Enhanced expression and activation of the alternative oxidase during infection of Arabidopsis with Pseudomonas syringae pv tomato.
(9/1096)
Cyanide-resistant ("alternative") respiration was studied in Arabidopsis during incompatible and compatible infection with Pseudomonas syringae pv tomato DC3000. Total leaf respiration increased as the leaves became necrotic, as did the cyanide-resistant component that was sensitive to salicylhydroxamic acid. Infiltration of leaves with an avirulent strain rapidly induced alternative oxidase (AOX) mRNA, whereas the increase was delayed in the compatible combination. The increase in mRNA correlated with the increase in AOX protein. Increased expression was confined to the infected leaves, in contrast to the pathogenesis-related protein-1, which was induced systemically. Virtually all of the AOX protein was in the reduced (high-activity) form. Using transgenic NahG and mutant npr1-1 and etr1-1 plants, we established that the rapid induction of the AOX was associated with necrosis and that ethylene, but not salicylic acid, was required for its induction. Increased pyruvate levels in the infected leaves suggested that increased substrate levels were respired through the alternative pathway; however, in the control leaves and the infected leaves, respiration was not inhibited by salicylhydroxamic acid alone. Increased respiration appeared to be associated primarily with symptom expression rather than resistance reactions. (+info)
Inactivation of brassinosteroid biological activity by a salicylate-inducible steroid sulfotransferase from Brassica napus.
(10/1096)
Recent discoveries from brassinosteroid-deficient mutants led to the recognition that plants, like animals, use steroids to regulate their growth and development. We describe the characterization of one member of a Brassica napus sulfotransferase gene family coding for an enzyme that catalyzes the O-sulfonation of brassinosteroids and of mammalian estrogenic steroids. The enzyme is specific for the hydroxyl group at position 22 of brassinosteroids with a preference for 24-epicathasterone, an intermediate in the biosynthesis of 24-epibrassinolide. Enzymatic sulfonation of 24-epibrassinolide abolishes its biological activity in the bean second internode bioassay. This mechanism of hormone inactivation by sulfonation is similar to the modulation of estrogen biological activity observed in mammals. Furthermore, the expression of the B. napus steroid sulfotransferase genes was found to be induced by salicylic acid, a signal molecule in the plant defense response. This pattern of expression suggests that, in addition to an increased synthesis of proteins having antimicrobial properties, plants respond to pathogen infection by modulating steroid-dependent growth and developmental processes. (+info)
Salicylic acid induction-deficient mutants of Arabidopsis express PR-2 and PR-5 and accumulate high levels of camalexin after pathogen inoculation.
(11/1096)
In Arabidopsis, systemic acquired resistance against pathogens has been associated with the accumulation of salicylic acid (SA) and the expression of the pathogenesis-related proteins PR-1, PR-2, and PR-5. We report here the isolation of two nonallelic mutants impaired in the pathway leading to SA biosynthesis. These SA induction-deficient (sid) mutants do not accumulate SA after pathogen inoculation and are more susceptible to both virulent and avirulent forms of Pseudomonas syringae and Peronospora parasitica. However, sid mutants are not as susceptible to these pathogens as are transgenic plants expressing the nahG gene encoding an SA hydroxylase that degrades SA to catechol. In contrast to NahG plants, only the expression of PR-1 is strongly reduced in sid mutants, whereas PR-2 and PR-5 are still expressed after pathogen attack. Furthermore, the accumulation of the phytoalexin camalexin is normal. These results indicate that SA-independent compensation pathways that do not operate in NahG plants are active in sid mutants. One of the mutants is allelic to eds5 (for enhanced disease susceptibility), whereas the other mutant has not been described previously. (+info)
An early salicylic acid-, pathogen- and elicitor-inducible tobacco glucosyltransferase: role in compartmentalization of phenolics and H2O2 metabolism.
(12/1096)
Treatment of tobacco cell suspension cultures with a fungal elicitor of defense responses resulted in an early accumulation of the phenylpropanoid glucosyltransferase TOGT, along with the rapid synthesis and secretion of scopolin, the glucoside of scopoletin. Elicitor-triggered extracellular accumulation of the aglycone scopoletin and of free caffeic and ferulic acids could only be revealed in the presence of diphenylene iodonium, an inhibitor of extracellular H2O2 production. Our results strongly support a role for TOGT in the elicitor-stimulated production of transportable phenylpropanoid glucosides, followed by the release of free antioxidant phenolics into the extracellular medium and subsequent H2O2 scavenging. (+info)
Hydrogen peroxide from the oxidative burst is neither necessary nor sufficient for hypersensitive cell death induction, phenylalanine ammonia lyase stimulation, salicylic acid accumulation, or scopoletin consumption in cultured tobacco cells treated with elicitin.
