the vasculature. Again, we are still ignorant of the identity of the initial mechanosensor and
associated signaling events that should be coupled to such a hydraulic signaling system.
One general theme emerging from all these studies on thigmonastic responses is that
the most rapid changes monitored upon mechanical stimulation are generally the electri-
cal changes thought to transmit the touch information to the motor response. These elec-
trical changes reflect ion transport phenomena at the plasma membrane, highlighting the
central role of ionic signaling in the early phase of a plant mechanoresponse.
5.2.2 Thigmomorphogenesis and thigmotropism
Even in plants with no obvious specialized mechanosensory structures, mechanical per-
turbation leads to morphogenetic changes that are highly adaptive. Thus, in shoots, me-
chanical stimulation causes inhibition of elongation and radial swelling (Biddington
1986; Telewski and Jaffe 1986). Such changes can be local; for example, trees will
strengthen regions of the trunk or branches undergoing compression or tension with the
production of reaction wood (compression and tension wood; Hellgren et al. 2004). These
modified areas are rich in cells exhibiting strengthened, often highly lignified cell walls
to reinforce mechanically stressed tissues.
However, changes in growth habit upon localized mechanical perturbation can also be
systemic (Biddington 1986; Coutand et al. 2000), suggesting that a mobile signal is inte-
grating the overall thigmomorphogenetic response. Although mechanosensitive channels,
Ca2+-dependent signaling cascades (see below), ethylene, and perhaps GA (Mitchell
1996) are strong candidates for components of the mechanosensory signal transduction
system regulating and integrating these changes in growth, we still have remarkably lit-
tle molecular data on how these thigmomorphogenetic responses occur.
In addition to this general change in form upon mechanical stimulation, plants also
show highly oriented changes in growth where the direction of response is determined by
the direction of the stimulus (i.e., a thigmotropic response). Thigmotropism can be seen
in many parts of the plant but is perhaps most familiar in the specialized touch-
responsive organs known as tendrils. Many plants use leaves or shoots modified into ten-
drils to secure themselves to supports to allow increased height without the need for ex-
tensive deposition of metabolically expensive strengthening agents such as lignin. These
tendrils are highly touch-sensitive, responding to stimuli of as little as 250 μg (Simons
1992). Upon sustained mechanical stimulation they rapidly (often within seconds) begin
to exhibit differential growth across the organ, leading to coiling around the contacted ob-
ject (Jaffe and Galston 1968). In Bryonia dioica,octadecanoids and auxin seem to regu-
late this cell expansion (Stelmach et al. 1999). The direction of coiling is often deter-
mined by the direction of the mechanical stimulus, leading to a thigmotropic response of
the organ. However, thigmotropism is not limited to such highly specialized touch-
sensitive organs. For example, thigmotropism is also exhibited by roots growing into ob-
stacles in the soil, a response we will discuss in more detail later in this chapter.
Again, although there is an extensive literature describing the physiology and develop-
ment of the growth response in tendrils and roots, we are largely ignorant of the initial
mechanosensory events making these organs so responsive to touch stimulation. How-
ever, there are many clues from other organisms as to the basic features that we should