Genetic transformation of ectomycorrhizal fungi mediated by Agrobacterium tumefaciens

Tools to make Stachybotrys chartarum genetically amendable: Key to unlocking cryptic biosynthetic gene clusters

The soil and indoor fungus Stachybotrys chartarum can induce respiratory disorders, collectively referred to as stachybotryotoxicosis, owing to its prolific production of diverse bioactive secondary metabolites (SMs) or mycotoxins. Although many of these toxins responsible for the harmful effects on animals and humans have been identified in the genus Stachybotrys, however a number of SMs remain elusive. Through in silico analyses, we have identified 37 polyketide synthase (PKS) genes, highlighting that the chemical profile potential of Stachybotrys is far from being fully explored. Additionally, by leveraging phylogenetic analysis of known SMs produced by non-reducing polyketide synthases (NR-PKS) in other filamentous fungi, we showed that Stachybotrys possesses a rich reservoir of untapped SMs. To unravel natural product biosynthesis in S. chartarum, genetic engineering methods are crucial. For this purpose, we have developed a reliable protocol for the genetic transformation of S. chartarum and applied it to the ScPKS14 biosynthetic gene cluster. This cluster is homologous to the already known Claviceps purpurea CpPKS8 BGC, responsible for the production of ergochromes. While no novel SMs were detected, we successfully applied genetic tools, such as the generation of deletionand overexpression strains of single cluster genes. This toolbox can now be readily employed to unravel not only this particular BGC but also other candidate BGCs present in S. chartarum, making this fungus accessible for genetic engineering.

Agrobacterium tumefaciens-Mediated Genetic Transformation of the Ect-endomycorrhizal Fungus Terfezia boudieri

Mycorrhizal desert truffles such as Terfezia boudieri, Tirmania nivea, and Terfezia claveryi, form mycorrhizal associations with plants of the Cistaceae family. These valued truffles are still collected from the wild and not cultivated under intensive farming due to the lack of basic knowledge about their biology at all levels. Recently, several genomes of desert truffles have been decoded, enabling researchers to attempt genetic manipulations to enable cultivation. To execute such manipulations, the development of molecular tools for genes transformation into truffles is needed. We developed an Agrobacterium tumefaciens-mediated genetic transformation system in T. boudieri. This system was optimized for the developmental stage of the mycelia explants, bacterial optical density, infection and co-cultivation durations, and concentrations of the selection antibiotics. The pFPL-Rh plasmid harboring hph gene conferring hygromycin resistance as a selection marker and the red fluorescent protein gene were used as visual reporters. The optimal conditions were incubation with 200 μM of acetosyringone, attaining a bacterial optical density of 0.3 OD₆₀₀; transfer time of 45 min; and co-cultivation for 3 days. This is the first report on a transformation system for T. boudieri, and the proposed protocol can be adapted for the transformation of other important desert truffles as well as ectomycorrhizal species.

Agrobacterium-Mediated Transformation of Yeast and Fungi

Two decades ago, it was discovered that the well-known plant vector Agrobacterium tumefaciens can also transform yeasts and fungi when these microorganisms are co-cultivated on a solid substrate in the presence of a phenolic inducer such as acetosyringone. It is important that the medium has a low pH (5-6) and that the temperature is kept at room temperature (20-25 °C) during co-cultivation. Nowadays, Agrobacterium-mediated transformation (AMT) is the method of choice for the transformation of many fungal species; as the method is simple, the transformation efficiencies are much higher than with other methods, and AMT leads to single-copy integration much more frequently than do other methods. Integration of T-DNA in fungi occurs by non-homologous end-joining (NHEJ), but also targeted integration of the T-DNA by homologous recombination (HR) is possible. In contrast to AMT of plants, which relies on the assistance of a number of translocated virulence (effector) proteins, none of these (VirE2, VirE3, VirD5, VirF) are necessary for AMT of yeast or fungi. This is in line with the idea that some of these proteins help to overcome plant defense. Importantly, it also showed that VirE2 is not necessary for the transport of the T-strand into the nucleus. The yeast Saccharomyces cerevisiae is a fast-growing organism with a relatively simple genome with reduced genetic redundancy. This yeast species has therefore been used to unravel basic molecular processes in eukaryotic cells as well as to elucidate the function of virulence factors of pathogenic microorganisms acting in plants or animals. Translocation of Agrobacterium virulence proteins into yeast was recently visualized in real time by confocal microscopy. In addition, the yeast 2-hybrid system, one of many tools that have been developed for use in this yeast, was used to identify plant and yeast proteins interacting with the translocated Agrobacterium virulence proteins. Dedicated mutant libraries, containing for each gene a mutant with a precise deletion, have been used to unravel the mode of action of some of the Agrobacterium virulence proteins. Yeast deletion mutant collections were also helpful in identifying host factors promoting or inhibiting AMT, including factors involved in T-DNA integration. Thus, the homologous recombination (HR) factor Rad52 was found to be essential for targeted integration of T-DNA by HR in yeast. Proteins mediating double-strand break (DSB) repair by end-joining (Ku70, Ku80, Lig4) turned out to be essential for non-homologous integration. Inactivation of any one of the genes encoding these end-joining factors in other yeasts and fungi was employed to reduce or totally eliminate non-homologous integration and promote efficient targeted integration at the homologous locus by HR. In plants, however, their inactivation did not prevent non-homologous integration, indicating that T-DNA is captured by different DNA repair pathways in plants and fungi.

