Cell wall assembly in Saccharomyces cerevisiae

Functional analysis of MoSnf7 in Magnaporthe oryzae

Snf7 is the core subunit protein of the yeast endosomal sorting complex required for transport (ESCRT) complex, which plays important roles in endocytosis and autophagy. In this study, we characterized MoSnf7 in Magnaporthe oryzae, a homolog of yeast Snf7, the core protein of ESCRT-III subcomplex. Like Snf7, MoSnf7 also localizes next to the vacuoles. Deletion of MoSNF7 resulted in significant decrease in vegetative growth and pathogenicity. Further analyses of ΔMosnf7 mutants showed that they were defective in endocytosis, sexual and asexual development, turgor pressure maintenance of appressorium at hyphal tips, and cell wall integrity. Additional assays for the localization and degradation of GFP-MoAtg8 in ΔMosnf7 mutants showed that they were defective in autophagy pathway. Based on the roles of yeast Snf7 in endocytosis and autophagy, we propose that the decreased vegetative growth and pathogenicity of ΔMosnf7 rice blast fungus M. oryzae, was partly due to the conservative roles of MoSnf7 in vesicle trafficking and autophagy pathway.

Intracellular mechanism by which genotoxic stress activates yeast SAPK Mpk1

Stress-activated MAP kinases (SAPKs) respond to a wide variety of stressors. In most cases, the pathways through which specific stress signals are transmitted to the SAPKs are not known. The Saccharomyces cerevisiae SAPK Mpk1 (Slt2) is a well-characterized component of the cell-wall integrity (CWI) signaling pathway, which responds to physical and chemical challenges to the cell wall. However, Mpk1 is also activated in response to genotoxic stress through an unknown pathway. We show that, in contrast to cell-wall stress, the pathway for Mpk1 activation by genotoxic stress does not involve the stimulation of the MAP kinase kinases (MEKs) that function immediately upstream of Mpk1. Instead, DNA damage activates Mpk1 through induction of proteasomal degradation of Msg5, the dual-specificity protein phosphatase principally responsible for maintaining Mpk1 in a low-activity state in the absence of stress. Blocking Msg5 degradation in response to genotoxic stress prevented Mpk1 activation. This work raises the possibility that other Mpk1-activating stressors act intracellularly at different points along the canonical Mpk1 activation pathway.

Fungal Resistance to Echinocandins and the MDR Phenomenon in Candida glabrata

Candida glabrata has thoroughly adapted to successfully colonize human mucosal membranes and survive in vivo pressures. prior to and during antifungal treatment. Out of all the medically relevant Candida species, C. glabrata has emerged as a leading cause of azole, echinocandin, and multidrug (MDR: azole + echinocandin) adaptive resistance. Neither mechanism of resistance is intrinsic to C. glabrata, since stable genetic resistance depends on mutation of drug target genes, FKS1 and FKS2 (echinocandin resistance), and a transcription factor, PDR1, which controls expression of major drug transporters, such as CDR1 (azole resistance). However, another hallmark of C. glabrata is the ability to withstand drug pressure both in vitro and in vivo prior to stable "genetic escape". Additionally, these resistance events can arise within individual patients, which underscores the importance of understanding how this fungus is adapting to its environment and to drug exposure in vivo. Here, we explore the evolution of echinocandin resistance as a multistep model that includes general cell stress, drug adaptation (tolerance), and genetic escape. The extensive genetic diversity reported in C. glabrata is highlighted.

Impact of Fungal MAPK Pathway Targets on the Cell Wall

The fungal cell wall is an extracellular organelle that provides structure and protection to cells. The cell wall also influences the interactions of cells with each other and surfaces. The cell wall can be reorganized in response to changing environmental conditions and different types of stress. Signaling pathways control the remodeling of the cell wall through target proteins that are in many cases not well defined. The Mitogen Activated Protein Kinase pathway that controls filamentous growth in yeast (fMAPK) was required for normal growth in media containing the cell wall perturbing agent Calcofluor White (CFW). A mass spectrometry (MASS-SPEC) approach and analysis of expression profiling data identified cell wall proteins and modifying enzymes whose levels were influenced by the fMAPK pathway. These include Flo11p, Flo10p, Tip1p, Pry2p and the mannosyltransferase, Och1p. Cells lacking Flo11p or Och1p were sensitive to CFW. The identification of cell wall proteins controlled by a MAPK pathway may provide insights into how signaling pathways regulate the cell wall.

Roles of Three HSF Domain-Containing Proteins in Mediating Heat-Shock Protein Genes and Sustaining Asexual Cycle, Stress Tolerance, and Virulence in Beauveria bassiana

Heat-shock transcription factors (HSFs) with a HSF domain are regulators of fungal heat-shock protein (HSP) genes and many others vectoring heat-shock elements, to which the domain binds in response to heat shock and other stress cues. The fungal insect pathogen Beauveria bassiana harbors three HSF domain-containing orthologous to Hsf1, Sfl1, and Skn7 in many fungi. Here, we show that the three proteins are interrelated at transcription level, play overlapping or opposite roles in activating different families of 28 HSP genes and mediate differential expression of some genes required for asexual developmental and intracellular Na⁺ homeostasis. Expression levels of skn7 and sfl1 largely increased in Δhsf1, which is evidently lethal in some other fungi. Hsf1 was distinct from Sfl1 and Skn7 in activating most HSP genes under normal and heat-shocked conditions. Sfl1 and Skn7 played overlapping roles in activating more than half of the HSP genes under heat shock. Each protein also activated a few HSP genes not targeted by two others under certain conditions. Deletion of sfl1 resulted in most severe growth defects on rich medium and several minimal media at optimal 25°C while such growth defects were less severe in Δhsf1 and minor in Δskn7. Conidiation level was lowered by 76% in Δskn7, 62% in Δsfl1, and 39% in Δhsf1. These deletion mutants also showed differential changes in cell wall integrity, antioxidant activity, virulence and cellular tolerance to osmotic salt, heat shock, and UV-B irradiation. These results provide a global insight into vital roles of Hsf1, Sfl1, and Skn7 in B. bassiana adaptation to environment and host.

Overexpression or Deletion of Ergosterol Biosynthesis Genes Alters Doubling Time, Response to Stress Agents, and Drug Susceptibility in Saccharomyces cerevisiae

Ergosterol (ERG) is a critical sterol in the cell membranes of fungi, and its biosynthesis is tightly regulated by 25 known enzymes along the ERG production pathway. The effects of changes in expression of each ERG biosynthesis enzyme in Saccharomyces cerevisiae were analyzed by the use of gene deletion or plasmid-borne overexpression constructs. The strains overexpressing the ERG pathway genes were examined for changes in doubling time and responses to a variety of stress agents. In addition, ERG gene overexpression strains and ERG gene deletion strains were tested for alterations in antifungal drug susceptibility. The data show that disruptions in ergosterol biosynthesis regulation can affect a diverse set of cellular processes and can cause numerous phenotypic effects. Some of the phenotypes observed include dramatic increases in doubling times, respiratory deficiencies on glycerol media, cell wall insufficiencies on Congo red media, and disrupted ion homeostasis under iron or calcium starvation conditions. Overexpression or deletion of specific enzymes in the ERG pathway causes altered susceptibilities to a variety of classes of antifungal ergosterol inhibitors, including fluconazole, fenpropimorph, lovastatin, nystatin, amphotericin B, and terbinafine. This analysis of the effect of perturbations to the ERG pathway caused by systematic overexpression of each of the ERG pathway genes contributes significantly to the understanding of the ergosterol biosynthetic pathway and its relationship to stress response and basic biological processes. The data indicate that precise regulation of ERG genes is essential for cellular homeostasis and identify several ERG genes that could be exploited in future antifungal development efforts.IMPORTANCE A common target of antifungal drug treatment is the fungal ergosterol biosynthesis pathway. This report helps to identify ergosterol biosynthesis enzymes that have not previously been appreciated as drug targets. The effects of overexpression of each of the 25 ERG genes in S. cerevisiae were analyzed in the presence of six stress agents that target essential cellular processes (cell wall biosynthesis, protein translation, respiration, osmotic/ionic stress, and iron and calcium homeostasis), as well as six antifungal inhibitors that target ergosterol biosynthesis. The importance of identifying cell perturbations caused by gene overexpression or deletion is emphasized by the prevalence of gene expression alterations in many pathogenic and drug-resistant clinical isolates. Genes whose altered expression causes the most extensive phenotypic alterations in the presence of stressors or inhibitors have the potential to be drug targets.

