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Role of chitin synthase genes in Fusarium oxysporum

Magdalena Martín-Udíroz

Departamento de Genética, Campus Rabanales C5, Universidad de Córdoba, E-14071 Córdoba, Spain

Correspondence
M. Isabel G. Roncero
ge1gorom{at}uco.es

Three structural chitin synthase genes, chs1, chs2 and chs3, were identified in the genome of Fusarium oxysporum f. sp. lycopersici, a soilborne pathogen causing vascular wilt disease in tomato plants. Based on amino acid identities with related fungal species, chs1, chs2 and chs3 encode structural chitin synthases (CSs) of class I, class II and class III, respectively. A gene (chs7) encoding a chaperone-like protein was identified by comparison of the deduced protein with Chs7p from Saccharomyces cerevisiae, an endoplasmic reticulum (ER) protein required for the export of ScChs3p (class IV) from the ER. So far no CS gene belonging to class IV has been isolated from F. oxysporum, although it probably contains more than one gene of this class, based on the genome data of the closely related species Fusarium graminearum. F. oxysporum chs1-, chs2- and chs7-deficient mutants were constructed through targeted gene disruption by homologous recombination. No compensatory mechanism seems to exist between the CS genes studied, since chitin content determination and expression analysis of the chs genes showed no differences between the disruption mutants and the wild-type strain. By fluorescence microscopy using Calcofluor white and DAPI staining, the wild-type strain and chs2 and chs7 mutants showed similar septation and even nuclear distribution, with each hyphal compartment containing only one nucleus, whereas the chs1 mutant showed compartments containing up to four nuclei. Pathogenicity assays on tomato plants indicated reduced virulence of chs2 and chs7 null mutants. Stress conditions affected normal development in chs2 but not in chs1 or chs7 disruptants, and the three chs-deficient mutants showed increased hyphal hydrophobicity compared to the wild-type strain when grown in sorbitol-containing medium. The chitin synthase mutants will be useful for elucidating cell wall biogenesis in F. oxysporum and the relationship between fungal cell wall integrity and pathogenicity.

The GenBank/EMBL/DDBJ accession numbers for the chs1, chs2, chs3 and chs7 gene sequences reported in this paper are AY572421, AY572422, AY572423 and AY572424, respectively.

Present address: Australian Centre for Necrotrophic Fungal Pathogens, Murdoch University, Perth, WA 6150, Australia.

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