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

Abstract

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.

Keywords

• Plant action potential
• Ca2+ channels
• Nitellopsis obtusa
• Inositol trisphosphate
• Inositol hexakisphosphate
• Excitation threshold