The pleiotropic phenotype of FlbA of Aspergillus niger is explained in part by the activity of seven of its downstream-regulated transcription factors

Inactivation of flbA in Aspergillus niger results in thinner cell walls, increased cell lysis, abolished sporulation, and an increased secretome complexity. A total of 36 transcription factor (TF) genes are differentially expressed in ΔflbA. Here, seven of these genes (abaA, aslA, aslB, azf1, htfA, nosA, and srbA) were inactivated. Inactivation of each of these genes affected sporulation and, with the exception of abaA, cell wall integrity and protein secretion. The impact on secretion was strongest in the case of ΔaslA and ΔaslB that showed increased pepsin, cellulase, and amylase activity. Biomass was reduced of agar cultures of ΔabaA, ΔaslA, ΔnosA, and ΔsrbA, while biomass was higher in liquid shaken cultures of ΔaslA and ΔaslB. The ΔaslA and ΔhtfA strains showed increased resistance to H2O2, while ΔaslB was more sensitive to this reactive oxygen species. Together, inactivation of the seven TF genes impacted biomass formation, sporulation, protein secretion, and stress resistance, and thereby these genes explain at least part of the pleiotropic phenotype of ΔflbA of A. niger.

Inheritable CRISPR based epigenetic modification in a fungus

The CRISPRoff system was recently introduced as a programmable epigenetic memory writer that can be used to silence genes in human cells. The system makes use of a dead Cas9 protein (dCas9) that is fused with the ZNF10 KRAB, Dnmt3A, and Dnmt3L protein domains. The DNA methylation resulting from the CRISPRoff system can be removed by the CRISPRon system that consists of dCas9 fused to the catalytic domain of Tet1. Here, the CRISPRoff and CRISPRon systems were applied for the first time in a fungus. The CRISPRoff system resulted in an inactivation up to 100 % of the target genes flbA and GFP in Aspergillus niger. Phenotypes correlated with the degree of gene silencing in the transformants and were stable when going through a conidiation cycle, even when the CRISPRoff plasmid was removed from the flbA silenced strain. Introducing the CRISPRon system in a strain in which the CRISPRoff plasmid was removed fully reactivated flbA showing a phenotype similar to that of the wildtype. Together, the CRISPRoff and CRISPRon systems can be used to study gene function in A. niger.

Comparison of cell wall polysaccharides in Schizophyllum commune after changing phenotype by mutation

The Agaricomycetes fungi produce various compounds with pharmaceutical, medicinal, cosmetic, environmental and biotechnological properties. In addition, some polysaccharides extracted from the fungal cell wall have antitumor and immunomodulatory actions. The aim of this study was to use genetic modification to transform Schizophyllum commune and identify if the phenotype observed (different from the wild type) resulted in changes of the cell wall polysaccharides. The plasmid pUCHYG-GPDGLS, which contains the Pleurotus ostreatus glucan synthase gene, was used in S. commune transformations. Polysaccharides from cell wall of wild (ScW) and mutants were compared in this study. Polysaccharides from the biomass and culture broth were extracted with hot water. One of the mutants (ScT4) was selected for further studies and, after hydrolysis/acetylation, the GLC analysis showed galactose as the major component in polysaccharide fraction from the mutant and glucose as the major monomer in the wild type. Differences were also found in the elution profiles from HPSEC and NMR analyses. From the monosaccharide composition it was proposed that mannogalactans are components of S. commune cell wall for both, wild and mutant, but in different proportions. To our knowledge, this is the first time that mannogalactans are isolated from S. commune liquid culture.

