Abstract
Upon nutrient limitation, budding yeasts like Saccharomyces cerevisiae can be induced to adopt alternate filament-like growth patterns called diploid pseudohyphal or invasive haploid growth. Here, we report a novel constitutive pseudohyphal growth state, sharing some characteristics with classic forms of filamentous growth, but differing in crucial aspects of morphology, growth conditions and genetic regulation. The constitutive pseudohyphal state is observed in fus3 mutants containing various septin assembly defects, which we refer to as sadF growth (septin assembly defect induced filamentation) to distinguish it from classic filamentation pathways. Similar to other filamentous states, sadF cultures comprise aggregated chains of highly elongated cells. Unlike the classic pathways, sadF growth occurs in liquid rich media, requiring neither starvation nor the key pseudohyphal proteins, Flo8p and Flo11p. Moreover sadF growth occurs in haploid strains of S288C genetic background, which normally cannot undergo pseudohyphal growth. The sadF cells undergo highly polarized bud growth during prolonged G2 delays dependent on Swe1p. They contain septin structures distinct from classical pseudo-hyphae and FM4-64 labeling at actively growing tips similar to the Spitzenkörper observed in true hyphal growth. The sadF growth state is induced by synergism between Kss1p-dependent signaling and septin assembly defects; mild disruption of mitotic septins activates Kss1p-dependent gene expression, which exacerbates the septin defects, leading to hyper-activation of Kss1p. Unlike classical pseudo-hyphal growth, sadF signaling requires Ste5, Ste4 and Ste18, the scaffold protein and G-protein β and γ subunits from the pheromone response pathway, respectively. A swe1 mutation largely abolished signaling, breaking the positive feedback that leads to amplification of sadF signaling. Taken together, our findings show that budding yeast can access a stable constitutive pseudohyphal growth state with very few genetic and regulatory changes.
Highlights
Many fungal pathogens undergo a developmental transition from unicellular to multicellular filamentous forms that are important for the invasion of host tissue and virulence [1]
Stable Pseudohyphal Growth in Budding Yeast strains of the nonpathogenic budding yeast Saccharomyces cerevisiae are capable of forming invasive pseudohyphal filaments in nutrient poor conditions, which has served as a model system for the study of filamentous fungal pathogens
Stable pseudohyphal growth arises as a result of only two mutations, neither of which causes pseudohyphal growth on their own
Summary
Many fungal pathogens undergo a developmental transition from unicellular to multicellular filamentous forms that are important for the invasion of host tissue and virulence [1]. Certain strains of nonpathogenic Saccharomyces cerevisiae are capable of developing filament-like growth under starvation conditions, which is thought to serve as a foraging mechanism. Haploid yeast undergoes a similar morphological transition called “haploid invasive growth” in response to glucose depletion. Under these conditions, haploid cells penetrate the agar, but do not become as elongated as cells in diploid pseudo-hyphal cells and do not form extensive filaments on the agar surface [4]
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