The multifaceted functions of lipopolysaccharide in plant-bacteria interactions

PGPR: Key to Enhancing Crop Productivity and Achieving Sustainable Agriculture

Due to the burgeoning global population and the advancement of economies, coupled with human activities leading to the degradation of soil ecosystems and the depletion of non-renewable resources, concerns have arisen regarding food security and human survival. In order to address these adverse impacts, the spotlight has been cast upon plant growth-promoting rhizobacteria (PGPR), driven by a strong environmental consciousness. PGPR possesses the capability to foster plant growth and amplify crop yield through both direct and indirect mechanisms. By expediting plant growth, augmenting nutrient assimilation, heightening crop yield and caliber, and fortifying stress resilience, the application of PGPR can mitigate reliance on chemical fertilizers and pesticides while diminishing ecological perils. This exposition delves into the function of PGPR in modulating plant hormones, fostering nutrient solubilization, and fortifying plant resistance against biotic and abiotic stressors. This review offers valuable insights into the intricate interplay between PGPR and plants, elucidating uncertainties ripe for further investigation. Profound comprehension and judicious utilization of PGPR are indispensable for attaining sustainable agricultural progression, making substantial contributions to resolving the conundrums of global food security and environmental conservation.

Genomic insights into novel Erwinia bacteriophages: unveiling their Henunavirus membership and host infection strategies

Erwinia amylovora, the primary causative agent of blight disease in rosaceous plants, poses a significant threat to agricultural yield worldwide, with limited effective countermeasures. The emergence of sustainable alternative agents such as bacteriophages is a promising solution for fire blight that specifically targets Erwinia. In this study, we isolated pEp_SNUABM_01 and pEa_SNUABM_55 from a South Korean apple orchard soil, analyzed their genomic DNA sequences, and performed a comprehensive comparative analysis of Hena1 in four distinct sections. This study aimed to unveil distinctive features of these phages, with a focus on host recognition, which will provide valuable insights into the evolution and characteristics of Henunavirus bacteriophages that infect plant pathogenic Erwinia spp. By elucidating the distinct genomic features of these phages, particularly in terms of host recognition, this study lays a foundation for their potential application in mitigating the risks associated with fire blight in Rosaceae plants on a global scale.

Green leaf volatiles co-opt proteins involved in molecular pattern signalling in plant cells

The green leaf volatiles (GLVs) Z-3-hexen-1-ol (Z3-HOL) and Z-3-hexenyl acetate (Z3-HAC) are airborne infochemicals released from damaged plant tissues that induce defenses and developmental responses in receiver plants, but little is known about their mechanism of action. We found that Z3-HOL and Z3-HAC induce similar but distinctive physiological and signaling responses in tomato seedlings and cell cultures. In seedlings, Z3-HAC showed a stronger root growth inhibition effect than Z3-HOL. In cell cultures, the two GLVs induced distinct changes in MAP kinase (MAPK) activity and proton fluxes as well as rapid and massive changes in the phosphorylation status of proteins within 5 min. Many of these phosphoproteins are involved in reprogramming the proteome from cellular homoeostasis to stress and include pattern recognition receptors, a receptor-like cytoplasmic kinase, MAPK cascade components, calcium signaling proteins and transcriptional regulators. These are well-known components of damage-associated molecular pattern (DAMP) signaling pathways. These rapid changes in the phosphoproteome may underly the activation of defense and developmental responses to GLVs. Our data provide further evidence that GLVs function like DAMPs and indicate that GLVs coopt DAMP signaling pathways.

Structure, Physicochemical Properties and Biological Activity of Lipopolysaccharide from the Rhizospheric Bacterium Ochrobactrum quorumnocens T1Kr02, Containing d-Fucose Residues

Lipopolysaccharides (LPSs) are major components of the outer membranes of Gram-negative bacteria. In this work, the structure of the O-polysaccharide of Ochrobactrum quorumnocens T1Kr02 was identified by nuclear magnetic resonance (NMR), and the physical-chemical properties and biological activity of LPS were also investigated. The NMR analysis showed that the O-polysaccharide has the following structure: →2)-β-d-Fucf-(1→3)-β-d-Fucp-(1→. The structure of the periplasmic glucan coextracted with LPS was established by NMR spectroscopy and chemical methods: →2)-β-d-Glcp-(1→. Non-stoichiometric modifications were identified in both polysaccharides: 50% of d-fucofuranose residues at position 3 were O-acetylated, and 15% of d-Glcp residues at position 6 were linked with succinate. This is the first report of a polysaccharide containing both d-fucopyranose and d-fucofuranose residues. The fatty acid analysis of the LPS showed the prevalence of 3-hydroxytetradecanoic, hexadecenoic, octadecenoic, lactobacillic, and 27-hydroxyoctacosanoic acids. The dynamic light scattering demonstrated that LPS (in an aqueous solution) formed supramolecular particles with a size of 72.2 nm and a zeta-potential of -21.5 mV. The LPS solution (10 mkg/mL) promoted the growth of potato microplants under in vitro conditions. Thus, LPS of O. quorumnocens T1Kr02 can be recommended as a promoter for plants and as a source of biotechnological production of d-fucose.

