Protoplast production and transformation of morphological mutants of the Quorn® myco-protein fungus, Fusarium graminearum A3/5, using the hygromycin B resistance plasmid pAN7–1

Stable overexpression and targeted gene deletion of the causative agent of ash dieback Hymenoscyphus fraxineus

BACKGROUND

Due to the infection with the invasive ascomycete Hymenoscyphus fraxineus, which has been replacing the closely related and non-pathogenic native Hymenoscyphus albidus, the European ashes, Fraxinus excelsior (also known as the common ash), Fraxinus angustifolia (also known as narrow-leaved ash) and Fraxinus ornus (also known as the manna ash) are at risk. Hymenoscyphus fraxineus is the causative agent of ash dieback of the European ashes, but is non-pathogenic to the native Asian ash Fraxinus mandshurica (also known as the Manchurian ash). Even though the invasion of H. fraxineus is a great threat for ashes in Europe, the fungal biology is still poorly understood. By the use of live cell imaging and targeted gene knock-out, the fungal life cycle and host-pathogen interaction can be studied in more detail.

RESULTS

Here, we developed a protocol for the preparation of protoplasts from mycelium of H. fraxineus, for their regeneration and for stable transformation with reporter genes and targeted gene knock-out by homologous recombination. We obtained mutants with various levels of reporter gene expression which did not correlate with the number of integrations. In an in vitro infection assay, we demonstrated the suitability of reporter gene overexpression for fungal detection in plant tissue after inoculation. As a proof of principle for targeted gene knock-out, the hygromycin resistance cassette of a reporter gene-expressing mutant was replaced with a geneticin resistance cassette.

CONCLUSIONS

The invasive fungal pathogen H. fraxineus is threatening the European ashes. To develop strategies for pest management, a better understanding of the fungal life cycle and its host interaction is crucial. Here, we provide a protocol for stable transformation of H. fraxineus to obtain fluorescence reporter strains and targeted gene knock-out mutants. This protocol will help future investigations on the biology of this pathogen.

Genetic Transformation of the Filamentous Fungus Pseudogymnoascus verrucosus of Antarctic Origin

Cold-adapted fungi isolated from Antarctica, in particular those belonging to the genus Pseudogymnoascus, are producers of secondary metabolites with interesting bioactive properties as well as enzymes with potential biotechnological applications. However, at genetic level, the study of these fungi has been hindered by the lack of suitable genetic tools such as transformation systems. In fungi, the availability of transformation systems is a key to address the functional analysis of genes related with the production of a particular metabolite or enzyme. To the best of our knowledge, the transformation of Pseudogymnoascus strains of Antarctic origin has not been achieved yet. In this work, we describe for the first time the successful transformation of a Pseudogymnoascus verrucosus strain of Antarctic origin, using two methodologies: the polyethylene glycol (PEG)-mediated transformation, and the electroporation of germinated conidia. We achieved transformation efficiencies of 15.87 ± 5.16 transformants per μg of DNA and 2.67 ± 1.15 transformants per μg of DNA for PEG-mediated transformation and electroporation of germinated conidia, respectively. These results indicate that PEG-mediated transformation is a very efficient method for the transformation of this Antarctic fungus. The genetic transformation of Pseudogymnoascus verrucosus described in this work represents the first example of transformation of a filamentous fungus of Antarctic origin.

Protoplast transformation as a potential platform for exploring gene function in Verticillium dahliae

BACKGROUND

Large efforts have focused on screening for genes involved in the virulence and pathogenicity of Verticillium dahliae, a destructive fungal pathogen of numerous plant species that is difficult to control once the plant is infected. Although Agrobacterium tumefaciens-mediated transformation (ATMT) has been widely used for gene screening, a quick and easy method has been needed to facilitate transformation.

