High frequency gene conversion among benomyl resistant transformants in the entomopathogenic fungus Metarhizium anisopliae

Strong preference for the integration of transforming DNA via homologous recombination in Trichoderma atroviride

Trichoderma species play important roles in nature as plant growth promotors and antagonists of phytopathogenic fungi, and are used as models to study photomorphogenesis. Molecular tools have been implemented to manipulate and improve these fungi. However, instability of transformants or very low frequency of homologous recombination has been reported. Here, we report the fate of transforming DNA, demonstrating that it can follow two different fates. When a vector contains sequences also present in the Trichodermaatroviride genome, it mainly integrates by homologous recombination generating stable recombinant strains. In contrast, vectors with no sequence homology to the T. atroviride genome generate unstable transformants, losing the transforming DNA in the first generation of conidia produced without selection where, surprisingly, the vector behaves as autoreplicative. Integration by homologous recombination was demonstrated when transformants were generated with a truncated version of the blr2 gene, resulting in insertional mutants with phenotypes identical to those of knockout mutants. Our results indicate that T. atroviride is highly efficient in integrating DNA by homologous recombination and that plasmid vectors with no sequence homology to the genome are maintained for several generations in T. atroviride if kept under selective pressure even though they lacked fungal autonomous replication sequences.

Development of Transformation System of Verticillium lecanii (Lecanicillium spp.) (Deuteromycotina: Hyphomycetes) Based on Nitrate Reductase Gene of Aspergillus nidulans

A heterologous transformation system was developed for V. lecanii based on the complementation of a nitrate reductase mutant. Nitrate reductase mutants were obtained by resistance to chlorate in a rate of 23.24% when compared to other mutations that lead to the chlorate resistance. Mutant no. 01 and 04 was chosen for the transformation experiments. Plasmid pBT was used as transformation vector containing the Aspergillus nidulans nitrate reductase gene. A frequency of approximately 3 transformants/μg DNA was obtained using the circular vector pBT. The establishment of a transformation system for V. lecanii is fundamental for genetic manipulation of this microorganism.

In vivo trans-specific gene silencing in fungal cells by in planta expression of a double-stranded RNA

BACKGROUND

Self-complementary RNA transcripts form a double-stranded RNA (dsRNA) that triggers a sequence-specific mRNA degradation, in a process known as RNA interference (RNAi), leading to gene silencing. In vascular plants, RNAi molecules trafficking occur between cells and systemically throughout the plant. RNAi signals can spread systemically throughout a plant, even across graft junctions from transgenic to non-transgenic stocks. There is also a great interest in applying RNAi to pathogenic fungi. Specific inhibition of gene expression by RNAi has been shown to be suitable for a multitude of phytopathogenic filamentous fungi. However, double-stranded (ds)RNA/small interfering (si)RNA silencing effect has not been observed in vivo.

RESULTS

This study demonstrates for the first time the in vivo interference phenomenon in the pathogenic fungus Fusarium verticillioides, in which expression of an individual fungal transgene was specifically abolished by inoculating mycelial cells in transgenic tobacco plants engineered to express siRNAs from a dsRNA corresponding to the particular transgene.

CONCLUSION

The results provide a powerful tool for further studies on molecular plant-microbe and symbiotic interactions. From a biotechnological perspective, silencing of fungal genes by generating siRNAs in the host provides a novel strategy for the development of broad fungi-resistance strategies in plants and other organisms.

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.

Development of a simple and rapid Agrobacterium tumefaciens-mediated transformation system for the entomopathogenic fungus Metarhizium anisopliae var. acridum

AIMS

To examine the ability of Agrobacterium to attach to Metarhizium anisopliae var. acridum strain CG423 under co-cultivation and to develop an Agrobacterium-mediated method of gene delivery into strain CG423, a promising agent for biological control of grasshoppers.

METHODS AND RESULTS

The co-cultivation of Agrobacterium tumefaciens and M. anisopliae var. acridum was analysed under scanning electron microscopy. We observed that Agrobacterium attached to and formed aggregates around Metarhizium conidia and germ tubes. We also observed the occurrence of fibril-like structures connecting neighbouring bacterial-fungal cells. The Agrobacterium-mediated transformation was applied using two binary vectors carrying a benomyl resistance gene as a selection marker. The efficiency of transformation was up to 53 transformants per 10(5) target conidia. High mitotic stability of the transformants (89-97%) was demonstrated after five successive transfers on non-selective media. Molecular analysis revealed the occurrence of high frequency of gene conversion.

CONCLUSIONS

In our study, we report that A. tumefaciens strain AGL-1 attaches to and genetically transforms the entomopathogenic fungus Metarhizium anisopliae var. acridum.

