Disruption of the subtilase gene, albin1, in Ophiostoma piliferum

Establishment of invasive and non-invasive reporter systems to investigate American elm-Ophiostoma novo-ulmi interactions

Dutch elm disease (DED), caused by ascomycete fungi in the Ophiostoma genus, is the most devastating disease of American elm (Ulmus americana) trees. Cerato ulmin (CU), a hydrophobin secreted by the fungus, has been implicated in the development of DED, but its role in fungal pathogenicity and virulence remains uncertain and controversial. Here, we describe reporter systems based on the CU promoter and three reporter proteins (GFP, GUS and LUC), developed as research tools for quantitative and qualitative studies of DED in vitro, in vivo and in planta. A strain of the aggressive species Ophiostoma novo-ulmi was transformed with the reporter constructs using Agrobacterium-mediated transformation and the fungal transformants, namely M75-GFP, M75-GUS and M75-LUC, were examined for mitotic stability after repeated subcultures. The intensity of GFP fluorescence was strong in M75-GFP spores and hyphae, allowing microscopic investigations of spore structure, fungal morphogenesis and fungal development. The interaction of M75-GFP and U. americana callus cells was explored with scanning laser confocal microscopy facilitating qualitative studies on fungal strategies for the invasion and penetration of elm cells. M75-GUS was generated to provide an invasive, yet quantitative approach to study fungal-plant interactions in vitro and in planta. The generation of M75-LUC transformants was aimed at providing a non-destructive quantitative approach to study the role of CU in vivo. The sensitivity, low background signal and linearity of LUC assays all predict a very reliable approach to investigate and re-test previously claimed roles of this CU in fungal pathogenicity. These reporter systems provide new tools to investigate plant-pathogen interactions in this complex pathosystem and may aid in better understanding the development of DED.

Functional categorization of unique expressed sequence tags obtained from the yeast-like growth phase of the elm pathogen Ophiostoma novo-ulmi

Background

The highly aggressive pathogenic fungus Ophiostoma novo-ulmi continues to be a serious threat to the American elm (Ulmus americana) in North America. Extensive studies have been conducted in North America to understand the mechanisms of virulence of this introduced pathogen and its evolving population structure, with a view to identifying potential strategies for the control of Dutch elm disease. As part of a larger study to examine the genomes of economically important Ophiostoma spp. and the genetic basis of virulence, we have constructed an expressed sequence tag (EST) library using total RNA extracted from the yeast-like growth phase of O. novo-ulmi (isolate H327).

Results

A total of 4,386 readable EST sequences were annotated by determining their closest matches to known or theoretical sequences in public databases by BLASTX analysis. Searches matched 2,093 sequences to entries found in Genbank, including 1,761 matches with known proteins and 332 matches with unknown (hypothetical/predicted) proteins. Known proteins included a collection of 880 unique transcripts which were categorized to obtain a functional profile of the transcriptome and to evaluate physiological function. These assignments yielded 20 primary functional categories (FunCat), the largest including Metabolism (FunCat 01, 20.28% of total), Sub-cellular localization (70, 10.23%), Protein synthesis (12, 10.14%), Transcription (11, 8.27%), Biogenesis of cellular components (42, 8.15%), Cellular transport, facilitation and routes (20, 6.08%), Classification unresolved (98, 5.80%), Cell rescue, defence and virulence (32, 5.31%) and the unclassified category, or known sequences of unknown metabolic function (99, 7.5%). A list of specific transcripts of interest was compiled to initiate an evaluation of their impact upon strain virulence in subsequent studies.

Conclusions

This is the first large-scale study of the O. novo-ulmi transcriptome. The expression profile obtained from the yeast-like growth phase of this species will facilitate a multigenic approach to gene expression studies to assess their role in the determination of pathogenicity for this species. The identification and evaluation of gene targets in such studies will be a prerequisite to the development of biological control strategies for this pathogen.

