Nuclear Magnetic Resonance (NMR) Studies on the Biosynthesis of Fusaric Acid from Fusarium oxysporum f. sp vasinfectum

Fusarium oxysporum is a fungal pathogen that attacks many important plants. Uniquely pathogenic strains of F. oxysporum f. sp. vasinfectum were inadvertently imported into the United States on live cottonseed for dairy cattle feed. These strains produce exceptionally high concentrations of the phytotoxin fusaric acid. Thus, fusaric acid may be a critical component in the pathogenicity of these biotypes. This study investigated the biosynthesis of fusaric acid using (13)C-labeled substrates including [1,2- (13)C(2)] acetate as well as (13)C- and (15)N-labeled aspartate and [(15)N]glutamine. The incorporation of labeled substrates is consistent with the biosynthesis of fusaric acid from three acetate units at C5-C6, C7-C8, and C9-C10, with the remaining carbons being derived from aspartate via oxaloacetate and the TCA cycle; the oxaloacetate originates in part by transamination of aspartate, but most of the oxaloacetate is derived by deamination of aspartate to fumarate by aspartase. The nitrogen from glutamine is more readily incorporated into fusaric acid than that from aspartate.