Cycling in degradation of organic polymers and uptake of nutrients by a litter-degrading fungus

Wood and litter degrading fungi are the main decomposers of lignocellulose and thus play a key role in carbon cycling in nature. Here, we provide evidence for a novel lignocellulose degradation strategy employed by the litter degrading fungus Agaricus bisporus (known as the white button mushroom). Fusion of hyphae allows this fungus to synchronize the activity of its mycelium over large distances (50 cm). The synchronized activity has a 13-h interval that increases to 20 h before becoming irregular and it is associated with a 3.5-fold increase in respiration, while compost temperature increases up to 2°C. Transcriptomic analysis of this burst-like phenomenon supports a cyclic degradation of lignin, deconstruction of (hemi-) cellulose and microbial cell wall polymers, and uptake of degradation products during vegetative growth of A. bisporus. Cycling in expression of the ligninolytic system, of enzymes involved in saccharification, and of proteins involved in nutrient uptake is proposed to provide an efficient way for degradation of substrates such as litter.

Interruption of an MSH4 homolog blocks meiosis in metaphase I and eliminates spore formation in Pleurotus ostreatus

Pleurotus ostreatus, one of the most widely cultivated edible mushrooms, produces high numbers of spores causing severe respiratory health problems for people, clogging of filters and spoilage of produce. A non-sporulating commercial variety (SPOPPO) has been successfully introduced into the market in 2006. This variety was generated by introgression breeding of a natural mutation into a commercial variety. Our cytological studies revealed that meiosis in the natural and derived sporeless strains was blocked in metaphase I, apparently resulting in a loss of spore formation. The gene(s) underlying this phenotype were mapped to an 80 kb region strongly linked to sporelessness and identified by transformation of wild type genes of this region into a sporeless strain. Sporulation was restored by re-introduction of the DNA sequence encoding the P. ostreatus meiotic recombination gene MSH4 homolog (poMSH4). Subsequent molecular analysis showed that poMSH4 in the sporeless P. ostreatus was interrupted by a DNA fragment containing a region encoding a CxC5/CxC6 cysteine cluster associated with Copia-type retrotransposons. The block of meiosis in metaphase I by a poMSH4 null mutant suggests that this protein plays an essential role in both Class I and II crossovers in mushrooms, similar to animals (mice), but unlike in plants. MSH4 was previously shown to be a target for breeding of sporeless varieties in P. pulmonarius, and the null mutant of the MSH4 homolog of S. commune (scMSH4) confers an extremely low level of spore formation. We propose that MSH4 homologs are likely to be a breeding target for sporeless strains both within Pleurotus sp. and in other Agaricales.

High-throughput targeted gene deletion in the model mushroom Schizophyllum commune using pre-assembled Cas9 ribonucleoproteins

Efficient gene deletion methods are essential for the high-throughput study of gene function. Compared to most ascomycete model systems, gene deletion is more laborious in mushroom-forming basidiomycetes due to the relatively low incidence of homologous recombination (HR) and relatively high incidence of non-homologous end-joining (NHEJ). Here, we describe the use of pre-assembled Cas9-sgRNA ribonucleoproteins (RNPs) to efficiently delete the homeodomain transcription factor gene hom2 in the mushroom-forming basidiomycete Schizophyllum commune by replacing it with a selectable marker. All components (Cas9 protein, sgRNA, and repair template with selectable marker) were supplied to wild type protoplasts by PEG-mediated transformation, abolishing the need to optimize the expression of cas9 and sgRNAs. A Δku80 background further increased the efficiency of gene deletion. A repair template with homology arms of 250 bp was sufficient to efficiently induce homologous recombination. This is the first report of the use of pre-assembled Cas9 RNPs in a mushroom-forming basidiomycete and this approach may also improve the genetic accessibility of non-model species.

Exploring molecular tools for transformation and gene expression in the cultivated edible mushroom Agrocybe aegerita

