Bacterial lipopolysaccharides induce defense responses associated with programmed cell death in rice cells

PCAP-1a, an exopolysaccharide from Pectobacterium actinidiae, exerts the dual role of immunogenicity and virulence in plants

Plant defense mechanisms begin with the recognition of microbe-associated molecular patterns or pathogen-associated molecular patterns (MAMPs/PAMPs). Several carbohydrates, such as chitin, were reported to induce plant defenses, acting as elicitors. Regrettably, the structures of polysaccharide elicitors have rarely been characterized, and their recognition receptors in plants remain unknown. In the present study, PCAP-1a, an exopolysaccharide (PCAP-1a) purified from Pectobacterium actinidiae, was characterized and found to induce rapid cell death of dicotyledons, acting as a polysaccharide elicitor to induce plant immunity. A series of pattern-triggered immunity (PTI) responses were triggered, including reactive oxygen species production, phosphorylation of mitogen-activated protein kinases and gene transcriptional reprogramming. Moreover, we confirmed that CERK1 is probably one of the immune coreceptors for plants to recognize PCAP-1a. Notably, PCAP-1a also promotes the infection caused by P. actinidiae. In conclusion, our study supports the potential of PCAP-1a as a toxin that plays a dual role of virulence and immune induction in pathogen-plant interactions.

Effects of LPS from Rhodobacter sphaeroides, a Purple Non-Sulfur Bacterium (PNSB), on the Gene Expression of Rice Root

The effects of lipopolysaccharide (LPS) from Rhodobacter sphaeroides, a purple non-sulfur bacterium (PNSB), on the gene expression of the root of rice (Oryza sativa) were investigated by next generation sequencing (NGS) RNA-seq analysis. The rice seeds were germinated on agar plates containing 10 pg/mL of LPS from Rhodobacter sphaeroides NBRC 12203 (type culture). Three days after germination, RNA samples were extracted from the roots and analyzed by RNA-seq. The effects of dead (killed) PNSB cells of R. sphaeroides NBRC 12203^T at the concentration of 10¹ cfu/mL (ca. 50 pg cell dry weight/mL) were also examined. Clean reads of NGS were mapped to rice genome (number of transcript ID: 44785), and differentially expressed genes were analyzed by DEGs. As a result of DEG analysis, 300 and 128 genes, and 86 and 8 genes were significantly up- and down-regulated by LPS and dead cells of PNSB, respectively. The plot of logFC (fold change) values of the up-regulated genes of LPS and PNSB dead cells showed a significant positive relationship (r² = 0.6333, p < 0.0001), indicating that most of the effects of dead cell were attributed to those of LPS. Many genes related to tolerance against biotic (fungal and bacterial pathogens) and abiotic (cold, drought, and high salinity) stresses were up-regulated, and the most strikingly up-regulated genes were those involved in the jasmonate signaling pathway, and the genes of chalcone synthase isozymes, indicating that PNSB induced defense response against biotic and abiotic stresses via the jasmonate signaling pathway, despite the non-pathogenicity of PNSB.

Ligand recognition and signal transduction by lectin receptor-like kinases in plant immunity

Lectin receptor-like kinases (LecRKs) locate on the cell membrane and play diverse roles in perceiving environmental factors in higher plants. Studies have demonstrated that LecRKs are involved in plant development and response to abiotic and biotic stresses. In this review, we summarize the identified ligands of LecRKs in Arabidopsis, including extracellular purine (eATP), extracellular pyridine (eNAD⁺), extracellular NAD⁺ phosphate (eNADP⁺) and extracellular fatty acids (such as 3-hydroxydecanoic acid). We also discussed the posttranslational modification of these receptors in plant innate immunity and the perspectives of future research on plant LecRKs.

[Pathophysiology of sepsis]

Up to now, sepsis is one of the most threatening diseases and its therapy remains challenging. Sepsis is currently defined as a severely dysregulated immune response to an infection resulting in organ dysfunction. The pathophysiology is mainly driven by exogenous PAMPs ("pathogen-associated molecular patterns") and endogenous DAMPs ("damage-associated molecular patterns"), which can activate PRRs ("pattern recognition receptors") on different cell types (mainly immune cells), leading to the initiation of manifold downstream pathways and a perpetuation of patients' immune response. Sepsis is neither an exclusive pro- nor an anti-inflammatory disease: both processes take place in parallel, resulting in an individual immunologic disease state depending on the severity of each component at different time points. Septic shock is a complex disorder of the macro- and microcirculation, provoking a severe lack of oxygenation further aggravating sepsis defining organ dysfunctions. An in-depth knowledge of the heterogeneity and the time-dependency of the septic immunopathology will be essential for the design of future sepsis trials and therapy planning in patients with sepsis. The big aim is to achieve a more individualized treatment strategy in patients suffering from sepsis or septic shock.

Integrating Biochar, Bacteria, and Plants for Sustainable Remediation of Soils Contaminated with Organic Pollutants

The contamination of soil with organic pollutants has been accelerated by agricultural and industrial development and poses a major threat to global ecosystems and human health. Various chemical and physical techniques have been developed to remediate soils contaminated with organic pollutants, but challenges related to cost, efficacy, and toxic byproducts often limit their sustainability. Fortunately, phytoremediation, achieved through the use of plants and associated microbiomes, has shown great promise for tackling environmental pollution; this technology has been tested both in the laboratory and in the field. Plant-microbe interactions further promote the efficacy of phytoremediation, with plant growth-promoting bacteria (PGPB) often used to assist the remediation of organic pollutants. However, the efficiency of microbe-assisted phytoremediation can be impeded by (i) high concentrations of secondary toxins, (ii) the absence of a suitable sink for these toxins, (iii) nutrient limitations, (iv) the lack of continued release of microbial inocula, and (v) the lack of shelter or porous habitats for planktonic organisms. In this regard, biochar affords unparalleled positive attributes that make it a suitable bacterial carrier and soil health enhancer. We propose that several barriers can be overcome by integrating plants, PGPB, and biochar for the remediation of organic pollutants in soil. Here, we explore the mechanisms by which biochar and PGPB can assist plants in the remediation of organic pollutants in soils, and thereby improve soil health. We analyze the cost-effectiveness, feasibility, life cycle, and practicality of this integration for sustainable restoration and management of soil.

Effects of Seed Bio-Priming by Purple Non-Sulfur Bacteria (PNSB) on the Root Development of Rice

The effects of seed bio-priming (seed soaking) with purple non-sulfur bacteria (PNSB) on the grain productivity and root development of rice were examined by a field study and laboratory experiments, respectively. Two PNSB strains, Rhodopseudomonas sp. Tsuru2 and Rhodobacter sp. Tsuru3, isolated from the paddy field of the study site were used for seed bio-priming. For seed bio-priming in the field study, the rice seeds were soaked for 1 day in water containing a 1 × 10⁵ colony forming unit (cfu)/mL of PNSB cells, and the rice grain productivities at the harvest time were 420, 462 and 504 kg/are for the control, strain Tsuru2-primed, and strain Tsuru3-primed seeds, respectively. The effects of seed priming on the root development were examined with cell pot cultivation experiments for 2 weeks. The total root length, root surface area, number of tips and forks were evaluated with WinRhizo, an image analysis system, and strains Tsuru2- and Tsuru3-primed seeds showed better root development than the control seeds. The effects of seed priming with the dead (killed) PNSB cells were also examined, and the seed priming with the dead cells was also effective, indicating that the effects were attributed to some cellular components. We expected the lipopolysaccharide (LPS) of PNSB as the effective component of PNSB and found that seed priming with LPS of Rhodobacter sphaeroides NBRC 12203 (type culture) at the concentrations of 5 ng/mL and 50 ng/mL enhanced the root development.

