Fusaric acid, a Fusarium verticillioides metabolite, antagonistic to the endophytic biocontrol bacterium Bacillus mojavensis

An endophytic bacterium, Bacillus mojavensis Roberts, Nakamura & Cohan, was patented as a nonpathogenic biocontrol for plant diseases. However, before this bacterium can be used as a biocontrol agent, it must be evaluated against homologous competing organisms, some of which are equally successful endophytes, such as species of Fusarium that are symptomless endophytes, especially on maize. Preliminary field trials using this bacterium as a biocontrol agent against production of the fumonisin mycotoxins caused by infection of maize with Fusarium verticillioides (Sacc.) Nirenberg (= Fusarium moniliforme Sheldon) was less than that observed with greenhouse studies. Fusarium verticillioides and other species produce fusaric acid. Fusaric acid at concentrations as low as 22 μmol/L accounted for a 41% reduction in CFU compared with the control group, while concentrations of 223 μmol/L and higher resulted in total toxicity to the bacterium. Mutants of F. verticillioides that produced low concentrations of fusaric acid did not affect the endophytic CFU of the bacterium in seedlings. These results suggest that fusaric acid accounted for the reduction of bacterial colonization and the resulting poor biocontrol activity and suggested its importance to the fungus is as an antibiotic, which assists in the in planta competition for the intercellular niche colonized by F. verticillioides during its endotrophic state.