Octadecanoid Signaling Pathway Research Articles

Avoidance of the Plant Hormone Cis-Jasmone by Aedes aegypti Depends On Mosquito Age in Both Plant and Human Odor Backgrounds

Adults of many mosquito species feed on plants to obtain metabolic energy and to enhance reproduction. Mosquitoes primarily rely on olfaction to locate plants and are known to respond to a range of plant volatiles. We studied the olfactory response of the yellow fever mosquito Aedes aegypti to methyl jasmonate (MeJA) and cis-jasmone (CiJA), volatile compounds originating from the octadecanoid signaling pathway that plays a key role in plant defense against herbivores. Specifically, we investigated how Ae. aegypti of different ages responded to elevated levels of CiJA in two attractive odor contexts, either derived from Lima bean plants or human skin. Aedes aegypti females landed significantly less often on a surface with CiJA and MeJA compared to the solvent control, CiJA exerting a stronger reduction in landing than MeJA. Odor context (plant or human) had no significant main effect on the olfactory responses of Ae. aegypti females to CiJA. Mosquito age significantly affected the olfactory response, older females (7–9 d) responding more strongly to elevated levels of CiJA than young females (1–3 d) in either odor context. Our results show that avoidance of CiJA by Ae. aegypti is independent of odor background, suggesting that jasmonates are inherently aversive cues to these mosquitoes. We propose that avoidance of plants with elevated levels of jasmonates is adaptive to mosquitoes to reduce the risk of encountering predators that is higher on these plants, i.e. by avoiding ‘enemy-dense-space’.

Hydrogen Peroxide Acts as a Signaling Molecule for the Methyl Jasmonate-Induced Antioxidant Defense in Wheat Callus to Promote Enhanced Drought Tolerance

Drought stress is a major challenge in agriculture, causing severe loss of crops throughout the world. Methyl jasmonate (MeJA) regulates a variety of plant developmental process and responds to many biotic and abiotic stresses. To explore the physiological mechanisms of drought stress mitigated by exogenous MeJA, we subjected callus of wheat (Triticum aestivum L.) to drought induced medium after pretreatment with 10 ml sterile solution that contained the following singly or in combinations: MeJA (0, 0.25, and 2.5µM) and the lipoxygenase (LOX) inhibitor salicylhydroxamic acid (SHAM; 0 and 0.5 mM) for 24 hours. The endogenous jasmonic acid (JA) content, activity of LOX, hydrogen peroxide (H2O2) content, antioxidant enzymes activities, levels of reactive oxygen species (ROS), content of malondialdehyde (MDA), and cell viability were measured. The results showed that exogenous MeJA induced the activity of LOX and resulted in rapid increase in endogenous JA levels in a concentration-dependent manner; JA declined gradually after reaching its maximum. The level of H2O2 increased with increases in LOX activity and endogenous JA level, which are involved in the octadecanoid signaling pathway. However, SHAM inhibited the LOX activity, the endogenous JA level, and the accumulation of H2O2 in wheat callus. In addition, the activities of several antioxidant enzymes increased after MeJA-pretreatment, but this effect was also inhibited by SHAM. The cumulative results suggest that H2O2 was generated through the octadecanoid signaling pathway, and some antioxidant defense genes were provoked by the accumulation of H2O2. Therefore, we inferred that H2O2 might act as a signaling molecule for the MeJA-induced antioxidant defense. Drought stress can be improved by MeJA-pretreatment, leading to decrease in endogenous MDA and ROS contents, which are normally induced by drought conditions. Both drought stress and exogenous MeJA-pretreatment increased in LOX activity, endogenous JA level, and antioxidant enzymes. Combinatorial treatments showed an apparent synergistic effect on the activities of these antioxidant enzymes and resulted in improved cell viability. These results confirmed the hypothesis that H2O2 acts as a signaling molecule for the MeJA-induced antioxidant defense and could alleviate the negative effects of drought stress on wheat callus.