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.

Untargeted Metabolomics Profiling of Arabidopsis WT, lbr-2-2 and bak1-4 Mutants Following Treatment with Two LPS Chemotypes

Plants perceive pathogenic threats from the environment that have evaded preformed barriers through pattern recognition receptors (PRRs) that recognise microbe-associated molecular patterns (MAMPs). The perception of and triggered defence to lipopolysaccharides (LPSs) as a MAMP is well-studied in mammals, but little is known in plants, including the PRR(s). Understanding LPS-induced secondary metabolites and perturbed metabolic pathways in Arabidopsis will be key to generating disease-resistant plants and improving global plant crop yield. Recently, Arabidopsis LPS-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI)-related proteins (LBP/BPI related-1) and (LBP/BPI related-2) were shown to perceive LPS from Pseudomonas aeruginosa and trigger defence responses. In turn, brassinosteroid insensitive 1 (BRI1)-associated receptor kinase 1 (BAK1) is a well-established co-receptor for several defence-related PRRs in plants. Due to the lack of knowledge pertaining to LPS perception in plants and given the involvement of the afore-mentioned proteins in MAMPs recognition, in this study, Arabidopsis wild type (WT) and mutant (lbr2-2 and bak1-4) plants were pressure-infiltrated with LPSs purified from Pseudomonas syringae pv. tomato DC3000 (Pst) and Xanthomonas campestris pv. campestris 8004 (Xcc). Metabolites were extracted from the leaves at four time points over a 24 h period and analysed by UHPLC-MS, generating distinct metabolite profiles. Data analysed using unsupervised and supervised multivariate data analysis (MVDA) tools generated results that reflected time- and treatment-related variations after both LPS chemotypes treatments. Forty-five significant metabolites were putatively annotated and belong to the following groups: glucosinolates, hydroxycinnamic acid derivatives, flavonoids, lignans, lipids, oxylipins, arabidopsides and phytohormones, while metabolic pathway analysis (MetPA) showed enrichment of flavone and flavanol biosynthesis, phenylpropanoid biosynthesis, alpha-linolenic acid metabolism and glucosinolate biosynthesis. Distinct metabolite accumulations depended on the LPS chemotype and the genetic background of the lbr2-2 and bak1-4 mutants. This study highlights the role of LPSs in the reprogramming Arabidopsis metabolism into a defensive state, and the possible role of LBR and BAK1 proteins in LPSs perception and thus plant defence against pathogenic bacteria.

An Aminobutyric Acid Transaminase in Zea mays Interacts With Rhizoctonia solani Cellulase to Participate in Disease Resistance

Corn sheath blight, caused by AG1-IA, a fusion group of Rhizoctonia solani, which acts as a kind of necrotrophic fungal pathogen, poses a global threat to the production of Zea mays. Although cellulase plays a crucial role in R. solani infections, how plants respond to it is still poorly understood. In this study, we identified a gamma-aminobutyric acid transaminase (GABA-T), ZmGABA-T, in Z. mays that interacts with a cell wall-degrading enzyme (CWDE), EG1, in the cell membrane, using yeast two-hybrid assay, co-immunoprecipitation (Co-IP), and bimolecular fluorescence complementation assays. We found that the combination of EG1 and ZmGABA-T suppressed the allergic necrosis induced by EG1. We also found that the substrate of GABA-T-GABA, can inhibit the transcription of EG1. Transient expression of ZmGABA-T inhibited R. solani infection in Nicotiana benthamiana. The homolog in Oryza sativa, OsGABA-T, could also interact with EG1 to suppress the allergic necrosis induced by EG1. The OsGABA-T knocked out plants displayed enhanced susceptibility to R. solani and showed larger lesions. In conclusion, our results suggest that ZmGABA-T inhibits allergic necrosis induced by EG1 based on the combination with EG1, producing resistance to R. solani infection.

Pathogen-Associated Molecular Pattern Active Sites of GH45 Endoglucanohydrolase from Rhizoctonia solani

A 207-amino-acid residue endoglucanohydrolase (EG1) belonging to the glycoside hydrolase 45 (GH45) from Rhizoctonia solani acts as a pathogen-associated molecular pattern (PAMP). However, the mechanism of EG1 inducing plant immunity is unclear. Here, we found that EG1 contains two domains related to its PAMP function. Transient expression showed that EG1-1, the mutation deleting 60 amino acid residues from the N-terminal, still reserved the PAMP function. Further truncation of EG1-1 obtained two truncating mutations: EG1-2, deleting seven amino acid residues from the N-terminal of EG1-1 (SPWAVND), and EG1-3, deleting five amino acid residues from the C-terminal of EG1-1 (GCSRK). Transient expression showed that the two truncating mutations EG1-2 and EG1-3 all lost the PAMP function. Site-directed mutagenesis of EG1-1 showed that the three amino acid residues (P, W, and D) in the region SPWAVND and the two amino acid residues (C and R) in the region GCSRK were involved in the PAMP function. The homology model showed that the two regions were located at a surface on the EG1 and structurally independent. These results demonstrate that there are two functional regions for the plant immune function of the EG1 released by R. solani, and the two functional regions are independent of each other.

Azospirillum brasilense Sp245 lipopolysaccharides induce target of rapamycin signaling and growth in Arabidopsis thaliana

The Target of Rapamycin (TOR) protein kinase plays a pivotal role in metabolism and gene expression, which enables cell proliferation, growth and development. Lipopolysaccharides (LPS) are a class of complex glycolipids present in the cell surface of Gram-negative bacteria and mediate plant-bacteria interactions. In this study, we examined whether LPS from Azospirillum brasilense Sp245 affect Arabidopsis thaliana growth via a mechanism involving TOR. A. thaliana plants were treated with LPS and plant growth and development were analyzed in mature plants. Morphological and molecular changes as well as TOR expression and activity were analyzed in root tissues. LPS increased total fresh weight, root length and TOR::GUS expression in the root meristem. Phosphorylation of S6k protein, a downstream target of TOR, increased following LPS treatment, which correlated with increased or decreased expression of CycB1;1::GUS protein upon treatment with LPS or TOR inhibitor AZD-8055, respectively. Long term LPS treatment further increased the rosette size as well as the number of stems and siliques per plant, indicating an overall phytostimulant effect for these signaling molecules. Taken together, the results suggest that A. brasilense LPS play probiotic roles in plants influencing TOR-mediated processes.

