Action Potential in Charophytes

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The plant action potential (AP) has been studied for more than half a century. The experimental system was provided mainly by the large charophyte cells, which allowed insertion of early large electrodes, manipulation of cell compartments, and inside and outside media. These early experiments were inspired by the Hodgkin and Huxley (HH) work on the squid axon and its voltage clamp techniques. Later, the patch clamping technique provided information about the ion transporters underlying the excitation transient. The initial models were also influenced by the HH picture of the animal AP. At the turn of the century, the paradigm of the charophyte AP shifted to include several chemical reactions, second messenger‐activated channel, and calcium ion liberation from internal stores. Many aspects of this new model await further clarification. The role of the AP in plant movements, wound signaling, and turgor regulation is now well documented. Involvement in invasion by pathogens, chilling injury, light, and gravity sensing are under investigation.

Introduction

The action potential (AP) is also referred to as excitation transient or just excitation. The AP involves rapid decrease (depolarization) of the negative membrane potential difference (PD) followed by a slower repolarization (Fig. 1). Sufficient similarities exist between the animal and plant AP to suggest that we are looking at related phenomena:

  • AP is elicited, when the membrane PD is depolarized to a definite threshold level.

  • Once the threshold PD is reached, the AP form and amplitude are independent of the amplitude of the stimulus (all‐or‐none response; Fig. 2A).

  • There is a refractory period following an AP, when a new AP cannot be stimulated (Fig. 2B).

  • The AP initiated in one part of the cell propagates along the cell, sometimes several cells.

  • An increase in temperature makes the AP faster (Fig. 3).

The Nobel Prize winning Hodgkin and Huxley (HH) model of the squid axon (Hodgkin 1952a, Hodgkin 1952b, Hodgkin 1952c, Hodgkin 1952d) has influenced the early analysis of the plant AP. The AP in the nerve is generated by two opposing ionic fluxes: Sodium (Na+) inflow and Potassium (K+) outflow. The increase of the Na+ conductance is the initial response to the stimulus, which depolarizes the membrane PD. The delayed increase in K+ conductance and the spontaneous decrease in Na+ conductance repolarize the membrane back to the resting state. The mathematical model and its application to Chara AP are outlined in Section II.J.3. The HH modeling was made possible by the technique of voltage clamping, in which the membrane PD is held at a selected level by passing a current through the membrane. The AP was also fixed in space by a thin current‐supplying electrode placed along the axis of the cylindrical cell. Similar technology was applied to charophytes (Section II.A.1).

However, there are also some marked differences between excitation in plant and animal cells. The time scale in plants is longer by a factor of 103 (compare Fig. 1, Fig. 1). In plants there are two membranes, the outer plasmalemma and the inner tonoplast. Both membranes usually undergo excitation (Fig. 4A). The contributions from two membranes and the variation of ion concentration in both cytoplasmic and vacuolar compartments are the probable reasons for the shape of the plant AP being more variable (Fig. 1A). The evolutionary pressures on the animal AP are greater to keep the shape constant (Johnson et al., 2002). The plant AP peak remains at negative PDs, crossing into positive region only under unusual circumstances (Beilby 1984, Findlay 1962). The AP peak in the squid axon usually becomes positive. (This is not apparent in Fig. 1B, as both Hodgkin and Huxley and Cole replotted APs by relabeling the negative membrane resting PD as zero and treating the AP as a positive change in PD.) The outflow of chloride ions instead of inflow of sodium ions is responsible for the depolarizing stage in plants. Calcium ion plays an important part in the AP of most charophytes (see 1 Calcium Channels, 2 Chloride Channels).

The HH picture of the axon excitation is so good, that it dominated our approach to analysis of charophyte (and other plant) excitation for almost half a century. It is becoming apparent that other mechanisms underlie the plant AP and their intricacies are now emerging from the shadow of the animal AP (Biskup 1999, Thiel 1990, Wacke 2001, Wacke 2003).

Because of their large cell size, charophytes have been the subject of many electrophysiological studies, leading to an extensive body of information about them. The “green plants” or Viridiplantae are now viewed as containing two evolutionary lineages or clades, the Charophyta and the Chlorophyta (Karol et al., 2001). The charophyte clade contains the charophyte algae and embryophytes (land plants). This close relationship indicates that knowledge gained on the charophyte system will be applicable to a wide range of land plants.

APs play an important role in signal transmission, not only in animal tissue such as nerve and muscle, but also in plant systems. Touch‐sensitive plants (Mimosa pudica) and carnivorous plants (Dionea) respond to a mechanical stimulus, which evoke APs that propagate to motor tissues, where turgor‐aided movement is initiated (Pickard 1973, Sibaoka 1969, Simons 1981). AP‐like prolonged response to hypotonic stress enables salt‐tolerant charophyte Lamprothamnium to regulate its turgor (Beilby and Shepherd, 2006). APs are also implicated in transmission of wound signals (Shimmen 1996, Shimmen 1997a, Shimmen 1997b, Shimmen 1997c). The stoppage of the cytoplasmic streaming triggered by the AP prevents leakage of cytoplasm from injured cells (Kamitsubo 1992, Kamitsubo 1989). Further, Davies (1987) suggested that most plants are capable of producing APs and that these play a major role in intercellular and intracellular communication alongside hormonal and other chemical signaling. The changes in ion concentrations, turgor, and water flow may result in modified activities of enzymes in the cell wall and changes in the membranes and the cytoplasm. There is a likely role for APs in chilling injury, invasion by pathogens, and light and gravity sensing. These electrical signaling cascades await future research.

Section snippets

Voltage Clamp

The voltage clamp technique was adopted in charophytes independently by Findlay 1961, Kishimoto 1961, Kishimoto 1964. The results became easier to interpret, once the space‐clamp was introduced and the two membranes were voltage clamped separately (Findlay, 1964b).

In four‐terminal voltage clamp, one pair of electrodes is used to measure the membrane PD, while the other pair of electrodes delivers the current. The measured PD is fed into an input of a comparator operational amplifier, while the

IP3 (Inositol 1,4,5, ‐triphosphate) and Ca2+ from Internal Stores

The experiments with Mn2+ as a quencher of fura‐2 fluorescence (Plieth et al., 1998) confirmed the importance of internal stores as the main source of Ca2+ at the time of the AP (see Section II.E.1). Further, the elevation of inositol‐1,4,5,‐triphosphate (IP3) in the cytoplasm was able to elicit APs (Thiel et al., 1990). The enzyme phospholipase C (PLC) is responsible for mobilizing IP3 from its membrane‐bound precursor phosphatidyl inositol 4,5‐biphosphate (PIP2). Inhibitors of PLC, Neomycin,

Summary

My scientific career started by investigating the charophyte AP (Beilby 1976, Beilby 1979a, Beilby 1979b, Beilby 1979c). I have moved on to other transporters in charophyte and other giant‐celled organisms, but I kept up my interest in the AP research. So, it was a great pleasure for me to refresh in my memory all the elegant experiments investigating excitation over many years. I hope that the readers will get the same enjoyment, following the intricate story of the charophyte AP. I have

Acknowledgments

Dedicated to G. P. Findlay, whose work continues to provide me with a lot of inspiration.

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