HCf-6, a novel class II hydrophobin from Cladosporium fulvum

Identification and Functional Analysis of a Novel Hydrophobic Protein VdHP1 from Verticillium dahliae

Verticillium dahliae could cause destructive vascular wilt disease on hundreds of plant species around the world, including cotton. In this study, we characterized the function of a hydrophobin gene VdHP1 in pathogen development and pathogenicity. Results showed that VdHP1 could induce cell death and activate plant immune responses. The VdHP1 deletion mutants (ΔVdHP1) and the complement mutants (C-ΔVdHP1) were obtained by the homologous recombination method. The VdHP1 deletion mutants exhibited increased hydrophilicity, inhibited microsclerotial formation, and reduced spore smoothness. In addition, the deletion mutants were more sensitive to NaCl, while relatively insensitive to KCl and sorbitol. Mutants also had greater resistance to Congo red, UV radiation, and high temperature, which suggested that ΔVdHP1 strains have stronger resistance to abiotic stress in general. Different carbon source assays showed that the utilization ability of skim milk, cellulose, and starch was greatly enhanced in ΔVdHP1, compared with that of WT and complemented strains. Furthermore, VdHP1 did not affect mycelium penetration on cellophane but contributed to mycelium growth on surface of the living plant cells. The pathogenicity test found that the crude toxin content, colonization, and dispersal of ΔVdHP1 was significantly increased compared with the WT and complementary strains. In addition, cotton seedlings showed more severe wilting symptoms after inoculation with ΔVdHP1 strains. These results suggested that the hydrophobin VdHP1 negatively regulated the virulence of V. dahliae, and played an important role in development, adaptability, and pathogenicity in V. dahliae, which maybe provide a new viewpoint to further understand the molecular mechanisms of pathogen virulence. IMPORTANCE Verticillium dahliae is a soilborne fungal pathogen that causes a destructive vascular disease on a large number of plant hosts, resulting in great threat to agricultural production. In this study, it was illustrated that the hydrophobin VdHP1 could induce cell death and activate plant immune responses. VdHP1 affected the hydrophobicity of V. dahliae, and negatively regulated the strains resistant to stress, and the utilization ability of different carbon sources. In addition, VdHP1 did not affect mycelium penetration on cellophane but contributed to mycelium growth on surface of the living plant cells. The VdHP1 gene negatively regulated the total virulence, colonization, and dispersal of V. dahliae, with enhanced pathogenicity of mutant strains in this gene. These results suggested that the hydrophobin VdHP1 played an importance in development, adaptability, and pathogenicity in V. dahliae, and would provide a new viewpoint to further understand the molecular mechanisms of pathogen virulence.

Specific Hypersensitive Response-Associated Recognition of New Apoplastic Effectors from Cladosporium fulvum in Wild Tomato

Tomato leaf mold disease is caused by the biotrophic fungus Cladosporium fulvum. During infection, C. fulvum produces extracellular small secreted protein (SSP) effectors that function to promote colonization of the leaf apoplast. Resistance to the disease is governed by Cf immune receptor genes that encode receptor-like proteins (RLPs). These RLPs recognize specific SSP effectors to initiate a hypersensitive response (HR) that renders the pathogen avirulent. C. fulvum strains capable of overcoming one or more of all cloned Cf genes have now emerged. To combat these strains, new Cf genes are required. An effectoromics approach was employed to identify wild tomato accessions carrying new Cf genes. Proteomics and transcriptome sequencing were first used to identify 70 apoplastic in planta-induced C. fulvum SSPs. Based on sequence homology, 61 of these SSPs were novel or lacked known functional domains. Seven, however, had predicted structural homology to antimicrobial proteins, suggesting a possible role in mediating antagonistic microbe-microbe interactions in planta. Wild tomato accessions were then screened for HR-associated recognition of 41 SSPs, using the Potato virus X-based transient expression system. Nine SSPs were recognized by one or more accessions, suggesting that these plants carry new Cf genes available for incorporation into cultivated tomato.

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.

Localization of Cladosporium fulvum hydrophobins reveals a role for HCf-6 in adhesion

Hydrophobins are amphipathic molecules which form part of fungal cell walls and extracellular matrices and perform a variety of roles in fungal growth and development. The tomato pathogen Cladosporium fulvum has six hydrophobin genes, HCf-1 to -6. We have devised an epitope tagging approach for establishing hydrophobin localization during growth in culture and in plants. In this paper we localize HCf-2, -3, -4 and -5 and compare the data to our previous observations for HCf-1 and -6. In culture, HCf-1, -2, -3 and 4 localize to conidia and also appear on aerial hyphae. HCf-4 is unique in that it appears on submerged hyphae. HCf-5 expression is tightly regulated and appears on aerial hyphae early on during growth. Only HCf-1, -3 and -6 were observed during infection; HCf-3 appears on both conidia and emerging germ tubes. We also show that HCf-6 is secreted and coats surfaces under and around growing hyphae and demonstrate the effect of deleting HCf-6 on the adhesion of germinating C. fulvum conidia to glass slides.