(13/1096)
H(2)O(2) from the oxidative burst, cell death, and defense responses such as the production of phenylalanine ammonia lyase (PAL), salicylic acid (SA), and scopoletin were analyzed in cultured tobacco (Nicotiana tabacum) cells treated with three proteinaceous elicitors: two elicitins (alpha-megaspermin and beta-megaspermin) and one glycoprotein. These three proteins have been isolated from Phytophthora megasperma H20 and have been previously shown to be equally efficient in inducing a hypersensitive response (HR) upon infiltration into tobacco leaves. However, in cultured tobacco cells these elicitors exhibited strikingly different biological activities. beta-Megaspermin was the only elicitor that caused cell death and induced a strong, biphasic H(2)O(2) burst. Both elicitins stimulated PAL activity similarly and strongly, while the glycoprotein caused only a slight increase. Only elicitins induced SA accumulation and scopoletin consumption, and beta-megaspermin was more efficient. To assess the role of H(2)O(2) in HR cell death and defense response expression in elicitin-treated cells, a gain and loss of function strategy was used. Our results indicated that H(2)O(2) was neither necessary nor sufficient for HR cell death, PAL activation, or SA accumulation, and that extracellular H(2)O(2) was not a direct cause of intracellular scopoletin consumption. (+info)
The gain-of-function Arabidopsis acd6 mutant reveals novel regulation and function of the salicylic acid signaling pathway in controlling cell death, defenses, and cell growth.
(14/1096)
We isolated a dominant gain-of-function Arabidopsis mutant, accelerated cell death 6 (acd6), with elevated defenses, patches of dead and enlarged cells, reduced stature, and increased resistance to Pseudomonas syringae. The acd6-conferred phenotypes are suppressed by removing a key signaling molecule, salicylic acid (SA), by using the nahG transgene, which encodes SA hydroxylase. This suppression includes phenotypes that are not induced by application of SA to wild-type plants, indicating that SA acts with a second signal to cause many acd6-conferred phenotypes. acd6-nahG plants show hyperactivation of all acd6-conferred phenotypes after treatment with a synthetic inducer of the SA pathway, benzo(1,2, 3)thiadiazole-7-carbothioic acid (BTH), suggesting that SA acts with and also modulates the levels and/or activity of the second defense signal. acd6 acts partially through a NONEXPRESSOR OF PR 1 (NPR1) gene-independent pathway that activates defenses and confers resistance to P. syringae. Surprisingly, BTH-treated acd6-nahG plants develop many tumor-like abnormal growths, indicating a possible role for SA in modulating cell growth. (+info)
Salicylic acid and aspirin inhibit the activity of RSK2 kinase and repress RSK2-dependent transcription of cyclic AMP response element binding protein- and NF-kappa B-responsive genes.
(15/1096)
Sodium salicylate (NaSal) and other nonsteroidal anti-inflammatory drugs (NSAIDs) coordinately inhibit the activity of NF-kappa B, activate heat shock transcription factor 1 and suppress cytokine gene expression in activated monocytes and macrophages. Because our preliminary studies indicated that these effects could be mimicked by inhibitors of signal transduction, we have studied the effects of NSAIDs on signaling molecules potentially downstream of LPS receptors in activated macrophages. Our findings indicate that ribosomal S6 kinase 2 (RSK2), a 90-kDa ribosomal S6 kinase with a critical role as an effector of the RAS-mitogen-activated protein kinase pathway and a regulator of immediate early gene transcription is a target for inhibition by the NSAIDs. NSAIDs inhibited the activity of purified RSK2 kinase in vitro and of RSK2 in mammalian cells and suppressed the phosphorylation of RSK2 substrates cAMP response element binding protein (CREB) and I-kappa B alpha in vivo. Additionally, NaSal inhibited the phosphorylation by RSK2 of CREB and I-kappa B alpha on residues crucial for their transcriptional activity in vivo and thus repressed CREB and NF-kappa B-dependent transcription. These experiments suggest that RSK2 is a target for NSAIDs in the inhibition of monocyte-specific gene expression and indicate the importance of RSK2 and related kinases in cell regulation, indicating a new area for anti-inflammatory drug discovery. (+info)
Arabidopsis thaliana PAD4 encodes a lipase-like gene that is important for salicylic acid signaling.
(16/1096)
The Arabidopsis PAD4 gene previously was found to be required for expression of multiple defense responses including camalexin synthesis and PR-1 gene expression in response to infection by the bacterial pathogen Pseudomonas syringae pv. maculicola. This report describes the isolation of PAD4. The predicted PAD4 protein sequence displays similarity to triacyl glycerol lipases and other esterases. The PAD4 transcript was found to accumulate after P. syringae infection or treatment with salicylic acid (SA). PAD4 transcript levels were very low in infected pad4 mutants. Treatment with SA induced expression of PAD4 mRNA in pad4-1, pad4-3, and pad4-4 plants but not in pad4-2 plants. Induction of PAD4 expression by P. syringae was independent of the regulatory factor NPR1 but induction by SA was NPR1-dependent. Taken together with the previous observation that pad4 mutants have a defect in accumulation of SA upon pathogen infection, these results suggest that PAD4 participates in a positive regulatory loop that increases SA levels, thereby activating SA-dependent defense responses. (+info)