Arabidopsis cell culture for comparable physiological and genetic studies

Cell cultures established from various plant species have been used for a range of physiological and biochemical studies. Homogeneity of cell types and size of clusters in the cell culture often gave a clearer and simpler results compared to those obtained with the whole plant. On the other hand, possible variability of physiological conditions and responsiveness to external stimuli between the cell lines could be problematic for comparative studies. Aiming at combining the usefulness of plant cell culture with the rich information and genetic resources of Arabidopsis, we systemically examined the methods/conditions to establish cell lines for comparative studies, which could be applicable to a variety of genetic resources. Arabidopsis cell lines thus established from the meristem of mature seeds showed reproducible and comparable MAMP responses such as ROS generation and defense-related gene expression. MAMP responses of the cultured cells showed the specificity depending on the presence/absence of the corresponding MAMP receptor. Pharmacological study with a protein kinase inhibitor, K252a, also showed the usefulness of the cell culture for such studies. These results indicated the usefulness of the method to establish Arabidopsis cell lines, which are useful for comparative studies between genetic resources.

Prospects of Gene Knockouts in the Functional Study of MAMP-Triggered Immunity: A Review

Plants depend on both preformed and inducible defence responses to defend themselves against biotic stresses stemming from pathogen attacks. In this regard, plants perceive pathogenic threats from the environment through pattern recognition receptors (PRRs) that recognise microbe-associated molecular patterns (MAMPs), and so induce plant defence responses against invading pathogens. Close to thirty PRR proteins have been identified in plants, however, the molecular mechanisms underlying MAMP perception by these receptors/receptor complexes are not fully understood. As such, knockout (KO) of genes that code for PRRs and co-receptors/defence-associated proteins is a valuable tool to study plant immunity. The loss of gene activity often causes changes in the phenotype of the model plant, allowing in vivo studies of gene function and associated biological mechanisms. Here, we review the functions of selected PRRs, brassinosteroid insensitive 1 (BRI1) associated receptor kinase 1 (BAK1) and other associated defence proteins that have been identified in plants, and also outline KO lines generated by T-DNA insertional mutagenesis as well as the effect on MAMP perception—and triggered immunity (MTI). In addition, we further review the role of membrane raft domains in flg22-induced MTI in Arabidopsis, due to the vital role in the activation of several proteins that are part of the membrane raft domain theory in this regard.

Elicitor and Receptor Molecules: Orchestrators of Plant Defense and Immunity

Pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), herbivore-associated molecular patterns (HAMPs), and damage-associated molecular patterns (DAMPs) are molecules produced by microorganisms and insects in the event of infection, microbial priming, and insect predation. These molecules are then recognized by receptor molecules on or within the plant, which activates the defense signaling pathways, resulting in plant’s ability to overcome pathogenic invasion, induce systemic resistance, and protect against insect predation and damage. These small molecular motifs are conserved in all organisms. Fungi, bacteria, and insects have their own specific molecular patterns that induce defenses in plants. Most of the molecular patterns are either present as part of the pathogen’s structure or exudates (in bacteria and fungi), or insect saliva and honeydew. Since biotic stresses such as pathogens and insects can impair crop yield and production, understanding the interaction between these organisms and the host via the elicitor–receptor interaction is essential to equip us with the knowledge necessary to design durable resistance in plants. In addition, it is also important to look into the role played by beneficial microbes and synthetic elicitors in activating plants’ defense and protection against disease and predation. This review addresses receptors, elicitors, and the receptor–elicitor interactions where these components in fungi, bacteria, and insects will be elaborated, giving special emphasis to the molecules, responses, and mechanisms at play, variations between organisms where applicable, and applications and prospects.

iTRAQ proteomics reveals the regulatory response to Magnaporthe oryzae in durable resistant vs. susceptible rice genotypes

Rice blast disease caused by Magnaporthe oryzae (M. oryzae) is one of the most serious diseases. Although previous research using two-dimensional gel-based proteomics to assess the proteins related to the rice blast resistance had been done, few proteins were identified. Here, we used the iTRAQ method to detect the differentially expressed proteins (DEPs) in the durable resistant rice variety Gangyuan8 (GY8) and the susceptible rice variety Lijiangxintuanheigu (LTH) in response to M. oryzae invasion, and then transcriptome sequencing was used to assist analysis A total of 193 and 672 DEPs were specifically identified in GY8 and LTH, respectively, with only 46 similarly expressed DEPs being shared by GY8 and LTH.39 DEPs involved in plant-pathogen interaction, plant hormone signal transduction, fatty acid metabolism and peroxisome biosynthesis were significantly different between compatible interaction (LTH) and incompatible interaction (GY8). Some proteins participated in peroxide signal transduction and biosynthesis was down-regulated in GY8 but up-regulated in LTH. A lot of genes encoding pathogenesis-related gene (PR), such as chitinase and glucanase, were significantly up-regulated at both the transcriptome and proteome levels at 24 hours post-inoculation in GY8, but up-regulated at the transcriptome level and down-regulated at the proteome level in LTH. Our study reveals that the pathogen-associated molecular pattern (PAMP)-triggered immunity defense system may be activated at the transcriptome level but was inhibited at the protein level in susceptible rice varieties after inoculation. The results may facilitate future studies of the molecular mechanisms of rice blast resistance.

Handmade leaf cutter for efficient and reliable ROS assay

Reactive oxygen species generation is one of the most popular index of plant immune responses. Leaf disk assay has been commonly used for MAMP/elicitor-induced ROS analysis by many groups. However, the reproducibility of the leaf disk assay relies on the skills of the people engaged in the experiments and the experiment itself seems not suitable for some plant species, which had a tough leaf structure and lower penetration efficiency of MAMPs/elicitors. In this study, we prepared a handmade leaf cutter to cut out the leaf fragments with uniform size and slits. The use of such fragments obtained by the new leaf cutter as well as the increase of the number of leaf fragments for each experiment improved the reliability and reproducibility of the leaf disk assay. This cutter was also successfully applied to rice leaf disk assay, indicating the applicability to other plant spices.

A T-DNA mutant screen that combines high-throughput phenotyping with the efficient identification of mutated genes by targeted genome sequencing

BACKGROUND

Nitrogen dioxide (NO₂) triggers hypersensitive response (HR)-like cell death in Arabidopsis thaliana. A high-throughput mutant screen was established to identify genes involved in this type of programmed cell death.

RESULTS

Altogether 14,282 lines of SALK T-DNA insertion mutants were screened. Growing 1000 pooled mutant lines per tray and simultaneous NO₂ fumigation of 4 trays in parallel facilitated high-throughput screening. Candidate mutants were selected based on visible symptoms. Sensitive mutants showed lesions already after fumigation for 1 h with 10 ppm (ppm) NO₂ whereas tolerant mutants were hardly damaged even after treatment with 30 ppm NO₂. Identification of T-DNA insertion sites by adapter ligation-mediated PCR turned out to be successful but rather time consuming. Therefore, next generation sequencing after T-DNA-specific target enrichment was tested as an alternative screening method. The targeted genome sequencing was highly efficient due to (1.) combination of the pooled DNA from 124 candidate mutants in only two libraries, (2.) successful target enrichment using T-DNA border-specific 70mer probes, and (3.) stringent filtering of the sequencing reads. Seventy mutated genes were identified by at least 3 sequencing reads. Ten corresponding mutants were re-screened of which 8 mutants exhibited NO₂-sensitivity or -tolerance confirming that the screen yielded reliable results. Identified candidate genes had published functions in HR, pathogen resistance, and stomata regulation.

