Generation of the volatile spiroketals conophthorin and chalcogran by fungal spores on polyunsaturated fatty acids common to almonds and pistachios

Authors: Beck, John; Mahoney, Noreen; Gee, Wai; Cook, Daniel

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2012
Publication Date: 11/15/2012
Citation: Beck, J.J., Mahoney, N.E., Gee, W.S., Cook, D. 2012. Generation of the volatile spiroketals conophthorin and chalcogran by fungal spores on polyunsaturated fatty acids common to almonds and pistachios. Journal of Agricultural and Food Chemistry. 60(48):11869-11876.

Interpretive Summary: The compound (E)-conophthorin has recently been reported as an odor used in chemical communication for the navel orangeworm moth, a major insect pest of California pistachios and almonds. Conophthorin is a spiroketal, a class of compounds characterized by two rings each containing an oxygen that are joined to one carbon and forming a bicyclic structure. Conophthorin and the isomeric spiroketal chalcogran are most commonly known as odors that serve as a form of chemical communication (semiochemicals) for several scolytid beetles. Conophthorin is both an insect- and plant-produced semiochemical widely recognized as a non-host plant volatile from the bark of several flowering plant (angiosperm) species. Chalcogran is the principal odor component that signals aggregation of the six-spined spruce bark beetle. Recent research has shown conophthorin is produced by almonds undergoing hull-split and both spiroketals are produced by mechanically damaged almonds. To better understand the origin of these spiroketals the odor emission profiles of orchard fungal spores grown on fatty acids common to both pistachios and almonds were evaluated. The volatile emission for the first 14 days of germinating spores was monitored. The spores investigated were Aspergillus flavus (toxigenic), A. flavus (atoxigenic), A. niger, A. parasiticus, Penicillium glabrum, and Rhizopus stolonifer. The fatty acids used as growth media were palmitic, oleic, linoleic, and linolenic. Spores grown on linoleic acid produced both spiroketals, on linolenic acid produced only chalcogran, and on palmitic and oleic acid did not produce either spiroketal. A total of 62 odor components were detected from the range of spores grown on the reported fatty acids. This is the first report of the spiroketals conophthorin and chalcogran from a microbial source.

Technical Abstract: The spiroketal (E)-conophthorin has recently been reported as a semiochemical of the navel orangeworm moth, a major insect pest of California pistachios and almonds. Conophthorin and the isomeric spiroketal chalcogran are most commonly known as semiochemicals of several scolytid beetles. Conophthorin is both an insect- and plant-produced semiochemical widely recognized as a non-host plant volatile from the bark of several angiosperm species. Chalcogran is the principal aggregation pheromone component of the six-spined spruce bark beetle. Recent research has shown conophthorin is produced by almonds undergoing hull-split and both spiroketals are produced by mechanically damaged almonds. To better understand the origin of these spiroketals the volatile emissions of orchard fungal spores on fatty acids common to both pistachios and almonds were evaluated. The volatile emission for the first 13 days of spores placed on a fatty acid was monitored. The spores investigated were Aspergillus flavus (atoxigenic), A. flavus (toxigenic), A. niger, A. parasiticus, Penicillium glabrum, and Rhizopus stolonifer. The fatty acids used as growth media were palmitic, oleic, linoleic, and linolenic. Spores on linoleic acid produced both spiroketals, on linolenic acid produced only chalcogran, and on palmitic and oleic acid did not produce either spiroketal. This is the first report of the spiroketals conophthorin and chalcogran from a fungal source.