Volatile Emission in Bracken Fern Is Induced by Jasmonates but Not by Spodoptera littoralis or Strongylogaster multifasciata Herbivory

Jasmonate-mediated regulation of VOC emission has been extensively investigated in higher plants, however, only little is known about VOC production and its regulation in ferns. Here, we investigate whether the emission of VOCs from bracken fern Pteridium aquilinum is triggered by herbivory and if so - whether it is regulated by the octadecanoid signaling pathway. Interestingly, feeding of both generalist (Spodoptera littoralis) and specialist (Strongylogaster multifasciata) herbivores as well as application of singular and continuous mechanical wounding of fronds induced only very low levels of VOC emission. In contrast, treatment with jasmonic acid (JA) led to the emission of a blend of VOCs that was mainly comprised of terpenoids. Likewise, treatment with the JA precursor 12-oxo-phytodienoic acid (OPDA) and α-linolenic acid also induced VOC emission, albeit to a lower intesity than the JA treatment. Accumulation of endogenous JA was low in mechanically wounded fronds and these levels were unaffected by the application of oral secretions from both generalist or specialist herbivores. The emission of terpenoids upon JA treatment could be blocked with fosmidomycin and mevinolin, which are inhibitors of the MEP- and MVA pathways, respectively. These results indicate that similar to higher plants, terpenoid VOCs are produced via these pathways in bracken fern and that these pathways are JA-responsive. However, the very low amounts of terpenoids released after herbivory or mechanical damage are in stark contrast to what is known from higher plants. We speculate that S. multifasciata and S. littoralis feeding apparently did not induce the threshold levels of JA required for activating the MEP and MVA pathways and the subsequent volatile emission in bracken fern.

Selective inhibition of jasmonic acid accumulation by a small α, β-unsaturated carbonyl and phenidone reveals different modes of octadecanoid signalling activation in response to insect elicitors and green leaf volatiles in Zea mays

BackgroundPlants often release a complex blend of volatile organic compounds (VOC) in response to insect herbivore damage. Among those blends of VOC green leaf volatiles (GLV) have been demonstrated to function as defence signals between plants, thereby providing protection against impending herbivory. A problem in understanding the mode of action of these 6-carbon aldehydes, alcohols, and esters is caused by their structural diversity. Besides different degrees of oxidation, E-2- as well as Z-3-configured isomers are often released. This study was therefore initiated to determine the structural requirement necessary to exhibit biological activity measured as jasmonic acid (JA) accumulation in Zea mays seedlings.FindingsThe structure/function analysis of green leaf volatiles and related compounds revealed that an olefinic bond in position 2 or 3 and a size of 6-8 carbons is required for biological activity in maize. Also, it was found that the presence of an α, β-unsaturated carbonyl is not a prerequisite for activity. However, by treating plants first with volatile acrolein it was discovered that this smallest α, β-unsaturated carbonyl inhibits JA accumulation in response to insect elicitor treatment, but not after GLV exposure. This selective inhibitory effect was also found for phenidone, an inhibitor of lipoxygenases. These findings led to the discovery of a pool of protein-associated 12-oxo-phytodienoic acid, a biosynthetic precursor of JA, which appeared to be rapidly converted into JA upon exposure to GLV.ConclusionsThe structure/function analysis of GLV demonstrates a high degree of correlation between the compounds released by wounded plants in nature and their biological activity. The selective inhibitory effects of acrolein and phenidone on insect elicitor- and GLV-induced JA accumulation in maize led to the discovery of a pool of protein-associated precursor, which is rapidly activated and transformed to JA after exposure to GLV. This novel mechanism for JA accumulation sheds new light on the biosynthetic variability of the octadecanoid signalling pathway and explains the observed differences in the response of maize seedling to inhibitors of JA accumulation.

The octadecanoid signaling pathway participates in the chilling-induced transcription of ω-3 fatty acid desaturases in Arabidopsis

The jasmonate signaling pathway is known to be involved in various stress responses in plants, but little is known specifically on the role of the octadecanoid pathway in the regulation of ω-3 fatty acid desaturase (FAD) genes in response to low temperature. To uncover this problem, the expression level of three ω-3 FAD genes (AtFAD3, AtFAD7, and AtFAD8) and the content of linolenic acid (C18:3) in both wild type (WT) and mutant (or WT suppressed) plants disrupted in the octadecanoid pathway were analyzed under normal conditions and the chilling treatment (4°C), respectively. As compared with WT plants, chilling-induced organ-specific expressions of ω-3 FAD genes significantly changed when the octadecanoid pathway was blocked or suppressed, and presented either the "suppressed increase" or the "enhanced decrease" tendency. Meanwhile, chilling-induced increases in the content of C18:3 found in WT plants, obviously reduced in both leaves and roots, and even turned to a decrease in stems when the octadecanoid pathway was disrupted. Overall, the transcription of ω-3 FAD genes and the proportion of C18:3 in the whole plant both of aos mutants and salicylic acid (SA)-treated plants under chilling stress were evidently lower than those of WT plants, except the expression of AtFAD3 gene in leaves. All these findings indicate that the octadecanoid pathway does participate in the chilling-induced transcription of ω-3 FAD genes in Arabidopsis.

