Mass flow and pressure-driven hyphal extension in Neurospora crassa

Abstract

Mass flow of cytoplasm in Neurospora crassa trunk hyphae was directly confirmed by injecting oil droplets into the hyphae. The droplets move in a manner similar to cytoplasmic particles and vacuoles within the hyphae. The direction of mass flow is towards the growing hyphal tips at the colony edge. Based on flow velocities (about 5 μm s−1), hyphal radius and estimates of cytoplasm viscosity, the Reynolds number is about 10−4, indicating that mass flow is laminar. Therefore, the Poiseulle equation can be used to calculate the pressure gradient required for mass flow: 0·0005–0·1 bar cm−1 (depending on the values used for septal pore radius and cytoplasmic viscosity). These values are very small compared to the normal hydrostatic pressure of the hyphae (4–5 bar). Mass flow stops after respiratory inhibition with cyanide, or creation of an extracellular osmotic gradient. The flow is probably caused by internal osmotic gradients created by differential ion transport along the hyphae. Apical cytoplasm migrates at the same rate as tip extension, as do oil droplets injected near the tip. Thus, in addition to organelle positioning mediated by molecular motors, pressure-driven mass flow may be an integral part of hyphal extension.

Growth-induced mass flows in fungal networks

“…) matches the rate of tip extension (Lew 2005). Such movement would be consistent with mass flow driven by the continuous subapical water influx required to sustain volume increases at the tip during growth.…”

A pressure gradient facilitates mass flow in the oomycete Achlya bisexualis

“…In fungi, in addition to forcing the tip to yield, turgor may play an additional role in that if there are pressure gradients along the hypha, then mass flow may move the cytoplasm in an anterograde direction. Mass flow, as indicated by the movement of injected oil droplets, has previously been shown in N. crassa hyphae and the velocity of these movements was dependent on the pressure (Lew, 2005). Calculations suggest that mass flow could be facilitated by only a small pressure gradient (of the order of 0.0005-0.1 bar cm 21 ) (Lew, 2011).…”

“…The value of viscosity that we used was based on that reported by Fushimi & Verkman (1991) for the fluid cytoplasm of fibroblasts. This value was also used by Lew (2005) for calculating the pressure gradients that would be needed to sustain mass flow rates in N. crassa hyphae. However, the oomycete hyphae are likely to be more viscous.…”

“…In Neurospora crassa hyphae, Robertson & Rizvi (1968) indirectly estimated a turgor gradient of 5 bar between the tip and basal regions. Mass flow has been shown in N. crassa hyphae by the observation of injected oil droplets and calculations suggest that only a small pressure gradient (of the order of 0.0005-0.1 bar cm 21 ) would be required for such movement (Lew, 2005). These gradients could be set up by ion transport in different parts of the hyphae (Lew, 2011).…”

Transient responses during hyperosmotic shock in the filamentous fungus Neurospora crassa

“…Many walled cells maintain their turgor by the biosynthesis of osmotically active metabolites (Jennings, 1995;Bohnert & Jensen, 1996) and ion uptake (Lew et al, 2006). Turgor is believed to drive cellular expansion, and intrahyphal pressure gradients may move cytoplasm forward with the growing tip (Lew, 2005), but not all hyphal organisms regulate turgor (Kaminskyj et al, 1992;Lew et al, 2004), and there are examples of amoeboid fungal cells (the slime mutant of N. crassa) and oomycetes (Money & Harold, 1993) that grow in the absence of turgor. Thus, tip-growing organisms can access alternative pathways to growth and morphogenesis, allowing longterm survival.…”