Synthesis of Rhizobial Exopolysaccharides and Their Importance for Symbiosis with Legume Plants

Abstract

Rhizobia dwell and multiply in the soil and represent a unique group of bacteria able to enter into a symbiotic interaction with plants from the Fabaceae family and fix atmospheric nitrogen inside de novo created plant organs, called nodules. One of the key determinants of the successful interaction between these bacteria and plants are exopolysaccharides, which represent species-specific homo- and heteropolymers of different carbohydrate units frequently decorated by non-carbohydrate substituents. Exopolysaccharides are typically built from repeat units assembled by the Wzx/Wzy-dependent pathway, where individual subunits are synthesized in conjunction with the lipid anchor undecaprenylphosphate (und-PP), due to the activity of glycosyltransferases. Complete oligosaccharide repeat units are transferred to the periplasmic space by the activity of the Wzx flippase, and, while still being anchored in the membrane, they are joined by the polymerase Wzy. Here we have focused on the genetic control over the process of exopolysaccharides (EPS) biosynthesis in rhizobia, with emphasis put on the recent advancements in understanding the mode of action of the key proteins operating in the pathway. A role played by exopolysaccharide in Rhizobium–legume symbiosis, including recent data confirming the signaling function of EPS, is also discussed.

In vitroandin vivoanti‐inflammatory activity of a succinoglycan Riclin fromAgrobacteriumsp. ZCC3656

“…Microbial EPSs are important multifunctional biopolymers that have great applications in the food, cosmetic, pharmaceutical and other industries. Many rhizobial bacteria are able to produce commercially useful EPSs that also have beneficial physiological effects, such as favouring cell adaptation to unfavourable environmental conditions and promoting plant–bacteria interactions (Skorupska et al ; Marczak et al ). In this study, Agrobacterium sp.…”

Glycoside Hydrolase Genes Are Required for Virulence of Agrobacterium tumefaciens on Bryophyllum daigremontiana and Tomato

“…Such alterations are known to be involved in the interaction of some bacteria with plant hosts. For example, in Sinorhizobium meliloti, the extracellular digestion of highmolecular-weight succinoglycan into low-molecular-weight succinoglycan plays a role in the interaction of the bacteria with the plant (29)(30)(31). It is also possible that some of the proteins encoded by these genes play a role in modifying the plant responses to bacteria.…”

PssJ Is a Terminal Galactosyltransferase Involved in the Assembly of the Exopolysaccharide Subunit in Rhizobium leguminosarum bv. Trifolii

“…trifolii RBL5515 strain producing EPS repeating units lacking terminal galactose in the side chain was described [ 9 ]; however, direct evidence demonstrating engagement of putative galactosyltransferase encoded by pssJ in this process was not provided. Likewise, the information concerning interactions between glycosyltransferases within the postulated multiprotein complex and proteins active in the later stages of the subunit synthesis is still fragmentary [ 30 ]. For example, from among the PssR, PssM, and PssK proteins, most probably engaged in non-sugar modification of EPS, only PssM has been experimentally proved to serve the ketal pyruvate transferase function and modify the subterminal sugar residue in the repeating unit [ 31 ].…”

“…In contrast to the poorly recognized stage of EPS subunit biosynthesis, steps related to EPS polymerization and export have been well defined in R. leguminosarum bv. trifolii , which allows proposing a robust model for exopolysaccharide secretion (see: [ 30 ] for review). In this Wzx/Wzy-dependent pathway, EPS subunits are transferred from the cytoplasm to the periplasm by Wzx-type flippase PssL and polymerized by Wzy-type polymerase PssT.…”