Resting and action potentials
Like animals, plants use action potentials (changes in cell membrane potential) to transmit information within and between cells. However, to understand these action potentials, we first need to understand the origin of these membrane potentials.
If we separate a strong potassium chloride (KCl) solution from pure water by a membrane, ions will flow down their concentration gradients into the pure water by diffusion. However, most biological membranes are differentially permeable to different ions, e.g. K+ will flow more readily than Cl−. This will generate a voltage across the membrane.
The concentrations of potassium ions on either side of the membrane will only differ by a tiny amount because as soon as any ions have crossed the membrane, they will set up an electrical gradient that would tend to attract them back. No further flow occurs when the (free energy change associated with) the concentration and electrical gradients are balanced, hence bulk solutions are still neutral.
Transmembrane portion of a potassium channel, showing three potassium
ions. The regulatory units have been omitted.
However, only a tiny number of ions need cross, to generate a large voltage: the two solutions need only differ by 1 ion in a million to generate a 100 mV potential difference. The membrane potential in plant cells is generally negative on the inside, by about -100 mV. The Nernst equation can be used to determine the equilibrium concentration of ions across a membrane, given a known membrane potential.
∆Em = ( R T ⁄ z F ) ln [X]out ⁄ [X]in
It can also be used to calculate the membrane potential you'd expect from the equilibrium distribution of a single ion. If you measure a membrane potential of -59 mV, and a distribution of 150 mM K+ inside and 15 mM K+ outside, this is consistent with the membrane being freely permeable to potassium ions, and to no other ions. The Goldman equation is used to deal with membranes that are permeable to more than one ion.
- ∆Em, membrane potential (V).
- R, universal gas constant, 8.314472 J K−1 mol−1.
- T, temperature (K).
- z, charge on the ion X.
- F, Faraday constant (change on a mole of protons), 96485.34 C mol−1.
- [X]out, concentration of X (e.g. K+) outside the cell (M).
- [X]in, concentration of X inside the cell (M).
Ion concentrations
|
Ion |
[X]out mM |
[X]in mM (predicted) |
[X]in mM (observed) |
|---|---|---|---|
|
K+ |
1 |
74 |
75 |
|
Na+ |
1 |
74 |
8 |
|
Mg2+ |
0.25 |
1340 |
3 |
|
Ca2+ |
1 |
5360 |
2 |
|
NO3− |
2 |
0.0272 |
28 |
|
Cl− |
1 |
0.0136 |
7 |
|
H2PO4− |
1 |
0.0136 |
21 |
|
SO42− |
0.25 |
0.00005 |
19 |
The ions are not at equilibrium: all but potassium are in fact very far from equilibrium. Plant cells actively pump certain ions across their membranes: All anions are actively imported into the cytosol, and all cations (except K+) are actively expelled from cytoplasm.
An action potential occurs when the voltage across the membrane is destroyed. We call this depolarisation, and for this to occur, ions must redistribute rapidly across the membrane. For ions to move across a membrane rapidly, they must pass through ion channels.
Plant cells show action potentials that in many ways resemble the nerve impulses of animals. They can be detected by placing electrodes on the plant surface. Patch clamping can also be carried out in protoplasts.
Action potentials are usually started (initiated) by a fall in the membrane potential below a threshold value. This may be caused by stretch-gated ion channels (e.g. in statolith cells in the root), or initiated by light-induced chemical changes (as in the mammalian eye). Whatever the cause, this local change in membrane potential opens nearby voltage-gated ion channels, which allow even more ions to redistribute. This 'short-circuits' the membrane, and causes complete depolarisation.
Neighbouring voltage-gated ion channels then open, and the action potential propagates like a wave over the cell surface. The speed with which this happens means that it can act as a rapid means of communication.
The channels are programmed to close after a more or less fixed interval, and the original voltage is restored by active transport (pumps or porters).
Action potentials were first detected in plants showing rapid movements (thigmonastic plants): such as Dionaea muscipula (Venus flytrap), Drosera spp. (sundew); and Mimosa pudica, (sensitive plant). The APs are initiated in touch-sensitive organs, propagate through ordinary cells, and bring about movements in other regions.
APs have since been detected in ordinary plants, usually in response to injury, e.g. in Pelargonium stems when a leaf is burned. Sometimes there is a biochemical response: tomato seedlings respond to insect injury by producing protease (insect-digestion) inhibitors in other parts of the plant. Algal action potentials are easier to study than land plants. In Chara (stoneworts, the group of algae most closely related to land plants), the plants respond to mechanical stimulation: mechanically (stretch) gated ion channels in cell membranes lead to the production of an action potential which makes the cytoplasm gel and prevents it from leaking out.
