Abstract
Several atoxigenic Aspergillus flavus isolates, including some being used as biocontrol agents, and one toxigenic isolate were surveyed for the ability to produce extracellular xylanolytic and pectinolytic hydrolases. All of the tested isolates displayed good production of endoxylanases when grown on a medium utilizing larch xylan as a sole carbon substrate. Four of the tested isolates produced reasonably high levels of esterase activity, while the atoxigenic biocontrol agent NRRL 21882 isolate esterase level was significantly lower than the others. Atoxigenic A. flavus isolates 19, 22, K49, AF36 (the latter two are biocontrol agents) and toxigenic AF13 produced copious levels of pectinolytic activity when grown on a pectin medium. The pectinolytic activity levels of the atoxigenic A. flavus 17 and NRRL 21882 isolates were significantly lower than the other tested isolates. In addition, A. flavus isolates that displayed high levels of pectinolytic activity in the plate assay produced high levels of endopolygalacturonase (pectinase) P2c, as ascertained by isoelectric focusing electrophoresis. Isolate NRRL 21882 displayed low levels of both pectinase P2c and pectin methyl esterase. A. flavus appears capable of producing these hydrolytic enzymes irrespective of aflatoxin production. This ability of atoxigenic isolates to produce xylanolytic and pectinolytic hydrolases mimics that of toxigenic isolates and, therefore, contributes to the ability of atoxigenic isolates to occupy the same niche as A. flavus toxigenic isolates.
Highlights
Aspergillus flavus is a ubiquitous saprophytic fungus commonly found in tropical and semitropical climes [1]
Biomass production for the tested A. flavus isolates fell within the expected range for fungal growth utilizing xylan as a sole carbon substrate
There were no significant differences observed for biomass production among the atoxigenic isolates and the toxigenic A. flavus isolate (AF13) (Table 1), though the K49 isolate biomass yield appeared to be somewhat low
Summary
Aspergillus flavus is a ubiquitous saprophytic fungus commonly found in tropical and semitropical climes [1]. This fungus is capable of opportunistic pathogenesis of oilseed commodities (maize, cotton, peanuts, tree nuts). One of the major thrusts for control of aflatoxin contamination has evolved from the use of atoxigenic isolates of A. flavus. Stated, this control strategy employs the introduction of an atoxigenic isolate, which is thought to competitively exclude the proliferation of toxigenic isolates, thereby reducing the levels of aflatoxin contamination. A. flavus K49, isolated from maize, has been registered for aflatoxin biocontrol use [6]
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