Studies on Saprolegnia ferax suggest the general importance of the cytoplasm in determining hyphal morphology

Tip growth of hyphae is the dominant growth form of the vast majority of fungi, and of their morphological allies, the oomycetes. Tip growth has been considered to result from the balance between the extensibility properties of the apical cell wall and the pressure generated by turgor. This model places regulation of tip growth in the extracellular domain, outside of the direct influence of the normal intracellular regulatory systems, which is at odds with the extraordinary level of regulatory precision needed to produce the typically even-diameter tube characteristic of hyphae. An alternative model, wherein regulation of tip extensibility resides in a part of the cytoskeleton linked to the plasma membrane, places control of hyphal morphology in the cytoplasm. This review focuses on the diverse processes of tip growth as they relate to the latter model, drawing heavily on our results from of the oomycete, Saprolegnia ferax. The concept of tensegrity is suggested as a possible integrator of these processes.