Agrocybe aegerita is a cultivated edible mushroom in numerous countries, which also serves as a model basidiomycete to study fruiting body formation. Aiming to create an easily expandable customised molecular toolset for transformation and constitutive gene of interest expression, we first created a homologous dominant marker for transformant selection. Progeny monokaryons of the genome-sequenced dikaryon A. aegerita AAE-3 used here were identified as sensitive to the systemic fungicide carboxin. We cloned the wild-type gene encoding the iron-sulphur protein subunit of succinate dehydrogenase AaeSdi1 including its up- and downstream regions, and introduced a single-point mutation (His237 to Leu) to make it confer carboxin resistance. PEG-mediated transformation of protoplasts derived from either oidia or vegetative monokaryotic mycelium with the resulting carboxin resistance marker (Cbx^R) plasmid pSDI1E3 yielded carboxin-resistant transformants in both cases. Plasmid DNA linearised within the selection marker resulted in transformants with ectopic multiple insertions of plasmid DNA in a head-to-tail repeat-like fashion. When circular plasmid was used, ectopic single integration into the fungal genome was favoured, but also gene conversion at the homologous locus was seen in 1 out of 11 analysed transformants. Employing Cbx^R as selection marker, two versions of a reporter gene construct were assembled via Golden Gate cloning which allows easy recombination of its modules. These consisted of an eGFP expression cassette controlled by the native promoter P_AaeGPDII and the heterologous terminator T_nos, once with and once without an intron in front of the eGFP start codon. After protoplast transformation with either construct as circular plasmid DNA, GFP fluorescence was detected with either transformants, indicating that expression of eGFP is intron-independent in A. aegerita. This paves the way for functional genetics approaches to A. aegerita, e.g., via constitutive expression of fruiting-related genes.

Production of α-1,3-L-arabinofuranosidase active on substituted xylan does not improve compost degradation by Agaricus bisporus

Agaricus bisporus consumes carbohydrates contained in wheat straw based compost used for commercial mushroom production. Double substituted arabinoxylan is part of the ~40% of the compost polysaccharides that are not degraded by A. bisporus during its growth and development. Genes encoding α-1,3-l-arabinofuranosidase (AXHd3) enzymes that act on xylosyl residues doubly substituted with arabinosyl residues are absent in this mushroom forming fungus. Here, the AXHd3 encoding hgh43 gene of Humicola insolens was expressed in A. bisporus with the aim to improve its substrate utilization and mushroom yield. Transformants secreted active AXHd3 in compost as shown by the degradation of double substituted arabinoxylan oligomers in an in vitro assay. However, carbohydrate composition and degree of arabinosyl substitution of arabinoxylans were not affected in compost possibly due to inaccessibility of the doubly substituted xylosyl residues.

H2O2 as a candidate bottleneck for MnP activity during cultivation of Agaricus bisporus in compost

Degradation of lignin by fungi enhances availability of cellulose and hemicellulose in plant waste and thereby increases the amount of carbon source available to these microorganisms. The button mushroom Agaricus bisporus degrades only about half of the lignin in compost and about 40% of the carbohydrates remain unutilized during mushroom cultivation. Here it was assessed whether over-expression of the manganese peroxidase gene mnp1 improves lignin degradation and, as a consequence, carbohydrate breakdown by A. bisporus. Transformants expressing mnp1 under the control of actin regulatory sequences produced MnP activity in malt extract medium, while the parental strain A15 did not. MnP activity was increased 0.3- and 3-fold at casing and after the 2nd flush of a semi-commercial cultivation, respectively, when compared to strain A15. Pyrolysis-GC-MS showed that overexpression of MnP decreased phenylmethane and phenylethane type lignin relative to the phenylpropane type after the 2nd flush. However, it neither affected the syringyl/guaiacyl derived residue ratio nor the ratio of oxidized to non-oxidized lignin residues. Moreover, the carbohydrate content and accessibility was not affected in compost. Notably, the capacity of compost extract to consume the MnP co-factor H₂O₂ was 4- to 8-fold higher than its production. This may well explain why over-expression of mnp1 did not improve carbohydrate degradation in compost. In fact, availability of H₂O₂ may limit lignin degradation by wild-type A. bisporus.

