Plants develop an enhanced defensive capacity against a broad spectrum of plant pathogens after colonization of the roots by selected strains of nonpathogenic biocontrol bacteria. In Arabidopsis thaliana, this induced systemic resistance (ISR) functions independently of salicylic acid but requires an intact response to the plant hormones jasmonic acid (JA) and ethylene. To further investigate the roles of JA and ethylene in the ISR signalling pathway, the levels of these signalling molecules were determined in A. thaliana upon induction of ISR by Pseudomonas¯uorescens WCS417r and subsequent challenge inoculation with Pseudomonas syringae pv. tomato DC3000. Upon treatment of the roots with ISR-inducing WCS417r bacteria, neither the JA content, nor the level of ethylene evolution was altered in systemically resistant leaves. In®ltration of leaves with WCS417r triggered the JA-and ethylene-dependent ISR pathway, but did not cause local changes in the production of either of these signalling molecules. These results indicate that rhizobacteria-mediated ISR is not based on the induction of changes in the biosynthesis of either JA or ethylene. However, in ISR-expressing plants the capacity to convert 1-aminocyclopropane-1-carboxylate (ACC) to ethylene was signi®cantly enhanced, providing a greater potential to produce ethylene upon pathogen attack.*
The free radical oxidation of arachidonic acid is known to generate complex metabolites, termed isoprostanes, that share structural features of prostaglandins and exert potent receptor-mediated biological activities. In the present study, we show that ␣-linolenic acid can undergo a similar oxidation process, resulting in a series of isomeric dinor isoprostanes E 1 . E-ring dinor isoprostane formation from linolenate was found to be catalyzed by soybean lipoxygenase. The main enzymatic products were 13-and 9-hydroperoxylinolenate but in addition, two dinor isoprostane E 1 regioisomers were formed with a yield of 0.31%.Identification and quantification of two dinor isoprostane E 1 regioisomers in plant cell cultures was achieved by a negative chemical ionization gas chromatographymass spectrometry method using [18 O]dinor isoprostanes E 1 as internal standards. Endogenous levels of these compounds were determined in four taxonomically distant plant species and found to be in the range of 4.5 to 60.9 ng/g of dry weight.Thus analogous pathways in animals and plants exist, each leading to a family of prostaglandin-like compounds derived from polyunsaturated fatty acids. It remains to be shown whether the dinor isoprostanes exert biological activities in plants as has been demonstrated for their C20 congeners in mammals.
Jasmonic acid (jA) is rapidly biosynthesized from a-linolenic acid in plants upon contact with pathogens or wounding, and triggers gene activation, leading to the synthesis of defensive secondary metabolites and proteins. Despite the recent finding that its precursor, 1 2-0x0-phytodienoic acid (PDA), is a more powerful inducer of gene activation, interest has focused so far almost exclusively on jA.A validated negative chemical ionization-gas chromatography-mass spectrometry method has been developed that allows the simultaneous quantification of endogenous 1 2-0x0-PDA and JA in plant tissues. In six out of eight plant species tested maximal levels of 12-0x0-PDA exceeded peak levels of JA by approximately 3-to 5-fold after elicitation with a yeast cell wall preparation or when plants were wounded. These experiments support the hypothesis that 12-0x0-PDA acts as the predominant jasmonate signal in most plants, whereas JA remains an active metabolite of its precursor. Furthermore, JA but not 12-0x0-PDA was shown to be secreted into the medium from cultured plant cells, suggesting that jA may also act as an intercellular signal.
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