An insecticidal GroEL protein with chitin binding activity from Xenorhabdus nematophila

Evaluation and Characterization of the Insecticidal Activity and Synergistic Effects of Different GroEL Proteins from Bacteria Associated with Entomopathogenic Nematodes on Galleria mellonella

GroEL is a chaperonin that helps other proteins fold correctly. However, alternative activities, such as acting as an insect toxin, have also been discovered. This work evaluates the chaperonin and insecticidal activity of different GroEL proteins from entomopathogenic nematodes on G. mellonella. The ability to synergize with the ExoA toxin of Pseudomonas aeruginosa was also investigated. The GroELXn protein showed the highest insecticidal activity among the different GroELs. In addition, it was able to significantly activate the phenoloxidase system of the target insects. This could tell us about the mechanism by which it exerts its toxicity on insects. GroEL proteins can enhance the toxic activity of the ExoA toxin, which could be related to its chaperonin activity. However, there is a significant difference in the synergistic effect that is more related to its alternative activity as an insecticidal toxin.

Comparative Genomics and Pathogenicity Analysis of Two Bacterial Symbionts of Entomopathogenic Nematodes: The Role of the GroEL Protein in Virulence

Bacteria of the genera Xenorhabdus and Photorhabdus are symbionts of entomopathogenic nematodes. Despite their close phylogenetic relationship, they show differences in their pathogenicity and virulence mechanisms in target insects. These differences were explored by the analysis of the pangenome, as it provides a framework for characterizing and defining the gene repertoire. We performed the first pangenome analysis of 91 strains of Xenorhabdus and Photorhabdus; the analysis showed that the Photorhabdus genus has a higher number of genes associated with pathogenicity. However, biological tests showed that whole cells of X. nematophila SC 0516 were more virulent than those of P. luminescens HIM3 when both were injected into G. mellonella larvae. In addition, we cloned and expressed the GroEL proteins of both bacteria, as this protein has been previously indicated to show insecticidal activity in the genus Xenorhabdus. Among these proteins, Cpn60-Xn was found to be the most toxic at all concentrations tested, with an LC50 value of 102.34 ng/larva. Sequence analysis suggested that the Cpn60-Xn toxin was homologous to Cpn60-Pl; however, Cpn60-Xn contained thirty-five differentially substituted amino acid residues that could be responsible for its insecticidal activity.

The differential strain virulence of the candidate toxins of Photorhabdus akhurstii can be correlated with their inter-strain gene sequence diversity

Photorhabdus akhurstii is an insect-parasitic bacterium that symbiotically associates with the nematode, Heterorhabditis indica. The bacterium possesses several pathogenicity islands that aids in conferring toxicity to different insects. Herein, we constructed the plasmid clones of coding sequences of four toxin genes (pirA, tcaA, tccA and tccC; each was isolated from four P. akhurstii strains IARI-SGMG3, IARI-SGGJ2, IARI-SGHR2 and IARI-SGMS1) in Escherichia coli and subsequently, their biological activity were investigated against the fourth-instar larvae of the model insect, Galleria mellonella via intra-hemocoel injection. Bioinformatics analyses indicated inter-strain amino acid sequence difference at several positions of the candidate toxins. In corroboration, differential insecticidal activity of the identical toxin protein (PirA, TcaA, TccA and TccC conferred 15-59, 27-100, 25-100 and 33-98% insect mortality, respectively, across the strains) derived from the different bacterial strains was observed, suggesting that the diverse gene pool in Indian strains of P. akhurstii leads to strain-specific virulence in this bacterium. These toxin candidates appear to be an attractive option to deploy them in biopesticide development for managing the insect pests globally.

Novel insecticidal chitinase from the insect pathogen Xenorhabdus nematophila

Xenorhabdus nematophila strain ATCC 19061 is an insect pathogen that produces various protein toxins which intoxicate and kill its larval host. In the present study, we have described the cloning, expression and characterization of a 76-kDa chitinase protein of X. nematophila. A 1.9 kb DNA sequence encoding the chitinase gene was PCR amplified and cloned. Further, the chitinase protein was expressed in Escherichia coli and purified by using affinity chromatography. Two highly conserved domains were identified GH18 and ChiA. The purified chitinase protein showed chitobiosidase activity, β-N-acetylglucosaminidase and endochitinase activity, when enzyme activity was measured using respective substrates. The purified chitinase protein was found to be orally toxic to the larvae of a major crop pest, Helicoverpa armigera when fed to the larvae mixed with artificial diet. It also had adverse effect on the growth and development of the surviving larvae. Surviving larvae showed 9-fold reduction in weight, as a result the transformation of larvae into pupae was adversely affected. Our results demonstrated that the chitinase protein of X. nematophila has insecticidal property and can prove to be a potent candidate for pest control in plants.

