Molecular identification and characterization of the tomato flagellin receptor LeFLS2, an orthologue of Arabidopsis FLS2 exhibiting characteristically different perception specificities

Interplay of flg22-induced defence responses and nodulation in Lotus japonicus

In this study the interplay between the symbiotic and defence signalling pathways in Lotus japonicus was investigated by comparing the responses to Mesorhizobium loti, the symbiotic partner of L. japonicus, and the elicitor flg22, a conserved peptide motif present in flagellar protein of a wide range of bacteria. It was found that defence and symbiotic pathways overlap in the interaction between L. japonicus and M. loti since similar responses were induced by the mutualistic bacteria and flg22. However, purified flagellin from M. loti did not induce any response in L. japonicus, which suggests the production of other elicitors by the symbiotic bacteria. Defence responses induced by flg22 caused inhibition of rhizobial infection and delay in nodule organogenesis, as demonstrated by the negative effect of flg22 in the formation of spontaneous nodules in the snf1 L. japonicus mutant, and the inhibition of NSP1 and NSP2 genes. This indicates the antagonistic effect of the defence pathway on the nodule formation in the initial rhizobium-legume interaction. However, the fact that flg22 did not affect the formation of new nodules once the symbiosis was established indicates that after the colonization of the host plant by the symbiotic partner, the symbiotic pathway has prevalence over the defensive response. This result is also supported by the down-regulation of the expression levels of the flg22 receptor FLS2 in the nodular tissue.

Plant innate immunity: perception of conserved microbial signatures

Plants and animals sense conserved microbial signatures through receptors localized to the plasma membrane and cytoplasm. These receptors typically carry or associate with non-arginine-aspartate (non-RD) kinases that initiate complex signaling networks cumulating in robust defense responses. In plants, coregulatory receptor kinases have been identified that not only are critical for the innate immune response but also serve an essential function in other regulatory signaling pathways.

The plant pathogen Pseudomonas syringae pv. tomato is genetically monomorphic and under strong selection to evade tomato immunity

Recently, genome sequencing of many isolates of genetically monomorphic bacterial human pathogens has given new insights into pathogen microevolution and phylogeography. Here, we report a genome-based micro-evolutionary study of a bacterial plant pathogen, Pseudomonas syringae pv. tomato. Only 267 mutations were identified between five sequenced isolates in 3,543,009 nt of analyzed genome sequence, which suggests a recent evolutionary origin of this pathogen. Further analysis with genome-derived markers of 89 world-wide isolates showed that several genotypes exist in North America and in Europe indicating frequent pathogen movement between these world regions. Genome-derived markers and molecular analyses of key pathogen loci important for virulence and motility both suggest ongoing adaptation to the tomato host. A mutational hotspot was found in the type III-secreted effector gene hopM1. These mutations abolish the cell death triggering activity of the full-length protein indicating strong selection for loss of function of this effector, which was previously considered a virulence factor. Two non-synonymous mutations in the flagellin-encoding gene fliC allowed identifying a new microbe associated molecular pattern (MAMP) in a region distinct from the known MAMP flg22. Interestingly, the ancestral allele of this MAMP induces a stronger tomato immune response than the derived alleles. The ancestral allele has largely disappeared from today's Pto populations suggesting that flagellin-triggered immunity limits pathogen fitness even in highly virulent pathogens. An additional non-synonymous mutation was identified in flg22 in South American isolates. Therefore, MAMPs are more variable than expected differing even between otherwise almost identical isolates of the same pathogen strain.

Engineering plant resistance by constructing chimeric receptors that recognize damage-associated molecular patterns (DAMPs)

An efficient sensing of danger and a rapid activation of the immune system are crucial for the survival of plants. Conserved pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) and endogenous molecular patterns, which are present only when the tissue is infected or damaged (damage-associated molecular patterns or DAMPs), can act as danger signals and activate the plant immune response. These molecules are recognized by surface receptors that are indicated as pattern recognition receptors (PRRs). In this paper we summarize recent information on oligogalacturonides (OGs), a class of DAMPs that is released from the extracellular matrix of the plant cell during pathogen attack or wounding. We also describe the characteristics of the Arabidopsis Wall-Associated Kinase 1 (WAK1), a PRR recently identified as a receptor of OGs and discuss the use of WAK1, PRRs and chimeric receptors to engineer resistance in crop plants.

