Native Fusarium species from indigenous fynbos soils of the Western Cape

[摘要] The genus Fusarium contains members that are phytopathogens of a number ofagricultural commodities causing severe diseases such as wilts and rots. Fusariumspecies also secrete mycotoxins that have devastating effects on humans and animals.The ability of Fusarium species to change their genetic makeup in response to theirimmediate environment allows these fungi to exist in diverse habitats. Due to theubiquitous nature of Fusarium, it forms part of the fungal communities in bothagricultural and native soils. Fynbos is the major vegetation type of the Cape FloristicRegion (CFR), which is a region that is renowned for its high plant species diversity andendemism. In this study, the occurrence and distribution of Fusarium species inindigenous fynbos soils and associated plant debris is investigated. In addition, thephylogenetic relationships between Fusarium species occurring in this particular habitatare evaluated.Fusarium isolates were recovered from soils and associated plant debris, andidentified based on morphological characteristics. The morphological identification ofisolates was confirmed using Polymerase Chain Reaction (PCR) based restrictionfragment length polymorphism (RFLP) analyses of the translation elongation factor 1alpha (TEF-1α) and internal transcribed spacer (ITS) regions. Furthermore, phylogeneticrelationships between Fusarium species were based on the TEF-1α, ITS and β-tubulingene regions.One-hundred-and-twenty-two (122) Fusarium strains were isolated from thefynbos soils in the Cape Peninsula area (Western Cape). Based on both morphologicaland molecular identification, the most prevalent Fusarium species in the fynbos soils were F. oxysporum Schlecht. emend. Snyd. and Hans., F. solani (Martius) Appel andWollenw. emend. Snyd. and Hans., F. equiseti (Corda) Sacc. and an undescribedFusarium species. Fusarium oxysporum was the dominant species in fynbos soils andstrains of this species displayed significant genetic variability. Some strains of bothF. oxysporum and F. solani showed close phylogenetic affinities to formae speciales(strains pathogenic to specific plant hosts) in the phylogenetic analyses. However, nodiseased plants were observed in and within the vicinity of our sampling sites.In the third chapter, the undescribed Fusarium strains are described as Fusariumpeninsulae prov. nom. Morphologically these strains are characterized by falcatemacroconidia produced from brown sporodochia. The macroconidia are pedicellate,falcate to curved with hooked apical cells. Also, this fungus produces apedicellatemesoconidia on polyphialides in the aerial mycelium and forms microconidia sparsely.Chlamydospores are formed abundantly on aerial mycelium and submerged hyphae. Allthese morphological characteristics closely relate this fungus to F. camptoceras speciescomplex in Fusarium section Arthrosporiella. However, phylogenetic analysis based onthe ITS sequences differentiate these strains from F. camptoceras and other relatedspecies in section Arthrosporiella.Considering the fact that both as phytopathogens and saprophytic fungi, Fusariumspecies secrete a variety of cell wall degrading enzymes such as cellulases and xylanases.These enzymes allow the fungi to degrade the plant cell wall components to obtainnutrients. In Fusarium, notably endoxylanases play a role in phytopathogenesis of thesefungi. Endoxylanase enzymes from F. oxysporum f. sp. lycopersici, F. verticillioides andF. graminearum have been characterized. In this final chapter, the use of the endoxylanase encoding gene, as a molecular marker in phylogenetic analysis wasevaluated using F. graminearum (Fg) clade species as model. Degenerated primers weredesigned and the endoxylanase region amplified by PCR, cloned and sequenced. PAUPgeneratedneighbour-joining analysis of the endoxylanase (XYL) region enabled allspecies to be distinguished and was as informative as the analysis generated with UTPammonialigase (URA), phosphate permase (PHO), reductase (RED) and trichothecene 3-О-acetyltransferase (TRI101). Furthermore, the results of the phylogenetic analysis ofXYL showed better species resolution in comparison to the analysis of the structuralgenes (TEF-1α and histone H3). Overall, the results demonstrated that phylogeneticanalysis of XYL combined with other functional genes (URA, PHO, RED and TRI101)clearly distinguished between the Fg clade species far better than the analysis ofstructural genes (TEF-1α and histone H3).

[发布机构] Stellenbosch University