Molecular architecture of fungal cell walls revealed by solid-state NMR

The high mortality of invasive fungal infections, and the limited number and inefficacy of antifungals necessitate the development of new agents with novel mechanisms and targets. The fungal cell wall is a promising target as it contains polysaccharides absent in humans, however, its molecular structure remains elusive. Here we report the architecture of the cell walls in the pathogenic fungus Aspergillus fumigatus. Solid-state NMR spectroscopy, assisted by dynamic nuclear polarization and glycosyl linkage analysis, reveals that chitin and α-1,3-glucan build a hydrophobic scaffold that is surrounded by a hydrated matrix of diversely linked β-glucans and capped by a dynamic layer of glycoproteins and α-1,3-glucan. The two-domain distribution of α-1,3-glucans signifies the dual functions of this molecule: contributing to cell wall rigidity and fungal virulence. This study provides a high-resolution model of fungal cell walls and serves as the basis for assessing drug response to promote the development of wall-targeted antifungals.

Necrotic cell death induced by dithianon on Saccharomyces cerevisiae

Dithianon is a broad-spectrum anthraquinone fungicide used to control several diseases of grapes, apples, and other fruits and vegetables. Its mode of action is described as multi-site and associated to thiol-reactivity. As other fungicides can affect non-phytopathogenic organisms as yeasts and bacteria, with impact on microbial population, diversity, and fermentation processes. In this context, we study the effect of dithianon on the model organism and fermentative yeast Saccharomyces cerevisiae in order to elucidate the mechanisms involved in yeast cell death., and explain its interference on wine fermentation kinetics. Thus for, we analyzed cellular protein and non-protein thiols, membrane and cell wall integrity, reactive oxygen species accumulation, mitochondrial membrane potential, and phosphatidylserine externalization. The results showed that when exponentially aerobic growing cells of S. cerevisiae are submitted to acute dithianon treatment they loss cell wall and membrane integrity, dying by necrosis, and this behavior is associated to a depletion of reduced proteic and non-proteic thiol groups. We also detected an important increase of cellular reactive oxygen species (ROS) associated to mitochondrial membrane potential modifications on dithianon treated cells. ROS accumulation was not associated to apoptotic cell death, but can be responsible for intracellular damages. Moreover, necrotic cell death induced by dithianon explains its effect on the kinetics of wine fermentations.

A Molecular Window into the Biology and Epidemiology of Pneumocystis spp

Pneumocystis, a unique atypical fungus with an elusive lifestyle, has had an important medical history. It came to prominence as an opportunistic pathogen that not only can cause life-threatening pneumonia in patients with HIV infection and other immunodeficiencies but also can colonize the lungs of healthy individuals from a very early age. The genus Pneumocystis includes a group of closely related but heterogeneous organisms that have a worldwide distribution, have been detected in multiple mammalian species, are highly host species specific, inhabit the lungs almost exclusively, and have never convincingly been cultured in vitro, making Pneumocystis a fascinating but difficult-to-study organism. Improved molecular biologic methodologies have opened a new window into the biology and epidemiology of Pneumocystis. Advances include an improved taxonomic classification, identification of an extremely reduced genome and concomitant inability to metabolize and grow independent of the host lungs, insights into its transmission mode, recognition of its widespread colonization in both immunocompetent and immunodeficient hosts, and utilization of strain variation to study drug resistance, epidemiology, and outbreaks of infection among transplant patients. This review summarizes these advances and also identifies some major questions and challenges that need to be addressed to better understand Pneumocystis biology and its relevance to clinical care.

Yeast Cell Wall Chitin Reduces Wine Haze Formation

Protein haze formation in bottled wines is a significant concern for the global wine industry, and wine clarification before bottling is therefore a common but expensive practice. Previous studies have shown that wine yeast strains can reduce haze formation through the secretion of certain mannoproteins, but it has been suggested that other yeast-dependent haze protective mechanisms exist. On the other hand, the addition of chitin has been shown to reduce haze formation, likely because grape chitinases have been shown to be the major contributors to haze. In this study, Chardonnay grape must fermented by various yeast strains resulted in wines with different protein haze levels, indicating differences in haze-protective capacities of the strains. The cell wall chitin levels of these strains were determined, and a strong correlation between cell wall chitin levels and haze protection capability was observed. To further evaluate the mechanism of haze protection, Escherichia coli-produced green fluorescent protein (GFP)-tagged grape chitinase was shown to bind efficiently to yeast cell walls in a cell wall chitin concentration-dependent manner, while commercial chitinase was removed from synthetic wine in quantities that also correlated with the cell wall chitin levels of the strains. Our findings suggest a new mechanism of reducing wine haze, and we propose a strategy for optimizing wine yeast strains to improve wine clarification.IMPORTANCE In this study, we establish a new mechanism by which wine yeast strains can impact the protein haze formation of wines, and we demonstrate that yeast cell wall chitin binds grape chitinase in a chitin concentration-dependent manner. We also show that yeast can remove this haze-forming protein from wine. Chitin has in the past been shown to efficiently reduce wine haze formation when added to the wine in high concentration as a clarifying agent. Our data suggest that the selection of yeast strains with high levels of cell wall chitin can reduce protein haze. We also investigate how yeast cell wall chitin levels are affected by environmental conditions.

Temporal regulation of morphogenetic events in Saccharomyces cerevisiae

Tip growth in fungi involves highly polarized secretion and modification of the cell wall at the growing tip. The genetic requirements for initiating polarized growth are perhaps best understood for the model budding yeast Saccharomyces cerevisiae. Once the cell is committed to enter the cell cycle by activation of G1 cyclin/cyclin-dependent kinase (CDK) complexes, the polarity regulator Cdc42 becomes concentrated at the presumptive bud site, actin cables are oriented toward that site, and septin filaments assemble into a ring around the polarity site. Several minutes later, the bud emerges. Here, we investigated the mechanisms that regulate the timing of these events at the single-cell level. Septin recruitment was delayed relative to polarity establishment, and our findings suggest that a CDK-dependent septin “priming” facilitates septin recruitment by Cdc42. Bud emergence was delayed relative to the initiation of polarized secretion, and our findings suggest that the delay reflects the time needed to weaken the cell wall sufficiently for the cell to bud. Rho1 activation by Rom2 occurred at around the time of bud emergence, perhaps in response to local cell-wall weakening. This report reveals regulatory mechanisms underlying the morphogenetic events in the budding yeast.