Advances in dissecting endomembrane trafficking with small molecules

Chemical genomics is relatively new to plant biology in academia; however, the ability of this approach to present novel discoveries is being demonstrated clearly. One particularly suitable application of this approach is plant endomembrane trafficking. The rapid and dynamic nature of vesicular trafficking plus genetic redundancy has hampered effective study of this complex network. The ability of small molecules to act quickly to inhibit or arrest vesicular trafficking should permit the association of specific vesicles, especially endosome compartments, with their cargoes, particularly those destined for the plasma membrane. This approach and the large target space presented by the endomembrane trafficking network require the discovery of many new bioactive molecules. Advances in high-throughput chemical screening in plants are making this a reality. However, successful chemical genomic approaches in plants must be coupled with improvements in automated microscopy, image analysis, and target identification. In addition, the ability to correlate specific molecules with complex phenotypic data will be crucial. The data obtained from these experiments will be composed of a matrix of intracellular markers displaying complex chemically induced phenotypes as well as whole plant and perhaps data generated by genomics, proteomics, and metabolomics. In this manner, it should be possible to view endomembrane trafficking and its interactions as a systems-based network.

A domain swap approach reveals a role of the plant wall-associated kinase 1 (WAK1) as a receptor of oligogalacturonides

Oligogalacturonides (OGs) released from the plant cell wall are active both as damage-associated molecular patterns (DAMPs) for the activation of the plant immune response and regulators of plant growth and development. Members of the Wall-Associated Kinase (WAK) family are candidate receptors of OGs, due to their ability to bind in vitro these oligosaccharides. Because lethality and redundancy have hampered the study of WAKs by reverse genetics, we have adopted a chimeric receptor approach to elucidate the role of Arabidopsis WAK1. In a test-of-concept study, we first defined the appropriate chimera design and demonstrated that the Arabidopsis pattern recognition receptor (PRR) EFR is amenable to the construction of functional and resistance-conferring chimeric receptors carrying the ectodomain of another Arabidopsis PRR, FLS2. After, we analyzed chimeras derived from EFR and WAK1. Our results show that, upon stimulation with OGs, the WAK1 ectodomain is capable of activating the EFR kinase domain. On the other hand, upon stimulation with the cognate ligand elf18, the EFR ectodomain activates the WAK1 kinase, triggering defense responses that mirror those normally activated by OGs and are effective against fungal and bacterial pathogens. Finally, we show that transgenic plants overexpressing WAK1 are more resistant to Botrytis cinerea.

Characterization of receptor proteins using affinity cross-linking with biotinylated ligands

The plant genome encodes a wide range of receptor-like proteins but the function of most of these proteins is unknown. We propose the use of affinity cross-linking of biotinylated ligands for a ligand-based survey of the corresponding receptor molecules. Biotinylated ligands not only enable the analysis of receptor-ligand interactions without the use of radioactive compounds but also the isolation and identification of receptor molecules by a simple affinity trapping method. We successfully applied this method for the characterization, isolation and identification of the chitin elicitor binding protein (CEBiP). A biocytin hydrazide conjugate of N-acetylchitooctaose (GN8-Bio) was synthesized and used for the detection of CEBiP in the plasma or microsomal membrane preparations from rice and carrot cells. Binding characteristics of CEBiP analyzed by inhibition studies were in good agreement with the previous results obtained with the use of a radiolabeled ligand. The biotin-tagged CEBiP could be purified by avidin affinity chromatography and identified by LC-MALDI-MS/MS after tryptic digestion. We also used this method to detect OsFLS2, a rice receptor-like kinase for the perception of the peptide elicitor flg22, in membrane preparations from rice cells overexpressing OsFLS2. This work demonstrates the applicability of this method to the purification and identification of plant receptor proteins.

