Hydrophobins: multipurpose proteins

Elasticity of interfacial rafts of hard particles with soft shells

We study an elasticity model for compressed protein monolayers or particle rafts at a liquid interface. Based on the microscopic view of hard-core particles with soft shells, a bead-spring model is formulated and analyzed in terms of continuum elasticity theory. The theory can be applied, for example, to hydrophobin-coated air-water interfaces or, more generally, to liquid interfaces coated with an adsorbed monolayer of interacting hard-core particles. We derive constitutive relations for such particle rafts and describe the buckling of compressed planar liquid interfaces as well as their apparent Poisson ratio. We also use the constitutive relations to obtain shape equations for pendant or buoyant capsules attached to a capillary, and to compute deflated shapes of such capsules. A comparison with capsules obeying the usual Hookean elasticity (without hard cores) reveals that the hard cores trigger capsule wrinkling. Furthermore, it is shown that a shape analysis of deflated capsules with hard-core/soft-shell elasticity gives apparent elastic moduli which can be much higher than the original values if Hookean elasticity is assumed.

Transcriptome changes in Fusarium verticillioides caused by mutation in the transporter-like gene FST1

BACKGROUND

Fusarium verticillioides causes an important seed disease on maize and produces the fumonisin group of mycotoxins, which are toxic to humans and livestock. A previous study discovered that a gene (FST1) in the pathogen affects fumonisin production and virulence. Although the predicted amino acid sequence of FST1 is similar to hexose transporters, previous experimental evidence failed to prove function.

RESULTS

Three new phenotypes were identified that are associated with the FST1 mutant of F. verticillioides (Δfst1), namely reduction in macroconidia production, increased sensitivity to hydrogen peroxide, and reduced mycelial hydrophobicity. A transcriptome comparison of the wild type and strain Δfst1 grown on autoclaved maize kernels for six days identified 2677 genes that were differentially expressed. Through gene ontology analysis, 961 genes were assigned to one of 12 molecular function categories. Sets of down-regulated genes in strain Δfst1 were identified that could account for each of the mutant phenotypes.

CONCLUSION

The study provides evidence that disruption of FST1 causes several metabolic and developmental defects in F. verticillioides. FST1 appears to connect the expression of several gene networks, including those involved in secondary metabolism, cell wall structure, conidiogenesis, virulence, and resistance to reactive oxygen species. The results support our hypothesis that FST1 functions within the framework of environmental sensing.

(1)H, (13)C and (15)N resonance assignments of the RodA hydrophobin from the opportunistic pathogen Aspergillus fumigatus

Hydrophobins are fungal proteins characterised by their amphipathic properties and an idiosyncratic pattern of eight cysteine residues involved in four disulphide bridges. The soluble form of these proteins spontaneously self-assembles at hydrophobic/hydrophilic interfaces to form an amphipathic monolayer. The RodA hydrophobin of the opportunistic pathogen Aspergillus fumigatus forms an amyloid layer with a rodlet morphology that covers the surface of fungal spores. This rodlet layer bestows hydrophobicity to the spores facilitating their dispersal in the air and rendering the conidia inert relative to the human immune system. As a first step in the analysis of the solution structure and self-association of RodA, we report the (1)H, (13)C and (15)N resonance assignments of the soluble monomeric form of RodA.

A model for growth of a single fungal hypha based on well-mixed tanks in series: simulation of nutrient and vesicle transport in aerial reproductive hyphae

Current models that describe the extension of fungal hyphae and development of a mycelium either do not describe the role of vesicles in hyphal extension or do not correctly describe the experimentally observed profile for distribution of vesicles along the hypha. The present work uses the n-tanks-in-series approach to develop a model for hyphal extension that describes the intracellular transport of nutrient to a sub-apical zone where vesicles are formed and then transported to the tip, where tip extension occurs. The model was calibrated using experimental data from the literature for the extension of reproductive aerial hyphae of three different fungi, and was able to describe different profiles involving acceleration and deceleration of the extension rate. A sensitivity analysis showed that the supply of nutrient to the sub-apical vesicle-producing zone is a key factor influencing the rate of extension of the hypha. Although this model was used to describe the extension of a single reproductive aerial hypha, the use of the n-tanks-in-series approach to representing the hypha means that the model has the flexibility to be extended to describe the growth of other types of hyphae and the branching of hyphae to form a complete mycelium.

The fungal cerato-platanin protein EPL1 forms highly ordered layers at hydrophobic/hydrophilic interfaces

Cerato-platanin proteins (CPPs) and hydrophobins are two classes of small, secreted proteins that are exclusively found in fungi. CPPs are known as chitin-binding proteins, and were recently also shown to form protein layers at air/water interfaces, but the features of these layers were not investigated on the molecular level yet. In this study, by means of atomic force microscopy (AFM), EPL1, a member of the CPP family was shown to form highly ordered monolayers at a hydrophobic surface/liquid-interface. Furthermore, two new hydrophobins were analysed, and the influence of EPL1 on hydrophobin layers was studied in situ. Hydrophobins are amphiphilic proteins that are able to self-assemble at hydrophobic/hydrophilic interfaces, thereby inverting the polarity of the surface. This renders fungal growth structures such as spores water repellent. The combination of AFM data and wettability experiments led to the conclusion that in presence of both, hydrophobins and EPL1, a previously unknown hybrid layer is formed. This mixed protein layer is on one hand not inverting but enhancing the hydrophobicity of HOPG (highly oriented pyrolytic graphite), typical for EPL1, and on the other hand, it is stable and water insoluble, which is reminiscent of hydrophobin layers.