Induced biosynthesis of insect semiochemicals in plants.

Plants under attack by a herbivore may emit characteristic volatiles that are implicated in the attraction of the natural enemies of the herbivore. The signal cascade between leaf damage and the volatile production is stimulated by high- or low-molecular-weight elicitors from the secretions of the herbivore. Besides compounds from the octadecanoid signalling pathway, several structurally non-related amino acid conjugates such as the bacterial phytotoxin coronatine, the synthetic indanoyl-isoleucine, or amino acid conjugates of linolenic acid likewise induce volatile biosynthesis. Minor changes in the amino acid moiety may result in different volatile profiles (sesqui- and diterpenoids), attributing to the amino acid substructure a specific role for the recognition and the selective induction. Volatile terpenoids (mono- and diterpenoids) are synthesised de novo along the novel deoxy-D-xylulose (DOX) pathway, while the biosynthesis of sesquiterpenes may be fuelled from both the DOX- and the mevalonate pathway. This finding may be of importance for the plant defence in case of introduction of inhibitors together with the salivary secretion of herbivores into the leaf tissue.

Insect Elicitors and Exposure to Green Leafy Volatiles Differentially Upregulate Major Octadecanoids and Transcripts of 12-Oxo Phytodienoic Acid Reductases in Zea mays

The induction of jasmonic acid (JA) is one of the major signaling events in plants in response to insect herbivore damage and leads to the activation of direct and indirect defensive measures. Green leafy volatiles, which constitute a major portion of volatile organic compounds, often are released in response to insect herbivore attack and have been shown to significantly activate JA production in exposed corn (Zea mays) seedlings, thereby priming these plants specifically against subsequent herbivore attack. To explore the factors determining the specificity of the octadecanoid signaling pathway in corn, we analyzed qualitative and quantitative changes in major octadecanoids. The time course and the amount of induced JA and 12-oxophytodienoic acid levels in corn seedlings were strikingly different after wounding, application of caterpillar regurgitant, or treatment with cis-3-hexenyl acetate (Z-3-6:AC). Exposure to Z-3-6:AC induced accumulation of transcripts encoded by three putative 12-oxophytodienoate10,11-reductase genes (ZmOPR1/2, ZmOPR5, and ZmOPR8). Although changes in ZmOPR5 RNAs were detected only after exposure to Z-3-6:AC, ZmOPR1/2 RNAs and ZmOPR8 RNAs also were abundant after treatment with crude regurgitant elicitor or mechanical damage. The physiological implications of these findings in the context of plant-insect interactions are discussed.

The rolB and rolC genes activate synthesis of anthraquinones in Rubia cordifolia cells by mechanism independent of octadecanoid signaling pathway

Several studies have recently revealed an interesting ability of the rolC and rolB oncogenes of Agrobacterium rhizogenes to stimulate secondary metabolite production in plant cells. We have studied the role of the octadecanoid signaling pathway in the signal transduction initiated by the rol genes in transformed Rubia cordifolia cell cultures. None of the octadecanoid pathway inhibitors tested (diethyldithiocarbamate, propyl gallate, salicylhydroxamic acid and piroxicam) inhibited anthraquinone accumulation in the normal and transformed cultures. This result indicates that the octadecanoid pathway is not involved in the rolB and rolC gene-mediated increase of anthraquinone production. Summarizing the data of previously reported and present investigations, we suggest that the rolB and rolC genes act downstream of Ca 2+ signaling, H 2O 2-generating NADPH oxidase, as well as downstream of salicylic acid-mediated and octadecanoid pathways or perturb secondary metabolism by unknown for plants way. An unusual effect of piroxicam, a member of non-steroidal anti-inflammatory drugs on anthraquinone production was detected: this inhibitor stimulated anthraquinone production in a greater extent then methyl jasmonate.

Dihydrocoronatine, promising candidate for a chemical probe to study coronatine-, jasmonoid- and octadecanoid-binding protein.

