pH regulation of gene expression in fungi

Interconnections between the Cation/Alkaline pH-Responsive Slt and the Ambient pH Response of PacC/Pal Pathways in Aspergillus nidulans

In the filamentous ascomycete Aspergillus nidulans, at least three high hierarchy transcription factors are required for growth at extracellular alkaline pH: SltA, PacC and CrzA. Transcriptomic profiles depending on alkaline pH and SltA function showed that pacC expression might be under SltA regulation. Additional transcriptional studies of PacC and the only pH-regulated pal gene, palF, confirmed both the strong dependence on ambient pH and the function of SltA. The regulation of pacC expression is dependent on the activity of the zinc binuclear (C6) cluster transcription factor PacX. However, we found that the ablation of sltA in the pacX- mutant background specifically prevents the increase in pacC expression levels without affecting PacC protein levels, showing a novel specific function of the PacX factor. The loss of sltA function causes the anomalous proteolytic processing of PacC and a reduction in the post-translational modifications of PalF. At alkaline pH, in a null sltA background, PacC72kDa accumulates, detection of the intermediate PacC53kDa form is extremely low and the final processed form of 27 kDa shows altered electrophoretic mobility. Constitutive ubiquitination of PalF or the presence of alkalinity-mimicking mutations in pacC, such as pacCc14 and pacCc700, resembling PacC53kDa and PacC27kDa, respectively, allowed the normal processing of PacC but did not rescue the alkaline pH-sensitive phenotype caused by the null sltA allele. Overall, data show that Slt and PacC/Pal pathways are interconnected, but the transcription factor SltA is on a higher hierarchical level than PacC on regulating the tolerance to the ambient alkalinity in A. nidulans.

"You Are What You Eat": How Fungal Adaptation Can Be Leveraged toward Myco-Material Properties

Fungi adapt to their surroundings, modifying their behaviors and composition under different conditions like nutrient availability and environmental stress. This perspective examines how a basic understanding of fungal genetics and the different ways that fungi can be influenced by their surroundings can be leveraged toward the production of functional mycelium materials. Simply put, within the constraints of a given genetic script, both the quality and quantity of fungal mycelium are shaped by what they eat and where they grow. These two levers, encompassing their global growth environment, can be turned toward different materials outcomes. The final properties of myco-materials are thus intimately shaped by the conditions of their growth, enabling the design of new biobased and biodegradable material constructions for applications that have traditionally relied on petroleum-based chemicals.This perspective highlights aspects of fungal genetics and environmental adaptation that have potential materials science implications, along the way touching on key studies, both to situate the state of the art within the field and to punctuate the viewpoints of the authors. Finally, this work ends with future perspectives, reinforcing key topics deemed important to consider in emerging myco-materials research.

Insights and Perspectives on the Role of Proteostasis and Heat Shock Proteins in Fungal Infections

Fungi are a diverse group of eukaryotic organisms that infect humans, animals, and plants. To successfully colonize their hosts, pathogenic fungi must continuously adapt to the host's unique environment, e.g., changes in temperature, pH, and nutrient availability. Appropriate protein folding, assembly, and degradation are essential for maintaining cellular homeostasis and survival under stressful conditions. Therefore, the regulation of proteostasis is crucial for fungal pathogenesis. The heat shock response (HSR) is one of the most important cellular mechanisms for maintaining proteostasis. It is activated by various stresses and regulates the activity of heat shock proteins (HSPs). As molecular chaperones, HSPs participate in the proteostatic network to control cellular protein levels by affecting their conformation, location, and degradation. In recent years, a growing body of evidence has highlighted the crucial yet understudied role of stress response circuits in fungal infections. This review explores the role of protein homeostasis and HSPs in fungal pathogenicity, including their contributions to virulence and host-pathogen interactions, as well as the concerted effects between HSPs and the main proteostasis circuits in the cell. Furthermore, we discuss perspectives in the field and the potential for targeting the components of these circuits to develop novel antifungal therapies.

Strain improvement of Trichoderma harzianum for enhanced biocontrol capacity: Strategies and prospects

In the control of plant diseases, biocontrol has the advantages of being efficient and safe for human health and the environment. The filamentous fungus Trichoderma harzianum and its closely related species can inhibit the growth of many phytopathogenic fungi, and have been developed as commercial biocontrol agents for decades. In this review, we summarize studies on T. harzianum species complex from the perspective of strain improvement. To elevate the biocontrol ability, the production of extracellular proteins and compounds with antimicrobial or plant immunity-eliciting activities need to be enhanced. In addition, resistance to various environmental stressors should be strengthened. Engineering the gene regulatory system has the potential to modulate a variety of biological processes related to biocontrol. With the rapidly developing technologies for fungal genetic engineering, T. harzianum strains with increased biocontrol activities are expected to be constructed to promote the sustainable development of agriculture.