Multiple roles and effects of a novel Trichoderma hydrophobin

Fungi belonging to the genus Trichoderma are among the most active and ecologically successful microbes found in natural environments, because they are able to use a variety of substrates and affect the growth of other microbes and virtually any plant species. We isolated and characterized a novel type II hydrophobin secreted by the biocontrol strain MK1 of Trichoderma longibrachiatum. The corresponding gene (Hytlo1) has a multiple role in the Trichoderma-plant-pathogen three-way interaction, while the purified protein displayed a direct antifungal as well as a microbe-associated molecular pattern and a plant growth promotion (PGP) activity. Leaf infiltration with the hydrophobin systemically increased resistance to pathogens and activated defense-related responses involving reactive oxygen species, superoxide dismutase, oxylipin, phytoalexin, and pathogenesis-related protein formation or activity. The hydrophobin was found to enhance development of a variety of plants when applied at very low doses. It particularly stimulated root formation and growth, as demonstrated also by transient expression of the encoding gene in tobacco and tomato. Targeted knock-out of Hytlo1 significantly reduced both antagonistic and PGP effect of the wild-type strain. We conclude that this protein represents a clear example of a molecular factor developed by Trichoderma spp. to establish a mutually beneficial interaction with the colonized plant.

Applications of hydrophobins: current state and perspectives

Hydrophobins are proteins exclusively produced by filamentous fungi. They self-assemble at hydrophilic-hydrophobic interfaces into an amphipathic film. This protein film renders hydrophobic surfaces of gas bubbles, liquids, or solid materials wettable, while hydrophilic surfaces can be turned hydrophobic. These properties, among others, make hydrophobins of interest for medical and technical applications. For instance, hydrophobins can be used to disperse hydrophobic materials; to stabilize foam in food products; and to immobilize enzymes, peptides, antibodies, cells, and anorganic molecules on surfaces. At the same time, they may be used to prevent binding of molecules. Furthermore, hydrophobins have therapeutic value as immunomodulators and can been used to produce recombinant proteins.

Kinetics of biofilm formation by drinking water isolated Penicillium expansum

Current knowledge on drinking water (DW) biofilms has been obtained mainly from studies on bacterial biofilms. Very few reports on filamentous fungi (ff) biofilms are available, although they can contribute to the reduction in DW quality. This study aimed to assess the dynamics of biofilm formation by Penicillium expansum using microtiter plates under static conditions, mimicking water flow behaviour in stagnant regions of drinking water distribution systems. Biofilms were analysed in terms of biomass (crystal violet staining), metabolic activity (resazurin, fluorescein diacetate and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide [MTT]) and morphology (epifluorescence [calcofluor white M2R, FUN-1, FDA and acridine orange] and bright-field microscopies). Biofilm development over time showed the typical sigmoidal curve with noticeable different phases in biofilm formation (induction, exponential, stationary, and sloughing off). The methods used to assess metabolic activity provided similar results. The microscope analysis allowed identification of the involvement of conidia in initial adhesion (4 h), germlings (8 h), initial monolayers (12 h), a monolayer of intertwined hyphae (24 h), mycelial development, hyphal layering and bundling, and development of the mature biofilms (≥48 h). P. expansum grows as a complex, multicellular biofilm in 48 h. The metabolic activity and biomass of the fungal biofilms were shown to increase over time and a correlation between metabolism, biofilm mass and hyphal development was found.

Morphogenesis of Streptomyces in submerged cultures

Members of the genus Streptomyces are mycelial bacteria that undergo a complex multicellular life cycle and propagate via sporulation. Streptomycetes are important industrial microorganisms, as they produce a plethora of medically relevant natural products, including the majority of clinically important antibiotics, as well as a wide range of enzymes with industrial application. While development of Streptomyces in surface-grown cultures is well studied, relatively little is known of the parameters that determine morphogenesis in submerged cultures. Here, growth is characterized by the formation of mycelial networks and pellets. From the perspective of industrial fermentations, such mycelial growth is unattractive, as it is associated with slow growth, heterogeneous cultures, and high viscosity. Here, we review the current insights into the genetic and environmental factors that determine mycelial growth and morphology in liquid-grown cultures. The genetic factors include cell-matrix proteins and extracellular polymers, morphoproteins with specific roles in liquid-culture morphogenesis, with the SsgA-like proteins as well-studied examples, and programmed cell death. Environmental factors refer in particular to those dictated by process engineering, such as growth media and reactor set-up. These insights are then integrated to provide perspectives as to how this knowledge can be applied to improve streptomycetes for industrial applications.

Hydrophobins as aqueous lubricant additive for a soft sliding contact

Two type II fungal hydrophobins, HFBI and FpHYD5, have been studied as aqueous lubricant additive at a nonpolar, compliant sliding contact (self-mated poly(dimethylsiloxane) (PDMS) contact) at two different concentrations, 0.1 mg/mL and 1.0 mg/mL. The two hydrophobins are featured as non-glycosylated (HFBI, m.w. ca. 7 kDa) vs glycosylated (FpHYD5, m.w. ca. 10 kDa) proteins. Far UV CD spectra of the two hydrophobins were very similar, suggesting overall structural similarity, but showed a noticeable difference according to the concentration. This is proposed to be related to the formation of multimers at 1.0 mg/mL. Despite 10-fold difference in the bulk concentration, the adsorbed masses of the hydrophobins onto PDMS surface obtained from the two solutions (0.1 and 1.0 mg/mL) were nearly identical, suggesting that a monolayer of the hydrophobins are formed from 0.1 mg/mL solution. PDMS-PDMS sliding interface was effectively lubricated by the hydrophobin solutions, and showed a reduction in the coefficient of friction by as much as ca. two orders of magnitude. Higher concentration solution (1.0 mg/mL) provided a superior lubrication, particularly in low-speed regime, where boundary lubrication characteristic is dominant via 'self-healing' mechanism. FpHYD5 revealed a better lubrication than HFBI presumably due to the presence of glycans and improved hydration of the sliding interface. Two type II hydrophobins function more favorably compared to a synthetic amphiphilic copolymer, PEO-PPO-PEO, with a similar molecular weight. This is ascribed to higher amount of adsorption of the hydrophobins to hydrophobic surfaces from aqueous solution.

