The growth of many cell types combines polarized elongation with directional responses to external cues. We have previously linked Ca2+ influx with directional growth in fungi and show here that Ca2+ influx can rescue phenotypes caused by genetic disruption of two Cdc42 GTPase plasma-membrane trafficking pathways that are required for polarity establishment, hence restoring directional polarization. Constitutive activation of Cdc42 reversed the direction of polarization, which was also enhanced by the provision of exogenous Ca2+. Our model proposes that Ca2+ transport amplifies weak directional growth signals specified by activated Cdc42 by promoting Cdc42 trafficking to the plasma membrane, thereby enhancing its directional regulation of polarized growth.

Brand, A. C., Morrison, E., Milne, S., Gonia, S., Gale, C. A., & Gow, N. A. (2014). Cdc42 GTPase dynamics control directional growth responses. Proceedings of the National Academy of Sciences, 111(2), 811-816.