Recent advances in immuno-based methods for the detection of Ralstonia solanacearum

Ralstonia solanacearum (RS) is a widely recognized phytopathogenic bacterium which is responsible for causing devastating losses in a wide range of economically significant crops. Timely and accurate detection of this pathogen is pivotal to implementing effective disease management strategies and preventing crop losses. This review provides a comprehensive overview of recent advances in immuno-based detection methods for RS. The review begins by introducing RS, highlighting its destructive potential and the need for point-of-care detection techniques. Subsequently, it explores traditional detection methods and their limitations, emphasizing the need for innovative approaches. The main focus of this review is on immuno-based detection methods and it discusses recent advancements in serological detection techniques. Furthermore, the review sheds light on the challenges and prospects of immuno-based detection of RS. It emphasizes the importance of developing rapid, field-deployable assays that can be used by farmers and researchers alike. In conclusion, this review provides valuable insights into the recent advances in immuno-based detection methods for RS.

Lipopolysaccharides from a Shiraia fruiting body-associated bacterium elicit host fungal hypocrellin A biosynthesis through nitric oxide generation

Hypocrellin A (HA) is an excellent perylenequinone photosensitizer from Shiraia fruiting bodies. A dominant bacterium Pseudomonas fulva SB1 in the fruiting body was found to promote HA biosynthesis. The bacterial LPS were purified and the O-specific polysaccharide (OPS) consisted of rhamnose (Rha), galactose (Gal) and N-acetyl-galactosamine (GalNAc) with an average molecular weight of 282.8 kDa. Although the OPS composing of Rhap and Galp backbone showed elicitation capability on fungal HA accumulation, the highest HA production (303.76 mg/L) was achieved by LPS treatment at 20 μg/mL on day 3 of the mycelium culture. The generation of nitric oxide (NO) in Shiraia mycelia was triggered by LPS, which was partially blocked by inhibitors of nitric oxide synthase (NOS) and nitrate reductase (NR), leading to the depressed HA production. Transcriptome analysis revealed that NO mediated LPS-induced HA production via upregulating the expressions of critical genes associated with central carbon metabolism and downstream HA biosynthesis genes. This is the first report of LPS-induced NO to regulate fungal secondary metabolite production, which provides new insights on the role of bacterial LPS in bacterium-fungus interactions and an effective strategy to enhance hypocrellin production.

Combining lipid-mimicking-enabled transition metal and enzyme-mediated catalysis at the cell surface of E. coli

Being an essential multifunctional platform and interface to the extracellular environment, the cell membrane constitutes a valuable target for the modification and manipulation of cells and cellular behavior, as well as for the implementation of artificial, new-to-nature functionality. While bacterial cell surface functionalization via expression and presentation of recombinant proteins has extensively been applied, the corresponding application of functionalizable lipid mimetics has only rarely been reported. Herein, we describe an approach to equip E. coli cells with a lipid-mimicking, readily membrane-integrating imidazolium salt and a corresponding NHC-palladium complex that allows for flexible bacterial membrane surface functionalization and enables E. coli cells to perform cleavage of propargyl ethers present in the surrounding cell medium. We show that this approach can be combined with already established on-surface functionalization, such as bacterial surface display of enzymes, i.e. laccases, leading to a new type of cascade reaction. Overall, we envision the herein presented proof-of-concept studies to lay the foundation for a multifunctional toolbox that allows flexible and broadly applicable functionalization of bacterial membranes.

Lipopolysaccharides from Ralstonia solanacearum induce a broad metabolomic response in Solanum lycopersicum

Ralstonia solanacearum, one of the most destructive crop pathogens worldwide, causes bacterial wilt disease in a wide range of host plants. The major component of the outer membrane of Gram-negative bacteria, lipopolysaccharides (LPS), has been shown to function as elicitors of plant defense leading to the activation of signaling and defense pathways in several plant species. LPS from a R. solanacearum strain virulent on tomato (LPSR. sol.), were purified, chemically characterized, and structurally elucidated. The lipid A moiety consisted of tetra- to hexa-acylated bis-phosphorylated disaccharide backbone, also decorated by aminoarabinose residues in minor species, while the O-polysaccharide chain consisted of either linear tetrasaccharide or branched pentasaccharide repeating units containing α-L-rhamnose, N-acetyl-β-D-glucosamine, and β-L-xylose. These properties might be associated with the evasion of host surveillance, aiding the establishment of the infection. Using untargeted metabolomics, the effect of LPSR. sol. elicitation on the metabolome of Solanum lycopersicum leaves was investigated across three incubation time intervals with the application of UHPLC-MS for metabolic profiling. The results revealed the production of oxylipins, e.g., trihydroxy octadecenoic acid and trihydroxy octadecadienoic acid, as well as several hydroxycinnamic acid amide derivatives, e.g., coumaroyl tyramine and feruloyl tyramine, as phytochemicals that exhibit a positive correlation to LPSR. sol. treatment. Although the chemical properties of these metabolite classes have been studied, the functional roles of these compounds have not been fully elucidated. Overall, the results suggest that the features of the LPSR. sol. chemotype aid in limiting or attenuating the full deployment of small molecular host defenses and contribute to the understanding of the perturbation and reprogramming of host metabolism during biotic immune responses.

