The Induction and Modulation of Plant Defense Responses by Bacterial Lipopolysaccharides

Plant Biostimulant as an Environmentally Friendly Alternative to Modern Agriculture

Ensuring the safety of crop production presents a significant challenge to humanity. Pesticides and fertilizers are commonly used to eliminate external interference and provide nutrients, enabling crops to sustain growth and defense. However, the addition of chemical substances does not meet the environmental standards required for agricultural production. Recently, natural sources such as biostimulants have been found to help plants with growth and defense. The development of biostimulants provides new solutions for agricultural product safety and has become a widely utilized tool in agricultural. The review summarizes the classification of biostimulants, including humic-based biostimulant, protein-based biostimulant, oligosaccharide-based biostimulant, metabolites-based biostimulants, inorganic substance, and microbial inoculant. This review attempts to summarize suitable alternative technology that can address the problems and analyze the current state of biostimulants, summarizes the research mechanisms, and anticipates future technological developments and market trends, which provides comprehensive information for researchers to develop biostimulants.

Lipopolysaccharides of Herbaspirillum species and their relevance for bacterium-host interactions

The lipopolysaccharides of Herbaspirillum lusitanum P6-12T (HlP6-12T) and H. frisingense GSF30T (HfGSF30T) was isolated by phenol-water extraction from bacterial cells and was characterized using chemical analysis and SDS-PAGE. It was shown that these bacteria produce LPSs that differ in their physicochemical properties and macromolecular organization. In this paper, the lipid A structure of the HlP6-12T LPS, was characterized through chemical analyses and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. To prove the effect of the size of micelles on their bioavailability, we examined the activity of both LPSs toward the morphology of wheat seedlings. Analysis of the HlP6-12T and HfGSF30T genomes showed no significant differences between the operons that encode proteins involved in the biosynthesis of the lipids A and core oligosaccharides. The difference may be due to the composition of the O-antigen operon. HfGSF30T has two copies of the rfb operon, with the main one divided into two fragments. In contrast, the HlP6-12T genome contains only a single rfb-containing operon, and the other O-antigen operons are not comparable at all. The integrity of O-antigen-related genes may also affect LPS variability of. Specifically, we have observed a hairpin structure in the middle of the O-antigen glycosyltransferase gene, which led to the division of the gene into two fragments, resulting in incorrect protein synthesis and potential abnormalities in O-antigen production.

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.

The Rice Endophyte-Derived α-Mannosidase ShAM1 Degrades Host Cell Walls To Activate DAMP-Triggered Immunity against Disease

Endophytes play an important role in shaping plant growth and immunity. However, the mechanisms for endophyte-induced disease resistance in host plants remain unclear. Here, we screened and isolated the immunity inducer ShAM1 from the endophyte Streptomyces hygroscopicus OsiSh-2, which strongly antagonizes the pathogen Magnaporthe oryzae. Recombinant ShAM1 can trigger rice immune responses and induce hypersensitive responses in various plant species. After infection with M. oryzae, blast resistance was dramatically improved in ShAM1-inoculated rice. In addition, the enhanced disease resistance by ShAM1 was found to occur through a priming strategy and was mainly regulated through the jasmonic acid-ethylene (JA/ET)-dependent signaling pathway. ShAM1 was identified as a novel α-mannosidase, and its induction of immunity is dependent on its enzyme activity. When we incubated ShAM1 with isolated rice cell walls, the release of oligosaccharides was observed. Notably, extracts from the ShAM1-digested cell wall can enhance the disease resistance of the host rice. These results indicated that ShAM1 triggered immune defense against pathogens by damage-associated molecular pattern (DAMP)-related mechanisms. Our work provides a representative example of endophyte-mediated modulation of disease resistance in host plants. The effects of ShAM1 indicate the promise of using active components from endophytes as plant defense elicitors for the management of plant disease. IMPORTANCE The specific biological niche inside host plants allows endophytes to regulate plant disease resistance effectively. However, there have been few reports on the role of active metabolites from endophytes in inducing host disease resistance. In this study, we demonstrated that an identified α-mannosidase protein, ShAM1, secreted by the endophyte S. hygroscopicus OsiSh-2 could activate typical plant immunity responses and induce a timely and cost-efficient priming defense against the pathogen M. oryzae in rice. Importantly, we revealed that ShAM1 enhanced plant disease resistance through its hydrolytic enzyme (HE) activity to digest the rice cell wall and release damage-associated molecular patterns. Taken together, these findings provide an example of the interaction mode of endophyte-plant symbionts and suggest that HEs derived from endophytes can be used as environmentally friendly and safe prevention agent for plant disease control.

Tn-seq identifies Ralstonia solanacearum genes required for tolerance of plant immunity induced by exogenous salicylic acid

Ralstonia solanacearum, the causal agent of the devastating bacterial wilt disease, is of particular interest to the scientific community. The repertoire of type III effectors plays an important role in the evasion of plant immunity, but tolerance to plant immunity is also crucial for the survival and virulence of R. solanacearum. Nevertheless, a systematic study of R. solanacearum tolerance to plant immunity is lacking. In this study, we used exogenous salicylic acid (SA) to improve the immunity of tomato plants, followed by transposon insertion sequencing (Tn-seq) analysis and the identification of R. solanacearum genes associated with tolerance to plant immunity. Target gene deletion revealed that the lipopolysaccharide (LPS) production genes RS_RS02830, RS_RS03460, and RS_RS03465 are essential for R. solanacearum tolerance to plant immunity, and their expression is induced by plant immunity, thereby expanding our knowledge of the pathogenic function of R. solanacearum LPS. SA treatment increased the relative abundance of transposon insertion mutants of four genes, including two genes with unknown function, RS_RS11975 and RS_RS07760. Further verification revealed that deletion of RS_RS11975 or RS_RS07760 resulted in reduced in vivo competitive indexes but increased tolerance to plant immunity induced by SA treatment, suggesting that these two genes contribute to the trade-off between tolerance to plant immunity and fitness cost. In conclusion, this work identified and validated R. solanacearum genes required for tolerance to plant immunity and provided essential information for a more complete view of the interaction between R. solanacearum and the host plant.