Coronatine (1), its synthetic analogs (6-13) and jasmonic acid induced various volatiles in rice leaves. In the range of 0.01-0.1 mM, dihydrocoronatine (7) exhibited 4-687 times higher activity for linalool emission than that of 1. The radioactive derivative of 7, [4,5-3H]-7, was employed to identify the putative coronatine-binding protein in rice leaves. 7 would be a promising candidate for a chemical probe to study cornatine-binding protein related to the jasmonoid and octadecanoid signaling pathway in higher plants. A detailed study of coronatine-binding protein in rice leaves and cell culture with [4,5-3H]-7 is now in progress.

Resistance of cultivated tomato to cell content-feeding herbivores is regulated by the octadecanoid-signaling pathway.

The octadecanoid signaling pathway has been shown to play an important role in plant defense against various chewing insects and some pathogenic fungi. Here, we examined the interaction of a cell-content feeding arachnid herbivore, the two-spotted spider mite (Tetranychus urticae Koch), with cultivated tomato (Lycopersicon esculentum) and an isogenic mutant line (defenseless-1 [def-1]) that is deficient in the biosynthesis of the octadecanoid pathway-derived signal, jasmonic acid (JA). Spider mite feeding and fecundity on def-1 plants was significantly greater than on wild-type plants. Decreased resistance of def-1 plants was correlated with reduced JA accumulation and expression of defensive proteinase inhibitor (PI) genes, which were induced in mite-damaged wild-type leaves. Treatment of def-1 plants with methyl-JA restored resistance to spider mite feeding and reduced the fecundity of female mites. Plants expressing a 35S::prosystemin transgene that constitutively activates the octadecanoid pathway in a Def-1-dependent manner were highly resistant to attack by spider mites and western flower thrips (Frankliniella occidentalis), another cell-content feeder of economic importance. These findings indicate that activation of the octadecanoid signaling pathway promotes resistance of tomato to a broad spectrum of herbivores. The techniques of amplified fragment length polymorphism (AFLP) and bulk segregant analysis were used to map the Def-1 gene to a region on the long arm of chromosome 3 that is genetically separable from the map position of known JA biosynthetic genes. Tight linkage of Def-1 to a T-DNA insertion harboring the maize (Zea mays) Dissociation transposable element suggests a strategy for directed transposon tagging of the gene.

Hydrogen Peroxide Acts as a Second Messenger for the Induction of Defense Genes in Tomato Plants in Response to Wounding, Systemin, and Methyl Jasmonate

The systemic accumulation of both hydrogen peroxide (H(2)O(2)) and proteinase inhibitor proteins in tomato leaves in response to wounding was inhibited by the NADPH oxidase inhibitors diphenylene iodonium (DPI), imidazole, and pyridine. The expression of several defense genes in response to wounding, systemin, oligosaccharides, and methyl jasmonate also was inhibited by DPI. These genes, including those of four proteinase inhibitors and polyphenol oxidase, are expressed within 4 to 12 hr after wounding. However, DPI did not inhibit the wound-inducible expression of genes encoding prosystemin, lipoxygenase, and allene oxide synthase, which are associated with the octadecanoid signaling pathway and are expressed 0.5 to 2 hr after wounding. Accordingly, treatment of plants with the H(2)O(2)-generating enzyme glucose oxidase plus glucose resulted in the induction of only the later-expressed defensive genes and not the early-expressed signaling-related genes. H(2)O(2) was cytochemically detected in the cell walls of vascular parenchyma cells and spongy mesophyll cells within 4 hr after wounding of wild-type tomato leaves, but not earlier. The cumulative results suggest that active oxygen species are generated near cell walls of vascular bundle cells by oligogalacturonide fragments produced by wound-inducible polygalacturonase and that the resulting H(2)O(2) acts as a second messenger for the activation of defense genes in mesophyll cells. These data provide a rationale for the sequential, coordinated, and functional roles of systemin, jasmonic acid, oligogalacturonides, and H(2)O(2) signals for systemic signaling in tomato plants in response to wounding.

Ion channel-forming alamethicin is a potent elicitor of volatile biosynthesis and tendril coiling. Cross talk between jasmonate and salicylate signaling in lima bean.