Factors Required for Adhesion of Salmonella enterica Serovar Typhimurium to Lactuca sativa (Lettuce)

Salmonella enterica serovar Typhimurium is a major cause of foodborne gastroenteritis. Recent outbreaks of infections by S. enterica serovar Typhimurium are often associated with non-animal-related food, i.e., vegetables, fruits, herbs, sprouts, and nuts. One main problem related to the consumption of fresh produce is the minimal processing, especially for leafy green salads. In this study, we focused on butterhead lettuce (Lactuca sativa) to which S. enterica serovar Typhimurium adheres at higher rates compared to Valerianella locusta, resulting in prolonged persistence. Here, we systematically analyzed factors contributing to adhesion of S. enterica serovar Typhimurium to L. sativa leaves. Application of a reductionist, synthetic approach, including the controlled surface expression of specific adhesive structures of S. enterica serovar Typhimurium, one at a time, enabled the identification of relevant fimbrial and nonfimbrial adhesins, the O-antigen of lipopolysaccharide, the flagella, and chemotaxis being involved in binding to L. sativa leaves. The analyses revealed contributions of Lpf fimbriae, Sti fimbriae, autotransported adhesin MisL, T1SS-secreted BapA, intact lipopolysaccharide (LPS), and flagella-mediated motility to adhesion of S. enterica serovar Typhimurium to L. sativa leaves. In addition, we identified BapA as a potential adhesin involved in binding to V. locusta and L. sativa leaf surfaces. IMPORTANCE The number of produce-associated outbreaks by gastrointestinal pathogens is increasing and underlines the relevance to human health. The mechanisms involved in the colonization of, persistence on, and transmission by, fresh produce are poorly understood. Here, we investigated the contribution of adhesive factors of S. enterica serovar Typhimurium in the initial phase of plant colonization, i.e., the binding to the plant surface. We used the previously established reductionist, synthetic approach to identify factors that contribute to the surface binding of S. enterica serovar Typhimurium to leaves of L. sativa by expressing all known adhesive structures by remote control expression system.

Putative transcription antiterminator RfaH contributes to Erwinia amylovora virulence

The gram-negative bacterium Erwinia amylovora causes fire blight disease of apple and pear trees. The exopolysaccharide amylovoran and lipopolysaccharides are essential E. amylovora virulence factors. Production of amylovoran and lipopolysaccharide is specified in part by genes that are members of long operons. Here, we show that full virulence of E. amylovora in apple fruitlets and tree shoots depends on the predicted transcription antiterminator RfaH. RfaH reduces pausing in the production of long transcripts having an operon polarity suppressor regulatory element within their promoter region. In E. amylovora, only the amylovoran operon and a lipopolysaccharide operon have such regulatory elements within their promoter regions and in the correct orientation. These operons showed dramatically increased polarity in the ΔrfaH mutant compared to the wild type as determined by RNA sequencing. Amylovoran and lipopolysaccharide production in vitro was reduced in rfaH mutants compared to the wild type, which probably contributes to the rfaH mutant virulence phenotype. Furthermore, type VI secretion cluster 1, which contributes to E. amylovora virulence, showed reduced expression in ΔrfaH compared to the wild type, although without an increase in polarity. The data suggest that E. amylovora RfaH directly, specifically, and exclusively suppresses operon polarity in the amylovoran operon and a lipopolysaccharide operon.

Breaking down the cell wall: Still an attractive antibacterial strategy

Since the advent of penicillin, humans have known about and explored the phenomenon of bacterial inhibition via antibiotics. However, with changes in the global environment and the abuse of antibiotics, resistance mechanisms have been selected in bacteria, presenting huge threats and challenges to the global medical and health system. Thus, the study and development of new antimicrobials is of unprecedented urgency and difficulty. Bacteria surround themselves with a cell wall to maintain cell rigidity and protect against environmental insults. Humans have taken advantage of antibiotics to target the bacterial cell wall, yielding some of the most widely used antibiotics to date. The cell wall is essential for bacterial growth and virulence but is absent from humans, remaining a high-priority target for antibiotic screening throughout the antibiotic era. Here, we review the extensively studied targets, i.e., MurA, MurB, MurC, MurD, MurE, MurF, Alr, Ddl, MurI, MurG, lipid A, and BamA in the cell wall, starting from the very beginning to the latest developments to elucidate antimicrobial screening. Furthermore, recent advances, including MraY and MsbA in peptidoglycan and lipopolysaccharide, and tagO, LtaS, LspA, Lgt, Lnt, Tol-Pal, MntC, and OspA in teichoic acid and lipoprotein, have also been profoundly discussed. The review further highlights that the application of new methods such as macromolecular labeling, compound libraries construction, and structure-based drug design will inspire researchers to screen ideal antibiotics.