Lipopolysaccharides and plant innate immunity

Plants posses an innate immune system that has many parallels with those found in mammals and insects. A range of molecules of microbial origin called Microbe Associated Molecular Patterns (MAMPs) act to trigger basal defense responses in plants. These elicitors include lipopolysaccharides (LPS) from diverse Gram-negative bacteria. Both core oligosaccharide and the lipid A moieties of LPS as well as synthetic O-antigen oligosaccharides have activity in inducing defense responses in the model plant Arabidopsis thaliana. Very little is known of the mechanism of LPS perception by plants, although plant receptors for other MAMPs such as flagellin have been described. Recent work has implicated the Arabidopsis syntaxin PEN1 as a potential actor in LPS induction of plant defenses, which may suggest a role for vesicle trafficking in the signalling process.

Pattern recognition receptors require N-glycosylation to mediate plant immunity

N-Glycans attached to the ectodomains of plasma membrane pattern recognition receptors constitute likely initial contact sites between plant cells and invading pathogens. To assess the role of N-glycans in receptor-mediated immune responses, we investigated the functionality of Arabidopsis receptor kinases EFR and FLS2, sensing bacterial translation elongation factor Tu (elf18) and flagellin (flg22), respectively, in N-glycosylation mutants. As revealed by binding and responses to elf18 or flg22, both receptors tolerated immature N-glycans induced by mutations in various Golgi modification steps. EFR was specifically impaired by loss-of-function mutations in STT3A, a subunit of the endoplasmic reticulum resident oligosaccharyltransferase complex. FLS2 tolerated mild underglycosylation occurring in stt3a but was sensitive to severe underglycosylation induced by tunicamycin treatment. EFR accumulation was significantly reduced when synthesized without N-glycans but to lesser extent when underglycosylated in stt3a or mutated in single amino acid positions. Interestingly, EFR(N143Q) lacking a single conserved N-glycosylation site from the EFR ectodomain accumulated to reduced levels and lost the ability to bind its ligand and to mediate elf18-elicited oxidative burst. However, EFR-YFP protein localization and peptide:N-glycosidase F digestion assays support that both EFR produced in stt3a and EFR(N143Q) in wild type cells correctly targeted to the plasma membrane via the Golgi apparatus. These results indicate that a single N-glycan plays a critical role for receptor abundance and ligand recognition during plant-pathogen interactions at the cell surface.

Virus-induced gene silencing reveals the involvement of ethylene-, salicylic acid- and mitogen-activated protein kinase-related defense pathways in the resistance of tomato to bacterial wilt

Bacterial wilt (BW), caused by Ralstonia solanacearum, is a devastating vascular disease of tomato worldwide. However, information on tomato's defense mechanism against infection by this soil-borne bacterium is limited. In this study, virus-induced gene silencing (VIGS) was employed to decipher signaling pathways involved in the resistance of tomato to this pathogen. Defined sequence fragments derived from a group of genes known or predicted to be involved in ethylene (ET) and salicylic acid (SA) signaling transduction pathways and mitogen-activated protein kinase (MAPK) cascades were subjected to VIGS in 'Hawaii 7996', a tomato cultivar with stable resistance to BW, and their effect on resistance was determined. The results indicated that silencing of ACO1/3, EIN2, ERF3, NPR1, TGA2.2, TGA1a, MKK2, MPK1/2 and MPK3 caused significant increase in bacterial proliferation in stembases and/or mid-stems. Partial wilting symptoms appeared on plants in which TGA2.2, TGA2.1a, MKK2 and MPK1/2 were silenced. These results suggested that ET-, SA- and MAPK-related defense signaling pathways are involved in the resistance of tomato to BW. This is the first report elucidating the multiple layers of defense governing the resistance of tomato to BW. The results are discussed to enlighten an important and complex interaction between tomato and a soil-borne vascular pathogen.

