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Plant Innate Immunity Advanced article Jens Staal, Department of Molecular Biomedical Research, VIB, Ghent University, Ghent, Belgium Article Contents . Introduction Christina Dixelius, Department of Plant Biology and Forest Genetics, SLU, Uppsala, Sweden . Plant Host–Pathogen Models . Entering Plant Cells The ability to discriminate between self- and nonself-molecules is characteristic of all . How Plants Identify Different Parasites through General living organisms. This feature forms the basis for the activation of innate immune Patterns responses upon microbial attack. If microbes bypass the external physical barrier, plants . Effector-Triggered Susceptibility and Effector-Triggered have evolved two classes of immune receptors to detect nonself-molecules to prevent Immunity . Receptors Recognizing Pathogens, Blurring the further pathogen progress. One class consists of membrane-resident pattern- Borderline between PTI and ETI recognition receptors that sense molecules from microbes, the so-called microbe- . Stabilization, Signalling and Degradation of R Protein associated molecular patterns (MAMPs). The second class consists of plant resistance (R) Complexes proteins that have capacity to detect directly or indirectly isolate-specific pathogen . Hypersensitive Response and Cell Death Signalling effectors encoded by avirulence genes. These receptors are mainly intracellular. An . Defence Signal Transduction alternative route is effector molecules that act as transcription factors. Signal . Remembering Previous Attacks transduction cascades link recognition and defence responses through second . Acknowledgements messengers, transcription factors and crosstalk between plant hormones to fine-tune Online posting date: 15th March 2009 the overall response. Introduction specific recognition events must occur which determines the host response. Furthermore, different pathogens are Innate immunity is an ancient trait where various recog- acquiring their nutrients from the host in different manners nition systems distinguish between ‘self’ and ‘nonself’. This and a defence strategy that may be efficient against one distinction provides the fundamental basis for altruistic pathogen could actually benefit another (Spoel et al., 2007), behaviour in ‘social’ microbes and ultimately how multi- which implies that the plant also needs to monitor the type cellular organisms could arise. Like all living systems, of attack and determine its defence response accordingly. plants need to protect themselves from parasites. Large, Plants, unlike mammals, lack mobile defender cells and a multicellular organisms with a comparably slow genera- somatic adaptive immune system. Instead, they rely on in- tion time must race against small and rapidly evolving nate (nonadaptive) immunity of each cell and on systemic parasites. Despite this, plants and other multicellular or- signals emanating from infection sites. The first barrier of ganisms are not fighting a losing battle and infectious dis- defence against pathogens is by preventing access to the ease is more an exception than a rule. In fact, the immune host through various physical barriers on the outer sur- response itself is a double-edged sword and must be tightly faces, mainly the plant cell wall. This is however not enough controlled in order not to cause disease itself or otherwise and specific detection systems have evolved to cope with be detrimental for the organism. See also: Eukaryotes and rapidly evolving pathogens, which will be the main focus of Multicells: Origin; Innate Immune Mechanisms: Nonself this article. One solution of the problem is to evolve rec- Recognition ognition of general ‘patterns’, which are indispensable for It is also important for the plant, just like for any other various types of pathogens. The challenge of pathogens is organism, to distinguish between parasites and other mi- probably one reason why sexual reproduction has been croorganisms that are commensalistic. Sometimes the dis- maintained throughout evolution despite its fitness costs crimination between a pathogen and a commensalistic (Kover and Cacedo, 2001). microorganism can be very indistinct and both often ac- quire nutrients from the plant in a very similar manner (Holub, 2006; Soto et al., 2006). To make this distinction, Plant Host–Pathogen Models The majority of our current knowledge about plant innate immune receptors and signalling components originate ELS subject area: Plant Science from genetic analyses from the model organism Arabido- psis thaliana.InArabidopsis, some of the most important How to cite: model pathogens where most of the genetics and under- Staal, Jens; and, Dixelius, Christina (March 2009) Plant Innate Immunity. standing of plant innate immunity has been delineated are In: Encyclopedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Pseudomonas syringae (a bacterium), Hyaloperonospera Chichester. parasitica (an oomycete) and Alternaria brassicicola DOI: 10.1002/9780470015902.a0020114 (a fungus). Importantly, many of the models presented ENCYCLOPEDIA OF LIFE SCIENCES & 2009, John Wiley & Sons, Ltd. www.els.net 1 Plant Innate Immunity here are valid in Arabidopsis but there are observations that defence responses leading to basal or nonhost resistance some signalling may occur differently in other plant (Figure 1a), or more commonly denoted PAMP-triggered species. Systems where additional important information immunity (PTI). To date, all known PRRs in plants are are generated are the interactions between the leaf mould plasma membrane-resident proteins, allowing the percep- fungus Cladosporium fulvum on tomato, the rust fungus tion of MAMPs to occur at the cell surface (He et al., 2007). Melampsora lini on flax, the powdery mildew causing MAMPs represent a broad category of compounds but fungus Blumeria graminis f. sp. hordei on barley, the rice typically all are essential for microbial life. These include blast fungus Magnaporthe grisea and the potato late blight chitin, ergosterol and xylanse from fungi and b-glucans oomycete Phytophthora infestans. See also: Arabidopsis from oomycetes. Similarly, lipopolysaccharides flagellin, thaliana as an Experimental Organism cold shock protein and elongation factor Tu (EF-Tu) all from bacteria act as MAMPs in plants (Zipfel, 2008). Interestingly, MAMPs such as glucans, chitin, lipopoly- Entering Plant Cells saccharides and flagellin can act in both plants and animals, but the individual epitopes that are recognized To invade a plant, phytopathogenic bacteria, fungi and differ. For example, the Arabidopsis flagellin insensitive oomycetes have evolved strategies to subvert host immu- 2 (FLS2) protein recognize a 22-amino acid motif in flag- nity and actively penetrate plant tissue. Wounded plants ellin, whereas mammalian toll-like receptor 5 recognize an are sensitive since wounds constitute an easy entrance for epitope formed by the N- and C-termini of the flagellin many types of pathogens. Many bacteria such as Ps. peptide chain (Zipfel, 2008). The FLS2, EFR (EF-Tu re- syringae swim towards openings like stomata and hydath- ceptor) and LeEix1/2 receptors display extracellular leu- odes and enter the apoplastic space of plant tissue. From cine-rich repeat (LRR) domains for either direct or indirect there, bacterial effectors can be injected into the cytoplasm recognition of the MAMPs. The chitin receptor LysM via various secretion systems, like the type III secretion RLK1/CERK1, however, has a different extracellular do- system in Pseudomonas and the type IV secretion system main (Miya et al., 2007; Wan et al., 2008). The Arabidopsis found in Agrobacterium. Several fungi have spores that LysM chitin receptor also has an intracellular kinase do- upon germination on a host form an appressorium. This is main analogously to that of EFR and FLS2 and signalling a fungal structure that exerts high pressure on the plant cell appear to converge partly with components downstream of walls allowing the fungal hyphae to enter and invaginate those two receptors of bacterial MAMPs. This is in con- cell membranes. At this stage pathogens need either to trast to the rice LysM chitin receptor CEBiP that lack in- bypass or suppress pathogen-associated molecular pat- tracellular domains. terns (PAMP) triggered immune responses to proliferate In addition, knowledge on gene silencing and thereby and colonize the host tissue. resistance mechanisms in plants derives from studies on The plant endomembrane system exhibits a much higher degradation of viral ribonucleic acid (RNA) (Hamilton degree of complexity than that of mammals or yeast and Baulcombe, 1999) which contributed to the discovery (Jurgens, 2004). This is accompanied by an increased of short RNA species. The research on small noncoding complexity of genes coding for proteins regulating vesicle RNA has expanded enormously the last 10 years and we trafficking. In plants N-ethylmaleimide-sensitive factor have learnt much of their important roles in gene expres- adaptor protein receptors or SNARE (soluble N-ethyl- sion not least in plant immunity including PAMP and PRR maleimide-sensitive factor attachment protein receptor)- recognition affecting PTI, and R gene recognition that mediated vesicle trafficking are of outmost importance in modify effector-triggered immunity (ETI) (Jin, 2008). Re- basal plant defence. Much of our current understanding of cently, evidence was also presented where bacteria-like vi- these events derives from studies of Bl. graminis f. sp. hordei ruses were shown to have evolved mechanisms

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