Agriculture Reference
In-Depth Information
wounding; see references in the review by Stratmann (2003). Following
Baydoun and Fry (1985), it has generally been assumed that oligosaccha-
rides act only locally, but the later work of Rigby et al. (1994) showed
translocation in the phloem of an oligogalacturonide that induced PI syn-
thesis. Thus, oligosaccharides remain as possible candidates for a systemic
signal. Both oligosaccharides (Thain et al. 1995) and systemin (Moyen and
Johannes 1996) produce transient membrane depolarisation, which could
be a factor in the electrical activity associated with severe wounds.
Possible physical signals include a hydraulic pressure wave that arises
when a wound releases the tension in the xylem of a transpiring plant. Such
apressurewavepropagatesthroughtheplantfromthewoundsiteathigh
speeds, 100 mm s −1 or more; see the review by Malone (1996). A pressure
wave could bring about its effects either by the activation of mechano-
sensitive (stretch-activated) channels (Stankovic and Davies 1997) or by
the effect of signal molecules. Malone and co-workers showed, in tomato,
that pressure pulses could be induced without significant wounding by
submerged excision of a single leaflet through its submerged petiole; such
a pressure pulse did not induce PI synthesis in tomato leaves. Instead,
release of tension in the xylem sets up a reverse flow of xylem sap from
a wound site into the rest of the plant, a phenomenon which they termed
hydraulic dispersal ;whenthisflowcarriedsapfromwoundedleaftissue
(e.g. a heat wound), systemic PI synthesis ensued, presumably owing to the
presence of chemical elicitors in the sap.
Wounding also gives rise to electrical events that have usually been de-
tected with surface-contact electrodes, and that appear to move through the
plantfromthewoundsite.Theseobservationshaveledtothesuggestion,
e.g. Wildon et al. (1992), that the systemic signal for induction of PI syn-
thesis is an electrical one, i.e. a self-propagating action potential similar to
those in nervous conduction or in animal epithelial conduction: cell-to-cell
conduction of action potentials (Mackie 1965). If that were true it should be
possible to initiate the signal by depolarisation of the membrane potential
of cells in the signalling pathway, preferably by injection of current via
intracellular microelectrodes. It would also be expected that the electrical
signals would show other characteristics typical of action potentials such
as the all-or-nothing response, a strength-duration relationship for the
initiating current, and a refractory period.
Evidence for the occurrence of action potentials in plants has been re-
viewed by Thain and Wildon (1996). There is clear evidence for action
potentials in the charophyte algae, and in a number of higher plant species,
which suggests that the ability to generate action potentials is widespread
in the plant kingdom. Electrical stimulation has been used to induce PI
gene expression (Herde et al. 1995; Peña-Cortès et al. 1995; Stankovic and
Davies 1996, 1997), although intracellular electrodes were not used either
 
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