ThpacC Acts as a Positive Regulator of Homodimericin A Biosynthesis and Antifungal Activities of Trichoderma harzianum 3.9236

The Pal/Rim pathway and its key transcription factor PacC play important roles in fungal adaptation to ambient pH regarding growth, secondary metabolism, and virulence. However, the effect of PacC on the secondary metabolism of the important biocontrol fungus Trichoderma harzianum remains elusive. To answer this question, ThpacC deletion (KO-ThpacC) and overexpression (OE-ThpacC) mutants of T. harzianum 3.9236 were constructed. Transcriptomic analysis of T. harzianum and KO-ThpacC suggested that ThpacC acted as both a positive and a negative regulator for secondary metabolite (SM) production. Further investigation revealed that deletion of ThpacC abolished homodimericin A and 8-epi-homodimericin A production. Moreover, ThpacC plays a role in the antagonism of T. harzianum against Sclerotinia sclerotiorum. 8-epi-Homodimericin A demonstrated moderate inhibitory activity against S. sclerotiorum. Our results contribute to a deeper understanding of the ThpacC function on SM production and the antifungal activity of T. harzianum.

LED control of gene expression in a nanobiosystem composed of metallic nanoparticles and a genetically modified E. coli strain

BACKGROUND

Within the last decade, genetic engineering and synthetic biology have revolutionized society´s ability to mass-produce complex biological products within genetically-modified microorganisms containing elegantly designed genetic circuitry. However, many challenges still exist in developing bioproduction processes involving genetically modified microorganisms with complex or multiple gene circuits. These challenges include the development of external gene expression regulation methods with the following characteristics: spatial-temporal control and scalability, while inducing minimal permanent or irreversible system-wide conditions. Different stimuli have been used to control gene expression and mitigate these challenges, and they can be characterized by the effect they produce in the culture media conditions. Invasive stimuli that cause permanent, irreversible changes (pH and chemical inducers), non-invasive stimuli that cause partially reversible changes (temperature), and non-invasive stimuli that cause reversible changes in the media conditions (ultrasound, magnetic fields, and light).

METHODS

Opto-control of gene expression is a non-invasive external trigger that complies with most of the desired characteristics of an external control system. However, the disadvantage relies on the design of the biological photoreceptors and the necessity to design them to respond to a different wavelength for every bioprocess needed to be controlled or regulated in the microorganism. Therefore, this work proposes using biocompatible metallic nanoparticles as external controllers of gene expression, based on their ability to convert light into heat and the capacity of nanotechnology to easily design a wide array of nanostructures capable of absorbing light at different wavelengths and inducing plasmonic photothermal heating.

RESULTS

Here, we designed a nanobiosystem that can be opto-thermally triggered using LED light. The nanobiosystem is composed of biocompatible gold nanoparticles and a genetically modified E. coli with a plasmid that allows mCherry fluorescent protein production at 37 °C in response to an RNA thermometer.

CONCLUSIONS

The LED-triggered photothermal protein production system here designed offers a new, cheaper, scalable switchable method, non-destructive for living organisms, and contribute toward the evolution of bioprocess production systems.

The Effect of Environmental pH during Trichothecium roseum (Pers.:Fr.) Link Inoculation of Apple Fruits on the Host Differential Reactive Oxygen Species Metabolism

Trichothecium roseum is an important postharvest pathogen, belonging to an alkalizing group of pathogens secreting ammonia during fungal growth and colonization of apple fruits. Fungal pH modulation is usually considered a factor for improving fungal gene expression, contributing to its pathogenicity. However, the effects of inoculation with T. roseum spore suspensions at increasing pH levels from pH 3 up to pH 7, on the reactive oxygen species (ROS) production and scavenging capability of the apple fruits, affecting host susceptibility, indicate that the pH regulation by the pathogens also affects host response and may contribute to colonization. The present results indicate that the inoculation of T. roseum spores at pH 3 caused the lowest cell membrane permeability, and reduced malondialdehyde content, NADPH oxidases activity, O₂^●− and H₂O₂ production in the colonized fruit. Observations of the colonized area on the 9th day after inoculation at pH 3, showed that the rate of O₂^●− production and H₂O₂ content was reduced by 57% and 25%, compared to their activities at pH 7. In contrast, antioxidative activities of superoxide dismutase, catalase and peroxidases of fruit tissue inoculated with spores’ suspension in the presence of a solution at pH 3.0 showed their highest activity. The catalase and peroxidases activities in the colonized tissue at pH 3 were higher by almost 58% and 55.9%, respectively, on the 6th day after inoculation compared to inoculation at pH 7. The activities of key enzymes of the ascorbate-glutathione (AsA-GSH) cycle and their substrates and products by the 9th day after fruit inoculation at pH 3 showed 150%, 31%, 16%, and 110% higher activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase, respectively, compared to pH 7. A similar pattern of response was also observed in the accumulation of ascorbic acid and dehydroascorbate which showed a higher accumulation at pH 3 compared to the colonization at pH 7. The present results indicate that the metabolic regulation of the pH environment by the T. roseum not only modulates the fungal pathogenicity factors reported before, but it induces metabolic host changes contributing both together to fungal colonization.

