References

326 J. Fromm, S. Lautner
22.5
Ion Channels of the Phloem
The propagation of electrical signals along sieve tubes is achieved by opening
and closing movements of ion channels in their plasma membranes.
Since calcium, chloride and potassium fluxes are involved in the generation
of action potentials in plants (Fig. 22.1), some of their corresponding
channels were identified. Most of the work focuses on K + channels. The
membrane potential of the sieve tubes, measured by means of the aphid
technique, was shown to be dominated by K + conductance (Ache et al.
2001). Corresponding AKT2/3-like channels expressed in the phloem were
identified in several species such as Arabidopsis, maize and broad bean
(Marten et al. 1999; Deeken et al. 2000; Bauer et al. 2000; Lacombe et al.
2000). AKT2/3 is capable of mediating both uptake and release of K + in response
to changes in membrane potential in a calcium- and pH-dependent
fashion. Since AKT2/3 loss-of-function mutants (akt2/3–1) from Arabidopsis
thaliana, lacking the phloem channels of the AKT2/3 type, possessed
only half the sucrose content of the wild type, the authors assumed that the
channel is involved in the loading of sugar into the phloem (Deeken et al.
2002). Furthermore, the akt2/3–1 mutant exhibited a reduced K + dependence
of the phloem potential. Most likely, the channel is also involved in
the generation of electrical signals, making it the subject of further studies.
Concerning calcium, dihydropyridine-type Ca 2+ channels were localized
in the phloem of leaf veins from Nicotiana tabacum and Pistia stratiotes
by immunolabeling techniques at the light and electron microscopic level
(Volk and Franceschi 2000). The results indicate that sieve elements may
be enriched with Ca 2+ channels which may be involved in long-distance
electrical signalling.
22.6
Functions of Electrical Signals in Higher Plants
There are numerous functions of short-distance electrical signalling via
plasmodesmata in plants. For instance, insectivorous plants that live in
nitrogen-depleted areas use electrical signals to capture insects in order
to secure their nitrogen supply. When the outer Drosera leaf tentacles are
touched by insects, the plasma membrane of the cells of the sensitive tip
is depolarized (Williams and Pickard 1972a). Once depolarization exceeds
a certain threshold, a series of action potentials is generated and propagated
at a rate of 5 mm s −1 along the tentacle stalk. As soon as the base of the
tentacle is reached, it is induced to wrap itself around the insect. Action
potentials are also propagated to neighbouring tentacles and cause them