Efficient transformation of filamentous fungus Pleurotus ostreatus using single-strand carrier DNA

Chaga mushroom: a super-fungus with countless facets and untapped potential

Inonotus obliquus (Chaga mushroom) is an inexpensive fungus with a broad range of traditional and medicinal applications. These applications include therapy for breast, cervix, and skin cancers, as well as treating diabetes. However, its benefits are virtually untapped due to a limited understanding of its mycochemical composition and bioactivities. In this article, we explore the ethnobotany, mycochemistry, pharmacology, traditional therapeutic, cosmetic, and prospective agricultural uses. The review establishes that several secondary metabolites, such as steroids, terpenoids, and other compounds exist in chaga. Findings on its bioactivity have demonstrated its ability as an antioxidant, anti-inflammatory, antiviral, and antitumor agent. The study also demonstrates that Chaga powder has a long history of traditional use for medicinal purposes, pipe smoking rituals, and mystical future forecasts. The study further reveals that the applications of Chaga powder can be extended to industries such as pharmaceuticals, food, cosmetics, and agriculture. However numerous publications focused on the pharmaceutical benefits of Chaga with few publications on other applications. Overall, chaga is a promising natural resource with a wide range of potential applications and therefore the diverse array of therapeutic compounds makes it an attractive candidate for various applications such as plant biofertilizers and active ingredients in cosmetics and pharmaceutical products. Thus, further exploration of Chaga's potential benefits in agriculture and other industries could lead to exciting new developments and innovations.

A carnivorous mushroom paralyzes and kills nematodes via a volatile ketone

The carnivorous mushroom Pleurotus ostreatus uses an unknown toxin to rapidly paralyze and kill nematode prey upon contact. We report that small lollipop-shaped structures (toxocysts) on fungal hyphae are nematicidal and that a volatile ketone, 3-octanone, is detected in these fragile toxocysts. Treatment of Caenorhabditis elegans with 3-octanone recapitulates the rapid paralysis, calcium influx, and neuronal cell death arising from fungal contact. Moreover, 3-octanone disrupts cell membrane integrity, resulting in extracellular calcium influx into cytosol and mitochondria, propagating cell death throughout the entire organism. Last, we demonstrate that structurally related compounds are also biotoxic to C. elegans, with the length of the ketone carbon chain being crucial. Our work reveals that the oyster mushroom has evolved a specialized structure containing a volatile ketone to disrupt the cell membrane integrity of its prey, leading to rapid cell and organismal death in nematodes.

Establishment of an Efficient Polyethylene Glycol (PEG)-Mediated Transformation System in Pleurotus eryngii var. ferulae Using Comprehensive Optimization and Multiple Endogenous Promoters

Pleurotus eryngii var. ferulae, a fungus of the genus Pleurotus, efficiently degrades lignin, especially during co-cultivation with other fungi. However, low transformation efficiency and heterologous gene expression restrict systematic studies of the molecular mechanisms and metabolic control of natural products in this mushroom. In this study, the homologous resistance marker carboxin (cbx) was used to establish a polyethylene glycol-mediated transformation (PMT) system in P. eryngii var. ferulae. Optimization of the transformation process greatly improved the number of positive transformants. In particular, we optimized: (i) protoplast preparation and regeneration; (ii) screening methods; and (iii) transformation-promoting factors. The optimized transformation efficiency reached 72.7 CFU/μg, which is higher than the average level of Pleurotus sp. (10–40 CFU/μg). Moreover, three endogenous promoters (Ppfgpd1, Ppfgpd2, and Ppfsar1) were screened and evaluated for different transcription initiation characteristics. A controllable overexpression system was established using these three promoters that satisfied various heterologous gene expression requirements, such as strong or weak, varied, or stable expression levels. This study lays the foundation for recombinant protein expression in P. eryngii var. ferulae and provides a method to investigate the underlying molecular mechanisms and secondary metabolic pathway modifications.

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.

Optimization of Production Conditions for Protoplasts and Polyethylene Glycol-Mediated Transformation of Gaeumannomyces tritici

Take-all, caused by Gaeumannomyces tritici, is one of the most important wheat root diseases worldwide, as it results in serious yield losses. In this study, G. tritici was transformed to express the hygromycin B phosphotransferase using a combined protoplast and polyethylene glycol (PEG)-mediated transformation technique. Based on a series of single-factor experimental results, three major factors-temperature, enzyme lysis time, and concentration of the lysing enzyme-were selected as the independent variables, which were optimized using the response surface methodology. A higher protoplast yield of 9.83 × 10⁷ protoplasts/mL was observed, and the protoplast vitality was also high, reaching 96.27% after optimization. Protoplasts were isolated under the optimal conditions, with the highest transformation frequency (46⁻54 transformants/μg DNA). Polymerase chain reaction and Southern blotting detection indicated that the genes of hygromycin phosphotransferase were successfully inserted into the genome of G. tritici. An optimised PEG-mediated protoplast transformation system for G. tritici was established. The techniques and procedures described will lay the foundation for establishing a good mutation library of G. tritici and could be used to transform other fungi.