RNA-Seq analysis of global transcriptomic changes suggests a roles for the MAPK pathway and carbon metabolism in cell wall maintenance in a Saccharomyces cerevisiae FKS1 mutant

FKS1 encodes a β-1,3-glucan synthase, which is a key player in cell wall assembly in Saccharomyces cerevisiae. Here we analyzed the global transcriptomic changes in the FKS1 mutant to establish a correlation between the changes in the cell wall of the FKS1 mutant and the molecular mechanism of cell wall maintenance. These transcriptomic profiles showed that there are 1151 differentially expressed genes (DEGs) in the FKS1 mutant. Through KEGG pathway analysis of the DEGs, the MAPK pathway and seven pathways involved in carbon metabolism were significantly enriched. We found that the MAPK pathway is activated for FKS1 mutant survival and the synthesis of cell wall components are reinforced in the FKS1 mutant. Our results confirm that the FKS1 mutant has a β-1,3-glucan defect that affects the cell wall and partly elucidate the molecular mechanism responsible for cell wall synthesis. Our greater understanding of these mechanisms helps to explain how the FKS1 mutant survives, has useful implications for the study of similar pathways in other fungi, and increases the theoretical foundation for the regulation of the cell wall in S. cerevisiae.

High temperature induced disruption of the cell wall integrity and structure in Pleurotus ostreatus mycelia

Fungal cells are surrounded by a tight cell wall to protect them from harmful environmental conditions and to resist lysis. The synthesis and assembly determine the shape, structure, and integrity of the cell wall during the process of mycelial growth and development. High temperature is an important abiotic stress, which affects the synthesis and assembly of cell walls. In the present study, the chitin and β-1,3-glucan concentrations in the cell wall of Pleurotus ostreatus mycelia were changed after high-temperature treatment. Significantly higher chitin and β-1,3-glucan concentrations were detected at 36 °C than those incubated at 28 °C. With the increased temperature, many aberrant chitin deposition patches occurred, and the distribution of chitin in the cell wall was uneven. Moreover, high temperature disrupts the cell wall integrity, and P. ostreatus mycelia became hypersensitive to cell wall-perturbing agents at 36 °C. The cell wall structure tended to shrink or distorted after high temperature. The cell walls were observed to be thicker and looser by using transmission electron microscopy. High temperature can decrease the mannose content in the cell wall and increase the relative cell wall porosity. According to infrared absorption spectrum, high temperature broke or decreased the glycosidic linkages. Finally, P. ostreatus mycelial cell wall was easily degraded by lysing enzymes after high-temperature treatment. In other words, the cell wall destruction caused by high temperature may be a breakthrough for P. ostreatus to be easily infected by Trichoderma.

Improved Polysaccharide Production by Homologous Co-overexpression of Phosphoglucomutase and UDP Glucose Pyrophosphorylase Genes in the Mushroom Coprinopsis cinerea

Coprinopsis polysaccharides exhibit hypoglycemic and antioxidant activities. In this report, increases in polysaccharide production by homologous co-overexpression or individual homologous overexpression of phosphoglucomutase and UDP glucose pyrophosphorylase gene in Coprinopsis cinerea, which participate in polysaccharide biosynthesis. The transcription levels of the target genes were upregulated significantly in the oePGM-UGP strain when compared with the oePGM or oeUGP strain. The maximum intracellular polysaccharide content obtained in the oePGM-UGP strain was 1.49-fold higher than that of the WT strain, whereas a slight improvement in polysaccharide production was obtained in the oePGM and oeUGP strains. Extracellular polysaccharide production was enhanced by 75% in the oePGM-UGP strain when compared with that of the WT strain, whereas improvements of 30% and 16% were observed for the oePGM and oeUGP strains, respectively. These results show that multiple interventions in polysaccharide biosynthesis pathways of Basidiomycetes might improve polysaccharide yields when compared with that of single interventions.

Osw2 is required for proper assembly of glucan and/or mannan layers of the yeast spore wall

OSW2 is a meiotically-induced gene required for spore wall formation. osw2Δ spores are sensitive to ether treatment. Except for this phenotype, the mutants do not show obvious sporulation defects; thus, its function remains elusive. We found that deletion of both OSW2 and CHS3 results in a synthetic sporulation defect. The spore wall is composed of four layers, and chs3Δ spores lack the outer two (chitosan and dityrosine) layers. Thus, Osw2 is involved in the assembly of the inner (glucan and mannan) layers. In agreement with this notion, a glycosylphosphatidylinositol-anchored protein reporter mislocalizes in osw2Δ spores. The osw2Δ mutation also exhibited a severe synthetic sporulation defect when combined with the deletion of a ß-1,6-glucan synthesis-related gene, BIG1. Osw2 is localized to the prospore membrane during sporulation. However, it disappears in mature spores, indicating that it is not a structural component of the spore wall. Given that Osw2 contains a probable 2-dehydropantoate 2-reductase domain, it may mediate an enzymatic reaction. Osw2 shows a weak similarity to other 2-dehydropantoate 2-reductase domain-containing proteins, Svl3 and Pam1. A pam1Δsvl3Δ mutant exhibits vegetative cell and spore wall defects. Thus, the 2-dehydropantoate 2-reductase domain-containing proteins may have a similar function in glucan and/or mannan layer assembly.

Genomics-driven discovery of a novel self-resistance mechanism in the echinocandin-producing fungus Pezicula radicicola

The echinocandins are antifungal lipopeptides targeting fungi via noncompetitive inhibition of the β-1,3-d-glucan synthase FKS1 subunit. A novel echinocandin resistance mechanism involving an auxiliary copy of FKS1 in echinocandin-producing fungus Pezicula radicicola NRRL 12192 was discovered. We sequenced the genome of NRRL 12192 and predicted two FKS1-encoding genes (prfks1n and prfks1a), rather than a single FKS1 gene typical of filamentous ascomycetes. The prfks1a gene sits immediately adjacent to an echinocandin (sporiofungin) gene cluster, which was confirmed by disruption of prnrps4 and abolishment of sporiofungin production. Disruption of prfks1a dramatically increased the strain's sensitivity to exogenous echinocandins. In the absence of echinocandins, transcription levels of prfks1a relative to β-tubulin in the wild type and in Δprnrps4 stains were similar. Moreover, prfks1a is consistently transcribed at low levels and is upregulated in the presence of exogenous echinocandin, but not during growth conditions promoting endogenous production of sporiofungin. Therefore, we conclude that prfks1a is primarily responsible for protecting the fungus against extracellular echinocandin toxicity. The presence of unclustered auxiliary copies of FKS1 with high similarity to prfks1a in two other echinocandin-producing strains suggests that this previously unrecognized resistance mechanism may be common in echinocandin-producing fungi of the family Dermataceae of the class Leotiomycetes.