Silencing of six hydrophobins in Cladosporium fulvum: complexities of simultaneously targeting multiple genes

In this study, we have constructed and expressed inverted repeat chimeras from the first exons of the six known hydrophobins of the fungus Cladosporium fulvum, the causal agent of tomato leaf mold. We used quantitative PCR to measure specifically the expression levels of the hydrophobins. The targeted genes are silenced to different degrees, but we also detected clear changes in the expression levels of nontargeted genes. This work highlights the difficulties that are likely to be encountered when attempting to silence more than one gene in a multigene family.

TasHyd1, a new hydrophobin gene from the biocontrol agent Trichoderma asperellum, is involved in plant root colonization

SUMMARY A hydrophobin-like clone (TasHyd1) was isolated during a PCR differential mRNA display analysis conducted on Trichoderma asperellum mycelia interacting with plant roots. The open reading frame encodes a 145-amino-acid protein showing similarity to Pbhyd1, a Class I hydrophobin from the dimorphic fungus Paracoccidioides brasiliensis. TasHyd1 expression was detected in planta up to 5 days after Trichoderma root inoculation. TasHyd1 is constitutively expressed at low levels in mycelia in young cultures but gene expression is not detected in sporulating hyphae or in non-germinating spores. Carbon limitation stimulates expression of TasHyd1 whereas nitrogen or phosphate starvation down-regulate expression. TasHyd1 fused to an HA tag was over-expressed in Trichoderma and the protein was detected with an anti-HA antibody in the trifluoroacetic-acid-soluble fraction of mycelial cell walls. Over-expressor mutants were not affected in their mycoparasitic activity when tested in vitro against the plant pathogen Rhizoctonia solani and retained root colonization capacity comparable with that of the wild-type. TasHyd1 deletion mutants had no significant reduction in in vitro mycoparasitic activity but were altered in their wettability and were severely impaired in root attachment and colonization. These phenotypes were recovered by complementation of TasHyd1, indicating that the protein is a new hydrophobin that contributes to Trichoderma interaction with the plant.

Interaction and Comparison of a Class I Hydrophobin from Schizophyllum commune and Class II Hydrophobins from Trichoderma reesei

Hydrophobins fulfill a wide spectrum of functions in fungal growth and development. These proteins self-assemble at hydrophilic-hydrophobic interfaces into amphipathic membranes. Hydrophobins are divided into two classes based on their hydropathy patterns and solubility. We show here that the properties of the class II hydrophobins HFBI and HFBII of Trichoderma reesei differ from those of the class I hydrophobin SC3 of Schizophyllum commune. In contrast to SC3, self-assembly of HFBI and HFBII at the water-air interface was neither accompanied by a change in secondary structure nor by a change in ultrastructure. Moreover, maximal lowering of the water surface tension was obtained instantly or took several minutes in the case of HFBII and HFBI, respectively. In contrast, it took several hours in the case of SC3. Oil emulsions prepared with HFBI and SC3 were more stable than those of HFBII, and HFBI and SC3 also interacted more strongly with the hydrophobic Teflon surface making it wettable. Yet, the HFBI coating did not resist treatment with hot detergent, while that of SC3 remained unaffected. Interaction of all the hydrophobins with Teflon was accompanied with a change in the circular dichroism spectra, indicating the formation of an alpha-helical structure. HFBI and HFBII did not affect self-assembly of the class I hydrophobin SC3 of S. commune and vice versa. However, precipitation of SC3 was reduced by the class II hydrophobins, indicating interaction between the assemblies of both classes of hydrophobins.