SIGNIFICANCE AND IMPACT OF THE STUDY

We report for the first time, the attachment of Agrobacterium to fungal cells opening new avenues for the study of this essential step of the T-DNA transfer process. Considering the efficiency of the transformation protocol herein described, this is a useful tool for gene disruption in M. anisopliae var. acridum.

Transformation of Metarhizium anisopliae with benomyl resistance and green fluorescent protein genes provides a tag for genetically engineered strains

A plasmid, pBGFP, carrying green fluorescent protein (gfp) and benomyl-resistance genes was constructed and transformed into Metarhizium anisopliae. The transformants grew normally and GFP fluorescence was detected. No change was found in virulence for the transformants. Fluorescence was detected in hyphae from the haemolymph of the infected locust, and the benomyl-resistance was maintained. Results suggested that the two markers provided a useful tool for screening and monitoring the engineered strains even after infection.

Approaches to functional genomics in filamentous fungi

The study of gene function in filamentous fungi is a field of research that has made great advances in very recent years. A number of transformation and gene manipulation strategies have been developed and applied to a diverse and rapidly expanding list of economically important filamentous fungi and oomycetes. With the significant number of fungal genomes now sequenced or being sequenced, functional genomics promises to uncover a great deal of new information in coming years. This review discusses recent advances that have been made in examining gene function in filamentous fungi and describes the advantages and limitations of the different approaches.

Development of an expression vector for Metarhizium anisopliae based on the tef-1alpha homologous promoter

The high-conserved translation elongation factor 1 alpha (tef-1alpha) gene from the enthomopathogenic fungus Metarhizium anisopliae was characterized to select the promoter region. A 640-bp DNA fragment upstream to the start codon was employed to drive the expression of the reporter protein sGFP or a dominant selectable marker, the gene bar (resistance to ammonium glufosinate). Transformants carrying this homologous promoter system showed no difference in virulence bioassays against the cattle tick Boophilus microplus comparing to the M. anisopliae wild-type strain. Moreover, GFP fluorescence was detected during tick infection bioassay.

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-mediated insertional mutagenesis (AIM) of the entomopathogenic fungus Beauveria bassiana

Agrobacterium tumefaciens was used to stably transform the entomopathogenic deuteromycete Beauveria bassiana to hygromycin B resistance by integration of the hph gene of Escherichia coli into the fungal genome. The transformation protocol was optimized to generate a library of insertion mutants of Beauveria. Transformation frequencies around 10(-4) and suppression of background growth were achieved. Over 90% of the AIM mutants investigated contained single-copy T-DNA integrations at different chromosomal locations. Integrated T-DNAs were re-isolated from ten transformants by a marker rescue approach. When the sequences flanking these T-DNAs were compared with the corresponding locations of the wild-type genome, truncations of T-DNA borders were found to be common, while none of the sites of integration had suffered deletion or rearrangement. Thus, AIM can be considered a promising tool for insertional mutagenesis studies of entomopathogenic filamentous fungi.

Pinto F, Fungaro MHP. Transformation of the entomopathogenic fungusMetarhizium flavovirideto high resistance to benomyl

A Brazilian isolate of Metarhizium flavoviride, which has been developed as a biocontrol agent against the grasshopper Rhammatocerus schistocercoides, was stably transformed to be resistant to benomyl (beta-tubulin gene). Highly resistant transformants were obtained which grew in benomyl concentrations greater than 30 times (200 µg mL-1) the concentration that inhibits wild type proliferation. These transformants were mitotically stable after 20 successive transfers on non-selective media. No significant differences in conidia yield were observed between stably transformed strains and wild type (CG423). Chymoelastase (Pr1) secretion was greater in some transformants than in the wild type. In the presence of benomyl, appressoria differentiation occurred at similar rates in CG423 and transformants. However, the percentage of conidial germination in the transformants was higher than in the wild type, indicating the potential use of these transformants along with benomyl. Additionally, the resistance levels of the transformants observed in the present study demonstrate the potential use of these transformants for assessing the persistence of a particular isolate in fields without this fungicide.Key words: Metarhizium flavoviride, genetic transformation, benomyl resistance, biocontrol.

Biological control of agricultural pests by filamentous fungi

Agricultural use of chemical pesticides has polluted the environment and resulted in resistance among the target organisms. The chemical strategies of pest control are dangerous to both the nontarget organisms in natural habitats and human health. Biological control is an attractive less dangerous possibility for controlling plant pathogens.Some methods of biological control are becoming now commercially available against plant parasitic fungi, nematods and insects. Among filamentous fungi many candidates with biocontrol potential can be found. Fungal biocontrol agents are less effective and reliable than the synthetic pesticides therefore their use in the agricultural practice requires genetic improvement.