Protein phosphorylation in Nicotiana tabacum cells in response to perception of lipopolysaccharides from Burkholderia cepacia

Invited review: Priming, induction and modulation of plant defence responses by bacterial lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) have multiple roles in plant—microbe interactions. LPS contributes to the low permeability of the outer membrane, which acts as a barrier to protect bacteria from plant-derived antimicrobial substances. Conversely, perception of LPS by plant cells can lead to the triggering of defence responses or to the priming of the plant to respond more rapidly and/or to a greater degree to subsequent pathogen challenge. LPS from symbiotic bacteria can have quite different effects on plants to those of pathogens. Some details are emerging of the structures within LPS that are responsible for induction of these different plant responses. The lipid A moiety is not solely responsible for all of the effects of LPS in plants; core oligosaccharide and O-antigen components can elicit specific responses. Here, we review the effects of LPS in induction of defence-related responses in plants, the structures within LPS responsible for eliciting these effects and discuss the possible nature of the (as yet unidentified) LPS receptors in plants.

Deciphering the dual effect of lipopolysaccharides from plant pathogenic Pectobacterium

Lipopolysaccharides (LPS) are a component of the outer cell surface of almost all Gram-negative bacteria and play an essential role for bacterial growth and survival. Lipopolysaccharides represent typical microbe-associated molecular pattern (MAMP) molecules and have been reported to induce defense-related responses, including the expression of defense genes and the suppression of the hypersensitive response in plants. However, depending on their origin and the challenged plant, LPS were shown to have complex and different roles. In this study we showed that LPS from plant pathogens Pectobacterium atrosepticum and Pectobacterium carotovorum subsp. carotovorum induce common and different responses in A. thaliana cells when compared to those induced by LPS from non-phytopathogens Escherichia coli and Pseudomonas aeruginosa. Among common responses to both types of LPS are the transcription of defense genes and their ability to limit of cell death induced by Pectobacterium carotovorum subsp carotovorum. However, the differential kinetics and amplitude in reactive oxygen species (ROS) generation seemed to regulate defense gene transcription and be determinant to induce programmed cell death in response to LPS from the plant pathogenic Pectobacterium. These data suggest that different signaling pathways could be activated by LPS in A. thaliana cells.

Identification of three elicitins and a galactan-based complex polysaccharide from a concentrated culture filtrate of Phytophthora infestans efficient against Pectobacterium atrosepticum

The induction of plant immunity by Pathogen Associated Molecular Patterns (PAMPs) constitutes a powerful strategy for crop protection. PAMPs indeed induce general defense responses in plants and thus increase plant resistance to pathogens. Phytophthora infestans culture filtrates (CCFs) are known to induce defense responses and decrease the severity of soft rot due to Pectobacterium atrosepticum in potato tubers. The aim of this study was to identify and characterize the active compounds from P. infestans filtrate. The filtrate was fractionated by gel filtration, and the protection effects against P. atrosepticum and the ability to induce PAL activity were tested for each fraction. The fraction active in protection (F1) also induced PAL activity, as did the whole filtrate. Three elicitins (INF1, INF4 and INF5) were identified in F1b, subfraction of F1, by MALDI-TOF-MS and MS/MS analyses. However, deproteinized F1b still showed biological activity against the bacterium, revealing the presence of an additional active compound. GC-MS analyses of the deproteinized fraction highlighted the presence of a galactan-based complex polysaccharide. These experiments demonstrate that the biological activity of the CCF against P. atrosepticum results from a combined action of three elicitins and a complex polysaccharide, probably through the activation of general defense responses.

Ergosterol, an orphan fungal microbe-associated molecular pattern (MAMP)

Fungal pathogens continue to pose a significant threat to crop production and food supply. The early stages of plant-fungus interactions are mostly mediated by microbe-associated molecular pattern (MAMP) molecules, perceived by plant pattern recognition receptors (PRRs). Currently, the identified fungal MAMP molecules include chitin, chitosan, β-glucans, elicitins and ergosterol. Although the molecular battles between host plants and infecting fungal phytopathogens have been studied extensively, many aspects still need to be investigated to obtain a holistic understanding of the intrinsic mechanisms, which is paramount in combating fungal plant diseases. Here, an overview is given of the most recent findings concerning an 'orphan' fungal MAMP molecule, ergosterol, and we present what is currently known from a synopsis of different genes, proteins and metabolites found to play key roles in induced immune responses in plant-fungus interactions. Clearly, integrative investigations are still needed to provide a comprehensive systems-based understanding of the dynamics associated with molecular mechanisms in plant-ergosterol interactions and associated host responses.

