type. The high sensitivity of the surface cells of the root cap fits well with their position,
being the first cells to encounter obstacles as the root forces itself through the soil.
It is important to note that many other stimuli have been shown to elicit increases in
intracellular Ca2+, ranging from cold shock to symbiont elicitors (reviewed in
Hetherington and Brownlee 2004). These Ca2+changes often occur in exactly the same
cell as touch-induced Ca2+transients (Figure 5.5). One possible explanation for this re-
sponse to multiple stimuli is that Ca2+is simply acting as an “on switch” informing the
cell that something has happened, with the specificity of the response being encoded by
other signaling systems (Scrase-Field and Knight 2003; Plieth 2005). Alternatively, the
spatial and/or temporal footprint of the Ca2+change (the so-called Ca2+signature) may
encode information about the stimulus evoking the response.
There has long been literature supporting the informational content of Ca2+changes in
animal cells (Dolmetsch et al. 1997, 1998) and there is some evidence supporting a sim-
ilar phenomenon in plants. For example, in stomatal guard cells, Ca2+spiking is seen in
response to stimuli that induce closure of the stomatal aperture (McAinsh et al. 1995;
Allen et al. 1999). The frequency of spiking appears to be critical, with Ca2+transients
that occur either too frequently or too slowly being less effective in triggering the re-
sponse (Allen et al. 2001). These observations suggest that, at least in the guard cell, the
temporal character of a Ca2+increase is important for the response that is elicited.
Similarly, in the touch response the magnitude of Ca2+increase has been reported to cor-
relate with the magnitude of mechanical stimulation (Haley et al. 1995), consistent with
the idea that the Ca2+change could be carrying information about the kind of mechani-
cal stimulation that the cell is experiencing. However, we clearly need more detailed
analysis to distinguish a role for Ca2+in the “signature” versus “on switch” modes of ac-
tion in plants in general and the mechanoresponse in particular.
A role for Ca2+in signaling touch response appears to extend to the specialized touch-
sensitive systems such as the tendril described at the beginning of this chapter. For exam-
ple, a Gd3+-sensitive, voltage-dependent Ca2+release channel (BCC1) has been electro-
physiologically identified in ER isolated from the tendrils ofBryonia dioica(Klusener et
104 PLANT TROPISMS
Figure 5.5. Touch- and cold-induced calcium signatures in root cap cells. a. An outline of the cells in an
Arabidopsisroot tip; box indicates area of the root cap observed in (b) and (c). b. Changes in cytoplasmic
calcium in a root cap peripheral cell following a touch stimulus. c. Changes in cytoplasmic calcium in the
root cap following cold shock. In both (b) and (c), cells were expressing YC 3.6, a GFP-based calcium indi-
cator, and fluorescence was monitored using a Zeiss 510 confocal microscope. Scale bars = 10 microns.