5 Touch Sensing and Thigmotropism
Gabriele B. Monshausen, Sarah J. Swanson, and Simon Gilroy*915.1 Introduction
The sessile nature of the plant lifestyle requires exquisite sensitivity to environmental sig-
nals. These stimuli provide the information that controls much of plant behavior, ranging
from decisions about when to grow or reproduce to whether to mount a defense response
against a pathogen. The directional cue of gravity is clearly a central component of this
wealth of information that regulates normal plant development. However, other mechan-
ical stimuli, ranging from the buffeting by wind and rain, impedance of the soil, and even
the weight of an organ itself provide similarly important information that governs plant
morphogenesis (Figure 5.1).
Indeed, plants are highly sensitive to mechanical cues in their environment. This is per-
haps most obvious when seeing the rapid mechanoresponse in organs specialized for
touch sensing, such as the closing of the Venus fly trap (Dionaea muscipula) triggered by
the mechanical stimulus of an insect alighting on the leaf, or the twining of a tendril in
response to the touch signal from contacting a support. However, plants exhibit many and
varied responses to touch. In general, plants grown with mechanical stimulation develop
a shorter stature, more robust and stronger support tissues, and altered organ architecture
and growth habit (Braam 2005 and references therein). Similarly, the directional cues of-
fered by mechanical stimulation (be it touch or gravity) lead to highly controlled direc-
tional growth responses manifested as thigmo- and gravitropism. Considering the physi-
cal nature of both the gravity and touch stimulus, it seems likely that they share common
mechanotransduction elements. It has even been proposed that gravity sensing is derived
from an ancestral touch perception apparatus (Trewavas and Knight 1994).
In this chapter we will therefore describe some of the broad classes of mechanore-
sponse seen in plants, discuss some of the models for how mechanosensing is likely op-
erating at a cell and molecular level, and then ask how the plant integrates multiple stim-
uli, in this case touch and gravity, to generate the appropriate tropic response.
5.2 Plant mechanoresponses
Plant mechanoresponses can be divided into two broad categories: those associated with
highly specialized mechanosensory organs such as tendrils or the traps of carnivorous
plants, and those reflecting a more ubiquitous mechanosensory system that seems to af-
fect most parts of the plant. The specialized touch sensory systems have clearly evolved
to trigger a single specialized response, such as the twining of a tendril. The more “gen-
eral” touch sensitivity, however, may well relate to the systems required to monitor and
control the mechanical stresses inherent in normal turgor-driven cell expansion.
*Corresponding author