Bacterial outer membrane vesicles induce a transcriptional shift in arabidopsis towards immune system activation leading to suppression of pathogen growth in planta

Gram-negative bacteria form spherical blebs on their cell periphery, which later dissociate from the bacterial cell wall to form extracellular vesicles. These nano scale structures, known as outer membrane vesicles (OMVs), have been shown to promote infection and disease and can induce typical immune outputs in both mammal and plant hosts. To better understand the broad transcriptional change plants undergo following exposure to OMVs, we treated Arabidopsis thaliana (Arabidopsis) seedlings with OMVs purified from the Gram-negative plant pathogenic bacterium Xanthomonas campestris pv. campestris and performed RNA-seq analysis on OMV- and mock-treated plants at 2, 6 and 24 h post challenge. The most pronounced transcriptional shift occurred at the first two time points tested, as reflected by the number of differentially expressed genes and the average fold change. OMVs induce a major transcriptional shift towards immune system activation, upregulating a multitude of immune-related pathways including a variety of immune receptors. Comparing the response of Arabidopsis to OMVs and to purified elicitors, revealed that OMVs induce a similar suite of genes and pathways as single elicitors, however, pathways activated by OMVs and not by other elicitors were detected. Pretreating Arabidopsis plants with OMVs and subsequently infecting with a bacterial pathogen led to a significant reduction in pathogen growth. Mutations in the plant elongation factor receptor (EFR), flagellin receptor (FLS2), or the brassinosteroid-insensitive 1-associated kinase (BAK1) co-receptor, did not significantly affect the immune priming effect of OMVs. All together these results show that OMVs induce a broad transcriptional shift in Arabidopsis leading to upregulation of multiple immune pathways, and that this transcriptional change may facilitate resistance to bacterial infection.

The right microbe-associated molecular patterns for effective recognition by plants

Plants are constantly exposed to diverse microbes and thus develop a sophisticated perceive system to distinguish non-self from self and identify non-self as friends or foes. Plants can detect microbes in apoplast via recognition of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) on the cell surface to activate appropriate signaling in response to microbes. MAMPs are highly conserved but essential molecules of microbes and often buried in microbes’ complex structure. Mature MAMPs are released from microbes by invasion-induced hydrolytic enzymes in apoplast and accumulate in proximity of plasma membrane-localized PRRs to be perceived as ligands to activate downstream signaling. In response, microbes developed strategies to counteract these processing. Here, we review how the form, the concentration, and the size of mature MAMPs affect the PRR-mediated immune signaling. In particular, we describe some potential applications and explore potential open questions in the fields.

Functional Characterization of the M36 Metalloprotease FgFly1 in Fusarium graminearum

Fungalysin metallopeptidase (M36), a hydrolase, catalyzes the hydrolysis of alanine, glycine, etc. Normally, it is considered to play an important role in the progress of fungal infection. However, the function of fungalysin metallopeptidase (M36) in Fusarium graminearum has not been reported. In this study, we explored the biological functions of FgFly1, a fungalysin metallopeptidase (M36) of F. graminearum. We found that ΔFgFly1 did not affect the ability to produce DON toxin, although it inhibited spore germination during asexual reproduction and reduction in pathogenicity compared with PH-1. Therefore, we speculated that FgFly1 affects the pathogenicity of F.graminearum by affecting pathways related to wheat disease resistance. Target protein TaCAMTA (calmodulin-binding transcription activator) was selected by a yeast two-hybrid (Y2H) system. Then, the interaction between FgFly1 and TaCAMTA was verified by bimolecular fluorescent complimentary (BiFC) and luciferase complementation assay (LCA). Furthermore, compared with wild-type Arabidopsis thaliana, the morbidity level of ΔAtCAMTA was increased after infection with F.graminearum, and the expression level of NPR1 was significantly reduced. Based on the above results, we concluded that FgFly1 regulated F. graminearum pathogenicity by interacting with host cell CAMTA protein.

Fungal oxysterol-binding protein-related proteins promote pathogen virulence and activate plant immunity

Oxysterol-binding protein-related proteins (ORPs) are a conserved class of lipid transfer proteins that are closely involved in multiple cellular processes in eukaryotes, but their roles in plant-pathogen interactions are mostly unknown. We show that transient expression of ORPs of Magnaporthe oryzae (MoORPs) in Nicotiana benthamina plants triggered oxidative bursts and cell death; treatment of tobacco Bright Yellow-2 suspension cells with recombinant MoORPs elicited the production of reactive oxygen species. Despite ORPs being normally described as intracellular proteins, we detected MoORPs in fungal culture filtrates and intercellular fluids from barley plants infected with the fungus. More importantly, infiltration of Arabidopsis plants with recombinant Arabidopsis or fungal ORPs activated oxidative bursts, callose deposition, and PR1 gene expression, and enhanced plant disease resistance, implying that ORPs may function as endogenous and exogenous danger signals triggering plant innate immunity. Extracellular application of fungal ORPs exerted an opposite impact on salicylic acid and jasmonic acid/ethylene signaling pathways. Brassinosteroid Insensitive 1-associated Kinase 1 was dispensable for the ORP-activated defense. Besides, simultaneous knockout of MoORP1 and MoORP3 abolished fungal colony radial growth and conidiation, whereas double knockout of MoORP1 and MoORP2 compromised fungal virulence on barley and rice plants. These observations collectively highlight the multifaceted role of MoORPs in the modulation of plant innate immunity and promotion of fungal development and virulence in M. oryzae.

'Candidatus Liberibacter asiaticus'-Encoded BCP Peroxiredoxin Suppresses Lipopolysaccharide-Mediated Defense Signaling and Nitrosative Stress In Planta

The lipopolysaccharides (LPS) of gram-negative bacteria trigger a nitrosative and oxidative burst in both animals and plants during pathogen invasion. Liberibacter crescens strain BT-1 is a surrogate for functional genomic studies of the uncultured pathogenic 'Candidatus Liberibacter' spp. that are associated with severe diseases such as citrus greening and potato zebra chip. Structural determination of L. crescens LPS revealed the presence of a very long chain fatty acid modification. L. crescens LPS pretreatment suppressed growth of Xanthomonas perforans on nonhost tobacco (Nicotiana benthamiana) and X. citri subsp. citri on host orange (Citrus sinensis), confirming bioactivity of L. crescens LPS in activation of systemic acquired resistance (SAR). L. crescens LPS elicited a rapid burst of nitric oxide (NO) in suspension cultured tobacco cells. Pharmacological inhibitor assays confirmed that arginine-utilizing NO synthase (NOS) activity was the primary source of NO generation elicited by L. crescens LPS. LPS treatment also resulted in biological markers of NO-mediated SAR activation, including an increase in the glutathione pool, callose deposition, and activation of the salicylic acid and azelaic acid (AzA) signaling networks. Transient expression of 'Ca. L. asiaticus' bacterioferritin comigratory protein (BCP) peroxiredoxin in tobacco compromised AzA signaling, a prerequisite for LPS-triggered SAR. Western blot analyses revealed that 'Ca. L. asiaticus' BCP peroxiredoxin prevented peroxynitrite-mediated tyrosine nitration in tobacco. 'Ca. L. asiaticus' BCP peroxiredoxin (i) attenuates NO-mediated SAR signaling and (ii) scavenges peroxynitrite radicals, which would facilitate repetitive cycles of 'Ca. L. asiaticus' acquisition and transmission by fecund psyllids throughout the limited flush period in citrus.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Search for host defense markers uncovers an apple agglutination factor corresponding with fire blight resistance

