HAU16008-07-3 - Cell Mol Life Sci 64(2007 2726 2732...

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Visions & Reflections (Minireview) How plants recognize pathogens and defend themselves P. J. G. M. de Wit Wageningen University, Laboratory of Phytopathology and Wageningen Centre for Biosystems Genomics, Binnenhaven 5, Wageningen (The Netherlands), Fax: + 31317483412, e-mail: [email protected] Received 24 June 2007; received after revision 18 July 2007; accepted 15 August 2007 Online First 17 September 2007 Abstract. Plants have an innate immunity system to defend themselves against pathogens. With the pri- mary immune system, plants recognize microbe- associated molecular patterns (MAMPs) of potential pathogens through pattern recognition receptors (PRRs) that mediate a basal defense response. Plant pathogens suppress this basal defense response by means of effectors that enable them to cause disease. With the secondary immune system, plants have gained the ability to recognize effector-induced per- turbations of host targets through resistance proteins (RPs) that mediate a strong local defense response that stops pathogen growth. Both primary and secon- dary immune responses in plants depend on germ line- encoded PRRs and RPs. During induction of local immune responses, systemic immune responses also become activated, which predispose plants to become more resistant to subsequent pathogen attacks. This review gives an update on recent findings that have enhanced our understanding of plant innate immunity and the arms race between plants and their pathogens. Keywords. MAMP, plant innate immunity, microbial effector, infection, local and systemic resistance, guard model. Biotrophic and necrotrophic plant pathogens Plants become infected by pathogens with different lifestyles [1]. Biotrophic pathogens are specialized to feed on living plant tissues, and some have developed an intimate relationship with their host plant, co- evolving into obligate biotrophs that cannot be cultured on synthetic media. Non-obligate biotrophs can be cultured on synthetic media, but neither obligate nor non-obligate biotrophs can grow as saprophytes. Biotrophs have a narrow host range, and strains of these pathogens have often adapted to a specific line of a given plant species. Many biotrophs live in the intercellular space between leaf mesophyl cells, and some produce haustoria as feeding struc- tures that invaginate the plasma membrane of host cells, enabling them to create a specific micro- environment for retrieval of nutrients [2, 3]. Necrotrophic pathogens are less specialized and have a much less intimate relationship with their host plants. They often grow on plant tissues that are wounded, weakened or senescent and frequently produce toxins to kill host tissue prior to colonization. Necrotrophs can also grow outside the host as saprophytes and can easily be cultured on synthetic media [1].
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