Effect of Medium pH and Light on Quinidine Production in Cinchona calisaya Wedd. Endophytic Fungi

Objectives

Quinidine has pharmaceutical importance as an antimalarial, antiarrhythmia, antimicrobial, anticancer, antioxidant, astringent, and bitter flavoring agent. Quinidine is in high demand, yet its production from the bark of the quina tree (Cinchona calisaya) is limited. Quinidine production from quina tree fungal endophytes, namely Aspergillus sydowii, Diaporthe sp., Diaporthe lithicola, Fusarium oxysporum, and F. solani is lower than the quinidine content of the tree bark. This study attempted to increase quinidine production from these fungi. This research aimed to determine the optimum culture conditions for quinidine production from endophytic fungi.

Materials and Methods

Quinidine was produced by in vitro culturing of the fungal endophytes in potato dextrose broth (PDB) medium under different culture conditions, i.e., a combination of an initial medium pH of 6.2 or 6.8, with or without light, in a static condition for 21 days of incubation at room temperature. Production under natural daylight in PDB medium without pH modification was used as the control. At the end of the incubation period, the mycelial mass was separated from the filtrate. The dried biomass and chloroform-extracted filtrate were weighed. Quinidine in the extract was analyzed qualitatively and quantitatively using high-performance liquid chromatograph.

Results

Quinidine production was affected by both light and the initial pH of the medium, depending on the fungal strain used. A significant increment in quinidine production, approximately 1.1-9.3-fold relative to its respective control was obtained from all fungi under their optimum conditions. Quinidine production in most of the fungi was significantly correlated with their biomass production but not with their extract production. Of those five fungi, F. solani that was cultured in PDB medium with an initial pH of 6.2 and incubated under continuous light produced the highest concentration of quinidine with low biomass.

Conclusion

The quinidine production of all fungal endophytes studied was affected by the culture conditions. F. solani is the most promising fungus for use as a quinidine production agent.

Candida albicans Virulence Factors and Pathogenicity for Endodontic Infections

Candida albicans (C. albicans) is the fungus most frequently isolated from endodontic root canal infections. Although recognized by dental pulp and periradicular tissue cells that elicit immune responses, it eludes host defenses and elicits cell death. Then, C. albicans binds tooth dentin, forms biofilms, and invades dentinal tubules to resist intracanal disinfectants and endodontic treatments. Insensitive to most common medicaments, it survives sequestered within biofilms and intratubular dentin. Thus, C. albicans has been associated with cases of persistent or refractory root canal infections. Its treatment strategies may require alternative intracanal irrigants, intracanal medicaments such as chlorhexidine gel or human beta defensin-3 (HBD3), Ca-Si-based obturating materials, and microsurgical procedures.

Penicillium purpurogenum produces a novel, acidic, GH3 beta-xylosidase: Heterologous expression and characterization of the enzyme

Xylan, a component of plant cell walls, is composed of a backbone of β-1,4-linked xylopyranosyl units with a number of substituents. The complete degradation of xylan requires the action of several enzymes, among them β-xylosidase. The fungus Penicillium purpurogenum secretes a number of enzymes participating in the degradation of xylan. In this study, a β-xylosidase from this fungus was expressed in Pichia pastoris, and characterized. This enzyme (Xyl2) is a member of glycoside hydrolase family 3; it consists of a sequence of 792 residues including a signal peptide of 20 residues, with a theoretical molecular mass for the mature protein of 84.2 KDa and an isoelectric point of 5.07. The highest identity with a characterized fungal enzyme, is with a β-xylosidase from Aspergillus oryzae (70%). The optimal activity of Xyl2 is found at pH 2.0 and 28 °C. The enzyme is most stable at pH 2.0 and conserves 40% of activity at 42 °C (after 1h incubation). The kinetic parameters for p-nitrophenyl-β-d-xylopyranoside are: K_M 0.53 mM, kcat 1*10⁷ s^-1 and kcat/K_M 1.9*10¹⁰ M^-1 s^-1. The enzyme is about 10% active on p-nitrophenyl-α-l-arabinofuranoside. Xyl2 exhibits a high hydrolytic activity on xylooligosaccharides; it liberates xylose from beechwood and birchwood glucuronoxylan and it acts synergistically with endoxylanases in the degradation of xylan. Its low pH optimum make this enzyme particularly useful in potential applications requiring a low pH such as increasing the flavor of wine.

