Fusarium graminearum gene deletion mutants map1 and tri5 reveal similarities and differences in the pathogenicity requirements to cause disease on Arabidopsis and wheat floral tissue

The Ascomycete pathogen Fusarium graminearum can infect all cereal species and lower grain yield, quality and safety. The fungus can also cause disease on Arabidopsis thaliana. In this study, the disease-causing ability of two F. graminearum mutants was analysed to further explore the parallels between the wheat (Triticum aestivum) and Arabidopsis floral pathosystems. Wild-type F. graminearum (strain PH-1) and two isogenic transformants lacking either the mitogen-activated protein kinase MAP1 gene or the trichodiene synthase TRI5 gene were individually spray- or point-inoculated onto Arabidopsis and wheat floral tissue. Disease development was quantitatively assessed both macroscopically and microscopically and deoxynivalenol (DON) mycotoxin concentrations determined by enzyme-linked immunosorbent assay (ELISA). Wild-type strain inoculations caused high levels of disease in both plant species and significant DON production. The map1 mutant caused minimal disease and DON accumulation in both hosts. The tri5 mutant, which is unable to produce DON, exhibited reduced pathogenicity on wheat ears, causing only discrete eye-shaped lesions on spikelets which failed to infect the rachis. By contrast, the tri5 mutant retained full pathogenicity on Arabidopsis floral tissue. This study reveals that DON mycotoxin production is not required for F. graminearum to colonize Arabidopsis floral tissue.