CONCLUSIONS

The presented NO₂ dead-or-alive screen combined with next-generation sequencing after T-DNA-specific target enrichment was highly efficient. Two researchers finished the screen within 3 months. Moreover, the target enrichment approach was cost-saving because of the limited number of DNA libraries and sequencing runs required. The experimental design can be easily adapted to other screening approaches e.g. involving high-throughput treatments with abiotic stressors or phytohormones.

A LysM Receptor Heteromer Mediates Perception of Arbuscular Mycorrhizal Symbiotic Signal in Rice

Symbiotic microorganisms improve nutrient uptake by plants. To initiate mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, plants perceive Myc factors, including lipochitooligosaccharides (LCOs) and short-chain chitooligosaccharides (CO4/CO5), secreted by AM fungi. However, the molecular mechanism of Myc factor perception remains elusive. In this study, we identified a heteromer of LysM receptor-like kinases consisting of OsMYR1/OsLYK2 and OsCERK1 that mediates the perception of AM fungi in rice. CO4 directly binds to OsMYR1, promoting the dimerization and phosphorylation of this receptor complex. Compared with control plants, Osmyr1 and Oscerk1 mutant rice plants are less sensitive to Myc factors and show decreased AM colonization. We engineered transgenic rice by expressing chimeric receptors that respectively replaced the ectodomains of OsMYR1 and OsCERK1 with those from the homologous Nod factor receptors MtNFP and MtLYK3 of Medicago truncatula. Transgenic plants displayed increased calcium oscillations in response to Nod factors compared with control rice. Our study provides significant mechanistic insights into AM symbiotic signal perception in rice. Expression of chimeric Nod/Myc receptors achieves a potentially important step toward generating cereals that host nitrogen-fixing 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.

Lipopolysaccharide-induced priming enhances NO-mediated activation of defense responses in pearl millet challenged with Sclerospora graminicola

Lipopolysaccharide (LPS) elicitors isolated from Pseudomonas fluorescens UOM SAR 14 effectively induced systemic and durable resistance against pearl millet downy mildew disease caused by the oomycete Sclerospora graminicola. Rapid and increased callose deposition and H2O2 accumulation were evidenced in downy mildew susceptible seeds pre-treated with LPS (SLPS) in comparison with the control seedlings, which also correlated with expression of various other defense responses. Biochemical analysis of enzymes and quantitative real-time polymerase chain reaction data suggested that LPS protects pearl millet against downy mildew through the activation of plant defense mechanisms such as generation of nitric oxide (NO), increased expression, and activities of defense enzymes and proteins. Elevation of NO concentrations was shown to be essential for LPS-mediated defense manifestation in pearl millet and had an impact on the other downstream defense responses like enhanced activation of enzymes and pathogen-related (PR) proteins. Temporal expression analysis of defense enzymes and PR-proteins in SLPS seedlings challenged with the downy mildew pathogen revealed that the activity and expression of peroxidase, phenylalanine ammonia lyase, and the PR-proteins (PR-1 and PR-5) were significantly enhanced compared to untreated control. Higher gene expression and protein activities of hydroxyproline-rich glycoproteins (HRGPs) were observed in SLPS seedlings which were similar to that of the resistant check. Collectively, our results suggest that, in pearl millet-downy mildew interaction, LPS pre-treatment affects defense signaling through the central regulator NO which triggers the activities of PAL, POX, PR-1, PR-5, and HRGPs.

The multifaceted functions of lipopolysaccharide in plant-bacteria interactions

In Gram-negative bacteria, the cell envelope largely consists of lipopolysaccharide (LPS), a class of heterogeneous glycolipids. As a fundamental component of the outer membrane, LPS provides stability to the bacterial cell and forms a protective cover shielding it from hostile environments. LPS is not only fundamental to bacterial viability, but also makes a substantial contribution both directly and indirectly to multiple aspects of inter-organismic interactions. During infection of animal and plant hosts, LPS promotes bacterial virulence but simultaneously betrays bacteria to the host immune system. Moreover, dynamic remodulation of LPS structures allows bacteria to fine-tune OM properties and quickly adapt to diverse and often hostile environments, such as those encountered in host tissues. Here, we summarize recent insights into the multiple functions of LPS in plant-bacteria interactions and discuss what we can learn from the latest advances in the field of animal immunity. We further pinpoint open questions and future challenges to unravel the different roles of LPS in the dynamic interplay between bacteria and plant hosts at the mechanistic level.

Disease Resistance Mechanisms in Plants

Plants have developed a complex defense system against diverse pests and pathogens. Once pathogens overcome mechanical barriers to infection, plant receptors initiate signaling pathways driving the expression of defense response genes. Plant immune systems rely on their ability to recognize enemy molecules, carry out signal transduction, and respond defensively through pathways involving many genes and their products. Pathogens actively attempt to evade and interfere with response pathways, selecting for a decentralized, multicomponent immune system. Recent advances in molecular techniques have greatly expanded our understanding of plant immunity, largely driven by potential application to agricultural systems. Here, we review the major plant immune system components, state of the art knowledge, and future direction of research on plant⁻pathogen interactions. In our review, we will discuss how the decentralization of plant immune systems have provided both increased evolutionary opportunity for pathogen resistance, as well as additional mechanisms for pathogen inhibition of such defense responses. We conclude that the rapid advances in bioinformatics and molecular biology are driving an explosion of information that will advance agricultural production and illustrate how complex molecular interactions evolve.

Genome-wide identification and expression analysis of chitinase gene family in Brassica rapa reveals its role in clubroot resistance

Chitinases, a category of pathogenesis-related proteins, are responsible for catalyzing the hydrolysis of chitin into the N-acetyl-d-glucosamine. Therefore, chitinases are believed to function as a guardian against chitin-containing pathogens. Here, we examined the role of the Brassica rapa chitinase family genes in clubroot disease. A total of 33 chitinase genes were identified and grouped into five classes based on their conserved domain. They were distributed unevenly across eight chromosomes in B. rapa, and 31 of them contained few introns (≤2). In addition, the expression of these genes was organ-specific, and 14 genes were expressed differentially in response to Plasmodiophora brassicae challenge of clubroot-susceptible (CS NIL) and resistant (CR NIL) lines. Furthermore, reduced pathogen DNA content and clubroot symptoms were observed in the CS NILs after their treatment with chitin oligosaccharides 24 h prior to inoculation with P. brassicae. The findings indicate that chitinases play a crucial role in pathogen resistance of the host plants. The results offer an insight into the role of chitinase in B. rapa-P. brassicae interaction.