Understanding the regulation of extracellular protease gene expression in fungi: a key step towards their biotechnological applications

The secretion of proteases by certain species of yeast and filamentous fungi is of importance not only for their biological function and survival, but also for their biotechnological application to various processes in the food, beverage, and bioprocessing industries. A key step towards understanding the role that these organisms play in their environment, and how their protease-secreting ability may be optimally utilised through industrial applications, involves an evaluation of those factors which influence protease production. The objective of this review is to provide an overview of the findings from investigations directed at elucidating the regulatory mechanisms underlying extracellular protease secretion in yeast and filamentous fungi, and the environmental stimuli that elicit these responses. The influence of nitrogen-, carbon-, and sulphur-containing compounds, as well as proteins, temperature, and pH, on extracellular protease regulation, which is frequently exerted at the transcriptional level, is discussed in particular depth. Protease-secreting organisms of biotechnological interest are also presented in this context, in an effort to explore the areas of industrial significance that could possibly benefit from such knowledge. In this way, the establishment of a platform of existing knowledge regarding fungal protease regulation is attempted, with the particular goal of aiding in the practical application of these organisms to processes that require secretion of this enzyme.

The pH sensing receptor AopalH plays important roles in the nematophagous fungus Arthrobotrys oligospora

There is well-conserved PacC/Rim101 signaling among ascomycete fungi to mediate environmental pH sensing. For pathogenic fungi, this pathway not only enables fungi to grow over a wide pH range, but it also determines whether these fungi can successfully colonize and invade the targeted host. Within the pal/PacC pathway, palH is a putative ambient pH sensor with a seven-transmembrane domain. To characterize the function of a palH homolog, AopalH, in the nematophagous fungus Arthrobotrys oligospora, we knocked out the encoding gene of AopalH through homologous recombination, and the transformants exhibited slower growth rates, greater sensitivities to cationic and hyperoxidation stresses, as well as reduced conidiation and reduced trap formation, suggesting that the pH regulatory system has critical functions in nematophagous fungi. Our results provide novel insights into the mechanisms of pH response and regulation in fungi.

pH-Signaling Transcription Factor AopacC Regulates Ochratoxin A Biosynthesis in Aspergillus ochraceus

In Aspergillus and Penicillium species, an essential pH-response transcription factor pacC is involved in growth, pathogenicity, and toxigenicity. To investigate the connection between ochratoxin A (OTA) biosynthesis and ambient pH, the AopacC in Aspergillus ochraceus was functionally characterized using a loss-of-function mutant. The mycelium growth was inhibited under pH 4.5 and 10.0, while the sporulation increased under alkaline condition. A reduction of mycelium growth and an elevation of sporulation was observed in Δ AopacC mutant. Compared to neutral condition, OTA contents were respectively reduced by 71.6 and 79.8% under acidic and alkaline conditions. The expression of AopacC increased with the elevated pH, and deleting AopacC dramatically decreased OTA production and biosynthetic genes Aopks expression. Additionally, the Δ AopacC mutant exhibited attenuated infection ability toward pear fruits. These results suggest that AopacC is an alkaline-induced regulator responsible for growth and OTA biosynthesis in A. ochraceus and this regulatory mechanism might be pH-dependent.

Quantitative multiplexed profiling of Penicillium funiculosum secretome grown on polymeric cellulase inducers and glucose

Filamentous fungi respond to the need to secure utilisable carbon from their growth milieu by secreting unique extracellular proteins depending upon the types of polymeric substrates. We have here profiled the variations in the secretome pattern of a non-model hypercellulolytic fungus - Penicillium funiculosum, grown in minimal media containing four different polymeric cellulase inducers, i.e., Avicel, wheat bran, ammonium-pretreated wheat straw and Avicel & wheat bran, and glucose over its early and late log phases of growth. Of the 137 secreted proteins validated at 1% FDR, we identified the quantified proteins in three clusters as early, persistently or lately expressed. The type of carbon substrate present in the culture media significantly affected the levels of cellulolytic enzymes expression by the fungus. The top abundant proteins quantified in the secretome for Avicel and wheat bran were cellobiohydrolaseI [GH7-CBM1], cellobiohydrolaseII [GH6-CBM1], β-glucosidase [GH3], arabinofuranosidase [GH51] and β-xylosidase [GH3], with bicupin being highest in case of wheat straw. Our results further suggested that the fungus secreted the extracellular proteins in waves, such that the initial responders act to hydrolyse the composite substrates in the culture environment before the second wave of proteins which tend to be more tailored to the specific substrate in the cultivating media.

