Polarized growth in fungi: Symmetry breaking and hyphal formation

Cell shape is a critical determinant for function. The baker's yeast Saccharomyces cerevisiae changes shape in response to its environment, growing by budding in rich nutrients, forming invasive pseudohyphal filaments in nutrient poor conditions and pear shaped shmoos for growth towards a partner during mating. The human opportunistic pathogen Candida albicans can switch from budding to hyphal growth, in response to numerous environmental stimuli to colonize and invade its host. Hyphal growth, typical of filamentous fungi, is not observed in S. cerevisiae. A number of internal cues regulate when and where yeast cells break symmetry leading to polarized growth and ultimately distinct cell shapes. This review discusses how cells break symmetry using the yeast S. cerevisiae paradigm and how polarized growth is initiated and maintained to result in dramatic morphological changes during C. albicans hyphal growth.

► Symmetry breaking in budding yeast requires actin dependent processes, scaffold proteins and Cdc42 GTP hydrolyses.
► Modeling has revealed that Cdc42 diffusion in the plasma membrane is a defining parameter for symmetry breaking and singularity.
► Initiation of hyphal growth is a cell cycle independent process, which requires restricted activation of Cdc42.
► Hyphal shape is dictated by sustained apical G-protein activation, directed secretion and inhibition of cell separation.