Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis with Legumes

Abstract

Rhizobia are soil proteobacteria able to engage in a nitrogen-fixing symbiotic interaction with legumes that involves the rhizobial infection of roots and the bacterial invasion of new organs formed by the plant in response to the presence of appropriate bacterial partners. This interaction relies on a complex molecular dialogue between both symbionts. Bacterial N-acetyl-glucosamine oligomers called Nod factors are indispensable in most cases for early steps of the symbiotic interaction. In addition, different rhizobial surface polysaccharides, such as exopolysaccharides (EPS), may also be symbiotically relevant. EPS are acidic polysaccharides located out of the cell with little or no cell association that carry out important roles both in free-life and in symbiosis. EPS production is very complexly modulated and, frequently, co-regulated with Nod factors, but the type of co-regulation varies depending on the rhizobial strain. Many studies point out a signalling role for EPS-derived oligosaccharides in root infection and nodule invasion but, in certain symbiotic couples, EPS can be dispensable for a successful interaction. In summary, the complex regulation of the production of rhizobial EPS varies in different rhizobia, and the relevance of this polysaccharide in symbiosis with legumes depends on the specific interacting couple.

Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the Rhizobium-Legume Symbioses

“…In rhizobia inducing determinate nodules, EPS mutants still induce effective nodules, with EPS playing a signaling role at the late stages of both infection thread initiation and bacterial release ( Kelly et al, 2013 ). However, in symbioses forming indeterminate nodules, EPS is absolutely essential for a successful interaction (reviewed by Fraysse et al, 2003 ; Acosta-Jurado et al, 2021 ). EPS-altered mutants (exhibiting conserved LPS) generated in an isogenic strain, R. loti PN184, able to nodulate Lotus pedunculatus (determinate nodules) and Leucaena leucocephala (indeterminate nodules) showed that they were fully effective on a determinate-nodulating host but ineffective on the indeterminate one ( Hotter and Scott, 1991 ).…”

Genetic diversity of microsymbionts nodulating Trifolium pratense in subpolar and temperate climate regions

“…Furthermore, they have an important role in crop rotation and are used for production of wood, tannins, oils, dyes, and medicines and in the horticultural trade 1 , 5 . Most species of Fabaceae plants can establish nitrogen-fixing symbioses with soil bacteria, collectively known as rhizobia 6 , 7 . This process, called biological nitrogen fixation (BNF), is an ecological and low-cost alternative providing nitrogen to legume crops.…”

Organic acids and glucose prime late-stage fungal biotrophy in maize

“…For example, obligate mycorrhizal fungi respond to lipids and sugars with proliferation and pre-spore formation, and rhizobia bacteria respond to dicarboxylates such as succinate and malate from legume hosts during nodulation and nitrogen fixation (27)(28)(29)(30)(31). Furthermore, features of bacterial nodulation such as extracellular polysaccharide production, malate metabolism, and oxygen sensing are shared with fungal tumor formation (32,33). The response to defined combinations of nutrients may therefore be a general theme in plant-microbe interactions.…”