BIOLOGICAL SIGNIFICANCE

In this article, we have comprehensively examined the dynamics of the secretome of a non-model hypercellulolytic fungus produced in response to model and composite cellulase inducers. Our study has provided additional insights into how the fungus enzyme machinery responds to the presence of different polymeric cellulase inducers over the two different growth phases (early growth and late growth phase). The comprehensive typing and quantification of the different proteins present in the secretomes of the cellulolytic fungal strains in response to diverse nutrient sources hold many prospects in understanding the fungus unique enzyme machinery and dynamics for the downstream biotechnological applications.

Spore Germination of Pathogenic Filamentous Fungi

Fungi, algae, plants, protozoa, and bacteria are all known to form spores, especially hardy and ubiquitous propagation structures that are also often the infectious agents of diseases. Spores can survive for thousands of years, frozen in the permafrost (Kochkina et al., 2012), with the oldest viable spores extracted after 250 million years from salt crystals (Vreeland, Rosenzweig, & Powers, 2000). Their resistance to high levels of UV, desiccation, pressure, heat, and cold enables the survival of spores in the harshest conditions (Setlow, 2016). For example, Bacillus subtilis spores can survive and remain viable after experiencing conditions similar to those on Mars (Horneck et al., 2012). Spores are disseminated through environmental factors. Wind, water, or animal carriage allow spores to be spread ubiquitously throughout the environment. Spores will break dormancy and begin to germinate once exposed to favorable conditions. Germination is the mechanism that converts the spore from a dormant biological organism to one that grows vegetatively and is capable of either sexual or asexual reproduction. The process of germination has been well studied in plants, moss, bacteria, and many fungi (Hohe & Reski, 2005; Huang & Hull, 2017; Vesty et al., 2016). Unfortunately, information on the complex signaling involved in the regulation of germination, particularly in fungi remains lacking. This chapter will discuss germination of fungal spores covering our current understanding of the regulation, signaling, outcomes, and implications of germination of pathogenic fungal spores. Owing to the morphological similarities between the spore-hyphal and yeast-hyphal transition and their relevance for disease progression, relevant aspects of fungal dimorphism will be discussed alongside spore germination in this chapter.

Adaptation to the Host Environment by Plant-Pathogenic Fungi

Many fungi can live both saprophytically and as endophyte or pathogen inside a living plant. In both environments, complex organic polymers are used as sources of nutrients. Propagation inside a living host also requires the ability to respond to immune responses of the host. We review current knowledge of how plant-pathogenic fungi do this. First, we look at how fungi change their global gene expression upon recognition of the host environment, leading to secretion of effectors, enzymes, and secondary metabolites; changes in metabolism; and defense against toxic compounds. Second, we look at what is known about the various cues that enable fungi to sense the presence of living plant cells. Finally, we review literature on transcription factors that participate in gene expression in planta or are suspected to be involved in that process because they are required for the ability to cause disease.

Production of dodecanedioic acid via biotransformation of low cost plant-oil derivatives using Candida tropicalis

Dodecanedioic acid (DDA) is highly useful to the chemical industry as a versatile precursor for producing the polyamide nylon-6,12, which is used for many technical applications, such as heat and chemical-resistant sheaths. However, DDA synthesis has several drawbacks, such as high energy input and cost-intensive removal of by-products. Therefore, alternative bio-based production routes are required due to increasing industrial demand for green chemicals and renewable products. Candida tropicalis converts petrochemical-based n-dodecanes to the corresponding dicarboxylic acids by targeted functionalization. To increase sustainability of the DDA production process, we tested dodecanoic acid methyl ester, which can be easily obtained from transesterification of coconut oil, in whole-cell biotransformation by C. tropicalis. By modifying selected process parameters, a final DDA concentration of 66 g/L was achieved using a highly reliable, small-scale bioreactor system. Crucial process development included a gradual pH shift, an optimized substrate feeding strategy, and monitoring the transcriptional profile.

Membrane lipids and soluble sugars dynamics of the alkaliphilic fungus Sodiomyces tronii in response to ambient pH

Alkaliphily, the ability of an organism to thrive optimally at high ambient pH, has been well-documented in several lineages: archaea, bacteria and fungi. The molecular mechanics of such adaptation has been extensively addressed in alkaliphilic bacteria and alkalitolerant fungi. In this study, we consider an additional property that may have enabled fungi to prosper at alkaline pH: altered contents of membrane lipids and cytoprotectant molecules. In the alkaliphilic Sodiomyces tronii, we showed that at its optimal growth pH 9.2, the fungus accumulates abundant cytosolic trehalose (4-10% dry weight) and phosphatidic acids in the membrane lipids, properties not normally observed in neutrophilic species. At a very high pH 10.2, the major carbohydrate, glucose, was rapidly substituted by mannitol and arabitol. Conversely, lowering the pH to 5.4-7.0 had major implications both on the content of carbohydrates and membrane lipids. It was shown that trehalose dominated at pH 5.4. Fractions of sphingolipids and sterols of plasma membranes rapidly elevated possibly indicating the formation of membrane structures called rafts. Overall, our results reveals complex dynamics of the contents of membrane lipids and cytoplasmic sugars in alkaliphilic S. tronii, suggesting their adaptive functionality against pH stress.