Expression and characterization of two chitinases with synergistic effect and antifungal activity from Xenorhabdus nematophila

Xenorhabdus nematophila HB310 secreted the insecticidal protein toxin complex. Two chitinase genes, chi60 and chi70, were found in X. nematophila toxin complex locus. In order to clarify the function of two chitinases, chi60 and chi70 genes were cloned and expressed in Escherichia coli Transetta (DE3). As a result, we found that the Chi60 and Chi70 belonged to glycoside hydrolases (GH) family 18 with a molecular mass of 65 kDa and 78 kDa, respectively. When colloidal chitin was treated as the substrate, Chi60 and Chi70 were proved to have the highest enzymatic activity at pH 6.0 and 50 °C. Chi60 and Chi70 had obvious growth inhibition effect against the second larvae of Helicoverpa armigera with growth inhibiting rate of 81.99% and 90.51%. Chi70 had synergistic effect with the insecticidal toxicity of Bt Cry 1Ac, but the Chi60 had no synergistic effect with Bt Cry 1Ac. Chi60 and Chi70 showed antifungal activity against Alternaria brassicicola, Verticillium dahliae and Coniothyrium diplodiella. The results increased our understanding of the chitinases produced by X. nematophila and laid a foundation for further studies on the mechanism of the chitinases.

TcaB, an insecticidal protein from Photorhabdus akhurstii causes cytotoxicity in the greater wax moth, Galleria mellonella

Photorhabdus akhurstii can produce a variety of proteins that aid this bacterium and its mutualistic nematode vector, Heterorhabditis indica to kill the insect host. Herein, we characterized (by heterologously expressing in E. coli) an open reading frame (1713 bp) of the toxin complex protein, TcaB from P. akhurstii strains IARI-SGHR2 and IARI-SGMS1 and assessed its toxic effect on G. mellonella larvae. The intra-hemocoel injection of purified TcaB (molecular weight-63 kDa) caused fourth instar larval bodies to blacken and die with LD₅₀ values of 67.25 (IARI-SGHR2) and 52.08 (IARI-SGMS1) ng per larva at 12 h. Additionally, oral administration of the toxin caused larval mortality with LD₅₀ values of 709.55 (IARI-SGHR2) and 598.44 (IARI-SGMS1) ng per g diet per larva at 7 days post feeding. Injection of purified TcaB caused loss of viability of fourth instar G. mellonella hemocytes at 6 h post incubation; cells displayed morphological changes typical of apoptosis, including cell shrinkage, membrane blebbing, nuclear condensation and disintegration. Injection of TcaB also elevated the phenoloxidase activity in insect hemolymph which triggers an extensive immune response that potentially leads to larval death. Similar to other bacterial toxins TcaB possesses potent biological activity which may enable it to be used as an efficient agent for pest management.

Proteomics as a tool for tapping potential of entomopathogens as microbial insecticides

Biopesticides are collective pest control harnessing the knowledge of the target pest and its natural enemies that minimize the risks of synthetic pesticides. A subset of biopesticides; bioinsecticides, are specifically used in controlling insect pests. Entomopathogens (EPMs) are micro-organisms sought after as subject for bioinsecticide development. However, lack of understanding of EPM mechanism of toxicity and pathogenicity slowed the progress of bioinsecticide development. Proteomics is a useful tool in elucidating the interaction of entomopathogenic fungi, entomopathogenic bacteria, and entomopathogenic virus with their target host. Collectively, proteomics shed light onto insect host response to EPM infection, mechanism of action of EPM's toxic proteins and secondary metabolites besides characterizing secreted and membrane-bound proteins of EPM that more precisely describe relevant proteins for host recognition and mediating pathogenesis. However, proteomics requires optimized protein extraction methods to maximize the number of proteins for analysis and availability of organism's genome for a more precise protein identification.

A Glimpse Into the Structure and Function of Atypical Type I Chaperonins

Chaperonins are a subclass of molecular chaperones that assist cellular proteins to fold and assemble into their native shape. Much work has been done on Type I chaperonins, which has elucidated their elegant mechanism. Some debate remains about the details in these mechanisms, but nonetheless the roles of these in helping protein folding have been understood in great depth. In this review we discuss the known functions of atypical Type I chaperonins, highlighting evolutionary aspects that might lead chaperonins to perform alternate functions.

