Agriculture Reference
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longitudinal connectivity, a functional plasma membrane, and low lateral
connectivity with surrounding tissues (van Bel 2003). However the phe-
nomenon of epithelial conduction in animals shows that action potentials
can propagate through tissue consisting of relatively unspecialised cells,
so it is not impossible that action potentials could propagate along other
tissues, as long as they have adequate electrical connectivity via plasmod-
esmata.
We now consider the possibilities for signals and signalling pathways
mainly in the light of results from our laboratory.
26.2
Patterns of Proteinase Inhibitor Activity
and Electrical Activity Following a Variety
of Wounding Protocols Applied to Tomato Seedlings
In our work (Rhodes et al. 1999) we used five different wounding protocols:
(1)arazorcutthroughacotyledoninair;(2)arazorcutsimultaneously
through a cotyledon and a droplet of Lucifer Yellow CH (LY) solution
placed on its surface (i.e. cut under water); (3) a small (area about 60 mm
2
)
mechanical wound; (4) a large (area about 150 mm
2
) mechanical wound;
(5) a heat wound using a hot spatula held under, but not touching, the
lamina. Wound-type 3 is referred to as a minor wound; wound-types 4
and5arereferredtoasseverewounds.Forallthewoundingprotocols,we
usedsurfaceelectrodesonleaves1and2(Fig.26.1)tocheckforelectrical
activity. Significant results are summarised in Table 26.1.
A razor cut through the lamina of a cotyledon in air (wound-type 1)
produced neither electrical activity at leaf 1, nor systemic induction of
PI activity (Table 26.1), as would be expected for a wound type in which
neither a hydraulic pressure wave, nor hydraulic dispersal occurs.
A razor cut through a droplet of water on the lamina of a cotyledon
(wound-type 2) induced systemic PI activity, but did not result in any
electrical activity (Table 26.1), showing that electrical activity is not caused
by reversed xylem flow alone. The pattern of induction of PI activity was
striking: when cotyledon 1 (see Fig. 26.1, legend) was cut, more PI activity
was seen in the right side of leaf 1 and the left side of leaf 2 (Fig. 26.1:
low
values indicate
high
PI activity). The pattern was reversed when cotyledon 2
was cut. To trace the pattern of flow in the xylem, the razor cut was made
throughadropletofLYsolutiononthesurfaceofthecotyledon.Theflow
of dye was observed in real time, and travelled with an average speed of
5mms
−1
; petiole sections confirmed that the flow was in the xylem. The
distribution of LY in leaves 1 and 2 depended on which cotyledon was cut:
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