Messerli’s Model of Oscillatory Growth

1. Messerli’s Model of Oscillatory Growth Presented by Cai Chunhui

2. Objective: • To study the phase relation between tip calcium oscillation and the elongation rate oscillation • Techniques • A newly developed, high temporal resolution system and the complementary use of both luminescent and fluorescent calcium reporters.

3. Results: • the average period was around 38.7s for both calcium and elongation rate oscillation • calcium oscillations lag growth oscillations by around 4.1s, 38o • calcium concentration is very high at the tip and decline rapidly down the shaft of the tube • the free calcium level in at the growing tip oscillates, on average, between 3 and 10 μM • all intracellular ionic changes, extracellular fluxes of specific ions, and net current influx are found to oscillate and have amplitudes that are reflective of the preceding growth oscillation

6. The proposed model: • A few assumptions: • The current influx marks the time when nonspecific, stretch-activated cation channels are opened • the Ca2+ that enters the tube tip raises the cytosolic Ca2+ to some critical threshold in order to release Ca2+ from intracellular stores – secretory vesicles • the added membrane, in this model, acts to reduce tension on the membrane and close the stretch-activated channels thus reducing net ionic current entry • the fusion of vesicles and release of cell wall material during the decrease in growth rate slacken the plasma membrane and produce a relatively thickened cell wall

7. Model description • Due to the surge in growth and the resultant increase in cytoplasmic volume, the turgor will have decreased • The turgor is partially restored by H+-driven K+ uptake • The generation of a critical threshold of turgor overcomes the cell wall strength to promote growth • After turgor has reached a critical threshold, the thickened cell wall yields, leading to another growth surge • As little secretion occurs at this time, any further elongation will stretch the existing cell wall and take the slack out of the membrane, thus tensing the membrane • Tension in the membrane then opens stretch-activated channels leading to the triggering amount of Ca2+ (H+,K+) that releases Ca2+ from intracellular stores.

9. Growth surge cell wall yields turgor increase H+, and H+ driven K+ tense the membrane open the stretch-activated channel produce added membrane to slacken the plasmic membrane produce a relatively thicken cell wall Ca2+ influx, leading to the release of Ca2+ intracellular stores membrane materials Ca2+ tip concentration increase secretion cell wall materials