Pathenogenesis-related (PR) proteins are extensively used as molecular markers to dissect the signaling cascades leading to plant defense responses. However, studies focusing on the biochemical or biological properties of these proteins remain rare. Here, we identify and characterize a class of apple (Malus domestica) PR proteins, named M. domestica AGGLUTININS (MdAGGs), belonging to the amaranthin-like lectin family. By combining molecular and biochemical approaches, we show that abundant production of MdAGGs in leaf tissues corresponds with enhanced resistance to the bacterium Erwinia amylovora, the causal agent of the disease fire blight. We also show that E. amylovora represses the expression of MdAGG genes by injecting the type 3 effector DspA/E into host cells and by secreting bacterial exopolysaccharides. Using a purified recombinant MdAGG, we show that the protein agglutinates E. amylovora cells in vitro and binds bacterial lipopolysaccharides at low pH, conditions reminiscent of the intercellular pH occurring in planta upon E. amylovora infection. We finally provide evidence that negatively charged polysaccharides, such as the free exopolysaccharide amylovoran progressively released by the bacteria, act as decoys relying on charge-charge interaction with the MdAGG to inhibit agglutination. Overall, our results suggest that the production of this particular class of PR proteins may contribute to apple innate immunity mechanisms active against E. amylovora.

Plant immunity inducers: from discovery to agricultural application

While conventional chemical fungicides directly eliminate pathogens, plant immunity inducers activate or prime plant immunity. In recent years, considerable progress has been made in understanding the mechanisms of immune regulation in plants. The development and application of plant immunity inducers based on the principles of plant immunity represent a new field in plant protection research. In this review, we describe the mechanisms of plant immunity inducers in terms of plant immune system activation, summarize the various classes of reported plant immunity inducers (proteins, oligosaccharides, chemicals, and lipids), and review methods for the identification or synthesis of plant immunity inducers. The current situation, new strategies, and future prospects in the development and application of plant immunity inducers are also discussed.

Tyrosine Kinase Self-Phosphorylation Controls Exopolysaccharide Biosynthesis in Gluconacetobacter diazotrophicus Strain Pal5

The biosynthesis of exopolysaccharides (EPSs) is essential for endophytic bacterial colonisation in plants bacause this exopolymer both protects bacterial cells against the defence and oxidative systems of plants and acts on the plant colonisation mechanism in Gluconacetobacter diazotrophicus. The pathway involved in the biosynthesis of bacterial EPS has not been fully elucidated, and several areas related to its molecular regulation mechanisms are still lacking. G. diazotrophicus relies heavily on EPS for survival indirectly by protecting plants from pathogen attack as well as for endophytic maintenance and adhesion in plant tissues. Here, we report that EPS from G. diazotrophicus strain Pal5 is a signal polymer that controls its own biosynthesis. EPS production depends on a bacterial tyrosine (BY) kinase (Wzc) that consists of a component that is able to phosphorylate a glycosyltranferase or to self-phosphorylate. EPS interacts with the extracellular domain of Wzc, which regulates kinase activity. In G. diazotrophicus strains that are deficient in EPS production, the Wzc is rendered inoperative by self-phosphorylation. The presence of EPS promotes the phosphorylation of a glycosyltransferase in the pathway, thus producing EPS. Wzc-mediated self-regulation is an attribute for the control of exopolysaccharide biosynthesis in G. diazotrophicus.

Bacteriophage-Mediated Control of Phytopathogenic Xanthomonads: A Promising Green Solution for the Future

Xanthomonads, members of the family Xanthomonadaceae, are economically important plant pathogenic bacteria responsible for infections of over 400 plant species. Bacteriophage-based biopesticides can provide an environmentally friendly, effective solution to control these bacteria. Bacteriophage-based biocontrol has important advantages over chemical pesticides, and treatment with these biopesticides is a minor intervention into the microflora. However, bacteriophages’ agricultural application has limitations rooted in these viruses’ biological properties as active substances. These disadvantageous features, together with the complicated registration process of bacteriophage-based biopesticides, means that there are few products available on the market. This review summarizes our knowledge of the Xanthomonas-host plant and bacteriophage-host bacterium interaction’s possible influence on bacteriophage-based biocontrol strategies and provides examples of greenhouse and field trials and products readily available in the EU and the USA. It also details the most important advantages and limitations of the agricultural application of bacteriophages. This paper also investigates the legal background and industrial property right issues of bacteriophage-based biopesticides. When appropriately applied, bacteriophages can provide a promising tool against xanthomonads, a possibility that is untapped. Information presented in this review aims to explore the potential of bacteriophage-based biopesticides in the control of xanthomonads in the future.

The N-terminus of an Ustilaginoidea virens Ser-Thr-rich glycosylphosphatidylinositol-anchored protein elicits plant immunity as a MAMP

Many pathogens infect hosts through specific organs, such as Ustilaginoidea virens, which infects rice panicles. Here, we show that a microbe-associated molecular pattern (MAMP), Ser-Thr-rich Glycosyl-phosphatidyl-inositol-anchored protein (SGP1) from U. virens, induces immune responses in rice leaves but not panicles. SGP1 is widely distributed among fungi and acts as a proteinaceous, thermostable elicitor of BAK1-dependent defense responses in N. benthamiana. Plants specifically recognize a 22 amino acid peptide (SGP1 N terminus peptide 22, SNP22) in its N-terminus that induces cell death, oxidative burst, and defense-related gene expression. Exposure to SNP22 enhances rice immunity signaling and resistance to infection by multiple fungal and bacterial pathogens. Interestingly, while SGP1 can activate immune responses in leaves, SGP1 is required for U. virens infection of rice panicles in vivo, showing it contributes to the virulence of a panicle adapted pathogen.

Ustilaginoidea virens is a fungal pathogen that infects rice via the panicles. Here, the authors show that U. virens SGP1, a conserved Ser-Thr-rich glycosyl-phosphatidyl-inositol-anchored protein, elicits immune responses in rice leaves while contributing to virulence in panicles.

