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

• IP 3 does not affect electrogenic parameters of Nitellopsis obtusa . • IP 6 hyperpolarises the action potential excitation threshold. • IP 6 shifted the reversal potentials of Ca 2+ and Cl – currents during excitation. Despite the importance of action potentials (APs) in plant stress physiology, the molecular identity of Ca 2+ channels that initiate APs by passing Ca 2+ into the cytoplasm is still unknown in Characean macroalgae. While the Thiel-Beilby mathematical model of AP generation proposes that Ca 2+ channels are activated by inositol 1,4,5-trisphosphate (IP 3 ), this hypothesis is controversial because plants do not possess animal IP 3 receptor gene homologues. In the present study, we employed the two-electrode current/voltage clamp technique to determine whether IP 3 and another inositol phosphate IP 6 could modulate the electrogenic parameters of an aquatic macrophyte Nitellopsis obtusa internodal cells. IP 3 had no significant effect, whereas IP 6 reversibly hyperpolarised the AP excitation threshold which is consistent with the activation of Ca 2+ channels. IP 6 also shifted the reversal potentials of the Ca 2+ and Cl – currents during excitation to negative membrane potential values, indicating altered calcium dynamics in the cytoplasm. These findings suggest the regulation of Ca 2+ channels during electrical excitation by IP 6 rather than IP 3 . IP 6 -induced shift of Ca 2+ channel voltage dependence allows a lower magnitude external stressor to initiate electrical signalling, thus turning on various downstream physiological responses.