Role of Endosymbionts in Insect-Parasitic Nematode Interactions

Endosymbiotic bacteria exist in many animals where they develop relationships that affect certain physiological processes in the host. Insects and their nematode parasites form great models for understanding the genetic and molecular basis of immune and parasitic processes. Both organisms contain endosymbionts that possess the ability to interfere with certain mechanisms of immune function and pathogenicity. This review summarizes recent information on the involvement of insect endosymbionts in the response to parasitic nematode infections, and the influence of nematode endosymbionts on specific aspects of the insect immune system. Analyzing this information will be particularly useful for devising endosymbiont-based strategies to intervene in insect immunity or nematode parasitism for the efficient management of noxious insects in the field.

A novel pilin subunit from Xenorhabdus nematophila, an insect pathogen, confers pest resistance in tobacco and tomato

KEY MESSAGE

Overexpression of insecticidal pilin subunit from Xenorhabdus nematophila protects transgenic tobacco and tomato plants against Helicoverpa armigera. Xenorhabdus nematophila is a pathogenic bacterium producing toxins that kill the larval host. Previously, we characterized a pilin subunit of X. nematophila which was found to be a pore-forming toxin and cytotoxic to the larval hemocytes of Helicoverpa armigera by causing agglutination and lysis of the cells. In the present study, we report the efficacy of the insecticidal pilin subunit expressed in transgenic tobacco and tomato plants for control against H. armigera. A 537 bp mrxA gene encoding the 17 kDa insecticidal pilin subunit was transferred into the genome of tobacco and tomato, respectively, via Agrobacterium-mediated transformation. The stable integration of the 537 bp mrxA gene in the transgenic plants was confirmed by Southern blot analysis and expression of mrxA gene was confirmed by RT-PCR and Western blot analyses. The transgenic plants appeared healthy and phenotypically normal but proved toxic to the insects in insect bioassays, showing 100% insect mortality and reduced damage of the transgenic plants. Based on these observations, it is suggested that pilin subunit can be used as a potential candidate for control of H. armigera and may open new strategies for pest control in agricultural plants.

Ectopic expression of GroEL from Xenorhabdus nematophila in tomato enhances resistance against Helicoverpa armigera and salt and thermal stress

The GroEL homolog XnGroEL protein of Xenorhabdus nematophila belongs to a highly conserved family of molecular chaperones/heat shock proteins (Hsps). XnGroEL was shown to possess oral insecticidal activity against a major crop pest Helicoverpa armigera. Under normal conditions, the Hsps/chaperones facilitate folding, assembly, and translocation of cellular proteins, while in stress conditions they protect proteins from denaturation. In this study, we describe generation of transgenic tomato plants overexpressing insecticidal XnGroEL protein and their tolerance to biotic and abiotic stresses. Presence of XnGroEL in the transgenic tomato lines conferred resistance against H. armigera showing 100% (p ≤ 0.001) mortality of neonates. In addition, XnGroEL provided thermotolerance and protection against high salt concentration to the tomato plants. Expression of XnGroEL minimized photo-oxidation of chlorophyll and reduced oxidative damage of cell membrane system of the plants under heat and salt stress. The enhanced tolerance to abiotic stresses correlated with increase in the anti-oxidative enzyme activity and reduced H2O2 accumulation in transgenic tomato plants. The variety of beneficial properties displayed by XnGroEL protein provides an opportunity for value addition and improvement of crop productivity.

A novel insecticidal GroEL protein from Xenorhabdus nematophila confers insect resistance in tobacco

Xenorhabdus nematophila is an entomopathogenic bacteria. It secretes a GroEL homolog, XnGroEL protein, toxic to its larval prey. GroEL belongs to the family of molecular chaperones and is required for proper folding of cellular proteins. Oral ingestion of insecticidal XnGroEL protein is toxic to Helicoverpa armigera, leading to cessation of growth and development of the larvae. In the present study, the insecticidal efficacy of XnGroEL against H. armigera has been evaluated in transgenic tobacco plant expressing the protein. A 1.7-kb gene encoding the 58-kDa XnGroEL protein was incorporated into the tobacco genome via Agrobacterium-mediated transformation. The stable integration of the transgene was confirmed by Southern blot analysis and its expression by RT-PCR and western blot analyses in transgenic plants. The transgenic lines showed healthy growth and were phenotypically normal. Insect bioassays revealed significant reduction of 100 % in the survival of larvae (p < 0.001) and 55-77 % reduction in plant damage (p < 0.05 and p < 0.001) compared to the untransformed and vector control plants. The results demonstrate that XnGroEL is a novel potential candidate for imparting insect resistance against H. armigera in plants.

