Submitted to: Journal of Applied Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/1/2004
Publication Date: 10/1/2004
Citation: Chang, P.-K., Wilson, R.A., Keller, N.P., Cleveland, T.E. 2004. Deletion of the delta 12-oleic acid desaturase gene of a non-aflatoxigenic Aspergillus parasiticus field isolate affects conidiation and sclerotial development. Journal of Applied Microbiology. 97:1178-1184. Interpretive Summary: Oil-rich crops such as corn, peanuts, cottonseed, and tree nuts are often infested before harvest by Aspergillus flavus and Aspergillus parasiticus. These fungi produce cancer-causing aflatoxins, which are strictly regulated for both animal feed and food destined for human consumption due to health concerns. Limited uses of contaminated agricultural commodities greatly reduce the profitability of farmers. One method for the reduction of aflatoxin contamination is through field application of atoxigenic Aspergillus strains to displace toxigenic strains. The success of the biocontrol method lies in the fitness of applied biopesticides in the field. Fungal fitness can be defined in terms of its ability to reproduce quickly, to disseminate widely, to colonize the host plants efficiently, and to survive harsh environments, such as heat, cold, arid soil conditions, and drastic temperature changes. In this study, we investigated how fatty acid metabolism affects the production and development of fungal propagules, conidia, and overwintering sclerotia. We showed that linoleic acid is a major determinant for the development of these propagules. Conditions that promote sclerotial development differ from those required to promote maximum conidia. Manipulation of content and availability of linoleic acid at different fungal growth phases might optimize conidial and sclerotial production hence increasing the efficacy of biocontrol Aspergillus species. Reduction of carcinogenic aflatoxins in agricultural commodities can greatly increase the profitability of farmers.
Technical Abstract: Aims: To investigate how linoleic acid affects conidial production and sclerotial development in a strictly mitotic Aspergillus parasiticus field isolate as related to improving biocompetitivity of atoxigenic Aspergillus species. Methods and Results: We disrupted A. parasiticus delta 12-oleic acid desaturase gene (odeA) responsible for the conversion of oleic acid to linoleic acid. We examined conidiation and sclerotial development of SRRC 2043 and three isogenic delta odeA mutants, either with or without supplementing linoleic acid, on one complex potato dextrose agar (PDA) medium and on two defined media: nitrate-containing Czapek agar (CZ) and Cove's ammonium medium (CVN). The delta odeA mutants produced much less conidia than the parental strain on all media. Linoleic acid supplementation (as sodium linoleate at 1- and 4 mM) restored the delta odeA conidial production comparable to or exceeding the parental level, and the effect was medium-dependent, with the highest increase on CVN and the least on PDA. SRRC 2043 and the delta odeA mutants were unable to produce sclerotia on CVN. On unsupplemented PDA and CZ, delta odeA sclerotial mass was comparable to that of SRRC 2043, but sclerotial number in odeA mutants increased significantly to 2- to 3-fold relative to SRRC 2043. Supplementing linoleic acid to media, in general, tended to decrease delta odeA sclerotial mass and sclerotial number, although some variations were observed. Conclusions: Linoleic acid stimulates conidial production, but it has an inhibitory effect on sclerotial development. The relationship between the two processes in A. parasiticus is complex and affected by multiple factors, such as fatty acid composition and nitrogen sources. Significant and Impact of Study: Conditions that promote sclerotial development differ from those required to promote maximum conidia. Manipulation of content and availability of linoleic acid at different fungal growth phases might optimize conidial and sclerotial production hence increasing the efficacy of biocompetitive Aspergillus species formulations.