Plant APs, whilst similar in general principle to animal nerve APs, exhibit several important differences: plant APs propagate c. 1000 times more slowly than animal ones and last c. 1000 times longer. They are transmitted by unspecialised cells, via plasmodesmata and through the pores in the sieve-plates of phloem. They also use different ions: Ca2+ influx, Cl− efflux and K+ efflux are important in plants; whereas Na+ influx and K+ efflux are characteristic of the stereotypical AP in animals. Finally, many action potentials are confined to the cell of origin and have no obvious function in communication.
Although not strictly related to APs, it is interesting to note than plants have (sometimes very close) analogues of animal neurotransmitters.
- Peptides: systemin.
- Prostaglandin (left): jasmonic acid (right).
.png)
Prostaglandin
Jasmonic acid - Serotonin (left): auxin (right).
Serotonin.png)
Auxin
When localised to a single cell, plant AP appear to be needed for membrane repair. This is because a −100 mV potential difference across a 10 nm membrane gives an enormous voltage gradient (10 MV m−1). If the voltage were not switched off by the AP, the flow of ions through a small injury would be too rapid to allow a repair to be made. This may have been their original role in unicells but, because of their rapid propagation, they were hi-jacked for communication by multicellular organisms.
When transmitted to other cells, APs seem to be most commonly associated with nastic responses. Nastic responses are (undirected) plant movements in response to stimuli (these are different to tropisms, which are directed plant growth responses to stimuli). Photonasty we have come across: e.g. flowers opening in response to light (or dark). Thigmonasty is movement triggered by touching.
Mimosa pudica, the 'sensitive plant', is a member of the pea family (Fabaceae or Leguminosae). Its leaflets respond to touch by folding. An AP travels at 5 cm s−1 down the petiole, closing leaflets, and finally making the leaf collapse. This reveals a mouthful of thorns to any would-be herbivores. Where the leaflets meets the petiole, there is a swelling called the pulvinus. When the AP hits this, the depolarisation causes potassium efflux. Water follows the potassium by osmosis, and the cell loses turgor, allowing the leaflet to collapse.
Dionaea muscipula, the Venus flytrap, is a member of the sundew family, most of which also exhibit (somewhat slower) movement. The leaf traps close in response to prey touching one of the three trigger hairs on each lobe twice within 30 seconds (the trigger hairs are the small hairs inside the trap, not the spikes around the lobes, which are there to act as a filter to let pathetically small prey escape). This avoids wasting energy on raindrops and falling leaves. Prey are digested for their nitrogen content, allowing the flytrap to grow in soils lacking in nutrients. The mechanism by which the trap snaps shut involves a complex interaction between elasticity, turgor and growth. In the open, untripped state, the lobes are convex (bent outwards), but in the closed state, the lobes are concave (forming a cavity). It is the rapid flipping of this bistable state that closes the trap, but the mechanism by which this occurs is still poorly understood. When the trigger hairs are stimulated, an action potential (mostly involving calcium ions) is generated, which propagates across the lobes and stimulates cells in the lobes and in the midrib between them. Exactly what this stimulation does is still debated: cells in the outer layers of the lobes and midrib may rapidly secrete protons into their cell walls, loosening them and allowing them to swell rapidly by osmosis and acid growth; alternatively, cells in the inner layers of the lobes and midrib may rapidly secrete other ions, allowing water to follow by osmosis (in the same way as we have seen in the sensitive plant), and the cells to collapse. Both, either or neither of these mechanisms may play a role. Later, further APs stimulate growth of the outer surface of the lobes, hermetically sealing the 'stomach'.
Test yourself
- If molecules flow down their concentration gradients, why do membrane potentials exist at all?
- How do plant and animal action potentials differ?
Bibliography
- Alberts, B., et al. (2002). Molecular biology of the cell. 4th edition. Garland Science, New York. 631-656. "Ion channels and the electrical properties of membranes"
- Fleurat-Lessard, P., et al. (1997). Distribution and activity of the plasma membrane H+-ATPase in Mimosa pudica L. in relation to ionic fluxes and leaf movements. Plant Physiology 113:747-754. http://www.plantphysiol.org/cgi/reprint/113/3/747
- Forterre, Y., et al. (2005). How the Venus flytrap snaps. Nature 433:421-425. http://dx.doi.org/10.1038/nature03185
- Hodick, D. and Sievers, A. (1988). The action potential of Dionaea muscipula Ellis. Planta 174:8-18. http://dx.doi.org/10.1007/BF00394867
- Hodick, D. and Sievers, A. (1989). On the mechanism of closure of Venus Flytrap (Dionaea muscipula Ellis). Planta 179:8-18. http://dx.doi.org/10.1007/BF00395768
- Roberts, K. (1992). Potential awareness of plants. Nature 360:14
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