Plant systems for recognition of pathogen-associated molecular patterns

Research of the last decade has revealed that plant immunity consists of different layers of defense that have evolved by the co-evolutional battle of plants with its pathogens. Particular light has been shed on PAMP- (pathogen-associated molecular pattern) triggered immunity (PTI) mediated by pattern recognition receptors. Striking similarities exist between the plant and animal innate immune system that point for a common optimized mechanism that has evolved independently in both kingdoms. Pattern recognition receptors (PRRs) from both kingdoms consist of leucine-rich repeat receptor complexes that allow recognition of invading pathogens at the cell surface. In plants, PRRs like FLS2 and EFR are controlled by a co-receptor SERK3/BAK1, also a leucine-rich repeat receptor that dimerizes with the PRRs to support their function. Pathogens can inject effector proteins into the plant cells to suppress the immune responses initiated after perception of PAMPs by PRRs via inhibition or degradation of the receptors. Plants have acquired the ability to recognize the presence of some of these effector proteins which leads to a quick and hypersensitive response to arrest and terminate pathogen growth.

Post-Golgi traffic in plants

Secretory and endocytic traffic through the post-Golgi endomembrane system regulates the abundance of plasma-membrane proteins such as receptors, transporters and ion channels, modulating the ability of a cell to communicate with its neighbours and to adapt to a changing environment. The major post-Golgi compartments are numerous and appear to be similar to their counterparts in animals. However, endosomes are rather ill defined morphologically but seem to be involved in specific trafficking pathways. Many plasma-membrane proteins cycle constitutively via endosomal compartments. The trans-Golgi network (TGN) appears to be an early endosome where secretory and endocytic traffic meet. Endocytosed proteins that are to be degraded are targeted to the vacuole via the multivesiculate prevacuolar compartment (PVC) whereas cycling proteins pass through recycling endosomes. The trafficking machinery involves the same classes of proteins as in other eukaryotes. However, there are modifications that match the specifics of post-Golgi traffic in plants. Although plants lack epithelia, some plasma-membrane proteins are located on specific faces of the cell which reflects polarized traffic and influences the physiological performance of the tissue. Plants also differentiate highly polarized tip-growing cells in which post-Golgi traffic is adapted to very high rates of targeted exocytosis, endocytosis and recycling.

Xanthomonas T3S Effector XopN Suppresses PAMP-Triggered Immunity and Interacts with a Tomato Atypical Receptor-Like Kinase and TFT1

XopN is a virulence factor from Xanthomonas campestris pathovar vesicatoria (Xcv) that is translocated into tomato (Solanum lycopersicum) leaf cells by the pathogen's type III secretion system. Xcv DeltaxopN mutants are impaired in growth and have reduced ability to elicit disease symptoms in susceptible tomato leaves. We show that XopN action in planta reduced pathogen-associated molecular pattern (PAMP)-induced gene expression and callose deposition in host tissue, indicating that XopN suppresses PAMP-triggered immune responses during Xcv infection. XopN is predicted to have irregular, alpha-helical repeats, suggesting multiple protein-protein interactions in planta. Consistent with this prediction, XopN interacted with the cytosolic domain of a Tomato Atypical Receptor-Like Kinase1 (TARK1) and four Tomato Fourteen-Three-Three isoforms (TFT1, TFT3, TFT5, and TFT6) in yeast. XopN/TARK1 and XopN/TFT1 interactions were confirmed in planta by bimolecular fluorescence complementation and pull-down analysis. Xcv DeltaxopN virulence defects were partially suppressed in transgenic tomato leaves with reduced TARK1 mRNA levels, indicating that TARK1 plays an important role in the outcome of Xcv-tomato interactions. These data provide the basis for a model in which XopN binds to TARK1 to interfere with TARK1-dependent signaling events triggered in response to Xcv infection.