Kluyveromyces marxianus, an Attractive Yeast for Ethanolic Fermentation in the Presence of Imidazolium Ionic Liquids

Imidazolium ionic liquids (ILs) are promising solvents for lignocellulosic biomass (LCB) pretreatment and allow the achievement of higher ethanolic yields after enzymatic hydrolysis and ethanolic fermentation. However, residual ILs entrapped in pretreated biomass are often toxic for fermentative microorganisms, but interaction mechanisms between ILs and cells are still unknown. Here we studied the effects of 1-ethyl-3-methylimidazolium acetate [Emim][OAc] and 1-ethyl-3-methylimidazolium methylphosphonate [Emim][MeO(H)PO₂] on Kluyveromyces marxianus, a thermotolerant ethanologenic yeast. Morphological impacts induced by ILs on K. marxianus were characterized by Scanning Electron Microscopy analysis and showed wrinkled, softened, and holed shapes. In Yeast-Malt-Dextrose (YMD) medium, K. marxianus tolerated IL additions up to 2% for [Emim][OAc] and 6% for [Emim][MeO(H)PO₂]. Below these thresholds, some IL concentrations enhanced ethanolic yields up to +34% by switching the metabolic status from respiratory to fermentative. Finally, K. marxianus fermentation was applied on several substrates pretreated with [Emim][OAc] or [Emim][MeO(H)PO₂] and enzymatically hydrolyzed: a model long fiber cellulose and two industrial LCBs, softwood (spruce) and hardwood (oak) sawdusts. The maximum ethanolic yields obtained were 1.8 to 3.9 times higher when substrates were pretreated with imidazolium ILs. Therefore K. marxianus is an interesting fermentative yeast in a second-generation bioethanol process implying IL pretreatment.

VdPLP, A Patatin-Like Phospholipase in Verticillium dahliae, Is Involved in Cell Wall Integrity and Required for Pathogenicity

The soil-borne ascomycete fungus Verticillium dahliae causes vascular wilt disease and can seriously diminish the yield and quality of important crops. Functional analysis of growth- and pathogenicity-related genes is essential for revealing the pathogenic molecular mechanism of V. dahliae. Phospholipase is an important virulence factor in fungi that hydrolyzes phospholipids into fatty acid and other lipophilic substances and is involved in hyphal development. Thus far, only a few V. dahliae phospholipases have been identified, and their involvement in V. dahliae development and pathogenicity remains unknown. In this study, the function of the patatin-like phospholipase gene in V. dahliae (VdPLP, VDAG_00942) is characterized by generating gene knockout and complementary mutants. Vegetative growth and conidiation of VdPLP deletion mutants (ΔVdPLP) were significantly reduced compared with wild type and complementary strains, but more microsclerotia formed. The ΔVdPLP mutants were very sensitive to the cell-wall-perturbing agents: calcofluor white (CFW) and Congo red (CR). The transcriptional level of genes related to the cell wall integrity (CWI) pathway and chitin synthesis were downregulated, suggesting that VdPLP has a pivotal role in the CWI pathway and chitin synthesis in V. dahliae. ΔVdPLP strains were distinctly impaired in in their virulence and ability to colonize Nicotiana benthamiana roots. Our results demonstrate that VdPLP regulates hyphal growth and conidial production and is involved in stabilizing the cell wall, thus mediating the pathogenicity of V. dahliae.

Transcriptional Response to Lactic Acid Stress in the Hybrid Yeast Zygosaccharomyces parabailii

Lactic acid has a wide range of applications starting from its undissociated form, and its production using cell factories requires stress-tolerant microbial hosts. The interspecies hybrid yeast Zygosaccharomyces parabailii has great potential to be exploited as a novel host for lactic acid production, due to high organic acid tolerance at low pH and a fermentative metabolism with a high growth rate. Here we used mRNA sequencing (RNA-seq) to analyze Z. parabailii's transcriptional response to lactic acid added exogenously, and we explore the biological mechanisms involved in tolerance. Z. parabailii contains two homeologous copies of most genes. Under lactic acid stress, the two genes in each homeolog pair tend to diverge in expression to a significantly greater extent than under control conditions, indicating that stress tolerance is facilitated by interactions between the two gene sets in the hybrid. Lactic acid induces downregulation of genes related to cell wall and plasma membrane functions, possibly altering the rate of diffusion of lactic acid into cells. Genes related to iron transport and redox processes were upregulated, suggesting an important role for respiratory functions and oxidative stress defense. We found differences in the expression profiles of genes putatively regulated by Haa1 and Aft1/Aft2, previously described as lactic acid responsive in Saccharomyces cerevisiae. Furthermore, formate dehydrogenase (FDH) genes form a lactic acid-responsive gene family that has been specifically amplified in Z. parabailii in comparison to other closely related species. Our study provides a useful starting point for the engineering of Z. parabailii as a host for lactic acid production.

IMPORTANCE Hybrid yeasts are important in biotechnology because of their tolerance to harsh industrial conditions. The molecular mechanisms of tolerance can be studied by analyzing differential gene expression under conditions of interest and relating gene expression patterns to protein functions. However, hybrid organisms present a challenge to the standard use of mRNA sequencing (RNA-seq) to study transcriptional responses to stress, because their genomes contain two similar copies of almost every gene. Here we used stringent mapping methods and a high-quality genome sequence to study the transcriptional response to lactic acid stress in Zygosaccharomyces parabailii ATCC 60483, a natural interspecies hybrid yeast that contains two complete subgenomes that are approximately 7% divergent in sequence. Beyond the insights we gained into lactic acid tolerance in this study, the methods we developed will be broadly applicable to other yeast hybrid strains.

Cinnamomum zeylanicum bark essential oil induces cell wall remodelling and spindle defects in Candida albicans

Background

Cinnamon (Cinnamomum zeylanicum) bark extract exhibits potent inhibitory activity against Candida albicans but the antifungal mechanisms of this essential oil remain largely unexplored.

Results

We analyzed the impact of cinnamon bark oil on C. albicans RSY150, and clinical strains isolated from patients with candidemia and candidiasis. The viability of RSY150 was significantly compromised in a dose dependent manner when exposed to cinnamon bark oil, with extensive cell surface remodelling at sub inhibitory levels (62.5 μg/mL). Atomic force microscopy revealed cell surface exfoliation, altered ultrastructure and reduced cell wall integrity for both RSY150 and clinical isolates exposed to cinnamon bark oil. Cell wall damage induced by cinnamon bark oil was confirmed by exposure to stressors and the sensitivity of cell wall mutants involved in cell wall organization, biogenesis, and morphogenesis. The essential oil triggered cell cycle arrest by disrupting beta tubulin distribution, which led to mitotic spindle defects, ultimately compromising the cell membrane and allowing leakage of cellular components. The multiple targets of cinnamon bark oil can be attributed to its components, including cinnamaldehyde (74%), and minor components (< 6%) such as linalool (3.9%), cinamyl acetate (3.8%), α-caryophyllene (5.3%) and limonene (2%). Complete inhibition of the mitotic spindle assembly was observed in C. albicans treated with cinnamaldehyde at MIC (112 μg/mL).

Conclusions

Since cinnamaldehyde disrupts both the cell wall and tubulin polymerization, it may serve as an effective antifungal, either by chemical modification to improve its specificity and efficacy or in combination with other antifungal drugs.

Electronic supplementary material

The online version of this article (10.1186/s40694-018-0046-5) contains supplementary material, which is available to authorized users.

The FKS family genes cause changes in cell wall morphology resulted in regulation of anti-autolytic ability in Saccharomyces cerevisiae

The aim of this study was to discuss the functions of FKS family genes which encode β-1, 3-glucan synthase regarding the viability and autolysis of yeast strain. Loss of FKS1 gene severely influences the viability and anti-autolytic ability of yeast. Mutation of FKS1 and FKS2 genes led to cell reconstruction, resulting in a sharp shrinkage of cell volume and decreased stress resistance, viability, and anti-autolytic ability. Deletion of FKS3 gene did not clearly influence the synthesis of β-1, 3-glucan of yeast but increased the strain's stress resistance, viability, and anti-autolytic ability. It is suggested that FKS3 would be the potential target for improving the stress resistance of yeast. The results revealed the relationship among FKS family genes and demonstrated their functions on yeast cell wall construction and anti-autolytic ability.

