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Salicylates – plant hormones and painkillers
Salicylic Acid Acetylsalicylic Acid ASPIRIN Named for Spiraea, the former name for meadowsweet Named for white willow Salix alba Photo credits: Geaugagrrl; Sten Porse
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How do salicylates work in plants?
SA’s effects in plants and animals are unrelated Expression of pathogen-induced genes Increased synthesis of SA Signal (e.g. Pathogen, UV light, developmental cues) DEFENSE Stress responses Developmental responses In plants SA’s main effect is to alter patterns of gene expression
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History of salicylate research
In 1990 SA was shown to be an endogenous signal in defense responses Hippocrates wrote about the use of willow to relieve pain ~ 2400 years ago Untreated side In 1987 endogenous SA was shown to be responsible for heat production in Arum lily flowers SA treated In 1979 White showed that pre-treatment of a leaf with aspirin or SA conferred resistance to tobacco mosaic virus (TMV) National Library of Medicine; Raskin, I. (1992). Salicylate, a new plant hormone. Plant Physiol. 99:
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Lecture outline Synthesis, conjugation and transport
Perception and signaling Salicylates in whole-plant processes Image credit: Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885, Gera, Germany
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There are two pathways for SA synthesis, via ICS and PAL
Cytoplasm ca. 5% total Chloroplast ca. 95% total The major route for the biosynthesis of SA (up to 95% of total) takes place in the chloroplast. One of the key enzymes is ICS, isochorismate synthase PAL ICS A small amount of SA is derived from cinnamic acid produced by PAL in the cytoplasm Reprinted with permission from Chen, Z., Zheng, Z., Huang, J., Lai, Z. and Fan, B. (2009). Biosynthesis of salicylic acid in plants. Plant Signaling & Behavior. 4: ; see also Reprinted from Métraux, J.-P. (2002). Recent breakthroughs in the study of salicylic acid biosynthesis. Trends in Plant Science 7: with permission from Elsevier. .
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In many plants ICS1 is upregulated by pathogen exposure
Expression of ICS1 increases upon exposure to a pathogen PR1 is an SA-induced pathogen responsive gene In the top experiment, -Erysiphe on the left is without pathogen, +Erysiphe on the right is with pathogen. The experiment was done in triplicate and shows results from each independent experiment three separate lanes. On the lower panel, hpi is hours post infection. Expresison of two genes, ICS1 (biosynthesis of SA) and PR1 (response to SA) are shown as solid and white bars respectively. Reprinted by permission from Macmillan Publishers Ltd. Wildermuth, M.C., Dewdney, J., Wu, G., and Ausubel, F.M. (2001). Isochorismate synthase is required to synthesize salicylic acid for plant defence. Nature 414: copyright 2001.
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Regulation of SA accumulation
Other stress (e.g. UV light) Bacterial pathogen Pathogen effectors SA synthesis is induced by pathogens or stress. Genetic studies have identified some of the signals that transduce pathogen perception to SA synthesis Fungal or oomycete pathogen ICS1 SA
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Perception and signaling
SA PR SA induces an enormous transcriptional response PR genes are pathogenesis related genes, and are involved in defense responses Ward, E.R., Uknes, S.J., Williams, S.C., Dincher, S.S., Wiederhold, D.L., Alexander, D.C., Ahl-Goy, P., Metraux, J.P. and Ryals, J.A. (1991). Coordinate gene activity in response to agents that induce systemic acquired resistance. Plant Cell. 3:
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NPR1 is a major activator of SA-mediated responses
Wild-type npr1 (aka nim1) NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1) is necessary for defense responses and at the core of SA signal transduction The plant on the right is supporting a lot of pathogen growth, which appears as a white fuzz on the leaf surfaces (the disease caused by this pathogen is called downy mildew). Plants were treated with SA, then three days later challenged with a fungal pathogen (Hyaloperonospora arabidopsidis) Delaney, T.P., Friedrich, L., and Ryals, J.A. (1995). Arabidopsis signal transduction mutant defective in chemically and biologically induced disease resistance. Proc. Natl. Acad. Sci. USA 92: copyright 1995 National Academy of Sciences USA.
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NPR1 oligomerizes via redox-sensitive cysteines
HS SH SS Reduction Oxidization SA accumulation contributes to a reducing environment, which causes multimeric NPR1 to monomerize NPR1 SA SA SA NPR1 Then panel on the right shows confocal images of NPR-GFP fluorescence in mesophyll cells, with GFP fluorescence shown in the green channel and differential interference contrast in the red channel. Monomeric NPR1 is imported into the nucleus (here it is GFP-labeled) Kinkema, M., Fan, W., and Dong, X. (2000). Nuclear localization of NPR1 is required for activation of PR gene expression. Plant Cell 12:
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NPR1, NPR3 and NPR4 have recently been reported as SA receptors
In mid-2012, two groups reported that members of the NPR family serve as SA receptors The two groups used different assays to characterize SA binding, leading to different conclusions NPR3/4 bind SA and regulate NPR1 NPR1 is regulated directly by SA binding and / or Fu, Z.Q., Yan, S., Saleh, A., Wang, W., Ruble, J., Oka, N., Mohan, R., Spoel, S.H., Zheng, N. and Dong, X. (2012) NPR3 and NPR4 are receptors for the immune signal salicylic acid in plants. Nature 486: 228–232.
