Introduction

Plant roots generate electrical currents and voltages in the rhizosphere that may influence the behaviour of the many pathogenic, symbiotic or commensal microorganisms that live in association with them. These electrical currents represent circulations of protons and other ions. Consequently, they also lead to the creation of substantial ionic and pH gradients whose affect on the infection and colonisation of the root is only now being explored. Here, we summarise briefly what is known about the electrical currents of plant roots and discuss the ways in which they may influence the root microflora. In particular, we focus on the swimming zoospores of Phytophthora species, which are exquisitely sensitive to electrical fields and may target their host roots using a combination of chemotaxis and electrotaxis.

Growth and electricity

It has been known for many years that plants generate electrical currents (Müller-Hettlingen, 1883; Lund & Kenyon, 1927; Lund, 1947). These were first measured using microelectrodes inserted into cells at different regions of a root or tissue. Voltage differences were found between different sites and electrical current was presumed to flow through the cells between the electrodes and through the extracellular medium to complete the circuit. Endogenous currents in the extracellular loop of the circuit can now be measured directly, without invading cells with intracellular microelectrodes. In the 1970s ultrasensitive, voltage-sensing vibrating electrodes were devised that are capable of detecting the minute electrical fields generated by individual cells or tissues (Jaffe & Nuccitelli, 1974).