Chaperone networking facilitates protein targeting to the bacterial cytoplasmic membrane

Nascent polypeptides emerging from the ribosome are assisted by a pool of molecular chaperones and targeting factors, which enable them to efficiently partition as cytosolic, integral membrane or exported proteins. Extensive genetic and biochemical analyses have significantly expanded our knowledge of chaperone tasking throughout this process. In bacteria, it is known that the folding of newly-synthesized cytosolic proteins is mainly orchestrated by three highly conserved molecular chaperones, namely Trigger Factor (TF), DnaK (HSP70) and GroEL (HSP60). Yet, it has been reported that these major chaperones are strongly involved in protein translocation pathways as well. This review describes such essential molecular chaperone functions, with emphasis on both the biogenesis of inner membrane proteins and the post-translational targeting of presecretory proteins to the Sec and the twin-arginine translocation (Tat) pathways. Critical interplay between TF, DnaK, GroEL and other molecular chaperones and targeting factors, including SecB, SecA, the signal recognition particle (SRP) and the redox enzyme maturation proteins (REMPs) is also discussed. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Role of Yersinia pestis toxin complex family proteins in resistance to phagocytosis by polymorphonuclear leukocytes

Yersinia pestis carries homologues of the toxin complex (Tc) family proteins, which were first identified in other Gram-negative bacteria as having potent insecticidal activity. The Y. pestis Tc proteins are neither toxic to fleas nor essential for survival of the bacterium in the flea, even though tc gene expression is highly upregulated and much more of the Tc proteins YitA and YipA are produced in the flea than when Y. pestis is grown in vitro. We show that Tc(+) and Tc(-) Y. pestis strains are transmitted equivalently from coinfected fleas, further demonstrating that the Tc proteins have no discernible role, either positive or negative, in transmission by the flea vector. Tc proteins did, however, confer Y. pestis with increased resistance to killing by polymorphonuclear leukocytes (PMNs). Resistance to killing was not the result of decreased PMN viability or increased intracellular survival but instead correlated with a Tc protein-dependent resistance to phagocytosis that was independent of the type III secretion system (T3SS). Correspondingly, we did not detect T3SS-dependent secretion of the native Tc proteins YitA and YipA or the translocation of YitA- or YipA-β-lactamase fusion proteins into CHO-K1 (CHO) cells or human PMNs. Thus, although highly produced by Y. pestis within the flea and related to insecticidal toxins, the Tc proteins do not affect interaction with the flea or transmission. Rather, the Y. pestis Tc proteins inhibit phagocytosis by mouse PMNs, independent of the T3SS, and may be important for subverting the mammalian innate immune response immediately following transmission from the flea.

Physiological levels of glucose induce membrane vesicle secretion and affect the lipid and protein composition of Yersinia pestis cell surfaces

Yersinia pestis grown with physiologic glucose increased cell autoaggregation and deposition of extracellular material, including membrane vesicles. Membranes were characterized, and glucose had significant effects on protein, lipid, and carbohydrate profiles. These effects were independent of temperature and the biofilm-related locus pgm and were not observed in Yersinia pseudotuberculosis.

An insecticidal protein from Xenorhabdus ehlersii stimulates the innate immune response in Galleria mellonella

The bacteria Xenorhabdus spp. are entomopathogenic symbionts that can produce several toxic proteins that interfere with the immune system of insects. Recently, we purified the insecticidal protein XeGroEL from Xenorhabdus ehlersii and discovered that injection of XeGroEL into larvae of Galleria mellonella triggers strong immune responses. In this study, we determined the level of induction of several immune-responsive proteins that were secreted into the hemolymph using comparative proteomic analyses of hemolymph proteins from XeGroEL-challenged larvae. Additionally, quantitative real-time reverse transcription-PCR analyses demonstrated increased transcriptional rates of immune-related genes at 5 h post-challenge with purified XeGroEL. Our results help to understand anti-microbial immune responses in G. mellonella, suggesting that the immune system recognizes exogenous proteins and pathogen-associated molecular patterns.

