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

Abstract

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.

Positive selection is the main driving force for evolution of citrus canker-causing Xanthomonas

“…Positive selection for evolution of Xac Y Zhang et al genes in this region, such as wxacO, rfbC, nlxA and wzt, have been demonstrated to be involved in O-antigen synthesis and transport and be associated with biofilm formation on the host and contribute to virulence in XacA (Casabuono et al, 2011;Li and Wang, 2011;Petrocelli et al, 2012;Yan et al, 2012). Among the three components, lipid A, core and O-antigen that are composed of the entire Xac LPS, the O-antigen region is probably the most important for host basal response during citrus-pathogen interaction (Casabuono et al, 2011).…”

Requirement of the Lipopolysaccharide O-Chain Biosynthesis Gene wxocB for Type III Secretion and Virulence of Xanthomonas oryzae pv. Oryzicola

“…Plants recognize certain LPSs as microbe-associated molecular patterns (MAMPs), which trigger innate defense responses (14)(15)(16)(17). In Arabidopsis, both the core and lipid A of Xanthomonas campestris LPS trigger independently the induction of the defense genes PR1 and PR2 (15).…”

“…citri lacking the O chain than by the wild type, suggesting a defense-eliciting role for the O chain as well; notably, this O chain has a structure somewhat distinct from that of X. campestris pv. campestris (17).…”

Host immune responses accelerate pathogen evolution

“…Xcc causes citrus canker disease, and has previously been chosen as a model for investigating the molecular determinants of host specialization (Brunings and Gabriel, 2003). Xcc contains typical pathogen-associated molecular patterns (Casabuono et al, 2011) and 25 type three secretion system effectors (da Silva et al, 2002). It is well documented that Meiwa kumquat (Fortunella crassifolia) is immune to Xcc and shows a hypersensitive response (HR) reaction to Xcc infection (Khalaf et al, 2008;Fu et al, 2012), whereas grapefruit (Citrus paradise) is one of the highly susceptible citrus varieties (Gottwald and Graham, 2000).…”