Functional characterisation of glyoxalase I from the fungal wheat pathogen Stagonospora nodorum

Stagonospora nodorum: from pathology to genomics and host resistance

Stagonospora nodorum is a major necrotrophic pathogen of wheat that causes the diseases S. nodorum leaf and glume blotch. A series of tools and resources, including functional genomics, a genome sequence, proteomics and metabolomics, host-mapping populations, and a worldwide collection of isolates, have enabled the dissection of pathogenicity mechanisms. Metabolic and signaling genes required for pathogenicity have been defined. Interaction with the host is dominated by interplay of fungal effectors that induce necrosis on wheat lines carrying specific sensitivity loci. As such, the pathogen has emerged as a model for the Pleosporales group of pathogens.

Malayamycin, a new streptomycete antifungal compound, specifically inhibits sporulation of Stagonospora nodorum (Berk) Castell and Germano, the cause of wheat glume blotch disease

BACKGROUND

Malayamycin is a novel perhydrofuropyran C-nucleoside isolated from Streptomyces malaysiensis that shows promising antifungal activity, fully controlling a range of diseases when applied to plants at 100 microg mL(-1). The goal of this study was to determine the mode of action.

RESULTS

Malayamycin exhibited in vitro antifungal activity against Stagonospora nodorum (Berk) Castell & Germano, the cause of stagonospora nodorum blotch of wheat. Growth in liquid minimum medium was merely delayed at 50 microg mL(-1), but sporulation was suppressed by more than 50% by 10 microg mL(-1) of malayamycin. When applied to wheat seedlings 36 h prior to infection, 10 microg mL(-1) of malayamycin reduced lesion size and significantly reduced pycnidiation to only 5% of the non-treated level. A transcription factor gene, Mrg1 (malayamycin response gene) whose expression was upregulated by application of malayamycin, was identified. Both Mrg1 knockout and overexpression strains were created. These strains were fully pathogenic, suggesting that the expression of Mrg1 did not affect pathogenicity. Interestingly, a strain that expressed Mrg1 50 times more than wild type showed a significant reduction in sporulation. However, all the tested knockout and overexpression strains retained sensitivity to malayamycin.

CONCLUSIONS

Malayamycin is a new type of antifungal compound that acts primarily by inhibiting sporulation. Although Mrg1 may be involved in the sporulation process, it is not the major contributor for sporulation inhibition caused by malayamycin treatment.

Characterization of the glyoxalase I gene from the vascular wilt fungus Verticillium dahliae

A glyoxalase I gene homologue (VdGLO1) was identified in the vascular wilt fungus Verticillium dahliae by sequence tag analysis of genes expressed during resting structure development. The results of the current study show that the gene encodes a putative 345 amino acid protein with high similarity to glyoxalase I, which produces S-D-lactoylglutathione from the toxic metabolic by-product methylglyoxal (MG). Disruption of the V. dahliae gene by Agrobacterium tumefaciens-mediated transformation resulted in enhanced sensitivity to MG. Mycelial growth of disruption mutants was severely reduced in the presence of 5 mmol/L MG. In contrast, spore production in liquid medium was abolished at 1 mmol/L MG, although not at physiologically relevant concentrations of ≤100 µmol/L. In this first report on the characterization of a glyoxalase I gene in a vascular wilt pathogen, we found that disruption of VdGLO1 had no discernable effect on the pathogenicity of V. dahliae. These data suggest that while the glyoxalase system is necessary for effectively dealing with catastrophic levels of MG, under normal conditions of growth and infection, other MG detoxification pathways in V. dahliae are able to compensate for the absence of the glyoxalase system.Key words: verticillium wilt, glycolytic methylglyoxal pathway, 2-oxoaldehydes.

Stagonospora nodorum: cause of stagonospora nodorum blotch of wheat

SUMMARY Stagonospora nodorum is an important pathogen of wheat and related cereals, causing both a leaf and glume blotch. This review summarizes recent advances in our understanding of taxonomy, control and pathogenicity of this species.

