SummaryLipid peroxidation may be initiated either by lipoxygenases or by reactive oxygen species (ROS). Enzymatic oxidation of a-linolenate can result in the biosynthesis of cyclic oxylipins of the jasmonate type while freeradical-catalyzed oxidation of a-linolenate may yield several classes of cyclic oxylipins termed phytoprostanes in vivo. Previously, we have shown that one of these classes, the E 1 -phytoprostanes (PPE 1 ), occurs ubiquitously in plants. In this work, it is shown that PPE 1 are converted to novel cyclopentenone A 1 -and B 1 -phytoprostanes (PPA 1 and PPB 1 ) in planta. Enhanced formation of PPE 1 , PPA 1 , and PPB 1 is observed after peroxide stress in tobacco cell cultures as well as after infection of tomato plants with a necrotrophic fungus, Botrytis cinerea. PPA 1 and PPB 1 display powerful biologic activities including activation of mitogen-activated protein kinase (MAPK) and induction of glutathione-S-transferase (GST), defense genes, and phytoalexins. Data collected so far infer that enhanced phytoprostane formation is a general consequence of oxidative stress in plants. We propose that phytoprostanes are components of an oxidant-injury-sensing, archaic signaling system that serves to induce several plant defense mechanisms.

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