Structure-Dependent Modulation of a Pathogen Response in Plants by Synthetic O-Antigen Polysaccharides

Xanthomonas axonopodis pv. citri (Xac) causes citrus canker, provoking defoliation and premature fruit drop with concomitant economical damage. In plant pathogenic bacteria, lipopolysaccharides are important virulence factors, and they are being increasingly recognized as major pathogen-associated molecular patterns for plants. In general, three domains are recognized in a lipopolysaccharide: the hydrophobic lipid A, the hydrophilic O-antigen polysaccharide, and the core oligosaccharide, connecting lipid A and O-antigen. In this work, we have determined the structure of purified lipopolysaccharides obtained from Xanthomonas axonopodis pv. citri wild type and a mutant of the O-antigen ABC transporter encoded by the wzt gene. High pH anion exchange chromatography and matrix-assisted laser desorption/ionization mass spectrum analysis were performed, enabling determination of the structure not only of the released oligosaccharides and lipid A moieties but also the intact lipopolysaccharides. The results demonstrate that Xac wild type and Xacwzt LPSs are composed mainly of a penta-or tetra-acylated diglucosamine backbone attached to either two pyrophosphorylethanolamine groups or to one pyrophosphorylethanolamine group and one phosphorylethanolamine group. The core region consists of a branched oligosaccharide formed by Kdo 2 Hex 6 GalA 3 Fuc3NAcRha 4 and two phosphate groups. As expected, the presence of a rhamnose homo-oligosaccharide as O-antigen was determined only in the Xac wild type lipopolysaccharide. In addition, we have examined how lipopolysaccharides from Xac function in the pathogenesis process. We analyzed the response of the different lipopolysaccharides during the stomata aperture closure cycle, the callose deposition, the expression of defense-related genes, and reactive oxygen species production in citrus leaves, suggesting a functional role of the O-antigen from Xac lipopolysaccharides in the basal response.

Section: Discussionmentioning

confidence: 99%

Structural Analysis and Involvement in Plant Innate Immunity of Xanthomonas axonopodis pv. citri Lipopolysaccharide

Casabuono

Petrocelli

Ottado

et al. 2011

“…This has also been demonstrated for flagellin, the protein subunit that builds up bacterial flagella: whereas the Arabidopsis flagellin receptor FLS2 recognizes the very conserved peptide Flg22, which is part of the so called Spike region of bacterial flagellin, a more central hypervariable peptide in the D1 region acts as PAMP in mammals (7,73,74). Moreover, in mammals lipid A as the invariable part of LPS is the most potent stimulator of innate immunity (75,76), whereas Arabidopsis perceives LPS via both the lipid A part as well as synthetic oligorhamnans, which are commonly found in the highly variable O-antigen of LPS (36,77). We suggest a model in which PGN was chosen as non-self determinant both in plants and animals because of its characteristics as a typical PAMP: it is widely found in bacteria, structurally stable, displayed on the cell surface and not found in eukaryotic cells.…”

Section: Discussionmentioning

confidence: 99%

Bacteria-derived Peptidoglycans Constitute Pathogen-associated Molecular Patterns Triggering Innate Immunity in Arabidopsis

Gust

Biswas

Lenz

et al. 2007

282

224

The innate immune system is a host defense mechanism that is evolutionarily conserved from insects to human and is mainly involved in the recognition and control of the early stage of infection in all animals (1). Over the last decade, it has become increasingly evident that also plants have acquired the ability to recognize "non self" via sensitive perception systems for components of microorganisms called pathogen-associated molecular patterns (PAMPs) 2 (2-4). As classically defined, PAMPs are highly characteristic of potentially infectious microbes, but are not present in the host. In addition, such patterns are often vital for microbial survival and are therefore not subject to mutational variation. PAMPs that trigger innate immune responses in various vertebrate and non-vertebrate organisms include lipopolysaccharides (LPS) from Gram-negative bacteria, eubacterial flagellin, viral, and bacterial nucleic acids, fungal cell wall-derived glucans, chitins, mannans, or proteins and peptidoglycans (PGN) from Gram-positive bacteria (5-8). Peptidoglycan (PGN) is an essential and unique component of the bacterial envelope that provides rigidity and structure to the bacterial cell. Virtually all bacteria contain a layer of PGN, but the amount, location, and specific composition vary. PGN is a polymer of alternating N-acetylglucosamine (GlcNAc) and N-acetyl-muramic acid (MurNAc) residues in ␤-1-4 linkage which are cross-linked by short peptides (9, 10). The glycan chains display little variation among different bacterial species while the peptide subunit and the interpeptide bridge reveal species specific differences. PGN from Staphylococcus aureus belongs to the L-lysine (Lys)-type, which is primarily found in Gram-positive bacteria whereas meso-diaminopimelate (Dap)-type PGN is typical for many Gram-negative bacteria.As PGNs are located on most bacterial surfaces they constitute excellent targets for recognition by the innate immune system. Indeed, PGN is known for a long time to promote an innate immune response in vertebrates and insects (11-13), and a breakdown product of PGN, muramyl dipeptide (MurNAc-L-Ala-D-Glu; MDP) was found to be the minimal chemical structure required for PAMP activity in mammals (14). PGN is perceived in animals via various pattern recognition receptors (PRRs), including scavenger receptors, nucleotide-binding oligomerization domain-containing proteins (NODs), a family of peptidoglycan recognition proteins (PGRPs), PGN-lytic enzymes and Toll-like receptor TLR2 (15-19).Remarkable similarities have been uncovered in the molecular mode of PAMP perception in animals and plants (2,20,21). Perception of flagellin in Arabidopsis was shown to be depend-* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