Microbial biomass in compost during colonization of Agaricus bisporus

Agaricus bisporus mushrooms are commercially produced on a microbe rich compost. Here, fungal and bacterial biomass was quantified in compost with and without colonization by A. bisporus. Chitin content, indicative of total fungal biomass, increased during a 26-day period from 576 to 779 nmol N-acetylglucosamine g^-1 compost in the absence of A. bisporus (negative control). A similar increase was found in the presence of this mushroom forming fungus. The fungal phospholipid-derived fatty acid (PLFA) marker C18:2ω6, indicative of the living fraction of the fungal biomass, decreased from 575 to 280 nmol g^-1 compost in the negative control. In contrast, it increased to 1200 nmol g^-1 compost in the presence of A. bisporus. Laccase activity was absent throughout culturing in the negative control, while it correlated with the fungal PLFA marker in the presence of A. bisporus. PLFA was also used to quantify living bacterial biomass. In the negative control, the bacterial markers remained constant at 3000-3200 nmol PLFA g^-1 compost. In contrast, they decreased to 850 nmol g^-1 compost during vegetative growth of A. bisporus, implying that bacterial biomass decreased from 17.7 to 4.7 mg g^-1 compost. The relative amount of the Gram positive associated PLFA markers a15:0 and a17:0 and the Gram negative PLFA associated markers cy17:0 and cy19:0 increased and decreased, respectively, suggesting that Gram negative bacteria are more suppressed by A. bisporus. Together, these data indicate that fungal biomass can make up 6.8% of the compost after A. bisporus colonization, 57% of which being dead. Moreover, results show that A. bisporus impacts biomass and composition of bacteria in compost.

Transcription factors of Schizophyllum commune involved in mushroom formation and modulation of vegetative growth

Mushrooms are the most conspicuous fungal structures. Transcription factors (TFs) Bri1 and Hom1 of the model fungus Schizophyllum commune are involved in late stages of mushroom development, while Wc-2, Hom2, and Fst4 function early in development. Here, it is shown that Bri1 and Hom1 also stimulate vegetative growth, while biomass formation is repressed by Wc-2, Hom2, and Fst4. The Δbri1Δbri1 and the Δhom1Δhom1 strains formed up to 0.6 fold less biomass when compared to wild-type, while Δwc-2Δwc-2, Δhom2Δhom2, and Δfst4Δfst4 strains formed up to 2.8 fold more biomass. Inactivation of TF gene tea1, which was downregulated in the Δwc-2Δwc-2, Δhom2Δhom2, and Δfst4Δfst4 strains, resulted in a strain that was severely affected in mushroom development and that produced 1.3 fold more biomass than the wild-type. In contrast, introducing a constitutive active version of hom2 that had 4 predicted phosphorylation motifs eliminated resulted in radial growth inhibition and prompt fructification in both Δhom2 and wild-type strains, even in sterile monokaryons. Together, it is concluded that TFs involved in mushroom formation also modulate vegetative growth. Among these TFs is the homeodomain protein Hom2, being the first time that this class of regulatory proteins is implicated in repression of vegetative growth in a eukaryote.

Hydrophilins in the filamentous fungus Neosartorya fischeri (Aspergillus fischeri) have protective activity against several types of microbial water stress

Hydrophilins are proteins that occur in all domains of life and protect cells and organisms against drought and other stresses. They include most of the late embryogenesis abundant (LEA) proteins and the heat shock protein (HSP) Hsp12. Here, the role of a predicted LEA-like protein (LeamA) and two Hsp12 proteins (Hsp12A and Hsp12B) of Neosartorya fischeri was studied. This filamentous fungus forms ascospores that belong to the most stress-resistant eukaryotic cells described to date. Heterologous expression of LeamA, Hsp12A and Hsp12B resulted in increased tolerance against salt and osmotic stress in Escherichia coli. These proteins were also shown to protect lactate dehydrogenase against dry heat and freeze-thaw cycles in vitro. Deletion of leamA caused diminished viability of sexual ascospores after drought and heat. This is the first report on functionality of Hsp12 and putative LeamA proteins derived from filamentous fungi, and their possible role in N. fischeri ascospore resistance against desiccation, high temperature and osmotic stress is discussed.

