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In an environment rich in potentially harmful microbes, plant survival depends on efficient microbe perception and fast defense responses. Contrary to the mammalian immune system composed of cells specialized for defense (e.g. lymphocytes), plant immunity relies on the ability of each cell to recognize pathogens. A first level of microbe recognition is performed by membrane proteins termed pattern recognition receptors (PRRs), which perceive molecular signatures characteristic of a whole class of microbes, termed pathogen-associated (or microbe-associated) molecular patterns (PAMPs; Medzhitov and Janeway, 1997). Perception of PAMPs by PRRs is common to all multicellular organisms and leads to an array of defense responses and redeployment of cellular energy in a fast, efficient, and multiresponse manner, which prevents further pathogen ingress. PAMP recognition leads to a chain of signaling events, broadly referred to as general defense responses in plants. PAMP perception also results in plant systemic acquired resistance (Mishina and Zeier, 2007b).
Faced with PAMP-triggered immunity (PTI), successful pathogens evolved secreted effectors targeting key PTI actors to interfere with plant defense. In turn, some plant cultivars have evolved resistance (R) proteins to directly or indirectly detect these effectors (previously termed avirulence or Avr proteins) according to the gene-to-gene theory and leading to effector-triggered immunity (ETI), which is often accompanied by the hypersensitive response, a form of programmed cell death. This model illustrates the dynamic coevolution between plants and pathogens (Chisholm et al., 2006; Jones and Dangl, 2006).
Gaining knowledge related to recognition and signaling in PTI constitutes a challenge in plant pathology research, as many of the underlying molecular mechanisms remain largely unknown. In this review, we summarize our knowledge of PTI with a special focus on recognition of bacteria.
PAMP RECOGNITION BY PRRS: THE PLANT SENTINELS ARE AT THE PLASMA MEMBRANE
PAMPs are molecular components highly conserved within a class of microbes, where they carry out an essential function for fitness or survival (Medzhitov and Janeway, 1997). Plants recognize a wide range of bacterial PAMPs, most of which are derived from structural components of the bacterial cell. Although the number of identified bacterial PAMPs recognized by plants is increasing constantly, very few plant PRRs have been discovered.
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