Touch-induced action potentials in Arabidopsis thaliana

Arabidopsis thaliana (col accession) leaves were touched with a brush (30 mm2) in the middle of the leaf (equiv. weight ~1-2 g, for ~ 2 s). This soft stress generated reversible depolarizations that had most of the characteristics of so-called action potentials (APs). These electrical events moved away from the stressed zone at a speed of 1.3 mm s-1. The AP duration was ~10-20 s. We measured the extracellular electric potential with electrodes that were very thin silver wires inserted into the plant tissues at different distances from the touched zone. The APs could be clearly distinguished from the eventual artifacts (e.g., due to a moving electrode) because the latter were recorded simultaneously with the touching, whereas genuine APs were always delayed. The recorded APs had characteristics (amplitude, duration and propagation speed) that were very similar to previously described APs induced in the same plant tissue by electrical stimulation or touching or by cold-induced APs. Although the signals appear to be uniform (peak shape), we encountered a significant proportion of measurements that had more complex dynamics in which the signal moved away from the leaf until it was near the end of the petiole, near the center of the rosette, and then returned back to the leaf. We named these signals going-coming APs (GCAPs). We suggest that such complexity might arise from the vascular organization of the plant (i.e., many vascular bundles are present at locations where the electrodes were inserted in the plant) and from the extracellular nature of the measurements. This work confirms the high sensitivity of A. thaliana (Columbia accession) plants to another abiotic stress : touch. Moreover, this light mechanical stimulation can elicit APs without the simultaneous presence of other electrophysiological signals such as variation potentials.