RESULTS

High-quality protoplasts, with excellent regeneration efficiency (65 %) in TB3 broth (yeast extract 30 g, casamino acids 30 g and 200g sucrose in 1L H20), were generated using driselase (Sigma D-9515) and transformed with the GFP plasmid or linear GFP cassette using PEG or electroporation. PEG-mediated transformation yielded 600 transformants per microgram DNA for the linear GFP cassette and 250 for the GFP plasmid; electroporation resulted in 29 transformants per microgram DNA for the linear GFP cassette and 24 for the GFP plasmid. To determine whether short interfering RNAs (siRNAs) can be delivered to the protoplasts and used for silencing genes, we targeted the GFP gene of Vd-GFP (V. dahliae GFP strain obtained in this study) by delivering one of four different siRNAs-19-nt duplex with 2-nt 3' overhangs (siRNA-gfp1, siRNA-gfp2, siRNA-gfp3 and siRNA-gfp4)-into the Vd-GFP protoplasts using PEG-mediated transformation. Up to 100 % silencing of GFP was obtained with siRNA-gfp4; the other siRNAs were less effective (up to 10 % silencing). Verticillium transcription activator of adhesion (Vta2) gene of V. dahliae was also silenced with four siRNAs (siRNA-vta1, siRNA-vta2, siRNA-vta3 and siRNA-vta4) independently and together using the same approach; siRNA-vta1 had the highest silencing efficiency as assessed by colony diameter and quantitative real time PCR (qRT-PCR) analysis.

CONCLUSION

Our quick, easy transformation method can be used to investigate the function of genes involved in growth, virulence and pathogenicity of V. dahliae.

Geneticin (G418) resistance and electroporation-mediated transformation of Fusarium graminearum and F. culmorum

Fusarium graminearum and F. culmorum are phytopathogenic species causing scab and root rot diseases in all small grain cereals worldwide including Turkey. In this study, resistance levels to geneticin (G418) of 14 F. graminearum and 24 F. culmorum isolates collected from cereals were determined. Fungal cultures were grown on potato dextrose agar medium supplemented with 0, 25, 50, 75 and 100 µg/mL of G418. Minimum inhibitory concentration was determined as 25 µg/mL. As a result, it was concluded that all isolates were highly sensitive to G418. Plasmid pFA6-kanmx4 containing geneticin resistance gene (kanmx) was introduced singly or co-electroporated with pEGFP75 plasmid, containing GFP gene, into fungal protoplast cultures obtained with lytic enzyme. Transformants were grown in media including 25 µg/mL G418. Transformation frequencies were 2.8 and 1.8 transformant per µg plasmid for F. graminearum and F. culmorum isolates, respectively. Transformation process was also confirmed by spectrofluorimetric assay. Relative fluorescence unit values in co-transformants were calculated as 1.87 ± 0.04 for F. graminearum and 2.26 ± 0.08 for F. culmorum. The results obtained from the study gave information about antibiotic resistance levels of two Fusarium species in Turkey. Moreover, it was shown that pFA6-kanmx4 plasmid was a suitable vector, which can be used in genetic manipulation studies of these two fungal species in particular suppression of endogenous and/or the expression of exogenous genes.

Isolation of fungal homokaryotic lines from heterokaryotic transformants by sonic disruption of mycelia

Fungal hyphae—and in some cases, spores—are multi-nucleate. During genetic transformation of these spores or mycelia, only one nucleus generally receives the transferred T-DNA generating heterokaryotic colonies. Characterization of genetic changes, such as the effects of gene disruption in the transformants, requires purified homokaryotic lines. Hyphal tip transfer has conventionally been used to isolate homokaryons. We developed an alternative method for purification of fungal homokaryons from transformed heterokaryotic lines of Sclerotinia sclerotiorum. Ultrasound pulses were used to generate bi-septate, unicellular hyphal fragments that regenerate under selection to produce homokaryotic lines that can be easily identified using colony PCR. This technique facilitates the purification of transformed lines, which allows for routine genome manipulation, and should be adaptable for other filamentous fungi.

Fusarin C biosynthesis in Fusarium moniliforme and Fusarium venenatum

Fragments of polyketide synthase (PKS) genes were amplified from complementary DNA (cDNA) of the fusarin C producing filamentous fungi Fusarium moniliforme and Fusarium venenatum by using degenerate oligonucleotides designed to select for fungal PKS C-methyltransferase (CMeT) domains. The PCR products, which were highly homologous to fragments of known fungal PKS CMeT domains, were used to probe cDNA and genomic DNA (gDNA) libraries of F. moniliforme and F. venenatum. A 4.0 kb cDNA clone from F. venenatum was isolated and used to prepare a hygromycin-resistance knockout cassette, which was used to produce a fusarin-deficient strain of F. venenatum (kb = 1000 bp). Similarly, a 26 kb genomic fragment, isolated on two overlapping clones from F. moniliforme, encoded a complete iterative Type I PKS fused to an unusual nonribosomal peptide synthase module. Once again, targeted gene disruption produced a fusarin-deficient strain, thereby proving that this synthase is responsible for the first steps of fusarin biosynthesis.