Detection of Oil Palm Root Penetration by Agrobacterium-Mediated Transformed Ganoderma boninense, Expressing Green Fluorescent Protein

A highly efficient and reproducible Agrobacterium-mediated transformation protocol for Ganoderma boninense was developed to facilitate observation of the early stage infection of basal stem rot (BSR). The method was proven amenable to different explants (basidiospore, protoplast, and mycelium) of G. boninense. The transformation efficiency was highest (62%) under a treatment combination of protoplast explant and Agrobacterium strain LBA4404, with successful expression of an hyg marker gene and gus-gfp fusion gene under the control of heterologous p416 glyceraldehyde 3-phosphate dehydrogenase promoter. Optimal transformation conditions included a 1:100 Agrobacterium/explant ratio, induction of Agrobacterium virulence genes in the presence of 250 μm acetosyringone, co-cultivation at 22°C for 2 days on nitrocellulose membrane overlaid on an induction medium, and regeneration of transformants on potato glucose agar prepared with 0.6 M sucrose and 20 mM phosphate buffer. Evaluated transformants were able to infect root tissues of oil palm plantlets with needle-like microhyphae during the penetration event. The availability of this model pathogen system for BSR may lead to a better understanding of the pathogenicity factors associated with G. boninense penetration into oil palm roots.

Agrobacterium-mediated insertional mutagenesis in the mycorrhizal fungus Laccaria bicolor

Agrobacterium-mediated gene transfer (AMT) is extensively employed as a tool in fungal functional genomics and accordingly, in previous studies we used AMT on a dikaryotic strain of the ectomycorrhizal basidiomycete Laccaria bicolor. The interest in this fungus derives from its capacity to establish a symbiosis with tree roots, thereby playing a major role in nutrient cycling of forest ecosystems. The ectomycorrhizal symbiosis is a highly complex interaction involving many genes from both partners. To advance in the functional characterization of fungal genes, AMT was used on a monokaryotic L. bicolor. A collection of over 1200 transgenic strains was produced, of which 200 randomly selected strains were analyzed for their genomic T-DNA insertion patterns. By means of insertional mutagenesis, a number of transgenic strains were obtained displaying differential growth features. Moreover, mating with a compatible strain resulted in dikaryons that retained altered phenotypic features of the transgenic monokaryon. The analysis of the T-DNA integration pattern revealed mostly similar results to those reported in earlier studies, confirming the usefulness of AMT on different genetic backgrounds of L. bicolor. Taken together, our studies display the great versatility and potentiality of AMT as a tool for the genetic characterization of L. bicolor.

Model systems to unravel the molecular mechanisms of heavy metal tolerance in the ericoid mycorrhizal symbiosis

Ericoid mycorrhizal plants dominate in harsh environments where nutrient-poor, acidic soil conditions result in a higher availability of potentially toxic metals. Although metal-tolerant plant species and ecotypes are known in the Ericaceae, metal tolerance in these plants has been mainly attributed to their association with ericoid mycorrhizal fungi. The mechanisms underlying plant protection by the fungal symbiont are poorly understood, whereas some insights have been achieved regarding the molecular mechanisms of heavy metal tolerance in the fungal symbiont. This review will briefly introduce the general features of heavy metal tolerance in mycorrhizal fungi and will then focus on the use of "omics" approaches and heterologous expression in model organisms to reveal the molecular bases of fungal response to heavy metals. Functional complementation in Saccharomyces cerevisiae has allowed the identification of several ericoid mycorrhizal fungi genes (i.e., antioxidant enzymes, metal transporters, and DNA damage repair proteins) that may contribute to metal tolerance in a metal-tolerant ericoid Oidiodendron maius isolate. Although a powerful system, the use of the yeast complementation assay to study metal tolerance in mycorrhizal symbioses has limitations. Thus, O. maius has been developed as a model system to study heavy metal tolerance mechanisms in mycorrhizal fungi, thanks to its high metal tolerance, easy handling and in vitro mycorrhization, stable genetic transformation, genomics, transcriptomic and proteomic resources.

Genetic transformation of extremophilic fungi Acidea extrema and Acidothrix acidophila

Intact, growing cells of strongly acidophilic fungi Acidea extrema and Acidothrix acidophila have been successfully transformed by introduction of heterologous DNA fragment (composed of the glyceraldehyde-phosphate-dehydrogenase gene promoter from Emericella nidulans, a metallothionein-coding gene AsMt1 from Amanita strobiliformis and glyceraldehyde-phosphate-dehydrogenase gene terminator from Colletotrichum gloeosporioides) with the length of 1690 bp. The transformation procedure was based on the DNA transfer mediated by Agrobacterium tumefaciens bearing disarmed helper plasmid pMP90 and binary vector pCambia1300 with inserted DNA fragment of interest. The transformants proved to be mitotically stable, and the introduced gene was expressed at least at the level of transcription. Our work confirms that metabolic adaptations of strongly acidophilic fungi do not represent an obstacle for genetic transformation using conventional methods and can be potentially used for production of heterologous proteins. A promising role of the fast growing A. acidophila as active biomass in biotechnological processes is suggested not only by the low susceptibility of the culture grown at low pH to contaminations but also by reduced risk of accidental leaks of genetically modified microorganisms into the environment because highly specialized extremophilic fungi can poorly compete with common microflora under moderate conditions.

