A Polyketide Synthase Gene and An Aspartate Kinase Like Gene Are Required for the Biosynthesis of Fusaric Acid In Fusarium oxysporum F. Sp. Vasinfectum

A genetically unique strain of the Fusarium wilt pathogen was first recognized in wilted and dead Upland cotton seedlings in Australia in 1993.  Since that time the disease spread rapidly despite stringent containment practices.   The Australian biotypes isolates of Fusarium oxysporum f. sp. vasinfectum (Fov) produced copious amount of fusaric acid in potato dextrose cultures and their virulence was directly correlated to amounts of fusaric acid. Though fusaric acid has also long been implicated in the pathogenesis of Fusarium wilt for a number other plant species including tomato, watermelon, and flax, direct evidence for the role of fusaric acid in the virulence is still lacking as the biosynthetic enzymes and genes have not been identified.  We set out to identify and clone the PKS gene involved in the fusaric acid biosynthesis through differential display. We identified a partial clone whose expression matched expected profile of the fusaric acid PKS gene. Using homology based strategy, we cloned a gene cluster containing a PKS gene and an amino acid kinase gene and the corresponding cDNAs. Targeted gene disruption of either of these genes in the Australian biotype Fov isolate resulted in complete blockage of fusaric acid production. These fusaric acid knockout mutants showed much weaker pathogenicity than their wild type progenitor toward tomato seedling in the seedling germination bioassay on agar plates.  These results indicate that fusaric acid play an important role in the pathogenicity of the Australian biotype isolates.