Genomic diversity and organization of complex polysaccharide biosynthesis clusters in the genus Dickeya

The pectinolytic genus Dickeya (formerly Erwinia chrysanthemi) comprises numerous pathogenic species which cause diseases in various crops and ornamental plants across the globe. Their pathogenicity is governed by complex multi-factorial processes of adaptive virulence gene regulation. Extracellular polysaccharides and lipopolysaccharides present on bacterial envelope surface play a significant role in the virulence of phytopathogenic bacteria. However, very little is known about the genomic location, diversity, and organization of the polysaccharide and lipopolysaccharide biosynthetic gene clusters in Dickeya. In the present study, we report the diversity and structural organization of the group 4 capsule (G4C)/O-antigen capsule, putative O-antigen lipopolysaccharide, enterobacterial common antigen, and core lipopolysaccharide biosynthesis clusters from 54 Dickeya strains. The presence of these clusters suggests that Dickeya has both capsule and lipopolysaccharide carrying O-antigen to their external surface. These gene clusters are key regulatory components in the composition and structure of the outer surface of Dickeya. The O-antigen capsule/group 4 capsule (G4C) coding region shows a variation in gene content and organization. Based on nucleotide sequence homology in these Dickeya strains, two distinct groups, G4C group I and G4C group II, exist. However, comparatively less variation is observed in the putative O-antigen lipopolysaccharide cluster in Dickeya spp. except for in Dickeya zeae. Also, enterobacterial common antigen and core lipopolysaccharide biosynthesis clusters are present mostly as conserved genomic regions. The variation in the O-antigen capsule and putative O-antigen lipopolysaccharide coding region in relation to their phylogeny suggests a role of multiple horizontal gene transfer (HGT) events. These multiple HGT processes might have been manifested into the current heterogeneity of O-antigen capsules and O-antigen lipopolysaccharides in Dickeya strains during its evolution.

Molecular mechanism of nanochitin whisker elicits plant resistance against Phytophthora and the receptors in plants

Rod-like nanochitin (NC) whisker with cationic nature has a strong synergistic effect with fungicides on inhibition of tobacco root rot disease. This study we explored the activity of NC against Phytophthora and the mechanism for eliciting plant defense response and the receptors in planta. P. capsici isolates, model Nicotiana benthamiana plants and Arabidopsis thaliana were treated with 0.005% of NC suspension and 1 μM of flg22. Infection control efficacy against P. capsici isolates, biosynthetic enzyme activities and the PR genes expression were determined at different hours post treatment in plant. The infection control efficacy, ROS generation, and PTI maker gene expression were re-analyzed in A. thaliana Col-0, bak1 and cerk1 mutants. The results showed that NC did not exhibit inhibitory effect on vegetative growth of P. capsici, but enhanced the resistance against P. capsici by systemically enhanced phenylalanine ammonia-lyase activity and PR gene expression. P. capsici resistance, PTI maker gene promotion, and ROS production in A. thaliana induced by NC depended not only on chitin receptor CERK1, but also BAK1. NC and flg22 induced oomycete immunity through a mechanism of a cross-microbe protection via the BAK1-CERK1 pathway in plant, pointing to the complexity of the plant immunity system.

Genome-Wide Identification of M35 Family Metalloproteases in Rhizoctonia cerealis and Functional Analysis of RcMEP2 as a Virulence Factor during the Fungal Infection to Wheat

Rhizoctonia cerealis is the causal pathogen of the devastating disease, sharp eyespot, of the important crop wheat (Triticum aestivum L.). In phytopathogenic fungi, several M36 metalloproteases have been implicated in virulence, but pathogenesis roles of M35 family metalloproteases are largely unknown. Here, we identified four M35 family metalloproteases from R. cerealis genome, designated RcMEP2–RcMEP5, measured their transcriptional profiles, and investigated RcMEP2 function. RcMEP2-RcMEP5 are predicted as secreted metalloproteases since each protein sequence contains a signal peptide and an M35 domain that includes two characteristic motifs HEXXE and GTXDXXYG. Transcription levels of RcMEP2-RcMEP5 markedly elevated during the fungus infection to wheat, among which RcMEP2 expressed with the highest level. Functional dissection indicated that RcMEP2 and its M35 domain could trigger H₂O₂ rapidly-excessive accumulation, induce cell death, and inhibit expression of host chitinases. This consequently enhanced the susceptibility of wheat to R. cerealis and the predicated signal peptide of RcMEP2 functions required for secretion and cell death-induction. These results demonstrate that RcMEP2 is a virulence factor and that its M35 domain and signal peptide are necessary for the virulence role of RcMEP2. This study facilitates a better understanding of the pathogenesis mechanism of metalloproteases in phytopathogens including R. cerealis.

Tyrosine phosphorylation of the lectin receptor-like kinase LORE regulates plant immunity

Plant pattern recognition receptors (PRRs) perceive pathogen-associated molecular patterns (PAMPs) to activate immune responses. Medium-chain 3-hydroxy fatty acids (mc-3-OH-FAs), which are widely present in Gram-negative bacteria, were recently shown to be novel PAMPs in Arabidopsis thaliana. The Arabidopsis PRR LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (LORE) is a G-type lectin receptor-like kinase that recognizes mc-3-OH-FAs and subsequently mounts an immune response; however, the mechanisms underlying LORE activation and downstream signaling are unexplored. Here, we report that one of the mc-3-OH-FAs, 3-OH-C10:0, induces phosphorylation of LORE at tyrosine residue 600 (Y600). Phosphorylated LORE subsequently trans-phosphorylates the receptor-like cytoplasmic kinase PBL34 and its close paralogs, PBL35 and PBL36, and therefore activates plant immunity. Phosphorylation of LORE Y600 is required for downstream phosphorylation of PBL34, PBL35, and PBL36. However, the Pseudomonas syringae effector HopAO1 targets LORE, dephosphorylating the tyrosine-phosphorylated Y600 and therefore suppressing the immune response. These observations uncover the mechanism by which LORE mediates signaling in response to 3-OH-C10:0 in Arabidopsis.

Apoplastic invasion patterns triggering plant immunity: plasma membrane sensing at the frontline

Plants are able to effectively cope with invading pathogens by activating an immune response based on the detection of invasion patterns (IPs) originating from the pathogen or released by the plant after infection. At a first level, this perception takes place at the plasma membrane through cell surface immune receptors and although the involvement of proteinaceous pattern recognition receptors (PRRs) is well established, increasing data are also pointing out the role of membrane lipids in the sensing of IPs. In this review, we discuss the evolution of various conceptual models describing plant immunity and present an overview of well-characterized IPs from different natures and origins. We summarize the current knowledge on how they are perceived by plants at the plasma membrane, highlighting the increasingly apparent diversity of sentinel-related systems in plants.

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.