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.

Modulation of ethanol stress tolerance by aldehyde dehydrogenase in the mycorrhizal fungus Tricholoma vaccinum

We report the first mycorrhizal fungal aldehyde dehydrogenase gene, ald1, which was isolated from the basidiomycete Tricholoma vaccinum. The gene, encoding a protein Ald1 of 502 amino acids, is up-regulated in ectomycorrhiza. Phylogenetic analyses using 53 specific fungal aldehyde dehydrogenases from all major phyla in the kingdom of fungi including Ald1 and two partial sequences of T. vaccinum were performed to get an insight in the evolution of the aldehyde dehydrogenase family. By using competitive and real-time RT-PCR, ald1 is up-regulated in response to alcohol and aldehyde-related stress. Furthermore, heterologous expression of ald1 in Escherichia coli and subsequent in vitro enzyme activity assay demonstrated the oxidation of propionaldehyde and butyraldehyde with different kinetics using either NAD(+) or NADP(+) as cofactors. In addition, overexpression of ald1 in T. vaccinum after Agrobacterium tumefaciens-mediated transformation increased ethanol stress tolerance. These results demonstrate the ability of Ald1 to circumvent ethanol stress, a critical function in mycorrhizal habitats.

SOD1-targeted gene disruption in the ericoid mycorrhizal fungus Oidiodendron maius reduces conidiation and the capacity for mycorrhization

The genome sequences of mycorrhizal fungi will provide new opportunities for studying the biology and the evolution underlying this symbiotic lifestyle. The generation of null mutants at the wild-type loci is one of the best methods for gene-function assignment in the post-genomic era. To our knowledge, the generation of superoxide dismutase 1 (SOD1)-null mutants in the ericoid mycorrhizal fungus Oidiodendron maius is the first example of a gene-targeted disruption via homologous recombination in a mycorrhizal fungus. The disruption of OmSOD1 by Agrobacterium-mediated transformation resulted in the presence of oxidative stress markers, even in the absence of external superimposed stresses, and an increased sensitivity to reactive oxygen species (ROS)-generating substances, especially to menadione. A reduction in conidiation and in the percentage of mycorrhization of Vaccinium myrtillus roots was also observed. The latter findings establish the pivotal role of SOD1 as an important factor in the relationship between O. maius and its symbiotic partner. The lack of this ROS-scavenger may cause an imbalance in the redox homeostasis during host colonization and an alteration in the delicate dialogue between the fungus and its host plant.

Establishing molecular tools for genetic manipulation of the pleuromutilin-producing fungus Clitopilus passeckerianus

We describe efficient polyethylene glycol (PEG)-mediated and Agrobacterium-mediated transformation systems for a pharmaceutically important basidiomycete fungus, Clitopilus passeckerianus, which produces pleuromutilin, a diterpene antibiotic. Three dominant selectable marker systems based on hygromycin, phleomycin, and carboxin selection were used to study the feasibility of PEG-mediated transformation of C. passeckerianus. The PEG-mediated transformation of C. passeckerianus protoplasts was successful and generated hygromycin-resistant transformants more efficiently than either phleomycin or carboxin resistance. Agrobacterium-mediated transformation with plasmid pBGgHg containing hph gene under the control of the Agaricus bisporus gpdII promoter led to hygromycin-resistant colonies and was successful when homogenized mycelium and fruiting body gill tissue were used as starting material. Southern blot analysis of transformants revealed the apparently random integration of the transforming DNA to be predominantly multiple copies for the PEG-mediated system and a single copy for the Agrobacterium-mediated system within the genome. C. passeckerianus actin and tubulin promoters were amplified from genomic DNA and proved successful in driving green fluorescent protein and DsRed expression in C. passeckerianus, but only when constructs contained a 5' intron, demonstrating that the presence of an intron is prerequisite for efficient transgene expression. The feasibility of RNA interference-mediated gene silencing was investigated using gfp as a target gene easily scored in C. passeckerianus. Upon transformation of gfp antisense constructs into a highly fluorescent strain, transformants were recovered that exhibited either reduced or undetectable fluorescence. This was confirmed by Northern blotting showing depletion of the target mRNA levels. This demonstrated that gene silencing is a suitable tool for modulating gene expression in C. passeckerianus. The molecular tools developed in this study should facilitate studies aimed at gene isolation or characterization in this pharmaceutically important species.