Multi-platform metabolomic analyses of ergosterol-induced dynamic changes in Nicotiana tabacum cells

Metabolomics is providing new dimensions into understanding the intracellular adaptive responses in plants to external stimuli. In this study, a multi-technology-metabolomic approach was used to investigate the effect of the fungal sterol, ergosterol, on the metabolome of cultured tobacco cells. Cell suspensions were treated with different concentrations (0-1000 nM) of ergosterol and incubated for different time periods (0-24 h). Intracellular metabolites were extracted with two methods: a selective dispersive liquid-liquid micro-extraction and a general methanol extraction. Chromatographic techniques (GC-FID, GC-MS, GC × GC-TOF-MS, UHPLC-MS) and (1)H NMR spectroscopy were used for quantitative and qualitative analyses. Multivariate data analyses (PCA and OPLS-DA models) were used to extract interpretable information from the multidimensional data generated from the analytical techniques. The results showed that ergosterol triggered differential changes in the metabolome of the cells, leading to variation in the biosynthesis of secondary metabolites. PCA scores plots revealed dose- and time-dependent metabolic variations, with optimal treatment conditions being found to be 300 nM ergosterol and an 18 h incubation period. The observed ergosterol-induced metabolic changes were correlated with changes in defence-related metabolites. The 'defensome' involved increases in terpenoid metabolites with five antimicrobial compounds (the bicyclic sesquiterpenoid phytoalexins: phytuberin, solavetivone, capsidiol, lubimin and rishitin) and other metabolites (abscisic acid and phytosterols) putatively identified. In addition, various phenylpropanoid precursors, cinnamic acid derivatives and - conjugates, coumarins and lignin monomers were annotated. These annotated metabolites revealed a dynamic reprogramming of metabolic networks that are functionally correlated, with a high complexity in their regulation.

Activation of camalexin biosynthesis in Arabidopsis thaliana in response to perception of bacterial lipopolysaccharides: a gene-to-metabolite study

Lipopolysaccharides (LPS), as lipoglycan microbe-associated molecular pattern molecules, trigger activation of signal transduction pathways involved in defence that generate an enhanced defensive capacity in plants. The transcriptional regulation of the genes for tryptophan synthase B, TSB1, and the cytochrome P450 monooxygenases CYP79B2 and CYP71B15, involved in the camalexin biosynthetic pathway, were investigated in response to LPS treatment. GUS-reporter assays for CYP71B15 and CYP79B2 gene promoter activation were performed on transgenic plants and showed positive histochemical staining in response to LPS treatment, indicating activation of the promoters. Quantitative PCR revealed that transcripts of TSB1, CYP79B2 and CYP71B15 exhibited differential, transient up-regulation. TSB1 transcript levels were up-regulated between 6 and 9 h after LPS-induction, while CYP71B15 and CYP79B2 both exhibited maxima at 12 h. To obtain information on the gene-to-metabolite network, the effect of the transcriptome changes on the metabolome was correlated to camalexin production. Increases in camalexin concentration were quantified by ultra pressure liquid chromatography-mass spectrometry and both absorbance spectra and elemental composition confirmed its identity. The concentrations increased from 0.03 to 3.7 μg g(-1) fresh weight over a 24-h time period, thus indicating that the up-regulation of the biosynthetic pathway in response to LPS was accompanied by a time-dependent increase in camalexin concentration. Metabolomic analysis through principal component analysis-derived scores plots revealed clusters of sample replicates for 0, 6, 12, 18 and 24 h while loadings plots for LPS data identified camalexin as a biomarker that clearly demonstrated the variability between samples.

Molecular characterisation and regulation of a Nicotiana tabacum S-domain receptor-like kinase gene induced during an early rapid response to lipopolysaccharides

The isolation, characterization and regulation of the first lipopolysaccharide (LPS)-responsive S-domain receptor-like kinase (RLK) in Nicotiana tabacum are reported. The gene, corresponding to a differentially expressed LPS-responsive EST, was fully characterised to investigate its involvement in LPS-induced responses. The full genomic sequence, designated Nt-Sd-RLK, encodes for a S-domain RLK protein containing conserved modules (B-lectin-, S- and PAN-domains) reported to function in mediating protein-protein and protein-carbohydrate interactions in its extracellular domain, as well as the molecular architecture to transduce signals intracellularly through a Ser/Thr kinase domain. Phylogenetic analysis clustered Nt-Sd-RLK with S-domain RLKs induced by bacteria, wounding and salicylic acid. Perception of LPS induced a rapid, bi-phasic response in Nt-Sd-RLK expression with a 17-fold up-regulation at 3 and 9h. A defence-related W-box cis element was found in the promoter region of Nt-Sd-RLK and the transient induction of Nt-Sd-RLK in cultured cells by LPS exhibited a pattern typical of early response defence genes. Nt-Sd-RLK was also responsive to salicylic acid induction and was expressed in differentiated leaf tissue, where LPS elicited local as well as systemic up-regulation. The results contribute new knowledge about the potential role that S-domain RLKs may play within interactive signal transduction pathways associated with immunity and defence.

