Characterization and symbiotic importance of acidic extracellular polysaccharides of Rhizobium sp. strain GRH2 isolated from acacia nodules

Abstract

RIizobium sp. wild-type strain GRH2 was originally isolated from root nodules of the leguminous tree Acacia cyanophylla and has a broad host range which includes herbaceous legumes, e.g., Trifolium spp. We examined the extracellular exopolysaccharides (EPSs) produced by strain GRH2 and found three independent glycosidic structures: a high-molecular-weight acidic heteropolysaccharide which is very similar to the acidic EPS produced by Rhizobium leguminosarum biovar trifolii ANU843, a low-molecular-weight native heterooligosaccharide resembling a dimer of the repeat unit of the high-molecular-weight EPS, and low-molecular-weight neutral 13(1,2)-glucans. A TnS insertion mutant derivative of GRH2 (exo-57) Leguminous trees proliferate in arid and semiarid regions of the world which are unsuited for crop production. These leguminous trees are of increasing interest since they provide wood, fiber, gum, and food for animal and human nutrition, protect the soil from erosion, and form nitrogenfixing nodules in symbiotic association with bacteria (2,5,19,24,31,41). Recent studies revealed that leguminous trees are infected by both fast-growing rhizobia and slowly growing bradyrhizobia. Some rhizobial strains isolated from the nodules have a wide host range, even including herbaceous legumes (15,16,24,30,45). This is the case for fast-growing Rhizobium sp. strain GRH2, originally isolated from Acacia cyanophylla (24). Many reports suggest involvement of Rhizobium acidic heteropolysaccharides in infection of legume roots hairs, invasion of nodules, and nitrogen fixation (1,6,11,21,28,32,35,37) and a possible contribution to determination of the bacterial host range (13,14,20,36).Rhizobia with broad host ranges are very useful for investigations of the molecular mechanisms of symbiotic interaction. Rhizobium sp. strain NGR234 is one of the most extensively studied strains with a broad host range, which includes several tropical legumes and the nonlegume Parasponia spp. This strain shows functional and evolutionary genetic equivalence to Rhizobium meliloti (44). In contrast, studies of restriction fragment length polymorphism suggest that broad-host-range Rhizobium sp. strain GRH2 is more closely related to R. leguminosarum (30a). The aim of the present work was to evaluate the extracellular exopolysaccharide (EPS) of strain GRH2 and its role in the root nodule symbiosis between the bacterial symbiont and its leguminous hosts. We show that the EPSs produced by Rhizobium sp. strain GRH2 are very closely related to those produced by wild-type R. leguminosarum bv. tnfolii strain ANU843. * Corresponding author.In addition, a Tn5-induced Exo-mutant derivative of GRH2 which does not produce acidic heteropolysaccharides was obtained and found to be unable to induce nodules on several herbaceous legumes hosts. However, it still nodulated Acacia sp. but infected fewer host cells in the nodule. MATERIALS AND METHODSBacterial strains and genetic manipulations. Rhizobium sp. strain GRH2 was originally isolated from root nodules ofA. cya...

The symbiotic phenotypes of exopolysaccharide-defective mutants of Rhizobium sp. strain TAL1145 do not differ on determinate- and indeterminate-nodulating tree legumes

“…This is in contrast exo genes of Rhizobium sp. strain TAL1145 ~- (Borthakur et al, 1986(Borthakur et al, , 1988Chen et al, 1985;Diebold & Noel, 1989;Gray et al, 1991;Hotter 8;: Scott, 1991;Leigh et al, 1985;Long et al, 1988;Lopez-Lara et at., 1993). It is possible that the small amount of rhe medium-molecular-mass EPS present in these mutants substitutes for the absence of the low-and high-molecular-mass EPS.…”

“…Specific Chen et al, 1985;Diebold & Noel, 1989;Hotter & Scott, 1991;KO & Gadya, 1990;Lopez-Lara et al, 1993). Empty nodules, devoid of bacteroids, are formed in these legumes by the EPS-defective mutants of different Rhizobium spp., indicating that the infection is aborted at an early stage of symbiosis.…”

The Brucella abortus Cyclic β-1,2-Glucan Virulence Factor Is Substituted with O-Ester-Linked Succinyl Residues

“…The degree of substitution varies among species; for example, cyclic glucans produced by the broad-host-range Rhizobium sp. strain GRH2 and by certain strains of Rhizobium leguminosarum contain no anionic substituents (30,45). Bradyrhizobium japonicum cyclic ␤-1,6-␤-1,3-glucans are neutral but substituted with the zwitterionic phosphocholine (35).…”

Periplasmic glucans isolated from Proteobacteria

“…The degree of substitution (DS) of the PGs varies among bacterial strains, but some species often contain no negative-charged substituents as in the case of cyclic β-(1,2)-glucans produced by Sinorhizobium sp. strain GRH2, Rhizobium trifoli, Rhizobium phaseoli and certain strains of S. leguminosarum that synthesize Family II PGs (9,24,25). The growth stage of bacterial strains has also been shown to affect the DS of PGs.…”