An Asparagine-Rich Protein Nbnrp1 Modulate Verticillium dahliae Protein PevD1-Induced Cell Death and Disease Resistance in Nicotiana benthamiana

PevD1 is a fungal protein secreted by Verticillium dahliae. Our previous researches showed that this protein could induce hypersensitive responses-like necrosis and systemic acquired resistance (SAR) in cotton and tobacco. To understand immune activation mechanisms whereby PevD1 elicits defense response, the yeast two-hybrid (Y2H) assay was performed to explore interacting protein of PevD1 in Arabidopsis thaliana, and a partner AtNRP (At5g42050) was identified. Here, AtNRP homolog in Nicotiana benthamiana was identified and designated as Nbnrp1. The Nbnrp1 could interact with PevD1 via Y2H and bimolecular fluorescence complementation (BiFC) analyses. Moreover, truncated protein binding assays demonstrated that the C-terminal 132 amino acid (development and cell death, DCD domain) of Nbnrp1 is required for PevD1-Nbnrp1 interaction. To further investigate the roles of Nbnrp1 in PevD1-induced defense response, Nbnrp1-overexpressing and Nbnrp1-silence transgenic plants were generated. The overexpression of Nbnrp1 conferred enhancement of PevD1-induced necrosis activity and disease resistance against tobacco mosaic virus (TMV), bacterial pathogen Pseudomonas syringae pv. tabaci and fungal pathogen V. dahliae. By contrast, Nbnrp1-silence lines displayed attenuated defense response compared with the wild-type. It is the first report that an asparagine-rich protein Nbnrp1 positively regulated V. dahliae secretory protein PevD1-induced cell death response and disease resistance in N. benthamiana.

Early plant growth and biochemical responses induced by Azospirillum brasilense Sp245 lipopolysaccharides in wheat (Triticum aestivum L.) seedlings are attenuated by procyanidin B2

This study analyzes the effects of procyanidin B2 on early wheat plant growth and plant biochemical responses promoted by lipopolysaccharides (LPS) derived from the rhizobacteria Azospirillum brasilense Sp245. Measurements of leaf, root length, fresh weight, and dry weight showed in vitro plant growth stimulation 4 days after treatment with A. brasilense as well as LPS. Superoxide anion (O₂^·-) and hydrogen peroxide (H₂O₂) levels increased in seedling roots treated with LPS (100 μg mL^-1). The chlorophyll content in leaf decreased while the starch content increased 24 h after treatment in seedling roots. The LPS treatment induced a high increase in total peroxidase (POX) (EC 1.11.1.7) activity and ionically bound cell wall POX content in roots, when compared to respective controls. Early plant growth and biochemical responses observed in wheat seedlings treated with LPS were inhibited by the addition of procyanidin B2 (5 μg mL^-1), a B type proanthocyanidin (PAC), plant-derived polyphenolic compound with binding properties of LPS. All results suggest first that the ionically bound cell wall POX enzymes could be a molecular target of A. brasilense LPS, and second that the recognition or association of LPS by plant cells is required to activate plant responses. This last event could play a critical role during plant growth regulation by A. brasilense LPS.

OsCERK1 plays a crucial role in the lipopolysaccharide-induced immune response of rice

Plant cell surface receptor-like kinases (RLKs) mediate the signals from microbe-associated molecular patterns (MAMPs) that induce immune responses. Lipopolysaccharide (LPS), the major constituent of the outer membrane of gram-negative bacteria, is a common MAMP perceived by animals and plants; however, the plant receptors/co-receptors are unknown except for LORE, a bulb-type lectin S-domain RLK (B-lectin SD1-RLK) in Arabidopsis. OsCERK1 is a multifunctional RLK in rice that contains lysin motifs (LysMs) and is essential for the perception of chitin, a fungal MAMP, and peptidoglycan, a bacterial MAMP. Here, we analyzed the relevance of OsCERK1 to LPS perception in rice. Using OsCERK1-knockout mutants (oscerk1), we evaluated hydrogen peroxide (H₂ O₂ ) production and gene expression after LPS treatment. We also examined the LPS response in knockout mutants for the B-lectin SD1-RLK genes in rice and for all LysM-protein genes in Arabidopsis. Compared with wild-type rice cells, LPS responses in oscerk1 cells were mostly diminished. By contrast, rice lines mutated in either of three B-lectin SD1-RLK genes and Arabidopsis lines mutated in the LysM-protein genes responded normally to LPS. From these results, we conclude that OsCERK1 is an LPS receptor/co-receptor and that the LPS perception systems of rice and Arabidopsis are significantly different.

Lipopolysaccharide O-antigen delays plant innate immune recognition of Xylella fastidiosa

Lipopolysaccharides (LPS) are among the known pathogen-associated molecular patterns (PAMPs). LPSs are potent elicitors of PAMP-triggered immunity (PTI), and bacteria have evolved intricate mechanisms to dampen PTI. Here we demonstrate that Xylella fastidiosa (Xf), a hemibiotrophic plant pathogenic bacterium, possesses a long chain O-antigen that enables it to delay initial plant recognition, thereby allowing it to effectively skirt initial elicitation of innate immunity and establish itself in the host. Lack of the O-antigen modifies plant perception of Xf and enables elicitation of hallmarks of PTI, such as ROS production specifically in the plant xylem tissue compartment, a tissue not traditionally considered a spatial location of PTI. To explore translational applications of our findings, we demonstrate that pre-treatment of plants with Xf LPS primes grapevine defenses to confer tolerance to Xf challenge.

Transcriptome analysis reveals key roles of AtLBR-2 in LPS-induced defense responses in plants

BACKGROUND

Lipopolysaccharide (LPS) from Gram-negative bacteria cause innate immune responses in animals and plants. The molecules involved in LPS signaling in animals are well studied, whereas those in plants are not yet as well documented. Recently, we identified Arabidopsis AtLBR-2, which binds to LPS from Pseudomonas aeruginosa (pLPS) directly and regulates pLPS-induced defense responses, such as pathogenesis-related 1 (PR1) expression and reactive oxygen species (ROS) production. In this study, we investigated the pLPS-induced transcriptomic changes in wild-type (WT) and the atlbr-2 mutant Arabidopsis plants using RNA-Seq technology.

RESULTS

RNA-Seq data analysis revealed that pLPS treatment significantly altered the expression of 2139 genes, with 605 up-regulated and 1534 down-regulated genes in WT. Gene ontology (GO) analysis on these genes showed that GO terms, "response to bacterium", "response to salicylic acid (SA) stimulus", and "response to abscisic acid (ABA) stimulus" were enriched amongst only in up-regulated genes, as compared to the genes that were down-regulated. Comparative analysis of differentially expressed genes between WT and the atlbr-2 mutant revealed that 65 genes were up-regulated in WT but not in the atlbr-2 after pLPS treatment. Furthermore, GO analysis on these 65 genes demonstrated their importance for the enrichment of several defense-related GO terms, including "response to bacterium", "response to SA stimulus", and "response to ABA stimulus". We also found reduced levels of pLPS-induced conjugated SA glucoside (SAG) accumulation in atlbr-2 mutants, and no differences were observed in the gene expression levels in SA-treated WT and the atlbr-2 mutants.

CONCLUSION

These 65 AtLBR-2-dependent up-regulated genes appear to be important for the enrichment of some defense-related GO terms. Moreover, AtLBR-2 might be a key molecule that is indispensable for the up-regulation of defense-related genes and for SA signaling pathway, which is involved in defense against pathogens containing LPS.

