In contrast to animals, plants lack specialized mobile cells that can be readily produced when required for defense and an adaptive immune system that creates immunological memory. To fight pathogens, plants must therefore rely on a combination of germ-line encoded cellular innate immunity and the generation of mobile signals that travel from the infection site to prime resistance in systemic tissues. Microbial pathogens able to pass preformed structural and chemical barriers in the plants’ cell periphery as for example the
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cell wall, deposition of cutin and suberin in the cuticle or constitutively produced antimicrobial substances face two layers of inducible defense responses (Nürnberger and Brunner, 2002; Heath, 2000; Veronese et al., 2003). Typically, activated defense responses require an enhanced need for energy and therefore induction is strictly regulated and takes place only upon pathogen attack (Boller and He, 2009). Prerequisite for efficient defense reactions is the recognition of potential pathogens by the plants’
surveillance system. A crucial first step in non-self recognition that contributes to plant non-host resistance is the perception of pathogens at the cell surface by specialized pattern recognition receptors (PRRs) at the plasma membrane that perceive so called PAMPs (pathogen-associated molecular patterns), leading to the activation of defense signaling cascades and subsequent initiation of PAMP-triggered immunity (PTI, Figure 1.1 (1). PAMPs are slowly evolving molecules that are indispensable for microbial life but are not present in the host organism. Thus, PAMPs usually are structurally conserved within a class of microbes. Prominent examples for PAMPs are the epitope flg22 of bacterial flagellin which is recognized by the receptor FLAGELLIN SENSITIVE 2 (FLS2), the elongation factor thermo unstable (EF-Tu) peptide elf18 which is recognized by the EF-Tu receptor (EFR) and the fungal cell wall component chitin which is recognized by the CHITIN RECEPTOR KINASE 1 (CERK1, Gomez-Gomez and Boller, 2000; Zipfel et al., 2006;
Miya et al., 2007; Petutschnig et al., 2010; Zipfel et al., 2004). PAMP-recognition by PRRs typically triggers the production of reactive oxygen species (ROS), changes in ion fluxes at the plasma membrane, activation of calcium-dependent protein kinases (CDPKs) and mitogen-activated protein kinase (MAPK) cascades among other responses (Boller and Felix, 2009; Dodds and Rathjen, 2010; Schwessinger and Zipfel, 2008). For example, perception of flg22 by FLS2 results in activation of the MAP kinases MPK3 and MPK6 and subsequent activation of downstream WRKY-type transcription factors for increased expression of defense genes (Asai et al., 2002).
PAMP-triggered basal immune responses usually serve as a sufficient protection against non-adapted pathogens. Host-adapted pathogens, however, evolved effector molecules (also called virulence (vir) factors) that are secreted by the pathogen to evade recognition by the host or to suppress host defense responses in order to circumvent PTI, resulting in effector-triggered susceptibility (ETS, Boller and He, 2009; Panstruga and
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Dodds, 2009). Interaction of effectors with host proteins can take place at various locations including the host cell cytoplasm (Figure 1.1 (2). However, several effector proteins are targeted to host cell nuclei (Caillaud et al., 2012a; Caillaud et al., 2012b; Rivas and Deslandes, 2013; Deslandes et al., 2003; Schornack et al., 2010). The presence of predicted NLS motifs in some of these effectors indicate that the host cells’ nuclear import machinery is exploited for nuclear translocation (Chisholm et al., 2006; Schornack et al., 2010; Boch and Bonas, 2010).
Figure 1.1 Schematic illustration of the plant immune system. All pathogens expose PAMPs to their surroundings. 1) Plants perceive PAMPs via membrane bound Pattern Recognition Receptors (PRRs) and initiate PAMP triggered immunity (PTI). To counteract PTI, pathogens deliver virulence effectors (2) to the plant cell cytoplasm. Effector proteins translocate to specific subcellular locations where they can suppress PTI (3). This results in effector triggered susceptibility (ETS). 4) Intracellular R proteins (NLRs) can recognize effectors by direct interaction (4 a), by interaction with a decoy (4 b) or by guarding an effector target (4 c).
R protein activation leads to strong induction of defense responses and thus effector triggered immunity (ETI, 5). Notably, all layers of immunity require nucleocytoplasmic transport across the nuclear envelope through nuclear pore complexes. Figure from Dangl et al. (2013).
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To counteract ETS, plants have evolved intracellular Resistance (R) proteins, most of which are nucleotide-binding/leucine-rich repeat immune sensors NB-LRRs (or NLRs) to directly or indirectly recognize the presence of effector molecules (Figure 1.1 (4). Effector recognition leads to a strong defense response termed effector triggered immunity (ETI) that typically involves a ROS burst and local cell death execution in form of a hypersensitive response (HR). ETI and the HR are effective against biotrophic pathogens that depend on living tissue. Necrotrophic pathogens, in contrast, kill and feed on the dead host plants’ tissue in the course of infection. Because of the contribution of R proteins, ETI is also called R protein-mediated resistance. Effector recognition can take place via direct interaction (Figure 1.1 (4), Ueda et al., 2006; Dodds et al., 2006) or indirectly through a mechanism where the R protein guards the host cell effector target or a decoy protein (Mackey et al., 2002; Van Der Biezen, Erik A. and Jones, 1998; Dangl and Jones, 2001).
NB-LRR receptors are the most common R protein variants and are related to NLRs known from the animal immune system (Kanneganti et al., 2007b; Ronald and Beutler, 2010). NB-LRR-type R proteins usually contain three distinct domains: a central nucleotide-binding (NB) domain, C-terminal leucine-rich-repeats (LRRs) and either a coiled-coil (CC) or toll interleukin-1 receptor (TIR) domain at the N-terminus (Dangl and Jones, 2001). The two subclasses usually employ different downstream signaling components. Signals from TIR-NB-LRRs (TNLs) converge on the lipase-like protein EDS1 (ENHANCED DISEASE SUSCEPTIBILITY 1), whereas CC-NB-LRRs (CNLs) signaling requires the plasma membrane localized NDR1 (NON-RACE SPECIFIC DISEASE RESISTANCE 1, García et al., 2010; Aarts et al., 1998). Interestingly, the balance of EDS1 proteins present in the cytosol and nucleus is important for efficient immunity (García et al., 2010).
Notably, several R proteins have also been shown to be nuclear localized. One example is the EDS1-dependent nucleocytoplasmic TNL R protein RPS4 (RESISTANCE TO PSEUDOMONAS SYRINGAE 4), which accumulates in the nucleus after perception of its corresponding effector avrRps4 (Wirthmueller et al., 2007; Heidrich et al., 2011).
The local defense responses described above also confer elevated resistance of distal, uninfected tissues against subsequent attack by a broad spectrum of pathogens in a process called systemic acquired resistance (SAR, Durrant and Dong, 2004). Importantly,
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both, PRR and R protein activation results in transcriptional reprogramming of host cells that depends on defense signal transduction into the nucleus and nuclear export of defense-related mRNAs. Hence, communication between the cytoplasm and the nucleus is required for both, PTI and ETI.