Insights into the Pathogenic Role of Fusaric Acid in Fusarium oxysporum Infection of Brassica oleracea through the Comparative Transcriptomic, Chemical, and Genetic Analyses

Fusarium, a genus of fungi renowned for its plant-pathogenic
capabilities,
is capable of producing a myriad of structurally diverse secondary
metabolites, among which are phytotoxins that play a significant role
in the etiology of plant diseases. The particular strain Fusarium oxysporum f. sp. conglutinans (FOC), known as the instigator of Fusarium wilt in cabbage (Brassica oleracea), has been found to secrete an array of toxins and the identities
of which have largely remained elusive. In this study, we evaluated
the phytotoxicity of crude extracts from the pathogenic FOC strain
(FOCr1) and the nonpathogenic F. oxysporum strain (FOcs20) using the cabbage seed phytotoxicity bioassays.
Results showed that the crude extract of FOCr1 significantly inhibited
seed germination and seedling elongation. Comparative transcriptome
analysis and quantitative real-time PCR (qPCR) revealed higher expression
levels of a mycotoxin fusaric acid (FA) biosynthetic gene cluster
in FOCr1 under host-like conditions (cabbage medium). High-performance
liquid chromatography mass spectrometry (HPLC-MS) analysis detected
a higher yield FA in the crude extract of FOCr1 but is absent in the
FOcs20 strain. Deleting the key gene FUB8 in FOCr1’s
FA biosynthetic gene cluster delayed wilt symptoms. Moreover, FA treatment
was correlated with an uptick in H2O2 levels
within seedlings, underscoring its potential as a virulence amplifier.
These results suggest that FA acts as a positive virulence factor
in FOC.