Clarifying intercellular signalling in yeast: Saccharomyces cerevisiae does not undergo a quorum sensing-dependent switch to filamentous growth

Saccharomyces cerevisiae can alter its morphology to a filamentous form associated with unipolar budding in response to environmental stressors. Induction of filamentous growth is suggested under nitrogen deficiency in response to alcoholic signalling molecules through a quorum sensing mechanism. To investigate this claim, we analysed the budding pattern of S. cerevisiae cells over time under low nitrogen while concurrently measuring cell density and extracellular metabolite concentration. We found that the proportion of cells displaying unipolar budding increased between local cell densities of 4.8x106 and 5.3x107 cells/ml. However, the observed increase in unipolar budding could not be reproduced when cells were prepared at the critical cell density and in conditioned media. Removing the nutrient restriction by growth under high nitrogen conditions also resulted in an increase in unipolar budding between local cell densities of 5.2x106 and 8.2x107 cells/ml, but there were differences in metabolite concentration compared to the low nitrogen conditions. This suggests that neither cell density, metabolite concentration, nor nitrogen deficiency were necessary or sufficient to increase the proportion of unipolar budding cells. It is therefore unlikely that quorum sensing is the mechanism controlling the switch to filamentous growth in S. cerevisiae. Only a high concentration of the putative signalling molecule, 2-phenylethanol resulted in an increase in unipolar budding, but this concentration was not physiologically relevant. We suggest that the compound 2-phenylethanol acts through a toxicity mechanism, rather than quorum sensing, to induce filamentous growth. IMPORTANCEInvestigating dimorphism in the model organism Saccharomyces cerevisiae has been instrumental in understanding the signalling pathways that control hyphal growth and virulence in human pathogenic fungi. Quorum sensing was proposed to signal morphogenesis in S. cerevisiae populations. This mechanism requires the switch to filamentous growth to occur at a critical quorum sensing molecule concentration corresponding to a critical cell density. However, evidence for this mechanism is sparse and limited by the use of non-physiologically relevant concentrations of signalling metabolites. Our study designed a methodology to address this gap and may be applied to further studies of dimorphism in other types of yeasts. A significant implication of our findings is that morphogenesis does not occur in S. cerevisiae via a quorum sensing mechanism, and this important definition needs to be corrected. Mechanistic studies to understand dimorphism in yeasts, by considering metabolite concentrations, will further shed light onto this important cellular behaviour.