al. 2002; Muday and Murphy 2002; Friml 2003). Auxin plays an important role in tropic
response (see Chapter 3) and actin may affect polar auxin transport by changes in cyto-
plasmic pH. Actin may also operate through altering the distribution, targeting, and
turnover of auxin efflux and uptake transporters.
How might touch stimulation affect all these features of the gravity signaling system,
such as actin dynamics or pH fluxes? The touch-induced Ca2+increase described above
has the potential to directly affect many cellular activities, including actin structure
(Blancaflor 2002), proton pumping (Kinoshita et al. 1995), and auxin transport/signal
transduction (Benjamins et al. 2003). Thus, the ionic signaling associated with both touch
and gravity signaling may well form a nexus at which information from both systems is
incorporated to control pH and auxin flux and so generate an integrated tropic response.
5.7 Conclusion and Perspectives
It is clear that plants integrate a tremendous amount of environmental information to dic-
tate the appropriate growth response. Under laboratory settings, the gravitropic response
can represent an extremely powerful and often dominating influence on growth habit.
However, the integrative nature of plant signaling means that many other factors will
likely influence growth in the field. Thus, thigmotropic (this chapter) and hydrotropic
(Chapter 6) signals are known to modulate gravitropic response through reduction in
gravitropic sensitivity. For roots, where touch stimulation is likely almost constant, these
other stimuli may dominate, with gravitropism perhaps providing a default directional
cue to orient growth in the absence of other stimuli. Indeed, a putative gravitropic sensor
reported in the elongation zone of the maize root (Wolverton et al. 2002; also see Chapter
- could well reflect a mechanical strain sensor eliciting a thigmotropic response to the
stress from the mass of the unsupported root in these experiments.
Although much work has been directed to analysis of the interactions of gravitropism
with these other stimuli in primary roots, it will be very informative to understand how
signals such as touch interact with the mechanisms that define gravitational set-point
angles of lateral organs where growth is not simply directed straight up or down
(Chapter 2).
One other major question remaining about thigmotropic response is the molecular
identity of the plant mechanosensors(s). It seems highly likely that the touch receptor is
a mechanosensitive channel, and there is a wealth of electrophysiological data suggesting
that stretch-activated channels exist in the plant plasma membrane. However, apart from
theMSLgenes, there are no other clear homologs of mechanosensitive channels from
other kingdoms represented in the sequenced plant genomes. The clear challenge for
those working on touch and thigmotropic response is to robustly define the primary sen-
sor with molecular precision. At present, the MSL proteins hold great promise in defin-
ing putative mechanosensors, but we must await their eletrophysiological characteriza-
tion to know whether the elusive plant mechanosensors have truly been found. Once these
sensors are defined at the molecular level, we can anticipate an equivalent rapid increase
in understanding of plant mechanotransduction as that which accompanied the identifi-
cation of the Msc channels in bacteria or the Mec channel complex in C. elegans.