Current data from bacterial pathogens of animals and from bacterial symbionts of plants support some of the more general proposed functions for lipopolysaccharides (LPS) and underline the importance of LPS structural versatility and adaptability. Most of the structural heterogeneity of LPS molecules is found in the O-antigen polysaccharide. In this review, the role and mechanisms of this striking flexibility in molecular structure of the O-antigen in bacterial pathogens and symbionts are illustrated by some recent findings. The variation in O-antigen that gives rise to an enormous structural diversity of O-antigens lies in the sugar composition and the linkages between monosaccharides. The chemical composition and structure of the O-antigen is strain-specific (interstrain LPS heterogeneity) but can also vary within one bacterial strain (intrastrain LPS heterogeneity). Both LPS heterogeneities can be achieved through variations at different levels. First of all, O-polysaccharides can be modified non-stoichiometrically with sugar moieties, such as glucosyl and fucosyl residues. The addition of non-carbohydrate substituents, i.e. acetyl or methyl groups, to the O-antigen can also occur with regularity, but in most cases these modifications are again non-stoichiometric. Understanding LPS structural variation in bacterial pathogens is important because several studies have indicated that the composition or size of the O-antigen might be a reliable indicator of virulence potential and that these important features often differ within the same bacterial strain. In general, O-antigen modifications seem to play an important role at several (at least two) stages of the infection process, including the colonization (adherence) step and the ability to bypass or overcome host defense mechanisms. There are many reports of modifications of O-antigen in bacterial pathogens, resulting either from altered gene expression, from lysogenic conversion or from lateral gene transfer followed by recombination. In most cases, the mechanisms underlying these changes have not been resolved. However, in recent studies some progress in understanding has been made. Changes in O-antigen structure mediated by lateral gene transfer, O-antigen conversion and phase variation, including fucosylation, glucosylation, acetylation and changes in O-antigen size, will be discussed. In addition to the observed LPS heterogeneity in bacterial pathogens, the structure of LPS is also altered in bacterial symbionts in response to signals from the plant during symbiosis. It appears to be part of a molecular communication between bacterium and host plant. Experiments ex planta suggest that the bacterium in the rhizosphere prepares its LPS for its roles in symbiosis by refining the LPS structure in response to seed and root compounds and the lower pH at the root surface. Moreover, modifications in LPS induced by conditions associated with infection are another indication that specific structures are important. Also during the differentiation from bacterium to bacteroid, the LPS of Rhizobium undergoes changes in the composition of the O-antigen, presumably in response to the change of environment. Recent findings suggest that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPS that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. However, the genetic mechanisms by which the LPS epitope changes are regulated remain unclear. Finally, the possible roles of O-antigen variations in symbiosis will be discussed.

http://dzumenvis.nic.in/Microbes%20and%20Metals%20Interaction/pdf/O-antigen%20structural%20variation.pdf

O-antigen structural variation: mechanisms and possible roles in animal/plant-microbe interactions.

Plant Lectins: Molecular and Biological Aspects

https://mmbr.asm.org/content/mmbr/50/2/193.full.pdf

Signal exchange in plant-microbe interactions.

Role of lectins (and rhizobial exopolysaccharides) in legume nodulation.

Attachment of bacteria to plant cells is one of the earliest steps in many plant-bacterium interactions. This review covers the current knowledge on one of the best-studied examples of bacterium-plant attachment, namely the molecular mechanism by which Rhizobium bacteria adhere to plant roots. Despite differences in several studies with regard to growth conditions of bacteria and plants and to methods used for measuring attachment, an overall consensus can be drawn from the available data. Rhizobial attachment to plant root hairs appears to be a two-step process. A bacterial Ca(2+)-binding protein, designated as rhicadhesin, is involved in direct attachment of bacteria to the surface of the root hair cell. Besides this step, there is another step which results mainly in accumulation and anchoring of the bacteria to the surface of the root hair. This leads to so-called firm attachment. Depending on the growth conditions of the bacteria, the latter step is mediated by plant lectins and/or by bacterial appendages such as cellulose fibrils and fimbriae. The possible role of these adhesions in root nodule formation is discussed.

Molecular mechanisms of attachment of Rhizobium bacteria to plant roots.

Phase-contrast and fluorescence microscopy observations showed that pea symbiont R. leguminosarum adsorbed to pea root hairs, but non-symbiont rhizobial strains only adsorbed to a small extent. 14C-labeled cells were used to assay the number of rhizobial cells adsorbed to a pea root. Capsular polysaccharides or lipopolysaccharides obtained from R. leguminosarum specifically inhibited the adsorption of 14C-R. leguminosarum cells to a pea root and specifically adsorbed to pea root hairs. Also, they reacted specifically with pea seed lectins. These results suggest that capsular polysaccharides or lipopolysaccharides play an important role in host-specific adsorption. The interaction between the polysaccharides and pea lectins could be the key to determining host specificity in the infection process of Rhizobium-pea symbiosis.

Role of Bacterial Polysaccharides in the Adsorption Process of the Rhizobium-Pea. Symbiosis

Involvement of Lectins in Rhizobium-Pea Recognition

Rhizobium japonicum and R. leguminosarum combined specifically with lectins of host plants, and lipopolysaccharides with the strongest inhibitory activity towards the hemagglutinating reaction of host plants lectins were extracted and purified from the two Rhizobial species. Moreover, they inhibited the binding reaction between lectins and bacterial cells. These results support the lectin theory that the interaction between lectins of host plants and Rhizobium is the key to the mechanism of the host specificity in Rhizobium-legume symbiosis, and that the lectin-receptor of Rhizobial cells exists in the cell surface lipopolysaccharides.

Role of lectins and lipopolysaccharides in the recognition process of specific legume-Rhizobium symbiosis.

A minor carotenoid in integuments of orange pupae of a swallowtail, Papilio xuthus, was found also in carapaces of a crab, Paralithodes brevipes (hanasakigani). This new carotenoid showed a visible absorption spectrum similar to that of papilioerythrin (=α-doradexanthin) and was named papilioerythrinone. Reduction of this compound with sodium borohydride occurred in 2 steps and gave products identical with the reduction products of papilioerythrin. Therefore, the structure 3-hydroxy-β,e-carotene-4,3′-dione (IV) is ascribed to papilioerythrinone.

Papilioerythrinone: A New Ketocarotenoid in Integuments of Orange Pupae of a Swallowtail, Papilio xuthus, and Carapaces of a Crab, Paralithodes brevipes (Hanasakigani in Japanese)

Goro Kato

Papers

Role of Bacterial Polysaccharides in the Adsorption Process of the Rhizobium-Pea. Symbiosis

Involvement of Lectins in Rhizobium-Pea Recognition

Role of lectins and lipopolysaccharides in the recognition process of specific legume-Rhizobium symbiosis.

Papilioerythrinone: A New Ketocarotenoid in Integuments of Orange Pupae of a Swallowtail, Papilio xuthus, and Carapaces of a Crab, Paralithodes brevipes (Hanasakigani in Japanese)