Development of pseudohyphae by embedded haploid and diploid yeast

Aromatic Amino Acid-Derived Compounds Induce Morphological Changes and Modulate the Cell Growth of Wine Yeast Species

Yeasts secrete a large diversity of compounds during alcoholic fermentation, which affect growth rates and developmental processes, like filamentous growth. Several compounds are produced during aromatic amino acid metabolism, including aromatic alcohols, serotonin, melatonin, and tryptamine. We evaluated the effects of these compounds on growth parameters in 16 different wine yeasts, including non-Saccharomyces wine strains, for which the effects of these compounds have not been well-defined. Serotonin, tryptamine, and tryptophol negatively influenced yeast growth, whereas phenylethanol and tyrosol specifically affected non-Saccharomyces strains. The effects of the aromatic alcohols were observed at concentrations commonly found in wines, suggesting a possible role in microbial interaction during wine fermentation. Additionally, we demonstrated that aromatic alcohols and ethanol are able to affect invasive and pseudohyphal growth in a manner dependent on nutrient availability. Some of these compounds showed strain-specific effects. These findings add to the understanding of the fermentation process and illustrate the diversity of metabolic communication that may occur among related species during metabolic processes.

The Histone Acetyltransferase Gcn5 Regulates ncRNA-ICR1 and FLO11 Expression during Pseudohyphal Development in Saccharomyces cerevisiae

Filamentous growth is one of the key features of pathogenic fungi during the early infectious phase. The pseudohyphal development of yeast Saccharomyces cerevisiae shares similar characteristics with hyphae elongation in pathogenic fungi. The expression of FLO11 is essential for adhesive growth and filament formation in yeast and is governed by a multilayered transcriptional network. Here we discovered a role for the histone acetyltransferase general control nonderepressible 5 (Gcn5) in regulating FLO11-mediated pseudohyphal growth. The expression patterns of FLO11 were distinct in haploid and diploid yeast under amino acid starvation induced by 3-amino-1,2,4-triazole (3AT). In diploids, FLO11 expression was substantially induced at a very early stage of pseudohyphal development and decreased quickly, but in haploids, it was gradually induced. Furthermore, the transcription factor Gcn4 was recruited to the Sfl1-Flo8 toggle sites at the FLO11 promoter under 3AT treatment. Moreover, the histone acetylase activity of Gcn5 was required for FLO11 induction. Finally, Gcn5 functioned as a negative regulator of the noncoding RNA ICR1, which is known to suppress FLO11 expression. Gcn5 plays an important role in the regulatory network of FLO11 expression via Gcn4 by downregulating ICR1 expression, which derepresses FLO11 for promoting pseudohyphal development.

Saccharomyces unisporus: Biotechnological Potential and Present Status

 The yeast species of the Saccharomyces genus have a long history of traditional applications and beneficial effects. Among these presence of the Saccharomyces unisporus has been documented in various dairy products and has become a subject of interest and great importance. S. unisporus has shown a significant role in the ripening of cheese and production of fermented milk products such as kefir and koumiss. The absence of pseudohyphae during the life cycle of S. unisporus is an indication of nonpathogenicity. Significance has been laid on the presence of S. unisporus in food-grade products and a close proximity of S. unisporus to S. florentinus and both of these species are accepted by the International Dairy Federation and the European Food and Feed Cultures Association for food and feed applications. Since over the years, S. unisporus has already become a part of various dairy products, S. unisporus can be considered as a potential candidate for generally regarded as safe status. S. unisporus has the capacity to convert ketoisophorone to levodione, which is an important pharmaceutical precursor. S. unisporus are considered as the potential producers of farnesol which eventually controls filamentation of pathogenic microorganisms. Apart from that, S. unisporus produces certain omega unsaturated fatty acids which combat diseases. Henceforth, the areas which S. unisporus can be possibly exploited for its useful intermediates are the enzymes and fatty acids it produces. In this context, this review attempts to describe and discuss the ubiquity of S. unisporus in food products, cellular composition, regulatory pathways, and its synthesis of fatty acids and enzymes.