The Candida albicans ENO1 gene encodes a transglutaminase involved in growth, cell division, morphogenesis, and osmotic protection

Candida albicans is an opportunistic fungus that is part of the normal microflora commonly found in the human digestive tract and the normal mucosa or skin of healthy individuals. However, in immunocompromised individuals, it becomes a serious health concern and a threat to their lives and is ranked as the leading fungal infection in humans worldwide. As existing treatments for this infection are non-specific or under threat of developing resistance, there is a dire necessity to find new targets for designing specific drugs to defeat this fungus. Some authors reported the presence of the transglutaminase activity in Candida and Saccharomyces, but its identity remains unknown. We report here the phenotypic effects produced by the inhibition of transglutaminase enzymatic activity with cystamine, including growth inhibition of yeast cells, induction of autophagy in response to damage caused by cystamine, alteration of the normal yeast division pattern, changes in cell wall, and inhibition of the yeast-to-mycelium transition. The latter phenomenon was also observed in the C. albicans ATCC 26555 strain. Growth inhibition by cystamine was also determined in other Candida strains, demonstrating the importance of transglutaminase in these species. Finally, we identified enolase 1 as the cell wall protein responsible for TGase activity. After studying the inhibition of enzymatic activities with anti-CaEno1 antibodies and through bioinformatics studies, we suggest that the enolase and transglutaminase catalytic sites are localized in different domains of the protein. The aforementioned data indicate that TGase/Eno1 is a putative target for designing new drugs to control C. albicans infection.

Functional analysis of Mpk1-mediated cell wall integrity signaling pathway in the thermotolerant methylotrophic yeast Hansenula polymorpha

Understanding the characteristics and regulation mechanisms of cell wall integrity (CWI) in yeast is important not only for basic research but also in biotechnological applications. We found significantly different CWIs in two representative strains of the thermotolerant methylotrophic yeast Hansenula polymorpha. Compared to the A16 strain (classified as Ogataea polymorpha), the DL1-L strain (classified as Ogataea parapolymorpha) has a thinner cell wall that was found to be more fragile following long-term cultivation and more sensitive to zymolyase. To gain a deeper insight into this difference, we compared the characteristics of the Mpk1pmediated CWI signaling pathway in the two strains. While a DL1-L mutant deficient in Mpk1p (mpk1Δ) showed severe growth retardation at both normal and high growth temperatures and in the presence of cell-wall disrupting agents, the A16 mpk1Δ mutant displayed only a mild defect in cell growth. Sorbitol effect on rescuing growth retardation was different in the two mpk1Δ strains, which could partly be ascribed to subtle differences in the activation of HOG pathway. Among the cell wall disruptors evaluated, only caffeine clearly increased phosphorylation of Mpk1p in DL1-L, but not in A16. A transcriptome analysis of the DL1-L strain revealed that caffeine significantly increased the expression of a subset of cell-wall related genes in an Mpk1p-dependent manner, but not the expected Rlm1-target genes. Taken together, our data support an essential role for Mpk1p in maintaining CWI in H. polymorpha, although the requirement for Mpk1p and its regulation under diverse stress conditions varies depending on the strain background.

The CWI Pathway: Regulation of the Transcriptional Adaptive Response to Cell Wall Stress in Yeast

Fungi are surrounded by an essential structure, the cell wall, which not only confers cell shape but also protects cells from environmental stress. As a consequence, yeast cells growing under cell wall damage conditions elicit rescue mechanisms to provide maintenance of cellular integrity and fungal survival. Through transcriptional reprogramming, yeast modulate the expression of genes important for cell wall biogenesis and remodeling, metabolism and energy generation, morphogenesis, signal transduction and stress. The yeast cell wall integrity (CWI) pathway, which is very well conserved in other fungi, is the key pathway for the regulation of this adaptive response. In this review, we summarize the current knowledge of the yeast transcriptional program elicited to counterbalance cell wall stress situations, the role of the CWI pathway in the regulation of this program and the importance of the transcriptional input received by other pathways. Modulation of this adaptive response through the CWI pathway by positive and negative transcriptional feedbacks is also discussed. Since all these regulatory mechanisms are well conserved in pathogenic fungi, improving our knowledge about them will have an impact in the developing of new antifungal therapies.

β-(1→6)-D-glucan secreted during the optimised production of exopolysaccharides by Paecilomyces variotii has immunostimulatory activity

Paecilomyces variotii is a filamentous fungus that occurs worldwide in soil and decaying vegetation. Optimization of the fermentation process for exopolysaccharide (EPS) production from the fungus P. variotii, structure determination and immuno-stimulating activity of EPS were performed. Response surface methodology (RSM) coupled with central composite design (CCD) was used to optimize the physical and chemical factors required to produce EPS in submerged fermentation. Preliminary investigations to choose the three factors for the present work were made using a factorial experimental design. Glucose, ammonium nitrate (NH₄NO₃) and pH were used as variables for which, with constant temperature of 28 °C and agitation of 90 rpm, the optimal process parameters were determined as glucose values of 0.96%, NH₄NO₃ 0.26% and pH 8.0. The three parameters presented significant effects. In this condition of culture, the main composition of the isolated EPS was a linear β-(1 → 6)-linked-D-glucan, as determined by Nuclear Magnetic Resonance (NMR) and methylation analysis. This polysaccharide is a very unusual as an EPS from fungi, especially a filamentous fungus such as P. variotii. Murine peritoneal macrophages cultivated with β-glucan for 6 and 48 h showed an increase in TNF-α, IL-6 and nitric oxide release with increased polysaccharide concentrations. Therefore, we conclude that the β-(1 → 6)-linked-D-glucan produced in optimised conditions of P. variotii cultivation has an immune-stimulatory activity on murine macrophages.

Cell wall biosynthesis impairment affects the budding lifespan of the Saccharomyces cerevisiae yeast

The Saccharomyces cerevisiae yeast is one of the most widely used model in studies of cellular and organismal biology, including as aging and proliferation. Although several constraints of aging and budding lifespan have been identified, these processes have not yet been fully understood. Previous studies of aging in yeast have focused mostly on the molecular basics of the underlying mechanisms, while physical aspects, particularly those related to the cell wall, were rather neglected. In this paper, we examine for the first time, to our knowledge, the impact of cell wall biosynthesis disturbances on the lifespan in the budding yeast. We have used a set of cell wall mutants, including knr4Δ, cts1Δ, chs3Δ, fks1Δ and mnn9Δ, which affect biosynthesis of all major cell wall compounds. Our results indicated that impairment of chitin biosynthesis and cell wall protein mannosylation reduced the budding lifespan, while disruption in the 1,3-β-glucan synthase activity had no adverse effect on that parameter. The impact varied in the severity and the most notable effect was observed for the mnn9Δ mutant. What was interesting, in the case of the dysfunction of the Knr4 protein playing the role of the transcriptional regulator of cell wall chitin and glucan synthesis, the lifespan increased significantly. We also report the phenotypic characteristics of cell wall-associated mutants as revealed by imaging of the cell wall using transmission electron microscopy, scanning electron microscopy and atomic force microscopy. In addition, our findings support the conviction that achievement of the state of hypertrophy may not be the only factor that determines the budding lifespan.