Conidia of the insect pathogenic fungus, Metarhizium anisopliae, fail to adhere to mosquito larval cuticle

Adhesion of conidia of the insect pathogenic fungus, Metarhizium anisopliae, to the arthropod host cuticle initially involves hydrophobic forces followed by consolidation facilitated by the action of extracellular enzymes and secretion of mucilage. Gene expression analysis and atomic force microscopy were used to directly quantify recognition and adhesion between single conidia of M. anisopliae and the cuticle of the aquatic larval stage of Aedes aegypti and a representative terrestrial host, Tenebrio molitor. Gene expression data indicated recognition by the pathogen of both hosts; however, the forces for adhesion to the mosquito were approximately five times lower than those observed for Tenebrio. Although weak forces were recorded in response to Aedes, Metarhizium was unable to consolidate firm attachment. An analysis of the cuticular composition revealed an absence of long-chain hydrocarbons in Aedes larvae which are thought to be required for fungal development on host cuticle. This study provides, to our knowledge, the first evidence that Metarhizium does not form firm attachment to Ae. aegypti larvae in situ, therefore preventing the normal route of invasion and pathogenesis from occuring.

The functional role of Cys3-Cys4 loop in hydrophobin HGFI

Hydrophobins are a large group of low-molecular weight proteins. These proteins are highly surface-active and can form amphipathic membranes by self-assembling at hydrophobic-hydrophilic interfaces. Based on physical properties and hydropathy profiles, hydrophobins are divided into two classes. Upon the analysis of amino acid sequences and higher structures, some models suggest that the Cys3-Cys4 loop regions in class I and II hydrophobins can exhibit remarkable difference in their alignment and conformation, and have a critical role in the rodlets structure formation. To examine the requirement for the Cys3-Cys4 loop in class I hydrophobins, we used protein fusion technology to obtain a mutant protein HGFI-AR by replacing the amino acids between Cys3 and Cys4 of the class I hydrophobin HGFI from Grifola frondosa with those ones between Cys3 and Cys4 of the class II hydrophobin HFBI from Trichoderma reesei. The gene of the mutant protein HGFI-AR was successfully expressed in Pichia pastoris. Water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS) measurements demonstrated that the purified HGFI-AR could form amphipathic membranes by self-assembling at mica and hydrophobic polystyrene surfaces. This property enabled them to alter the surface wettabilities of polystyrene and mica and change the elemental composition of siliconized glass. In comparison to recombinant class I hydrophobin HGFI (rHGFI), the membranes formed on hydrophobic surfaces by HGFI-AR were not robust enough to resist 1 % hot SDS washing. Atomic force microscopy (AFM) measurements indicated that unlike rHGFI, no rodlet structure was observed on the mutant protein HGFI-AR coated mica surface. In addition, when compared to rHGFI, no secondary structural change was detected by Circular Dichroism (CD) spectroscopy after HGFI-AR self-assembled at the water-air interface. HGFI-AR could not either be deemed responsible for the fluorescence intensity increase of Thioflavin T (THT) and the Congo Red (CR) absorption spectra shift (after the THT(CR)/HGFI-AR mixed aqueous solution was drastically vortexed). Remarkably, replacement of the Cys3-Cys4 loop could impair the rodlet formation of the class I hydrophobin HGFI. So, it could be speculated that the Cys3-Cys4 loop plays an important role in conformation and functionality, when the class I hydrophobin HGFI self-assembles at hydrophobic-hydrophilic interfaces.

Differential gene expression by Moniliophthora roreri while overcoming cacao tolerance in the field

Frosty pod rot (FPR) of Theobroma cacao (cacao) is caused by the hemibiotrophic fungus Moniliophthora roreri. Cacao clones tolerant to FPR are being planted throughout Central America. To determine whether M. roreri shows a differential molecular response during successful infections of tolerant clones, we collected field-infected pods at all stages of symptomatology for two highly susceptible clones (Pound-7 and CATIE-1000) and three tolerant clones (UF-273, CATIE-R7 and CATIE-R4). Metabolite analysis was carried out on clones Pound-7, CATIE-1000, CATIE-R7 and CATIE-R4. As FPR progressed, the concentrations of sugars in pods dropped, whereas the levels of trehalose and mannitol increased. Associations between symptoms and fungal loads and some organic and amino acid concentrations varied depending on the clone. RNA-Seq analysis identified 873 M. roreri genes that were differentially expressed between clones, with the primary difference being whether the clone was susceptible or tolerant. Genes encoding transcription factors, heat shock proteins, transporters, enzymes modifying membranes or cell walls and metabolic enzymes, such as malate synthase and alternative oxidase, were differentially expressed. The differential expression between clones of 43 M. roreri genes was validated by real-time quantitative reverse transcription polymerase chain reaction. The expression profiles of some genes were similar in susceptible and tolerant clones (other than CATIE-R4) and varied with the biotrophic/necrotropic shift. Moniliophthora roreri genes associated with stress metabolism and responses to heat shock and anoxia were induced early in tolerant clones, their expression profiles resembling that of the necrotrophic phase. Moniliophthora roreri stress response genes, induced during the infection of tolerant clones, may benefit the fungus in overcoming cacao defense mechanisms.

