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      Chemical Diversity and Defence Metabolism: How Plants Cope with Pathogens and Ozone Pollution

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          Chemical defences represent a main trait of the plant innate immune system. Besides regulating the relationship between plants and their ecosystems, phytochemicals are involved both in resistance against pathogens and in tolerance towards abiotic stresses, such as atmospheric pollution. Plant defence metabolites arise from the main secondary metabolic routes, the phenylpropanoid, the isoprenoid and the alkaloid pathways. In plants, antibiotic compounds can be both preformed (phytoanticipins) and inducible (phytoalexins), the former including saponins, cyanogenic glycosides and glucosinolates. Chronic exposure to tropospheric ozone (O 3) stimulates the carbon fluxes from the primary to the secondary metabolic pathways to a great extent, inducing a shift of the available resources in favour of the synthesis of secondary products. In some cases, the plant defence responses against pathogens and environmental pollutants may overlap, leading to the unspecific synthesis of similar molecules, such as phenylpropanoids. Exposure to ozone can also modify the pattern of biogenic volatile organic compounds (BVOC), emitted from plant in response to herbivore feeding, thus altering the tritrophic interaction among plant, phytophagy and their natural enemies. Finally, the synthesis of ethylene and polyamines can be regulated by ozone at level of S-adenosylmethionine (SAM), the biosynthetic precursor of both classes of hormones, which can, therefore, mutually inhibit their own biosynthesis with consequence on plant phenotype.

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          Stress-Induced Phenylpropanoid Metabolism.

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            Isochorismate synthase is required to synthesize salicylic acid for plant defence.

            Salicylic acid (SA) mediates plant defences against pathogens, accumulating in both infected and distal leaves in response to pathogen attack. Pathogenesis-related gene expression and the synthesis of defensive compounds associated with both local and systemic acquired resistance (LAR and SAR) in plants require SA. In Arabidopsis, exogenous application of SA suffices to establish SAR, resulting in enhanced resistance to a variety of pathogens. However, despite its importance in plant defence against pathogens, SA biosynthesis is not well defined. Previous work has suggested that plants synthesize SA from phenylalanine; however, SA could still be produced when this pathway was inhibited, and the specific activity of radiolabelled SA in feeding experiments was often lower than expected. Some bacteria such as Pseudomonas aeruginosa synthesize SA using isochorismate synthase (ICS) and pyruvate lyase. Here we show, by cloning and characterizing an Arabidopsis defence-related gene (SID2) defined by mutation, that SA is synthesized from chorismate by means of ICS, and that SA made by this pathway is required for LAR and SAR responses.
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              Tropospheric ozone: Seasonal behavior, trends, and anthropogenic influence


                Author and article information

                Int J Mol Sci
                International Journal of Molecular Sciences
                Molecular Diversity Preservation International (MDPI)
                August 2009
                30 July 2009
                : 10
                : 8
                : 3371-3399
                Università degli Studi di Milano, Dipartimento di Produzione Vegetale, Sezione di Patologia Vegetale, Via Celoria 2, 20133 Milano, Italy
                Author notes
                [* ]Author to whom correspondence should be addressed; E-Mail: marcello.iriti@ ; Tel. +39-02-503-16766; Fax: +39-02-503-16781
                © 2009 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.

                This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (



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