Foisc1 regulates growth, conidiation, sensitivity to salicylic acid, and pathogenicity of Fusarium oxysporum f. sp. cubense tropical race 4

The secreted isochorismatases derived from certain filamentous pathogens play vital roles in the infection of host plants by lowering salicylic acid (SA) levels and suppressing SA-mediated defense pathway. However, it remains unclear whether the fungus Fusarium oxysporum f. sp. cubense tropical race 4 (FocTR4), which causes vascular wilt in bananas, utilizes isochorismatases to modulate SA levels in the host and subvert the banana defense system for successful infection. In the current study, we selected and functionally characterized the foisc1 gene, one of 10 putative isochorismatase-encoding genes in FocTR4 that showed significant upregulation during early stages of infection. Deletion of foisc1 resulted in enhanced vegetative growth and conidiation, increased sensitivity to SA, reduced colonization within host plants, as well as impaired pathogenicity. Conversely, complementation restored phenotypes similar to those observed in the wild-type strain. Furthermore, deletion of foisc1 led to a notable rise in activities of defense-related enzymes such as catalase, peroxidase, and phenylalnine ammonialyase; along with an upregulated expression of several defense-related genes including PR genes and NPR1 genes within hosts' tissues. The non-secretory nature of Foisc1 protein was confirmed and its absence did not affect SA levels within host plants. Transcriptome analysis revealed that deletion of foisc1 resulted in decreased expression levels for numerous genes associated with pathogenicity including those involved in fusaric acid biosynthesis and effector genes as well as a catechol 1,2-dioxygenase gene essential for SA degradation; while increasing expression levels for numerous genes associated with hyphal growth and conidiation were observed instead. Therefore, our findings suggest that Foisc1 may influence hyphal growth, conidiation, sensitivity to SA, and pathogenicity of FocTR4 through modulation of various genes implicated in these processes. These findings provide valuable insights into the pathogenesis of FocTR4, and create a groundwork for the future development of innovative control strategies targeting vascular wilt disease of banana.