Hydrodynamics and cell volume oscillations in the pollen tube apical region are integral components of the biomechanics of Nicotiana tabacum pollen tube growth

Pollen tube growth is localized at the apex and displays oscillatory dynamics. It is thought that a balance between intracellular turgor pressure (hydrostatic pressure, reflected by the cell volume) and cell wall loosening is a critical factor driving pollen tube growth. We previously demonstrated that water flows freely into and out of the pollen tube apical region dependent on the extracellular osmotic potential, that cell volume changes reflect changes in the intracellular pressure, and that cell volume changes differentially induce, increases or decreases in specific phospholipid signals. This article shows that manipulation of the extracellular osmotic potential rapidly induces modulations in pollen tube growth rate frequencies, demonstrating that changes in the intracellular pressure are sufficient to reset the pollen tube growth oscillator. This indicates a direct link between intracellular hydrostatic pressure and pollen tube growth. Altering hydrodynamic flow through the pollen tube by replacing extracellular H₂O with ²H₂O adversely affects both cell volume and growth rate oscillations and induces aberrant morphologies. Normal growth and cell morphology are rescued by replacing ²H₂O with H₂O. Further studies revealed that the cell volume oscillates in the pollen tube apical region. These cell volume oscillations were not from changes in cell shape at the tip and were detectable up to 30 μm distal to the tip (the longest length measured). Cell volume in the apical region oscillates with the same frequency as growth rate oscillations but surprisingly the cycles are phase-shifted by 180°. Raman microscopy yields evidence that hydrodynamic flow out of the apex may be part of the biomechanics that drive cellular expansion. The combined results suggest that hydrodynamic loading/unloading in the apical region induces cell volume oscillations and has a role in driving cell elongation and pollen tube growth.