Biosynthesis of phytooxylipins: the Peroxygenase pathway

Although plants lack the intricate defense mechanisms provided by the immune system in mammals, plant cells are capable of resisting against attack by constitutive defenses and induced responses. which tend to limit the invasion but also to weaken and/or even destroy the aggressor. During the past several years, increasing interest in phytooxylipins (oxygenated fatty acids) has been generated as these metabolites are considered to be involved in such plant responses. Such phytooxylipins derive mainly from C18 unsaturated fatty acids through the so-called oxylipin or lipoxygenase pathway. Lipoxygenases catalyze oxidation of unsaturated C18 fatty acids into either 9- or 13-hydroperoxyoctadecadi(tri)enoic acids, or a mixture of both. depending on the source of the enzymes: these highly reactive aliphatic molecules are then rapidly metabolized by plant cells into a variety of physiologically active derivatives. Two well characterized enzymes have been shown to cut or dehydrate the carbon skeleton. respectively, a lyase and an allene oxide synthase. We have established a new fate for fatty acid hydroperoxides: the peroxygenase pathway, which involves two enzymes i) a peroxygenase, which catalyzes an intramolecular transfer of oxygen from hydroperoxides yielding epoxyalcohols, and (or) intermolecular oxygen transfer (cooxidation reactions) resulting for example in the epoxidation of double bonds of unsaturated fatty acids, ii) an epoxide hydrolase, which hydrates preferentially the epoxides formed by the peroxygenase. The products resulting from these latter reactions are also involved in the response of the plant to aggression, since they are cutin monomers (cutin is the framework of cuticle), or natural pesticides, called phytoalexins.