expect from such a sensor. Therefore, in the next section we will discuss the principles of
mechanoperception that can be derived from the touch sensors seen across the kingdoms
and ask how well the molecular insight gained from these animals and bacteria might
translate to identification of a plant mechano- or gravisensory system.
5.3 General principles of touch perception
Any protein embedded in a membrane experiences mechanical force exerted by the lipid
bilayer. All bilayers have a characteristic lateral pressure profile, with outward (positive)
directed pressure in the hydrophobic core of the membrane and large tension at the
membrane–water interfaces, though the specific distribution and magnitude of pressures
vary considerably with lipid composition (Poolman et al. 2004) (Figure 5.2). In the rest-
ing state, a transmembrane protein adopts a conformation that is at equilibrium with the
surrounding mechanical forces of the lipid bilayer. However, when the membrane is
stretched or deformed by a mechanical stimulus, the distribution of forces is altered and
the equilibrium perturbed. In a mechanosensitive protein, this change in force is thought
to trigger a conformational change, which results in activation (or deactivation) of the
protein (Hamill and Martinac 2001; Janmey and Weitz 2004; Kung 2005) (Figure 5.2).
Though in-plane membrane forces may conceivably activate any number of transmem-
brane proteins, so far only the mechanosensitive ion channels of bacteria and the TRPC1
channel of Xenopusoocytes have been shown to be directly gated by membrane tension
(Maroto et al. 2005; Moe and Blount 2005) (see below). However, several other candi-
date mechanosensitive ion channels from animals and yeast have recently been identified
(TRPY1, TREK1 and 2) (Bang et al. 2000; Chemin et al. 2005; Zhou et al. 2005).
How relevant is this model of mechanoperception for plant cells where the flexion of
the plasma membrane must go hand-in-hand with the deformation of the cell wall? The
cell wall is under hydrostatic pressure from the protoplast, so that cell wall deformation
would require the application of an external force large enough to overcome turgor pres-
sure. Given that turgor is in the range of 1 to 40 bars (Tomos and Leigh 1999; Franks
2003), it may at first glance seem that only extreme forms of mechanical stimulation
would provide sufficient force to be detected via changes in membrane tension. However,
even subtle stresses such as a gentle breeze can make leaves or stems sway and such
organ bending can only occur when at least a subset of cells change shape (i.e., when the
cell wall and plasma membrane are deformed). This is possible because parts of the plant
act as levers. In this way, even very moderate mechanical forces can be sufficiently
amplified and focused onto the responding cells to exceed turgor pressure, flex the
plasma membrane, and thus directly gate transmembrane proteins via changes in lipid
force distribution.
A mechanosensitive protein can also be displaced within the pressure profile of the
membrane by tethering it to force-transmitting elements of the cytoskeleton or the extra-
cellular matrix (ECM). When these components are deformed during a mechanical stim-
ulation, they could pull on the attached transmembrane protein and move it relative to the
lipid bilayer (elevator model) (Kung 2005; Orr et al. 2006). Although the physical repo-
sitioning of the protein is thus achieved via a different mechanism than that described