13(S)-HpOTrE is a monohydroperoxy polyunsaturated fatty acid produced in soybeans by the action of soybean LO-2 on esterified α-linolenic acid.[1] Incubation of soybean seedling biomembranes with soybean LO-2 catalyzes the formation of both 9- and 13-HpOTrE in a molar ratio of 10:1.1 In plants, 13(S)-HpOTrE can be metabolized by the hydroperoxide lyase pathway producing aldehyde and oxoacid fragments, or by the hydroperoxide dehydratase pathway producing jasmonic acid.[2],[3],[4] Treatment of tomato leaves with 13-HpOTrE causes induction of proteinase inhibitors, simulating the normal response to wounding.5 This data suggests that in plants 13(S)-HpOTrE may participate in a lipid-based signalling system initiated by insect and pathogen attack.