Investigating dominant selection markers for Coprinopsis cinerea: a carboxin resistance system and re-evaluation of hygromycin and phleomycin resistance vectors

Dominant selectable markers are beneficial for transformation of many fungi, particularly those model species where repeated transformations may be required. A carboxin resistance allele of the Coprinopsis cinerea sdi1 gene, encoding the iron-sulphur protein subunit of succinate dehydrogenase, was developed by introducing a suitable point mutation in the histidine block responsible for binding of the associated iron ion. This modified gene was used successfully to confer carboxin resistance upon transformation of C. cinerea protoplasts. Plasmids previously used to establish hygromycin transformation systems of several basidiomycete species, such as pAN7-1 and phph004, failed to give rise to hygromycin-resistant transformants of C. cinerea, whilst pPHT1 was successful. Sequencing of these constructs showed that the hygromycin resistance gene in pAN7-1 and phph004 had been modified removing the codons encoding two lysine residues following the N-terminal methionine. Replacement of the deleted 6 bp (AAA AAG) in the truncated hph gene led to generation of hygromycin-resistant transformants indicating the importance of these two codons for expression in C. cinerea. Phleomycin-resistant (ble) transformants were also obtained, but only with the intron-containing construct pblei004, showing that an intron is necessary to obtain phleomycin-resistant C. cinerea. This contrasts with hygromycin-resistance, where introns are not required for expression, emphasising the variability in importance of these elements.

pHg/pSILBAγ vector system for efficient gene silencing in homobasidiomycetes: optimization of ihpRNA - triggering in the mycorrhizal fungus Laccaria bicolor

pSILBAγ silencing vector was constructed for efficient RNA silencing triggering in the model mycorrhizal fungus Laccaria bicolor. This cloning vector carries the Agaricus bisporus gpdII promoter, two multiple cloning sites separated by a L. bicolor nitrate reductase intron and the Aspergillus nidulans trpC terminator. pSILBAγ allows an easy oriented two‐step PCR cloning of hairpin sequences to be expressed in basidiomycetes. With one further cloning step into pHg, a pCAMBIA1300‐based binary vector carrying a hygromycin resistance cassette, the pHg/pSILBAγ plasmid is used for Agrobacterium‐mediated transformation. The pHg/pSILBAγ system results in predominantly single integrations of RNA silencing triggering T‐DNAs in the fungal genome and the integration sites of the transgenes can be resolved by plasmid rescue. pSILBAγ construct and two other pSILBA plasmid variants (pSILBA and pSILBAα) were evaluated for their capacity to silence Laccaria nitrate reductase gene. While all pSILBA variants tested resulted in up to 65–76% of transformants with reduced growth on nitrate, pSILBAγ produced the highest number (65%) of strongly affected fungal strains. The strongly silenced phenotype was shown to correlate with T‐DNA integration in transcriptionally active genomic sites. pHg/pSILBAγ was shown to produce T‐DNAs with minimum CpG methylation in transgene promoter regions which assures the maximum silencing trigger production in Laccaria. Methylation of the target endogene was only slight in RNA silencing triggered with constructs carrying an intronic spacer hairpin sequence. The silencing capacity of the pHg/pSILBAγ was further tested with Laccaria inositol‐1,4,5‐triphosphate 5‐phosphatase gene. Besides its use in silencing triggering, the herein described plasmid system can also be used for transgene expression in Laccaria. pHg/pSILBAγ silencing system is optimized for L. bicolor but it should be highly useful also for other homobasidiomycetes, group of fungi currently lacking molecular tools for RNA silencing.

The dominant Hc.Sdh (R) carboxin-resistance gene of the ectomycorrhizal fungus Hebeloma cylindrosporum as a selectable marker for transformation

In an attempt to get a marker gene suitable for genetical transformation of the ectomycorrhizal fungus Hebeloma cylindrosporum, the gene Hc.Sdh (R) that confers carboxin-resistance was isolated from a UV mutant of this fungus. It encodes a mutant allele of the Fe-S subunit of the succinate dehydrogenase gene that carries a single amino acid substitution known to confer carboxin-resistance. This gene was successfully used as the selective marker to transform, via Agrobacterium tumefaciens, monokaryotic and dikaryotic strains of H. cylindrosporum. We also successfully transformed hygromycin-resistant insertional mutants. Transformation yielded mitotically stable carboxin-resistant mycelia. This procedure produced transformants, the growth of which was not affected by 2 microg l(-1) carboxin, whereas wild-type strains were unable to grow in the presence of 0.1 microg l(-1) of this fungicide. This makes the carboxin-resistance cassette much more discriminating than the hygromycin-resistance one. PCR amplification and Southern blot hybridisation indicated that more than 90% of the tested carboxin-resistant mycelia contained the Hc.Sdh (R) cassette, usually as a single copy. The AGL-1 strain of A. tumefaciens was a much less efficient donor than LBA 1126; the former yielded ca. 0-30% transformation frequency, depending on fungal strain and resistance cassette used, whereas the latter yielded ca. 60-95%.

T-DNA insertion, plasmid rescue and integration analysis in the model mycorrhizal fungus Laccaria bicolor

