Formation of lipoxygenase-pathway-derived aldehydes in barley leaves upon methyl jasmonate treatment.
In barley leaves, the application of jasmonates leads to dramatic alterations of gene expression. Among the up-regulated gene products lipoxygenases occur abundantly. Here, at least four of them were identified as 13-lipoxygenases exhibiting acidic pH optima between pH 5.0 and 6.5. (13S,9Z,11E,15Z)-13-hydroxy-9,11,15-octadecatrienoic acid was found to be the main endogenous lipoxygenase-derived polyenoic fatty acid derivative indicating 13-lipoxygenase activity in vivo. Moreover, upon methyl jasmonate treatment > 78% of the fatty acid hydroperoxides are metabolized by hydroperoxide lyase activity resulting in the endogenous occurrence of volatile aldehydes. (2E)-4-Hydroxy-2-hexenal, hexanal and (3Z)- plus (2E)-hexenal were identified as 2,4-dinitro-phenylhydrazones using HPLC and identification was confirmed by GC/MS analysis. This is the first proof that (2E)-4-hydroxy-2-hexenal is formed in plants under physiological conditions. Quantification of (2E)-4-hydroxy-2-hexenal, hexanal and hexenals upon methyl jasmonate treatment of barley leaf segments revealed that hexenals were the major aldehydes peaking at 24 h after methyl jasmonate treatment. Their endogenous content increased from 1.6 nmol.g-1 fresh weight to 45 nmol.g-1 fresh weight in methyl-jasmonate-treated leaf segments, whereas (2E)-4-hydroxy-2-hexenal, peaking at 48 h of methyl jasmonate treatment increased from 9 to 15 nmol.g-1 fresh weight. Similar to the hexenals, hexanal reached its maximal amount 24 h after methyl jasmonate treatment, but increased from 0.6 to 3.0 nmol.g-1 fresh weight. In addition to the classical leaf aldehydes, (2E)-4-hydroxy-2-hexenal was detected, thereby raising the question of whether it functions in the degradation of chloroplast membrane constituents, which takes place after methyl jasmonate treatment. (+info)
Inverse relationship between systemic resistance of plants to microorganisms and to insect herbivory.
Pre-inoculation of plants with a pathogen that induces necrosis leads to the development of systemic acquired resistance (SAR) to subsequent pathogen attack . The phenylpropanoid-derived compound salicylic acid (SA) is necessary for the full expression of both local resistance and SAR  . A separate signaling pathway involving jasmonic acid (JA) is involved in systemic responses to wounding and insect herbivory  . There is evidence both supporting and opposing the idea of cross-protection against microbial pathogens and insect herbivores  . This is a controversial area because pharmacological experiments point to negative cross-talk between responses to systemic pathogens and responses to wounding   , although this has not been demonstrated functionally in vivo. Here, we report that reducing phenylpropanoid biosynthesis by silencing the expression of phenylalanine ammonialyase (PAL) reduces SAR to tobacco mosaic virus (TMV), whereas overexpression of PAL enhances SAR. Tobacco plants with reduced SAR exhibited more effective grazing-induced systemic resistance to larvae of Heliothis virescens, but larval resistance was reduced in plants with elevated phenylpropanoid levels. Furthermore, genetic modification of components involved in phenylpropanoid synthesis revealed an inverse relationship between SA and JA levels. These results demonstrate phenylpropanoid-mediated cross-talk in vivo between microbially induced and herbivore-induced pathways of systemic resistance. (+info)
EIN2, a bifunctional transducer of ethylene and stress responses in Arabidopsis.
Ethylene regulates plant growth, development, and responsiveness to a variety of stresses. Cloning of the Arabidopsis EIN2 gene identifies a central component of the ethylene signaling pathway. The amino-terminal integral membrane domain of EIN2 shows similarity to the disease-related Nramp family of metal-ion transporters. Expression of the EIN2 CEND is sufficient to constitutively activate ethylene responses and restores responsiveness to jasmonic acid and paraquat-induced oxygen radicals to mutant plants. EIN2 is thus recognized as a molecular link between previously distinct hormone response pathways. Plants may use a combinatorial mechanism for assessing various stresses by enlisting a common set of signaling molecules. (+info)
The jasmonate-induced 60 kDa protein of barley exhibits N-glycosidase activity in vivo.