Ergosterol-induced sesquiterpenoid synthesis in tobacco cells

Plants have the ability to continuously respond to microbial signals in their environment. One of these stimuli is a steroid from fungal membranes, ergosterol, which does not occur in plants, but acts as a pathogen-associated molecular pattern molecule to trigger defence mechanisms. Here we investigated the effect of ergosterol on the secondary metabolites in tobacco (Nicotiana tabacum) cells by profiling the induced sesquiterpenoids. Suspensions of tobacco cells were treated with different concentrations (0-1,000 nM) of ergosterol and incubated for different time periods (0-24 h). Metabolites were extracted with a selective dispersive liquid-liquid micro-extraction method. Thin layer chromatography was used as a screening method for identification of sesquiterpenoids in tobacco extracts. Liquid chromatography coupled to mass spectrometry was used for quantitative and qualitative analyses. The results showed that ergosterol triggered differential changes in the metabolome of tobacco cells, leading to variation in the biosynthesis of secondary metabolites. Metabolomic analysis through principal component analysis-scores plots revealed clusters of sample replicates for ergosterol treatments of 0, 50, 150, 300 and 1,000 nM and time-dependent variation at 0, 6, 12, 18 and 24 h. Five bicyclic sesquiterpenoid phytoalexins, capsidiol, lubimin, rishitin, solavetivone and phytuberin, were identified as being ergosterol-induced, contributing to the altered metabolome.

Distinct carbohydrate and lipid-based molecular patterns within lipopolysaccharides from Burkholderia cepacia contribute to defense-associated differential gene expression in Arabidopsis thaliana

Lipopolysaccharides are structural components within the cell walls of Gram-negative bacteria. The LPSs as microbe-associated molecular pattern (MAMP) molecules can trigger defense-related responses involved in MAMP-triggered immunity and basal resistance in plants, presumably from an initial perception event. LPS from Burkholderia cepacia as well as two fragments, the glycolipid, lipid A and the polysaccharide (OPS-core) chain, were used to treat Arabidopsis thaliana seedlings to evaluate the eliciting activities of the individual LPS sub-domains by means of Annealing Control Primer-based Differential Display transcript profiling. Genes found to be up-regulated encode for proteins involved in signal perception and transduction, transcriptional regulation and defense - and stress responses. Furthermore, genes encoding proteins involved in chaperoning, secretion, protein-protein interactions and protein degradation were differentially expressed. It is concluded that intact LPS, as well as the two sub-components, induced the expression of a broad range of genes associated with perception and defense as well as metabolic reprogramming of cellular activities in support of immunity and basal resistance. Whilst the lipid A and OPS moieties were able to up-regulate sub-sets of defense-associated genes over the same spectrum of categories as intact LPS, the up-regulation observed with intact LPS was the more comprehensive, suggesting that the lipid A and glycan molecular patterns of the molecule act as partial agonists, but that the intact LPS structure is required for full agonist activity.

Post-translation modification of proteins; methodologies and applications in plant sciences

Proteins have the potential to undergo a variety of post-translational modifications and the different methods available to study these cellular processes has advanced rapidly with the continuing development of proteomic technologies. In this review we aim to detail five major post-translational modifications (phosphorylation, glycosylaion, lipid modification, ubiquitination and redox-related modifications), elaborate on the techniques that have been developed for their analysis and briefly discuss the study of these modifications in selected areas of plant science.