Proteomics-based identification of differentially abundant proteins reveals adaptation mechanisms of Xanthomonas citri subsp. citri during Citrus sinensis infection

BACKGROUND

Xanthomonas citri subsp. citri (Xac) is the causal agent of citrus canker. A proteomic analysis under in planta infectious and non-infectious conditions was conducted in order to increase our knowledge about the adaptive process of Xac during infection.

RESULTS

For that, a 2D-based proteomic analysis of Xac at 1, 3 and 5 days after inoculation, in comparison to Xac growth in NB media was carried out and followed by MALDI-TOF-TOF identification of 124 unique differentially abundant proteins. Among them, 79 correspond to up-regulated proteins in at least one of the three stages of infection. Our results indicate an important role of proteins related to biofilm synthesis, lipopolysaccharides biosynthesis, and iron uptake and metabolism as possible modulators of plant innate immunity, and revealed an intricate network of proteins involved in reactive oxygen species adaptation during Plants` Oxidative Burst response. We also identified proteins previously unknown to be involved in Xac-Citrus interaction, including the hypothetical protein XAC3981. A mutant strain for this gene has proved to be non-pathogenic in respect to classical symptoms of citrus canker induced in compatible plants.

CONCLUSIONS

This is the first time that a protein repertoire is shown to be active and working in an integrated manner during the infection process in a compatible host, pointing to an elaborate mechanism for adaptation of Xac once inside the plant.

Functional and Evolutionary Characterization of a UDP-Xylose Synthase Gene from the Plant Pathogen Xylella fastidiosa, Involved in the Synthesis of Bacterial Lipopolysaccharide

Xylella fastidiosa is a plant-infecting bacillus, responsible for many important crop diseases, such as Pierce's disease of vineyards, citrus variegated chlorosis, and coffee leaf scorch (CLS), among others. Recent genomic comparisons involving two CLS-related strains, belonging to X. fastidiosa subsp. pauca, revealed that one of them carries a frameshift mutation that inactivates a gene encoding an oxidoreductase of the short-chain dehydrogenase/reductase (SDR) superfamily, which may play important roles in determining structural variations in bacterial glycans and glycoconjugates. However, the exact nature of this SDR has been a matter of controversy, as different annotations of X. fastidiosa genomes have implicated it in distinct reactions. To confirm the nature of this mutated SDR, a comparative analysis was initially performed, suggesting that it belongs to a subgroup of SDR decarboxylases, representing a UDP-xylose synthase (Uxs). Functional assays, using a recombinant derivative of this enzyme, confirmed its nature as XfUxs, and carbohydrate composition analyses, performed with lipopolysaccharide (LPS) molecules obtained from different strains, indicate that inactivation of the X. fastidiosa uxs gene affects the LPS structure among CLS-related X. fastidiosa strains. Finally, a comparative sequence analysis suggests that this mutation is likely to result in a morphological and evolutionary hallmark that differentiates two subgroups of CLS-related strains, which may influence interactions between these bacteria and their plant and/or insect hosts.

Rice Leaf Transcriptional Profiling Suggests a Functional Interplay Between Xanthomonas oryzae pv. oryzae Lipopolysaccharide and Extracellular Polysaccharide in Modulation of Defense Responses During Infection

Treatment of rice leaves with isolated Xanthomonas oryzae pv. oryzae lipopolysaccharide (LPS) induces the production of callose deposits, reactive oxygen species, and enhanced resistance against subsequent bacterial infection. Expression profiling of X. oryzae pv. oryzae LPS-treated rice (Oryza sativa subsp. indica) leaves showed that genes involved in the biosynthetic pathways for lignins, phenylpropanoids, chorismate, phenylalanine, salicylic acid, and ethylene, as well as a number of pathogenesis-related proteins are up-regulated. Gene ontology categories like cell-wall organization, defense response, stress response, and protein phosphorylation/kinases were found to be upregulated, while genes involved in photosynthesis were down-regulated. Coinfiltration with xanthan gum, the xanthomonas extracellular polysaccharide (EPS), suppressed LPS-induced callose deposition. Gene expression analysis of rice leaves that are treated with an EPS-deficient mutant of X. oryzae pv. oryzae indicated that a number of defense-regulated functions are up-regulated during infection. These transcriptional responses are attenuated in rice leaves treated with an EPS-deficient mutant that is also deficient in the O-antigen component of LPS. Overall, these results suggest that the O-antigen component of X. oryzae pv. oryzae LPS induces rice defense responses during infection and that these are suppressed by bacterial EPS.

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.

Modulation of plant defense responses to herbivores by simultaneous recognition of different herbivore-associated elicitors in rice

Induced plant defense responses against insect herbivores are triggered by wounding and/or perception of herbivore elicitors from their oral secretions (OS) and/or saliva. In this study, we analyzed OS isolated from two rice chewing herbivores, Mythimna loreyi and Parnara guttata. Both types of crude OS had substantial elicitor activity in rice cell system that allowed rapid detection of early and late defense responses, i.e. accumulation of reactive oxygen species (ROS) and defense secondary metabolites, respectively. While the OS from M. loreyi contained large amounts of previously reported insect elicitors, fatty acid-amino acid conjugates (FACs), the elicitor-active P. guttata's OS contained no detectable FACs. Subsequently, elicitor activity associated with the high molecular mass fraction in OS of both herbivores was identified, and shown to promote ROS and metabolite accumulations in rice cells. Notably, the application of N-linolenoyl-Gln (FAC) alone had only negligible elicitor activity in rice cells; however, the activity of isolated elicitor fraction was substantially promoted by this FAC. Our results reveal that plants integrate various independent signals associated with their insect attackers to modulate their defense responses and reach maximal fitness in nature.

Ectopic expression of a grape aspartic protease gene, AP13, in Arabidopsis thaliana improves resistance to powdery mildew but increases susceptibility to Botrytis cinerea

The grape aspartic protease gene, AP13 was previously reported to be responsive, in Chinese wild Vitis quinquangularis cv. 'Shang-24', to infection by Erysiphe necator, the causal agent of powdery mildew disease, as well as to treatment with salicylic acid in V. labrusca×V. vinifera cv. 'Kyoho'. In the current study, we evaluated the expression levels of AP13 in 'Shang-24' in response to salicylic acid (SA), methyl jasmonate (MeJA) and ethylene (ET) treatments, as well as to infection by the necrotrophic fungus, Botrytis cinerea, and the transcript levels of VqAP13 decreased after B. cinerea infection and MeJA treatment, but increased following ET and SA treatments. Transgenic Arabidopsis thaliana lines over-expressing VqAP13 under the control of a constitutive promoter showed enhanced resistance to powdery mildew and to the bacterium Pseudomonas syringae pv. tomato DC3000, and accumulated more callose than wild type plants, while the resistance of transgenic A. thaliana lines to B. cinerea inoculation was reduced. In addition, the expression profiles of various disease resistance- related genes in the transgenic A. thaliana lines following infection by different pathogens were compared to the equivalent profiles in the wild type plants. The results suggest that VqAP13 action promotes the SA dependent signal transduction pathway, but suppresses the JA signal transduction pathway.

