In biosynthesis of octadecanoids and jasmonate (JA), the naturally occurring enantiomer is established in a step catalysed by the gene cloned recently from tomato as a single-copy gene (Ziegler et al., 2000). Based on sequence homology, four full-length cDNAs were isolated from Arabidopsis thaliana ecotype Columbia coding for proteins with AOC activity. The expression of AOCgenes was transiently and differentially up-regulated upon wounding both locally and systemically and was induced by JA treatment. In contrast, AOC protein appeared at constitutively high basal levels and was slightly increased by the treatments. Immunohistochemical analyses revealed abundant occurrence of AOC protein as well as of the preceding enzymes in octadecanoid biosynthesis, lipoxygenase (LOX) and allene oxide synthase (AOS), in fully developed tissues, but much less so in 7-day old leaf tissues. Metabolic profiling data of free and esterified polyunsaturated fatty acids and lipid peroxidation products including JA and octadecanoids in wild-type leaves and the jasmonate-deficient mutant OPDA reductase 3 (opr3) revealed preferential activity of the AOS branch within the LOX pathway. 13-LOX products occurred predominantly as esterified derivatives, and all 13-hydroperoxy derivatives were below the detection limits. There was a constitutive high level of free 12-oxo-phytodienoic acid (OPDA) in untreated wild-type and opr3 leaves, but an undetectable expression of AOC. Upon wounding opr3 leaves exhibited only low expression of AOC, wounded wild-type leaves, however, accumulated JA and AOC mRNA. These and further data suggest regulation of JA biosynthesis by OPDA compartmentalization and a positive feedback by JA during leaf development.

Hypersensitive cell death is an important defense reaction of plants to pathogen infection and is accompanied by lipid peroxidation processes. These may occur non-enzymatically by the action of reactive oxygen species or may be catalyzed by enzymes such as α-dioxygenases, lipoxygenases, or peroxidases. Correlative data showing increases in 9-lipoxygenase products in hyper-sensitively reacting cells have so far suggested that a large part of lipid peroxidation is mediated by a specific set of 9-lipoxygenases. To address the significance of 9-lipoxygenases for this type of pathogen response in potato, RNA interference constructs of a specific pathogen-induced potato 9-lipoxygenase were transferred to potato plants. Significantly reduced 9-lipoxygenase transcript levels were observed in transgenic plants after pathogen treatment. In addition, 9-lipoxygenase activity was hardly detectable, and levels of 9-lipoxygenase-derived oxylipins were reduced up to 12-fold after pathogen infection. In contrast to wild type plants, high levels of non-enzymatically as well as 13-lipoxygenase-derived oxylipins were present in 9-lipoxygenase-deficient plants. From this we conclude that during the normal hypersensitive response in potato, lipid peroxidation may occur as a controlled and directed process that is facilitated by the action of a specific 9-lipoxygenase. If 9-lipoxygenase-mediated formation of hydroperoxides is repressed, autoxidative lipid peroxidation processes and 13-lipoxygenase-mediated oxylipins synthesis become prominent. The unaltered timing and extent of necrosis formation suggests that the origin of lipid hydroperoxides does not influence pathogen-induced cell death in potato.

Polyunsaturated fatty acids (PUFAs) are a source of numerous oxidation products, the oxylipins. In leaves, α-linolenic acid (α-LeA) is the preferential substrate for lipid peroxidation reactions. This reaction may be catalyzed either by a 9-lipoxygenase (9-LOX) or by a 13-LOX and oxygen is inserted regioselectively as well as stereospecifically leading to formation of 13S- or 9S-hydroperoxy octadecatrienoic acid (13-/9-HPOT; Brash, 1999). At least, seven different enzyme families or reaction branches within the LOX pathway can use these HPOTs or other hydroperoxy PUFAs leading to (i) keto-PUFAs (LOX); (ii) epoxy hydroxy-PUFAs (epoxy alcohol synthase, EAS); (iii) octadecanoids and jasmonates (allene oxide synthase, AOS); (iv) leaf aldehydes and leaf alcohols (hydroperoxide lyase, HPL); (v) hydroxy PUFAs (reductase); (vi) divinyl ether PUFAs (divinyl ether synthase, DES); and (vii) epoxy- or dihydrodiol-PUFAs (peroxygenase, PDX; Fig. 1; Feussner and Wasternack, 2002).

Mycorrhizas are by far the most frequent occurring beneficial symbiotic interactions between plants and fungi. Species in >80% of extant plant families are capable of establishing an arbuscular mycorrhiza (AM). In relation to the development of the symbiosis the first molecular modifications are those associated with plant defense responses, which seem to be locally suppressed to levels compatible with symbiotic interaction (Gianinazzi-Pearson, 1996). AM symbiosis can, however, reduce root disease caused by several soil-borne pathogens. The mechanisms underlying this protective effect are still not well understood. In plants, products of the enzyme lipoxygenase (LOX) and the corresponding downstream enzymes, collectively named LOX pathway (Fig. 1B), are involved in wound healing, pest resistance, and signaling, or they have antimicrobial and antifungal activity (Feussner and Wasternack, 2002). The central reaction in this pathway is catalyzed by LOXs leading to formation of either 9- or 13-hydroperoxy octadeca(di/trien)oic acids (9/13-HPO(D/T); Brash, 1999). Thus LOXs may be divided into 9- and 13-LOXs (Fig. 1A). Seven different reaction branches within this pathway can use these hydroperoxy polyenoic fatty acids (PUFAs) leading to (i) keto PUFAs by a LOX; (ii) epoxy hydroxy-fatty acids by an epoxy alcohol synthase (EAS); (iii) octadecanoids and jasmonates via allene oxide synthase (AOS); (iv) leaf aldehydes and leaf alcohols via fatty acid hydroperoxide lyase (HPL); (v) hydroxy PUFAs (reductase); (vi) divinyl ether PUFAs via divinyl ether synthase (DES); and (vii) epoxy- or dihydrodiolPUFAs via peroxygenase (PDX; Feussner and Wasternack, 2002). AOS, HPL and DES belong to one subfamily of P450-containing enzymes, the CYP74 family (Feussner and Wasternack, 2002). Here, the involvement of this CYP74 enzyme family in mycorrhizal roots of M. truncatula during early stages of AM symbiosis formation was analyzed.

Jasmonic acid (JA) and its precursor 12-oxo phytodienoic acid (OPDA) are lipid-derived signals in plant stress responses and development (Wasternack and Hause, 2002). Within the wound-response pathway of tomato, a local response of expression of defense genes such as the proteinase inhibitor 2 gene (PIN2) is preceded by a rise in JA (Herde et al., 1996; Howe et al., 1996) and ethylene (O’Donnell et al., 1996). Mutants affected in JA biosynthesis such as defl (Howe et al., 1996) or spr-2 (Li et al., 2002) clearly indicated that JA biosynthesis is an ultimate part of wound signaling. It is less understood, however, how the rise in JA is regulated.