Cladosporium fulvum (syn. Passalora fulva), a highly specialized plant pathogen as a model for functional studies on plant pathogenic Mycosphaerellaceae

SUMMARY Taxonomy: Cladosporium fulvum is an asexual fungus for which no sexual stage is currently known. Molecular data, however, support C. fulvum as a member of the Mycosphaerellaceae, clustering with other taxa having Mycosphaerella teleomorphs. C. fulvum has recently been placed in the anamorph genus Passalora as P. fulva. Its taxonomic disposition is supported by its DNA phylogeny, as well as the distinct scars on its conidial hila, which are typical of Passalora, and unlike Cladosporium s.s., which has teleomorphs that reside in Davidiella, and not Mycosphaerella. Host range and disease symptoms: The presently known sole host of C. fulvum is tomato (members of the genusLycopersicon). C. fulvum is mainly a foliar pathogen. Disease symptoms are most obvious on the abaxial side of the leaf and include patches of white mould that turn brown upon sporulation. Due to stomatal clogging, curling of leaves and wilting can occur, leading to defoliation. C. fulvum as a model pathogen: The interaction between C. fulvum and tomato is governed by a gene-for-gene relationship. A total of eight Avr and Ecp genes, and for four of these also the corresponding plant Cf genes, have been cloned. Obtaining conclusive evidence for gene-for-gene relationships is complicated by the poor availability of genetic tools for most Mycosphaerellaceae-plant interactions. Newly developed tools, including Agrobacterium-mediated transformation and RNAi, added to the genome sequence of its host tomato, which will be available within a few years, render C. fulvum attractive as a model species for plant pathogenic Mycosphaerellaceae.

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Stage-specific cellular localisation of two hydrophobins during plant infection by the pathogenic fungus Cladosporium fulvum

Hydrophobins are central to developmental processes of filamentous fungi. HCf-1 and HCf-6 are two of the six hydrophobins identified in the plant pathogenic fungus Cladosporium fulvum. We have fused the viral epitope V5 to HCf-1 and HCf-6, introduced the recombinant genes into C. fulvum strains that lack the two genes, and localised the tagged proteins by immunofluorescence microscopy. HCf-1(V5) is abundant on conidia and aerial structures formed in vitro and emerging from disease lesions on infected tomato plants. This is consistent with the proposed function of HCf-1 in aerial development and dissemination of conidia. HCf-6(V5) is secreted onto the growth substrate by the hyphae and during invasion of plant tissues, which suggests a function in adhesion and infection. This was not supported by the phenotypic analysis of DeltaHCf-6 strains. Hydrophobins may play distinct roles due to precisely regulated spatial localisation during infection-related development of C. fulvum.

Pbhyd1 and Pbhyd2: two mycelium-specific hydrophobin genes from the dimorphic fungus Paracoccidioides brasiliensis

Paracoccidioides brasiliensis, the etiologic agent of paracoccidioidomycosis, is a dimorphic fungus which is found as mycelia (M) at 26 degrees C and as yeasts (Y) at 37 degrees C, or after the invasion of host tissues. Although the dimorphic transition in P. brasiliensis and other dimorphic fungi is an essential step in the establishment of infection, the molecular events regulating this process are yet poorly understood. Since the differential gene expression is a well-known mechanism which plays a central role in the dimorphic transition as well as in other biological process, in this work we describe the identification and characterization of two differentially expressed P. brasiliensis hydrophobin cDNAs (Pbhyd1 and Pbhyd2). Hydrophobins are small hydrophobic proteins related to a variety of important functions in fungal biology, including cell growth, development, infection, and virulence. These two hydrophobin genes are present as single copy in P. brasiliensis genome and Northern blot analysis revealed that both mRNAs are mycelium-specific and highly accumulated during the first 24 h of M to Y transition.

Constitutively activated Galpha negatively regulates virulence, reproduction and hydrophobin gene expression in the chestnut blight fungus Cryphonectria parasitica

Disruption of the gene encoding Galpha subunit cpg-1 in the chestnut blight fungus Cryphonectria parasitica reduces growth and pigmentation and abolishes reproduction and virulence. We now report the consequences of mutations designed to constitutively activate (Q204-L and R178-C) CPG-1-mediated signaling. Introduction of cpg-1-QL or cpg-1-RC into wild type, Deltacpg-1 and Deltacpgb-1 (Gbeta) mutant strains resulted in a dominant phenotype characterized by a complete absence of aerial hyphae, pigmentation, conidia production and virulence. Opposing responses of Deltacpg-1 and activated mutant strains to chronic heat, hyperosmolarity and oxidative stress suggested that CPG-1 plays a role in mediating stress response. Growth of the cpg-1 mutant strains proceeded at wild-type level in rich liquid medium, but was severely curtailed on solid medium and absent in chestnut tissue, indicating the importance of CPG-1 mediated signaling under these harsher conditions. Both cpg-1 deletion and activating CPG-1 mutations resulted in post-transcriptional alterations in the accumulation of CPG-1 and/or CPGB-1, providing evidence for extensive post-transcriptional regulation of G-protein subunit accumulation in C. parasitica. Finally, the absence of aerial hyphae and the easily wettable phenotype exhibited by the QL and RC mutants correlated with reduced expression of the gene encoding cryparin, suggesting G-protein-mediated regulation of a fungal hydrophobin.