Functional analysis of hydrophobin genes in sexual development of Botrytis cinerea

Hydrophobins are small secreted fungal proteins that play roles in growth and development of filamentous fungi, i.e. in the formation of aerial structures and the attachment of hyphae to hydrophobic surfaces. In Botrytis cinerea, three hydrophobin genes have been identified. Studies by Mosbach et al. (2011) showed that hydrophobins are neither involved in conferring surface hydrophobicity to conidia and aerial hyphae of B. cinerea, nor are they required for virulence. The present study investigated the role of hydrophobins in sclerotium and apothecium development. Expression analysis revealed high expression of the Bhp1 gene during different stages of apothecium development. Two Bhp1 splice variants were detected that differ by an internal stretch of 13 amino acid residues. Seven different mutants in which either a single, two or three hydrophobin genes were knocked out, as well as two wild type strains of opposite mating types, were characterized for sclerotium and apothecium development. No aberrant morphology was observed in sclerotium development when single deletion mutants in hydrophobin genes were analyzed. Sclerotia of double knock out mutant ΔBhp1/ΔBhp3 and the triple knock out mutant, however, showed easily wettable phenotypes. For analyzing apothecium development, a reciprocal crossing scheme was setup. Morphological aberrations were observed in crosses with two hydrophobin mutants. When the double knock out mutant ΔBhp1/ΔBhp2 and the triple knock out mutant were used as the maternal parent (sclerotia), and fertilized with wild type microconidia, the resulting apothecia were swollen, dark brown in color and had a blotched surface. After initially growing upwards toward the light source, the apothecia in many cases collapsed due to loss of structural integrity. Aberrant apothecium development was not observed in the reciprocal cross, when these same mutants were used as the paternal parent (microconidia). These results indicate that the presence of hydrophobins in maternal tissue is important for normal development of apothecia of B. cinerea.

Interspecific variability of class II hydrophobin GEO1 in the genus Geosmithia

The genus Geosmithia Pitt (Ascomycota: Hypocreales) comprises cosmopolite fungi living in the galleries built by phloeophagous insects. Following the characterization in Geosmithia species 5 of the class II hydrophobin GEO1 and of the corresponding gene, the presence of the geo1 gene was investigated in 26 strains derived from different host plants and geographic locations and representing the whole phylogenetic diversity of the genus. The geo1 gene was detected in all the species tested where it maintained the general organization shown in Geosmithia species 5, comprising three exons and two introns. Size variations were found in both introns and in the first exon, the latter being due to the presence of an intragenic tandem repeat sequence corresponding to a stretch of glycine residues in the deduced proteins. At the amino acid level the deduced proteins had 44.6 % identity and no major differences in the biochemical parameters (pI, GRAVY index, hydropathy plots) were found. GEO1 release in the fungal culture medium was also assessed by turbidimetric assay and SDS-PAGE, and showed high variability between species. The phylogeny based on the geo1 sequences did not correspond to that generated from a neutral marker (ITS rDNA), suggesting that sequence similarities could be influenced by other factors than phylogenetic relatedness, such as the intimacy of the symbiosis with insect vectors. The hypothesis of a strong selection pressure on the geo1 gene was sustained by the low values (<1) of non synonymous to synonymous nucleotide substitutions ratios (Ka/Ks), which suggest that purifying selection might act on this gene. These results are compatible with either a birth-and-death evolution scenario or horizontal transfer of the gene between Geosmithia species.

Pseudomonas protegens Pf-5 causes discoloration and pitting of mushroom caps due to the production of antifungal metabolites

Bacteria in the diverse Pseudomonas fluorescens group include rhizosphere inhabitants known for their antifungal metabolite production and biological control of plant disease, such as Pseudomonas protegens Pf-5, and mushroom pathogens, such as Pseudomonas tolaasii. Here, we report that strain Pf-5 causes brown, sunken lesions on peeled caps of the button mushroom (Agaricus bisporus) that resemble brown blotch symptoms caused by P. tolaasii. Strain Pf-5 produces six known antifungal metabolites under the control of the GacS/GacA signal transduction system. A gacA mutant produces none of these metabolites and did not cause lesions on mushroom caps. Mutants deficient in the biosynthesis of the antifungal metabolites 2,4-diacetylphloroglucinol and pyoluteorin caused less-severe symptoms than wild-type Pf-5 on peeled mushroom caps, whereas mutants deficient in the production of lipopeptide orfamide A caused similar symptoms to wild-type Pf-5. Purified pyoluteorin and 2,4-diacetylphloroglucinol mimicked the symptoms caused by Pf-5. Both compounds were isolated from mushroom tissue inoculated with Pf-5, providing direct evidence for their in situ production by the bacterium. Although the lipopeptide tolaasin is responsible for brown blotch of mushroom caused by P. tolaasii, P. protegens Pf-5 caused brown blotch-like symptoms on peeled mushroom caps through a lipopeptide-independent mechanism involving the production of 2,4-diacetylphloroglucinol and pyoluteorin.

Cell wall remodeling in mycorrhizal symbiosis: a way towards biotrophism

Cell walls are deeply involved in the molecular talk between partners during plant and microbe interactions, and their role in mycorrhizae, i.e., the widespread symbiotic associations established between plant roots and soil fungi, has been investigated extensively. All mycorrhizal interactions achieve full symbiotic functionality through the development of an extensive contact surface between the plant and fungal cells, where signals and nutrients are exchanged. The exchange of molecules between the fungal and the plant cytoplasm takes place both through their plasma membranes and their cell walls; a functional compartment, known as the symbiotic interface, is thus defined. Among all the symbiotic interfaces, the complex intracellular interface of arbuscular mycorrhizal (AM) symbiosis has received a great deal of attention since its first description. Here, in fact, the host plasma membrane invaginates and proliferates around all the developing intracellular fungal structures, and cell wall material is laid down between this membrane and the fungal cell surface. By contrast, in ectomycorrhizae (ECM), where the fungus grows outside and between the root cells, plant and fungal cell walls are always in direct contact and form the interface between the two partners. The organization and composition of cell walls within the interface compartment is a topic that has attracted widespread attention, both in ecto- and endomycorrhizae. The aim of this review is to provide a general overview of the current knowledge on this topic by integrating morphological observations, which have illustrated cell wall features during mycorrhizal interactions, with the current data produced by genomic and transcriptomic approaches.

