Ionic Mechanism of Mechano-perception in Characeae

A force of nature: molecular mechanisms of mechanoperception in plants

The ability to sense and respond to a wide variety of mechanical stimuli-gravity, touch, osmotic pressure, or the resistance of the cell wall-is a critical feature of every plant cell, whether or not it is specialized for mechanotransduction. Mechanoperceptive events are an essential part of plant life, required for normal growth and development at the cell, tissue, and whole-plant level and for the proper response to an array of biotic and abiotic stresses. One current challenge for plant mechanobiologists is to link these physiological responses to specific mechanoreceptors and signal transduction pathways. Here, we describe recent progress in the identification and characterization of two classes of putative mechanoreceptors, ion channels and receptor-like kinases. We also discuss how the secondary messenger Ca(2+) operates at the centre of many of these mechanical signal transduction pathways.

Involvement of protein synthesis in recovery from refractory period of electrical depolarization induced by osmotic stimulation in Chara corallina

Upon addition of sorbitol to the external medium of an internodal cell of Chara corallina, a transient depolarization is induced at its nodal end (Shimmen in Plant Cell Physiol 44:1215-1224, 2003). In the present study, refractory period was found to be very long, 2-4 h. Recovery from refractoriness was completely inhibited by inhibitors of eukaryote-type protein synthesis, cycloheximide or anisomysin, but not by inhibitors of prokaryote-type protein synthesis. This suggested that proteinous factor(s) responsible for generation of the depolarization is lost or inactivated upon depolarization and synthesized during the resting state. Low temperature, which is supposed to inhibit protein synthesis, also inhibited recovery from refractoriness. When unstimulated internodal cells were incubated in the medium containing an inhibitor of eukaryote-type protein synthesis, generation of the depolarization was almost completely inhibited. This result suggested that the factor is slowly turning over even in the absence of osmotic stimulation.

Membrane stretching triggers mechanosensitive Ca2+ channel activation in Chara

In order to confirm that mechanosensitive Ca(2+) channels are activated by membrane stretching, we stretched or compressed the plasma membrane of Chara by applying osmotic shrinkage or swelling of the cell by varying the osmotic potential of the bathing medium. Aequorin studies revealed that treatments causing membrane stretching induced a transient but large increase in cytoplasmic concentration of Ca(2+) (Delta[Ca(2+)](c)). However, the observed Delta[Ca(2+)](c) decreased during the treatments, resulting in membrane compression. A second experiment was carried out to study the relationship between changes in membrane potential (DeltaE(m)) and stretching or compression of the plasma membrane. Significant DeltaE(m) values, often accompanied by an action potential, were observed during the initial exchange of the bathing medium from a hypotonic medium to a hypertonic one (plasmolysis). DeltaE(m) appears to be triggered by a partial stretching of the membrane as it was peeled from the cell wall. After plasmolysis, other exchanges from hypertonic to hypotonic media, with their accompanying membrane stretching, always induced large DeltaE(m) values and were often accompanied by an action potential. By contrast, action potentials were scarcely observed during other exchanges from hypotonic to hypertonic solutions (=membrane compression). Thus, we concluded that activation of the mechanosensitive channels is triggered by membrane stretching in Chara.

Mechano-perception in Chara cells: the influence of salinity and calcium on touch-activated receptor potentials, action potentials and ion transport

This paper investigates the impact of increased salinity on touch-induced receptor and action potentials of Chara internodal cells. We resolved underlying changes in ion transport by current/voltage analysis. In a saline medium with a low Ca(2+) ion concentration [(Ca(2+))(ext)], the cell background conductance significantly increased and proton pump currents declined to negligible levels, depolarizing the membrane potential difference (PD) to the excitation threshold [action potential (AP)(threshold)]. The onset of spontaneous repetitive action potentials further depolarized the PD, activating K(+) outward rectifying (KOR) channels. K(+) efflux was then sustained and irrevocable, and cells were desensitized to touch. However, when [Ca(2+)](ext) was high, the background conductance increased to a lesser extent and proton pump currents were stimulated, establishing a PD narrowly negative to AP(threshold). Cells did not spontaneously fire, but became hypersensitive to touch. Even slight touch stimulus induced an action potential and further repetitive firing. The duration of each excitation was extended when [Ca(2+)](ext) was low. Cell viability was prolonged in the absence of touch stimulus. Chara cells eventually depolarize and die in the saline media, but touch-stimulated and spontaneous excitation accelerates the process in a Ca(2+)-dependent manner. Our results have broad implications for understanding the interactions between mechano-perception and salinity stress in plants.

Primary effect of bromoxynil to induce plant cell death may be cytosol acidification

Bromoxynil, 3,5-dibromo-4-hydroxybenzonitrile, is a commonly used herbicide and is also used as a tool to trigger rapid cell death in basic botany. However, the primary effect inducing cell death is not known. Bromoxynil inhibited the cytoplasmic streaming and killed cells in Chara corallina when it was applied in the acidic external medium. At higher pH, bromoxynil was inert even at high concentrations. It was speculated that bromoxynil in the protonated form enters the cell and acidifies the cytosol by releasing H(+). Experiments using analogues of bromoxynil supported this possibility. Acidification of the cytosol by bromoxynil was confirmed by experiments using pollen tubes. Based on the acidity of the apoplast, the herbicide action of bromoxynil in higher plants was discussed.

Electrophysiological characterization of the node in Chara corallina: functional differentiation for wounding response

Electrical characteristics of the node were analyzed in comparison with those of the flank of the internodal cell in Chara corallina. The dependence of the membrane potential of the node on pH and K+ concentration was almost the same as that of the flank. In the flank, the increase in the Ca2+ concentration stopped the depolarization in the presence of 100 mM KCl. In the node, however, Ca2+ could not stop the depolarization induced by 100 mM KCl. It has been reported that the node has a function to tranduce the signal of osmotic shock into a transient depolarization. In combination with osmotic shock, 10 mM K+ could induce a long-lasting depolarization of the node. These electrical characteristics of the node were suggested to be responsible for the electrical response to wounding in Characeae.

The effect of aluminium on bioelectrical activity of the Nitellopsis obtusa cell membrane after H+-ATPase inhibition

Aluminium induced membrane potential (Em) changes and potential changes during repolarization phase of the action potential (AP) in the internodal cells of Nitellopsis obtusa after blocking H+-ATPase activity by DCCD were investigated. Micromolar concentrations of DCCD are sufficient to give complete and irreversible inhibition of proton pumping. The membrane potential was measured by conventional glass-microelectrode technique. We found that the half-amplitude pulse duration differs significantly between standard conditions, after DCCD application, and after H+-ATPase blocking and subsequent Al3+ treatment: 4.9, 7.7 and 17.2 seconds, respectively. We propose that in the short term (2 hours) treatment of Al3+, the decrease in membrane potential was compensated for by H+-ATPase activity. Blocking H+-ATPase activity by DCCD can enhance the influence of Al3+ on the bioelectrical activity of cell membranes.