The Role of Mycotoxins in Interactions between Fusarium graminearum and F. verticillioides Growing in Saprophytic Cultures and Co-Infecting Maize Plants

Abstract

Fusarium graminearum (FG) and Fusarium verticillioides (FV) co-occur in infected plants and plant residues. In maize ears, the growth of FV is stimulated while FG is suppressed. To elucidate the role of mycotoxins in these effects, we used FG mutants with disrupted synthesis of nivalenol (NIV) and deoxynivalenol (DON) and a FV mutant with disrupted synthesis of fumonisins to monitor fungal growth in mixed cultures in vitro and in co-infected plants by real-time PCR. In autoclaved grains as well as in maize ears, the growth of FV was stimulated by FG regardless of the production of DON or NIV by the latter, whereas the growth of FG was suppressed. In autoclaved grains, fumonisin-producing FV suppressed FG more strongly than a fumonisin-nonproducing strain, indicating that fumonisins act as interference competition agents. In co-infected maize ears, FG suppression was independent of fumonisin production by FV, likely due to heterogeneous infection and a lower level of fumonisins in planta. We conclude that (i) fumonisins are agents of interference competition of FV, and (ii) trichothecenes play no role in the interaction between FG and FV. We hypothesize the following: (i) In vitro, FG stimulates the FV growth by secreting hydrolases that mobilize nutrients. In planta, suppression of plant defense by FG may additionally play a role. (ii) The biological function of fumonisin production in planta is to protect kernels shed on the ground by accumulating protective metabolites before competitors become established. Therefore, to decipher the biological function of mycotoxins, the entire life history of mycotoxin producers must be considered.

Mycoviruses Increase the Attractiveness of Fusarium graminearum for Fungivores and Suppress Production of the Mycotoxin Deoxynivalenol

“…The strains of Fusarium graminearum used are listed in Table 3 . F. graminearum 266 MV, deposited in the DSMZ culture collection (Braunschweig, Germany) as F. graminearum DSM 116490, was isolated in 2018 from a naturally infected maize ear grown in Pocking (Bavaria, Germany), with a disease rating of 5% according to a modified symptom scale [ 60 ], as previously described [ 61 ]. F. graminearum 668 (DSM 113709), kindly provided by Wilhelm Schäfer (University of Hamburg, Hamburg, Germany), is a zearalenone-deficient mutant (△ZEN) generated from wildtype strain F. graminearum 1003 by disrupting the polyketide synthase gene PKS4 [ 62 ].…”

“…Fungal biomass in grains was determined using quantitative PCR (qPCR), as described [ 67 ], using two replicates of densely spaced standards (see Figure A2 in [ 60 ]). The biomass of fungal colonies growing on agar was determined directly, as described in Section 4.7 .…”

Mechanism of Fumonisin Self-Resistance: Fusarium verticillioides Contains Four Fumonisin B1-Insensitive-Ceramide Synthases

“…F. graminearum and F. verticillioides can co-occur and compete in infected maize ears. In a recent study [ 28 ], no significant difference between wild-type and fum1 mutants in disease severity or amount of fungal DNA in the inoculated maize line was found. Yet, it was demonstrated that wild-type F. verticillioides could suppress the growth of F. graminearum in a co-culture on autoclaved kernels more strongly than a fumonisin-nonproducing strain.…”