Absence of induced resistance in Agaricus bisporus against Lecanicillium fungicola

Lecanicillium fungicola causes dry bubble disease and is an important problem in the cultivation of Agaricus bisporus. Little is known about the defense of mushrooms against pathogens in general and L. fungicola in particular. In plants and animals, a first attack by a pathogen often induces a systemic response that results in an acquired resistance to subsequent attacks by the same pathogen. The development of functionally similar responses in these two eukaryotic kingdoms indicates that they are important to all multi-cellular organisms. We investigated if such responses also occur in the interaction between the white button mushroom and L. fungicola. A first infection of mushrooms of the commercial A. bisporus strain Sylvan A15 by L. fungicola did not induce systemic resistance against a subsequent infection. Similar results were obtained with the A. bisporus strain MES01497, which was demonstrated to be more resistant to dry bubble disease. Apparently, fruiting bodies of A. bisporus do not express induced resistance against L. fungicola.

The blue light receptor complex WC-1/2 of Schizophyllum commune is involved in mushroom formation and protection against phototoxicity

Blue light is necessary for initiation of mushroom formation in Schizophyllum commune. The genome of this basidiomycete contains homologues of the blue light receptor genes wc-1 and wc-2 of Neurospora crassa. Here, it is shown that inactivation of either or both of these genes in S. commune results in a blind phenotype. Mushroom formation was abolished in dikaryons and they formed symmetrical instead of asymmetrical colonies. Development was restored in a temperature dependent way in a Δwc-2Δwc-2 strain by introducing a construct encompassing the wc-2 gene under control of the promoter of the heat shock gene hsp3. A genome-wide expression analysis showed that the transcription factor genes c2h2 and hom1 as well as many hydrophobin genes are downregulated in light-grown colonies of the Δwc-2Δwc-2 mutant when compared with the wild-type dikaryon. Inactivation of wc-1 and/or wc-2 also resulted in sensitivity of the mycelium to intense light. Monokaryotic mutant strains only survived exposure to 6500 lux of light by growing into the agar. Expression analysis indicates that the photosensitivity of the Δwc-1 and Δwc-2 strains is due to lower levels of photolyase and ferrochelatase.

Germination of Lecanicillium fungicola in the mycosphere of Agaricus bisporus

Dry bubble disease is a major problem in the commercial cultivation of the white button mushroom Agaricus bisporus and is caused by the ascomycete Lecanicillium fungicola. In the casing layer, germination of L. fungicola spores is inhibited by the microflora, a phenomenon known as fungistasis. Fungistasis is annulled when the casing is colonized by A. bisporus hyphae. We demonstrated that addition of A. bisporus-associated sugars, similarly annulled the casing fungistasis. However, casing fungistasis does not seem to be based on competition for resources as L. fungicola spores germinate regardless of nutrient availability. Pseudomonas bacteria are a dominant group of bacteria in the casing and have previously been implied to be essential for the development of fungistasis in soils. Antibiotics produced by model strain Pseudomonas fluorescens CHA0 inhibited L. fungicola spore germination. Addition of glucose desensitized spores of L. fungicola, which resulted in germination in the presence of antibiotics. We conclude that antibiotics produced by the microflora most likely cause fungistasis.

Transcription factor genes of Schizophyllum commune involved in regulation of mushroom formation

Mushrooms represent the most conspicuous structures of fungi. Their development is being studied in the model basidiomycete Schizophyllum commune. The genome of S. commune contains 472 genes encoding predicted transcription factors. Of these, fst3 and fst4 were shown to inhibit and induce mushroom development respectively. Here, we inactivated five additional transcription factor genes. This resulted in absence of mushroom development (in the case of deletion of bri1 and hom2), in arrested development at the stage of aggregate formation (in the case of c2h2) and in the formation of more but smaller mushrooms (in the case of hom1 and gat1). Moreover, strains in which hom2 and bri1 were inactivated formed symmetrical colonies instead of irregular colonies like the wild type. A genome-wide expression analysis identified several gene classes that were differentially expressed in the strains in which either hom2 or fst4 was inactivated. Among the genes that were downregulated in these strains were c2h2 and hom1. Based on these results, a regulatory model of mushroom development in S. commune is proposed. This model most likely also applies to other mushroom-forming fungi and will serve as a basis to understand mushroom formation in nature and to enable and improve commercial mushroom production.

