Submitted to: Toxins
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2015
Publication Date: 8/27/2015
Citation: Mellon, J.E. 2015. Extracellular xylanolytic and pectinolytic hydrolase production by A. flavus isolates contributes to crop invasion. Toxins. 7(8):3257-3266. DOI: 10.3390/toxins.7083257.
Interpretive Summary: Aflatoxin is a very potent carcinogen and toxin that is produced by the fungus Aspergillus flavus. When this fungus infects target crops (corn, cotton, peanuts, tree nuts), the developing seed can be contaminated with this toxin, rendering the product unfit for additional agricultural uses. Pectins and xylans are plant polysaccharides that are important structural components of plant cell walls. Cottonseed contains high concentrations of xylans and is particularly susceptible to aflatoxin contamination. The fungus is capable of producing a number of hydrolytic enzymes, including xylanase that is crucial to breaking down xylans, and pectinase that digests pecitns. In the past 20 years, a methodology to control aflatoxin contamination has been developed using A. flavus isolates that do not produce aflatoxin (atoxigenic). Ability to digest xylans and pectins may be important attributes for efficiency as an atoxigenic biocontrol agent. A survey of a collection of atoxigenic A. flavus isolates, including some biocontrol agents, to determine the ability to produce xylanase and pectinase activities was undertaken. Some variability in pectinase production was observed. This research will benefit oilseed breeders, producers and pathologists, and will aid in the formulation of methods to prevent aflatoxin contamination of target crops.
Technical Abstract: Several atoxigenic Aspergillus flavus isolates, including some 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. Most of the tested isolates also produced reasonably high levels of esterase activity, although the NRRL 21882 isolate esterase level was significantly lower than the others. A. flavus isolates 19, 22, K49, AF36 and AF13 produced copious levels of pectinolytic activity when grown on a pectin medium. The pectinolytic activity levels of the 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 P2c, as ascertained by isoelectric focusing electrophoresis. A. flavus isolate 17 did not express detectable levels of pectinase P2c, but did produce moderate levels of pectin methyl esterase (PME). NRRL 21882 isolate displayed low levels of both pectinase P2c and PME. While the capacity for production of both xylanolytic and pectinolytic hydrolases may contribute to the effectiveness of A. flavus biocontrol agents, they are probably not the determinant factors controlling A. flavus biocontrol efficacy.