TAXONOMY

Stagonospora (syn. Septoria) nodorum (Berk.) Castell. and Germano [teleomorph: Phaeosphaeria (syn. Leptosphaeria) nodorum (Müll.) Hedjar.], kingdom Fungi, phylum Ascomycota, subphylum Euascomycota, class Dothideomycetes, order Pleosporales, family Phaeosphaeriaceae, genus Phaeosphaeria, species nodorum.

HOST RANGE

Wheat, Triticum aestivum, T. durum, Triticale, are the main hosts but other cereals and wild grasses have been reported to harbour S. nodorum. Disease symptoms are lens-shaped necrotic lesions on leaves, girdling necrosis on stems (especially the nodes, hence 'nodorum') and lesions on glumes. Mature lesions produce pycnidia scattered throughout the lesions, especially as tissue senesces.

USEFUL WEBSITES

http://ocid.nacse.org/research/deephyphae/htmls/asco_taxlist_spat.html (taxonomic information), http://ohioline.osu.edu/ac-fact/0002.html (disease information), http://wwwacnfp.murdoch.edu.au/ (ACNFP homepage), http://www.broad.mit.edu/annotation/fungi/stagonospora_nodorum/index.html (genome sequence homepage), http://cogeme.ex.ac.uk/efungi/ (genome sequence annotation and analysis).

A symbiosis expressed non-ribosomal peptide synthetase from a mutualistic fungal endophyte of perennial ryegrass confers protection to the symbiotum from insect herbivory

While much is known about the biosynthesis of secondary metabolites by filamentous fungi their biological role is often less clear. The assumption is these pathways have adaptive value to the organism but often the evidence to support this role is lacking. We provide the first genetic evidence that the fungal produced secondary metabolite, peramine, protects a host plant from insect herbivory. Peramine is a potent insect feeding deterrent synthesized by Epichloë/Neotyphodium mutualistic endophytes in association with their grass hosts. The structure of peramine, a pyrrolopyrazine, suggests that it is the product of a reaction catalysed by a two-module non-ribosomal peptide synthetase (NRPS). Candidate sequences for a peramine synthetase were amplified by reverse transcription polymerase chain reaction. Four unique NRPS products were identified, two of which were preferentially expressed in planta. One of these hybridized to known peramine producing strains. This clone was used to isolate an Epichloë festucae cosmid that contained a two-module NRPS, designated perA. Nine additional genes, which show striking conservation of microsynteny with Fusarium graminearum and other fungal genomes, were identified on the perA-containing cosmid. Associations between perennial ryegrass and an E. festucae mutant deleted for perA lack detectable levels of peramine. A wild-type copy of perA complemented the deletion mutant, confirming that perA is a NRPS required for peramine biosynthesis. In a choice bioassay, plant material containing the perA mutant was as susceptible to Argentine stem weevil (ASW) (Listronotus bonariensis) feeding damage as endophyte-free plants confirming that peramine is the E. festucae metabolite responsible for ASW feeding deterrent activity.

The Mak2 MAP kinase signal transduction pathway is required for pathogenicity in Stagonospora nodorum

A gene encoding a mitogen-activated protein kinase (MAPK) putatively orthologous to Pmk1 from Magnaporthe grisea was cloned and characterised from the wheat glume blotch pathogen Stagonospora nodorum. Protein sequence alignments showed the cloned gene, Mak2, is closely related to homologues from other dothideomycete fungi. Expression studies revealed Mak2 is up-regulated during in vitro growth upon nitrogen starvation but is not sensitive to carbon starvation or osmotic stress. Transcript analysis in planta showed Mak2 to be expressed throughout infection and up-regulated during the sporulation phase of the infection cycle. Fungal strains harbouring a disrupted Mak2 gene were created by homologous gene recombination. The mutant strains had a severely altered phenotype in vitro with reduced growth rate and failure to sporulate. Further phenotypic analysis revealed that the mutants had near-normal levels of secreted protease activity, were not hypersensitive to osmotic stress and appeared to have melanin synthesis intact. The mak2 strains were essentially non-pathogenic to wheat leaves. No penetration structures formed and although entry was observed through stomates, the infection rarely continued. The results within this study are discussed within the context of the differences in downstream regulation of the Mak2 MAPK pathway and the cAMP signal transduction pathway in S. nodorum; and differences are compared to mak2 mutant strains in other pathogenic fungi.