Lecanicillium fungicola: causal agent of dry bubble disease in white-button mushroom

Lecanicillium fungicola causes dry bubble disease in commercially cultivated mushroom. This review summarizes current knowledge on the biology of the pathogen and the interaction between the pathogen and its most important host, the white-button mushroom, Agaricus bisporus. The ecology of the pathogen is discussed with emphasis on host range, dispersal and primary source of infection. In addition, current knowledge on mushroom defence mechanisms is reviewed.

TAXONOMY

Lecanicillium fungicola (Preuss) Zare and Gams: Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Sordariomycetes; Subclass Hypocreales; Order Hypocreomycetidae; Family Cordycipitaceae; genus Lecanicillium.

HOST RANGE

Agaricus bisporus, Agaricus bitorquis and Pleurotus ostreatus. Although its pathogenicity for other species has not been established, it has been isolated from numerous other basidiomycetes.

DISEASE SYMPTOMS

Disease symptoms vary from small necrotic lesions on the caps of the fruiting bodies to partially deformed fruiting bodies, called stipe blow-out, or totally deformed and undifferentiated masses of mushroom tissue, called dry bubble. The disease symptoms and severity depend on the time point of infection. Small necrotic lesions result from late infections on the fruiting bodies, whereas stipe blow-out and dry bubble are the result of interactions between the pathogen and the host in the casing layer.

ECONOMIC IMPORTANCE

Lecanicillium fungicola is a devastating pathogen in the mushroom industry and causes significant losses in the commercial production of its main host, Agaricus bisporus. Annual costs for mushroom growers are estimated at 2-4% of total revenue. Reports on the disease originate mainly from North America and Europe. Although China is the main producer of white-button mushrooms in the world, little is known in the international literature about the impact of dry bubble disease in this region.

CONTROL

The control of L. fungicola relies on strict hygiene and the use of fungicides. Few chemicals can be used for the control of dry bubble because the host is also sensitive to fungicides. Notably, the development of resistance of L. fungicola has been reported against the fungicides that are used to control dry bubble disease. In addition, some of these fungicides may be banned in the near future.

USEFUL WEBSITES

http://www.mycobank.org; http://www.isms.biz; http://www.cbs.knaw.nl.

An efficient gene deletion procedure for the mushroom-forming basidiomycete Schizophyllum commune

Gene deletion in Schizophyllum commune is hampered by a low incidence of homologous integration. As a consequence, extensive screening is required to identify a transformant with the desired genotype. To alleviate this and to facilitate the assembly of deletion plasmids, vector pDelcas was constructed. This construct has a set of restriction sites, which allows for directional cloning of the flanking sequences at both sides of a nourseothricin resistance cassette. Moreover, it contains a phleomycin resistance cassette elsewhere in the plasmid, which is used to screen for transformants with an ectopic integration of the pDelcas derivative. The use of pDelcas derivatives in combination with an improved PCR screening protocol permitted the efficient identification of S. commune deletion strains. This procedure may also function in other basidiomycetes.

The septal pore cap is an organelle that functions in vegetative growth and mushroom formation of the wood-rot fungus Schizophyllum commune

Mushroom-forming basidiomycetes colonize large areas in nature. Their hyphae are compartmentalized by perforated septa, which are usually covered by a septal pore cap (SPC). Here, we describe, for the first time, the composition and function of SPCs using the model system Schizophyllum commune. The SPC of S. commune was shown to consist of a proteinaceous matrix covered by a lipid membrane. The matrix was demonstrated to define the ultrastructure of the SPC and to consist of two main proteins, Spc14 and Spc33. Gene spc14 encodes a protein of 86 amino acids, which lacks known domain, signal or localization sequences. Gene spc33 encodes a 239 and a 340 amino acid variant. Both forms contain a predicted signal anchor that targets them to the ER. Immuno-localization showed the presence of Spc33 in the SPC but not in ER. From this and previous reports it is concluded that the SPC is derived from this organelle. Inactivation of spc33 resulted in loss of SPCs and the inability to close septa. The latter may well explain why vegetative growth and mushroom formation were severely reduced in strains in which spc33 was inactivated.