“…Recent findings have suggested that the lipid A moiety may be at least partially responsible for LPS perception by Arabidopsis thaliana, leading to a rapid burst of NO, a hallmark of innate immunity in animals (9). Using synthetic O-antigen polysaccharides (oligorhamnans), it has been shown that the O-chain of LPS is recognized by Arabidopsis and that this recognition leads to elicitation of a specific gene transcription response associated with defense (10). These observations, together with the established influence of the core region on lipid A toxicity in animals, indicate that the elucidation of the structure and of the biological activity of both lipid A and core region are of high importance for a better understanding of LPS action in plants.…”

mentioning

confidence: 99%

The Elicitation of Plant Innate Immunity by Lipooligosaccharide of Xanthomonas campestris

Silipo¹,

Molinaro²,

Sturiale³

et al. 2005

Self Cite

180

164

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 about the mechanisms of perception of these molecules by plants and about the associated signal transduction pathways that trigger plant immunity. Here we address the issue of the molecular basis of elicitation of plant defenses through the structural determination of the LOS of the plant pathogen Xanthomonas campestris pv. campestris strain 8004 and examination of the effects of LOS and fragments obtained by chemical treatments on the immune response in Arabidopsis thaliana. The structure shows a strong accumulation of negatively charged groups in the lipid A-inner core region and has a number of novel features, including a galacturonyl phosphate attached at a 3-deoxy-D-manno-oct-2-ulosonic acid residue and a unique phosphoramide group in the inner core region. Intact LOS and the lipid A and core oligosaccharides derived from it were all able to induce the defense-related genes PR1 and PR2 in Arabidopsis and to prevent the hypersensitive response caused by avirulent bacteria. Although LOS induced defense-related gene transcription in two temporal phases, the core oligosaccharide induced only the earlier phase, and lipid A induced only the later phase. These findings suggest that plant cells can recognize lipid A and core oligosaccharide structures within LOS to trigger defensive cellular responses and that this may occur via two distinct recognition events. Lipopolysaccharides (LPSs)2 are ubiquitous and vital components of the cell surface of Gram-negative bacteria (1, 2). They are amphiphilic macromolecules composed of a hydrophilic heteropolysaccharide (comprising the core oligosaccharide and O-specific polysaccharide or O-chain) covalently linked to a lipophilic moiety termed lipid A, which anchors these macromolecules to the outer membrane. LPSs not possessing the O-chain are termed rough LPSs or lipooligosaccharides (LOSs).In animal and insect cells, innate immune defenses are triggered by the perception of pathogen-associated molecular patterns (PAMPs), conserved and generally indispensable microbial structures including LPSs. The recognition of PAMPs by these cells is often mediated by leucine-rich repeat proteins such as Toll in Drosophila and the Toll-like receptors in mammals (3-6). Recognition of LPSs occurs through the lipid A moiety, which is responsible for most of the biological effects of LPS in animals. Lipid A toxicity in animals strongly depends on its structure and is also influenced by the covalently linked core region, which possesses immunogenic properties (1, 2).LPSs apparently have diverse roles in bacterial pathogenesis of plants. As major components of the outer membrane, LPSs are involved in the protection of bacterial cell, contributing to reduce the membrane permeability and thus allowing growth of bacterium in the unfavorable conditi...