Effects of cre1 modification in the white-rot fungus Pleurotus ostreatus PC9: altering substrate preference during biological pretreatment

BACKGROUND

During the process of bioethanol production, cellulose is hydrolyzed into its monomeric soluble units. For efficient hydrolysis, a chemical and/or mechanical pretreatment step is required. Such pretreatment is designed to increase enzymatic digestibility of the cellulose chains inter alia by de-crystallization of the cellulose chains and by removing barriers, such as lignin from the plant cell wall. Biological pretreatment, in which lignin is decomposed or modified by white-rot fungi, has also been considered. One disadvantage in biological pretreatment, however, is the consumption of the cellulose by the fungus. Thus, fungal species that attack lignin with only minimal cellulose loss are advantageous. The secretomes of white-rot fungi contain carbohydrate-active enzymes (CAZymes) including lignin-modifying enzymes. Thus, modification of secretome composition can alter the ratio of lignin/cellulose degradation.

RESULTS

Pleurotus ostreatus PC9 was genetically modified to either overexpress or eliminate (by gene replacement) the transcriptional regulator CRE1, known to act as a repressor in the process of carbon catabolite repression. The cre1-overexpressing transformant demonstrated lower secreted cellulolytic activity and slightly increased selectivity (based on the chemical composition of pretreated wheat straw), whereas the knockout transformant demonstrated increased cellulolytic activity and significantly reduced residual cellulose, thereby displaying lower selectivity. Pretreatment of wheat straw using the wild-type PC9 resulted in 2.8-fold higher yields of soluble sugar compared to untreated wheat straw. The overexpression transformant showed similar yields (2.6-fold), but the knockout transformant exhibited lower yields (1.2-fold) of soluble sugar. Based on proteomic secretome analysis, production of numerous CAZymes was affected by modification of the expression level of cre1.

CONCLUSIONS

The gene cre1 functions as a regulator for expression of fungal CAZymes active against plant cell wall lignocelluloses, hence altering the substrate preference of the fungi tested. While the cre1 knockout resulted in a less efficient biological pretreatment, i.e., less saccharification of the treated biomass, the converse manipulation of cre1 (overexpression) failed to improve efficiency. Despite the inverse nature of the two genetic alterations, the expected "mirror image" (i.e., opposite regulatory response) was not observed, indicating that the secretion level of CAZymes, was not exclusively dependent on CRE1 activity.

Quantification of M13 and T7 bacteriophages by TaqMan and SYBR green qPCR

TaqMan and SYBR Green quantitative PCR (qPCR) methods were developed as DNA-based approaches to reproducibly enumerate M13 and T7 phages from phage display selection experiments individually and simultaneously. The genome copies of M13 and T7 phages were quantified by TaqMan or SYBR Green qPCR referenced against M13 and T7 DNA standard curves of known concentrations. TaqMan qPCR was capable of quantifying M13 and T7 phage DNA simultaneously with a detection range of 2.75*10¹-2.75*10⁸genome copies(gc)/μL and 2.66*10¹-2.66*10⁸ genome copies(gc)/μL respectively. TaqMan qPCR demonstrated an efficient amplification efficiency (E_s) of 0.97 and 0.90 for M13 and T7 phage DNA, respectively. SYBR Green qPCR was ten-fold more sensitive than TaqMan qPCR, able to quantify 2.75-2.75*10⁷gc/μL and 2.66*10¹-2.66*10⁷gc/μL of M13 and T7 phage DNA, with an amplification efficiency E_s of 1.06 and 0.78, respectively. Due to its superior sensitivity, SYBR Green qPCR was used to enumerate M13 and T7 phage display clones selected against a cell line, and quantified titers demonstrated accuracy comparable to titers from traditional double-layer plaque assay. Compared to enzyme linked immunosorbent assay, both qPCR methods exhibited increased detection sensitivity and reproducibility. These qPCR methods are reproducible, sensitive, and time-saving to determine their titers and to quantify a large number of phage samples individually or simultaneously, thus avoiding the need for time-intensive double-layer plaque assay. These findings highlight the attractiveness of qPCR for phage enumeration for applications ranging from selection to next-generation sequencing (NGS).

