References

22 Electrical signaling via the phloem 323
Fig.22.1. Intercellular electrical communication in plants: Short-distance electrical signalling
via plasmodesmata (below) and long-distance signalling along the sieve tube pathway
(above).Astimuluslikecold-shockortouch(star) induces calcium influx into a living
cell, e.g. a mesophyll cell (MC). After the membrane potential is depolarized below a certain
threshold level, an action potential is elicited by chloride and potassium efflux. The
signal is propagated over short distances through plasmodesmal (P)networksand,afterit
has passed the few plasmodesmata between sieve element/companion cells (SE/CCs) and
phloem parenchyma cells (PA), it will enter the SE/CC complex to be transmitted over
long distances. Sieve pores (SP)withtheirlargefunctionaldiameterspresentlow-resistance
corridors for the rapid propagation of electrical signals along the SE plasma membrane.
Such signals can leave the phloem pathway at any site via plasmodesmata to induce certain
physiological responses in the neighbouring tissue
22.3
Long-Distance Signalling via the Phloem
Apart from assimilate transport, long-distance signalling between various
organs by physical and chemical signals travelling along the sieve tubes
is a well-known process. Concerning electrical signals the transmission
of action potentials along the plasma membranes of phloem cells is also
an established phenomenon in plants which exhibit rapid leaf movements
such as Mimosa (Samejima and Sibaoka 1983; Fromm and Eschrich 1988b).
Previous studies using dye-filled microelectrodes reported that in Mimosa
petioles the excitable phloem cells are small parenchyma or companion