Regulation of polarity during morphological transitions in Aureobasidium pullulans

Many fungi are able to switch between different morphologies by regulating the behavior of the cytoskeleton. In turn, the cytoskeleton is regulated by Rho-family GTPases, whose differing spatiotemporal patterns of activity lead to different modes of growth. The polymorphic fungus Aureobasidium pullulans alternates between multiple growth modes, including multibudding, isotropic, and hyphal growth. Here, we describe hyphal growth and the yeast-to-hyphal transition in this organism, with a focus on cell polarity. Yeast cells switched between cycles of budding and isotropic expansion in a variable manner prior to initiating hyphal extension. Initial hyphal extension proceeded in a stop-and-start manner with frequent pauses, before switching to continuous growth. Hyphal branching occurred from the sides of hyphae, and septation at multiple sites occurred in more mature hyphae. Previous work showed that either of two Rho-family GTPases, Rac1 and Cdc42, were sufficient promote budding. However, while Rac1 was also able to drive hyphal extension without Cdc42, Cdc42 could not promote hyphal growth without Rac1. Our findings provide the basis for an investigation of how polarity components are re-purposed for different fungal growth modalities.