Current technologies and related issues for mushroom transformation

Mushroom transformation requires a series of experimental steps, including generation of host strains with a desirable selective marker, design of vector DNA, removal of host cell wall, introduction of foreign DNA across the cell membrane, and integration into host genomic DNA or maintenance of an autonomous vector DNA inside the host cell. This review introduces limitations and obstacles related to transformation technologies along with possible solutions. Current methods for cell wall removal and cell membrane permeabilization are summarized together with details of two popular technologies, Agrobacterium tumefaciens-mediated transformation and restriction enzyme-mediated integration.

A perspective on the use of Pleurotus for the development of convenient fungi-made oral subunit vaccines

This review provides an outlook of the medical applications of immunomodulatory compounds taken from Pleurotus and proposes this fungus as a convenient host for the development of innovative vaccines. Although some fungal species, such as Saccharomyces and Pichia, occupy a relevant position in the biopharmaceutical field, these systems are essentially limited to the production of conventional expensive vaccines. Formulations made with minimally processed biomass constitute the ideal approach for developing low cost vaccines, which are urgently needed by low-income populations. The use of edible fungi has not been explored for the production and delivery of low cost vaccines, despite these organisms' attractive features. These include the fact that edible biomass can be produced at low costs in a short period of time, its high biosynthetic capacity, its production of immunomodulatory compounds, and the availability of genetic transformation methods. Perspectives associated to this biotechnological application are identified and discussed.

Identification and functional analysis of pheromone and receptor genes in the B3 mating locus of Pleurotus eryngii

Pleurotus eryngii has recently become a major cultivated mushroom; it uses tetrapolar heterothallism as a part of its reproductive process. Sexual development progresses only when the A and B mating types are compatible. Such mating incompatibility occasionally limits the efficiency of breeding programs in which crossing within loci-shared strains or backcrossing strategies are employed. Therefore, understanding the mating system in edible mushroom fungi will help provide a short cut in the development of new strains. We isolated and identified pheromone and receptor genes in the B3 locus of P. eryngii and performed a functional analysis of the genes in the mating process by transformation. A genomic DNA library was constructed to map the entire mating-type locus. The B3 locus was found to contain four pheromone precursor genes and four receptor genes. Remarkably, receptor PESTE3.3.1 has just 34 amino acid residues in its C-terminal cytoplasmic region; therefore, it seems likely to be a receptor-like gene. Real-time quantitative RT-PCR (real-time qRT-PCR) revealed that most pheromone and receptor genes showed significantly higher expression in monokaryotic cells than dikaryotic cells. The pheromone genes PEphb3.1 and PEphb3.3 and the receptor gene PESTE3.3.1 were transformed into P5 (A3B4). The transformants were mated with a tester strain (A4B4), and the progeny showed clamp connections and a normal fruiting body, which indicates the proposed role of these genes in mating and fruiting processes. This result also confirms that PESTE3.3.1 is a receptor gene. In this study, we identified pheromone and receptor genes in the B3 locus of P. eryngii and found that some of those genes appear to play a role in the mating and fruiting processes. These results might help elucidate the mechanism of fruiting differentiation and improve breeding efficiency.

The white-rot fungus pleurotus ostreatus transformant overproduced intracellular cAMP and laccase

Transformation of Pleurotus ostreatus PC9 with the mutated heterotrimeric G protein alpha subunit (Gα) gene resulted in higher laccase (Lac) activity and intracellular cyclic adenosine monophosphate (cAMP) concentrations as compared to those in wild-type PC9. The transformant also exhibited higher Lac activity than the wild type when cultured in a medium containing known Lac inducers CuSO4 and ferulic acid.

Inactivation of a Pleurotus ostreatus versatile peroxidase-encoding gene (mnp2) results in reduced lignin degradation

Lignin biodegradation by white-rot fungi is pivotal to the earth's carbon cycle. Manganese peroxidases (MnPs), the most common extracellular ligninolytic peroxidases produced by white-rot fungi, are considered key in ligninolysis. Pleurotus ostreatus, the oyster mushroom, is a preferential lignin degrader occupying niches rich in lignocellulose such as decaying trees. Here, we provide direct, genetically based proof for the functional significance of MnP to P. ostreatus ligninolytic capacity under conditions mimicking its natural habitat. When grown on a natural lignocellulosic substrate of cotton stalks under solid-state culture conditions, gene and isoenzyme expression profiles of its short MnP and versatile peroxidase (VP)-encoding gene family revealed that mnp2 was predominately expressed. mnp2, encoding the versatile short MnP isoenzyme 2 was disrupted. Inactivation of mnp2 resulted in three interrelated phenotypes, relative to the wild-type strain: (i) reduction of 14% and 36% in lignin mineralization of stalks non-amended and amended with Mn(2+), respectively; (ii) marked reduction of the bioconverted lignocellulose sensitivity to subsequent bacterial hydrolyses; and (iii) decrease in fungal respiration rate. These results may serve as the basis to clarify the roles of the various types of fungal MnPs and VPs in their contribution to white-rot decay of wood and lignocellulose in various ecosystems.