Differential Functions of Two Metalloproteases, Mrmep1 and Mrmep2, in Growth, Sporulation, Cell Wall Integrity, and Virulence in the Filamentous Fungus Metarhizium robertsii

The Metarhizium genus of filamentous entomopathogenic fungi plays a pivotal role in regulating insect populations. Metalloproteases (MEPs) are a widely distributed and diverse family of hydrolytic enzymes that are important toxicity factors in the interactions between fungi and their hosts. Herein, we characterized two MEPs, Mrmep1 and Mrmep2, in Metarhizium robertsii using gene deletion. Growth rates of the resulting ΔMrmep1 and ΔMrmep2 mutants decreased by 16.2 and 16.5%, respectively, relative to the wild-type (WT) strain. Both mutants were less sensitive to cell wall-perturbing agents, sodium dodecyl sulfate and Congo red than the WT strain, whereas did not show any obvious changes in fungal sensitivity to ultraviolet B irradiation or heat stress. The conidial yield of ΔMrmep1, ΔMrmep2, and ΔMrmep1ΔMrmep2 mutants decreased by 56.0, 23, and 53%, respectively. Insect bioassay revealed that median lethal time values against Galleria mellonella increased by 25.5% (ΔMrmep1), 19% (ΔMrmep2), and 28.8% (ΔMrmep1ΔMrmep2) compared with the WT strain at a concentration of 1 × 10⁷ conidia mL^-1, suggesting attenuated fungal virulence in the ΔMrmep1, ΔMrmep2, and ΔMrmep1ΔMrmep2 strains. During fungal infection, transcription levels of Mrmep1 was 1.6-fold higher than Mrmep2 at 36 h post inoculation. Additionally, transcription levels of gallerimycin gene were 1.2-fold, 2.18-fold, and 2.5-fold higher in insects infected with the ΔMrmep1, ΔMrmep2, or ΔMrmep1ΔMrmep2 mutant than those infected with the WT strain, respectively. Our findings suggest that Mrmep1 and Mrmep2 are differentially contributed to the growth, sporulation, cell wall integrity, and virulence of M. robertsii.

Bacillus cereus AR156 Activates Defense Responses to Pseudomonas syringae pv. tomato in Arabidopsis thaliana Similarly to flg22

Bacillus cereus AR156 (AR156) is a plant growth-promoting rhizobacterium capable of inducing systemic resistance to Pseudomonas syringae pv. tomato in Arabidopsis thaliana. Here, we show that, when applied to Arabidopsis leaves, AR156 acted similarly to flg22, a typical pathogen-associated molecular pattern (PAMP), in initiating PAMP-triggered immunity (PTI). AR156-elicited PTI responses included phosphorylation of MPK3 and MPK6, induction of the expression of defense-related genes PR1, FRK1, WRKY22, and WRKY29, production of reactive oxygen species, and callose deposition. Pretreatment with AR156 still significantly reduced P. syringae pv. tomato multiplication and disease severity in NahG transgenic plants and mutants sid2-2, jar1, etr1, ein2, npr1, and fls2. This suggests that AR156-induced PTI responses require neither salicylic acid, jasmonic acid, and ethylene signaling nor flagella receptor kinase FLS2, the receptor of flg22. On the other hand, AR156 and flg22 acted in concert to differentially regulate a number of AGO1-bound microRNAs that function to mediate PTI. A full-genome transcriptional profiling analysis indicated that AR156 and flg22 activated similar transcriptional programs, coregulating the expression of 117 genes; their concerted regulation of 16 genes was confirmed by real-time quantitative polymerase chain reaction analysis. These results suggest that AR156 activates basal defense responses to P. syringae pv. tomato in Arabidopsis, similarly to flg22.

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.

Xanthomonas campestris pv. musacearum: a major constraint to banana, plantain and enset production in central and east Africa over the past decade

TAXONOMY

Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; currently classified as X. campestris pv. musacearum (Xcm). However, fatty acid methyl ester analysis and genetic and genomic evidence suggest that this pathogen is X. vasicola and resides in a separate pathovar.

ISOLATION AND DETECTION

Xcm can be isolated on yeast extract peptone glucose agar (YPGA), cellobiose cephalexin agar and yeast extract tryptone sucrose agar (YTSA) complemented with 5-fluorouracil, cephalexin and cycloheximide to confer semi-selectivity. Xcm can also be identified using direct antigen coating enzyme-linked immunosorbent assay (DAC-ELISA), species-specific polymerase chain reaction (PCR) using GspDm primers and lateral flow devices that detect latent infections.

HOST RANGE

Causes Xanthomonas wilt on plants belonging to the Musaceae, primarily banana (Musa acuminata), plantain (M. acuminata × balbisiana) and enset (Ensete ventricosum).

DIVERSITY

There is a high level of genetic homogeneity within Xcm, although genome sequencing has revealed two major sublineages.

SYMPTOMS

Yellowing and wilting of leaves, premature fruit ripening and dry rot, bacterial exudate from cut stems.

DISTRIBUTION

Xcm has only been found in African countries, namely Burundi, Ethiopia, Democratic Republic of the Congo, Kenya, Rwanda, Tanzania and Uganda.

ECOLOGY AND EPIDEMIOLOGY

Xcm is transmitted by insects, bats, birds and farming implements. Long-distance dispersal of the pathogen is by the transportation of latently infected plants into new areas.

MANAGEMENT

The management of Xcm has relied on cultural practices that keep the pathogen population at tolerable levels. Biotechnology programmes have been successful in producing resistant banana plants. However, the deployment of such genetic material has not as yet been achieved in farmers' fields, and the sustainability of transgenic resistance remains to be addressed.

Studies on lipid A isolated from Phyllobacterium trifolii PETP02T lipopolysaccharide

The structure of lipid A from lipopolysaccharide of Phyllobacterium trifolii PETP02T, a nitrogen-fixing symbiotic bacterium, was studied. It was found that the lipid A backbone was composed of two 2,3-diamino-2,3-dideoxy-D-glucose (GlcpN3N) residues connected by a β-(1 → 6) glycosidic linkage, substituted by galacturonic acid (GalpA) at position C-1 and partly decorated by a phosphate residue at C-4' of the non-reducing GlcpN3N. Both diaminosugars were symmetrically substituted by 3-hydroxy fatty acids (14:0(3-OH) and 16:0(3-OH)). Ester-linked secondary acyl residues [i.e. 19:0cyc and 28:0(27-OH) or 28:0(27-4:0(3-OMe))] were located in the distal part of lipid A. A high similarity between the lipid A of P. trifolii and Mesorhizobium was observed and discussed from the perspective of the genetic context of both genomes.

