Characteristics and Functions of Phloem-Transmitted Electrical Signals in Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 321

22 Characteristics and Functions

of Phloem-Transmitted Electrical
Signals in Higher Plants
Jörg Fromm, Silke Lautner

Abstract Electrical signalling along the phloem has been studied in a number of species
like maize, willow and Mimosa . It appears that sieve tubes with their large sieve pores are
used to transmit information over long distances, while plasmodesmata serve as a means for
the propagation of electrical signals over short distances between cells. By using the aphid
technique the phloem pathway has been shown to transmit action potentials with a velocity
up to 10 cm s −1 . With regard to the ion fluxes which create the conditions necessary for
the generation of an action potential, we found that calcium influx as well as potassium
and chloride efflux are involved. Some of their corresponding ion channels were identified.
AKT2/3-like channels, expressed in the phloem and capable of mediating both uptake and
release of K + in response to changes in membrane potential, were identified in several species
such as Arabidopsis , maize and broad bean. Concerning physiological functions of electrical
signalling, evidence was found for a link between the signals and photosynthetic response in
Mimosa , apart from the regulation of rapid leaf movements. In addition, electrical signals
in maize play a role in the regulation of phloem transport as well as in root-to-shoot
communication of entire plants.

22.1
Introduction

Almost the entire chemistry of the neuromotoric system in animals is also
available to plants. They do not possess nerves but neurotransmitters such
as acetylcholine, cellular messengers like calmodulin, cellular motors, e.g.
actin and myosin, voltage-gated ion channels and sensors for touch, light,
gravity and temperature. This exciting cellular chemistry raises the ques-
tion: Why do plants need cellular equipment similar to nerves? Although
the degree of development and complexity of plant cells is not comparable
to that of the nervous system in animals, plants seem to have developed
a simple neural network within the phloem which serves the communi-
cation between plant organs over long distances. They have presumably
developed such a system in order to cope in the best possible way with
environmental stress factors. Having sensed environmental stimuli, the
sensor region needs to transmit a signal to the responding region. The na-
ture of this signal may be chemical (e.g. hormonal), hydraulic (e.g. pressure
changes) or electrical (e.g. ionic). The electrical signals, in particular, have
the capacity to rapidly transmit information over long distances. However,
the conduction rate of most of the plant action potentials studied so far is