Arabidopsis LBP/BPI related-1 and -2 bind to LPS directly and regulate PR1 expression

Lipopolysaccharide (LPS) is a major constituent of the outer membrane of Gram-negative bacteria and acts as a pathogen-associated molecular pattern that triggers immune responses in both plants and animals. LPS-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI), which bind to LPS and play important roles in immunity of mammals, have been well studied. However, the molecule contributing to LPS binding in plants is mostly unknown. The Arabidopsis genome carries two genes encoding LBP/BPI-related proteins which we designated as AtLBP/BPI related-1 (AtLBR-1) and AtLBP/BPI related-2 (AtLBR-2). We found that their N-terminal domains were co-purified with cell wall-derived LPS when expressed in E. coli. Since this finding implied the direct binding of AtLBRs to LPS, we also confirmed binding by using LPS-free AtLBRs and purified LPS. AtLBRs directly bind to both rough and smooth types of LPS. We also demonstrated that LPS-treated atlbr mutant Arabidopsis exhibit a significant delay of induction of defence-related gene pathogenesis-related 1 (PR1) but no other PR genes. Furthermore, LPS-treated atlbr mutants showed defects in reactive oxygen species (ROS) generation. These results demonstrate that, as well as LBP and BPI of mammals, AtLBRs also play an important role in the LPS-induced immune response of plants.

Plant innate immunity in rice: a defense against pathogen infection

A large number of pathogenic microorganisms cause rice diseases that lead to enormous yield losses worldwide. Such losses are important because rice is a staple food for more than half of the world's population. Over the past two decades, the extensive study of the molecular interactions between rice and the fungal pathogen Magnaporthe oryzae and between rice and the bacterial pathogen Xanthomonas oryzae pv. oryzae has made rice a model for investigating plant–microbe interactions of monocotyledons. Impressive progress has been recently achieved in understanding the molecular basis of rice pathogen-associated molecular pattern-immunity and effector-triggered immunity. Here, we briefly summarize these recent advances, emphasizing the diverse functions of the structurally conserved fungal effectors, the regulatory mechanisms of the immune receptor complexes, and the novel strategies for breeding disease resistance. We also discuss future research challenges.

Lipopolysaccharides attenuates growth of HS cells through the JNK pathway

Rheumatoid arthritis is characterized by inflammation of the synovial membrane, which can result in joint destruction within the first few years after onset. Lipopolysaccharides (LPS) are glycolipids found in abundance on the outer membranes of all Gram-negative bacteria and incite a vigorous inflammatory response. We studied the potential involvement of mitogen-activated protein kinase serine/threonine kinases in LPS-induced growth of HS synovial cells. Various concentrations of LPS were applied to cultured HS cells and growth rate, as well as changes in the phosphorylation states of extracellular regulated kinase 1/2, Jun N-terminal kinase (JNK), and p38 were determined. As results, growth of LPS-treated HS cells was inhibited primarily by phosphorylated JNK and this phosphorylation was mediated by the LPS receptor. Our results suggest that LPS inhibits growth of HS cells primarily through the JNK pathway.

Surface polysaccharides and quorum sensing are involved in the attachment and survival of Xanthomonas albilineans on sugarcane leaves

Xanthomonas albilineans, the causal agent of sugarcane leaf scald, is a bacterial plant pathogen that is mainly spread by infected cuttings and contaminated harvesting tools. However, some strains of this pathogen are known to be spread by aerial means and are able to colonize the phyllosphere of sugarcane before entering the host plant and causing disease. The objective of this study was to identify the molecular factors involved in the survival or growth of X. albilineans on sugarcane leaves. We developed a bioassay to test for the attachment of X. albilineans on sugarcane leaves using tissue-cultured plantlets grown in vitro. Six mutants of strain XaFL07-1 affected in surface polysaccharide production completely lost their capacity to survive on the sugarcane leaf surface. These mutants produced more biofilm in vitro and accumulated more cellular poly-β-hydroxybutyrate than the wild-type strain. A mutant affected in the production of small molecules (including potential biosurfactants) synthesized by non-ribosomal peptide synthetases (NRPSs) attached to the sugarcane leaves as well as the wild-type strain. Surprisingly, the attachment of bacteria on sugarcane leaves varied among mutants of the rpf gene cluster involved in bacterial quorum sensing. Therefore, quorum sensing may affect polysaccharide production, or both polysaccharides and quorum sensing may be involved in the survival or growth of X. albilineans on sugarcane leaves.

Full Genome Sequence Analysis of Two Isolates Reveals a Novel Xanthomonas Species Close to the Sugarcane Pathogen Xanthomonas albilineans

Xanthomonas albilineans is the bacterium responsible for leaf scald, a lethal disease of sugarcane. Within the Xanthomonas genus, X. albilineans exhibits distinctive genomic characteristics including the presence of significant genome erosion, a non-ribosomal peptide synthesis (NRPS) locus involved in albicidin biosynthesis, and a type 3 secretion system (T3SS) of the Salmonella pathogenicity island-1 (SPI-1) family. We sequenced two X. albilineans-like strains isolated from unusual environments, i.e., from dew droplets on sugarcane leaves and from the wild grass Paspalum dilatatum, and compared these genomes sequences with those of two strains of X. albilineans and three of Xanthomonas sacchari. Average nucleotide identity (ANI) and multi-locus sequence analysis (MLSA) showed that both X. albilineans-like strains belong to a new species close to X. albilineans that we have named "Xanthomonas pseudalbilineans". X. albilineans and "X. pseudalbilineans" share many genomic features including (i) the lack of genes encoding a hypersensitive response and pathogenicity type 3 secretion system (Hrp-T3SS), and (ii) genome erosion that probably occurred in a common progenitor of both species. Our comparative analyses also revealed specific genomic features that may help X. albilineans interact with sugarcane, e.g., a PglA endoglucanase, three TonB-dependent transporters and a glycogen metabolism gene cluster. Other specific genomic features found in the "X. pseudalbilineans" genome may contribute to its fitness and specific ecological niche.

Lipopolysaccharide perception leads to dynamic alterations in the microtranscriptome of Arabidopsis thaliana cells and leaf tissues

BACKGROUND

MicroRNAs (miRNAs) are non-coding RNA molecules which have recently emerged as important gene regulators in plants and their gene expression analysis is becoming increasingly important. miRNAs regulate gene expression at the post-transcriptional level by translational repression or target degradation of specific mRNAs and gene silencing. In order to profile the microtranscriptome of Arabidopsis thaliana leaf and callus tissues in response to bacterial lipopolysaccharide (LPS), small RNA libraries were constructed at 0 and 3 h post induction with LPS and sequenced by Illumina sequencing technology.

RESULTS

Differential regulation of subset of miRNAs in response to LPS treament was observed. Small RNA reads were mapped to the miRNA database and 358 miRNAs belonging to 49 miRNA families in the callus tissues and 272 miRNAs belonging to 40 miRNA families in the leaf tissues were identified. Moreover, target genes for all the identified miRNAs families in the leaf tissues and 44 of the 49 miRNAs families in the callus tissues were predicted. The sequencing analysis showed that in both callus and leaf tissues, various stress regulated-miRNAs were differentially expressed and real time PCR validated the expression profile of miR156, miR158, miR159, miR169, miR393, miR398, miR399 and miR408 along with their target genes.

