Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2008
Publication Date: 8/30/2008
Citation: Wicklow, D.T., Poling, S.M., Summerbell, R. 2008. Occurrence of Pyrrocidine and Dihydroresorcyclide Production among Acremonium zeae Populations from Maize Grown in Different Regions. Canadian Journal of Plant Pathology. 30(3):425-433. Interpretive Summary: Acremonium zeae is a type of fungus called an endophyte that is commonly isolated from healthy corn plants or corn kernels showing no visible symptoms of disease. This fungus produces pyrrocidine antibiotics which exhibit potent activity against stalk and ear rot pathogens of maize that cause substantial annual losses to the U.S. corn crop. To determine if there are regional differences in the occurrence and amounts of pyrrocidines produced by A. zeae populations from maize, we examined nine A. zeae populations from the Midwestern corn belt (e.g., Illinois, Indiana, Iowa, Kentucky, Michigan, and Nebraska) and four populations from maize grown in regions where maize is vulnerable to pathogen attack following the damaging effects of drought and temperature stress (e.g., Arizona, California, and Texas). Nearly all (98%) of the 52 A. zeae isolates from regions associated with drought and temperature stress produced pyrrocidines. In contrast, only one of the 102 A. zeae isolates from within the Midwestern corn belt produced pyrrocidines. These results invite further studies to reveal those environmental conditions under which this protective endophyte becomes the dominant fungus in corn, as well as the potential use of A. zeae as a biocontrol agent to eliminate aflatoxin and fumonisin contamination in corn at harvest. The corn seed industry will benefit from the knowledge that A. zeae could represent a confounding variable impacting the results of resistance trials for aflatoxin/fumonisin as well as other fungal or bacterial pathogens.
Technical Abstract: Acremonium zeae is recognized as a protective endophyte of maize. It is thus a potential confounding variable in maize variety trials for resistance to pathogenic microbes and their mycotoxins. This fungus grows systemically in maize and produces pyrrocidines A and B, polyketide amino acid derived antibiotics exhibiting potent in vitro activity against major stalk and ear rot pathogens of maize. An evaluation of a number of A. zeae isolates accessioned by the ARS (NRRL) and CBS Culture Collections from 1969-1992 revealed that pyrrocidines were produced by 5/11 isolates from maize grown in regions with milder climates (e.g. Illinois, Nebraska, and Germany) and 10/12 isolates from maize grown in warmer regions (e.g., N. Carolina, Georgia, USA, Iran, and India). In regions where maize is particularly vulnerable to pathogen attack following the damaging effects of drought and temperature stress, selection may favor A. zeae endophytes that produce pyrrocidines as acquired chemical defenses. Cultures of A. zeae, representing 13 populations isolated from maize seeds harvested from different regions and years, were grown on potato dextrose agar in petri dishes. Excised portions of the developing colony and underlying agar (3g) were extracted with 7 ml acetonitrile. After filtering, 10-µl portions of the extracts were analyzed by liquid chromatography atmospheric pressure chemical ionization mass spectrometry (LC-APCI-MS). Among 52 isolates representing four A. zeae populations from regions associated with drought and temperature stress (e.g., Arizona, California, and Texas), 49 isolates (94%) produced pyrrocidine A in amounts ranging from 1 to 248 ng (29 ave.)/10µl acetonitrile, and 51 isolates (98%) produced pyrrocidine B in amounts ranging from 2 to 440 ng/10µl (average 137). In contrast, among 102 isolates representing nine A. zeae populations from the Midwestern corn belt (e.g., Illinois, Indiana, Iowa, Kentucky, Michigan, and Nebraska), only one isolate produced pyrrocidine A (280 ng/10ul) and pyrrocidine B (411/ng/10µl). During the population study, a new secondary metabolite of A. zeae, dihydroresorcylide, was discovered, and its distribution among the population is described. Clonal populations of A. zeae endophytes differing in their ability to produce pyrrocidines may be distributed with the seed of maize cultivars grown in commercial plantings.