Adaptation to high salt concentrations in halotolerant/halophilic fungi: a molecular perspective

Molecular studies of salt tolerance of eukaryotic microorganisms have until recently been limited to the baker's yeast Saccharomyces cerevisiae and a few other moderately halotolerant yeast. Discovery of the extremely halotolerant and adaptable fungus Hortaea werneckii and the obligate halophile Wallemia ichthyophaga introduced two new model organisms into studies on the mechanisms of salt tolerance in eukaryotes. H. werneckii is unique in its adaptability to fluctuations in salt concentrations, as it can grow without NaCl as well as in the presence of up to 5 M NaCl. On the other hand, W. ichthyophaga requires at least 1.5 M NaCl for growth, but also grows in up to 5 M NaCl. Our studies have revealed the novel and intricate molecular mechanisms used by these fungi to combat high salt concentrations, which differ in many aspects between the extremely halotolerant H. werneckii and the halophilic W. ichthyophaga. Specifically, the high osmolarity glycerol signaling pathway that is important for sensing and responding to increased salt concentrations is here compared between H. werneckii and W. ichthyophaga. In both of these fungi, the key signaling components are conserved, but there are structural and regulation differences between these pathways in H. werneckii and W. ichthyophaga. We also address differences that have been revealed from analysis of their newly sequenced genomes. The most striking characteristics associated with H. werneckii are the large genetic redundancy, the expansion of genes encoding metal cation transporters, and a relatively recent whole genome duplication. In contrast, the genome of W. ichthyophaga is very compact, as only 4884 protein-coding genes are predicted, which cover almost three quarters of the sequence. Importantly, there has been a significant increase in their hydrophobins, cell-wall proteins that have multiple cellular functions.

Six hydrophobins are involved in hydrophobin rodlet formation in Aspergillus nidulans and contribute to hydrophobicity of the spore surface

Hydrophobins are amphiphilic proteins able to self-assemble at water-air interphases and are only found in filamentous fungi. In Aspergillus nidulans two hydrophobins, RodA and DewA, have been characterized, which both localize on the conidiospore surface and contribute to its hydrophobicity. RodA is the constituent protein of very regularly arranged rodlets, 10 nm in diameter. Here we analyzed four more hydrophobins, DewB-E, in A. nidulans and found that all six hydrophobins contribute to the hydrophobic surface of the conidiospores but only deletion of rodA caused loss of the rodlet structure. Analysis of the rodlets in the dewB-E deletion strains with atomic force microscopy revealed that the rodlets appeared less robust. Expression of DewA and DewB driven from the rodA promoter and secreted with the RodA secretion signal in a strain lacking RodA, restored partly the hydrophobicity. DewA and B were able to form rodlets to some extent but never reached the rodlet structure of RodA. The rodlet-lacking rodA-deletion strain opens the possibility to systematically study rodlet formation of other natural or synthetic hydrophobins.

Emulsions with unique properties from proteins as emulsifiers

Many proteins are surface active molecules and form stable emulsions. In these emulsions, the protein covered oil droplets behave as sticky droplets even when they are ionically charged. As a result of the stickiness of the droplets the emulsions have gel-like properties. The stickiness is due to the multipolar nature of the proteins in contrast to the bipolar nature of surfactants or other amphiphilic compounds that form emulsions with repulsive droplets. Stable emulsions are also formed from particles like clays to which proteins are adsorbed. These hybrid compounds form even more stable emulsions with stronger elastic properties than clays and proteins on their own. These so called pickering emulsions have paste-like properties and do not flow. The scaffolding network of the crosslinked protein bilayers on the droplets is so strong that both the water and the oil can be removed from the emulsions by freeze drying without collapse of the scaffold. The resulting sponge can be used again for the uptake of both water and oil. Emulsions which are prepared from different proteins differ mainly in their elastic properties.

Significance of the class II hydrophobin FgHyd5p for the life cycle of Fusarium graminearum

Hydrophobins are small secreted proteins ubiquitously found in filamentous fungi. Some hydrophobins were shown to have functions in fungal development, while others lack known function. Class II hydrophobins from Fusarium graminearum and Fusarium culmorum are characterized by formation of low stability aggregates and their solubility in organic solvents. They are economically relevant to the brewing industry because they can induce beer gushing. Since cellular functions of Hyd5p's are still unknown, we analyzed the influence of FgHyd5p on growth and morphology of F. graminearum using FgΔhyd5 knock-out mutants expressing sGFP under the control of the hyd5 promoter and compared them with the performance of the parent wild type strain. Results demonstrate that FgHyd5p does not affect the colony and hyphal morphology. FgHyd5p affects the hydrophobicity of aerial mycelia but had no obvious function in penetration of hyphae through the water air interface. The hydrophobin affects the morphology of conidia, but not their fitness. Different sources of carbon and nitrogen as well as different pH have no effect on the expression of the hyd5 gene, which was demonstrated to be expressed upon growth of F. graminearum on hydrophobic surfaces.

Genome and secretome analysis of the hemibiotrophic fungal pathogen, Moniliophthora roreri, which causes frosty pod rot disease of cacao: mechanisms of the biotrophic and necrotrophic phases

BACKGROUND

The basidiomycete Moniliophthora roreri is the causal agent of Frosty pod rot (FPR) disease of cacao (Theobroma cacao), the source of chocolate, and FPR is one of the most destructive diseases of this important perennial crop in the Americas. This hemibiotroph infects only cacao pods and has an extended biotrophic phase lasting up to sixty days, culminating in plant necrosis and sporulation of the fungus without the formation of a basidiocarp.