Alamethicin (ALA), a voltage-gated, ion channel-forming peptide mixture from Trichoderma viride, is a potent elicitor of the biosynthesis of volatile compounds in lima bean (Phaseolus lunatus). Unlike elicitation with jasmonic acid or herbivore damage, the blend of substances emitted comprises only the two homoterpenes, 4,11-dimethylnona-1,3,7-triene and 4,8,12-trimethyltrideca-1,3,7,11-tetraene, and methyl salicylate. Inhibition of octadecanoid signaling by aristolochic acid and phenidone as well as mass spectrometric analysis of endogenous jasmonate demonstrate that ALA induces the biosynthesis of volatile compounds principally via the octadecanoid-signaling pathway (20-fold increase of jasmonic acid). ALA also up-regulates salicylate biosynthesis, and the time course of the production of endogenous salicylate correlates well with the appearance of the methyl ester in the gas phase. The massive up-regulation of the SA-pathway (90-fold) interferes with steps in the biosynthetic pathway downstream of 12-oxophytodienoic acid and thereby reduces the pattern of emitted volatiles to compounds previously shown to be induced by early octadecanoids. ALA also induces tendril coiling in various species like Pisum, Lathyrus, and Bryonia, but the response appears to be independent from octadecanoid biosynthesis, because inhibitors of lipoxygenase and phospholipase A(2) do not prevent the coiling reaction.

Lipid hydroperoxide levels in plant tissues

Hydroperoxides are the primary oxygenated products of polyunsaturated fatty acids and are key intermediates in the octadecanoid signalling pathway in plants. Lipid hydroperoxides (LHPO) were determined spectrophotometrically based on their reaction with an excess of Fe(2+)at low pH in the presence of the dye xylenol orange. Triphenylphosphine-mediated hydroxide formation was used to authenticate the signal generated by the hydroperoxides. The method readily detected lipid peroxidation in Phaseolus: microsomes, senescing potato leaves and in a range of other plant tissues including Phaseolus hypocotyls (26+/-5 nmol g(-1) FW), Alstroemeria floral tissues (sepals 66+/-13 nmol g(-1) FW petals 49+/-6 nmol g(-1) FW), potato leaves (334+/-75 nmol g(-1) FW), broccoli florets (568+/-68 nmol g(-1) FW) and Chlamydomonas cells (602+/-40 nmol g(-1) FW). Relative to the total fatty acid content of the tissues, the % LHPO was within the range of 0.6-1.7% for all tissue types (photosynthetic and non-photosynthetic) and represents the basal oxidation level of membrane fatty acids in plant cells. In order to relate the levels of LHPO to specific signalling pathways, transgenic potato plant lines were used in which lipoxygenase (LOX) (responsible for hydroperoxide biosynthesis) and hydroperoxide lyase (a route of hydroperoxide degradation) activities were largely reduced by an antisense-mediated approach. While the LHPO levels were similar to wild type in the individual LOX antisensed plants, basal LHPO levels, by contrast, were elevated by 38% in transgenic potato leaves antisensed in hydroperoxide lyase, indicating a role for this enzyme in the maintenance of cellular levels of LHPOs.

Lipid hydroperoxide levels in plant tissues

Hydroperoxides are the primary oxygenated products of polyunsaturated fatty acids and are key intermediates in the octadecanoid signalling pathway in plants. Lipid hydroperoxides (LHPO) were determined spectrophotometrically based on their reaction with an excess of Fe2+at low pH in the presence of the dye xylenol orange. Triphenylphosphine‐mediated hydroxide formation was used to authenticate the signal generated by the hydroperoxides. The method readily detected lipid peroxidation in Phaseolus microsomes, senescing potato leaves and in a range of other plant tissues including Phaseolus hypocotyls (26±5 nmol g−1 FW), Alstroemeria floral tissues (sepals 66±13 nmol g−1 FW; petals 49±6 nmol g−1 FW), potato leaves (334±75 nmol g−1 FW), broccoli florets (568±68 nmol g−1 FW) and Chlamydomonas cells (602±40 nmol g−1 FW). Relative to the total fatty acid content of the tissues, the % LHPO was within the range of 0.6–1.7% for all tissue types (photosynthetic and non‐photosynthetic) and represents the basal oxidation level of membrane fatty acids in plant cells. In order to relate the levels of LHPO to specific signalling pathways, transgenic potato plant lines were used in which lipoxygenase (LOX) (responsible for hydroperoxide biosynthesis) and hydroperoxide lyase (a route of hydroperoxide degradation) activities were largely reduced by an antisense‐mediated approach. While the LHPO levels were similar to wild type in the individual LOX antisensed plants, basal LHPO levels, by contrast, were elevated by 38% in transgenic potato leaves antisensed in hydroperoxide lyase, indicating a role for this enzyme in the maintenance of cellular levels of LHPOs.