Ste12 and Ste12-like proteins, fungal transcription factors regulating development and pathogenicity

Ste12 and Ste12-like proteins are transcription factors found exclusively in the fungal kingdom. In the yeast model Saccharomyces cerevisiae, where the first member was identified, Ste12p was shown to regulate mating and invasive/pseudohyphal growth. In recent literature, there have been several reports of Ste12-like factors in multiple fungal systems, yeasts or filamentous fungi, with saprophytic or parasitic life-styles. In all these models, Ste12 and Ste12-like factors are involved in the regulation of fungal development and pathogenicity. In this review, we discuss the features, the regulation, and the role of Ste12 and Ste12-like factors by highlighting the similarities and dissimilarities that occur within this group.

Role of the cell wall integrity and filamentous growth mitogen-activated protein kinase pathways in cell wall remodeling during filamentous growth

Many fungal species including pathogens exhibit filamentous growth (FG) as a means of foraging for nutrients. Genetic screens were performed to identify genes required for FG in the budding yeast Saccharomyces cerevisiae. Genes encoding proteins with established functions in transcriptional activation (MCM1, MATalpha2, PHD1, MSN2, SIR4, and HMS2), cell wall integrity (MPT5, WSC2, and MID2), and cell polarity (BUD5) were identified as potential regulators of FG. The transcription factors MCM1 and MATalpha2 induced invasive growth by promoting diploid-specific bipolar budding in haploid cells. Components of the cell wall integrity pathway including the cell surface proteins Slg1p/Wsc1p, Wsc2p, Mid2p, and the mitogen-activated protein kinase (MAPK) Slt2p/Mpk1p contributed to multiple aspects of the FG response including cell elongation, cell-cell adherence, and agar invasion. Mid2p and Wsc2p stimulated the FG MAPK pathway through the signaling mucin Msb2p and components of the MAPK cascade. The FG pathway contributed to cell wall integrity in parallel with the cell wall integrity pathway and in opposition with the high osmolarity glycerol response pathway. Mass spectrometry approaches identified components of the filamentous cell wall including the mucin-like proteins Msb2p, Flo11p, and subtelomeric (silenced) mucin Flo10p. Secretion of Msb2p, which occurs as part of the maturation of the protein, was inhibited by the ss-1,3-glucan layer of the cell wall, which highlights a new regulatory aspect to cell wall remodeling in this organism. Disruption of ss-1,3-glucan linkages induced mucin shedding and resulted in defects in cell-cell adhesion and invasion of cells into the agar matrix.

An interrelationship between autophagy and filamentous growth in budding yeast

Over the last 15 years, yeast pseudohyphal growth (PHG) has been the focus of intense research interest as a model of fungal pathogenicity. Specifically, PHG is a stress response wherein yeast cells deprived of nitrogen form filaments of elongated cells. Nitrogen limitation also induces autophagy, a ubiquitous eukaryotic stress response in which proteins are trafficked to the vacuole/lysosome for degradation and recycling. Although autophagy and filamentous growth are both responsive to nitrogen stress, a link between these processes has not been investigated to date. Here, we present several studies describing an interrelationship between autophagy and filamentous growth. By microarray-based expression profiling, we detect extensive upregulation of the pathway governing autophagy during early PHG and find both processes active under conditions of nitrogen stress in a filamentous strain of budding yeast. Inhibition of autophagy results in increased PHG, and autophagy-deficient yeast induce PHG at higher concentrations of available nitrogen. Our results suggest a model in which autophagy mitigates nutrient stress, delaying the onset of PHG; conversely, inhibition of autophagy exacerbates nitrogen stress, resulting in precocious and overactive PHG. This physiological connection highlights the central role of autophagy in regulating the cell's nutritional state and the responsiveness of PHG to that state.