Lipopolysaccharides of Pantoea agglomerans 7604 and 8674 with structurally related O-polysaccharide chains: Chemical identification and biological properties

Structurally related O-specific polysaccharide (O-antigen) and lipid A components were obtained by mild acid degradation of the lipopolysaccharides (LPSs) of two strains of bacteria Pantoea agglomerans, 7604 and 8674. Studies by sugar analysis along with 1D and 2D ¹H and ¹³C NMR spectroscopy enabled elucidation of the following structures of the O-polysaccharides, which differ only in the linkage configuration of a side-chain glucose residue: R=α-d-Glcp in strain 7604 or β-d-Glcp in strain 8674 Lipid A samples were studied by GC-MS and high-resolution ESI-MS and found to be represented by penta- and tetra-acyl species; lipid A of strain 8674 also included hexaacyl species. A peculiar feature of lipid A of both strains is the presence of the major cis-9-hexadecenoic (palmitoleic) acid, which has not been found in P. agglomerans strains studied earlier. The LPSs of both strains were pyrogenic, reduced the average adhesion and the index of adhesiveness and showed a relatively low level of lethal toxicity. O-antiserum against strain 7604 showed one-way cross-reactivity with the LPS of strain 8674, and O-antisera against both strains cross-reacted with LPSs of some other Р. agglomerans strains but more strains were serologically unrelated. These structural and serological data indicate immunochemical heterogeneity of Р. agglomerans strains and will find demand in classification of Р. agglomerans by O-antigens.

Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance

Plants are constantly exposed to would-be pathogens and pests, and thus have a sophisticated immune system to ward off these threats, which otherwise can have devastating ecological and economic consequences on ecosystems and agriculture. Plants employ receptor kinases (RKs) and receptor-like proteins (RLPs) as pattern recognition receptors (PRRs) to monitor their apoplastic environment and detect non-self and damaged-self patterns as signs of potential danger. Plant PRRs contribute to both basal and non-host resistances, and treatment with pathogen-/microbe-associated molecular patterns (PAMPs/MAMPs) or damage-associated molecular patterns (DAMPs) recognized by plant PRRs induces both local and systemic immunity. Here, we comprehensively review known PAMPs/DAMPs recognized by plants as well as the plant PRRs described to date. In particular, we describe the different methods that can be used to identify PAMPs/DAMPs and PRRs. Finally, we emphasize the emerging biotechnological potential use of PRRs to improve broad-spectrum, and potentially durable, disease resistance in crops.

Biomedical Properties and Origins of Sesquiterpene Lactones, with a Focus on Dehydroleucodine

Dehydroleucodine, a sesquiterpene lactone, belongs to the terpenoid class of secondary metabolites. Dehydroleucodine and other Artemisia-derived phytochemicals evolved numerous biodefenses that were first co-opted for human pharmacological use by traditional cultures in the Middle East, Asia, Europe and the Americas. Later, these phytochemicals were modified through the use of medicinal chemical techniques to increase their potency. All sesquiterpene lactones contain an α-methylene-γ-lactone group, which confers thiol reactivity, which is responsible, in part, for their therapeutic effects. A wide range of therapeutic uses of sequiterpene lactones has been found, including anti-adipogenic, cytoprotective, anti-microbial, anti-viral, anti-fungal, anti-malarial and, anti-migraine effects. Dehydroleucodine significantly inhibits differentiation of murine preadipocytes and also significantly decreases the accumulation of lipid content by a dramatic down regulation of adipogenic-specific transcriptional factors PPARγ and C-EBPα. Dehydroleucodine also inhibits secretion of matrix metalloprotease-2 (MMP-2), which is a known protease involved in migration and invasion of B16 cells. In addition to these anti-adipogenic and anti-cancer effects, dehydroleucodine effectively neutralizes several bacterial species, including Bacillus cereus, Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Helicobacter pylori, methicillin resistant Staphylococcus aueus (MRSA) and S. epidermis (MRSE). The compound also inhibits the growth and secretion of several toxins of Pseudomonas aeruginosa, possesses gastro-protective qualities and possesses anti-parasitic properties against Trypanosoma cruzi, responsible for Chagas disease. Other sesquiterpene lactones, such as parthenolide, costunolide, and helanin, also possess significant therapeutic utility.

Genetic dissection of the maize (Zea mays L.) MAMP response

Loci associated with variation in maize responses to two microbe-associated molecular patterns (MAMPs) were identified. MAMP responses were correlated. No relationship between MAMP responses and quantitative disease resistance was identified. Microbe-associated molecular patterns (MAMPs) are highly conserved molecules commonly found in microbes which can be recognized by plant pattern recognition receptors. Recognition triggers a suite of responses including production of reactive oxygen species (ROS) and nitric oxide (NO) and expression changes of defense-related genes. In this study, we used two well-studied MAMPs (flg22 and chitooctaose) to challenge different maize lines to determine whether there was variation in the level of responses to these MAMPs, to dissect the genetic basis underlying that variation and to understand the relationship between MAMP response and quantitative disease resistance (QDR). Naturally occurring quantitative variation in ROS, NO production, and defense genes expression levels triggered by MAMPs was observed. A major quantitative traits locus (QTL) associated with variation in the ROS production response to both flg22 and chitooctaose was identified on chromosome 2 in a recombinant inbred line (RIL) population derived from the maize inbred lines B73 and CML228. Minor QTL associated with variation in the flg22 ROS response was identified on chromosomes 1 and 4. Comparison of these results with data previously obtained for variation in QDR and the defense response in the same RIL population did not provide any evidence for a common genetic basis controlling variation in these traits.

Lipopolysaccharide of Pantoea agglomerans 7969: Chemical identification, function and biological activity

Lipopolysaccharide (LPS) of Pantoea agglomerans 7969 isolated from apple tree was purified and characterized chemically by sugar and fatty acid analysis. Lipid A was analysed by negative-ion mode ESI MS and found to consist mainly of hexa- and tetra-acyl species typical of E. coli lipid A. The O-specific polysaccharide of the LPS was studied by sugar analysis, Smith degradation, and one- and two-dimensional ¹H and ¹³C NMR spectroscopy. The polysaccharide is built up of linear tetrasaccharide repeating units, and about ∼25% repeats contain glycerol 1-phosphate on the GlcNAc residue: →3)-α-l-Rha p-(1→6)-α-d-Man p-(1→3)-α-d-Fuc p-(1→3)-β-d-Glc pNAc-(1→∼25% Gro-1-P-(O→6)^⌋ The LPS showed low levels of toxic and pyrogenic activities and reduced the average adhesion and the index of adhesiveness.

