SummaryAnalyses of emerging concepts indicate that parallels exist between selfincompatibility and pathogen recognition. In the case of surveillance of 'nonself', plant immune responses are triggered either by pattern recognition receptors (PRRs) that detect conserved pathogen-associated molecular patterns (PAMPs) or by resistance (R) proteins recognizing isolate-specific pathogen effectors. PAMP detection is an important component of innate immunity in plants and serves as an early warning system for the presence of potential pathogens and activation of plant defense mechanisms. In the Brassicaceae, the recognition of 'self' and self-incompatibility are components of a receptor-ligand based mechanism that utilizes an S receptor kinase (SRK) to perceive and reject 'self'-pollen. SRK is an S-domain receptor-like kinase (RLK), which in turn is part of the RLK family, some members of which represent PRRs involved in the detection of PAMPs. S-domain RLKs also occur in species that do not exhibit self-incompatibility and are up-regulated in response to wounding, PAMPs and pathogen recognition. Although evolution may have driven expansion of certain RLK families to serve roles in particular physiological processes, this may not exclude these receptor types from functioning in different programs. Recent findings on self/nonself recognition are reviewed and conceptual and mechanistic links between microbial recognition and self-incompatibility are discussed.
IntroductionThe two separate mechanisms of innate immunity and selfincompatibility (SI) are remarkably similar. The similarities and differences between the two mechanisms in terms of functions, functional outcomes, selective processes, responses, recognition molecules, recognition receptors, and signal transduction and perception are summarized herein. In order to elucidate how innate immunity fits into the global picture of overlapping and complex plant defense mechanisms, a short overview is presented first. In addition, an overview of SI is given to elucidate the molecular and biochemical mode of SI in the Brassicaceae. This is followed by a discussion of the role of receptor-like kinases (RLKs) in defense mechanisms and SI. While the role of S-domain RLKs in SI within the Brassicaceae is well described, the role of these receptors in pathogen perception and defense is not widely recognized.Plant innate immunity, with its associated defense mechanisms, exhibits similar characteristics to the mammalian and insect mechanisms (Nürnberger et al., 2004;Zipfel & Felix, 2005). Although they express an apparent passivity associated with their sedentary lifestyle, and are simultaneously exposed to evolving pathogens as well as environmental stresses, plants have evolved a unique metabolic plasticity that allows them to perceive pathogens and unleash effective defense strategies. The innate immune system in plants is unable to acquire or specifically adapt like the animal adaptive immune system (Goldsby et al., 2000) and relies on a spectrum of predetermined receptors ex...
The ability to distinguish 'self' from 'nonself' is the most fundamental aspect of any immune system. The evolutionary solution in plants to the problems of perceiving and responding to pathogens involves surveillance of nonself, damaged-self and altered-self as danger signals. This is reflected in basal resistance or non-host resistance, which is the innate immune response that protects plants against the majority of pathogens. In the case of surveillance of nonself, plants utilize receptor-like proteins or -kinases (RLP/Ks) as pattern recognition receptors (PRRs), which can detect conserved pathogen/microbe-associated molecular pattern (P/MAMP) molecules. P/MAMP detection serves as an early warning system for the presence of a wide range of potential pathogens and the timely activation of plant defense mechanisms. However, adapted microbes express a suite of effector proteins that often interfere or act as suppressors of these defenses. In response, plants have evolved a second line of defense that includes intracellular nucleotide binding leucine-rich repeat (NB-LRR)-containing resistance proteins, which recognize isolate-specific pathogen effectors once the cell wall has been compromised. This host-immunity acts within the species level and is controlled by polymorphic host genes, where resistance protein-mediated activation of defense is based on an 'altered-self' recognition mechanism.
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