RNA-mediated gene silencing in monokaryons and dikaryons of Schizophyllum commune

Disruption of genes by homologous recombination occurs at a low frequency in the basidiomycete Schizophyllum commune. For instance, the SC3 and SC15 genes were inactivated at frequencies of 1 and 5%, respectively. As an alternative to disruption, we used gene silencing through the introduction of a hairpin construct. The SC15 gene, which encodes an abundantly secreted structural protein, was silenced at a frequency of 80% in monokaryons of S. commune after introduction of a hairpin construct of the gene. Silencing also occurred in dikaryons in which one of the partners was not a silenced strain. The silencing mechanism resembles RNAi in other filamentous fungi and is a powerful tool for the functional analysis of genes expressed in monokaryons or dikaryons.

In situ hybridisation in filamentous fungi using peptide nucleic acid probes

Fluorescent DNA and peptide nucleic acid (PNA) probes were used for in situ hybridisations in colonies of Schizophyllum commune and Aspergillus niger. DNA probes for 18S rRNA did not diffuse through the cell wall after mild chemical fixation. After permeabilising the cell wall with lysing enzymes or slow freezing and embedding, hybridisation was still poor and not reproducible. In contrast, PNA probes did diffuse through the cell wall after mild chemical fixation and reproducible fluorescent signals were obtained. The rRNA signal was most intense in the apical compartment of hyphae of S. commune. Within this compartment, the signal was lower at the extreme apex. Apparently, ribosomes are unevenly distributed in hyphae. In S. commune, the mRNA of the SC3 gene was also detected with a PNA probe. The ratio between 18S rRNA and SC3 mRNA signals were variable between hyphae and their compartments. This is the first report of using PNA probes for in situ hybridisation of mRNA in fungi. The method provides a powerful tool to study gene expression.

The SC15 protein of Schizophyllum commune mediates formation of aerial hyphae and attachment in the absence of the SC3 hydrophobin

Disruption of the SC3 gene in the basidiomycete Schizophyllum commune affected not only formation of aerial hyphae but also attachment to hydrophobic surfaces. However, these processes were not completely abolished, indicating involvement of other molecules. We here show that the SC15 protein mediates formation of aerial hyphae and attachment in the absence of SC3. SC15 is a secreted protein of 191 aa with a hydrophilic N-terminal half and a highly hydrophobic C-terminal half. It is not a hydrophobin as it lacks the eight conserved cysteine residues found in these proteins. Besides being secreted into the medium, SC15 was localized in the cell wall and the mucilage that binds aerial hyphae together. In a strain in which the SC15 gene was deleted (DeltaSC15) formation of aerial hyphae and attachment were not affected. However, these processes were almost completely abolished when the SC15 gene was deleted in the DeltaSC3 background. The absence of aerial hyphae in the DeltaSC3DeltaSC15 strain can be explained by the inability of the strain to lower the water surface tension and to make aerial hyphae hydrophobic.

Introns are necessary for mRNA accumulation in Schizophyllum commune

The cDNA coding sequence of the Agaricus bisporus hydrophobin gene ABH1 under the regulation sequences of the Schizophyllum commune SC3 hydrophobin gene gave no expression in S. commune. In contrast, the genomic coding sequence (containing three introns) produced high levels of ABH1 mRNA when transformed to S. commune in the same configuration. Apparently, introns were needed for the accumulation of mRNAs from the ABH1 gene. When the effect of intron deletion on expression of the homologous genes SC3 and SC6 was examined, it was observed that only the genomic coding sequences were expressed in S. commune. Run-on analysis with nuclei harbouring intron-containing and intronless SC6 showed that this effect did not occur at the level of transcription initiation: genomic and cDNA sequences were equally active in this respect. When a 50 bp artificial intron containing the consensus splice and branch sites of S. commune introns, in addition to random-generated sequences, was introduced in the right orientation into the intronless SC3 transcriptional unit, accumulation of SC3 mRNA was restored. By polymerase chain reaction amplification, no unspliced SC3 mRNA species could be detected. Furthermore, the addition of an intron into the transcriptional unit of the gene for green fluorescent protein (GFP) effected clear fluorescence of the transgenic hyphae. Apparently, splicing is required for the normal processing of primary transcripts in S. commune.