Xenocin export by the flagellar type III pathway in Xenorhabdus nematophila

The xenocin operon of Xenorhabdus nematophila consists of xciA and ximB genes encoding a 64-kDa xenocin and 42-kDa immunity protein to kill competing microbes in the insect larva. The catalytic domain of xenocin has RNase activity and is responsible for its cytotoxicity. Under SOS conditions, xenocin is produced with immunity protein as a complex. Here, we show that xenocin and immunity protein complex are exported through the flagellar type III system of X. nematophila. Secretion of xenocin complex was abolished in an flhA strain but not in an fliC strain. The xenocin operon is not linked to the flagellar operon transcriptionally. The immunity protein is produced alone from a second, constitutive promoter and is targeted to the periplasm in a flagellum-independent manner. For stable expression of xenocin, coexpression of immunity protein was necessary. To examine the role of immunity protein in xenocin export, an enzymatically inactive protein was produced by site-directed mutagenesis in the active site of the catalytic domain. Toxicity was abolished in D535A and H538A variants of xenocin, which were expressed alone without an immunity domain and secreted in the culture supernatant through flagellar export. Secretion of xenocin through the flagellar pathway has important implications in the evolutionary success of X. nematophila.

A new gene from Xenorhabdus bovienii and its encoded protease inhibitor protein against Acyrthosiphon pisum

BACKGROUND

Aphids are insect pests with significant importance worldwide for agricultural and horticultural crops. The chemical pesticides used to control aphids could result in pesticide residues in agricultural and horticultural products as well as in negative effects on the environment. Therefore, alternative control methods are urgently needed. This study identified a new gene from strain BJFS526 of the symbiotic bacterium Xenorhabdus bovienii and expressed the protease inhibitor protein encoded by the gene. The effects of the protein on the pea aphids, Acyrthosiphon pisum, were also investigated.

RESULTS

The gene PIN1 encoding the protease inhibitor protein against aphids was successfully cloned from BJFS526. The study demonstrated that the protein had adverse effects on pea aphid survival, and that the activity of aphid aminopeptidase was significantly inhibited by the protein.

CONCLUSION

The results from this study suggest that this gene and the protease inhibitor protein encoded may offer an alternative method to control aphids in the future.

How the insect pathogen bacteria Bacillus thuringiensis and Xenorhabdus/Photorhabdus occupy their hosts

Insects are the largest group of animals on earth. Like mammals, virus, fungi, bacteria and parasites infect them. Several tissue barriers and defense mechanisms are common for vertebrates and invertebrates. Therefore some insects, notably the fly Drosophila and the caterpillar Galleria mellonella, have been used as models to study host-pathogen interactions for several insect and mammal pathogens. They are excellent tools to identify pathogen determinants and host tissue cell responses. We focus here on the comparison of effectors used by two different groups of bacterial insect pathogens to accomplish the infection process in their lepidopteran larval host: Bacillus thuringiensis and the nematode-associated bacteria, Photorhabdus and Xenorhabdus. The comparison reveals similarities in function and expression profiles for some genes, which suggest that such factors are conserved during evolution in order to attack the tissue encountered during the infection process.

An insecticidal protein from Xenorhabdus ehlersii triggers prophenoloxidase activation and hemocyte decrease in Galleria mellonella

The bacteria Xenorhabdus spp. are entomopathogenic symbionts that can produce several toxic proteins that interfere the immune system of insects. We purified an insecticidal protein from Xenorhabdus ehlersii, and designated it as XeGroEL with an estimated molecular mass of ~58 kDa. Galleria mellonella larva injected with XeGroEL presented prophenoloxidase activation and hemocyte decrease. XeGroEL can kill G. mellonella larva in 48 h with an LD(50) of 0.76 ± 0.08 μg/larva. Our results demonstrate that X. ehlersii possesses a toxic XeGroEL protein acting as a potential factor to activate proPO in host insect, which also provides a meaningful hypothesis to understand the interaction between nematode-symbiotic bacteria and host.

