The study of membrane excitability has primarily been shaped by investigations in animal cells, but the significant role of electrical excitation in plants has been recognized almost simultaneously. For a long time, it was believed that action potentials (APs) served similar functions in both plant and animal systems, a theory supported by the similarities observed in excitations between plant cells, like Nitella, and nerve cells, such as squid giant axons. In plants, environmental factors such as fluctuations in temperature, mechanical stimuli, and various signaling molecules—can trigger the generation of APs. Once an electrical signal is generated in response to a stimulus, it is transmitted to other regions of the plant, helping to coordinate cellular adaptations. This process influences essential physiological functions such as respiration and photosynthesis.
Among the many signaling molecules that can modulate AP parameters, glutamate (Glu) is one of the most universal. Previous studies have shown that Glu alters various AP parameters in Characean internodal cells. However, it remains unclear whether these changes affect the propagation and transmission characteristics of APs. Additionally, under standard environmental conditions, it is still unknown whether Characean macroalgae exhibit a directional disparity in AP transmission and which electrophysiological parameters influence the probability and velocity of AP transmission between internodal cells. To address these questions, this study aims to examine the link between AP transmission properties and the electrophysiological parameters of tandem internodal cells in Nitellopsis obtusa. The research employed the two-electrode current-clamp technique, with intracellular glass electrodes inserted into both basal and apical cells of each tandem (internodal cell-multicellular node-internodal cell). Three APs were elicited in one cell every 5 minutes by gradually increasing the direct current until the excitation threshold was reached. The membrane potential was consistently recorded in both cells. The protocol was repeated bidirectionally, first in a control solution (APW’) and then in a solution containing Glu dissolved in APW’.
The results revealed that, under standard conditions, the probability and velocity of AP transmission between internodal cells of N.obtusa were higher in the basal direction compared to the apical direction. In control conditions, a hyperpolarized excitation threshold, prolonged repolarization, and a depolarized resting potential were associated with a higher probability of transmission. Additionally, transmission velocity positively correlated with AP amplitude and the reduced difference between the membrane resting potential and the excitation threshold. Exposure of the entire tandem to 1 mM glutamate for 30 minutes resulted in a significant decrease in the percentage of transmitted APs between adjacent internodal cells and reduced the transmission velocity.