How a fungus escapes the water to grow into the air

Fungi are well known to the casual observer for producing water-repelling aerial moulds and elaborate fruiting bodies such as mushrooms and polypores. Filamentous fungi colonize moist substrates (such as wood) and have to breach the water-air interface to grow into the air. Animals and plants breach this interface by mechanical force. Here, we show that a filamentous fungus such as Schizophyllum commune first has to reduce the water surface tension before its hyphae can escape the aqueous phase to form aerial structures such as aerial hyphae or fruiting bodies. The large drop in surface tension (from 72 to 24 mJ m-2) results from self-assembly of a secreted hydrophobin (SC3) into a stable amphipathic protein film at the water-air interface. Other, but not all, surface-active molecules (that is, other class I hydrophobins and streptofactin from Streptomyces tendae) can substitute for SC3 in the medium. This demonstrates that hydrophobins not only have a function at the hyphal surface but also at the medium-air interface, which explains why fungi secrete large amounts of hydrophobin into their aqueous surroundings.

A hydrophobin (ABH3) specifically secreted by vegetatively growing hyphae of Agaricus bisporus (common white button mushroom)

Aerial mycelium and hyphal strands of Agaricus bisporus, strain U1, exhibited a rodlet pattern at their surfaces characteristic for assembled class I hydrophobins. An SDS-insoluble/trifluoroacetic-acid-soluble fraction from strands was found to contain one abundant protein with an apparent molecular mass on gel of 19 kDa. Two sequences for this protein (ABH3), typical of class I hydrophobins, could be deduced by sequencing cDNA clones obtained by RT-PCR. The two forms of the protein could be assigned to different alleles present in the two homokaryons that constitute the heterokaryotic U1 strain. ABH3 displays all the in vitro properties of a typical class I hydrophobin such as SC3 from Schizophyllum commune but is not glycosylated or otherwise post-translationally modified because the molecular mass values deduced from the amino acid sequence (9228 and 9271 Da) and derived from mass spectrometry were in good agreement. The ABH3 transcript was found to be present in the vegetative mycelium of both primary and secondary mycelium but not in the fruiting bodies, whereas the reverse was found for the ABH1 hydrophobin. Using an S. commune mutant with a disrupted SC3 gene it was found that ABH3 can substitute for SC3 in inducing formation of aerial hyphae, suggesting a role of ABH3 in the emergence of aerial hyphae and strands in A. bisporus.

An abundant hydrophobin (ABH1) forms hydrophobic rodlet layers in Agaricus bisporus fruiting bodies

The SDS-insoluble protein fraction of Agaricus bisporus fruiting bodies was solubilized with trifluoroacetic acid. On SDS-PAGE this fraction was found to contain one abundant protein with an apparent M(r) of 16 kDa. The N-terminal amino acid sequence of this protein was determined and RT-PCR used to isolate a cDNA clone which upon sequencing identified the protein as a typical class I hydrophobin (ABH1). The gene (ABH1) was isolated and sequenced, and a second hydrophobin gene (ABH2) was found about 2.5 kbp downstream of ABH1. Purified ABH1 self-assembled at hydrophobic-hydrophilic interfaces, producing the typical rodlet layer known from other hydrophobins. Similar rodlets were observed on the surface of the fruiting body, while immunological localization showed the hydrophobin to be particularly abundant at the outer surface of fruiting bodies, in the veil and in the core tissue of the stipe. Transcripts of ABH1 were found only in fruiting-body hyphae. The ABH1 hydrophobin is probably solely responsible for the hydrophobicity of the fruiting-body surface but may also line air channels within fruiting bodies.