Ferric ions accumulate in the walls of metabolically inactivating Saccharomyces cerevisiae cells and are reductively mobilized during reactivation

Mössbauer and EPR spectra of fermenting yeast cells before and after cell wall (CW) digestion revealed that CWs accumulated iron as cells transitioned from exponential to post-exponential growth. Most CW iron was mononuclear nonheme high-spin (NHHS) Fe(III), some was diamagnetic and some was superparamagnetic. A significant portion of CW Fe was removable by EDTA. Simulations using an ordinary-differential-equations-based model suggested that cells accumulate Fe as they become metabolically inactive. When dormant Fe-loaded cells were metabolically reactivated in Fe-deficient bathophenanthroline disulfonate (BPS)-treated medium, they grew using Fe that had been mobilized from their CWs AND using trace amounts of Fe in the Fe-deficient medium. When grown in Fe-deficient medium, Fe-starved cells contained the lowest cellular Fe concentrations reported for a eukaryotic cell. During metabolic reactivation of Fe-loaded dormant cells, Fe(III) ions in the CWs of these cells were mobilized by reduction to Fe(II), followed by release from the CW and reimport into the cell. BPS short-circuited this process by chelating mobilized and released Fe(II) ions before reimport; the resulting Fe(II)(BPS)3 complex adsorbed on the cell surface. NHHS Fe(II) ions appeared transiently during mobilization, suggesting that these ions were intermediates in this process. In the presence of chelators and at high pH, metabolically inactive cells leached CW Fe; this phenomenon probably differs from metabolic mobilization. The iron regulon, as reported by Fet3p levels, was not expressed during post-exponential conditions; Fet3p was maximally expressed in exponentially growing cells. Decreased expression of the iron regulon and metabolic decline combine to promote CW Fe accumulation.

Fission Yeast Cell Wall Analysis

The Schizosaccharomyces pombe cell wall is a rigid exoskeletal structure mainly composed of interlinked glucose polysaccharides and galactomannoproteins. It is essential for survival of the fission yeast, as it prevents cells from bursting from internal turgor pressure and protects them from mechanical injuries. Additionally, the cell wall determines the cell shape and, therefore, a better knowledge of cell wall structure and composition could provide valuable data in S. pombe morphogenetic studies. Here, we provide information about this structure and the current reliable methods for rapid analysis of the cell wall polymers by specific enzymatic and chemical degradations of purified cell walls.

The PHR Family: The Role of Extracellular Transglycosylases in Shaping Candida albicans Cells

Candida albicans is an opportunistic microorganism that can become a pathogen causing mild superficial mycosis or more severe invasive infections that can be life-threatening for debilitated patients. In the etiology of invasive infections, key factors are the adaptability of C. albicans to the different niches of the human body and the transition from a yeast form to hypha. Hyphal morphology confers high adhesiveness to the host cells, as well as the ability to penetrate into organs. The cell wall plays a crucial role in the morphological changes C. albicans undergoes in response to specific environmental cues. Among the different categories of enzymes involved in the formation of the fungal cell wall, the GH72 family of transglycosylases plays an important assembly role. These enzymes cut and religate β-(1,3)-glucan, the major determinant of cell shape. In C. albicans, the PHR family encodes GH72 enzymes, some of which work in specific environmental conditions. In this review, we will summarize the work from the initial discovery of PHR genes to the study of the pH-dependent expression of PHR1 and PHR2, from the characterization of the gene products to the recent findings concerning the stress response generated by the lack of GH72 activity in C. albicans hyphae.

Re-Evaluation of Imaging Methods of Reactive Oxygen and Nitrogen Species in Plants and Fungi: Influence of Cell Wall Composition

Developmental transitions and stress reactions in both eukaryotes and prokaryotes are tightly linked with fast and localized modifications in concentrations of reactive oxygen and nitrogen species (ROS and RNS). Fluorescent microscopic analyses are widely applied to detect localized production of ROS and RNS in vivo. In this mini-review we discuss the biological characteristics of studied material (cell wall, extracellular matrix, and tissue complexity) and its handling (concentration of probes, effect of pressure, and higher temperature) which influence results of histochemical staining with "classical" fluorochromes. Future perspectives of ROS and RNS imaging with newly designed probes are briefly outlined.

Cooperation between ER stress and calcineurin signaling contributes to the maintenance of cell wall integrity in Candida glabrata

Candida glabrata is the second most common source of Candida infections in humans. In this pathogen, the maintenance of cell wall integrity (CWI) frequently precludes effective pharmacological treatment by antifungal agents. In numerous fungi, cell wall modulation is reported to be controlled by endoplasmic reticulum (ER) stress, but how the latter affects CWI maintenance in C. glabrata is not clearly understood. Here, we characterized a C. glabrata strain harboring a mutation in the CNE1 gene, which encodes a molecular chaperone associated with nascent glycoprotein maturation in the ER. Disruption of cne1 induced ER stress and caused changes in the normal cell wall structure, specifically a reduction in the β-1,6-glucan content and accumulation of chitin. Conversely, a treatment with the typical ER stress inducer tunicamycin up-regulated the production of cell wall chitin but did not affect β-1,6-glucan content. Our results also indicated that C. glabrata features a uniquely evolved ER stress-mediated CWI pathway, which differs from that in the closely related species Saccharomyces cerevisiae. Furthermore, we demonstrated that ER stress-mediated CWI pathway in C. glabrata is also induced by the disruption of other genes encoding proteins that function in a correlated manner in the quality control of N-linked glycoproteins in the ER. These results suggest that calcineurin and ER quality control system act as a platform for maintaining CWI in C. glabrata.

De novo transcriptome sequencing of Isaria cateniannulata and comparative analysis of gene expression in response to heat and cold stresses

Isaria cateniannulata is a very important and virulent entomopathogenic fungus that infects many insect pest species. Although I. cateniannulata is commonly exposed to extreme environmental temperature conditions, little is known about its molecular response mechanism to temperature stress. Here, we sequenced and de novo assembled the transcriptome of I. cateniannulata in response to high and low temperature stresses using Illumina RNA-Seq technology. Our assembly encompassed 17,514 unigenes (mean length = 1,197 bp), in which 11,445 unigenes (65.34%) showed significant similarities to known sequences in NCBI non-redundant protein sequences (Nr) database. Using digital gene expression analysis, 4,483 differentially expressed genes (DEGs) were identified after heat treatment, including 2,905 up-regulated genes and 1,578 down-regulated genes. Under cold stress, 1,927 DEGs were identified, including 1,245 up-regulated genes and 682 down-regulated genes. The expression patterns of 18 randomly selected candidate DEGs resulting from quantitative real-time PCR (qRT-PCR) were consistent with their transcriptome analysis results. Although DEGs were involved in many pathways, we focused on the genes that were involved in endocytosis: In heat stress, the pathway of clathrin-dependent endocytosis (CDE) was active; however at low temperature stresses, the pathway of clathrin-independent endocytosis (CIE) was active. Besides, four categories of DEGs acting as temperature sensors were observed, including cell-wall-major-components-metabolism-related (CWMCMR) genes, heat shock protein (Hsp) genes, intracellular-compatible-solutes-metabolism-related (ICSMR) genes and glutathione S-transferase (GST). These results enhance our understanding of the molecular mechanisms of I. cateniannulata in response to temperature stresses and provide a valuable resource for the future investigations.

