Long-term root electrotropism reveals habituation and hysteresis.

Plant roots sense many physical and chemical cues in soil, such as gravity, humidity, light, and chemical gradients, and respond by redirecting their growth toward or away from the source of the stimulus. This process is called tropism. While gravitropism is the tendency to follow the gravitational field downwards, electrotropism is the alignment of growth with external electric fields and the induced ionic currents. Although root tropisms are at the core of their ability to explore large volumes of soil in search of water and nutrients, the molecular and physical mechanisms underlying most of them remain poorly understood. We have previously provided a quantitative characterization of root electrotropism in Arabidopsis (Arabidopsis thaliana) primary roots exposed for 5 h to weak electric fields, showing that auxin asymmetric distribution is not necessary for root electrotropism but that cytokinin biosynthesis is. Here, we extend that study showing that long-term electrotropism is characterized by a complex behavior. We describe overshoot and habituation as key traits of long-term root electrotropism in Arabidopsis and provide quantitative data about the role of past exposures in the response to electric fields (hysteresis). On the molecular side, we show that cytokinin, although necessary for root electrotropism, is not asymmetrically distributed during the bending. Overall, the data presented here represent a step forward toward a better understanding of the complexity of root behavior and provide a quantitative platform for future studies on the molecular mechanisms of electrotropism.