Large-scale analysis of yeast filamentous growth by systematic gene disruption and overexpression

Under certain conditions of nutrient stress, the budding yeast Saccharomyces cerevisiae initiates a striking developmental transition to a filamentous form of growth, resembling developmental transitions required for virulence in closely related pathogenic fungi. In yeast, filamentous growth involves known mitogen-activated protein kinase and protein kinase A signaling modules, but the full scope of this extensive filamentous response has not been delineated. Accordingly, we have undertaken the first systematic gene disruption and overexpression analysis of yeast filamentous growth. Standard laboratory strains of yeast are nonfilamentous; thus, we constructed a unique set of reagents in the filamentous Sigma1278b strain, encompassing 3627 integrated transposon insertion alleles and 2043 overexpression constructs. Collectively, we analyzed 4528 yeast genes with these reagents and identified 487 genes conferring mutant filamentous phenotypes upon transposon insertion and/or gene overexpression. Using a fluorescent protein reporter integrated at the MUC1 locus, we further assayed each filamentous growth mutant for aberrant protein levels of the key flocculence factor Muc1p. Our results indicate a variety of genes and pathways affecting filamentous growth. In total, this filamentous growth gene set represents a wealth of yeast biology, highlighting 84 genes of uncharacterized function and an underappreciated role for the mitochondrial retrograde signaling pathway as an inhibitor of filamentous growth.

Modulation of Congo-red-induced aberrations in the yeast Saccharomyces cerevisiae by the general stress response protein Hsp12p

Previous studies have shown that in Saccharomyces cerevisiae HSP12, which codes for the small cell wall heat shock protein Hsp12p, was induced upon exposure to cell-wall-damaging agents such as Congo red. Here, we demonstrate that Hsp12p decreases the interaction between Congo red and chitin. A Δhsp12 mutant strain displayed decreased viability, increased aggregation and sedimentation, as well as an altered morphology when grown in the presence of Congo red dye. The presence of Hsp12p was also necessary for the Congo-red-mediated invasion of agar plates.

Effects of low-shear modeled microgravity on cell function, gene expression, and phenotype in Saccharomyces cerevisiae

Only limited information is available concerning the effects of low-shear modeled microgravity (LSMMG) on cell function and morphology. We examined the behavior of Saccharomyces cerevisiae grown in a high-aspect-ratio vessel, which simulates the low-shear and microgravity conditions encountered in spaceflight. With the exception of a shortened lag phase (90 min less than controls; P < 0.05), yeast cells grown under LSMMG conditions did not differ in growth rate, size, shape, or viability from the controls but did differ in the establishment of polarity as exhibited by aberrant (random) budding compared to the usual bipolar pattern of controls. The aberrant budding was accompanied by an increased tendency of cells to clump, as indicated by aggregates containing five or more cells. We also found significant changes (greater than or equal to twofold) in the expression of genes associated with the establishment of polarity (BUD5), bipolar budding (RAX1, RAX2, and BUD25), and cell separation (DSE1, DSE2, and EGT2). Thus, low-shear environments may significantly alter yeast gene expression and phenotype as well as evolutionary conserved cellular functions such as polarization. The results provide a paradigm for understanding polarity-dependent cell responses to microgravity ranging from pathogenesis in fungi to the immune response in mammals.

Genetic/genomic evidence for a key role of polarized endocytosis in filamentous differentiation of S. cerevisiae

