A Role for Topoisomerase I in Fusarium graminearum and F. culmorum Pathogenesis and Sporulation

Fusarium graminearum and F. culmorum are the causal agents of Fusarium ear blight (FEB) in wheat. A forward genetics approach was taken to discover novel pathogenicity genes in the genome of E graminearum. A library of transformants created by random plasmid insertional mutagenesis was screened on wheat ears for virulence defects. Plasmid rescue on one of the reduced-virulence mutants revealed a single-copy plasmid insertion in the gene coding for the DNA interacting enzyme, topoisomerase I. Targeted topoisomerase I gene-deletion mutants were created in strains of both F. graminearum and F. culmorum. The topI mutants of both species exhibited greatly reduced virulence in wheat ear infection assays (GO:0009405 and GO:0044145). Detailed microscopy analyses revealed that topI hyphal growth was restricted to palea tissue whereas host responses were discernable 1,000 pm further away in the rachis node. Asexual sporulation was reduced in the E graminearum mutants and was absent from the F. culmorum mutants. The E graminearum mutant did not develop sexual spores when subjected to an in vitro perithecia production assay. During in vitro growth, the topI mutants of both species were still able to produce the trichothecene mycotoxin, deoxynivalenol.