Characterization of a Fusarium graminearum Salicylate Hydroxylase

Abstract

Salicylic acid (SA) plays an important role in regulating plant defense responses against pathogens. However, pathogens have evolved ways to manipulate plant SA-mediated defense signaling. <i>Fusarium graminearum</i> causes Fusarium head blight (FHB) and reduces crop yields and quality by producing various mycotoxins. In this study, we aimed to identify the salicylate hydroxylase in <i>F. graminearum</i> and determine its role in wheat head blight development. We initially identified a gene in <i>F. graminearum</i> strain NRRL 46422 that encodes a putative salicylate hydroxylase (designated <i>FgShyC</i>). However, the <i>FgShyC</i> deletion mutant showed a similar ability to degrade SA as wild-type strain 46422; nor did overexpression of FgShyC in <i>E. coli</i> convert SA to catechol. The results indicate that <i>FgShyC</i> is not involved in SA degradation. Further genome sequence analyses resulted in the identification of eight salicylate hydroxylase candidates. Upon addition of 1 mM SA, FGSG_03657 (designated <i>FgShy1</i>), was induced approximately 400-fold. Heterologous expression of FgShy1 in <i>E. coli</i> converted SA to catechol, confirming that FgShy1 is a salicylate hydroxylase. Deletion mutants of <i>FgShy1</i> were greatly impaired but not completely blocked in SA degradation. Expression analyses of infected tissue showed that <i>FgShy1</i> was induced during infection, but virulence assays revealed that deletion of FgShy1 alone was not sufficient to affect FHB severity. Although the <i>Fgshy1</i> deletion mutant did not reduce pathogenicity, we cannot rule out that additional salicylate hydroxylases are present in <i>F. graminearum</i> and characterization of these enzymes will be necessary to fully understand the role of SA-degradation in FHB pathogenesis.