Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2011
Publication Date: 6/1/2011
Publication URL: http://naldc.nal.usda.gov/download/55000/PDF
Citation: Bacon, C.W., Hinton, D.M. 2011. In planta reduction of maize seedling stalk lesions by the bacterial endophyte Bacillus mojavensis. Canadian Journal of Microbiology. 57:485-492. Interpretive Summary: The fungus Fusarium verticillioides infects corn internally as an endophyte where it produces the fumonisin toxins that are toxic to livestock, poultry and humans. Since this fungus resides internally in corn, we proposed a biocontrol utilizing the bacterium Bacillus mojavensis that is also an endophyte. Laboratory data indicates that this bacterium inhibits the growth and production of the fumonisin toxins but field data suggests that there is a negative interaction allowing the fungus to grow. Prior work by us identified fusaric acid produced by this fungus that upon analysis indicated that it was toxic to the bacterium. The question answered in this work is based on the question does fusaric acid inhibits the growth and inhibitory substance produced by the bacterium allowing the fungus to control the bacterium within the plant? Using an in vitro test based on corn seedlings infected by wild type and fusaric acidless mutants, we demonstrated that this test can be used to detect virulence by the fungus as evidenced by necrotic tissue in the seedlings. However, this test using a combination of the bacterium and the fusaric acidless mutants of the fungus also suggests that fusaric acid is not involved in the necrotic lesions observed in the seedling. Thus, other toxins are involved, suggesting that the fumonisins are the toxic substance to the corn, which re-enforces our earlier work suggesting that the fumonisins are toxic to corn seedlings.
Technical Abstract: Due to autoinfection and alloinfection, maize is susceptible to infection by Fusarium verticillioides resulting in diseases and contamination of maize kernels with the fumonisin mycotoxins. Attempts at controlling this fungus are currently being done with biocontrol agents such as bacteria and this includes bacterial endophytes such as Bacillus mojavensis. However, the fungus also produces fusaric acid that acts both as a phytotoxin and as an antibiotic as well as fumonisins that are phytotoxic and mycotoxic. The question now is can B. mojavensis reduce lesion development during the alloinfection process in maize, simulated by internode injection of the fungus? Strains of fusaric acid tolerant mutants of B. mojavensis were used in a growth room study to determine the development of stalk lesions indicative of maize seedling blight by co-inoculations with wild type and non-fusaric acid producing mutants of F. verticillioides. Lesions were measured on 14-day-old maize stalks consisting of treatment groups inoculated with and without mutants and wild type strains of bacteria and fungi. The results indicate that the fusaric acid tolerant B. mojavensis mutant reduced stalk lesions, suggesting an in planta role for this substance as an antibiotic. Further, lesion development occurred in maize infected with F. verticillioides mutants that are non-fusaric acid producers, indicating a role for other phytotoxins such as the fumonisins. Thus, additional pathological components should be examined before strains of B. mojavensis can be identified as being effective as a biocontrol agent, particularly for the control seedling disease of maize.