Integration of Biochemical, Biophysical and Transcriptomics Data for Investigating the Structural and Nanomechanical Properties of the Yeast Cell Wall

The yeast cell is surrounded by a cell wall conferring protection and resistance to environmental conditions that can be harmful. Identify the molecular cues (genes) which shape the biochemical composition and the nanomechanical properties of the cell wall and the links between these two parameters represent a major issue in the understanding of the biogenesis and the molecular assembly of this essential cellular structure, which may have consequences in diverse biotechnological applications. We addressed this question in two ways. Firstly, we compared the biochemical and biophysical properties using atomic force microscopy (AFM) methods of 4 industrial strains with the laboratory sequenced strain BY4743 and used transcriptome data of these strains to infer biological hypothesis about differences of these properties between strains. This comparative approach showed a 4–6-fold higher hydrophobicity of industrial strains that was correlated to higher expression of genes encoding adhesin and adhesin-like proteins and not to their higher mannans content. The second approach was to employ a multivariate statistical analysis to identify highly correlated variables among biochemical, biophysical and genes expression data. Accordingly, we found a tight association between hydrophobicity and adhesion events that positively correlated with a set of 22 genes in which the main enriched GO function was the sterol metabolic process. We also identified a strong association of β-1,3-glucans with contour length that corresponds to the extension of mannans chains upon pulling the mannosyl units with the lectin-coated AFM tips. This association was positively correlated with a group of 27 genes in which the seripauperin multigene family was highly documented and negatively connected with a set of 23 genes whose main GO biological process was sulfur assimilation/cysteine biosynthetic process. On the other hand, the elasticity modulus was found weakly associated with levels of β-1,6-glucans, and this biophysical variable was positively correlated with a set of genes implicated in microtubules polymerization, tubulin folding and mitotic organization.

Amino Acid Sensor Kinase Gcn2 Is Required for Conidiation, Secondary Metabolism, and Cell Wall Integrity in the Taxol-Producer Pestalotiopsis microspora

The canonical Gcn2/Cpc1 kinase in fungi coordinates the expression of target genes in response to amino acid starvation. To investigate its possible role in secondary metabolism, we characterized a gcn2 homolog in the taxol-producing fungus Pestalotiopsis microspora. Deletion of the gene led to severe physiological defects under amino acid starvation, suggesting a conserved function of gcn2 in amino acid sensing. The mutant strain Δgcn2 displayed retardation in vegetative growth. It generated dramatically fewer conidia, suggesting a connection between amino acid metabolism and conidiation in this fungus. Importantly, disruption of the gene altered the production of secondary metabolites by HPLC profiling. For instance, under amino acid starvation, the deletion strain Δgcn2 barely produced secondary metabolites including the known natural product pestalotiollide B. Even more, we showed that gcn2 played critical roles in the tolerance to several stress conditions. Δgcn2 exhibited a hypersensitivity to Calcofluor white and Congo red, implying a role of Gcn2 in maintaining the integrity of the cell wall. This study suggests that Gcn2 kinase is an important global regulator in the growth and development of filamentous fungi and will provide knowledge for the manipulation of secondary metabolism in P. microspora.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

β-1,6-glucan synthesis-associated genes are required for proper spore wall formation in Saccharomyces cerevisiae

The yeast spore wall is an excellent model to study the assembly of an extracellular macromolecule structure. In the present study, mutants defective in β-1,6-glucan synthesis, including kre1∆, kre6∆, kre9∆ and big1∆, were sporulated to analyse the effect of β-1,6-glucan defects on the spore wall. Except for kre6∆, these mutant spores were sensitive to treatment with ether, suggesting that the mutations perturb the integrity of the spore wall. Morphologically, the mutant spores were indistinguishable from wild-type spores. They lacked significant sporulation defects partly because the chitosan layer, which covers the glucan layer, compensated for the damage. The proof for this model was obtained from the effect of the additional deletion of CHS3 that resulted in the absence of the chitosan layer. Among the double mutants, the most severe spore wall deficiency was observed in big1∆ spores. The majority of the big1∆chs3∆ mutants failed to form visible spores at a higher temperature. Given that the big1∆ mutation caused a failure to attach a GPI-anchored reporter, Cwp2-GFP, to the spore wall, β-1,6-glucan is involved in tethering of GPI-anchored proteins in the spore wall as well as in the vegetative cell wall. Thus, β-1,6-glucan is required for proper organization of the spore wall. Copyright © 2017 John Wiley & Sons, Ltd.

Sfp1 and Rtg3 reciprocally modulate carbon source-conditional stress adaptation in the pathogenic yeast Candida albicans

The pathogenicity of the clinically important yeast, Candida albicans, is dependent on robust responses to host-imposed stresses. These stress responses have generally been dissected in vitro at 30°C on artificial growth media that do not mimic host niches. Yet host inputs, such as changes in carbon source or temperature, are known to affect C. albicans stress adaptation. Therefore, we performed screens to identify novel regulators that promote stress resistance during growth on a physiologically relevant carboxylic acid and at elevated temperatures. These screens revealed that, under these 'non-standard' growth conditions, numerous uncharacterised regulators are required for stress resistance in addition to the classical Hog1, Cap1 and Cta4 stress pathways. In particular, two transcription factors (Sfp1 and Rtg3) promote stress resistance in a reciprocal, carbon source-conditional manner. SFP1 is induced in stressed glucose-grown cells, whereas RTG3 is upregulated in stressed lactate-grown cells. Rtg3 and Sfp1 regulate the expression of key stress genes such as CTA4, CAP1 and HOG1 in a carbon source-dependent manner. These mechanisms underlie the stress sensitivity of C. albicans sfp1 cells during growth on glucose, and rtg3 cells on lactate. The data suggest that C. albicans exploits environmentally contingent regulatory mechanisms to retain stress resistance during host colonisation.

A novel connection between the Cell Wall Integrity and the PKA pathways regulates cell wall stress response in yeast

Fungal cells trigger adaptive mechanisms to survive in situations that compromise cell wall integrity. We show here that the global transcriptional response elicited by inhibition of the synthesis of β-1,3-glucan by caspofungin, encompasses a set of genes that are dependent on Slt2, the MAPK of the Cell Wall Integrity (CWI) pathway, and a broad group of genes regulated independently of Slt2. Genes negatively regulated by the cyclic AMP/Protein Kinase A (PKA) signaling pathway were overrepresented in the latter group. Moreover, cell wall stress mediated by inhibition of β-1,3-glucan synthesis, but not by other cell wall interfering compounds, negatively regulated PKA signaling as indicated by the nuclear localisation of Msn2, cellular glycogen accumulation, a decrease of intracellular cAMP levels and a severe decrease in both the activation of the small GTPase Ras2 and the phosphorylation of known substrates of PKA. All these effects relied on the plasma membrane-spanning sensor of the CWI pathway Wsc1. In addition, caspofungin induced a reduction in the cytosolic pH, which was dependent on the extracellular region of Wsc1. Therefore, alterations of the β-1,3-glucan network in the fungal cell wall, induce, through Wsc1, the activation of the CWI pathway and parallel inhibition of PKA signaling.

