Characteristics and Functions of Phloem-Transmitted Electrical Signals in Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 327

to wrap themselves around the insect too, so as to make a really secure
trap (Williams and Pickard 1972a,b). Subsequently, secretory cells exude
enzymes to digest the prey. Furthermore, the lobes of the trap leaf from
Dionaea close when an insect strikes two hairs or one hair twice (Williams
and Benett 1982; Hodick and Sievers 1989) to ensure that the trap only
closes around live prey. The catching process starts with a release of calcium
into the cytosol of the sensor cells (Hodick and Sievers 1988), induced by
mechanical pressure. Subsequently, a depolarization signal is propagated
at a speed of approximately 20 cm s −1 ,causingthesnappingofthetrap
(Sibaoka 1966). The fast closure of the trap is based on a snap-buckling
instability, the onset of which is controlled actively by the plant (Forterre
et al. 2005).
With regard to pollination, bioelectric potential changes were observed
in the style of flowers, presumably transmitted via plasmodesmata down
the style (Sinyukhin and Britikov 1967). Stimulation of the Hibiscus stigma
by pollen, heat or cold-shock evokes electrical potential changes in the style
which propagate towards the ovary at the rate of 1.3−1.5 cm s −1 ,affecting
its metabolism (Fromm et al. 1995). Self-pollination and cross-pollination
induce signals which cause a transient increase in the ovarian respiration
rate, indicating that the ovary metabolism was prepared for fertilization.
Long-distance electrical signalling via the phloem pathway is best known
in plants performing rapid leaf movements. After touching a leaf of Mimosa
anactionpotentialisevokedandtransmittedalongthesievetubes(Fromm
and Eschrich 1988b; Fromm 1991) to cause shifts in ions and water between
extensor and flexor cells in the pulvinus. As a result, the leaflets fold to-
gether and the petiole bends downwards making the leaf look dead and
unappealing to a would-be herbivore. In the Mimosa petiole the vascular
bundles are surrounded by a sclerenchyma sheath in order to restrict elec-
trical signalling to the phloem. When a signal reached a pulvinus, it can be
transmitted laterally via numerous plasmodesmata to the cells of the motor
cortex. The latter possess voltage-gated ion channels which respond to elec-
trical signals, causing ion influxes and effluxes associated with water fluxes
that lead to leaf movements (Fromm and Eschrich 1988a-c). In addition,
considerable amounts of photoassimilates are accumulated in the pulvini.
Apart from the role of action potentials in the regulation of leaf movements,
evidence was found for a link between electrical signalling and photosyn-
thetic response in Mimosa (Koziolek et al. 2004). Following flame-wounding
of a leaf pinna electrical signals travel rapidly into the neighbouring pinna
of the leaf to eliminate the net-CO 2 uptake rate. At the same time the photo-
system II quantum yield of electron transport is reduced. Two-dimensional
imaging analysis of the chlorophyll fluorescence signal revealed that this
yieldreductionspreadsacropetallythroughthepinnaandviatheveins
through the leaflets (Koziolek et al. 2004). Further photosynthetic research