Upon jasmonate treatment barley leaf segments express a putative ribosome-inactivating protein (JIP60). The influence of this protein on translation in planta has been analysed by using barley plants and tobacco plants transformed with a barley cDNA encoding JIP60. In both plant systems JIP60 exhibited N-glycosidase activity in vivo. The depurination of the 25S rRNA of tobacco and barley ribosomes led to accumulation of translationally inactive polysomes. (+info)
A novel jasmonate- and elicitor-responsive element in the periwinkle secondary metabolite biosynthetic gene Str interacts with a jasmonate- and elicitor-inducible AP2-domain transcription factor, ORCA2.
Jasmonate (JA) is an important plant stress hormone that induces various plant defense responses, including the biosynthesis of protective secondary metabolites. The induction of the secondary metabolite biosynthetic gene Strictosidine synthase (Str) in Catharanthus roseus (periwinkle) cells by elicitor requires JA as a second messenger. A 42 bp region in the Str promoter is both necessary and sufficient for JA- and elicitor-responsive expression. This region is unlike other previously identified JA-responsive regions, and contains a GCC-box-like element. Yeast one-hybrid screening identified cDNAs encoding two AP2-domain proteins. These octadecanoid-derivative responsive Catharanthus AP2-domain (ORCA) proteins bind in a sequence-specific manner the JA- and elicitor-responsive element. ORCA2 trans-activates the Str promoter and its expression is rapidly inducible with JA and elicitor, whereas Orca1 is expressed constitutively. The results indicate that a GCC-box-like element and ORCA2 play key roles in JA- and elicitor-responsive expression of the terpenoid indole alkaloid biosynthetic gene Str. (+info)
Oligogalacturonide-mediated induction of a gene involved in jasmonic acid synthesis in response to the cell-wall-degrading enzymes of the plant pathogen Erwinia carotovora.
Identification of Arabidopsis thaliana genes responsive to plant cell-wall-degrading enzymes of Erwinia carotovora subsp. carotovora led to the isolation of a cDNA clone with high sequence homology to the gene for allene oxide synthase, an enzyme involved in the biosynthesis of jasmonates. Expression of the corresponding gene was induced by the extracellular enzymes from this pathogen as well as by treatment with methyl jasmonate and short oligogalacturonides (OGAs). This suggests that OGAs are involved in the induction of the jasmonate pathway during plant defense response to E. carotovora subsp. carotovora attack. (+info)
Differential induction of plant volatile biosynthesis in the lima bean by early and late intermediates of the octadecanoid-signaling pathway.
Plants are able to respond to herbivore damage with de novo biosynthesis of an herbivore-characteristic blend of volatiles. The signal transduction initiating volatile biosynthesis may involve the activation of the octadecanoid pathway, as exemplified by the transient increase of endogenous jasmonic acid (JA) in leaves of lima bean (Phaseolus lunatus) after treatment with the macromolecular elicitor cellulysin. Within this pathway lima bean possesses at least two different biologically active signals that trigger different biosynthetic activities. Early intermediates of the pathway, especially 12-oxo-phytodienoic acid (PDA), are able to induce the biosynthesis of the diterpenoid-derived 4,8, 12-trimethyltrideca-1,3,7,11-tetraene. High concentrations of PDA result in more complex patterns of additional volatiles. JA, the last compound in the sequence, lacks the ability to induce diterpenoid-derived compounds, but is highly effective at triggering the biosynthesis of other volatiles. The phytotoxin coronatine and amino acid conjugates of linolenic acid (e.g. linolenoyl-L-glutamine) mimic the action of PDA, but coronatine does not increase the level of endogenous JA. The structural analog of coronatine, the isoleucine conjugate of 1-oxo-indanoyl-4-carboxylic acid, effectively mimics the action of JA, but does not increase the level of endogenous JA. The differential induction of volatiles resembles previous findings on signal transduction in mechanically stimulated tendrils of Bryonia dioica. (+info)
Requirement of functional ethylene-insensitive 2 gene for efficient resistance of Arabidopsis to infection by Botrytis cinerea.
Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola results in systemic induction of genes encoding a plant defensin (PDF1.2), a basic chitinase (PR-3), and an acidic hevein-like protein (PR-4). Pathogen-induced induction of these three genes is almost completely abolished in the ethylene-insensitive Arabidopsis mutant ein2-1. This indicates that a functional ethylene signal transduction component (EIN2) is required in this response. The ein2-1 mutants were found to be markedly more susceptible than wild-type plants to infection by two different strains of the gray mold fungus Botrytis cinerea. In contrast, no increased fungal colonization of ein2-1 mutants was observed after challenge with avirulent strains of either Peronospora parasitica or A. brassicicola. Our data support the conclusion that ethylene-controlled responses play a role in resistance of Arabidopsis to some but not all types of pathogens. (+info)