Deep Population Genomics Reveals Systematic and Parallel Evolution at a Lipopolysaccharide Biosynthetic Locus in Xanthomonas Pathogens That Infect Rice and Sugarcane

The advent of high-throughput sequencing and population genomics has enabled researchers to investigate selection pressure at hypervariable genomic loci encoding pathogen-associated molecular pattern (PAMP) molecules like lipopolysaccharide (LPS). Xanthomonas is a model and a major group of phytopathogenic bacteria that infect hosts in tissue-specific manner. Our in-depth population-based genomic investigation revealed the emergence of major lineages in two Xanthomonas pathogens that infect xylem of rice and sugarcane is associated with the acquisition and later large-scale replacement by distinct type of LPS cassettes. In the population of the rice xylem pathogen, Xanthomonas oryzae pv. oryzae (Xoo) and sugarcane pathogens Xanthomonas sacchari (Xsac) and Xanthomonas vasicola (Xvv), the BXO8 type of LPS cassette is replaced by a BXO1 type of cassette in Xoo and by Xvv type LPS cassette in Xsac and Xvv. These findings suggest a wave of parallel evolution at an LPS locus mediated by horizontal gene transfer (HGT) events during its adaptation and emergence. Aside from xylem pathogens, two closely related lineages of Xoo that infect parenchyma of rice and Leersia hexandra grass have acquired an LPS cassette from Xanthomonas pathogens that infect parenchyma of citrus, walnut, and strawberries, indicating yet another instance of parallel evolution mediated by HGT at an LPS locus. Our targeted and megapopulation-based genome dynamic studies revealed the acquisition and dominance of specific types of LPS cassettes in adaptation and success of a major group of phytopathogenic bacteria. IMPORTANCE Lipopolysaccharide (LPS) is a major microbe associated molecular pattern and hence a major immunomodulator. As a major and outer member component, it is expected that LPS is a frontline defense mechanism to deal with different host responses. Limited studies have indicated that LPS loci are also highly variable at strain and species level in plant-pathogenic bacteria, suggesting strong selection pressure from plants and associated niches. The advent of high-throughput genomics has led to the availability of a large set of genomic resources at taxonomic and population levels. This provides an exciting and important opportunity to carryout megascale targeted and population-based comparative genomic/association studies at important loci like those encoding LPS biosynthesis to understand their role in the evolution of the host, tissue specificity, and also predominant lineages. Such studies will also fill major gap in understanding host and tissue specificity in pathogenic bacteria. Our pioneering study uses the Xanthomonas group of phytopathogens that are known for their characteristic host and tissue specificity. The present deep phylogenomics of diverse Xanthomonas species and its members revealed lineage association and dominance of distinct types of LPS in accordance with their origin, host, tissue specificity, and evolutionary success.

Lipopolysaccharide and flagellin of Azospirillum brasilense Sp7 influence callus morphogenesis and plant regeneration in wheat

In vitro somatic callus culturing is used widely in plant biotechnology, but its effectiveness depends largely on the donor plant genotype. Bacteria or components of their cells are rarely used to activate morphogenesis. In this work, inoculation of explants from immature wheat (Triticum aestivum L.) embryos with a suspension of living cells of the bacterium Azospirillum brasilense Sp7 resulted in callus death after 7 days of growth, in contrast to explant treatment with a suspension of heat-killed whole cells of Sp7. The experiments used two wheat lines, LRht-B1a and LRht-B1c, which differ in morphogenic activity. Growing calluses with the lipopolysaccharide of A. brasilense Sp7 increased the yield of regenerated plants 2- to 3.5-fold in both lines. This increase was through the activation of regenerant formation from morphogenic calluses. We have demonstrated for the first time the effects of bacterial flagellin on plant tissue culture. The polar-flagellum flagellin of A. brasilense Sp7 leveled the genotypic differences in the morphogenic ability of callus tissue. Specifically, it increased the yield of morphogenic calluses in the weakly morphogenic line LRht-B1a to the yield value in the highly morphogenic line LRht-B1c but lowered the yield of regenerants in the highly morphogenic line LRht-B1c to the yield value in the weakly morphogenic line LRht-B1a. Thus, bacterial lipopolysaccharides and flagellins can be used to regulate the formation of morphogenic calluses and regenerants in plant tissue culturing in vitro.

