While brackish-water characean cells regulate their turgor pressure, fresh-water characean cells regulate their osmotic pressure instead (Kamiya and Kuroda 1956a). The transformation of the pressure signal into an electrical signal was studied further in Chara corallina by Shimmen (2003, 2006b). When the nodal cells are exposed to a sorbitol solution, their turgor pressure decreases quickly, while the turgor of the internode remains high. After a few seconds lag time, the membrane potential of the nodal region depolarizes. The pressure difference produced between the internodal cell and nodal cells causes a bending of the cell wall of the terminal end of the internodal cell (Shimmen 2003). The bending of the wall may activate the stretch-activated ion channels of the plasma membrane at the end of the internodal cell facing the node (Shimmen 2003).
Characeae algae provide a simple cell system for studying the wounding response in plants. A specimen consisting of two adjoining internodes with a node between them can be prepared. When one internode (victim cell) is cut, a very rapid depolarization is induced at the nodal end of the intact neighboring healthy cell (receptor cell). This is followed by a long-lasting depolarization, which is known as the receptor potential. The receptor potential often induces propagating action potentials that result from the activation of voltage-activated ion channels. The long-lasting receptor potential is induced by K+ ions released from the victim cell. The amplitude of the receptor potential decreases when the cell turgor is experimentally reduced. This indicates that the stretching of the plasma membrane of the nodal part is involved in the depolarization (Shimmen 2006a). Here again, the nodal complex plays a central role in generation of the wounding signal (Shimmen 2006b).
The nodal region of the internodal cell is electrically distinct from the flank of the cell. This electrophysiological differentiation was demonstrated by showing that Ca2+ prevents a large depolarization induced by 100 mM KCl along the flank, while Ca2+ is ineffective in preventing the KCl-induced depolarization at the node (Shimmen 2008). When an osmotic shock (200 mM sorbitol) is given to the node, a large and long-lasting depolarization is induced. The depolarized state of the node continues after the removal of sorbitol. These characteristics of the node are suggested to be responsible for electrical responses to wounding in Characeae (Shimmen 2008).
Cytoplasmic streaming is also sensitive to the hydrostatic pressure. A positive hydrostatic pressure applied to one end of the internode of Chara induces an acceleration of the streaming that is away from the applied pressure and deceleration of the streaming that is toward the applied pressure. A negative pressure applied to one cell end causes the reverse responses of the streaming (Staves et al. 1992). The sensitivity to the hydrostatic pressure was lost by ligating the cell end, indicating that the site of pressure sensing is located at the nodal region.
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