References

322 J. Fromm, S. Lautner
in the range 0.01−0.2 m s −1 ,i.e.muchslowerthantheconductionvelocity
of action potentials in nerves, which is between 0.4 and 42 m s −1 . Similar
to the velocities of action potentials in nerves are signals induced by
pentachlorophenol in soybean, reaching a speed of 30 m s −1 (Volkov et al.
2000).
22.2
Signal Perception and Short-Distance Electrical Signalling
Environmental stimuli such as changes in light, temperature, touch or
wounding can induce electrical signals at any site of the symplasm, and
are transmitted via plasmodesmata to all the other symplasmic cells (van
Bel and Ehlers 2005). For instance, mechanical stimulation of Chara cells
generates the depolarization of the plasma membrane known as a receptor
potential (Kishimoto 1968). Usually, the receptor potential lasts as long as
the stimulus is present, being an electrical replica of the initial stimulus.
Ifthestimulusissufficientlygreattocausethemembranepotentialto
depolarize below a certain threshold, this will cause an action potential
to be generated. It shows a large transient depolarization which is selfperpetuating
and therefore allows the rapid transmission of information
over long distances. To demonstrate intercellular coupling Spanswick and
Costerton (1967) injected a current into a Nitella cell and managed to detect
it using a microelectrode several cells away from the injected cell. Electrical
coupling was also shown in a variety of tissues such as Elodea leaves
and Avena coleoptiles (Spanswick 1972) as well as Lupinus stem phloem
(van Bel and van Rijen 1994). These studies suggest that plasmodesmata
are relays in a signalling network at the local level. If the information
has to be transmitted to other parts of the plant further away, electrical
communication via the phloem appears to be used (Fig. 22.1). Sieve tubes
maybeconsideredlow-resistancepathwaysforthepropagationofelectrical
signals over long distances. The few existing plasmodesmata between
sieve elements/companion cells and phloem parenchyma cells (Kempers
et al. 1998) may open up, making way for the electrical signals to move
laterally from neighbouring cells into the sieve elements/companion cells.
Therefore, the signalling cascade within the plant depends on the electrical
conductance of plasmodesmata in a lateral direction and on the high
degree of electrical coupling via the sieve pores in a longitudinal direction
(van Bel and van Rijen 1994).