A renaissance of elicitors: perception of microbe-associated molecular patterns and danger signals by pattern-recognition receptors

Microbe-associated molecular patterns (MAMPs) are molecular signatures typical of whole classes of microbes, and their recognition plays a key role in innate immunity. Endogenous elicitors are similarly recognized as damage-associated molecular patterns (DAMPs). This review focuses on the diversity of MAMPs/DAMPs and on progress to identify the corresponding pattern recognition receptors (PRRs) in plants. The two best-characterized MAMP/PRR pairs, flagellin/FLS2 and EF-Tu/EFR, are discussed in detail and put into a phylogenetic perspective. Both FLS2 and EFR are leucine-rich repeat receptor kinases (LRR-RKs). Upon treatment with flagellin, FLS2 forms a heteromeric complex with BAK1, an LRR-RK that also acts as coreceptor for the brassinolide receptor BRI1. The importance of MAMP/PRR signaling for plant immunity is highlighted by the finding that plant pathogens use effectors to inhibit PRR complexes or downstream signaling events. Current evidence indicates that MAMPs, DAMPs, and effectors are all perceived as danger signals and induce a stereotypic defense response.

Chemical dissection of endosomal pathways

Membrane trafficking and associated signal transduction pathways are critical for plant development and responses to environment. These transduction pathways, including those for brassinosteroids and auxins, require endocytosis to endosomes and recycling back to the plasma membrane. A major challenge toward understanding these processes and their biological roles has been the highly dynamic nature of endomembrane trafficking. To effectively study endocytosis and recycling, which occur in a time frame of minutes, bioactive chemicals provide a powerful and exacting tool. Pharmacological inhibitors such as Brefeldin A (BFA) and the newly identified Endosidin 1 (ES1) have been used to define endosome compartments. ES1 is a clear example of the ability of chemicals to dissect even distinct subpopulations of endosomes involved in trafficking and signal transduction. The ability to characterize and dissect such highly dynamic pathways in a temporal and spatial manner is possible only using pharmacological reagents which can act rapidly and reversibly.

Analysis of flagellin perception mediated by flg22 receptor OsFLS2 in rice

Plants have sensitive perception systems that recognize various pathogen-derived molecules. We previously reported that rice detects flagellin from a rice-incompatible strain of gram-negative phytopathogenic bacterium, Acidovorax avenae, which induces subsequent immune responses involving cell death. The mechanism of flagellin perception in rice, however, has remained obscure. In this study, we found that flg22, a peptide derived from the flagellin N-terminus, induced weak immune responses without cell death in cultured rice cells. To elucidate the mechanism by which flg22 induced signaling in rice, we characterized OsFLS2, the rice ortholog of AtFLS2, which mediates flg22 perception. Heterologous expression of OsFLS2 functions in Arabidopsis, showing the conservation of the flg22 signaling pathway across divergent plant taxa. OsFLS2-overexpressing rice cultured cells generated stronger immune responses with the induction of cell death following stimulation with flg22 and flagellin. However, examination of the growth rate of the compatible strain in inoculated OsFLS2-overexpressing rice could not confirm bacterial growth suppression compared with wild-type rice. These results suggest that rice possesses a conserved flagellin perception system utilizing the FLS2 receptor which, when upregulated, hardly affects resistance against compatible A. avenae.

Early responses of tobacco suspension cells to rhizobacterial elicitors of induced systemic resistance