Hydrogen peroxide is generated systemically in plant leaves by wounding and systemin via the octadecanoid pathway.

Hydrogen peroxide (H2O2) generated in response to wounding can be detected at wound sites and in distal leaf veins within 1 hr after wounding. The response is systemic and maximizes at about 4-6 hr in both wounded and unwounded leaves, and then declines. The timing of the response corresponds with an increase in wound-inducible polygalacturonase (PG) mRNA and enzyme activity previously reported, suggesting that oligogalacturonic acid (OGA) fragments produced by PG are triggering the H2O2 response. Systemin, OGA, chitosan, and methyl jasmonate (MJ) all induce the accumulation of H2O2 in leaves. Tomato plants transformed with an antisense prosystemin gene produce neither PG activity or H2O2 in leaves in response to wounding, implicating systemin as a primary wound signal. The antisense plants do produce both PG activity and H2O2 when supplied with systemin, OGA, chitosan, or MJ. A mutant tomato line compromised in the octadecanoid pathway does not exhibit PG activity or H2O2 in response to wounding, systemin, OGA, or chitosan, but does respond to MJ, indicating that the generation of H2O2 requires a functional octadecanoid signaling pathway. Among 18 plant species from six families that were assayed for wound-inducible PG activity and H2O2 generation, 14 species exhibited both wound-inducible PG activity and the generation of H2O2. Four species, all from the Fabaceae family, exhibited little or no wound-inducible PG activity and did not generate H2O2. The time course of wound-inducible PG activity and H2O2 in Arabidopsis thaliana leaves was similar to that found in tomato. The cumulative data suggest that systemic wound signals that induce PG activity and H2O2 are widespread in the plant kingdom and that the response may be associated with the defense of plants against both herbivores and pathogens.

Increase in free linolenic and linoleic acids associated with phospholipase D-mediated hydrolysis of phospholipids in wounded castor bean leaves

Stimulus-induced release of polyunsaturated fatty acids from membranes has been proposed to couple the processes of stimulus perception and oxylipin synthesis in the octadecanoid signaling pathway. This study investigated wound-induced changes in free fatty acids, diacylglycerol, and phospholipids at the site of wounding and at an unwounded area of the same wounded leaf in castor bean ( Ricinus communis L.). Increases in free fatty acids and diacylglycerol and decreases in phospholipids were relatively large and continuous at the site of wounding. The changes at the unwounded area were selective and transient, suggesting a regulated activation of lipid turnover in response to wounding. In unwounded cells, the free fatty acids that increased in the early phase of wounding were linolenate and linoleate, which peaked within 5 min after wounding. Diacylglycerols that increased in unwounded cells were the species containing linolenate and linoleate, not those with oleate and stearate. Within 5 min of wounding, the levels of phosphatidylcholine and phosphatidylglycerol, but not other phospholipids, decreased in unwounded cells. These results provide evidence for the wound-induced selective increase in linolenate and linoleate in unwounded cells. The varied susceptibility of different phospholipids to hydrolysis after wounding indicates that phosphatidylcholine and phosphatidylglycerol may serve as substrates that lead to the increase in linolenate and linoleate in the early phase of wound response. The pattern of increases in polyunsaturated fatty acids, diacylglycerol, and phosphatidic acid and of decreases in phospholipids suggests the activation of a PLD-initiated signaling pathway in response to wounding in castor bean.

Accumulation of feruloyltyramine and p-coumaroyltyramine in tomato leaves in response to wounding

The amounts of two phenolic compounds, E-feruloyltyramine and E- p-coumaroyltyramine, increased 10-fold in tomato leaves in response to mechanical wounding, and 25-fold in response to the oligosaccharide elicitor chitosan. Using fluorescence detection, feruloyltyramine was shown to accumulate in response to wounding, but not to jasmonic acid or systemin, two elicitors of systemic wound-inducible defense responses in tomato. Feruloyltyramine also accumulated in leaves of mutant tomato plants deficient in the octadecanoid signaling pathway in response to wounding, indicating that its synthesis is not regulated through this pathway. These data support a role for hydroxycinnamate-tyramine conjugates as part of the array of defense chemicals and protective biopolymers induced in leaves and other plant tissues by wounding, to protect the plants against pathogen and herbivore attacks.

