ABSTRACT

Several lines of accumulated evidence show that oxidized derivatives of lipids are involved in plant defense mechanisms, functioning as toxic compounds effective against pathogens, or as signaling molecules (jasmonate and its derivatives) for activation of defense genes. Such oxidized lipids (oxylipins) are synthesized via the lipoxygenase pathway (or the oxylipin pathway) (Fig. 1). Lipoxygenases (linoleate: oxygen oxidoreductase, E.C. 1.13.11.12) initiate the peroxidation of fatty acids that contain a (Z,Z)-1,4-pentadiene moiety. In higher plants, linolenic acid and linoleic acid, which are common in plasma membrane phospholipids and thylakoid membrane glycolipids, are substrates for lipoxygenases. Fatty acid hydroperoxides generated by lipoxygenation are substrates for lyase, peroxygenase, or allene oxide synthase. These metabolites are further transformed into various oxylipins as shown in Figure 1. Various oxylipins directly inhibit the growth of pathogens when exogenously applied. This suggests defensive roles of these oxylipins, such as hydroperoxide lyase products described in Chapter 9. Roles of the peroxygenase cascade, which leads to products relevant to plant defense mechanisms, are described in Chapter 12. Among the oxylipins, jasmonate and methyl jasmonate have been investigated mostly in terms of their role in plant defense, especially against insect attack or wounding,

as described in Chapter 11. This chapter deals with plant defense mechanisms against pathogens, as related to lipoxygenase function.