Lipopolysaccharide mobility in leaf tissue of Arabidopsis thaliana

Bacterial lipopolysaccharides (LPS) are triggers of defence responses in plants, and induce local as well as systemic acquired resistance. Arabidopsis thaliana plants pretreated with LPS show an increased resistance to the virulent bacterial plant pathogen Pseudomonas syringae pv. tomato DC3000. To investigate the mobilization and transport of LPS in Arabidopsis leaves, fluorescently labelled LPS (Alexa Fluor® 488 conjugate) from Salmonella minnesota was used. Leaves were pressure infiltrated with fluorescein-labelled LPS and fluorescence microscopy was used to follow the movement and localization of LPS as a function of time. The observation of leaves 1 h after supplementation with fluorescein-labelled LPS revealed a fluorescent signal in the intercellular space. Capillary zone electrophoresis was used for the detection and analysis of the labelled LPS in directly treated leaves and systemic leaves. In addition, gel electrophoresis was used to confirm LPS mobilization. The results indicated that LPS mobilization/translocation occurs through the xylem from local, treated leaves to systemic, untreated leaves. Consequently, care should be taken when ascribing the observed biochemical responses and induced resistance from LPS perception as being uniquely local or systemic, as these responses might overlap because of the mobility of LPS in the plant vascular system.

Responses of wheat seedlings to cadmium, mercury and trichlorobenzene stresses

The molecular response of wheat (Triticum aestivum L., cv. Yangmai 13) seedlings to heavy metal (Cd, Hg) and 1,2,4-trichlorobenzene (TCB) stresses were examined by two-dimensional gel electrophoresis, image analysis, and peptide mass fingerprinting. The results showed inhibitions of root and shoot growth by Cd, Hg, and TCB. These stresses led to water deficit and lipid phosphorylation in the seedling which also promoted protein phophorylation in the leaves. Hg stress inhibited protein synthesis while Cd and TCB stresses induced or up-regulated more proteins in the leaves. Most of these induced proteins played important roles in the biochemical reactions involved in tolerance of wheat to Cd and TCB stresses. The primary functions of Cd- and TCB-induced proteins included methionine metabolism, Rubisco modification, protein phosphorylation regulation, protein configuration protection, H+ transmembrane transportation and also the synthesis of ethylene, defense substances and cell wall compounds.

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.

Proteomic profiling of cellular targets of lipopolysaccharide-induced signalling in Nicotiana tabacum BY-2 cells

Plants constantly monitor for pathogen challenge and utilize a diverse array of adaptive defense mechanisms, including differential protein regulation, during pathogen attack. A proteomic analysis of Nicotiana tabacum BY-2 cells was performed in order to investigate the dynamic changes following perception of bacterial lipopolysaccharides. A multiplexed proteome analysis, employing two-dimensional difference-in-gel-electrophoresis with CyDye DIGE fluors, as well as Ruthenium II tris (bathophenanthroline disulfonate) fluorescence staining and Pro-Q Diamond phosphoprotein-specific gel staining, monitored over 1500 proteins and resulted in the identification of 88 differentially regulated proteins and phosphoproteins responsive to LPS(B.cep.)-elicitation. Functional clustering of the proteins both at the level of their abundance and phosphorylation status, revealed 9 proteins involved in transport, ion homeostasis and signal transduction. A large number of responsive proteins were found to be involved in metabolism- and energy-related processes (36), representing various metabolic pathways. Another abundant category corresponded to proteins classified as molecular chaperones and involved in protein destination/targeting (12). Other categories of proteins found to be LPS(B.cep.)-responsive and differentially regulated include cell structure- and cytoskeletal rearrangement proteins (8) and proteins involved in transcription and translation as well as degradation (11). The results indicate that LPS(B.cep.) induces metabolic reprogramming and changes in cellular activities supporting protein synthesis, -folding, vesicle trafficking and secretion; accompanied by changes to the cytoskeleton and proteosome function. Many of the identified proteins are known to be interconnected at various levels through a complex web of activation/deactivation, complex formation, protein-protein interactions, and chaperoning reactions. The presented data offers novel insights and further evidence for the biochemical action of LPS(B.cep.) as a resistance elicitor, a pathogen-associated molecular pattern molecule and triggering agent of defense responses associated with innate immunity.

The grateful dead: calcium and cell death in plant innate immunity

Plant cells sensing pathogenic microorganisms evoke defence systems that can confer resistance to infection. This innate immune reaction can include triggering of basal defence responses as well as programmed cell death, or hypersensitive response (HR). In both cases (basal defence and HR), pathogen perception is translated into elevated cytosolic Ca(2+) (mediated by plasma membrane and intracellular channels) as an early step in a signalling cascade. Cyclic nucleotide-gated channels contribute to this influx of Ca(2+) into the cell. The molecular nature of other transport proteins contributing to the Ca(2+) elevation is unclear. Pathogen recognition occurs at two levels: the perception of pathogen-associated molecular pattern (PAMP) molecules widely present in microorganisms, and an interaction between pathogen avirulence gene products (if present) and corresponding plant R (resistance) gene products. The Ca(2+) elevation occurring in response to PAMP perception or R gene interactions could occur due to phosphorylation events, G-protein signalling and/or an increase in cyclic nucleotides. Downstream from the initial Ca(2+) rise, the signalling cascade includes: activation of calmodulin and protein kinases, and nitric oxide and reactive oxygen species generation. Some of these downstream events amplify the Ca(2+) signal by further activation of Ca(2+) transporters.

