WHAT FORCES DRIVE CELL-WALL EXPANSION

Fungal hyphae characteristically extend at the apex, by the localized deposition of new cell wall and plasma membrane. That entails the perf ormance of work and raises the question, what forces drive hyphal exte nsion in the face of surface cohesion and external resistance? Convent ional 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 o omycetes Achlya bisexualis and Saprolegnia ferax do not regulate turgo r. When the osmotic pressure of the medium is raised by the addition o f sucrose or other osmolytes, the organisms produce a more plastic wal l and continue to grow. Saprolegnia ferax produces near-normal hyphae in the absence of any measurable turgor. Responses to variations in th e 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 meshwor k in the apical region plays an indispensable role. In the extreme cas e, 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 pre ssure is likely to be required to stress the wall, allowing it to expa nd and admit new wall material.