Colonization of roots by selected strains of fluorescent Pseudomonas spp. can trigger induced systemic resistance (ISR) against foliar pathogens in a plant species-specific manner. It has been suggested that early responses in cell suspension cultures in response to rhizobacterial elicitors, such as generation of active oxygen species (AOS) and extracellular medium alkalinization (MA), are linked to the development of ISR in whole plants. Perception of flagellin was demonstrated to elicit ISR in Arabidopsis, and bacterial lipopolysaccharides (LPS) have been shown to elicit several defense responses and to act as bacterial determinants of ISR in various plant species. In the present study, the LPS-containing cell walls, the pyoverdine siderophores, and the flagella of Pseudomonas putida WCS358, P. fluorescens WCS374, and P. fluorescens WCS417, which are all known to act as elicitors of ISR in selected plant species, were tested for their effects on the production of AOS, MA, elevation of cytoplasmic Ca(2+) ([Ca(2+)](cyt)), and defense-related gene expression in tobacco suspension cells. The LPS of all three strains, the siderophore of WCS374, and the flagella of WCS358 induced a single, transient, early burst of AOS, whereas the siderophores of WCS358 and WCS417 and the flagella of WCS374 and WCS417 did not. None of the compounds caused cell death. Once stimulated by the active compounds, the cells became refractory to further stimulation by any of the active elicitors, but not to the elicitor cryptogein from the oomycete Phytophthora cryptogea, indicating that signaling upon perception of the different rhizobacterial compounds rapidly converges into a common response pathway. Of all compounds tested, only the siderophores of WCS358 and WCS417 did not induce MA; the flagella of WCS374 and WCS417, although not active as elicitors of AOS, did induce MA. These results were corroborated by using preparations from relevant bacterial mutants. The active rhizobacterial elicitors led to a rapid increase in [Ca(2+)](cyt), peaking at 6 min, whereas the inactive siderophores of WCS358 and WCS417 elicited a single spike at 1 min. Elicitation of the cells by cell-wall LPS of WCS358 or the siderophore of WCS374 induced a weak, transient expression of several defense-related genes, including PAL and GST. The spectrum of early responses of the suspension cells was not matched by the expression of ISR in whole tobacco plants against Erwinia carotovora pv. carotovora. Of the live bacterial strains, only WCS358 elicited significant ISR, but application of the LPS or the siderophore of all three strains also elicited ISR. Notably, the absence of elicitation of AOS and MA in suspension-cultured cells but induction of ISR in whole plants by the siderophore of WCS358, which was lost upon treatment with the siderophore-minus mutant of WCS358, indicates that the early responses in suspension cells are not predictive of the ability to induce ISR in whole plants. Possible explanations for these discrepancies are discussed.

News from the frontline: recent insights into PAMP-triggered immunity in plants

Plants have developed a complex defence network to fight off invading pathogens. In recent years, the full importance of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) within this network became apparent. Several new PAMPs have been isolated and new pattern-recognition receptors (PRRs) identified. The discovery of the PRR-interacting protein BAK1 sheds light on the immediate downstream signalling events. Further, transcriptomic analyses identified a core set of approximately 100 PAMP-responsive genes. These studies also revealed a significant overlap with genes regulated during effector-triggered immunity (ETI). Strikingly, ETI seems to operate by alleviating the negative feedback regulation of PTI, leading to stronger defences. This review discusses recent findings in PTI recognition and signalling, and illustrates the need to discover new regulators of PTI responses for a full understanding of plant innate immunity.

Pseudomonas syringae effector AvrPto blocks innate immunity by targeting receptor kinases

Plants use receptor kinases, such as FLS2 and EFR, to perceive bacterial pathogens and initiate innate immunity. This immunity is often suppressed by bacterial effectors, allowing pathogen propagation. To counteract, plants have evolved disease resistance genes that detect the bacterial effectors and reinstate resistance. The Pseudomonas syringae effector AvrPto promotes infection in susceptible plants but triggers resistance in plants carrying the protein kinase Pto and the associated resistance protein Prf. Here we show that AvrPto binds receptor kinases, including Arabidopsis FLS2 and EFR and tomato LeFLS2, to block plant immune responses in the plant cell. The ability to target receptor kinases is required for the virulence function of AvrPto in plants. The FLS2-AvrPto interaction and Pto-AvrPto interaction appear to share similar sequence requirements, and Pto competes with FLS2 for AvrPto binding. The results suggest that the mechanism by which AvrPto recognizes virulence targets is linked to the evolution of Pto, which, in association with Prf, recognizes the bacterium and triggers strong resistance.