A Journey from Structure to Function of Bacterial Lipopolysaccharides

Lipopolysaccharide (LPS) is a crucial constituent of the outer membrane of most Gram-negative bacteria, playing a fundamental role in the protection of bacteria from environmental stress factors, in drug resistance, in pathogenesis, and in symbiosis. During the last decades, LPS has been thoroughly dissected, and massive information on this fascinating biomolecule is now available. In this Review, we will give the reader a third millennium update of the current knowledge of LPS with key information on the inherent peculiar carbohydrate chemistry due to often puzzling sugar residues that are uniquely found on it. Then, we will drive the reader through the complex and multifarious immunological outcomes that any given LPS can raise, which is strictly dependent on its chemical structure. Further, we will argue about issues that still remain unresolved and that would represent the immediate future of LPS research. It is critical to address these points to complete our notions on LPS chemistry, functions, and roles, in turn leading to innovative ways to manipulate the processes involving such a still controversial and intriguing biomolecule.

Pectobacterium parmentieri SCC 3193 Mutants with Altered Synthesis of Cell Surface Polysaccharides Are Resistant to N4-Like Lytic Bacteriophage ϕA38 (vB_Ppp_A38) but Express Decreased Virulence in Potato (Solanum tuberosum L.) Plants

Pectobacterium parmentieri is a Gram-negative plant-pathogenic bacterium able to infect potato (Solanum tuberosum L.). Little is known about lytic bacteriophages infecting P. parmentieri and how phage-resistance influences the environmental fitness and virulence of this species. A lytic phage vB_Ppp_A38 (ϕA38) has been previously isolated and characterized as a potential biological control agent for the management of P. parmentieri. In this study, seven P. parmentieri SCC 3193 Tn5 mutants were identified that exhibited resistance to infection caused by vB_Ppp_A38 (ϕA38). The genes disrupted in these seven mutants encoded proteins involved in the assembly of O-antigen, sugar metabolism, and the production of bacterial capsule exopolysaccharides. The potential of A38-resistant P. parmentieri mutants for plant colonization and pathogenicity as well as other phenotypes expected to contribute to the ecological fitness of P. parmentieri, including growth rate, use of carbon and nitrogen sources, production of pectinolytic enzymes, proteases, cellulases, and siderophores, swimming and swarming motility, presence of capsule and flagella as well as the ability to form biofilm were assessed. Compared to the wild-type P. parmentieri strain, all phage-resistant mutants exhibited a reduced ability to colonize and to cause symptoms in growing potato (S. tuberosum L.) plants. The implications of bacteriophage resistance on the ecological fitness of P. parmentieri are discussed.

Analysis of the Structure and Biosynthesis of the Lipopolysaccharide Core Oligosaccharide of Pseudomonas syringae pv. tomato DC3000

Lipopolysaccharide (LPS), the major component of the outer membrane of Gram-negative bacteria, is important for bacterial viability in general and host-pathogen interactions in particular. Negative charges at its core oligosaccharide (core-OS) contribute to membrane integrity through bridging interactions with divalent cations. The molecular structure and synthesis of the core-OS have been resolved in various bacteria including the mammalian pathogen Pseudomonas aeruginosa. A few core-OS structures of plant-associated Pseudomonas strains have been solved to date, but the genetic components of the underlying biosynthesis remained unclear. We conducted a comparative genome analysis of the core-OS gene cluster in Pseudomonas syringae pv. tomato (Pst) DC3000, a widely used model pathogen in plant-microbe interactions, within the P. syringae species complex and to other plant-associated Pseudomonas strains. Our results suggest a genetic and structural conservation of the inner core-OS but variation in outer core-OS composition within the P. syringae species complex. Structural analysis of the core-OS of Pst DC3000 shows an uncommonly high phosphorylation and presence of an O-acetylated sugar. Finally, we combined the results of our genomic survey with available structure information to estimate the core-OS composition of other Pseudomonas species.

Lipopolysaccharide Enhances Tanshinone Biosynthesis via a Ca2+-Dependent Manner in Salvia miltiorrhiza Hairy Roots