RESULTS

We sequenced and assembled 52.3 Mb into 3,298 contigs that represent the M. roreri genome. Of the 17,920 predicted open reading frames (OFRs), 13,760 were validated by RNA-Seq. Using read count data from RNA sequencing of cacao pods at 30 and 60 days post infection, differential gene expression was estimated for the biotrophic and necrotrophic phases of this plant-pathogen interaction. The sequencing data were used to develop a genome based secretome for the infected pods. Of the 1,535 genes encoding putative secreted proteins, 1,355 were expressed in the biotrophic and necrotrophic phases. Analysis of the data revealed secretome gene expression that correlated with infection and intercellular growth in the biotrophic phase and invasive growth and plant cellular death in the necrotrophic phase.

CONCLUSIONS

Genome sequencing and RNA-Seq was used to determine and validate the Moniliophthora roreri genome and secretome. High sequence identity between Moniliophthora roreri genes and Moniliophthora perniciosa genes supports the taxonomic relationship with Moniliophthora perniciosa and the relatedness of this fungus to other basidiomycetes. Analysis of RNA-Seq data from infected plant tissues revealed differentially expressed genes in the biotrophic and necrotrophic phases. The secreted protein genes that were upregulated in the biotrophic phase are primarily associated with breakdown of the intercellular matrix and modification of the fungal mycelia, possibly to mask the fungus from plant defenses. Based on the transcriptome data, the upregulated secreted proteins in the necrotrophic phase are hypothesized to be actively attacking the plant cell walls and plant cellular components resulting in necrosis. These genes are being used to develop a new understanding of how this disease interaction progresses and to identify potential targets to reduce the impact of this devastating disease.

Hydrophobins are required for conidial hydrophobicity and plant root colonization in the fungal biocontrol agent Clonostachys rosea

BACKGROUND

Filamentous fungi produce small cysteine rich surface active amphiphilic hydrophobins on the outer surface of cell walls that mediate interactions between the fungus and the environment. The role of hydrophobins in surface hydrophobicity, sporulation, fruit body formation, recognition and adhesion to host surface and virulence have been reported. The aim of the present study was to characterize the biological function of hydrophobins in the fungal biocontrol agent Clonostachys rosea in order to understand their potential roles in biocontrol mechanisms.

RESULTS

Based on the presence of hydrophobin domains, cysteine spacing patterns and hydropathy plots, we identified three class II hydrophobin genes in C. rosea. Gene expression analysis showed basal expression of Hyd1, Hyd2 and Hyd3 in all conditions tested with the exception of induced Hyd1 expression in conidiating mycelium. Interestingly, up-regulation of Hyd1, Hyd2 and Hyd3 was found during C. rosea self interaction compared to interactions with the fungal plant pathogens Botrytis cinerea or Fusarium graminearum in dual culture assays. Phenotypic analysis of C. rosea deletion and complementation strains showed that Hyd1 and Hyd3 are jointly required for conidial hydrophobicity, although no difference in mycelia hydrophobicity was found between wild type (WT) and mutant strains. Interestingly, mutant strains showed increased growth rates, conidiation and enhanced tolerances of conidia to abiotic stresses. Antagonism tests using in vitro dual culture and detached leaf assays showed that the mutant strains were more aggressive towards B. cinerea, F. graminearum or Rhizoctonia solani, and that aggression was partly related to earlier conidial germination and enhanced tolerance of mutant strains to secreted fungal metabolites. Furthermore, in vitro Arabidopsis thaliana root colonization assays revealed reduced root colonization ability of the ΔHyd3 strain, but not for the ΔHyd1 strain. Furthermore, enhanced root colonization ability for the ΔHyd1ΔHyd3 strain was found in comparison to WT.

CONCLUSIONS

These results show a role for hydrophobins in conidial hydrophobicity, control of conidial germination under stress conditions, and in root colonization in C. rosea. However, functional studies of Hyd2 remains to be performed in order to fully assess the role of hydrophobins in C. rosea.

Transcriptional reprogramming of wheat and the hemibiotrophic pathogen Septoria tritici during two phases of the compatible interaction

The disease septoria leaf blotch of wheat, caused by fungal pathogen Septoria tritici, is of worldwide concern. The fungus exhibits a hemibiotrophic lifestyle, with a long symptomless, biotrophic phase followed by a sudden transition to necrotrophy associated with host necrosis. Little is known about the systematic interaction between fungal pathogenicity and host responses at specific growth stages and the factors triggering the transition. In order to gain some insights into global transcriptome alterations in both host and pathogen during the two phases of the compatible interaction, disease transition was monitored using pathogenesis-related gene markers and H₂O₂ signature prior to RNA-Seq. Transcriptome analysis revealed that the slow symptomless growth was accompanied by minor metabolic responses and slightly suppressed defences in the host, whereas necrotrophic growth was associated with enhanced host responses involving energy metabolism, transport, signalling, defence and oxidative stress as well as a decrease in photosynthesis. The fungus expresses distinct classes of stage-specific genes encoding potential effectors, probably first suppressing plant defence responses/facilitating the symptomless growth and later triggering life style transition and inducing host necrosis/facilitating the necrotrophic growth. Transport, signalling, anti-oxidative stress mechanisms and apoplastic nutrient acquisition play important roles in the entire infection process of S. tritici. Our findings uncover systematic S. tritici-induced expression profiles of wheat related to specific fungal infection strategies and provide a transcriptome resource for studying both hosts and pathogens in plant-Dothideomycete interactions.

