Transformation of Fusarium oxysporum f. sp cubense, causal agent of Fusarium wilt of banana, with the green fluorescent protein (GFP) gene

Fusarium oxysporum f. sp. cubense (Foc) is the causal agent of Fusarium wilt (Panama disease) of bananas in most tropical and subtropical banana-producing regions of the world. The fungus infects through roots, colonises the rhizomes and eventually blocks the vascular system of the pseudostems, resulting in plant death. The green fluorescent protein (GFP) emits green fluorescence when excited by blue light, making it a useful tool to study early stages of fungal infection. The objective of this study was to transform Foc isolates with the GFP gene. Isolates representing 'subtropical' race 4 of the fungus were transformed with the sGFP derivative using hygromycin as a selectable marker. Efficiency and transformation of spheroplasts depended on mycelium age, the choice of enzymes and the temperature and duration of incubation. The transformed isolates did not differ markedly from the wild type isolates in growth and morphological characteristics in vitro. Fluorescence microscopy showed expression of the green fluorescent protein in fungal structures. The presence of the GFP DNA in the fungal cells was confirmed by PCR using a GFP-specific primer pair and Southern blot analysis. Pathogenicity tests showed that the transformation process did not alter pathogenicity of Foc isolates. Fungal hyphae within tissues of infected plants could be seen to fluoresce and the transformed fungus was re-isolated from artificially inoculated plants. Transformants of Foc will facilitate future infection studies with this pathogen on banana.