Tanshinones, the major bioactive components in Salvia miltiorrhiza Bunge (Danshen), are synthesized via the mevalonic acid (MVA) pathway or the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway and the downstream biosynthesis pathway. In this study, the bacterial component lipopolysaccharide (LPS) was utilized as a novel elicitor to induce the wild type hairy roots of S. miltiorrhiza. HPLC analysis revealed that LPS treatment resulted in a significant accumulation of cryptotanshinone (CT) and dihydrotanshinone I (DTI). qRT-PCR analysis confirmed that biosynthesis genes such as SmAACT and SmHMGS from the MVA pathway, SmDXS and SmHDR from the MEP pathway, and SmCPS, SmKSL and SmCYP76AH1 from the downstream pathway were markedly upregulated by LPS in a time-dependent manner. Furthermore, transcription factors SmWRKY1 and SmWRKY2, which can activate the expression of SmDXR, SmDXS and SmCPS, were also increased by LPS. Since Ca²⁺ signaling is essential for the LPS-triggered immune response, Ca²⁺ channel blocker LaCl₃ and CaM antagonist W-7 were used to investigate the role of Ca²⁺ signaling in tanshinone biosynthesis. HPLC analysis demonstrated that both LaCl₃ and W-7 diminished LPS-induced tanshinone accumulation. The downstream biosynthesis genes including SmCPS and SmCYP76AH1 were especially regulated by Ca²⁺ signaling. To summarize, LPS enhances tanshinone biosynthesis through SmWRKY1- and SmWRKY2-regulated pathways relying on Ca²⁺ signaling. Ca²⁺ signal transduction plays a key role in regulating tanshinone biosynthesis in S. miltiorrhiza.

Identification of MAMP-Responsive Plasma Membrane-Associated Proteins in Arabidopsis thaliana Following Challenge with Different LPS Chemotypes from Xanthomonas campestris

Lipopolysaccharides (LPS) are recognized as microbe-associated molecular patterns (MAMPs) responsible for eliciting defense-related responses and while the effects have been well-documented in mammals, there is a lack of knowledge regarding the mechanism of perception in plant systems and recognized structural moieties within the macromolecular lipoglycan structure. Thus, identification of the LPS plasma membrane (PM) receptor(s)/receptor complex in Arabidopsis thaliana through proteomics will contribute to a deeper understanding of induced defense responses. As such, structurally characterized LPS chemotypes from Xanthomonas campestris pv. campestris (Xcc) wild-type 8004 (prototypical smooth-type LPS) and mutant 8530 (truncated core with no O–chain) strains were utilized to pre-treat A. thaliana plants. The associated proteomic response/changes within the PM were compared over a 24 h period using mass spectrometry-based methodologies following three variants of LPS-immobilized affinity chromatography. This resulted in the identification of proteins from several functional categories, but importantly, those involved in perception and defense. The distinct structural features between wild-type and mutant LPS are likely responsible for the differential changes to the proteome profiles, and many of the significant proteins were identified in response to the wild-type Xcc LPS where it is suggested that the core oligosaccharide and O-chain participate in recognition by receptor-like kinases (RLKs) in a multiprotein complex and, notably, varied from that of the mutant chemotype.

Recognition Pattern, Functional Mechanism and Application of Chitin and Chitosan Oligosaccharides in Sustainable Agriculture

Background:

Application of chitin attracts much attention in the past decades as the second abundant polysaccharides in the world after cellulose. Chitin oligosaccharides (CTOS) and its deacetylated derivative chitosan oligosaccharides (COS) were shown great potentiality in agriculture by enhancing plant resistance to abiotic or biotic stresses, promoting plant growth and yield, improving fruits quality and storage, etc. Those applications have already served huge economic and social benefits for many years. However, the recognition mode and functional mechanism of CTOS and COS on plants have gradually revealed just in recent years.

Objective:

Recognition pattern and functional mechanism of CTOS and COS in plant together with application status of COS in agricultural production will be well described in this review. By which we wish to promote further development and application of CTOS and COS–related products in the field.

Chitosan oligosaccharide induces resistance to Pst DC3000 in Arabidopsis via a non-canonical N-glycosylation regulation pattern

Roles of protein N-glycosylation in chitosan oligosaccharide (COS) induced resistance were investigated in the present study. Results demonstrated that N-glycosylation deficient Arabidopsis mutants (stt3a and ManI) were more susceptible against Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) than wild type (WT) plants. Surprisingly, in stt3a and ManI, COS-induced resistance to Pst DC3000 was mostly intact, and the up-regulation effect on SA- and JA-mediated signalling pathways also similar like WT. Nucleotide sugars accumulation and N-glycosylation related genes expression were differently regulated after COS treatment. Global glycomics analysis quantified 157 N-glycan isomers, and 56.7, 50.3 and 47.1 % of them were significantly changed in COS, mock + Pst, and COS + Pst treated plants, respectively. Moreover, COS pretreatment could reverse the effect of Pst DC3000 on many N-glycans, suggesting that COS regulates protein N-glycosylation via a non-canonical pattern compared with plant defense, which may contribute to its obvious disease control effect when N-glycosylation impairment occurs.

