At the edge of a fungal colony, leading hyphae grow into new territory in search of food. Behind the colony edge, the hyphae interconnect to form a three-dimensional network that is optimized to extract nutrients from the surrounding medium in order to fuel continued exploration. Colony growth can be fast (about 10–100 μm min−1, depending on the organism, nutrient availability and temperature) and involves the continuous synthesis of all the cellular constituents that are necessary for rapid cell expansion. A major driving force for cell expansion is pressure.
Pressure is a thermodynamic state property that affects the life of all organisms. In cells that lack a cell wall, excessive pressure can result in cell lysis and death. In cells that do have a wall (most bacteria, algae, fungi and plants), an internal hydrostatic pressure (turgor) provides both mechanical support for free-standing structures and a force that drives cellular expansion, substrate penetration and other processes. Extreme examples from fungi are the projectile release of spores at >100,000 × g (g is the acceleration due to gravity at the Earth’s surface) in ascomycetes and zygomycetes.
This review describes the roles of turgor and pressure in fungal growth. How is turgor regulated? How does it affect tip growth? Do intra-hyphal pressure gradients play a part in fungal growth (such as in the transport of new materials to the growing tip)? These areas of active research are revealing the mechanisms of hyphal growth in filamentous fungi and are relevant to applied research on pathogenicity and the control of fungal diseases.
How does a hypha grow? The biophysics of pressurized growth in fungi. (2011) Nature Reviews Microbiology 9, 509-518 doi:10.1038/nrmicro2591
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 structures. Turgor is controlled by an osmotic mitogen-activated protein kinase cascade that causes de novo synthesis of osmolytes and uptake of ions from the external medium. However, as discussed in this Review, turgor and pressure have additional roles in hyphal growth, such as causing the mass flow of cytoplasm from the basal mycelial network towards the expanding hyphal tips at the colony edge.