Structures of Exopolysaccharides Involved in Receptor-mediated Perception of Mesorhizobium loti by Lotus japonicus

Abstract

In the symbiosis formed between Mesorhizobium loti strain R7A and Lotus japonicus Gifu, rhizobial exopolysaccharide (EPS) plays an important role in infection thread formation. Mutants of strain R7A affected in early exopolysaccharide biosynthetic steps form nitrogen-fixing nodules on L. japonicus Gifu after a delay, whereas mutants affected in mid or late biosynthetic steps induce uninfected nodule primordia. Recently, it was shown that a plant receptor-like kinase, EPR3, binds low molecular mass exopolysaccharide from strain R7A to regulate bacterial passage through the plant's epidermal cell layer ( Rhizobia are soil bacteria that are able to form nitrogen-fixing endosymbiotic relationships with their host legume plants. In this mutualistic interaction, the host plant secretes flavonoids that stimulate compatible rhizobia to synthesize specific lipochitooligosaccharide signaling molecules called Nod factors (1, 2). The host plant perceives these Nod factors using membrane-bound receptor-like kinases that contain extracellular LysM domains (3-5). Perception of the Nod factors results in deformation and curling of root hairs and also the induction of cortical cell division to form nodule primordia (6). The bacteria entrapped in the curled root hairs form microcolonies and then enter the host through infection threads that are formed by localized digestion of the plant cell wall within the curl, followed by inverted growth of the cell wall/plasma membrane down the length of the root hair (7,8). The rhizobia grow down the infection thread by cell division, and the infection thread ramifies as it reaches the growing nodule primordia. Eventually the rhizobia are released from the infection thread into the plant cell cytoplasm surrounded by a membrane of plant origin and develop into the nitrogenfixing bacteroid form. The nodules formed may be either determinate (e.g. Lotus spp., Phaseolus spp., and Glycine max) or indeterminate (e.g. Medicago spp., Vicia spp., and Trifolium spp.) depending on the host (9). Determinate nodules have a short-lived meristem, and the nodules grow by plant cell expansion and division, resulting in nodules progressing through well defined developmental stages. In contrast, indeterminate nodules have a persistent meristem, and infection is continuous, resulting in all developmental stages being present within a single nodule.Rhizobia produce a variety of cell surface-associated glycolipids and polysaccharides, as well as extracellular polysaccharides that also contribute to a successful symbiotic plant-*

Sinorhizobium fredii HH103 nolR and nodD2 mutants gain capacity for infection thread invasion of Lotus japonicus Gifu and Lotus burttii

“…The perception of Nod-factors by legume receptors (NFR) activates nodule initiation and development (Madsen et al, 2003;Radutoiu et al, 2007;Madsen et al, 2011;Broghammer et al, 2012). In addition to Nod-factors, there are other molecules such as surface polysaccharides and proteins secreted by bacterial secretion systems that also play a very important role in the establishment of an efficient symbiosis with the host plant (Janczarek et al, 2015;Kawaharada et al, 2015Kawaharada et al, , 2017López-Baena et al, 2016;Muszy nski et al, 2016).…”

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

“…(Ghosh and Maiti ). Two major symbiotically active types of EPS, succinoglycan and galactoglucan, are primarily derived from Sinorhizobium meliloti but have also been isolated from many other rhizobial bacteria (O'Neill et al ; Muszyński et al ). Succinoglycans are a class of acidic polymers composed of eight repeating sugar units containing galactose and glucose residues in a molar ratio of 1 : 7 that are linked by β‐1,3, β‐1,4 and β‐1,6 glycosidic bonds.…”

“…The ability to produce extracellular polysaccharide is widespread among rhizobial strains. A number of reports have revealed that rhizobial exopolysaccharide (EPS) plays an important role in symbiosis, including promoting their derived from Sinorhizobium meliloti but have also been isolated from many other rhizobial bacteria (O'Neill et al 1991;Muszy nski et al 2016). Succinoglycans are a class of acidic polymers composed of eight repeating sugar units containing galactose and glucose residues in a molar ratio of 1 : 7 that are linked by b-1,3, b-1,4 and b-1,6 glycosidic bonds.…”

Extracellular polysaccharide protects Rhizobium leguminosarum cells against zinc stress in vitro and during symbiosis with clover

“…The symbiosis is a complex process involving an exchange of many signals; among these, legume flavonoids and rhizobial lipochitooligosaccharides play crucial roles (Downie, 2010;L opez-Baena et al, 2016). Lowmolecular-weight (LMW) fraction of extracellular polysaccharide (EPS) secreted by rhizobia is a signal molecule involved in the early stages of symbiosis (Kelly et al, 2013;Kawaharada et al, 2015;Muszy nski et al, 2016). High-molecular-weight (HMW) EPS, which forms the most external layer surrounding the rhizobial cell, protects the bacteria against desiccation, salinity and other stresses (Broos et al, 2005;Jaszek et al, 2014).…”

“…EPS-deficient rhizobia (e.g., Sinorhizobium meliloti, R. leguminosarum bvs. trifolii and viciae and Mesorhizobium loti) induce empty nodule-like structures on compatible host plants, which are inefficient in nitrogen fixation (Rolfe et al, 1996;Cheng and Walker, 1998;Janczarek and Urbanik-Sypniewska, 2013;Muszy nski et al, 2016). However, the importance of EPS in the legume-rhizobium symbiosis depends on the type of the host plant, as indicated by a recent finding that EPS produced by Sinorhizobium fredii HH103 is not required for nodulation of Glycyrrhiza uralensis (Margaret-Oliver et al, 2012;L opez-Baena et al, 2016).…”