References

400 J.D.Rhodes,J.F.Thain,D.C.Wildon
rations of the electrical events recorded by the surface and bath electrodes
would normally lead to their classification as variation potentials with the
implication that they reflect depolarisations in the underlying tissues due
to chemicals that have travelled in the xylem from the wound site. However,
given our current ignorance of detailed mechanisms at the cell level,
they could reflect the summation of action potentials travelling at slightly
different times in the different STE/CC complexes of the petiole.
26.3
Conclusions and Future Prospects
For severe wounding, the results are consistent with the distribution of
elicitors of PI synthesis and electrical activity by hydraulic dispersal in the
xylem. We conclude that the electrical events are likely to be responses to
chemicals transported in the xylem by hydraulic dispersal from the wound
site, rather than action potentials propagated from the wound site.
Following a small crushing wound, the pattern of systemic PI synthesis
was consistent with the transport of a chemical elicitor in the phloem from
the wound site.
The lack of an electrical event following a small wound indicates that
electrical signals are not an essential part of the systemic signalling system
that induces PI synthesis.
The fact that a severe wound leads to large rapid action-potential-like
depolarisations in the cells of the STE/CC complexes in the petiole, but not
in other cell types, is intriguing. Do these depolarisations have a function?
In the absence of detailed studies at the cell level, the description of
wound-induced electrical events recorded with surface-contact electrodes
as either variation potentials or action potentials could carry misleading
implications about their cellular mechanisms.
A significant step forward in plant signalling would be the identification
of the cellular mechanism of wound-induced electrical events; this could be
achieved using a similar approach to that described here and in Rhodes et
al. (1996). However, since the mapping of its genome, Arabidopsis may be
the preferred model organism; Arabidopsis does show electrical events following
severe wounds (our unpublished observations). The identification
of the ion channels responsible for the electrical events could be studied by
the use of a range of agonists and inhibitors which could be introduced via
the xylem stream and their electrical consequences in the phloem could be
monitored using microelectrodes.
Acknowledgements. The work from our laboratory was funded by the
Biotechnology and Biological Sciences Research Council, UK.