Inositol hexakisphosphate (IP6) enhances the electrical excitability of Characean Nitellopsis obtusa

Despite the importance of action potentials (APs) in plant stress physiology, the molecular identity of Ca²⁺ channels that initiate APs by passing Ca²⁺ into the cytoplasm is still unknown in Characean macroalgae. While the Thiel-Beilby mathematical model of AP generation proposes that Ca²⁺ channels are activated by inositol 1,4,5-trisphosphate (IP₃), this hypothesis is controversial because plants do not possess animal IP₃ receptor gene homologues. In the present study, we employed the two-electrode current/voltage clamp technique to determine whether IP₃ and another inositol phosphate IP₆ could modulate the electrogenic parameters of an aquatic macrophyte Nitellopsis obtusa internodal cells. IP₃ had no significant effect, whereas IP₆ reversibly hyperpolarised the AP excitation threshold which is consistent with the activation of Ca²⁺ channels. IP₆ also shifted the reversal potentials of the Ca²⁺ and Cl^– currents during excitation to negative membrane potential values, indicating altered calcium dynamics in the cytoplasm. These findings suggest the regulation of Ca²⁺ channels during electrical excitation by IP₆ rather than IP₃. IP₆-induced shift of Ca²⁺ channel voltage dependence allows a lower magnitude external stressor to initiate electrical signalling, thus turning on various downstream physiological responses.