PfaH2: a novel hydrophobin from the ascomycete Paecilomyces farinosus

The pfah2 gene coding for a novel hydrophobin PfaH2 from the ascomycete Paecilomyces farinosus was identified during sequencing of random clones from a cDNA library. The corresponding protein sequence of PfaH2 deduced from the cDNA comprised 134 amino acids (aa). A 16 aa signal sequence preceded the N-terminus of the mature protein. PfaH2 belonged to the class Ia hydrophobins. The protein was isolated using trifluoroacetic acid extraction and purified via SDS-PAGE and high-performance liquid chromatography. The surface activity of the recently described PfaH1 and of PfaH2 was compared by the determination of contact angles (CAs) on glass slides and Teflon tape, and the CA of distilled water droplets was measured on glass slides coated with hydrophobin PfaH1 or PfaH2. Surprisingly, both hydrophobins adsorbed to hydrophilic surfaces and changed their physicochemical properties to a similar quantitative extent, although little aa sequence homology was found.

Evolutionary analysis of hydrophobin gene family in two wood-degrading basidiomycetes, Phlebia brevispora and Heterobasidion annosum s.l

Background

Hydrophobins are small secreted cysteine-rich proteins that play diverse roles during different phases of fungal life cycle. In basidiomycetes, hydrophobin-encoding genes often form large multigene families with up to 40 members. The evolutionary forces driving hydrophobin gene expansion and diversification in basidiomycetes are poorly understood. The functional roles of individual genes within such gene families also remain unclear. The relationship between the hydrophobin gene number, the genome size and the lifestyle of respective fungal species has not yet been thoroughly investigated. Here, we present results of our survey of hydrophobin gene families in two species of wood-degrading basidiomycetes, Phlebia brevispora and Heterobasidion annosum s.l. We have also investigated the regulatory pattern of hydrophobin-encoding genes from H. annosum s.s. during saprotrophic growth on pine wood as well as on culture filtrate from Phlebiopsis gigantea using micro-arrays. These data are supplemented by results of the protein structure modeling for a representative set of hydrophobins.

Results

We have identified hydrophobin genes from the genomes of two wood-degrading species of basidiomycetes, Heterobasidion irregulare, representing one of the microspecies within the aggregate H. annosum s.l., and Phlebia brevispora. Although a high number of hydrophobin-encoding genes were observed in H. irregulare (16 copies), a remarkable expansion of these genes was recorded in P. brevispora (26 copies). A significant expansion of hydrophobin-encoding genes in other analyzed basidiomycetes was also documented (1–40 copies), whereas contraction through gene loss was observed among the analyzed ascomycetes (1–11 copies). Our phylogenetic analysis confirmed the important role of gene duplication events in the evolution of hydrophobins in basidiomycetes. Increased number of hydrophobin-encoding genes appears to have been linked to the species’ ecological strategy, with the non-pathogenic fungi having increased numbers of hydrophobins compared with their pathogenic counterparts. However, there was no significant relationship between the number of hydrophobin-encoding genes and genome size. Furthermore, our results revealed significant differences in the expression levels of the 16 H. annosum s.s. hydrophobin-encoding genes which suggest possible differences in their regulatory patterns.

Conclusions

A considerable expansion of the hydrophobin-encoding genes in basidiomycetes has been observed. The distribution and number of hydrophobin-encoding genes in the analyzed species may be connected to their ecological preferences. Results of our analysis also have shown that H. annosum s.l. hydrophobin-encoding genes may be under positive selection. Our gene expression analysis revealed differential expression of H. annosum s.s. hydrophobin genes under different growth conditions, indicating their possible functional diversification.

Osmoadaptation strategy of the most halophilic fungus, Wallemia ichthyophaga, growing optimally at salinities above 15% NaCl

Wallemia ichthyophaga is a fungus from the ancient basidiomycetous genus Wallemia (Wallemiales, Wallemiomycetes) that grows only at salinities between 10% (wt/vol) NaCl and saturated NaCl solution. This obligate halophily is unique among fungi. The main goal of this study was to determine the optimal salinity range for growth of the halophilic W. ichthyophaga and to unravel its osmoadaptation strategy. Our results showed that growth on solid growth media was extremely slow and resulted in small colonies. On the other hand, in the liquid batch cultures, the specific growth rates of W. ichthyophaga were higher, and the biomass production increased with increasing salinities. The optimum salinity range for growth of W. ichthyophaga was between 15 and 20% (wt/vol) NaCl. At 10% NaCl, the biomass production and the growth rate were by far the lowest among all tested salinities. Furthermore, the cell wall content in the dry biomass was extremely high at salinities above 10%. Our results also showed that glycerol was the major osmotically regulated solute, since its accumulation increased with salinity and was diminished by hypo-osmotic shock. Besides glycerol, smaller amounts of arabitol and trace amounts of mannitol were also detected. In addition, W. ichthyophaga maintained relatively small intracellular amounts of potassium and sodium at constant salinities, but during hyperosmotic shock, the amounts of both cations increased significantly. Given our results and the recent availability of the genome sequence, W. ichthyophaga should become well established as a novel model organism for studies of halophily in eukaryotes.

Genome and transcriptome sequencing of the halophilic fungus Wallemia ichthyophaga: haloadaptations present and absent

Background

The basidomycete Wallemia ichthyophaga from the phylogenetically distinct class Wallemiomycetes is the most halophilic fungus known to date. It requires at least 10% NaCl and thrives in saturated salt solution. To investigate the genomic basis of this exceptional phenotype, we obtained a de-novo genome sequence of the species type-strain and analysed its transcriptomic response to conditions close to the limits of its lower and upper salinity range.

Results

The unusually compact genome is 9.6 Mb large and contains 1.67% repetitive sequences. Only 4884 predicted protein coding genes cover almost three quarters of the sequence. Of 639 differentially expressed genes, two thirds are more expressed at lower salinity. Phylogenomic analysis based on the largest dataset used to date (whole proteomes) positions Wallemiomycetes as a 250-million-year-old sister group of Agaricomycotina. Contrary to the closely related species Wallemia sebi, W. ichthyophaga appears to have lost the ability for sexual reproduction. Several protein families are significantly expanded or contracted in the genome. Among these, there are the P-type ATPase cation transporters, but not the sodium/ hydrogen exchanger family. Transcription of all but three cation transporters is not salt dependent. The analysis also reveals a significant enrichment in hydrophobins, which are cell-wall proteins with multiple cellular functions. Half of these are differentially expressed, and most contain an unusually large number of acidic amino acids. This discovery is of particular interest due to the numerous applications of hydrophobines from other fungi in industry, pharmaceutics and medicine.

