Recent Advances in Studies on Structure and Symbiosis-Related Function of Rhizobial K-Antigens and Lipopolysaccharides

Abstract

Exopolysaccharides (EPSs) and K polysaccharides (K-antigens, capsular polysaccharides, or KPSs) are important for the recognition of the symbiotic partner and the infection process, whereas lipopolysaccharides (LPSs) may function at a later stage of symbiosis. Recently, considerable progress has been made in the structural investigation of rhizobial K-antigens and LPSs. This structural data, together with the availability of more and more mutant data, allows new insights into the structure-function relationships of surface polysaccharides and the mode of their action on host cells. This review focuses on rhizobial LPSs and K-antigens. It gives a condensed overview of the recent developments in analysis of their structures and roles during symbiosis.

Molecular Determinants of a Symbiotic Chronic Infection

“…Rhizobial invasion of the host nodule via the IT is strongly influenced by a complex variety of bacterial polysaccharides in addition to NF, including secreted EPSs and K-antigens, secreted and periplasmic cyclic β glucans, and the outer membrane-localized LPSs (14,57,78,155,157). Similar to NF, several of these molecules exert their effects on symbiosis in a structurally dependent manner, arguing that they may function as signals between invading bacteria and their host.…”

“…Not surprisingly then, the bacterial cell surface plays an important role in promoting rhizobial intracellular adaptation, including the lipopolysaccharide (LPS) component of the gramnegative outer membrane (14,83,157). LPS is a complex macromolecule composed of a lipid A membrane anchor and an oligosaccharide core, which can be further modified by the addition of a variable O-antigen polysaccharide (135).…”

“…Rhizobia produce a lipid A with unique structural characteristics that distinguish it from the potent E. coli innate defense elicitor endotoxin (135), although there is variation in certain elements among the rhizobia (14,83). For instance, most rhizobia produce lipid A species lacking either one or both of the 1-and 4′-phosphate groups present on the β-(1,6)-glucosamine disaccharide of E. coli; Sinorhizobium lipid A is an exception to this trend as it is bisphosphorylated (Figure 2d).…”

“…The precise function of LPS in promoting symbiosis remains unclear (14). Defects in LPS can sensitize bacteria to membrane-disrupting agents and antimicrobial peptides so that it may provide a protective barrier against environmental stress and host defense responses.…”

Coupled Mass Spectrometric Strategies for the Determination of Carbohydrates at Very Low Concentrations: The Case of Polysaccharides Involved in the Molecular Dialogue Between Plants and Rhizobia

“…The role of polysaccharides from the rhizobia family during nitrogen fixing symbiosis has been demonstrated, as well as the activity of other bacterial polysaccharides during pathogen infection. However, little is known about their structure/activity relationships [108], which implies a long life for polysaccharide structural characterization. …”