Ecological Inducers of the Yeast Filamentous Growth Pathway Reveal Environment-Dependent Roles for Pathway Components

ABSTRACT Signaling modules, such as MAPK pathways, are evolutionarily conserved drivers of cell differentiation and stress responses. In many fungal species including pathogens, MAPK pathways control filamentous growth, where cells differentiate into an elongated cell type. The convenient model budding yeast Saccharomyces cerevisiae undergoes filamentous growth by the filamentous growth (fMAPK) pathway; however, the inducers of the pathway remain unclear, perhaps because pathway activity has been mainly studied in laboratory conditions. To address this knowledge gap, an ecological framework was employed, which uncovered new fMAPK pathway inducers, including pectin, a material found in plants, and the metabolic byproduct ethanol. We also show that induction by a known inducer of the pathway, the non-preferred carbon source galactose, required galactose metabolism and induced the pathway differently than glucose limitation or other non-preferred carbon sources. By exploring fMAPK pathway function in fruit, we found induction of the pathway led to visible digestion of fruit rind through a known target, PGU1 , which encodes a pectolytic enzyme. Different stimuli revealed different modes of pathway signaling. For example, combinations of inducers (galactose and ethanol) stimulated the pathway to near maximal levels, which showed dispensability of several fMAPK pathway components (e.g. mucin sensor, PAK), but not others (e.g. adaptor, MAPKKK) and required the Ras2-PKA pathway. This included a difference between the transcription factor binding partners for the pathway, as Tec1p, but not Ste12p, was partly dispensable for fMAPK pathway activity. Thus, by exploring ecologically-relevant stimuli, new modes of MAPK pathway signaling were uncovered, perhaps revealing how a pathway can respond differently to specific environments. Data Availability Statement All data are in the manuscript and/or supporting information files.