How does a hypha grow? The biophysics of pressurized growth in fungi

Key Points Turgor (hydrostatic pressure within the cell) is a major driving force for cell expansion in fungi and in other walled cells. It is created by the accumulation of solutes inside the cell. The thermodynamics of pressure, cell volume and solute uptake are complicated by the dynamic process of cell expansion; all three parameters must change in a coordinated fashion in order to create a steady-state equilibrium that maintains the driving forces required for continued growth, including water uptake. The rate of growth is controlled by changes in cell wall extensibility. Tip-localized Ca 2+ gradients play a part inside the cell during cell expansion. The maintenance of turgor during growth relies on signal transduction pathways, and particularly on an osmotic mitogen-activated protein kinase cascade that regulates the accumulation of solutes (including ion uptake from the external medium). Pressure gradients within the cell also have a role during cell growth, by moving cytoplasm towards the growing tip. Essentially, mass flow operates in a microfluidics environment that encompasses the mycelial network. This is separate from the intracellular transport that is mediated by the cytoskeleton and molecular motors. The rate of cytoplasmic flow depends on pressure gradients that are small in magnitude but affect flow over relatively long distances. External ionic currents that extend over long distances are a manifestation of the macroscale polar aspects of fungal growth, and they point to coordinated contributions from the mycelial network behind the growing colony edge. They may play a part in the creation of the trans-hyphal pressure gradients that are responsible for cytoplasmic migration to the colony edge. Internal hydrostatic pressure (turgor) is a major force driving the growth of hyphae in filamentous fungi. Here, Lew reviews the complex interplay of turgor, calcium gradients, transport of secretory vesicles by molecular motors, and mass flow of cytoplasm during fungal growth. The mechanisms underlying the growth of fungal hyphae are rooted in the physical property of cell pressure. Internal hydrostatic pressure (turgor) is one of the major forces driving the localized expansion at the hyphal tip which causes the characteristic filamentous shape of the hypha. Calcium gradients regulate tip growth, and secretory vesicles that contribute to this process are actively transported to the growing tip by molecular motors that move along cytoskeletal str