The pigment characteristics and productivity shifting in high cell density culture of Monascus anka mycelia

Background

Monascus mycelia and pigments are promising sources of food and medicine with their potential pharmaceutical values and health-improving functions. Using high cell density fermentation of Monascus spp. to achieve higher mycelium and yellow pigment production is worthy to be researched. In this study, the characteristics and productivity shifting of pigments in high cell density culture of Monascus anka GIM 3.592 were investigated.

Results

The high yield of Monascus mycelia up to 39.77 g/L dry cell weight (DCW), which was achieved by fed-batch fermentation with the feeding medium containing C, N, P and trace elements, was four times higher than that of conventional batch culture. But the total pigment production decreased by 14.6 %, which suggested non-coupled growth. Potential novel yellow pigments accumulated constantly at the late stage of the fed-batch culture, which resulted in a shift in pigment characteristics so that yellow pigments became the dominant pigments. Citrinin production was extremely low and independent of feeding ingredients.

Conclusions

This study provided a suitable fermentation strategy to produce functional Monascus mycelia with a high proportion of yellow pigments in high cell density culture. For the first time, it reported the pigment productivity and characteristics shifting in high cell density culture of Monascus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0183-3) contains supplementary material, which is available to authorized users.

Assessing medium constituents for optimal heterologous production of anhydromevalonolactone in recombinant Aspergillus oryzae

Anhydromevalonolactone (AMVL) is a bioactive natural product that arises from a molecular biology technique using Aspergillus oryzae as a heterologous host. AMVL has been used as a precursor for the synthesis of insect pest control reagents and has numerous applications in the biotechnological and medical industries. In this study, the Plackett-Burman Design and the Central Composite Design, which offer efficient and feasible approaches, were complemented to screen significant parameters and identify the optimal values for maximum AMVL production. The results suggested that sucrose, NaNO₃, yeast extract and K₂HPO₄ were the key factors affecting AMVL production in a complex medium, whereas the major components required for a defined medium were NaNO₃, K₂HPO₄, KH₂PO₄ and trace elements. These factors were subsequently optimized using the response surface methodology. Under optimal conditions, a maximum AMVL production of 250 mg/L in the complex medium and 200 mg/L in the defined medium was achieved, which represents an increase of approximately 3-4-fold compared to the commonly used malt extract medium.

Definition of culture conditions for Arxula adeninivorans, a rational basis for studying heterologous gene expression in this dimorphic yeast

The yeast Arxula adeninivorans is considered to be a promising producer of recombinant proteins. However, growth characteristics are poorly investigated and no industrial process has been established yet. Though of vital interest for strain screening and production processes, rationally defined culture conditions remain to be developed. A cultivation system was evolved based on targeted sampling and mathematical analysis of rationally designed small-scale cultivations in shake flasks. The oxygen and carbon dioxide transfer rates were analyzed as conclusive online parameters. Oxygen limitation extended cultivation and led to ethanol formation in cultures supplied with glucose. Cultures were inhibited at pH-values below 2.8. The phosphorus demand was determined as 1.55 g phosphorus per 100 g cell dry weight. Synthetic SYN6 medium with 20 g glucose l−1 was optimized for cultivation in shake flasks by buffering at pH 6.4 with 140 mmol MES l−1. Optimized SYN6 medium and operating conditions provided non-limited cultivations without by-product formation. A maximal specific growth rate of 0.32 h−1 and short fermentations of 15 h were achieved. A pH optimum curve was derived from the oxygen transfer rates of differently buffered cultures, showing maximal growth between pH 2.8 and 6.5. Furthermore, it was shown that the applied medium and cultivation conditions were also suitable for non-limiting growth and product formation of a genetically modified A. adeninivorans strain expressing a heterologous phytase.

Investigating Acid Stress Response in Different Saccharomyces Strains

Yeast cells need to respond to a variety of stresses found in such different conditions as gastrointestinal tract after probiotic ingestion or fermentation vat during ethanol production. In the present study, H+ neutralisation capacity, membrane fatty acid composition, H+-ATPase activity, and cytosolic Ca2+ concentration were evaluated in yeast cells used for probiotic (Saccharomyces boulardii) and laboratory (Saccharomyces cerevisiae W303) purposes, as well as in some W303 mutant strains for ENA1 gene and S. cerevisiae BY4741. Results show that the H+ internal concentration of yeast is regulated by several systems, including the plasma membrane H+-ATPase, and that Ena1p has an important but undefined role in the cellular response to acid. Membrane fatty acid composition of S. cerevisiae W303 strain was affected by exposure to acidic pH, but the presence of 86 mM NaCl prevented this effect, whereas membrane fatty acid composition of S. boulardii was unaffected by acidic pH. We also demonstrated that the acid stress response is dependent on calcium metabolism and blocked by FK 506.