Adaptive defence-related changes in the metabolome of Sorghum bicolor cells in response to lipopolysaccharides of the pathogen Burkholderia andropogonis

Plant cell suspension culture systems are valuable for the study of complex biological systems such as inducible defence responses and aspects of plant innate immunity. Perturbations to the cellular metabolome can be investigated using metabolomic approaches in order to reveal the underlying metabolic mechanism of cellular responses. Lipopolysaccharides from the sorghum pathogen, Burkholderia andropogonis (LPSB.a.), were purified, chemically characterised and structurally elucidated. The lipid A moiety consists of tetra- and penta-acylated 1,4'-bis-phosphorylated disaccharide backbone decorated by aminoarabinose residues, while the O-polysaccharide chain consists of linear trisaccharide repeating units of [→2)-α-Rha3CMe-(1 → 3)-α-Rha-(1 → 3)-α-Rha-(1 → ]. The effect of LPSB.a. in triggering metabolic reprogramming in Sorghum bicolor cells were investigated using untargeted metabolomics with liquid chromatography coupled to mass spectrometry detection. Cells were treated with LPSB.a. and the metabolic changes monitored over a 30 h time period. Alterations in the levels of phytohormones (jasmonates, zeatins, traumatic-, azelaic- and abscisic acid), which marked the onset of defence responses and accumulation of defence-related metabolites, were observed. Phenylpropanoids and indole alkaloids as well as oxylipins that included di- and trihydroxyoctadecedienoic acids were identified as signatory biomarkers, with marked secretion into the extracellular milieu. The study demonstrated that sorghum cells recognise LPSB.a. as a 'microbe-associated molecular pattern', perturbing normal cellular homeostasis. The molecular features of the altered metabolome were associated with phytohormone-responsive metabolomic reconfiguration of primary and secondary metabolites originating from various metabolic pathways, in support of defence and immunity.

Factors Required for Adhesion of Salmonella enterica Serovar Typhimurium to Corn Salad (Valerianella locusta)

Salmonella enterica is a foodborne pathogen often leading to gastroenteritis and is commonly acquired by consumption of contaminated food of animal origin. However, frequency of outbreaks linked to the consumption of fresh or minimally processed food of nonanimal origin is increasing. New infection routes of S. enterica by vegetables, fruits, nuts, and herbs have to be considered. This leads to special interest in S. enterica interactions with leafy products, e.g., salads, that are mainly consumed in a minimally processed form. The attachment of S. enterica to salad is a crucial step in contamination, but little is known about the bacterial factors required and mechanisms of adhesion. S. enterica possesses a complex set of adhesive structures whose functions are only partly understood. Potentially, S. enterica may deploy multiple adhesive strategies for adhering to various salad species and other vegetables. In this study, we systematically analyzed the contributions of the complete adhesiome, of lipopolysaccharide (LPS), and of flagellum-mediated motility of S. enterica serovar Typhimurium (STM) in adhesion to Valerianella locusta (corn salad). We deployed a reductionist, synthetic approach to identify factors involved in the surface binding of STM to leaves of corn salad, with particular regard to the expression of all known adhesive structures, using the Tet-on system. This work reveals the contribution of Saf fimbriae, type 1 secretion system-secreted BapA, an intact LPS, and flagellum-mediated motility of STM in adhesion to corn salad leaves.IMPORTANCE Transmission of gastrointestinal pathogens by contaminated fresh produce is of increasing relevance to human health. However, the mechanisms of contamination of, persistence on, and transmission by fresh produce are poorly understood. We investigated the contributions of the various adhesive structures of STM to the initial event in transmission, i.e., binding to the plant surface. A reductionist system was used that allowed experimentally controlled surface expression of individual adhesive structures and analyses of the contribution to binding to leave surfaces of corn salad under laboratory conditions. The model system allowed the determination of the relative contributions of fimbrial and nonfimbrial adhesins, the type 3 secretion systems, the O antigen of lipopolysaccharide, the flagella, and chemotaxis of STM to binding to corn salad leaves. Based on these data, future work could reveal the mechanism of binding and the relevance of interaction under agricultural conditions.

Proteomics analysis reveals the defense priming effect of chitosan oligosaccharides in Arabidopsis-Pst DC3000 interaction

Chitosan oligosaccharides (COS) worked effectively in multiple plant-pathogen interactions as plant immunity regulator, however, due to the complexity of the COS-induced immune signaling network, the topic requires further investigation. In the present study, quantitative analysis of proteins was performed to investigate the underlying mechanism of COS induced resistance to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) in Arabidopsis thaliana. 4303 proteins were successfully quantified, 186, 217 and 207 proteins were differently regulated in mock + Pst, COS, and COS + Pst treated plants, respectively, compared with mock plants. From detailed functional and hierarchical clustering analysis, a priming effect of COS on plant immune system by pre-regulated the key proteins related to signaling transduction, defense response, cell wall biosynthesis and modification, plant growth and development, gene transcription and translation, which confers enhanced resistance when Pst DC3000 infection in Arabidopsis. Moreover, RACK1B which has the potential to be the key kinase receptor for COS signals was found out by protein-protein interaction network analysis of COS responsive proteins. In conclusion, COS treatment enable plant to fine-tuning its defense mechanisms for a more rapid and stronger response to future pathogen attacks, which obviously enhances plants defensive capacity that makes COS worked effectively in multiple plant-pathogen interactions.

