Magnetic measurements in plant electrophysiology

Abstract

Plants show a huge range of dynamic electrical phenomena, including triggered (Umrath 1929) and autonomous action potentials (AP) (Gradmann et al. 1993), or stimulus-evoked slow transient depolarizations (Roblin and Bonnemain 1985). This electrical activity can comprise singular events (Umrath 1929), trains of periodic activity (Williams and Pickard 1972) or even long lasting periodic oscillations of the membrane voltage (Gradmann et al. 1993). Electrical activity can occur in single cells (Gradmann 1976; Bauer et al. 1997) as well as in complex plant tissues (Bentrup 1979). Many of these electrical phenomena resemble electrical activity in animal cells, and by pure analogy it has been proposed that plant cells may function like nerves, and that plants may even have the equivalent of a nervous system (Baluska et al. 2005). Irrational views like these are only possible on the background of a serious ignorance of the physics and molecular basis of electrical activity in plants. Currently we neither really understand the elementary mechanisms underlying electrical activity in plants on the same level as for example the action potential in animal cells, nor do we have clear-cut ideas on their physiological roles. In most cases also, the cellular connections and pathways that propagate electrical activity are not yet certain. Consequently, the mechanistic basis for propagation of electrical activity is also not resolved; it is still a matter of debate whether the signal is really propagated electrically like in nerves or the result of a traveling chemical wave. The application of new experimental methods such as patch clamp technology (Okihara et al. 1991; Homann and Thiel 1994) has in the past 2 decades brought some deeper insight into the molecular basis of membrane excitation in plants. Combination of classical electrophysiology with fluorescent markers (Rhodes et al. 1996) or molecular sensors (Pena-Cortes et al. 1995) is now also paving the way to address fundamental questions on long distance propagation. A new promising method to address many of these open questions is provided by the magnetic measurements of electrical activity in plants. This non-invasive method presents a tool for both highresolution recordings on the cellular level and time resolved imaging of electrical activity over a whole plant. The present review guides through the theoretical background of the method and shows its application in a few case studies.