Ectomycorrhiza is a mutualistic symbiosis formed between fine roots of trees and the mycelium of soil fungi. This symbiosis plays a key role in forest ecosystems for the mineral nutrition of trees and the biology of the fungal communities associated. The characterization of genes involved in developmental and metabolic processes is important to understand the complex interactions that control the ectomycorrhizal symbiosis. Agrobacterium-mediated gene transfer (AMT) in fungi is currently opening a new era for fungal research. As whole genome sequences of several fungi are being released studies about T-DNA integration patterns are needed in order to understand the integration mechanisms involved and to evaluate the AMT as an insertional mutagenesis tool for different fungal species. The first genome sequence of a mycorrhizal fungus, the basidiomycete Laccaria bicolor, became public in July 2006. Release of Laccaria genome sequence and the availability of AMT makes this fungus an excellent model for functional genomic studies in ectomycorrhizal research. No data on the integration pattern in Laccaria genome were available, thus we optimized a plasmid rescue approach for this fungus. To this end the transformation vector (pHg/pBSk) was constructed allowing the rescue of the T-DNA right border (RB)-genomic DNA junctions in Escherichia coli. Fifty-one Agrobacterium-transformed fungal strains, picked up at random from a larger collection of T-DNA tagged strains (about 500), were analysed. Sixty-nine per cent were successfully rescued for the RB of which 87% were resolved for genomic integration sequences. Our results demonstrate that the plasmid rescue approach can be used for resolving T-DNA integration sites in Laccaria. The RB was well conserved during transformation of this fungus and the integration analysis showed no clear sequence homology between different genomic sites. Neither obvious sequence similarities were found between these sites and the T-DNA borders indicating non-homologous integration of the transgenes. Majority (75%) of the integrations were located in predicted genes. Agrobacterium-mediated gene transfer is a powerful tool that can be used for functional gene studies in Laccaria and will be helpful along with plasmid rescue in searching for relevant fungal genes involved in the symbiotic process.

Molecular and functional characterization of a Rho GDP dissociation inhibitor in the filamentous fungus Tuber borchii

BACKGROUND

Small GTPases of the Rho family function as tightly regulated molecular switches that govern important cellular functions in eukaryotes. Several families of regulatory proteins control their activation cycle and subcellular localization. Members of the guanine nucleotide dissociation inhibitor (GDI) family sequester Rho GTPases from the plasma membrane and keep them in an inactive form.

RESULTS

We report on the characterization the RhoGDI homolog of Tuber borchii Vittad., an ascomycetous ectomycorrhizal fungus. The Tbgdi gene is present in two copies in the T. borchii genome. The predicted amino acid sequence shows high similarity to other known RhoGDIs. Real time PCR analyses revealed an increased expression of Tbgdi during the phase preparative to the symbiosis instauration, in particular after stimulation with root exudates extracts, that correlates with expression of Tbcdc42. In a translocation assay TbRhoGDI was able to solubilize TbCdc42 from membranes. Surprisingly, TbRhoGDI appeared not to interact with S. cerevisiae Cdc42, precluding the use of yeast as a surrogate model for functional studies. To study the role of TbRhoGDI we performed complementation experiments using a RhoGDI null strain of Dictyostelium discoideum, a model organism where the roles of Rho signaling pathways are well established. For comparison, complementation with mammalian RhoGDI1 and LyGDI was also studied in the null strain. Although interacting with Rac1 isoforms, TbRhoGDI was not able to revert the defects of the D. discoideum RhoGDI null strain, but displayed an additional negative effect on the cAMP-stimulated actin polymerization response.

CONCLUSION

T. borchii expresses a functional RhoGDI homolog that appears as an important modulator of cytoskeleton reorganization during polarized apical growth that antecedes symbiosis instauration. The specificity of TbRhoGDI actions was underscored by its inability to elicit a growth defect in S. cerevisiae or to compensate the loss of a D. discoideum RhoGDI. Knowledge of the cell signaling at the basis of cytoskeleton reorganization of ectomycorrhizal fungi is essential for improvements in the production of mycorrhized plant seedlings used in timberland extension programs and fruit body production.

Expression of the fluorescence markers DsRed and GFP fused to a nuclear localization signal in the arbuscular mycorrhizal fungus Glomus intraradices

Here, arbuscular mycorrhizal (AM) fungi were monitored in vivo introducing the fluorescent reporters DsRed and GFP (green fluorescent protein) in Glomus intraradices using a biolistic approach and Agrobacterium tumefaciens-mediated transformation. Both reporter genes were fused to the nuclear localization signal of the Aspergillus nidulans transcription factor StuA to target fluorescence to nuclei. Expression of DsRed was driven by two Glomus mosseae promoters highly expressed during early symbiosis, GmPMA1 and GmFOX2, while expression of GFP was driven by the A. nidulans gpd promoter. All promoters worked in G. intraradices as well as in A. nidulans. Red and green fluorescence was localized to nuclei of G. intraradices spores and hyphae 3 d after bombardment. However, expression was transient. The efficiency of the Agrobacterium-mediated transformation was very low. These results indicate that the biolistic method allows the expression of foreign DNA into G. intraradices with high frequency, but it is insufficient to render stable transformants. DsRed vs GFP is a more appropriate living reporter to be used in G. intraradices because of the lower autofluorescence in the red channel but targeted to the nucleus both marker genes can be visualized. This is the first report of fluorescent marker expression in an AM fungus driven by arbuscular mycorrhizal promoters.

Cadophora finlandia and Phialocephala fortinii: Agrobacterium-mediated transformation and functional GFP expression

Hygromycin B resistance was transferred to the sterile mycelia of Cadophora finlandia and Phialocephala fortinii by co-cultivation with Agrobacterium tumefaciens. Constitutively expressed green fluorescent protein (GFP) was also introduced using the same vector. Confocal laser scanning microscopy (CLSM) revealed strong fluorescence of transformants. Both traits were mitotically stable during one year of subculturing on non-selective growth medium. Southern blot analysis showed that the majority of the transformants contained single-copy integrations at random sites in the genome.