Redundancy among manganese peroxidases in Pleurotus ostreatus

Manganese peroxidases (MnPs) are key players in the ligninolytic system of white rot fungi. In Pleurotus ostreatus (the oyster mushroom) these enzymes are encoded by a gene family comprising nine members, mnp1 to -9 (mnp genes). Mn(2+) amendment to P. ostreatus cultures results in enhanced degradation of recalcitrant compounds (such as the azo dye orange II) and lignin. In Mn(2+)-amended glucose-peptone medium, mnp3, mnp4, and mnp9 were the most highly expressed mnp genes. After 7 days of incubation, the time point at which the greatest capacity for orange II decolorization was observed, mnp3 expression and the presence of MnP3 in the extracellular culture fluids were predominant. To determine the significance of MnP3 for ligninolytic functionality in Mn(2+)-sufficient cultures, mnp3 was inactivated via the Δku80 strain-based P. ostreatus gene-targeting system. In Mn(2+)-sufficient medium, inactivation of mnp3 did not significantly affect expression of nontargeted MnPs or their genes, nor did it considerably diminish the fungal Mn(2+)-mediated orange II decolorization capacity, despite the significant reduction in total MnP activity. Similarly, inactivation of either mnp4 or mnp9 did not affect orange II decolorization ability. These results indicate functional redundancy within the P. ostreatus MnP gene family, enabling compensation upon deficiency of one of its members.

Release of Pleurotus ostreatus versatile-peroxidase from Mn2+ repression enhances anthropogenic and natural substrate degradation

The versatile-peroxidase (VP) encoded by mnp4 is one of the nine members of the manganese-peroxidase (MnP) gene family that constitutes part of the ligninolytic system of the white-rot basidiomycete Pleurotus ostreatus (oyster mushroom). VP enzymes exhibit dual activity on a wide range of substrates. As Mn(2+) supplement to P. ostreatus cultures results in enhanced degradation of recalcitrant compounds and lignin, we examined the effect of Mn(2+) on the expression profile of the MnP gene family. In P. ostreatus (monokaryon PC9), mnp4 was found to be the predominantly expressed mnp in Mn(2+)-deficient media, whereas strongly repressed (to approximately 1%) in Mn(2+)-supplemented media. Accordingly, in-vitro Mn(2+)-independent activity was found to be negligible. We tested whether release of mnp4 from Mn(2+) repression alters the activity of the ligninolytic system. A transformant over-expressing mnp4 (designated OEmnp4) under the control of the β-tubulin promoter was produced. Now, despite the presence of Mn(2+) in the medium, OEmnp4 produced mnp4 transcript as well as VP activity as early as 4 days after inoculation. The level of expression was constant throughout 10 days of incubation (about 0.4-fold relative to β-tubulin) and the activity was comparable to the typical activity of PC9 in Mn(2+)-deficient media. In-vivo decolorization of the azo dyes Orange II, Reactive Black 5, and Amaranth by OEmnp4 preceded that of PC9. OEmnp4 and PC9 were grown for 2 weeks under solid-state fermentation conditions on cotton stalks as a lignocellulosic substrate. [(14)C]-lignin mineralization, in-vitro dry matter digestibility, and neutral detergent fiber digestibility were found to be significantly higher (about 25%) in OEmnp4-fermented substrate, relative to PC9. We conclude that releasing Mn(2+) suppression of VP4 by over-expression of the mnp4 gene in P. ostreatus improved its ligninolytic functionality.

