Aggregate Filamentous Growth Responses in Yeast

Abstract

Many fungal species, including pathogens, undergo a morphogenetic response called filamentous growth, where cells differentiate into a specialized cell type to promote nutrient foraging and surface colonization. Despite the fact that filamentous growth is required for virulence in some plant and animal pathogens, certain aspects of this behavior remain poorly understood. By examining filamentous growth in the budding yeast Saccharomyces cerevisiae and the opportunistic pathogen Candida albicans, we identify responses where cells undergo filamentous growth in groups of cells or aggregates. In S. cerevisiae, aggregate invasive growth was regulated by signaling pathways that control normal filamentous growth. These pathways promoted aggregation in part by fostering aspects of microbial cooperation. For example, aggregate invasive growth required cellular contacts mediated by the flocculin Flo11p, which was produced at higher levels in aggregates than cells undergoing regular invasive growth. Aggregate invasive growth was also stimulated by secreted enzymes, like invertase, which produce metabolites that are shared among cells. Aggregate invasive growth was also induced by alcohols that promote density-dependent filamentous growth in yeast. Aggregate invasive growth also required highly polarized cell morphologies, which may affect the packing or organization of cells. A directed selection experiment for aggregating phenotypes uncovered roles for the fMAPK and RAS pathways, which indicates that these pathways play a general role in regulating aggregate-based responses in yeast. Our study extends the range of responses controlled by filamentation regulatory pathways and has implications in understanding aspects of fungal biology that may be relevant to fungal pathogenesis. IMPORTANCE Filamentous growth is a fungal morphogenetic response that is critical for virulence in some fungal species. Many aspects of filamentous growth remain poorly understood. We have identified an aspect of filamentous growth in the budding yeast Saccharomyces cerevisiae and the human pathogen Candida albicans where cells behave collectively to invade surfaces in aggregates. These responses may reflect an extension of normal filamentous growth, as they share the same signaling pathways and effector processes. Aggregate responses may involve cooperation among individual cells, because aggregation was stimulated by cell adhesion molecules, secreted enzymes, and diffusible molecules that promote quorum sensing. Our study may provide insights into the genetic basis of collective cellular responses in fungi. The study may have ramifications in fungal pathogenesis, in situations where collective responses occur to promote virulence.

Biochemical and genetic analysis of Ecm14, a conserved fungal pseudopeptidase

“…Ecm14 has been identi ed in studies of several pathogenic fungi in recent years, and these studies continue to point toward a function in cell wall/extracellular matrix remodeling. A C. albicans screen for genes involved in aggregate invasive growth, a process related to bio lm formation, identi ed an ECM14 mutant showing fewer aggregates than the wild-type [44]. This hints at a role in aggregate invasion, although the equivalent experiment performed with an S. cerevisiae ecm14Δ mutant did not result in a phenotype [44], suggesting either a false positive in the rst screen or the use of inappropriate conditions for this assay with S. cerevisiae.…”

“…A C. albicans screen for genes involved in aggregate invasive growth, a process related to bio lm formation, identi ed an ECM14 mutant showing fewer aggregates than the wild-type [44]. This hints at a role in aggregate invasion, although the equivalent experiment performed with an S. cerevisiae ecm14Δ mutant did not result in a phenotype [44], suggesting either a false positive in the rst screen or the use of inappropriate conditions for this assay with S. cerevisiae. A transcriptomics study of the plant pathogen V. dahliae showed that the expression of Ecm14, along with a variety of cell wall degrading hydrolases, was upregulated in response to exudates from V. dahliae-susceptible cotton roots [45].…”

Cell Communications among Microorganisms, Plants, and Animals: Origin, Evolution and Interplays

“…Direct communications between yeasts occur through membrane proteins such as flocculins, which are lectins recognizing their partners' polysaccharide chains to form a solid mass (veil, biofilm, etc.) [24,25]. Yeasts also communicate via soluble molecules that can diffuse and affect the community's organization on a long-range (quorum sensing).…”

“…QS molecules identified in fungi include peptide pheromones, oxylipins, aromatic alcohols (such as tyrosol and farnesol [26]), α1-3 glucans [27] and pantothenic acid [28]. For example, C. Albicans cells develop as budding yeasts when inoculated at farnesol act through receptors acting as transcription factors, which respectively, accelerate and block the change from yeasts to hyphae morphology [24]. Moreover, farnesol inhibits the immune system of humans hosting C. Albicans' [29].…”

Cell Communications among Microorganisms, Plants, and Animals: Origin, Evolution and Interplays

“…Direct communications between yeasts occur through membrane proteins such as flocculins, which are lectins recognizing their partners' polysaccharide chains to form a solid mass (veil, biofilm, etc.) [34,35]. Yeasts also communicate via soluble molecules that can diffuse and affect the community's organization on a long-range (quorum sensing).…”