Franklin M. Harold

Two water molds can grow without measurable turgor pressure

The water molds Achlya bisexualis Coker and Saprolegnia ferax (Gruithuisen) Thuret (Class: Oomycetes) normally grow in the form of slender hyphae with up to 0.8 MPa (8 bar) of internal pressure. Models of plant cell growth indicate that this turgor pressure drives the expansion of 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 phase 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 microorganisms produce a very fluid wall in the region of surface growth, circumventing the usual requirement for turgor.

Biochemical Topology" From Vectorial Metabolism to Morphogenesis

In living cells, many biochemical processes are spatially organized: they have a location, and often a direction, in cellular space. In the hands of Peter Mitchell and Jennifer Moyle, the chemiosmotic formulation of this principle proved to be the key to understanding biological energy transduction and related aspects of cellular physiology. For H. E. Huxley and A. F. Huxley, it provided the basis for unravelling the mechanism of muscle contraction; and vectorial biochemistry continues to reverberate through research on cytoplasmic transport, motility and organization. The spatial deployment of biochemical processes serves here as a point of departure for an inquiry into morphogenesis and self-organization during the apical growth of fungal hyphae.

Electric and ionic dimensions of apical growth inAchlya hyphae

Hyphae of Achlya bisexualis generate endogenous electric currents, which are thought to represent the spatial organization of the chemiosmotic proton circuit. The experiments reported here bear on the relationship between currents and apical extension. The polarity of the current reverses as the colony ages, yet mature hyphae (as well as young ones) extend only at the apex. We also do not find an obligatory connection between inward current and the emergence of antheridial branches in Achlya ambisexualis . Applied electric fields orient hyphal extension, but whether the hyphae grow towards the anode or the cathode depends on the medium. We conclude that the hypothesis that electric currents localize outgrowth and determine polarity is no longer useful, and offer a reassessment of the currents nature and significance.