Molecular Immunology 41 (2004) 1055–1062 Review Plant perception systems for pathogen recognition and defence Lourdes G ´ omez-G´ omez * Secci´ on de Biotecnolog´ ıa, IDR, Campus Universitario s/n, E-02071 Albacete, Spain Available online 2 September 2004 Abstract Recognition of and defence against microbial infections are universal adaptations of multicellular organisms. Plants express a sophisticated molecular system for recognition of and response to potentially pathogenic microorganism. Although the environments of plant and animal cells are vastly different and present unique challenges to invading pathogens, the molecular basis of the innate immune response shows remarkable evolutionary conservation in both kingdoms. Even if more cellular components are continually being identified in plants, the mechanism, interactions and responses are only partly understood. Furthermore, most of this research has been performed in the model plant Arabidopsis thaliana and a significant effort will be needed to understand the above processes in crop species and in natural populations. In this review I will describe examples of the best characterized recognition systems in plants, that mediate pathogen perception either through the perception of highly variable and non-essential pathogen molecules or via conserved microbial structures called pathogen associated molecular patterns (PAMPs). © 2004 Elsevier Ltd. All rights reserved. Keywords: Plant perception systems; Pathogen recognition; Defence 1. Introduction Microorganisms are capable of causing disease in a myr- iad of plant hosts and are responsible for important economic damage to crops around the world. However, a relatively small proportion of these pathogens invades the plant host successfully and cause disease. Unlike animals, which can move from place to place to find the most accommodating surroundings, plants must do with the local circumstances dictated by where they sprouted. The surfaces of plant or- gans, both above and below ground, are continuously and permanently exposed to a diverse range of enemies, includ- ing microbial pathogens, nematodes and insects. Plants have no circulating cells; therefore, recognition of pathogens must be a capability of every cell in the organism. Plants, in fact are equipped with a variety of defence mechanisms, which include preformed defences such as waxes, cell wall compo- nents, and secondary metabolites. Upon pathogen detection plants activate a number of defences like the hypersensitive * Tel.: +34 967 599200 2612; fax: +34 967 599233 E-mail address: marialourdes.gomez@uclm.es (L. G ´ omez-G´ omez). response, increased expression of defence-related genes, pro- duction of antimicrobial compounds, lignin formation and the oxidative burst (Feys and Parker, 2000). Thus, resistance is due to a combination of physical and chemical barriers, which are either preformed or induced after the infection. By con- trast, disease results either from the failure of the recognition event or the ability of the pathogen to avoid or overcome the resistance response. 2. Pathogen recognition by plant cells Central to the activation of defence responses is timely perception of the pathogen by the plant. The pathogen must either penetrate the plant cell wall and/or the membrane, or locally diffuse effectors or elicitors that are perceived by the host. The simplest mode of elicitor perception by the plant cell would be a direct interaction of pathogen-derived elic- itors with specific receptors on host cells (Ebel and Cosio, 1994). The pathogen effectors seem to fall into two broad categories. The “virulent determinants” (Avr) act as elicitors that are specific and unique for a particular pathogen, and have been developed as a microbial adaptation to the unique 0161-5890/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.molimm.2004.06.008