Combined use of growth rate correlated and growth rate independent promoters for recombinant glucoamylase production in Fusarium venenatum

Fusarium venenatum JeRS 325, a strain which produces recombinant glucoamylase under control of a growth rate independent promoter was transformed with a plasmid carrying the Aspergillus niger glucoamylase gene under control of its own growth rate correlated promoter. Some disruption of the original recombinant genes occurred and at pH 5.8 the double transformant did not produce as much glucoamylase as JeRS 325 in batch culture. However, the double transformant still produced as much glucoamylase as JeRS 325 in fed-batch cultures, illustrating the potential for the combined use of growth rate independent and dependent promoters to improve production of recombinant proteins in fed-batch culture systems.

Protoplast preparation and regeneration from spores of the biocontrol fungus Pseudozyma flocculosa

This paper describes a specific protocol for yielding and regenerating protoplasts from spores of the recently described powdery mildew biocontrol agent Pseudozyma flocculosa. With this protocol, a large quantity of protoplasts was obtained from beta-mercaptoethanol-pretreated spores in 3-day-old cultures of P. flocculosa grown in YMPD. Enzymatic digestion was optimal with 0.5% Novozym 234 and 5% Glucanex prepared in 0.6 M KCl in 0.1 M citrate buffer. All liberated protoplasts fluoresced in the presence of fluorescein diacetate indicating that viability was nearly 100%. The regeneration rate was equally outstanding reaching 75% when 0.8 M sucrose was used as osmotic stabilizer in the regeneration medium. This protocol will find useful applications in genetic studies of this poorly characterized and understood biocontrol agent.

Sexual Recombination in Gibberella zeae

ABSTRACT We developed a method for inducing sexual outcrosses in the homothallic Ascomycete fungus Gibberella zeae (anamorph: Fusarium graminearum). Strains were marked with different nitrate nonutilizing (nit) mutations, and vegetative compatibility groups served as additional markers in some crosses. Strains with complementary nit mutations were cocultured on carrot agar plates. Ascospores from individual perithecia were plated on a minimal medium (MM) containing nitrate as the sole nitrogen source. Crosses between different nit mutants segregated in expected ratios (3:1 nit(-):nit(+)) from heterozygous perithecia. Analysis of vegetative compatibility groups of progeny of two crosses indicated two and three vegetative incompatibility (vic) genes segregating, respectively. For rapid testing of sexual recombination between nit mutants, perithecia were inverted over MM to deposit actively discharged ascospores. Development of proto-trophic wild-type colonies was taken as evidence of sexual recombination. Strains of G. zeae group 2 from Japan, Nepal, and South Africa, and from Indiana, Kansas, and Ohio in the United States were sexually interfertile. Four group 1 strains were not interfertile among themselves or with seven group 2 strains. Attempts to cross G. zeae with representatives of F. acuminatum, F. avenaceum, F. culmorum, F. crookwellense, F. oxysporum, and three mating populations of G. fujikuroi were not successful.

Extracellular proteases produced by the Quorn® myco-protein fungus Fusarium graminearum in batch and chemostat culture

Summary: Fusarium graminearum was grown in batch and continuous (chemostat) culture on a glucose-mineral salts medium in the presence and absence of casein. In the absence of casein no protease activity was detected in the culture filtrate from either batch or chemostat culture. For batch cultures grown on medium containing casein, most of the proteolytic activity detected in the supernatant during exponential growth had an optimum at ca pH 5.0. However, as the cultures passed from late exponential into stationary phase, the pH profile of the protease activity broadened until most of it was in the alkaline pH region. For glucose-limited chemostat cultures grown on media containing casein, protease activity had a narrow pH optimum with maximum activity at pH 5.0. For all concentrations of casein examined, protease activity was greater in chemostat culture than in batch culture. Extracellular proteases from batch and chemostat cultures were purified by bacitracin-Sepharose affinity chromatography. At least seven proteins were purified from batch cultures but chemostat cultures contained only a single aspartic protease with a molecular mass of 40 kDa.