While not as persistent as gravity, plants regularly encounter touch stimuli in their environment. For twining plants, the presence of a solid support induces differential growth of the tendrils, resulting in the familiar coiling response and the association with the support. For roots, touch sensing takes the form of avoiding obstacles in the path of elongation, resulting in the redirection of the root around the object. For stems, touch sensing may mean the perception of wind and other forces that shake the stem, leading to its increased radial thickening and rigidity. Without this rigidification in response to mechanical stimuli, plants would probably collapse under their own weight.
Touch stimulation results in both rapid changes within cells and longer-lived changes in development. One of the first events observed after a touch stimulus is an increase in intracellular calcium ions (Ca2+). The release of Ca2+ is probably linked to the temporary mechanical deformation of the plasma membrane, which contains mechanosensitive ion channels. One popular model holds that these channels open upon a mechanical stimulus, thus allowing the influx of Ca2+ leading to the elevation of intracellular concentrations.
The elevated Ca2+ concentration within the cell induces two other rapid changes in response to touch. High Ca2+ inhibits the plasma membrane H+-ATPase and probably also stimulates the opening of inward rectifying H+ channels, both of which drive an increase in intracellular H+ levels and an increasing alkalinization of the apoplast. Elevated Ca2+ also stimulates the production of reactive oxygen species outside the cell by activating an enzyme in the plasma membrane. Both ROS and alkalinization of the apoplast are likely associated with increasing the rigidity of the cell wall as a result of mechanical stimulation. Interestingly, both the acidification of the cytoplasm and ROS production can be induced by artificially increasing Ca2+ levels.
Longer-term responses to touch involve the synthesis of the gaseous plant hormone ethylene, as well as the upregulation of a variety of genes, including the TCH genes. Several of the TCH genes encode Ca2+ binding proteins such as calmodulins or proteins like calmodulins, which bind Ca2+ and mediate downstream responses. Another of the TCH genes (TCH4) encodes a xyloglucan endotransglycosylase, an enzyme involved in modifying cell wall polysaccharides.
Monshausen GB, Bibikova TN, Weisenseel MH, Gilroy S (2009). Ca2+ regulates reactive oxygen species production and pH during mechanosensing in Arabidopsis roots. The Plant Cell 21: 2341–2356