Cellulysin from the plant parasitic fungus Trichoderma viride elicits volatile biosynthesis in higher plants via the octadecanoid signalling cascade

Cellulysin, a crude cellulase from the plant parasitic fungus Trichoderma viride, induces the biosynthesis of volatiles in higher plants ( Nicotiana plumbaginifolia, Phaseolus lunatus, and Zea mays) when applied to cut petioles by the transpiration stream. The pattern of the emitted volatiles largely resembles that from a herbivore damage or treatment of the plants with jasmonic acid (JA) indicating that cellulysin acts via activation of the octadecanoid signalling pathway. The treatment with cellulysin raises the level of endogenous JA after 30 min and is followed by a transient emission of ethylene after 2–3 h. Volatile production becomes significant after 12–24 h. Inhibitors of the JA pathway effectively block the cellulysin-dependent volatile biosynthesis.

Molecular Cloning and Characterization of 12-Oxophytodienoate Reductase, an Enzyme of the Octadecanoid Signaling Pathway fromArabidopsis thaliana: STRUCTURAL AND FUNCTIONAL RELATIONSHIP TO YEAST OLD YELLOW ENZYME

Using partial amino acid sequence information for 12-oxophytodienoate-10,11-reductase obtained from Corydalis sempervirens we have cloned the homologous enzyme from Arabidopsis thaliana. The open reading frame of the cDNA encodes a polypeptide of 372 amino acids (Mr = 41,165) with significant similarity to the sequence of Old Yellow Enzyme from Saccharomyces carlsbergensis (Saito, K., Thiele, D. J., Davio, M., Lockridge, O., and Massey, V. (1991) J. Biol. Chem. 266, 20720-20724), a flavin (FMN)-protein catalyzing the NADPH-dependent reduction of the olefinic bond of alpha,beta-unsaturated carbonyls. Specifically, all residues required for binding of FMN in Old Yellow Enzyme are conserved in the A. thaliana sequence, as are all residues associated with catalytic activity. The enzyme was functionally expressed from its cDNA in Escherichia coli and thus proven to encode OPDA reductase. Further similarities of OPDA reductase and yeast Old Yellow Enzyme include their binding to and elution by reductant from N-(4-hydroxybenzoyl)aminohexyl-Sepharose, the immunoreactivity of yeast Old Yellow Enzyme with an antiserum raised against plant OPDA reductase and the demonstration that Old Yellow Enzyme is an active OPDA reductase. It is thus conceivable that the physiological role of Old Yellow Enzymes now known from bacteria, yeasts, and higher plants, is in oxylipin metabolism.

Jasmonate-Inducible Genes Are Activated in Rice by Pathogen Attack without a Concomitant Increase in Endogenous Jasmonic Acid Levels.

The possible role of the octadecanoid signaling pathway with jasmonic acid (JA) as the central component in defense-gene regulation of pathogen-attacked rice was studied. Rice (Oryza sativa L.) seedlings were treated with JA or inoculated with the rice blast fungus Magnaporthe grisea (Hebert) Barr., and gene-expression patterns were compared between the two treatments. JA application induced the accumulation of a number of pathogenesis-related (PR) gene products at the mRNA and protein levels, but pathogen attack did not enhance the levels of (-)-JA during the time required for PR gene expression. Pathogen-induced accumulation of PR1-like proteins was reduced in plants treated with tetcyclacis, a novel inhibitor of jasmonate biosynthesis. There was an additive and negative interaction between JA and an elicitor from M. grisea with respect to induction of PR1-like proteins and of an abundant JA-and wound-induced protein of 26 kD, respectively. Finally, activation of the octadecanoid signaling pathway and induction of a number of PR genes by exogenous application of JA did not confer local acquired resistance to rice. The data suggest that accumulation of nonconjugated (-)-JA is not necessary for induction of PR genes and that JA does not orchestrate localized defense responses in pathogen-attacked rice. Instead, JA appears to be embedded in a signaling network with another pathogen-induced pathway(s) and may be required at a certain minimal level for induction of some PR genes.