The Dual Activity Responsible for the Elongation and Branching of β-(1,3)-Glucan in the Fungal Cell Wall

β-(1,3)-Glucan, the major fungal cell wall component, ramifies through β-(1,6)-glycosidic linkages, which facilitates its binding with other cell wall components contributing to proper cell wall assembly. Using Saccharomyces cerevisiae as a model, we developed a protocol to quantify β-(1,6)-branching on β-(1,3)-glucan. Permeabilized S. cerevisiae and radiolabeled substrate UDP-(¹⁴C)glucose allowed us to determine branching kinetics. A screening aimed at identifying deletion mutants with reduced branching among them revealed only two, the bgl2Δ and gas1Δ mutants, showing 15% and 70% reductions in the branching, respectively, compared to the wild-type strain. Interestingly, a recombinant Gas1p introduced β-(1,6)-branching on the β-(1,3)-oligomers following its β-(1,3)-elongase activity. Sequential elongation and branching activity of Gas1p occurred on linear β-(1,3)-oligomers as well as Bgl2p-catalyzed products [short β-(1,3)-oligomers linked by a linear β-(1,6)-linkage]. The double S. cerevisiae gas1Δ bgl2Δ mutant showed a drastically sick phenotype. An ScGas1p ortholog, Gel4p from Aspergillus fumigatus, also showed dual β-(1,3)-glucan elongating and branching activity. Both ScGas1p and A. fumigatus Gel4p sequences are endowed with a carbohydrate binding module (CBM), CBM43, which was required for the dual β-(1,3)-glucan elongating and branching activity. Our report unravels the β-(1,3)-glucan branching mechanism, a phenomenon occurring during construction of the cell wall which is essential for fungal life.

The fungal cell wall is essential for growth, morphogenesis, protection, and survival. In spite of being essential, cell wall biogenesis, especially the core β-(1,3)-glucan ramification, is poorly understood; the ramified β-(1,3)-glucan interconnects other cell wall components. Once linear β-(1,3)-glucan is synthesized by plasma membrane-bound glucan synthase, the subsequent event is its branching event in the cell wall space. Using Saccharomyces cerevisiae as a model, we identified GH72 and GH17 family glycosyltransferases, Gas1p and Bgl2p, respectively, involved in the β-(1,3)-glucan branching. The sick phenotype of the double Scgas1Δ bgl2Δ mutant suggested that β-(1,3)-glucan branching is essential. In addition to ScGas1p, GH72 family ScGas2p and Aspergillus fumigatus Gel4p, having CBM43 in their sequences, showed dual β-(1,3)-glucan elongating and branching activity. Our report identifies the fungal cell wall β-(1,3)-glucan branching mechanism. The essentiality of β-(1,3)-glucan branching suggests that enzymes involved in the glucan branching could be exploited as antifungal targets.

Value-added probiotic development by high-solid fermentation of sweet potato with Saccharomyces boulardii

Controlled fermentation of Sweet potato (Ipomoea batatas) var. Beauregard by yeast, Saccharomyces boulardii (MAY 796) to enhance the nutritional value of sweet potato was investigated. An average 8.00 × 10¹⁰ Colony Forming Units (CFU)/g of viable cells were obtained over 5‐day high‐solid fermentation. Yeast cell viability did not change significantly over time at 4°C whereas the number of viable yeast cells reduced significantly at room temperature (25°C), which was approximately 40% in 12 months. Overall, the controlled fermentation of sweet potato by MAY 796 enhanced protein, crude fiber, neutral detergent fiber, acid detergent fiber, amino acid, and fatty acid levels. Development of value‐added sweet potato has a great potential in animal feed and human nutrition. S. boulardii‐ fermented sweet potato has great potential as probiotic‐enriched animal feed and/or functional food for human nutrition.

Differential Roles for Six P-Type Calcium ATPases in Sustaining Intracellular Ca2+ Homeostasis, Asexual Cycle and Environmental Fitness of Beauveria bassiana

A global insight into the roles of multiple P-type calcium ATPase (CA) pumps in sustaining the life of a filamentous fungal pathogen is lacking. Here we elucidated the functions of five CA pumps (Eca1, Spf1 and PmcA/B/C) following previous characterization of Pmr1 in Beauveria bassiana, a fungal insect pathogen. The fungal CA pumps interacted at transcriptional level, at which singular deletions of five CA genes depressed eca1 expression by 76–98% and deletion of spf1 resulted in drastic upregulation of four CA genes by 36–50-fold. Intracellular Ca²⁺ concentration increased differentially in most deletion mutants exposed to the stresses of Ca²⁺, EDTA chelator, and/or endoplasmic reticulum and calcineurin inhibitors, accompanied with their changed sensitivities to not only the mentioned agents but also Fe²⁺, Cu²⁺ and Zn²⁺. Liquid culture acidification was delayed in the Δspf1, Δpmr1 and ΔpmcA mutants, coinciding well with altered levels of their extracellular lactic and oxalic acids. Moreover, all deletion mutants showed differential defects in conidial germination, vegetative growth, conidiation capacity, antioxidant activity, cell wall integrity, conidial UV-B resistance and/or virulence. Our results provide the first global insight into differential roles for six CA pumps in sustaining intracellular Ca²⁺ level, asexual cycle and environmental fitness of B. bassiana.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

The Biology of Pichia membranifaciens Killer Toxins

The killer phenomenon is defined as the ability of some yeast to secrete toxins that are lethal to other sensitive yeasts and filamentous fungi. Since the discovery of strains of Saccharomyces cerevisiae capable of secreting killer toxins, much information has been gained regarding killer toxins and this fact has substantially contributed knowledge on fundamental aspects of cell biology and yeast genetics. The killer phenomenon has been studied in Pichia membranifaciens for several years, during which two toxins have been described. PMKT and PMKT2 are proteins of low molecular mass that bind to primary receptors located in the cell wall structure of sensitive yeast cells, linear (1→6)-β-d-glucans and mannoproteins for PMKT and PMKT2, respectively. Cwp2p also acts as a secondary receptor for PMKT. Killing of sensitive cells by PMKT is characterized by ionic movements across plasma membrane and an acidification of the intracellular pH triggering an activation of the High Osmolarity Glycerol (HOG) pathway. On the contrary, our investigations showed a mechanism of killing in which cells are arrested at an early S-phase by high concentrations of PMKT2. However, we concluded that induced mortality at low PMKT2 doses and also PMKT is indeed of an apoptotic nature. Killer yeasts and their toxins have found potential applications in several fields: in food and beverage production, as biocontrol agents, in yeast bio-typing, and as novel antimycotic agents. Accordingly, several applications have been found for P. membranifaciens killer toxins, ranging from pre- and post-harvest biocontrol of plant pathogens to applications during wine fermentation and ageing (inhibition of Botrytis cinerea, Brettanomyces bruxellensis, etc.).

Pgas, a Low-pH-Induced Promoter, as a Tool for Dynamic Control of Gene Expression for Metabolic Engineering of Aspergillus niger

The dynamic control of gene expression is important for adjusting fluxes in order to obtain desired products and achieve appropriate cell growth, particularly when the synthesis of a desired product drains metabolites required for cell growth. For dynamic gene expression, a promoter responsive to a particular environmental stressor is vital. Here, we report a low-pH-inducible promoter, Pgas, which promotes minimal gene expression at pH values above 5.0 but functions efficiently at low pHs, such as pH 2.0. First, we performed a transcriptional analysis of Aspergillus niger, an excellent platform for the production of organic acids, and we found that the promoter Pgas may act efficiently at low pH. Then, a gene for synthetic green fluorescent protein (sGFP) was successfully expressed by Pgas at pH 2.0, verifying the results of the transcriptional analysis. Next, Pgas was used to express the cis-aconitate decarboxylase (cad) gene of Aspergillus terreus in A. niger, allowing the production of itaconic acid at a titer of 4.92 g/liter. Finally, we found that Pgas strength was independent of acid type and acid ion concentration, showing dependence on pH only.IMPORTANCE The promoter Pgas can be used for the dynamic control of gene expression in A. niger for metabolic engineering to produce organic acids. This promoter may also be a candidate tool for genetic engineering.