Elicitin-responsive lectin-like receptor kinase genes in BY-2 cells

The inhibition of elicitor-induced plant defense responses by the protein kinase inhibitors K252a and staurosporine indicates that defense responses require protein phosphorylation. We isolated a cDNA clone encoding Nicotiana tabacum lectin-like receptor protein kinase 1 (NtlecRK1), an elicitor-responsive gene; in tobacco bright yellow (BY-2) cells by a differential display method. NtlecRK forms a gene family with at least three members in tobacco. All three NtlecRK genes potentially encode the N-terminal legume lectin domain, transmembrane domain and C-terminal Ser/Thr-type protein kinase domain. Green fluorescent protein (GFP) fusion showed that the NtlecRK1 protein was located on the plasma membrane. In addition, NtlecRK1 and 3 were responsive to INF1 elicitin and the bacterial elicitor harpin. These results indicate that NtlecRKs are membrane-located protein kinases that are induced during defense responses in BY-2 cells.

The O-chain structure from the LPS of the endophytic bacterium Burkholderia cepacia strain ASP B 2D

The O-chain polysaccharide of the lipopolysaccharide from the endophytic bacterium Burkholderia cepacia strain was characterized. The structure was studied by means of chemical analysis and 2D NMR spectroscopy and shown to be the following: -->2)-beta-D-Ribf-(1-->6)-alpha-D-Glcp-(1-->.

Activation of defence reactions in Solanaceae: where is the specificity?

When a potential pathogen attempts to infect a plant, biochemical and molecular communication takes place and leads to the induction of plant defence mechanisms. In the case of efficient defence, visible symptoms are restricted and the pathogen does not multiply (incompatible interaction); when defence is inefficient, the plant becomes rapidly infected (compatible interaction). During the last 30 years, a growing body of knowledge on plant-pathogen interactions has been gathered, and a large number of studies investigate the induction of various plant defence reactions by pathogens or by pathogen-derived compounds. However, as most papers focus on incompatible interactions, there is still a lack of understanding about the similarities and differences between compatible and incompatible situations. This review targets the question of specificity in Solanaceae-pathogen interactions, by comparing defence patterns in plants challenged with virulent or avirulent pathogens (or with pathogen-associated molecular patterns from these). A special emphasis is made on analysing whether defence reactions in Solanaceae depend primarily on the type of elicitor, on the plant genotype/species, or on the type of interaction (compatible or incompatible).

Lipopolysaccharide-responsive phosphoproteins in Nicotiana tabacum cells

Mounting evidence is merging to affirm the effectiveness of bacterial lipopolysaccharides (LPS) as biological control agents, inducers of innate immunity, and to stimulate/potentiate the development of defense responses in plants through protein phosphorylation-mediated signal perception/transduction responses. In vivo labeling of protein phosphorylation events during signal transduction indicated the rapid phosphorylation of several proteins. Substantial differences and de novo LPS-induced phosphorylation were also observed with two-dimensional analysis. In this study, qualitative and quantitative changes in phosphoproteins of Nicotiana tabacum suspension cells during elicitation by LPS from the Gram-negative bacteria, Burkholderia cepacia, were analyzed using two-dimensional electrophoresis in combination with a phosphoprotein-specific gel stain. Trypsin digested phosphoproteins were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS) and nano-electrospray-ionization liquid chromatography tandem mass spectrometry (nano-ESI-LC/MS/MS). A total of 27 phosphoproteins were identified from 23 excised gel spots. The identified phosphoproteins indicate that LPS(B.cep)-induced signal perception/transduction involves G-protein coupled receptor signaling, Ca(2+)/calmodulin-dependent signaling pathways, H(+)-ATPase regulation of intracellular pH, thioredoxin-mediated signaling and phosphorylation of 14-3-3 regulatory proteins. Other targets of LPS(B.cep)-responsive phosphorylation included NTP pool maintenance, heat shock proteins, protein biosynthesis and chaperones as well as cytoskeletal tubulin. The results add novel insights into the biochemical process of LPS perception and resulting signal transduction.