Fusaric acid is a virulence factor of Fusarium oxysporum f. sp. cubense on banana plantlets

Fusaric acid (FA) is highly phytotoxic and causes severe Fusarium oxysporum-induced vascular wilt. However, the molecular mechanisms of FA production in Fusarium oxysporum f. sp. cubense (FOC) are largely unknown. In this study, FoFUB4, a F. verticillioides FUB4 homolog was cloned from banana pathogenic fungus FOC and its deletion mutants were generated using homologous recombination. Our null mutation results showed that FoFUB4 did not affect fungal development and conidiation, but was essential for virulence on banana plantlets compared with wild type (WT) strain. Biochemical analysis showed that production of FA was not detected in mutant ΔFoFUB4 and FA production by WT was significantly reduced in Czapek Dox medium containing 6 compared with 120 mM NaNO3. Moreover, transcriptional analysis indicated that the expression levels of FoFUB4 were reduced in WT in Czapek Dox medium containing 6 compared with 120 mM NaNO3. Taken together, these results provide direct evidence for the vital roles of FA in virulence, and that nitrogen source regulated FA biosynthesis in FOC.