Predominance of a versatile-peroxidase-encoding gene, mnp4, as demonstrated by gene replacement via a gene targeting system for Pleurotus ostreatus

Pleurotus ostreatus (the oyster mushroom) and other white rot filamentous basidiomycetes are key players in the global carbon cycle. P. ostreatus is also a commercially important edible fungus with medicinal properties and is important for biotechnological and environmental applications. Efficient gene targeting via homologous recombination (HR) is a fundamental tool for facilitating comprehensive gene function studies. Since the natural HR frequency in Pleurotus transformations is low (2.3%), transformed DNA is predominantly integrated ectopically. To overcome this limitation, a general gene targeting system was developed by producing a P. ostreatus PC9 homokaryon Δku80 strain, using carboxin resistance complemented by the development of a protocol for hygromycin B resistance protoplast-based DNA transformation and homokaryon isolation. The Δku80 strain exhibited exclusive (100%) HR in the integration of transforming DNA, providing a high efficiency of gene targeting. Furthermore, the Δku80 strains produced showed a phenotype similar to that of the wild-type PC9 strain, with similar growth fitness, ligninolytic functionality, and capability of mating with the incompatible strain PC15 to produce a dikaryon which retained its resistance to the corresponding selection and was capable of producing typical fruiting bodies. The applicability of this system is demonstrated by inactivation of the versatile peroxidase (VP) encoded by mnp4. This enzyme is part of the ligninolytic system of P. ostreatus, being one of the nine members of the manganese-peroxidase (MnP) gene family, and is the predominantly expressed VP in Mn(2+)-deficient media. mnp4 inactivation provided a direct proof that mnp4 encodes a key VP responsible for the Mn(2+)-dependent and Mn(2+)-independent peroxidase activity under Mn(2+)-deficient culture conditions.

Pleurotus ostreatus manganese-dependent peroxidase silencing impairs decolourization of Orange II

Decolourization of azo dyes by Pleurotus ostreatus, a white-rot fungus capable of lignin depolymerization and mineralization, is related to the ligninolytic activity of enzymes produced by this fungus. The capacity of P. ostreatus to decolourize the azo dye Orange II (OII) was dependent and positively co-linear to Mn(2+) concentration in the medium, and thus attributed to Mn(2+)-dependent peroxidase (MnP) activity. Based on the ongoing P. ostreatus genome deciphering project we identified at least nine genes encoding for MnP gene family members (mnp 1-9), of which only four (mnp 1-4) were previously known. Relative real-time PCR quantification analysis confirmed that all the nine genes are transcribed, and that Mn(2+) amendment results in a drastic increase in the transcript levels of the predominantly expressed MnP genes (mnp 3 and mnp 9), while decreasing versatile peroxidase gene transcription (mnp 4). A reverse genetics strategy based on silencing the P. ostreatus mnp 3 gene by RNAi was implemented. Knock-down of mnp 3 resulted in the reduction of fungal OII decolourization capacity, which was co-linear with marked silencing of the Mn(2+)-dependent peroxidase genes mnp 3 and mnp 9. This is the first direct genetic proof of an association between MnP gene expression levels and azo dye decolourization capacity in P. ostreatus, which may have significant implication on understanding the mechanisms governing lignin biodegradation. Moreover, this study has proven the applicability of RNAi as a tool for gene function studies in Pleurotus research.

Agrobacterium tumefaciens-mediated genetic transformation of the phytopathogenic fungus Penicillium digitatum

Agrobacterium tumefaciens-mediated transformation (ATMT) system was assessed for conducting insertional mutagenesis in Penicillium digitatum, a major fungal pathogen infecting post-harvest citrus fruits. A transformation efficiency of up to 60 transformants per 10(6) conidia was achieved by this system. The integration of the hph gene into the fungal genome was verified by polymerase chain reaction (PCR) amplification and sequencing. These transformants tested were also shown to be mitotically stable. Southern blot analysis of 14 randomly selected transformants showed that the hph gene was randomly integrated as single copy into the fungal genome of P. digitatum. Thus, we conclude that ATMT of P. digitatum could be used as an alternatively practical genetic tool for conducting insertional mutagenesis in P. digitatum to study functional genomics.

Development of a transformation system for Crinipellis perniciosa, the causal agent of witches' broom in cocoa plants

Protoplasts of the pathogenic plant fungus, Crinipellis perniciosa, were transformed to hygromycin B resistance using the pAN7-1 plasmid, which contains the Escherichia coli hph gene under the control of Aspergillus nidulans regulatory sequences. The pAN7-1 plasmid was introduced by PEG/CaCl(2) treatment. Transformation frequencies of 1.6-2.5 transformants/microg of DNA were achieved. About 54% of the transformants were abortive and 40 analyzed transformants were mitotically stable and showed different hygromycin B resistance levels. The presence of the hph gene was checked by PCR in five transformants and the integration of multiple plasmid copies into different genome sites was observed by Southern analysis. This is the first report of a C. perniciosa transformation system and represents an important step for further research into genetic manipulation of this fungal plant pathogen.