Functional characterization of chicken TLR5 reveals species-specific recognition of flagellin

Mammalian Toll-like receptor 5 (TLR5) senses flagellin of several bacterial species and activates the innate immune system. The avian TLR repertoire exhibits considerable functional diversity compared to mammalian TLRs and evidence of a functional TLR5 in the avian species is lacking. In the present study we cloned and successfully expressed chicken TLR5 (chTLR5) in HeLa cells, as indicated by laser confocal microscopy. Infection of chTLR5 transfected cells with Salmonella enterica serovar Enteritidis activated NF-kappaB in a dose- and flagellin-dependent fashion. Similar NF-kappaB activation was observed with recombinant bacterial flagellin. Targeted mutagenesis of the proline residue at position 737 in the chTLR5-TIR domain was detrimental to chTLR5 function, confirming that the observed effects were conferred via chTLR5 and the MyD88 signaling pathway. Comparison of human, mouse and chicken TLR5 activation by flagellin of S. enterica serovar Typhimurium revealed that chTLR5 consistently yielded stronger responses than human but not mouse TLR5. This species-specific reactivity was not observed with flagellin of serovar Enteritidis. The species-specific TLR5 response was nullified after targeted mutagenesis of a single amino acid (Q89A) in serovar Typhimurium flagellin, while L415A and N100A substitutions had no effect. These results show that chickens express a functional TLR5 albeit with different flagellin sensing qualities compared to human TLR5. The finding that single amino acid substitutions in bacterial flagellin can alter the species-specific TLR5 response may influence the host range and susceptibility of infection.

Identification and mutational analysis of Arabidopsis FLS2 leucine-rich repeat domain residues that contribute to flagellin perception

Mutational, phylogenetic, and structural modeling approaches were combined to develop a general method to study leucine-rich repeat (LRR) domains and were used to identify residues within the Arabidopsis thaliana FLAGELLIN-SENSING2 (FLS2) LRR that contribute to flagellin perception. FLS2 is a transmembrane receptor kinase that binds bacterial flagellin or a flagellin-based flg22 peptide through a presumed physical interaction within the FLS2 extracellular domain. Double-Ala scanning mutagenesis of solvent-exposed beta-strand/beta-turn residues across the FLS2 LRR domain identified LRRs 9 to 15 as contributors to flagellin responsiveness. FLS2 LRR-encoding domains from 15 Arabidopsis ecotypes and 20 diverse Brassicaceae accessions were isolated and sequenced. FLS2 is highly conserved across most Arabidopsis ecotypes, whereas more diversified functional FLS2 homologs were found in many but not all Brassicaceae accessions. flg22 responsiveness was correlated with conserved LRR regions using Conserved Functional Group software to analyze structural models of the LRR for diverse FLS2 proteins. This identified conserved spatial clusters of residues across the beta-strand/beta-turn residues of LRRs 12 to 14, the same area identified by the Ala scan, as well as other conserved sites. Site-directed randomizing mutagenesis of solvent-exposed beta-strand/beta-turn residues across LRRs 9 to 15 identified mutations that disrupt flg22 binding and showed that flagellin perception is dependent on a limited number of tightly constrained residues of LRRs 9 to 15 that make quantitative contributions to the overall phenotypic response.

Pattern recognition receptors: from the cell surface to intracellular dynamics

Detection of potentially infectious microorganisms is essential for plant immunity. Microbial communities growing on plant surfaces are constantly monitored according to their conserved microbe-associated molecular patterns (MAMPs). In recent years, several pattern-recognition receptors, including receptor-like kinases and receptor-like proteins, and their contribution to disease resistance have been described. MAMP signaling must be carefully controlled and seems to involve receptor endocytosis. As a further surveillance layer, plants are able to specifically recognize microbial effector molecules via nucleotide-binding site leucine-rich repeat receptors (NB-LRR). A number of recent studies show that NB-LRR translocate to the nucleus in order to exert their activity. In this review, current knowledge regarding the recognition of MAMPs by surface receptors, receptor activation, signaling, and subcellular redistribution are discussed.