Salicylate degradation by the phytopathogen Fusarium graminearum.

Salicylic Acid (SA) is a major signalling hormone of the plant defence. Conversely, the cereal crop phytopathogen Fusarium graminearum (Fg) is able to utilize SA as a sole source of carbon, and degrade it through the -ketoadipate pathway. In this study, we selected four fungal SA responsive genes that are predicted to encode SA degrading enzymes and used a "loss of function" approach to assess them. We generated Fg-deletant () strains of each enzyme using a gene replacement method. Two of the selected genes shared a similar gene annotation and are predicted to encode for a Salicylate-1 monooxygenase (FGSG_03657 and FGSG_09063). The two other genes were a Catechol 1-2 dioxygenase (FGSG_03667) and a 2,3dihydroxybenzoic acid decarboxylase (FGSG_09061). Three isolates of each Fg-deletant strains were assayed for their catalytic activity. Thereafter, we emphasized on the FGSG_03667 strains and monitored the gene expression profiles in an in vitro and in planta growth assays. FGSG_03657 and FGSG_03667 were identified as the first two key steps of the classical SA degradation pathway. We also demonstrated by RT-qPCR analyses that expression of both genes was substrate dependent, and was regulated by feedback inhibition. Neither of those two genes, when disrupted, was shown to attenuate fungal virulence in the plant disease assay. Additional analyses revealed that FGSG_09061, predicted to catalyse a potential non-oxidative decarboxylation, also contributed to the SA biodegradation. These findings will bring new basis for future research on the SA catabolism in this important pathogenic fungus.