What forces drive cell wall expansion?

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Abstract
Fungal hyphae characteristically extend at the apex, by the localized deposition of new cell wall and plasma membrane. That entails the performance of work and raises the question, what forces drive hyphal extension in the face of surface cohesion and external resistance? Conventional wisdom credits turgor, i.e., hydrostatic pressure, with driving the tip forward and shaping it by plastic deformation. An experimental test of this hypothesis became possible with the discovery that the oomycetes Achlya bisexualis and Saprolegnia ferax do not regulate turgor. When the osmotic pressure of the medium is raised by the addition of sucrose or other osmolytes, the organisms produce a more plastic wall and continue to grow. Saprolegnia ferax produces near-normal hyphae in the absence of any measurable turgor. Responses to variations in the composition of the medium and to a range of inhibitors indicate that the processes responsible for growth are the same in normal hyphae (4 bars; 1 bar = 100 kPa) and in turgorless ones. Our observations imply that hyphal extension in oomycetes has much in common with pseudopod extension in animal cells, in that polymerization of the actin meshwork in the apical region plays an indispensable role. In the extreme case, when turgor is essentially zero and the wall is most plastic, actin polymerization may contribute substantially to the driving force for extension. But when turgor is high and the wall rigid, hydrostatic pressure is likely to be required to stress the wall, allowing it to expand and admit new wall material. Key words: hyphae, turgor, actin, apical growth, tip growth, cell wall.
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