Silenciamiento genetico

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Early molecular events in PAMP-triggered immunity
Cyril Zipfel
In plant innate immunity, the first line of microbial recognition leading to active defence responses relies on the perception of pathogen-associated molecular patterns (PAMPs) by patternrecognition receptors (PRRs). This recognition leads to PAMPtriggered immunity (PTI). Despite thenumerous PAMPs recognised by plants, only a handful of PRRs are characterised. For most, they correspond to transmembrane proteins with a ligand-binding ectodomain. PRRs interact with additional transmembrane proteins that act as signalling adapters or amplifiers to achieve full functionality. The crucial role of PRRs in anti-microbial immunity is demonstrated by the direct targeting of PRRs and theirassociated proteins by pathogenic virulence effectors.
Address The Sainsbury Laboratory, Colney Lane, Norwich NR4 7UH, UK Corresponding author: Zipfel, Cyril (cyril.zipfel@tsl.ac.uk)

the dynamic interplay between plants and their pathogens. The term PRR is sometimes used incorrectly to refer to any protein involved in plant innate immune recognition, whether they recognise PAMPs or effectors.The term PRR should be used only to refer to receptors that directly recognise molecules that are conserved among a large group or class of microbes. The definition of a PAMP relates to the conservation of the molecule, or epitope within, it is referring to. In rare examples, some virulent phytopathogenic bacteria are able to mask recognition of a PAMP (e.g. flagellin) by mutating residues within therecognised epitope [1]. This reflects a virulence strategy evolved by successful pathogens in addition to effector secretion. In other words, while flagellins (for example) from nearly all bacteria are recognised by plants, only a few from plant pathogenic bacteria are not. This cannot be compared with the hyper-variability (most often presence/absence) observed for most effectors [6]. Being atransmembrane receptor is not sufficient to define a PRR either. In mammals for example, some PRRs are transmembrane proteins whereas others PRRs are cytoplasmic proteins [7]. In plants, while a few previously defined R proteins are transmembrane proteins [8], the lack of knowledge on the identity and/or the conservation of their ligands (in case they mediate direct recognition) preclude theirclassification as PRR or ‘true’ R protein. In the present review, I will summarise and discuss recent findings on plant PRRs (Figure 1), their association with other signalling components, and their direct targeting by pathogenic virulence effectors.

Current Opinion in Plant Biology 2009, 12:414–420 This review comes from a themed issue on Biotic Interactions Edited by Xinnian Dong and Regine KahmannAvailable online 14th July 2009 1369-5266/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2009.06.003

Introduction
Plants are resistant to most microbes and rely entirely on innate immune responses for their defence. In plant, inducible defence responses are triggered by two levels of microbial recognition. The first recognises conserved microbial molecules, referredto as pathogen- (or microbe) associated molecular patterns (PAMPs/MAMPs), via pattern-recognition receptors (PRRs) [1]. While immunologists commonly use the term PAMPs, the term MAMPs is considered more accurate since they are not restricted to pathogens [2]. PAMP-triggered immunity (PTI) is key to plant innate immunity. PRR mutants are more susceptible to microbial infections and pathogens thatfail to avoid or suppress PTI are unable to cause disease [1]. In many examples, successful pathogens suppress PTI by secreting effectors in the apoplast or directly into the cytoplasm of host cells, leading to effector-triggered susceptibility [3]. Some plant cultivars have evolved resistance proteins (R proteins) to recognise particular effectors directly or indirectly, leading to...