Imaging mycorrhizal fungal transformants that express EGFP during ericoid endosymbiosis

Ericoid endomycorrhizal fungi form intracellular associations with the epidermal root cells of plants belonging to Ericales. In natural environments, these fungi increase the ability of their host plants to colonise soils polluted with toxic metals, although the underlying mechanisms are not clearly understood. Genetic transformation is a powerful tool to study the function of specific genes involved in the interaction of symbiotic fungi with the host plants and with the environment. Here, we investigated the possibility to genetically transform an ericoid endomycorrhizal strain. A metal tolerant mycorrhizal Oidiodendron maius strain isolated from a contaminated area was chosen to develop the transformation system. Two different protocols were used: protoplasts and Agrobacterium-mediated transformation. Stable transformants were obtained with both techniques. They remained competent for mycorrhizal formation and GFP-transformed fungi were visualised in planta. This is the first report of stable transformation of an ericoid endomycorrhizal fungus. The protocol set up could represent a good starting point for the identification of genes important in the ericoid mycorrhiza formation and in the understanding of how this symbiosis is established and functions. The success in the genetic transformation of this strain will allow us to better define its potential use in bioremediation strategies.

Metal induction of a Paxillus involutus metallothionein and its heterologous expression in Hebeloma cylindrosporum

* Metallothioneins are small polypeptides involved in metal tolerance of many eukaryotes. Here we characterized the Pimt1 gene, coding for a metallothionein from the ectomycorrhizal fungus Paxillus involutus. * Expression of Pimt1 in P. involutus under metal stress conditions was measured by northern blot and RT-PCR analyses. The full-length cDNA was used to perform functional complementation in yeast mutant strains and agrotransformation of Hebeloma cylindrosporum. * Heterologous expression in yeast showed that PiMT1 was able to complement the hypersensitivity of mutant strains to cadmium (Cd) and copper (Cu), but not to zinc (Zn). Transcripts were almost undetectable under control conditions, whereas Cu and Cd, but not Zn, strongly induced Pimt1 expression in P. involutus. Constitutive overexpression of Pimt1 in H. cylindrosporum conferred a higher copper tolerance. * The present study identified PiMT1 as a potential determinant in the response of mycorrhizal fungi to Cu and Cd stress. Additionally, we demonstrated the usefulness of mycorrhizal fungi transformation using Agrobacterium technology to approach gene function.

Functional expression of the green fluorescent protein in the ectomycorrhizal model fungus Hebeloma cylindrosporum

Hebeloma cylindrosporum is a model fungus for mycorrhizal studies because of its fast growth rate, simple nutritional requirements, and completion of its life cycle in vitro, and because it is amenable to transformation. To advance cell biological research during establishment of symbiosis, a tool that would enable the direct visualisation of fusion proteins in the different symbiotic tissues [namely, the expression of reporter genes such as Green Fluorescent Protein (GFP)] was still a missing tool. In the present study, H. cylindrosporum was transformed using Agrobacterium carrying the binary plasmid pBGgHg containing the Escherichia coli hygromycin B phosphotransferase (hph) and the EGFP genes, both under the control of the Agaricus bisporus glyceraldehyde-3-phosphate dehydrogenase promoter. EGFP expression was successfully detected in transformants. The fluorescence was uniformly distributed in the hyphae, while no significant background signal was detected in control hyphae. The suitability of EGFP for reporter gene studies in Hebeloma cylindrosporum was demonstrated opening up new perspectives in the Hebeloma genetics.

Transformation of the entomopathogenic fungus Paecilomyces fumosoroseus with Agrobacterium tumefaciens

AIMS

To test the suitability of the Agrobacterium tumefaciens-mediated transformation (AMT) method with Paecilomyces fumosoroseus, a fungal pathogen that causes diseases in a wide range of insects including whiteflies.

METHODS AND RESULTS

Conidia of P. fumosoroseus were successfully transformed to hygromycin B resistance using the hph gene of Escherichia coli as the selectable marker. Transformation frequencies were 58.3 +/- 18.5, 98.3 +/- 24.8 and 169.7 +/- 35.5 (+/-SEM) transformants per 10(5), 10(6) and 10(7) target conidia respectively. After confirmation by PCR, transformants were subjected to Southern analysis, and the results revealed that 45% (four of nine) of the transformants contained single-copy integration of the T-DNA.

CONCLUSIONS

In our AMT system, we efficiently transformed conidia of P. fumosoroseus. The employment of this method circumvents time-consuming protoplast preparation and allows the isolation of transformants containing single-copy integration of the T-DNA.

SIGNIFICANCE AND IMPACT OF THE STUDY

Considering the efficiency of Ag. tumefaciens-mediated transformation, this method represents a useful tool for insertional mutagenesis to characterize genes that are important for the pathogenicity of P. fumosoroseus.