Structural and functional analysis of an oligomeric hydrophobin gene from Claviceps purpurea

SUMMARY Fungal hydrophobins are small hydrophobic proteins containing eight cysteine residues at conserved positions which have the ability to form amphipathic polymers. We have characterized a gene from the phytopathogenic ascomycete Claviceps purpurea, cpph1, which encodes a modular-type hydrophobin. It consists of five units, each showing a significant homology to class II hydrophobins. The units are separated by GN-repeat regions, which could form amphipathic alpha-helices; the amino terminus contains a glycine-rich region which could be involved in attaching the protein to the cell wall. The presence of long direct repeats within cpph1, and the high homology of the three internal modules suggest a recent generation of this gene from a tripartite precursor. Although sequencing of cDNA clones indicated that recombination could be mediated via the direct repeats, the majority of the transcripts appear to be full-sized. This was confirmed by Northern blot analysis, which showed the presence of a full-sized transcript in axenic culture. The high molecular weight pentahydrophobin was detected by Western blot analysis, indicating that CPPH1 is not processed into monomeric subunits. Targeted deletion of cpph1 did not lead to differences in morphology, growth rate, sporulation, or hydrophobicity of spores. Furthermore, the cpph1 deletion mutants showed no reduction in virulence on rye.

Immunoglobulin E-binding and skin test reactivity to hydrophobin HCh-1 from Cladosporium herbarum, the first allergenic cell wall component of fungi

BACKGROUND

For many years, fungal spores have been recognized as potential causes of respiratory allergies. All fungal allergens cloned so far represent either secreted or cytoplasmatic proteins, but nothing is known about the involvement of fungal surface proteins in allergic diseases.

METHODS

A phage surface displayed cDNA-library from the mould Cladosporium herbarum was constructed and phage displaying IgE-binding proteins were selectively enriched with immobilized serum IgE from C. herbarum-sensitized individuals. Inserts encoding putative allergens were sequenced, subcloned and used to produce recombinant proteins. Allergenicity of the proteins was evaluated by IgE binding in Western blots, enzyme-linked immunosorbent assay (ELISA) and skin prick test in a total of 84 patients sensitized to either C. herbarum or Aspergillus fumigatus and three healthy controls.

RESULTS

After four rounds of affinity selection, the cDNA-library was enriched for clones displaying IgE-binding molecules. Sequencing of inserts showed that one clone contained an open reading frame predicting a protein of 105 amino acids and a calculated molecular weight of 10.5 kDa showing the classical signature of members of the hydrophobin family. The recombinant protein, termed HCh-1, was able to bind IgE from six patients sensitized to fungi in vitro. Two of those patients were also included in a skin prick test survey and showed strong type I skin reactions to HCh-1, demonstrating the allergenic nature of C. herbarum hydrophobin and indicating a prevalence of sensitization in the range of 8-9%. In contrast, the hydrophobin HYP1 from Aspergillus fumigatus was not recognized by the sera of the same patients and controls investigated with HCh-1.

CONCLUSION

C. herbarum hydrophobin represents the first component of the cell wall of fungi demonstrated to act as a rare but clinically relevant allergen in vitro and in vivo.

Hydrophobins and the interactions between fungi and plants

Summary Hydrophobins are small proteins thought to be ubiquitous in filamentous fungi. They are usually secreted and are found on the outer surfaces of cell walls of hyphae and conidia where they mediate interactions between the fungus and the environment. We review here what is currently known about the primary and secondary structure of these proteins, as well as their post-translational modifications. We also discuss the diverse functions of hydrophobins in biology and development, with particular attention to fungi involved in pathogenesis and symbiosis.

The hydrophobin HCf-1 of Cladosporium fulvum is required for efficient water-mediated dispersal of conidia

Six hydrophobin genes (HCf-1 to -6) have thus far been identified in the tomato pathogen Cladosporium fulvum. HCf-1 to -4 are Class I hydrophobins and HCf-5 and -6 are Class II hydrophobins. In this paper we describe the isolation of deletion mutants that lack HCf-1, HCf-2, or both these genes. Global down-regulation of the expression of Class I hydrophobins is achieved by homology-dependent gene silencing. Analysis of the mutant strains shows that HCf-1 confers hydrophilic character to the conidia and this facilitates the dissemination of conidia on the surface of water droplets. Other Class I hydrophobins, such as HCf-3 or HCf-4, may be involved in the development and germination of conidia.