Unicellular S. cerevisiae cells switch from the yeast form to pseudohyphal or filamentous form in response to environmental cues. We report that wild-type BY diploids (in which yeast ORFs have been systematically deleted) undergo normal HU-induced filamentous growth and discernable nitrogen starvation-induced filamentous growth, despite their perceived filamentation-deficient S288C genetic background. This finding allowed us to perform a genome-wide survey for non-essential genes that are required for filamentous growth with the homozygous deletion strains. We report that genes involved in endocytosis are required for both HU-induced and nitrogen starvation-induced filamentous growth. Surprisingly, no known genes involved in exocytosis are required. Despite the fact that polarized growth involves transport of vesicles to the site of growth, we failed to obtain genetic/genomic evidence that exocytosis plays an essential role in filamentous growth. A possible key role of polarized endocytosis (from the growth tip) is consistent with the proposed biological function of filamentous growth as a foraging behaviour. In addition, BUD8 that encodes the distal landmark in yeast-form bipolar budding is required for nitrogen starvation-induced but not HU-induced filamentous growth. Moreover, BUD5, SPA2, PEA2 and BUD6 that regulate bipolar bud site selection do not regulate the unipolar distal budding pattern in HU-induced filamentous growth.

The sensing of nutritional status and the relationship to filamentous growth in Saccharomyces cerevisiae

Heterotrophic organisms rely on the ingestion of organic molecules or nutrients from the environment to sustain energy and biomass production. Non-motile, unicellular organisms have a limited ability to store nutrients or to take evasive action, and are therefore most directly dependent on the availability of nutrients in their immediate surrounding. Such organisms have evolved numerous developmental options in order to adapt to and to survive the permanently changing nutritional status of the environment. The phenotypical, physiological and molecular nature of nutrient-induced cellular adaptations has been most extensively studied in the yeast Saccharomyces cerevisiae. These studies have revealed a network of sensing mechanisms and of signalling pathways that generate and transmit the information on the nutritional status of the environment to the cellular machinery that implements specific developmental programmes. This review integrates our current knowledge on nutrient sensing and signalling in S. cerevisiae, and suggests how an integrated signalling network may lead to the establishment of a specific developmental programme, namely pseudohyphal differentiation and invasive growth.

The roles of bud-site-selection proteins during haploid invasive growth in yeast

In haploid strains of Saccharomyces cerevisiae, glucose depletion causes invasive growth, a foraging response that requires a change in budding pattern from axial to unipolar-distal. To begin to address how glucose influences budding pattern in the haploid cell, we examined the roles of bud-site-selection proteins in invasive growth. We found that proteins required for bipolar budding in diploid cells were required for haploid invasive growth. In particular, the Bud8p protein, which marks and directs bud emergence to the distal pole of diploid cells, was localized to the distal pole of haploid cells. In response to glucose limitation, Bud8p was required for the localization of the incipient bud site marker Bud2p to the distal pole. Three of the four known proteins required for axial budding, Bud3p, Bud4p, and Axl2p, were expressed and localized appropriately in glucose-limiting conditions. However, a fourth axial budding determinant, Axl1p, was absent in filamentous cells, and its abundance was controlled by glucose availability and the protein kinase Snf1p. In the bud8 mutant in glucose-limiting conditions, apical growth and bud site selection were uncoupled processes. Finally, we report that diploid cells starved for glucose also initiate the filamentous growth response.

MST12 regulates infectious growth but not appressorium formation in the rice blast fungus Magnaporthe grisea

In the rice blast fungus Magnaporthe grisea, a mitogen-activated protein kinase gene, PMK1, is known to regulate appressorium formation and infectious hyphae growth. Since PMK1 is homologous to the FUS3 and KSS1 genes that regulate the transcription factor STE12 in yeast, we functionally characterized the STE12 homologue in M. grisea (MST12). A polymerase chain reaction-based approach was used to isolate the MST12 gene that is homologous to yeast STE12. Four mst12 deletion mutants were isolated by gene replacement. No obvious defect in vegetative growth, conidiation, or conidia germination was observed in mst12 mutants. However, mst12 mutants were nonpathogenic on rice and barley leaves. In contrast to pmk1 mutants that did not form appressoria, mst12 mutants produced typical dome-shaped and melanized appressoria. However, the appressoria formed by mst12 mutants failed to penetrate onion epidermal cells. When inoculated through wound sites, mst12 mutants failed to cause spreading lesions and appeared to be defective in infectious growth. These data indicate that MST12 may function downstream of PMK1 to regulate genes involved in infectious hyphae growth. A transcription factor or factors other than MST12 must exist in M. grisea and function downstream from PMK1 for appressorium formation.