Loss of wbpL disrupts O-polysaccharide synthesis and impairs virulence of plant-associated Pseudomonas strains

Despite its importance for membrane stability and pathogenicity of mammalian pathogens, functions of the O-polysaccharide (OPS) of lipopolysaccharide (LPS) remain unclear in plant-associated bacteria. Genetic information about OPS biosynthesis in these bacteria is largely missing. Genome analysis of various plant-associated Pseudomonas strains revealed that one of the two known OPS biosynthesis clusters from Pseudomonas aeruginosa PAO1, the common polysaccharide antigen (CPA) gene cluster, is only conserved in some strains of the Pseudomonas fluorescens group. For the O-specific antigen (OSA) biosynthesis cluster, the putative genomic position could be identified, but orthologues of most functional important OSA biosynthesis enzymes could not be detected. Nevertheless, orthologues of the glycosyltransferase WbpL, required for initiation of CPA and OSA synthesis in P. aeruginosa PAO1, could be identified in the analysed Pseudomonas genomes. Knockout mutations of wbpL orthologues in Pseudomonas syringae pv. tomato DC3000 (Pst) and Pseudomonas cichorii ATCC10857/DSM50259 (Pci) resulted in strains lacking the OPS. Infection experiments of Arabidopsis thaliana plants revealed a reduced entry into the leaf apoplast after spray inoculation and a reduced apoplastic amplification of Pst ∆wbpL. Stab and spray inoculation of lettuce (Lactuca sativa) leaves with Pci ∆wbpL causes reduced infection symptoms compared to the wild-type strain. Furthermore, swarming motility was reduced in ∆wbpL mutants of Pst and Pci. This might be a possible reason for reduced bacterial titres after surface inoculation and reduced bacterial amplification in the plant. Our results imply that the presence of lipopolysaccharide OPS is required for efficient host colonization and full virulence of plant-pathogenic Pseudomonas bacteria.

Stress Tolerance and Virulence-Related Roles of Lipopolysaccharide in Burkholderia glumae

The lipopolysaccharide (LPS) composed of lipid A, core, and O-antigen is the fundamental constituent of the outer membrane in gram-negative bacteria. This study was conducted to investigate the roles of LPS in Burkholderia glumae, the phytopathogen causing bacterial panicle blight and seedling rot in rice. To study the roles of the core oligosaccharide (OS) and the O-antigen region, mutant strains targeting the waaC and the wbiFGHI genes were generated. The LPS profile was greatly affected by disruption of the waaC gene and slight reductions were observed in the O-antigen region following wbiFGHI deletions. The results indicated that disruption in the core OS biosynthesis-related gene, waaC, was associated with increased sensitivity to environmental stress conditions including acidic, osmotic, saline, and detergent stress, and to polymyxin B. Moreover, significant impairment in the swimming and swarming motility and attenuation of bacterial virulence to rice were also observed in the waaC-defective mutant. The motility and virulence of O-antigen mutants defective in any gene of the wbiFGHI operon, were not significantly different from the wild-type except in slight decrease in swimming and swarming motility with wbiH deletion. Altogether, the results of present study indicated that the LPS, particularly the core OS region, is required for tolerance to environmental stress and full virulence in B. glumae. To our knowledge, this is the first functional study of LPS in a plant pathogenic Burkholderia sp. and presents a step forward toward full understanding of B. glumae pathogenesis.

Recognition of Elicitors in Grapevine: From MAMP and DAMP Perception to Induced Resistance

In a context of a sustainable viticulture, the implementation of innovative eco-friendly strategies, such as elicitor-triggered immunity, requires a deep knowledge of the molecular mechanisms underlying grapevine defense activation, from pathogen perception to resistance induction. During plant-pathogen interaction, the first step of plant defense activation is ensured by the recognition of microbe-associated molecular patterns, which are elicitors directly derived from pathogenic or beneficial microbes. Vitis vinifera, like other plants, can perceive elicitors of different nature, including proteins, amphiphilic glycolipid, and lipopeptide molecules as well as polysaccharides, thanks to their cognate pattern recognition receptors, the discovery of which recently began in this plant species. Furthermore, damage-associated molecular patterns are another class of elicitors perceived by V. vinifera as an invader's hallmark. They are mainly polysaccharides derived from the plant cell wall and are generally released through the activity of cell wall-degrading enzymes secreted by microbes. Elicitor perception and subsequent activation of grapevine immunity end in some cases in efficient grapevine resistance against pathogens. Using complementary approaches, several molecular markers have been identified as hallmarks of this induced resistance stage. This review thus focuses on the recognition of elicitors by Vitis vinifera describing the molecular mechanisms triggered from the elicitor perception to the activation of immune responses. Finally, we discuss the fact that the link between elicitation and induced resistance is not so obvious and that the formulation of resistance inducers remains a key step before their application in vineyards.