2 WATER MOLDS CAN GROW WITHOUT MEASURABLE TURGOR PRESSURE

The water molds Achlya bisexualis Coker and Saprolegnia ferax (Gruithu isen) Thuret (Class: Oomycetes) normally grow in the form of slender h yphae with up to 0.8 MPa (8 bar) of internal pressure. Models of plant cell growth indicate that this turgor pressure drives the expansion o f the cell wall. However, under conditions of prolonged osmotic stress , these species were able to grow in the absence of measurable turgor. Unpressurized cells of A. bisexualis grew in the form of a plasmodium -like colony on solid media, and produced a multinucleate yeast-like p hase in liquid. By contrast, the morphology of S.ferax was unaffected by the loss of turgor, and the mold continued to generate tip-growing hyphae. Measurements of cell wall strength indicate that these microor ganisms produce a very fluid wall in the region of surface growth, cir cumventing the usual requirement for turgor.