Control of pseudohyphae formation in Saccharomyces cerevisiae

Pseudohyphal growth in both haploid and diploid strains of Saccharomyces cerevisiae reflects concerted changes in different cellular processes: budding pattern, cell elongation and cell adhesion. These changes are triggered by environmental signals and are controlled by several pathways which act in parallel. Nitrogen deprivation, and possibly other stresses, activate a MAP kinase cascade which has the transcription factor Ste12 as its final target. A cAMP-dependent pathway, in which the protein kinase Tpk2 plays a specific role, is also required for the morphogenetic switch. Both pathways contribute to modulate the expression of the MUC1/FLO11 gene which encodes a cell-surface flocculin required for pseudohyphal and invasive growth. The MAP kinase cascade could also control the activity of the cyclin/Cdc28 complexes which affect both the budding pattern of yeast and cell elongation. A further protein which stimulates filamentous growth in S. cerevisiae is Phd1; although its mode of action is unknown, it may be regulated by a cAMP-dependent protein kinase, as occurs with the homologous protein Efg1 from Candida albicans, which is required for the formation of true hyphae. Morphogenesis in different yeast genera share common elements, but there are also important differences. Although a complete picture cannot yet be drawn, partial models may be proposed for the interaction of the regulatory pathways, both in the case of S. cerevisiae and in that of C. albicans.

Polarization of cell growth in yeast. I. Establishment and maintenance of polarity states

The ability to polarize is a fundamental property of cells. The yeast Saccharomyces cerevisiae has proven to be a fertile ground for dissecting the molecular mechanisms that regulate cell polarity during growth. Here we discuss the signaling pathways that regulate polarity. In the second installment of this two-part commentary, which appears in the next issue of Journal of Cell Science, we discuss how the actin cytoskeleton responds to these signals and guides the polarity of essentially all events in the yeast cell cycle. During the cell cycle, yeast cells assume alternative states of polarized growth, which range from tightly focused apical growth to non-focused isotropic growth. RhoGTPases, and in particular Cdc42p, are essential to guiding this polarity. The distribution of Cdc42p at the cell cortex establishes cell polarity. Cyclin-dependent protein kinase, Ras, and heterotrimeric G proteins all modulate yeast cell polarity in part by altering the distribution of Cdc42p. In turn, Cdc42p generates feedback signals to these molecules in order to establish stable polarity states and coordinate cytoskeletal organization with the cell cycle. Given that many of these signaling pathways are present in both fungi and animals, they are probably ancient and conserved mechanisms for regulating polarity.

Filamentous growth of Candida albicans in response to physical environmental cues and its regulation by the unique CZF1 gene

Hyphal growth in the opportunistic fungal pathogen Candida albicans is believed to contribute to the virulence of the organism by promoting penetration of fungal cells into host tissue. In this study, stimulation of hyphal growth by a feature of the physical environment was demonstrated. Specifically, growth of cells embedded within a matrix promoted the formation of hyphae. The CZF1 gene, encoding a putative transcription factor, was shown to be involved in the regulation of hyphal growth under certain conditions, including embedded conditions. Ectopic expression of CZF1 in embedded cells promoted the rapid formation of hyphae. Elimination of CZF1 and CPH1, encoding a homologue of the Saccharomyces cerevisiae Ste12p transcription factor, led to a pronounced defect in filamentous growth of embedded cells. Elimination of CZF1 alone led to a moderate defect in hyphal growth under some conditions, including embedded conditions. Hyphal morphogenesis in response to matrix embedding may occur in the opportunistic pathogen, C. albicans, to promote invasion of fungal cells into host tissue.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.