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In the nucleus, SA-activated NPR1 promotes transcription
NO SA + SA -TGAs -WRKYs +TGAs +WRKYs In the absence of SA and activated NPR1, negative regulators repress defense genes In the presence of SA and activated nuclear NPR1, positive regulators activate defense genes
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NPR1 binds TGA transcription factors and promotes DNA binding
DNA binding of TGA2 is enhanced by NPR1 The Arabidopsis genome encodes 10 TGA factors. Some are positive and some negative regulators of defense genes NPR1 +TGAs NPR1 -TGAs -TGAs +TGAs Bound DNA Yeast-two hybrid assay showing NPR1/ TGA interaction +TGAs Free DNA Zhang, Y., Fan, W., Kinkema, M., Li, X., and Dong, X. (1999). Interaction of NPR1 with basic leucine zipper protein transcription factors that bind sequences required for salicylic acid induction of the PR-1 gene. Proc. Natl. Acad. Sci. USA 96: ; Despres, C., DeLong, C., Glaze, S., Liu, E., and Fobert, P.R. (2000). The Arabidopsis NPR1/NIM1 protein enhances the DNA binding activity of a subgroup of the TGA family of bZIP transcription factors. Plant Cell 12:
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Signaling downstream of SA - summary
Questions remain Questions remain ? NPR1 NPR3/4 Activating histone marks ? +WRKY -WRKY DNA recombination / repair? Kesarwani, M., Yoo, J. and Dong, X. (2007). Genetic interactions of TGA transcription factors in the regulation of pathogenesis-related genes and disease resistance in Arabidopsis. Plant Physiol. 144:
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Salicylates and whole plant physiology
Responses to pathogens Recognition Local and hypersensitive response Systemic acquired resistance Cross-talk Other functions Abiotic stress Thermogenesis
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Pathogen Triggered Immunity involves SA synthesis and response
1. PAMPs perceived by PRRs PAD4 / EDS1 SA ICS1 3. NPR1 mono-merization and nuclear import NPR1 2. Activation of ICS1 and increased SA synthesis NPR1
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Effector Triggered Immunity involves SA synthesis and response
R protein SA 4. Recognition of effectors directly or indirectly by R proteins and increased SA synthesis NPR1 NPR1
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Salicylates are indispensible signals for local and systemic resistance
SA-deficient plants are deficient in local and hypersensitive responses (HR) and systemic acquired resistance (SAR) SA Mobile signal HR
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Local responses – hypersensitive response
SA ROS accumulation Cell death, PR genes induced Wall fortification Pathogen perception triggers a local hypersensitive response that includes an oxidative burst, wall fortification, induction of PR genes and cell death
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Systemic acquired resistance was described in TMV-infected tobacco
This plant was not pre-inoculated with TMV and shows large lesions upon TMV inoculation This experiment demonstrates systemic signaling This plant was pre-inoculated with TMV on the three bottom leaves. Seven days later two upper leaves were inoculated. The upper leaves show tiny lesions indicating an enhanced resistance response Reprinted from Ross, A.F. (1961). Systemic acquired resistance induced by localized virus infections in plants. Virology 14: with permission from Elsevier.
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Systemic resistance may involve multiple signals
SA SAR MeSA AzA SA is necessary in systemic tissue for SAR, but the nature of the mobile signal(s) is still up in the air It is likely that multiple signals contribute to SAR 3PG 3PG XY DIR1, lipids
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Heat production in arum lilies involves SA synthesis
Blowflies on Helicodiceros muscivorus, the dead horse arum Some arum lilies smell like rotting flesh, attracting flies and carrion-eating insects as pollinators. They produce heat to carry the odor and mimic a decaying corpse There are some fun videos of arum lilies on the Teaching Tools YouTube channel ( Titan arum (Amorphophallus titanum) Photo credit: U.S. Botanic Garden
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Ongoing studies - Regulation of defense transcriptome
What is the inter-relationship between different SA receptors? How does cellular redox transmit SA signals, and vice versa? NPR1 NO SA + SA -TGAs -WRKYs +TGAs +WRKYs NPR1 NPR3 NPR4 SNI1 How does chromatin remodeling contribute to the regulation of SA-responsive genes? What about the many other genes that affect SA signaling and response?
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Ongoing studies- Systemic signals and signaling
SAR Systemic acquired resistance – What other signals are involved? How much information is conveyed? How is resistance maintained for long periods? SA Mobile signal(s) Local response – What are the respective roles of SA and redox in the hypersensitive response? HR
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