CONCLUSION

A. thaliana callus and leaf callus tissues respond to LPS as a microbe-associated molecular pattern molecule through dynamic changes to the microtranscriptome associated with differential transcriptional regulation in support of immunity and basal resistance.

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.

Plant cell death caused by fungal, bacterial, and viral elicitors: protective effect of mitochondria-targeted quinones

Chitosan (partially deacetylated chitin), a component of fungal cell walls, caused epidermal cell (EC) death in the leaves of pea (Pisum sativum L.) and tobacco Nicotiana tabacum or Nicotiana benthamiana detected by destruction of cell nuclei. The mitochondria-targeted quinone SkQ1 prevented the destruction of EC nuclei induced by chitosan. Chitosan increased and SkQ1 suppressed the activity of protein kinases in N. benthamiana and P. sativum and eliminated the effect of chitosan. Chitosan induced the generation of reactive oxygen species (ROS) in the guard cells (GC) of pea plants. Treatment with chitosan or H2O2 did not cause destruction of GC nuclei; however, it resulted in disruption of the permeability barrier of the plasma membrane detected by propidium iodide fluorescence. Treatment with bacterial lipopolysaccharide but not peptidoglycan caused destruction of pea EC nuclei, which was prevented by SkQ1. Leaves of tobacco plants containing the N gene responsible for resistance to tobacco mosaic virus (TMV) were infiltrated with Agrobacterium tumefaciens cells. These cells contained a genetic construct with the gene of the helicase domain of TMV replicase (p50); its protein product p50 is a target for the N-gene product. As a result, the hypersensitive response (HR) was initiated. The HR manifested itself in the death of leaves and was suppressed by SkQ3. Treatment of tobacco epidermal peels with the A. tumefaciens cells for the p50 gene expression stimulated the destruction of EC nuclei, which was inhibited by SkQ1 or SkQ3. The p50-lacking A. tumefaciens cells did not induce the destruction of EC nuclei. The protective effect of mitochondria-targeted antioxidants SkQ1 and SkQ3 demonstrates the involvement of mitochondria and their ROS in programmed cell death caused by pathogen elicitors.

Xanthomonas campestris lipooligosaccharides trigger innate immunity and oxidative burst in Arabidopsis

Plants lack the adaptive immunity mechanisms of jawed vertebrates, so they rely on innate immune responses to defense themselves from pathogens. The plant immune system perceives the presence of pathogens by recognition of molecules known as pathogen-associated molecular patterns (PAMPs). PAMPs have several common characteristics, including highly conserved structures, essential for the microorganism but absent in host organisms. Plants can specifically recognize PAMPs using a large set of receptors and can respond with appropriate defenses by activating a multicomponent and multilayered response. Lipopolysaccharides (LPSs) and lipooligosaccharides (LOSs) are major components of the cell surface of Gram-negative bacteria with diverse roles in bacterial pathogenesis of animals and plants that include elicitation of host defenses. Little is known on the mechanisms of perception of these molecules by plants and the associated signal transduction pathways that trigger plant immunity.Here we addressed the question whether the defense signaling pathway in Arabidopsis thaliana was triggered by LOS from Xanthomonas campestris pv. campestris (Xcc), using proteomic and transcriptomic approaches. By using affinity capture strategies with immobilized LOS and LC-MS/MS analyses, we identified 8 putative LOS protein ligands. Further investigation of these interactors led to the definition that LOS challenge is able to activate a signal transduction pathway that uses nodal regulators in common with salicylic acid-mediated pathway. Moreover, we proved evidence that Xcc LOS are responsible for oxidative burst in Arabidopsis either in infiltrated or systemic leaves. In addition, gene expression studies highlighted the presence of gene network involved in reactive oxygen species transduction pathway.

Peptidoglycan from fermentation by-product triggers defense responses in grapevine

Plants are constantly under attack from a variety of microorganisms, and rely on a series of complex detection and response systems to protect themselves from infection. Here, we found that a by-product of glutamate fermentation triggered defense responses in grapevine, increasing the expression of defense response genes in cultured cells, foliar chitinase activity, and resistance to infection by downy mildew in leaf explants. To identify the molecule that triggered this innate immunity, we fractionated and purified candidates extracted from Corynebacterium glutamicum, a bacterium used in the production of amino acids by fermentation. Using hydrolysis by lysozyme, a silkworm larva plasma detection system, and gel filtration analysis, we identified peptidoglycan as inducing the defense responses. Peptidoglycans of Escherichia coli, Bacillus subtilis, and Staphylococcus aureus also generated similar defensive responses.

Molecular effects of resistance elicitors from biological origin and their potential for crop protection

Plants contain a sophisticated innate immune network to prevent pathogenic microbes from gaining access to nutrients and from colonizing internal structures. The first layer of inducible response is governed by the plant following the perception of microbe- or modified plant-derived molecules. As the perception of these molecules results in a plant response that can provide efficient resistance toward non-adapted pathogens they can also be described as "defense elicitors." In compatible plant/microbe interactions, adapted microorganisms have means to avoid or disable this resistance response and promote virulence. However, this requires a detailed spatial and temporal response from the invading pathogens. In agricultural practice, treating plants with isolated defense elicitors in the absence of pathogens can promote plant resistance by uncoupling defense activation from the effects of pathogen virulence determinants. The plant responses to plant, bacterial, oomycete, or fungal-derived elicitors are not, in all cases, universal and need elucidating prior to the application in agriculture. This review provides an overview of currently known elicitors of biological rather than synthetic origin and places their activity into a molecular context.

Biosynthesis, elicitation and roles of monocot terpenoid phytoalexins

A long-standing goal in plant research is to optimize the protective function of biochemical agents that impede pest and pathogen attack. Nearly 40 years ago, pathogen-inducible diterpenoid production was described in rice, and these compounds were shown to function as antimicrobial phytoalexins. Using rice and maize as examples, we discuss recent advances in the discovery, biosynthesis, elicitation and functional characterization of monocot terpenoid phytoalexins. The recent expansion of known terpenoid phytoalexins now includes not only the labdane-related diterpenoid superfamily but also casbane-type diterpenoids and β-macrocarpene-derived sequiterpenoids. Biochemical approaches have been used to pair pathway precursors and end products with cognate biosynthetic genes. The number of predicted terpenoid phytoalexins is expanding through advances in cereal genome annotation and terpene synthase characterization that likewise enable discoveries outside the Poaceae. At the cellular level, conclusive evidence now exists for multiple plant receptors of fungal-derived chitin elicitors, phosphorylation of membrane-associated signaling complexes, activation of mitogen-activated protein kinase, involvement of phytohormone signals, and the existence of transcription factors that mediate the expression of phytoalexin biosynthetic genes and subsequent accumulation of pathway end products. Elicited production of terpenoid phytoalexins exhibit additional biological functions, including root exudate-mediated allelopathy and insect antifeedant activity. Such findings have encouraged consideration of additional interactions that blur traditionally discrete phytoalexin classifications. The establishment of mutant collections and increasing ease of genetic transformation assists critical examination of further biological roles. Future research directions include examination of terpenoid phytoalexin precursors and end products as potential signals mediating plant physiological processes.