Characteristics and Functions of Phloem-Transmitted Electrical Signals in Higher Plants

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.