Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2001
Publication Date: 7/1/2001
Interpretive Summary: Corn plants contain substances that are inhibitory to most fungi, bacteria, and insects. The chemical class that these substances belong to is called the benzoxazinoids. Corn plants are parasitized by a toxic fungus Fusarium moniliforme that grows inside the plants. The fungus can cause toxicity problems in pigs, poultry, cattle, and horses. If indeed the fungus grows inside the plant, why is it not inhibited by this class of compounds? Scientists within the Unit established that this fungus has the ability to degrade this class of compounds. We now show that nine other species of Fusarium have high tolerance to the benzoxazinoids. Based on the physiology of these fungi, a new medium for the semiselective isolation of tolerant fungi was developed as well as a new and novel technique to assay for tolerance to this class of compounds. The new assay technique is simple, rapid, and requires no costly hazardous extracting solvents. As a result of these discoveries, toxic Fusarium fungi can be isolated from field samples in much higher frequencies without interferences from the numerous other field fungi.
Technical Abstract: The preformed antimicrobial compounds produced by maize, DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one) and its desmethoxy derivative DIBOA, are highly reactive benzoxazinoids that quickly degrade to the antimicrobials MBOA (6-methoxy-2- benzoxazolinone) and BOA (2-benzoxazolinone), respectively. Fusarium verticillioides (syn., F. moniliforme) is highly tolerant of MBOA and BOA due to an ability to actively transform these compounds into non-toxic metabolites. We assessed tolerance to MBOA and BOA across a large selection of Fusarium species. Eleven of 29 species had some level of tolerance; the most tolerant, in decreasing order, were F. verticillioides, F. cerealis, F. subglutinans, and F. graminearum. A novel TLC assay indicated the differential tolerance among species was due to their ability or inability to detoxify the antimicrobials. The limited number of species having tolerance suggested the potential utility of these compounds as biologically active agents for inclusion within a semi-selective isolation medium. By replacing the PCNB in Nash & Snyder's medium with BOA, we developed a medium that resulted in superior isolation frequencies for F. verticillioides from corn while effectively suppressing competing fungi. The BOA medium should prove useful for surveys of F. verticillioides infection in field samples because previous isolation methods often result in low isolation frequencies due to numerous competing fungi.