Effect of cold and menthol on membrane potential in plants

Evolution of long-distance signalling upon plant terrestrialization: comparison of action potentials in Characean algae and liverworts

BACKGROUND

In this review, we summarize data concerning action potentials (APs) - long-distance electrical signals in Characean algae and liverworts. These lineages are key in understanding the mechanisms of plant terrestrialization. Liverworts are postulated to be pioneer land plants, whereas aquatic charophytes are considered the closest relatives to land plants. The drastic change of the habitat was coupled with the adaptation of signalling systems to the new environment.

SCOPE

APs fulfil the 'all-or-nothing' law, exhibit refractory periods and propagate with a uniform velocity. Their ion mechanism in the algae and liverworts consists of a Ca2+ influx (from external and internal stores) followed by/coincident with a Cl- efflux, which both evoke the membrane potential depolarization, and a K+ efflux leading to repolarization. The molecular identity of ion channels responsible for these fluxes remains unknown. Publication of the Chara braunii and Marchantia polymorpha genomes opened up new possibilities for studying the molecular basis of APs. Here we present the list of genes which can participate in AP electrogenesis. We also point out the differences between these plant species, e.g. the absence of Ca2+-permeable glutamate receptors (GLRs) and Cl--permeable SLAC1 channel homologues in the Chara genome. Both these channels play a vital role in long-distance signalling in liverworts and vascular plants. Among the common properties of APs in liverworts and higher plants is their duration (dozens of seconds) and the speed of propagation (mm s-1), which are much slower than in the algae (seconds, and dozens of mm s-1, respectively).

CONCLUSIONS

Future studies with combined application of electrophysiological and molecular techniques should unravel the ion channel proteins responsible for AP generation, their regulation and transduction of those signals to physiological responses. This should also help to understand the adaptation of the signalling systems to the land environment and further evolution of APs in vascular plants.

Cold- and menthol-evoked membrane potential changes in the moss Physcomitrella patens: influence of ion channel inhibitors and phytohormones

Microelectrode measurements carried out on leaf cells from Physcomitrella patens revealed that a sudden temperature drop and application of menthol evoked two types of different-shaped membrane potential changes. Cold stimulation evoked spike-type responses. Menthol depolarized the cell membrane with different rates. When it reached above 1 mV s^-1 , the full response was recorded. Characteristic for the full responses was also a few-minute plateau of the membrane potential recorded after depolarization. The influence of inhibitors of calcium channels (5 mM Gd³⁺ ), potassium channels (5 mM Ba²⁺ ), chloride channels (200 μM Zn²⁺ , 50 μM niflumic acid) and proton pumps (10 μM DES), an activator of calcium release from intracellular stores (Sr²⁺ ), calcium chelation (by 400 μM EGTA) and phytohormones (50 μM auxin, 50 μM abscisic acid (ABA), 500 μM salicylic acid) on cold- and menthol-evoked responses was tested. Both responses are different in respect to the ion mechanism: cold-evoked depolarizations were influenced by Ba²⁺ and DES; in turn, menthol-evoked potential changes were most effectively blocked by Zn²⁺ . Moreover, the effectiveness of menthol in generation of full responses was reduced after administration of auxin or ABA, i.e. phytohormones known for their participation in responses to cold and regulation of proton pumps. The effects of DES indicated that one of the main conditions for generation of menthol-evoked responses is inhibition of the proton pump activity. Our results indicate that perception of cold and menthol by plants proceeds in different ways due to the differences in ionic mechanism and hormone dependence of cold- and menthol-evoked responses.

Menthol-induced action potentials in Conocephalum conicum as a result of unspecific interactions between menthol and the lipid phase of the plasma membrane

Our previous study has shown that the liverwort Conocephalum conicum generates action potentials (APs) in response to both temperature drop and menthol, which are also activators of the TRPM8 (transient receptor potential melastatin 8) receptor in animals. Not only similarities but also differences between electrical reactions to menthol and cooling observed in the liverwort aroused our interest in the action of menthol at the molecular level. Patch-clamp investigations have shown that menthol causes a reduction of current flowing through slow vacuolar (SV) channels to 29 ± 10% of the initial value (n = 9); simultaneously, it does not influence magnitudes of currents passing through a single SV channel. This may point to an unspecific interaction between menthol and the lipid phase of the membrane. An influence of menthol on lipid organization in membranes was investigated in two-component monomolecular layers formed with menthol and dipalmitoylphosphatidylcholine (DPPC) at the argon-water interface. Analyses of the mean molecular area parameters vs the molar fraction of the menthol component have shown over-additivity (approximately 20 Å(2) ) in the region of high molar fractions of menthol. Infrared absorption spectroscopy studies have shown that menthol, most probably, induces breaking of a hydrogen bond network formed by ester carbonyl groups and water bridges in the lipid membrane and binds to the polar head group region of DPPC. We conclude that the disruption in the lipid phase of the membrane influences ion channels and/or pumps and subsequently causes generation of APs in excitable plants such as C. conicum.

Menthol-enhanced cytotoxicity of cigarette smoke demonstrated in two bioassay models

Background

Cigarette smoke is harmful to human health at both cellular and genetic levels. Recently, a unique bioassay for smoke cytotoxicity using air pollution-sensitive plant cells (tobacco) has been proposed.

Methods

Model plant cells (tobacco Bel-W3 cells) and human cells (alveolar epithelial A549 cells) suspended in fresh culture media were exposed to cigarette smoke sampled after lighting the tip of cigarettes (with vs. without menthol capsules) which were attached to a glass pipe connected to the cell-containing plastic tubes. Control cultures were also assessed.

Results

After exposing tobacco plant cells to cigarette smoke, cell death occurred in a dose-dependent manner. Cell death was significantly enhanced by mentholated smoke, while menthol alone was shown to be inert suggesting that menthol synergistically contributes to the enhancement of cell death, initiated by smoke-associated compounds. The enhanced toxicity of mentholated smoke was confirmed in human alveolar epithelial A549 cells.

Conclusions

Cigarette smoke cytotoxicity leading to cell death assessed in plant and human model cells was enhanced by menthol. Further research into these findings is encouraged.

Electrophysiological approach to examine a putative cold- and menthol- sensitive channel in the liverwort Conocephalum conicum

The mechanism of cold perception by plants is still poorly understood. It was found that temperature drop evokes changes in the activity of ion pumps and channels, which leads to plasma membrane depolarization. The nature of the primary step of its action (alteration in membrane composition, transient influx of Ca2+ etc.,) has not been elicited yet. Our electrophysiological experiments conducted on the liverwort Conocephalum conicum showed that its cells respond not only to sudden cooling but also to menthol, generating depolarization of the plasma membrane and action potentials (APs). Similar results are well documented in mammals; cold or “cooling compounds” including menthol cause activation of thermosenstitive channel TRPM8 permeable to Ca2+ and generation of AP series. TRP receptors are detected, among others, in green and brown algae. Possible existence of TRPM8-like channel-receptor in Conocephalum conicum is discussed here.