Conclusions

W. ichthyophaga is an extremophilic specialist that shows only low levels of adaptability and genetic recombination. This is reflected in the characteristics of its genome and its transcriptomic response to salt. No unusual traits were observed in common salt-tolerance mechanisms, such as transport of inorganic ions or synthesis of compatible solutes. Instead, various data indicate a role of the cell wall of W. ichthyophaga in its response to salt. Availability of the genomic sequence is expected to facilitate further research into this unique species, and shed more light on adaptations that allow it to thrive in conditions lethal to most other eukaryotes.

Formation of amphipathic amyloid monolayers from fungal hydrophobin proteins

The fungal hydrophobins are small proteins that are able to spontaneously self-assemble into amphipathic monolayers at hydrophobic:hydrophilic interfaces. These protein monolayers can reverse the wettability of a surface, making them suitable for increasing the biocompatibility of many hydrophobic nanomaterials. One subgroup of this family, the class I hydrophobins, forms monolayers that are composed of extremely robust amyloid-like fibrils, called rodlets. Here we describe protocols for the production and purification of recombinant hydrophobins and oxidative refolding to a biologically active, soluble, monomeric form. We describe methods to trigger self-assembly into the fibrillar rodlet state and techniques to characterize the physicochemical properties of the polymeric forms.

Gains of bacterial flagellar motility in a fungal world

The maintenance of energetically costly flagella by bacteria in non-water-saturated media, such as soil, still presents an evolutionary conundrum. Potential explanations have focused on rare flooding events allowing dispersal. Such scenarios, however, overlook bacterial dispersal along mycelia as a possible transport mechanism in soils. The hypothesis tested in this study is that dispersal along fungal hyphae may lead to an increase in the fitness of flagellated bacteria and thus offer an alternative explanation for the maintenance of flagella even in unsaturated soils. Dispersal along fungal hyphae was shown for a diverse array of motile bacteria. To measure the fitness effect of dispersal, additional experiments were conducted in a model system mimicking limited dispersal, using Pseudomonas putida KT2440 and its nonflagellated (ΔfliM) isogenic mutant in the absence or presence of Morchella crassipes mycelia. In the absence of the fungus, flagellar motility was beneficial solely under conditions of water saturation allowing dispersal, while under conditions limiting dispersal, the nonflagellated mutant exhibited a higher level of fitness than the wild-type strain. In contrast, in the presence of a mycelial network under conditions limiting dispersal, the flagellated strain was able to disperse using the mycelial network and had a higher level of fitness than the mutant. On the basis of these results, we propose that the benefit of mycelium-associated dispersal helps explain the persistence of flagellar motility in non-water-saturated environments.

Comparative genomics and evolutionary analysis of hydrophobins from three species of wood-degrading fungi

Hydrophobins are small, secreted proteins playing important roles at different stages of fungal life cycles. Their characteristic feature is the presence of eight highly conserved cysteine residues. Here we present an inventory and evolutionary analysis of hydrophobin genes from three wood-degrading basidiomycetes, Phlebia brevispora, Ganoderma sp. and Bjerkandera adusta. The genomes of the three analyzed species are characterized by the presence of high copy numbers of hydrophobin genes. Results of the phylogenetic analysis of the identified proteins revealed that many of them share a high degree of sequence similarity and probably originated from a series of duplication events. The presence of several clusters of adjacent copies of the hydrophobin gene in a particular location in the genome further supports the interpretation that gene duplication has played a role in the evolution of hydrophobins in the analyzed species.

Two novel class II hydrophobins from Trichoderma spp. stimulate enzymatic hydrolysis of poly(ethylene terephthalate) when expressed as fusion proteins

Poly(ethylene terephthalate) (PET) can be functionalized and/or recycled via hydrolysis by microbial cutinases. The rate of hydrolysis is however low. Here, we tested whether hydrophobins (HFBs), small secreted fungal proteins containing eight positionally conserved cysteine residues, are able to enhance the rate of enzymatic hydrolysis of PET. Species of the fungal genus Trichoderma have the most proliferated arsenal of class II hydrophobin-encoding genes among fungi. To this end, we studied two novel class II HFBs (HFB4 and HFB7) of Trichoderma. HFB4 and HFB7, produced in Escherichia coli as fusions to the C terminus of glutathione S-transferase, exhibited subtle structural differences reflected in hydrophobicity plots that correlated with unequal hydrophobicity and hydrophily, respectively, of particular amino acid residues. Both proteins exhibited a dosage-dependent stimulation effect on PET hydrolysis by cutinase from Humicola insolens, with HFB4 displaying an adsorption isotherm-like behavior, whereas HFB7 was active only at very low concentrations and was inhibitory at higher concentrations. We conclude that class II HFBs can stimulate the activity of cutinases on PET, but individual HFBs can display different properties. The present findings suggest that hydrophobins can be used in the enzymatic hydrolysis of aromatic-aliphatic polyesters such as PET.

Development in Aspergillus

The genus Aspergillus represents a diverse group of fungi that are among the most abundant fungi in the world. Germination of a spore can lead to a vegetative mycelium that colonizes a substrate. The hyphae within the mycelium are highly heterogeneous with respect to gene expression, growth, and secretion. Aspergilli can reproduce both asexually and sexually. To this end, conidiophores and ascocarps are produced that form conidia and ascospores, respectively. This review describes the molecular mechanisms underlying growth and development of Aspergillus.