Fig. 5.

The AGP–calcium oscillator. (A) Ca²⁺ release from periplasmic AGPs. The oscillator generates pulses of Ca²⁺ (green dots) whose influx coordinates exocytosis and rapid tip growth. This involves a pulse of H⁺ (black dots) releasing Ca²⁺ from periplasmic AGPs (red beads) via stretch-activated Ca²⁺ channels into the cytosol, and then sequestration by exocytotic Golgi vesicles containing AGPs (red dots). Diffusion of the initial H⁺ pulse to the wall domain restores the periplasmic pH. Ca²⁺ is recycled by fusion of vesicles with the plasma membrane. Ca²⁺ bound to periplasmic AGPs is now ready for the next oscillation. W, wall; P, periplasm; PM, plasma membrane; G, Golgi; GV, Golgi exocytotic vesicles. (B) Ca²⁺ current as a molecular clock: analogous to an electronic series RLC circuit where R is resistance, L is inductance and C is capacitance, with membrane-bound AGPs as the capacitor, C; Ca²⁺ sequestration as the inductance, L; vesicle exocytosis as the resistor, R, which limits the recycling rate. Hypothetically, C and L largely determine the oscillator frequency and amplitude of the Ca²⁺ current, I_Ca. Note the reported high efflux of Cl^– as a counterion may maintain electrical neutrality (Cosgrove and Hedrich, 1991; Zonia et al., 2002). [Reprinted from Lamport and Varnai (2013).]