Current state and perspectives of truffle genetics and sustainable biotechnology

Mycorrhizal fungi belonging to the genus Tuber produce, after the establishment of a productive interaction with a plant host, hypogeous fruitbodies of great economic value known as ''truffles''. This review summarizes the state of art on life cycle, genetic, and biotechnological investigations of Tuber spp. The ascocarp formation in truffles is a consequence of the activation of the sexual phase of the biological cycle. The formation of a dikaryotic secondary mycelium and the karyogamy in the ascal cell (followed by meiosis with ascospores formation) have been hypothesized by several authors but some doubts yet arise from the Tuber cycle by considering that a series of abnormalities have been pointed out in respect to other Ascomycetes. It is unclear if binucleated hyphal cells are derived from the fusion of mononucleated cells belonging to mycelia from different mating types or from one only. According to the karyotypes of Tuber melanosporum, Tuber magnatum, and Tuber borchii, the numbers of hyphal chromosomes suggest a chromosome number of eight (2n); these values are in the range of those of several Ascomycetes and observed for Tuber aestivum (2n=10). The importance and growth in interest during the last years in the fungi protoplasts isolation and transformation techniques can be related to current developments in Tuber genetics and biotechnology. T. borchii could be transformed through liposome-mediated delivery of genetic material as mycelial protoplasts isolation and fusion with liposomes has already been established. On the other hand, Agrobacterium-mediated transformation has been successfully established for T. borchii.

Development of a rapid and simpleAgrobacterium tumefaciens-mediated transformation system for the fungal pathogenHeterobasidion annosum

Heterobasidion annosum causes root and butt-rot in trees and is the most serious forest pathogen in the northern hemisphere. We developed a rapid and simple Agrobacterium-mediated method of gene delivery into H. annosum to be used in functional studies of candidate genes and for visualization of mycelial interactions. Heterobasidion annosum TC 32-1 was cocultivated at pH 5.6 and 20 degrees C in Hagems medium with Agrobacterium tumefaciens C58 carrying plasmids with hygromycin B resistance as the selectable marker and green fluorescent protein as a visual marker. We obtained 18 mitotically stable transformed isolates showing green fluorescence protein activity.

Agrobacterium-mediated transformation of the ectomycorrhizal symbiont Laccaria bicolor S238N

The development of an efficient transformation system is required to alter the expression of symbiosis-regulated genes and to develop insertional mutagenesis in the ectomycorrhizal basidiomycete Laccaria bicolor S238N. Vegetative mycelium of this fungus was transformed by Agrobacterium tumefaciens-mediated gene transfer. The selection marker was the hygromycin resistance gene of Escherichia coli (hph) under the control of the gpd promoter from Agaricus bisporus and the CaMV 35S terminator as part of the T-DNA. PCR amplification of hph and Southern blot analyses showed that the genome of the hygromycin-resistant transformants contained the cassette. The latter proved mostly single copy and random integration of part of the transgene into the fungal genome. A. tumefaciens-mediated gene transfer should facilitate future development of insertional mutagenesis, targeted gene disruption and RNA interference technology in L. bicolor.

Stable genetic transformation of the ectomycorrhizal fungus Pisolithus tinctorius

In the present work the genetic transformation and the expression of gene markers in transgenic Pisolithus tinctorius are reported. The ectomycorrhizae are facultative symbionts of plant roots, which are capable of affording mineral nutrients to its co-host in exchange of fixed carbon. Given the importance of this association (more than 80% of gymnosperms are associated with these fungi), its study from both basic and applied viewpoints is relevant. We have transformed this fungus with reporter genes and analyzed their expression in its saprophytic state. Genetic transformation was performed by microprojectile bombardment and Agrobacterium-mediated transformation. This last method proved to be the more efficient. Southern analysis of biolistic-transformed fungi revealed the random integration of the transgene into the genome. The accumulation of the transcript of the reporter gene was demonstrated by RT-PCR. The visualization of GFP-associated fluorescence in saprophytic mycelia confirmed the expression of the reporter gene. This is the first report on the stable transformation and expression of GFP in the ectomycorrhizal fungus P. tinctorius.

Antisense inhibition of xylitol dehydrogenase gene, xdh1 from Trichoderma reesei

AIMS

To inhibit xylitol dehydrogenase (XDH) in Trichoderma reesei by antisense inhibition strategy and construct novel strains capable of accumulating xylitol.

METHODS AND RESULTS

The xdh1 antisense expression plasmid pGTA-xdh was constructed by inserting xdh1 DNA fragment inversely between the gpdA promoter and the trpC terminator from Aspergillus nidulans into a pUC19 plasmid backbone. Trichoderma reesei protoplasts were co-transformated with pGTA-xdh and hygromycin B resistance plasmid pAN7-1. Of 20 transformants screened from the selective medium, one transformant with the highest xylitol accumulation, designated ZY15, showed a distinct reduction (c. 52%) in XDH activity compared with the original strain Rut-C30. The results of Southern hybridization and PCR assay showed that the antisense expression cassette of xdh1 was integrated into the genome of T. reesei. The RT-PCR analysis proved that antisense RNA effectively inhibited XDH expression (c. 65%). Xylitol accumulation (2.37 mg ml(-1)) of ZY15 was five times higher than that (0.46 mg ml(-1)) of the original strain Rut-C30.

CONCLUSIONS

Strain ZY15 successfully downregulated XDH production and exhibited xylitol accumulation in xylose liquid medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study contributed to the budding field of fungal genetics in two points. First, it confirmed that antisense RNA strategy could be used as a means of reducing gene expression in the filamentous fungus T. reesei. Secondly, it verified that the strategy appears most promising for creating novel filamentous fungi strains capable of accumulating intermediary metabolites.

