited plasmodesmatal
connections; visual comparison with the surface electrode and bath electrodetracessuggeststhatthesmallspikesapproximatelycoincideintime
with the large STE/CC ones.
The large spike depolarisations seen in the STE/CC complexes are the
main electrical events in the petiole following severe wounding. The origin
of these spikes is not known. From their shape and duration they could be
action potentials propagating along the STE/CC pathway. If they are action
potentials,thenpresumablytheyhaveafunction,butitwouldappearnotto
be the systemic signal for PI synthesis (Table 26.1). Alternatively they could
be local responses to chemicals, as yet unidentified, but possibly oligosaccharides
or systemin, carried in the xylem by hydraulic dispersal from the
wound site. This latter possibility would lead to the following conclusions:
firstly that, of all the cell types in the petiole, only the STE/CC complexes
are sensitive to these chemicals; secondly that the STE/CC complexes have
a mechanism for rapidly restoring their membrane potential to its normal
value, presumably to protect them from the damaging effects of the large
depolarisation. Without more detailed knowledge of the membrane events
that cause these spikes, it is not possible to decide whether they are true
action potentials.
While each experiment using intracellular electrodes allowed us to
record the electrical events in only one STE/CC complex, these large depolarisations
must have been happening in many of the STE/CC complexes in
thepetiole,eithersimultaneouslyoratslightlydifferenttimes.Therecords
from the surface and bath electrodes represent the summation of all the
membrane currents flowing in the underlying tissues but, as noted before,
these are primarily in the phloem STE/CC complexes. The shapes and du-