Strategies utilized by trophically diverse fungal species for Pinus sylvestris root colonization

Physiological changes in host plants in response to the broad spectrum of fungal modes of infection are still not well understood. The current study was conducted to better understand the infection of in vitro cultures of Pinus sylvestris L. seedlings by three trophically diverse fungal species, Fusarium oxysporum E. F. Sm. & Swingle, Trichoderma harzianum Rifai and Hebeloma crustuliniforme (Bull.) Quél. Biochemical methods and microscopy were utilized to determine (i) which factors (apoplastic and cellular pH, reactive oxygen species, glutathione and cell death) play a role in the establishment of pathogenic, saprotrophic and mycorrhizal fungi, and (ii) whether cell death is a common response of conifer seedling tissues when they are exposed to trophically diverse fungi. Establishment of the pathogen, F. oxysporum, was observed more frequently in the meristematic region of root tips than in the elongation zone, which was in contrast to T. harzianum and H. crustuliniforme. Ectomycorrhizal (ECM) hyphae, however, were occasionally observed in the studied root zone and caused small changes in the studied factors. Colonization of the meristematic zone occurred due to host cell death. Independently of the zone, changes in cellular pH resulting in an acidic cytoplasm conditioned the establishment of F. oxysporum. Additionally, cell death was negatively correlated with hydrogen peroxide (H2O2) in roots challenged by a pathogenic fungus. Cell death was the only factor uniquely associated with the colonization of host roots by a saprotrophic fungus. The mechanism may differ, however, between the zones since apoplastic pH was negatively correlated with cell death in the elongation zone, whereas in the meristematic zone, none of the studied factors explained cell death. Colonization by the ECM fungus, H. crustuliniforme, was associated with a decreasing number of cells with acidic apoplast and by production of H2O2 in the elongation zone resulting in cell death. Saprotrophic and ECM fungi had a greater effect on cell acidification in the meristematic zone than the pathogenic fungus.

Accumulation of the mycotoxin patulin in the presence of gluconic acid contributes to pathogenicity of Penicillium expansum

Penicillium expansum, the causal agent of blue mold rot, causes severe postharvest fruit maceration through secretion of D-gluconic acid (GLA) and secondary metabolites such as the mycotoxin patulin in colonized tissue. GLA involvement in pathogenicity has been suggested but the mechanism of patulin accumulation and its contribution to P. expansum pathogenicity remain unclear. The roles of GLA and patulin accumulation in P. expansum pathogenicity were studied using i) glucose oxidase GOX2-RNAi mutants exhibiting decreased GOX2 expression, GLA accumulation, and reduced pathogenicity; ii) IDH-RNAi mutants exhibiting downregulation of IDH (the last gene in patulin biosynthesis), reduced patulin accumulation, and no effect on GLA level; and iii) PACC-RNAi mutants exhibiting downregulation of both GOX2 and IDH that reduced GLA and patulin production. Present results indicate that conditions enhancing the decrease in GLA accumulation by GOX2-RNAi and PACC-RNAi mutants, and not low pH, affected patulin accumulation, suggesting GLA production as the driving force for further patulin accumulation. Thus, it is suggested that GLA accumulation may modulate patulin synthesis as a direct precursor under dynamic pH conditions modulating the activation of the transcription factor PACC and the consequent pathogenicity factors, which contribute to host-tissue colonization by P. expansum.

Virulence regulation of phytopathogenic fungi by pH

SIGNIFICANCE

Postharvest pathogens can start its attack process immediately after spores land on wounded tissue, whereas other pathogens can forcibly breach the unripe fruit cuticle and then remain quiescent for months until fruit ripens and then cause major losses.

RECENT ADVANCES

Postharvest fungal pathogens activate their development by secreting organic acids or ammonia that acidify or alkalinize the host ambient surroundings.

CRITICAL ISSUES

These fungal pH modulations of host environment regulate an arsenal of enzymes to increase fungal pathogenicity. This arsenal includes genes and processes that compromise host defenses, contribute to intracellular signaling, produce cell wall-degrading enzymes, regulate specific transporters, induce redox protectant systems, and generate factors needed by the pathogen to effectively cope with the hostile environment found within the host. Further, evidence is accumulating that the secreted molecules (organic acids and ammonia) are multifunctional and together with effect of the ambient pH, they activate virulence factors and simultaneously hijack the plant defense response and induce program cell death to further enhance their necrotrophic attack.

FUTURE DIRECTIONS

Global studies of the effect of secreted molecules on fruit pathogen interaction, will determine the importance of these molecules on quiescence release and the initiation of fungal colonization leading to fruit and vegetable losses.

