5.4 Signal transduction in touch and gravity perception
5.4.1 Ionic signaling
From the above discussion, it is clear that a major theme emerging from our current un-
derstanding of mechanosensors in animals and microbes is that channels and ionic fluxes
play a central role in the transduction of external force to intracellular biochemical sig-
nal. There is also an increasing body of evidence that ionic signals are intimately associ-
ated with the touch and gravity responses in plants. Principal among these mechanically
related signals are changes in Ca2+and pH.
5.4.2 Ca2+signaling in the touch and gravity response
Changes in the levels of cytosolic Ca2+are recognized as a ubiquitous regulatory system
and are also among the most widely reported initial changes in response to mechanostim-
ulation (Gillespie and Walker 2001). For plants there is a wide body of literature support-
ing the idea that mechanical stimuli, ranging from wind and rain to localized mechano-
stimulation of a single cell, elicit complex patterns of Ca2+change. Most of these
measurements have been made noninvasively using transgenic plants expressing the lu-
minescent Ca2+-sensitive protein aequorin. Such analyses confirm touch-related Ca2+in-
creases in plants ranging from Arabidopsisand tobacco (Knight et al. 1991; Knight et al.
1992; Knight et al. 1993; Plieth and Trewavas 2002) to the mossPhyscomitrella patens
(Haley et al. 1995; Russell et al. 1996) and even the characean algae (Blancaflor and
Gilroy, unpublished), suggesting the presence of an ancient mechanosensory system.
Ca2+chelation attenuates the stunting of growth associated with thigmomorphogenesis
(Jones and Mitchell 1989), tentatively suggesting a functional role for such Ca2+changes.
The site of these Ca2+fluxes (trans-plasma membrane versus release from intracellu-
lar stores) remains to be unequivocally determined. Thus, Ca2+chelators and channel
blockers that should inhibit influx at the plasma membrane have been reported to either
block or fail to affect touch-induced Ca2+increases (Haley et al. 1995; Legue et al. 1997).
Further, evidence for release from intracellular stores comes largely from the ability of
ruthenium red to inhibit Ca2+changes (Knight et al. 1992; Legue et al. 1997). Ruthenium
red is thought to block channel-mediated Ca2+release from mitochondria and the ER
(Denton et al. 1980; Campbell 1983) but its action on Ca2+channels in plants is not well
characterized. Ruthenium red is, however, known to affect other plant processes; for ex-
ample, it binds strongly to unesterified pectins (Moffatt et al. 2002). In addition, in some
reports (Haley et al. 1995) no effect of ruthenium red was observed on the mechanically
induced Ca2+transients.
Despite this uncertainty as to its precise source, the mechanically induced Ca2+increase
has been confirmed in plants using sensors other than aequorin. Thus, plants expressing the
Ca2+-sensitive, green fluorescent, protein-based sensor cameleon (Allen et al. 1999) show
mechanically induced Ca2+transients (Figure 5.5 and Color Section), as do plants loaded
with the Ca2+-sensitive dye Indo-1 (Legue et al. 1997). In this latter study, the apical cells
of the root cap were found to be approximately twice as sensitive to touch stimulation as
those more basal to the tip, suggesting that touch sensitivity may vary dependent on cell