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      Differential innate immune signalling via Ca 2+ sensor protein kinases

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          Abstract

          Innate immunity represents the first line of inducible defense against microbial infection in plants and animals 13. In both kingdoms, recognition of pathogen- or microbe-associated molecular patterns (PAMPs or MAMPs), such as flagellin, initiates convergent signalling pathways involving MAP kinase (MAPK) cascades and global transcriptional changes to boost immunity 14. Although Ca 2+ has long been recognized as an essential and conserved primary mediator in plant defense responses, how Ca 2+ signals are sensed and relayed into early MAMP signalling is unknown 5, 6. Here, we use a functional genomic screen and genome-wide gene expression profiling to show that four calcium-dependent protein kinases (CDPKs) are Ca 2+ sensor PKs critical to transcriptional reprogramming in plant innate immune signalling. Unexpectedly, CDPKs and MAPK cascades act differentially in four MAMP-mediated regulatory programs to control early genes involved in synthesis of defense peptides and metabolites, cell wall modifications and redox signalling. Transcriptome profile comparison suggests that CDPKs are the convergence point of signalling triggered by most MAMPs. Double, triple and quadruple cpk mutant plants display progressively diminished oxidative burst and gene activation induced by flg22, as well as compromised pathogen defense. In contrast to negative roles of calmodulin (CAM) and a CAM-activated transcription factor in plant defense 7, 8, the present study reveals Ca 2+ signalling complexity and demonstrates key positive roles of specific CDPKs in initial MAMP signalling.

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          Most cited references 31

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          A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence.

          Plants sense potential microbial invaders by using pattern-recognition receptors to recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, the leucine-rich repeat receptor kinases flagellin-sensitive 2 (FLS2) (ref. 2) and elongation factor Tu receptor (EFR) (ref. 3) act as pattern-recognition receptors for the bacterial PAMPs flagellin and elongation factor Tu (EF-Tu) (ref. 5) and contribute to resistance against bacterial pathogens. Little is known about the molecular mechanisms that link receptor activation to intracellular signal transduction. Here we show that BAK1 (BRI1-associated receptor kinase 1), a leucine-rich repeat receptor-like kinase that has been reported to regulate the brassinosteroid receptor BRI1 (refs 6,7), is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellin-triggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. We provide evidence that FLS2 and BAK1 form a complex in vivo, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through the plant hormone receptor BRI1 (refs 6,7), but also has a functional role in PRR-dependent signalling, which initiates innate immunity.
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            Bacterial disease resistance in Arabidopsis through flagellin perception.

            Plants and animals recognize microbial invaders by detecting pathogen-associated molecular patterns (PAMPs) such as flagellin. However, the importance of flagellin perception for disease resistance has, until now, not been demonstrated. Here we show that treatment of plants with flg22, a peptide representing the elicitor-active epitope of flagellin, induces the expression of numerous defence-related genes and triggers resistance to pathogenic bacteria in wild-type plants, but not in plants carrying mutations in the flagellin receptor gene FLS2. This induced resistance seems to be independent of salicylic acid, jasmonic acid and ethylene signalling. Wild-type and fls2 mutants both display enhanced resistance when treated with crude bacterial extracts, even devoid of elicitor-active flagellin, indicating the existence of functional perception systems for PAMPs other than flagellin. Although fls2 mutant plants are as susceptible as the wild type when bacteria are infiltrated into leaves, they are more susceptible to the pathogen Pseudomonas syringae pv. tomato DC3000 when it is sprayed on the leaf surface. Thus, flagellin perception restricts bacterial invasion, probably at an early step, and contributes to the plant's disease resistance.
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              Glucosinolate metabolites required for an Arabidopsis innate immune response.

              The perception of pathogen or microbe-associated molecular pattern molecules by plants triggers a basal defense response analogous to animal innate immunity and is defined partly by the deposition of the glucan polymer callose at the cell wall at the site of pathogen contact. Transcriptional and metabolic profiling in Arabidopsis mutants, coupled with the monitoring of pathogen-triggered callose deposition, have identified major roles in pathogen response for the plant hormone ethylene and the secondary metabolite 4-methoxy-indol-3-ylmethylglucosinolate. Two genes, PEN2 and PEN3, are also necessary for resistance to pathogens and are required for both callose deposition and glucosinolate activation, suggesting that the pathogen-triggered callose response is required for resistance to microbial pathogens. Our study shows that well-studied plant metabolites, previously identified as important in avoiding damage by herbivores, are also required as a component of the plant defense response against microbial pathogens.
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                Author and article information

                Journal
                0410462
                6011
                Nature
                Nature
                0028-0836
                1476-4687
                9 February 2010
                17 February 2010
                18 March 2010
                18 September 2010
                : 464
                : 7287
                : 418-422
                Affiliations
                Department of Genetics, Harvard Medical School, and Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, MA 02114, USA
                Author notes
                [* ]Tel: 617-726-5963, Fax: 617-643-3050, sheen@ 123456molbio.mgh.harvard.edu
                [*]

                present address: Institut des Sciences du Végétal, UPR2355, 1 avenue de la Terrasse, 91198 Gif s/Yvette cedex, France

                [#]

                present address: Department of Biology, University of Pennsylvania, 433 South University Avenue, Philadelphia, Pennsylvania, PA 19104, USA

                [§]

                present address: Institute for Plant Genomics and Biotechnology, Texas A&M University, College Station, TX 77843, USA

                [+]

                Deceased

                Article
                nihpa169264
                10.1038/nature08794
                2841715
                20164835

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                Funding
                Funded by: National Institute of General Medical Sciences : NIGMS
                Award ID: R01 GM092893-01 ||GM
                Funded by: National Institute of General Medical Sciences : NIGMS
                Award ID: R01 GM070567-05 ||GM
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