be activated by mechanical force transmitted through their linkage to the cytoskeleton
(e.g., TRPA1) (Lin and Corey 2005) and/or the ECM (e.g., MEC channel complex; see
below), though the precise mechanism of gating is not yet fully understood.
In plant cells, any force large enough to overcome turgor and flex load-bearing ele-
ments of the cell wall and alter cellular shape will also deform the cytoskeleton. However,
some plant organs are known to be extraordinarily sensitive to mechanical stimuli, re-
sponding to as little as a 10-μN weight (~1 mg) with altered growth (Jaffe and Galston
1968). Such weak forces are unlikely to change cell shape but may conceivably deform
non-load-bearing elements embedded in the wall matrix, such as cell wall proteins.If
these elements are linked via transmembrane proteins to the cytoskeleton, force may be
transmitted from the cell exterior not only to plasma membrane mechanosensors, but—
because the cytoskeleton is ideally suited to long-distance transfer of stresses—to
mechanosensitive proteins in endomembranes as well (Ingber 2003, 2006).
Most research has focused on ion channels as the principal transducers/sensors of me-
chanical stress because of their capacity to rapidly amplify a signal. Such amplification
occurs because a single channel can transport millions of ions in seconds, altering the
electrical (membrane potential) and chemical (ion concentration) cellular environment.
However, there is increasing evidence for a role of nonchannel proteins in mechanosens-
CHAPTER 5 TOUCH SENSING AND THIGMOTROPISM 97
Figure 5.3. Mechanical force may elicit biochemical changes through inducing conformational changes in
target proteins. Gating may be directly, through force transmissive linkages (e.g., in the “trap door” model
where mechanical force directly opens a gating domain in the channel) (a), or more indirectly though other
signaling proteins or cytoskeletal interactions. Shear at the membrane surface may drag channels relative to
their tethers, with the stretching of the tether providing the mechanical gating force (b). Similarly, a separate
mechanoresponsive protein may trigger conformational change and gating in the channel (c).
Mechanotransduction does not a priorirequire channels; for example, a mechanosensor may induce struc-
tural changes in proteins bringing enzyme and substrate into close proximity (d), or revealing binding sites
on target transduction elements. Force may also act to unravel a protein, domain-by-domain, revealing new
activation or binding sites (e).