Reduction of the degradation activity of umami-enhancing purinic ribonucleotide supplement in miso by the targeted suppression of acid phosphatases in the Aspergillus oryzae starter culture

Miso (fermented soybean paste) is a traditional Japanese fermented food, and is now used worldwide. The solid-state culture of filamentous fungus, Aspergillus oryzae, grown on rice is known as rice-koji, and is important as a starter for miso fermentation because of its prominent hydrolytic enzyme activities. Recently, commercial miso products have been supplemented with purinic ribonucleotides, such as inosine monophosphate (IMP) and guanine monophosphate, to enhance the characteristic umami taste of glutamate in miso. Because the purinic ribonucleotides are degraded by enzymes such as acid phosphatases in miso, heat inactivation is required prior to the addition of these flavorings. However, heat treatment is a costly process and reduces the quality of miso. Therefore, an approach to lower acid phosphatase activities in koji culture is necessary. Transcriptional analysis using an A. oryzae KBN8048 rice-koji culture showed that eight of the 13 acid phosphatase (aph) genes were significantly down-regulated by the addition of phosphoric acid in the preparation of the culture in a concentration-dependent manner, while aphC expression was markedly up-regulated under the same conditions. The eight down-regulated genes might be under the control of the functional counterpart of the Saccharomyces cerevisiae transcriptional activator Pho4, which specifically regulates phosphatase genes in response to the ambient phosphate availability. However, the regulatory mechanism of aphC was not clear. The IMP dephosphorylation activities in rice-koji cultures of KBN8048 and the aphC deletion mutant (ΔaphC) were reduced by up to 30% and 70%, respectively, in cultures with phosphoric acid, while protease and amylase activity, which is important for miso fermentation, was minimally affected. The miso products fermented using the rice-koji cultures of KBN8048 and ΔaphC prepared with phosphoric acid had reductions in IMP dephosphorylation activity of 80% and 90%, respectively, without any adverse effects on amylase and protease activities. Thus, preparing the A. oryzae rice-koji culture under phosphate-sufficient conditions preferentially produces a fermentation starter of miso exhibiting low purinic ribonucleotide dephosphorylation activity. Moreover, aphC is a potential breeding target to reduce purinic ribonucleotide degradation activity further in commercial miso products.

Differential regulation by ambient pH of putative virulence factor secretion by the phytopathogenic fungus Botrytis cinerea

Abstract The fungal pathogen Botrytis cinerea is capable of developing on a wide variety of host plants that differ greatly in their pH values and biochemical defences. To evaluate whether the pH of the host tissue can regulate the production of pathogenicity factors by this fungus, we examined the ability of two isolates of B. cinerea that originated from different plant species to secrete putative virulence elements on synthetic media buffered at pH 2.0 to pH 7.0. Even though differing in the intensity of their responses, both isolates reacted similarly to their ambient pH. The production of extracellular polysaccharides and oxalic acid was detectable above pH 4.0 and pH 5.0 respectively. Conversely, the production of aspartic acid proteases could only be seen between pH 3.0 and 4.0. Finally, the secretion of polygalacturonase and laccase activity was found to exhibit two maxima, one around pH 3.1 and one around pH 6.0. Thus, pathogenicity factor production was found to be minimal between pH 4.5 and 5.5 and a different set of factors was produced at pH 3.1 and 6.0, two values that were found to correspond respectively to the average host fruit and leaf pH. These results demonstrate that ambient pH differentially regulates the synthesis of pathogenicity factors by Botrytis and may act as a novel regulatory element to assist this fungus in tuning its virulence machinery to the composition of its host tissue.

Extracellular proteinase formation in carbon starving Aspergillus nidulans cultures--physiological function and regulation

Extracellular proteinase formation in carbon depleted cultures of the model filamentous fungus Aspergillus nidulans was studied to elucidate its regulation and possible physiological function. As demonstrated by gene deletion, culture optimization, microbial physiological and enzymological experiments, the PrtA and PepJ proteinases of A. nidulans did not appear to play a decisive role in the autolytic decomposition of fungal cells under the conditions we tested. However, carbon starvation induced formation of the proteinases observable in autolytic cultures. Similar to other degradative enzymes, production of proteinase was regulated by FluG-BrlA asexual developmental signaling and modulated by PacC-dependent pH-responsive signaling. Under the same carbon starved culture conditions, alterations of CreA, MeaB or heterotrimeric G protein mediated signaling pathways caused less significant changes in the formation of extracellular proteinases. Taken together, these results indicate that while the accumulation of PrtA and PepJ is tightly coupled to the initiation of autolysis, they are not essential for autolytic cell wall degradation in A. nidulans. Thus, as Aspergillus genomes contain a large group of genes encoding proteinases with versatile physiological functions, selective control of proteinase production in fungal cells is needed for the improved industrial use of fungi.