Agrobacterium-mediated transformation as a tool for functional genomics in fungi

In the era of functional genomics, the need for tools to perform large-scale targeted and random mutagenesis is increasing. A potential tool is Agrobacterium-mediated fungal transformation. A. tumefaciens is able to transfer a part of its DNA (transferred DNA; T-DNA) to a wide variety of fungi and the number of fungi that can be transformed by Agrobacterium-mediated transformation (AMT) is still increasing. AMT has especially opened the field of molecular genetics for fungi that were difficult to transform with traditional methods or for which the traditional protocols failed to yield stable DNA integration. Because of the simplicity and efficiency of transformation via A. tumefaciens, it is relatively easy to generate a large number of stable transformants. In combination with the finding that the T-DNA integrates randomly and predominantly as a single copy, AMT is well suited to perform insertional mutagenesis in fungi. In addition, in various gene-targeting experiments, high homologous recombination frequencies were obtained, indicating that the T-DNA is also a useful substrate for targeted mutagenesis. In this review, we discuss the potential of the Agrobacterium DNA transfer system to be used as a tool for targeted and random mutagenesis in fungi.

Agrobacterium-mediated gene transfer and enhanced green fluorescent protein visualization in the mycorrhizal ascomycete Tuber borchii: a first step towards truffle genetics

Mycorrhizal ascomycetes are ecologically and commercially important fungi that have proved impervious to genetic transformation so far. We report here on the successful transient transformation of Tuber borchii, an ectomycorrhizal ascomycete that colonizes a variety of trees and produces highly prized hypogeous fruitbodies known as "truffles". A hypervirulent Agrobacterium tumefaciens strain bearing the binary plasmid pBGgHg was used for transformation. The genes for hygromycin resistance and the enhanced green fluorescent protein (EGFP), both under the control of vector-borne promoters, were employed as selection markers. Patches of dark and fluorescent hyphae were observed upon fluorescence microscopic examination of hygromycin-resistant mycelia. The presence of EGFP was confirmed by both confocal microscopy and PCR analysis. The lack in the transformed mycelia of the DNA coding for kanamicin resistance (a trait encoded by a vector-borne gene located outside of the T-DNA region) indicates that Agrobacterium-mediated gene transfer correctly occurred in T. borchii.

Agrobacterium tumefaciens-mediated transformation of the antitumor clavaric acid-producing basidiomycete Hypholoma sublateritium

The basidiomycete Hypholoma sublateritium produces clavaric acid, an antitumor isoprenoid compound. Arthrospores of this fungus were transformed by Agrobacterium tumefaciens-mediated conjugation. Five plasmids carrying different regulatory sequences to drive expression of the hph (hygromycin phosphotransferase) gene were tested. The promoter used was critically important in order to express heterologous genes in H. sublateritium. Constructions carrying the Agaricus bisporus glyceraldehyde-3-phosphate dehydrogenase promoter (P gpd) showed a good transformation efficiency, whereas constructions with the gpd promoter from ascomycetes were ineffective. Transformant clones showed a random integration pattern of plasmid DNA. Most transformants showed a single integrated copy of the transforming plasmid, but about 1.5% showed double or multiple integrations. All the analyzed transformants were mitotically stable and maintained the integrated exogenous DNA in the absence of antibiotic. The green fluorescent protein gene was expressed from the A. bisporus gpd promoter, as shown by RT-PCR studies, but no significant fluorescence was observed. Transformation of H. sublateritium opens the way for the genetic manipulation of clavaric acid biosynthesis in this fungus.

Agrobacterium tumefaciens-mediated genetic transformation of the entomopathogenic fungus Beauveria bassiana

Agrobacterium tumefaciens-mediated transformation (agro-transformation) was successfully applied to the entomogenous fungus Beauveria bassiana. Conidia of B. bassiana were transformed to hygromycin B resistance using the hph gene of Escherichia coli as the selective trait, under the control of a heterologous fungal promoter and the Aspergillus nidulans trpC terminator. The efficiency of transformation was up to 28 and 96 transformants per 10(4) and 10(5) target conidia, respectively, using three distinct vectors. High mitotic stability of the transformants (80-100%) was demonstrated after five successive transfers on a nonselective medium. Abortive transformants were observed for all the hph(r) vectors used. Putative transformants were analysed for the presence of the hph gene by PCR and Southern analysis. The latter analysis revealed the integration of two or more copies of the hph gene in the genome. The agro-transformation method was found to be effective for the isolation of B. bassiana hygromycin resistant transformants and may represent a useful tool for insertional mutagenesis studies in this fungus.

Agrobacterium tumefaciens-mediated transformation as a tool for insertional mutagenesis in the symbiotic ectomycorrhizal fungus Hebeloma cylindrosporum

We transformed haploid mycelium of Hebeloma cylindrosporum via Agrobacterium tumefaciens and optimised the procedure to develop a new tool for insertional mutagenesis in this fungus. Southern blot analysis of 83 randomly selected transformants showed that they all contained plasmid inserts. Each of them showed a unique hybridisation pattern, suggesting that integration was random in the fungal genome. Sixty percent of transformants obtained in the presence of bacteria pre-treated with acetosyringone integrated a single transferred DNA copy. Thermal asymmetric interlaced polymerase chain reaction allowed us to recover the left border and the right border junctions in 85% and 15% of transformants analysed, respectively. Results show that A. tumefaciens-mediated transformation may be a powerful tool for insertional mutagenesis in H. cylindrosporum.