The Lipopolysaccharide-Induced Metabolome Signature in Arabidopsis thaliana Reveals Dynamic Reprogramming of Phytoalexin and Phytoanticipin Pathways

Lipopolysaccharides (LPSs), as MAMP molecules, trigger the activation of signal transduction pathways involved in defence. Currently, plant metabolomics is providing new dimensions into understanding the intracellular adaptive responses to external stimuli. The effect of LPS on the metabolomes of Arabidopsis thaliana cells and leaf tissue was investigated over a 24 h period. Cellular metabolites and those secreted into the medium were extracted with methanol and liquid chromatography coupled to mass spectrometry was used for quantitative and qualitative analyses. Multivariate statistical data analyses were used to extract interpretable information from the generated multidimensional LC-MS data. The results show that LPS perception triggered differential changes in the metabolomes of cells and leaves, leading to variation in the biosynthesis of specialised secondary metabolites. Time-dependent changes in metabolite profiles were observed and biomarkers associated with the LPS-induced response were tentatively identified. These include the phytohormones salicylic acid and jasmonic acid, and also the associated methyl esters and sugar conjugates. The induced defensive state resulted in increases in indole-and other glucosinolates, indole derivatives, camalexin as well as cinnamic acid derivatives and other phenylpropanoids. These annotated metabolites indicate dynamic reprogramming of metabolic pathways that are functionally related towards creating an enhanced defensive capacity. The results reveal new insights into the mode of action of LPS as an activator of plant innate immunity, broadens knowledge about the defence metabolite pathways involved in Arabidopsis responses to LPS, and identifies specialised metabolites of functional importance that can be employed to enhance immunity against pathogen infection.

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.

A highly conserved metalloprotease effector enhances virulence in the maize anthracnose fungus Colletotrichum graminicola

Colletotrichum graminicola causes maize anthracnose, an agronomically important disease with a worldwide distribution. We have identified a fungalysin metalloprotease (Cgfl) with a role in virulence. Transcriptional profiling experiments and live cell imaging show that Cgfl is specifically expressed during the biotrophic stage of infection. To determine whether Cgfl has a role in virulence, we obtained null mutants lacking Cgfl and performed pathogenicity and live microscopy assays. The appressorium morphology of the null mutants is normal, but they exhibit delayed development during the infection process on maize leaves and roots, showing that Cgfl has a role in virulence. In vitro chitinase activity assays of leaves infected with wild-type and null mutant strains show that, in the absence of Cgfl, maize leaves exhibit increased chitinase activity. Phylogenetic analyses show that Cgfl is highly conserved in fungi. Similarity searches, phylogenetic analysis and transcriptional profiling show that C. graminicola encodes two LysM domain-containing homologues of Ecp6, suggesting that this fungus employs both Cgfl-mediated and LysM protein-mediated strategies to control chitin signalling.

Low Agrobacterium tumefaciens inoculum levels and a long co-culture period lead to reduced plant defense responses and increase transgenic shoot production of sunflower (Helianthus annuus L.)

Agrobacterium-mediated plant transformation is typically conducted by inoculating plant tissues with an Agrobacterium suspension containing approximately 108-109 bacteria mL-1, followed by a 2-3-d co-culture period. Use of longer co-culture periods could potentially increase transformation efficiencies by allowing more time for Agrobacterium to interact with plant cells, but bacterial overgrowth is likely to occur, leading to severe tissue browning and reduced transformation and regeneration. Low bacterial inoculum levels were therefore evaluated as a means to reduce the negative outcomes associated with long co-culture. The use of low inoculum bacterial suspensions (approximately 6 × 102 bacteria mL-1) followed by long co-culture (15 d) led to the production of an average of three transformed sunflower shoots per explant while the use of high inoculum (approximately 6 × 108 bacteria mL-1) followed by short co-culture (3 d) led to no transformed shoots. Low inoculum and long co-culture acted synergistically, and both were required for the improvement of sunflower transformation. Gene expression analysis via qRT-PCR showed that genes related to plant defense response were generally expressed at lower levels in the explants treated with low inoculum than those treated with high inoculum during 15 d of co-culture, suggesting that low inoculum reduced the induction of plant defense responses. The use of low inoculum with long co-culture (LI/LC) led to large increases in sunflower transformation efficiency. This method has great potential for improving transformation efficiencies and expanding the types of target tissues amenable for transformation of different plant species.

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.

Overlapping Yet Response-Specific Transcriptome Alterations Characterize the Nature of Tobacco-Pseudomonas syringae Interactions

In this study transcriptomic alterations of bacterially induced pattern triggered immunity (PTI) were compared with other types of tobacco-Pseudomonas interactions. In addition, using pharmacological agents we blocked some signal transduction pathways (Ca(2+) influx, kinases, phospholipases, proteasomic protein degradation) to find out how they contribute to gene expression during PTI. PTI is the first defense response of plant cells to microbes, elicited by their widely conserved molecular patterns. Tobacco is an important model of Solanaceae to study resistance responses, including defense mechanisms against bacteria. In spite of these facts the transcription regulation of tobacco genes during different types of plant bacterial interactions is not well-described. In this paper we compared the tobacco transcriptomic alterations in microarray experiments induced by (i) PTI inducer Pseudomonas syringae pv. syringae type III secretion mutant (hrcC) at earlier (6 h post inoculation) and later (48 hpi) stages of defense, (ii) wild type P. syringae (6 hpi) that causes effector triggered immunity (ETI) and cell death (HR), and (iii) disease-causing P. syringae pv. tabaci (6 hpi). Among the different treatments the highest overlap was between the PTI and ETI at 6 hpi, however, there were groups of genes with specifically altered activity for either type of defenses. Instead of quantitative effects of the virulent P. tabaci on PTI-related genes it influenced transcription qualitatively and blocked the expression changes of a special set of genes including ones involved in signal transduction and transcription regulation. P. tabaci specifically activated or repressed other groups of genes seemingly not related to either PTI or ETI. Kinase and phospholipase A inhibitors had highest impacts on the PTI response and effects of these signal inhibitors on transcription greatly overlapped. Remarkable interactions of phospholipase C-related pathways with the proteasomal system were also observable. Genes specifically affected by virulent P. tabaci belonged to various previously identified signaling routes, suggesting that compatible pathogens may modulate diverse signaling pathways of PTI to overcome plant defense.

