References

19 Electrical Signals in Long-Distance Communication in Plants 283
A significant role of the proton pump in AP electrogenesis was demonstrated
in Cucurbita pepo by Opritov et al. (2002). Recently Fisahn et
al. (2004) provided evidence that the AP in Solanaum tuberosum is accompanied
by a substantial increase in [Ca 2+ ]cyt. The data suggest that
ryanodyne-receptor-coupled internal stores release Ca 2+ during the AP. In
marine diatoms (Odontella sinensis)fast(milliseconds)Na + -based APs, like
in animal cells, were registered (Taylor and Brownlee 2004). The abundance
of sodium in seawater might be a reason why such a scheme of excitation
is frequent in marine plants.
Investigation of the detailed nature of APs in terrestrial higher plants
is still necessary. Very little is known about the regulation of excitability.
Characterization of ion channels on a molecular level would help us to
understand such processes.
19.1.3
Ways of Action Potential Transmission
The problem of electrical signal transmission in plants has recently been
discussed by Dziubinska (2003). APs spread within cells on the basis of
local electrical circuits. The mechanism is the same as AP transmission
along axons. Cylindrical cells of Characean algae are suitable objects to
demonstrate the principles of local circuit and cable theories. Longitudinal
phloem cells of higher plants can also serve as a good analogy of axons. In
many plants, like Aldrovanda, Dionaea and Conocephalum, transmission
of APs resembles epithelial transmission of excitation in animals (Mackie
2004). Cells connected with plasmodesmata constitute a network which is
able to transmit APs in different directions. Plasmodesmata or sieve pores
in phloem serve as low-resistance bridges allowing AP transmission from
cell to cell. In higher plants phloem, phloem parenchyma or protoxylem
wereshowntobesuitableforAPtransmission.TheAPmaygraduallycover
all living tissues in the shoot (Zawadzki and Trebacz 1982; Dziubinska et al.
2001). APs are sometimes limited to certain organs or parts of plants. For
instance, in Dionaea all cells in the leaf converted into a trap can generate
APs (Hodick and Sievers 1988), whereas the remaining parts of the leaf are
totally unexcitable. In Mimosa the AP travels along the leaf and the petiole
tothemainpulvinus,whereitstops.InLupinus angustifolius the AP is
transmitted along the shoot, both acropetally and basipetally, but does not
enter leaves and roots (Zawadzki 1980). There are reports showing spontaneous
generation of APs (Zawadzki et al. 1995). Extracellular electrodes
recorded APs of different amplitudes transmitted along limited distances.
It was concluded that spontaneous APs can be transmitted only down the
cell lines having low-resistance plasmodesmata connections.