Figure 10.
Model of Early Signaling Events upon Mechanical Stimulation.
Plants can be mechanically perturbed exogenously, for example, during wind stress and by obstacles in the soil, but plants also generate mechanical stresses internally, for example, as tissue tension or during cell elongation. Perception of these stresses likely occurs through sensing the deformation of the cell wall and appressed plasma membrane via activation of mechanosensors, such as stretch-gated Ca2+ permeable channels (1) and/or putative cell wall integrity sensors (2). This activation leads directly (3) or indirectly (4) to an influx of Ca2+ into the cytoplasm. Within the cytoplasm, the Ca2+ signal is amplified, for example, through Ca2+-dependent mobilization of Ca2+ from internal stores (5). At the plasma membrane, elevated Ca2+ levels activate plasma membrane NADPH oxidases through the Ca2+ binding EF hand motifs, resulting in NADPH oxidation (6) and extracellular superoxide (ROS) production (7). In the cell wall, superoxide is dismutated to H2O2 (8), which can easily diffuse back into the cytoplasm. Elevated Ca2+ levels also lead to cytosolic acidification and cell wall alkalinization: both Ca2+-dependent activation of NADPH oxidation (6) and inhibition of plasma membrane H+ ATPases (9) contribute to this pH change; however, the rapidity and magnitude of the pH changes suggest the involvement of other transporters, such as unselective cation or anion channels (10), which may mediate H+ or OH− fluxes to quickly alter extra- and intracellular pH. Cytosolic ROS, acidification, and Ca2+ increase are all known to elicit signaling events. Similarly, elevated wall pH and ROS are known to rigidify the cell wall matrix. Intriguingly, similar signaling cascades have also been described during defense responses (Garcia-Brugger et al., 2006), suggesting that this suite of coupled changes in cytosolic and wall environment (alterations in ROS, pH, and Ca2+) represents a fundamental response cassette that is used by plants to protect themselves against a range of environmental insults but has also been recruited to monitor and regulate cellular expansion. ER, endoplasmic reticulum; V, vacuole.