The main virulence determinant of Yersinia entomophaga MH96 is a broad-host-range toxin complex active against insects

Through transposon mutagenesis and DNA sequence analysis, the main disease determinant of the entomopathogenic bacterium Yersinia entomophaga MH96 was localized to an ~32-kb pathogenicity island (PAI) designated PAI(Ye₉₆). Residing within PAI(Ye₉₆) are seven open reading frames that encode an insecticidal toxin complex (TC), comprising not only the readily recognized toxin complex A (TCA), TCB, and TCC components but also two chitinase proteins that form a composite TC molecule. The central TC gene-associated region (~19 kb) of PAI(Ye₉₆) was deleted from the Y. entomophaga MH96 genome, and a subsequent bioassay of the ΔTC derivative toward Costelytra zealandica larvae showed it to be innocuous. Virulence of the ΔTC mutant strain could be restored by the introduction of a clone containing the entire PAI(Ye₉₆) TC gene region. As much as 0.5 mg of the TC is released per 100 ml of Luria-Bertani broth at 25°C, while at 30 or 37°C, no TC could be detected in the culture supernatant. Filter-sterilized culture supernatants derived from Y. entomophaga MH96, but not from the ΔTC strain grown at temperatures of 25°C or less, were able to cause mortality. The 50% lethal doses (LD₅₀s) of the TC toward diamondback moth Plutella xylostella and C. zealandica larvae were defined as 30 ng and 50 ng, respectively, at 5 days after ingestion. Histological analysis of the effect of the TC toward P. xylostella larva showed that within 48 h after ingestion of the TC, there was a general dissolution of the larval midgut.

Serine protease-mediated host invasion by the parasitic nematode Steinernema carpocapsae

Steinernema carpocapsae is an insect parasitic nematode used in biological control, which infects insects penetrating by mouth and anus and invading the hemocoelium through the midgut wall. Invasion has been described as a key factor in nematode virulence and suggested to be mediated by proteases. A serine protease cDNA from the parasitic stage was sequenced (sc-sp-1); the recombinant protein was produced in an Escherichia coli system, and a native protein was purified from the secreted products. Both proteins were confirmed by mass spectrometry to be encoded by the sc-sp-1 gene. Sc-SP-1 has a pI of 8.7, a molecular mass of 27.3 kDa, a catalytic efficiency of 22.2 × 10(4) s(-1) m(-1) against N-succinyl-Ala-Ala-Pro-Phe-pNA, and is inhibited by chymostatin (IC 0.07) and PMSF (IC 0.73). Sc-SP-1 belongs to the chymotrypsin family, based on sequence and biochemical analysis. Only the nematode parasitic stage expressed sc-sp-1. These nematodes in the midgut lumen, prepared to invade the insect hemocoelium, expressed higher levels than those already in the hemocoelium. Moreover, parasitic nematode sense insect peritrophic membrane and hemolymph more quickly than they do other tissues, which initiates sc-sp-1 expression. Ex vivo, Sc-SP-1 was able to bind to insect midgut epithelium and to cause cell detachment from basal lamina. In vitro, Sc-SP-1 formed holes in an artificial membrane model (Matrigel), whereas Sc-SP-1 treated with PMSF did not, very likely because it hydrolyzes matrix glycoproteins. These findings highlight the S. carpocapsae-invasive process that is a key step in the parasitism thus opening new perspectives for improving nematode virulence to use in biological control.

Direct binding of a plant LysM receptor-like kinase, LysM RLK1/CERK1, to chitin in vitro

Plants induce immune responses against fungal pathogens by recognition of chitin, which is a component of the fungal cell wall. Recent studies have revealed that LysM receptor-like kinase 1/chitin elicitor receptor kinase 1 (LysM RLK1/CERK1) is a critical component for the immune responses to chitin in Arabidopsis thaliana. However, the molecular mechanism of the chitin recognition by LysM RLK1 still remains unknown. Here, we present the first evidence for direct binding of LysM RLK1 to chitin. We expressed LysM RLK1 fused with yeast-enhanced green fluorescent protein (LysM RLK1-yEGFP) in yeast cells. Binding studies using the solubilized LysM RLK1-yEGFP and several insoluble polysaccharides having similar structures showed that LysM RLK1-yEGFP specifically binds to chitin. Subsequently, the fluorescence microscopic observation of the solubilized LysM RLK1-yEGFP binding to chitin beads revealed that the binding was saturable and had a high affinity, with a K(d) of approximately 82 nm. This binding was competed by the addition of soluble glycol chitin or high concentration of chitin oligosaccharides having 4-8 residues of N-acetyl glucosamine. However, the competition of these chitin oligosaccharides is weaker than that of glycol chitin. These data suggest that LysM RLK1 has a higher affinity for chitin having a longer residue of N-acetyl glucosamine. We also found that LysM RLK1-yEGFP was autophosphorylated in vitro and that chitin does not affect the phosphorylation of LysM RLK1-yEGFP. Our results provide a new dimension to chitin elicitor perception in plants.