Agrobacterium-meditated gene disruption using split-marker in Grosmannia clavigera, a mountain pine beetle associated pathogen

Grosmannia clavigera is a fungal pathogen associated with the mountain pine beetle (Dendroctonus ponderosae) which is devastating large areas of western Canada's conifer forests. This fungus also produces a dark melanin pigment that discolors pine sapwood. We have generated the draft genome of G. clavigera. However, functional characterization of genes identified in the genome sequence requires an efficient gene disruption method. In this work, we report a gene replacement strategy for G. clavigera using the Agrobacterium-mediated transformation in conjunction with linear or split-marker deletion cassettes. In addition, we used long flanking regions up to 3 kb from both sides of the targeted genes in our deletion cassettes. We assessed this gene disruption method with two genes from the melanin biosynthesis pathway that produce easily detectable white and red/brown mutant phenotypes: polyketide synthase and scytalone dehydratase. The approach yielded G. clavigera gene replacements with homologous recombination rates between 65 and 82%. For filamentous fungi, this is the first report showing that split-markers can be used with Agrobacterium-mediated transformation to generate appropriate mutants. This method can now be applied to efficiently identify genes involved in G. clavigera fungal pathogenicity and will facilitate understanding how the fungus overcomes the host defence system.

An expanded family of fungalysin extracellular metallopeptidases of Coprinopsis cinerea

Proteolytic enzymes, particularly secreted proteases of fungal origin, are among the most important of industrial enzymes, yet the biochemical properties and substrate specificities of these proteins have been difficult to characterize. Genomic sequencing offers a powerful tool to identify potentially novel proteases. The genome of the model basidiomycete Coprinopsis cinereus was found to have an unusually high number of metalloproteases that closely match the M36 peptidase family known as fungalysins. The eight predicted C. cinereus fungalysins divide into two groups upon comparison with fungalysins from other fungi. One member, CcMEP1, is most similar to the single representative fungalysins from the basidiomycetes Phanerochaete chrysosporium, Cryptococcus neoformans, and Ustilago maydis, and to the fungalysin type-protein from Aspergillus fumigatus. The remaining seven C. cinereus predicted fungalysins form a group with similarity to three predicted M36 peptidases of Laccaria bicolor. All eight of the C. cinereus enzymes contain both the signature M36 Pfam domain and the FTP propeptide domain. All contain large propeptides with considerable sequence conservation near a proposed cleavage site. The predicted mature enzymes range in size from 37-46 kDa and have isoelectric points that are mildly acidic to neutral. The proximity of these genes to telomeres and/or to transposable elements may have contributed to the expansion of this gene family in C. cinereus.

Gene inactivation mediated by Agrobacterium tumefaciens in the filamentous fungi Metarhizium anisopliae

The list of fungal species with known complete genome and/or expressed sequence tag collections is extending rapidly during the last couple of years. Postgenomic gene function assignment is an obvious follow-up and depends on methodologies to test gene function in vivo. One of such methods is the generation of null mutants via homologous recombination at the wild-type loci by using inactivation cassettes. In this paper, the ability of Agrobacterium tumefaciens to genetically transform filamentous fungi was exploited to drive homologous recombination at the trp1 locus of the enthomopathogenic fungus Metarhizium anisopliae. The trp1 disruptants exhibited a clearly distinguishable phenotype from wild-type cells and were recovered with high efficiency of homologous recombination (22%). The complementation of such mutants with the wild-type gene generates only transformants with homologous integration.

A case of promiscuity: Agrobacterium's endless hunt for new partners

Agrobacterium tumefaciens is a phytopathogenic bacterium that induces the 'crown gall' disease in plants by transfer and integration of a segment of its tumor-inducing (Ti) plasmid DNA into the genome of numerous plant species that represent most of the higher plant families. Recently, it has been shown that, under laboratory conditions, the host range of Agrobacterium can be extended to non-plant eukaryotic organisms. These include yeast, filamentous fungi, cultivated mushrooms and human cultured cells. In this article, we present Agrobacterium-mediated transformation of non-plant organisms as a source of new protocols for genetic transformation, as a unique tool for genomic studies (insertional mutagenesis or targeted DNA integration) and as a useful model system to study bacterium-host cell interactions. Moreover, better knowledge of the DNA-transfer mechanisms from bacteria to eukaryotic organisms can also help in understanding horizontal gene transfer--a driving force throughout biological evolution.

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.