Structure of the O-polysaccharide of the lipopolysaccharide of Pseudomonas chlororaphis subsp. aureofaciens UCM B-306

Structure of the O-specific polysaccharide from Pseudomonas chlororaphis subsp. aureofaciens UCM B-306 was elucidated by sugar analysis along with 1D and 2D (1)H and (13)C NMR spectroscopy. The polysaccharide is built up of trisaccharide repeats containing D-rhamnose, 2,4-diacetamido-2,4,6-trideoxy-D-glucose (D-QuiNAc4NAc), and 2-acetamido-2-deoxy-D-galacturonic acid (D-GalNAcA), which is amidated in ∼40% repeats. It was suggested that the O-polysaccharide has a blockwise structure, which can be presented as follows: -[→3)-α-D-Rhap-(1→4)-α-D-GalpNAcA-(1→3)-α-D-QuipNAc4NAc-(1-]n→ and -[→3)-α-D-Rhap-(1→4)-α-D-GalpNAcAN-(1→3)-α-D-QuipNAc4NAc-(1-]m→, where GalNAcAN indicates 2-acetamido-2-deoxy-D-galacturonamide, n:m=∼3:2.

Lipopolysaccharides as Microbe-associated Molecular Patterns: A Structural Perspective

The lipopolysaccharide (LPS) macromolecule is the major constituent of the external leaflet of the Gram-negative outer membrane, exerting a plethora of biological activities in animals and plants. Among all, it represents a defensive barrier which helps bacteria to resist antimicrobial compounds and external stress factors and is involved in most aspects of host–bacterium interactions such as recognition, adhesion and colonization. One of the most interesting and studied LPS features is its key role in the pathogenesis of Gram-negative infections potentially causing fever or circulatory shock. On the other hand, the LPS acts as a beneficial factor for the host since it is recognized by specific receptors of the host innate immune system; this recognition activates the host defenses culminating, in most cases, in destruction of the pathogen. Most of the biological roles of the LPS are strictly related to its primary structure; thus knowledge of the structural architecture of such a macromolecule, which is different even among bacterial strains belonging to the same species, is a first step but is essential in order to understand the molecular bases of the wide variety of biological activities exerted by LPSs.

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.

Occurrence of an unusual hopanoid-containing lipid A among lipopolysaccharides from Bradyrhizobium species

The chemical structures of the unusual hopanoid-containing lipid A samples of the lipopolysaccharides (LPS) from three strains of Bradyrhizobium (slow-growing rhizobia) have been established. They differed considerably from other Gram-negative bacteria in regards to the backbone structure, the number of ester-linked long chain hydroxylated fatty acids, as well as the presence of a tertiary residue that consisted of at least one molecule of carboxyl-bacteriohopanediol or its 2-methyl derivative. The structural details of this type of lipid A were established using one- and two-dimensional NMR spectroscopy, chemical composition analyses, and mass spectrometry techniques (electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry and MALDI-TOF-MS). In these lipid A samples the glucosamine disaccharide characteristic for enterobacterial lipid A was replaced by a 2,3-diamino-2,3-dideoxy-d-glucopyranosyl-(GlcpN3N) disaccharide, deprived of phosphate residues, and substituted by an α-d-Manp-(1→6)-α-d-Manp disaccharide substituting C-4' of the non-reducing (distal) GlcpN3N, and one residue of galacturonic acid (d-GalpA) α-(1→1)-linked to the reducing (proximal) amino sugar residue. Amide-linked 12:0(3-OH) and 14:0(3-OH) were identified. Some hydroxy groups of these fatty acids were further esterified by long (ω-1)-hydroxylated fatty acids comprising 26-34 carbon atoms. As confirmed by mass spectrometry techniques, these long chain fatty acids could form two or three acyloxyacyl residues. The triterpenoid derivatives were identified as 34-carboxyl-bacteriohopane-32,33-diol and 34-carboxyl-2β-methyl-bacteriohopane-32,33-diol and were covalently linked to the (ω-1)-hydroxy group of very long chain fatty acid in bradyrhizobial lipid A. Bradyrhizobium japonicum possessed lipid A species with two hopanoid residues.

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.

Chemistry of lipid A: at the heart of innate immunity

In many Gram-negative bacteria, lipopolysaccharide (LPS) and its lipid A moiety are pivotal for bacterial survival. Depending on its structure, lipid A carries the toxic properties of the LPS and acts as a potent elicitor of the host innate immune system via the Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) receptor complex. It often causes a wide variety of biological effects ranging from a remarkable enhancement of the resistance to the infection to an uncontrolled and massive immune response resulting in sepsis and septic shock. Since the bioactivity of lipid A is strongly influenced by its primary structure, a broad range of chemical syntheses of lipid A derivatives have made an enormous contribution to the characterization of lipid A bioactivity, providing novel pharmacological targets for the development of new biomedical therapies. Here, we describe and discuss the chemical aspects regarding lipid A and its role in innate immunity, from the (bio)synthesis, isolation and characterization to the molecular recognition at the atomic level.

Pattern triggered immunity (PTI) in tobacco: isolation of activated genes suggests role of the phenylpropanoid pathway in inhibition of bacterial pathogens

BACKGROUND

Pattern Triggered Immunity (PTI) or Basal Resistance (BR) is a potent, symptomless form of plant resistance. Upon inoculation of a plant with non-pathogens or pathogenicity-mutant bacteria, the induced PTI will prevent bacterial proliferation. Developed PTI is also able to protect the plant from disease or HR (Hypersensitive Response) after a challenging infection with pathogenic bacteria. Our aim was to reveal those PTI-related genes of tobacco (Nicotiana tabacum) that could possibly play a role in the protection of the plant from disease.

METHODOLOGY/PRINCIPAL FINDINGS

Leaves were infiltrated with Pseudomonas syringae pv. syringae hrcC- mutant bacteria to induce PTI, and samples were taken 6 and 48 hours later. Subtraction Suppressive Hybridization (SSH) resulted in 156 PTI-activated genes. A cDNA microarray was generated from the SSH clone library. Analysis of hybridization data showed that in the early (6 hpi) phase of PTI, among others, genes of peroxidases, signalling elements, heat shock proteins and secondary metabolites were upregulated, while at the late phase (48 hpi) the group of proteolysis genes was newly activated. Microarray data were verified by real time RT-PCR analysis. Almost all members of the phenyl-propanoid pathway (PPP) possibly leading to lignin biosynthesis were activated. Specific inhibition of cinnamic-acid-4-hydroxylase (C4H), rate limiting enzyme of the PPP, decreased the strength of PTI--as shown by the HR-inhibition and electrolyte leakage tests. Quantification of cinnamate and p-coumarate by thin-layer chromatography (TLC)-densitometry supported specific changes in the levels of these metabolites upon elicitation of PTI.

CONCLUSIONS/SIGNIFICANCE

We believe to provide first report on PTI-related changes in the levels of these PPP metabolites. Results implicated an actual role of the upregulation of the phenylpropanoid pathway in the inhibition of bacterial pathogenic activity during PTI.