Low molecular weight EPS II of Rhizobium meliloti allows nodule invasion in Medicago sativa.

Abstract

Effective invasion of alfalfa by Rhizobium meliloti RmlO21 normally requires the presence of succinoglycan, an exopolysaccharide (EPS) produced by the bacterium. However, RmlO21 has the ability to produce a second EPS (EPS II) that can suppress the symbiotic defects of succinoglycan-deficient strains. EPS II is a polymer of modified glucose-(f8-1,3)-galactose subunits and is produced by RmlO21 derivatives carrying either an expRiO0 or mucR mutation. If the ability to synthesize succinoglycan is blocked genetically, expRiOl derivatives of Rm1O21 are nodulationproficient, whereas mucR derivatives of RmlO21 are not. The difference in nodulation proficiency between these two classes of EPS 11-producing strains is due to the specific production of a low molecular weight form of EPS H by expR101 strains. A low molecular weight EPS II fraction consisting of 15-20 EPS II disaccharide subunits efficiently allows nodule invasion by noninfective strains when present in amounts as low as 7 pmol per plant, suggesting that low molecular weight EPS II may act as a symbiotic signal during infection.The soil bacterium Rhizobium meliloti fixes nitrogen in symbiotic association with the leguminous plant Medicago sativa (alfalfa). Molecular analyses have revealed that early steps in establishment of this symbiosis depend upon an exchange of small signaling molecules between the two partners (1, 2). Bacteria invade root nodules via a tube of plant origin called the infection thread. This process depends upon the production of exopolysaccharides (EPSs) by K meliloti (3), but the role of EPSs in invasion is not yet clear.Studies focusing on the acidic Calcofluor-binding EPS (succinoglycan) of R. meliloti strain RmlO21 have clearly shown that the EPS is normally required for effective nodulation (4). Succinoglycan is a polymer of octasaccharide repeating units, each consisting of a backbone of three glucoses and one galactose, a side chain of four glucoses, and succinyl, acetyl, and 1-carboxyethylidene (pyruvyl) modifications in a ratio of approximately 1:1:1 (5-7). R meliloti RmlO21 mutants that fail to synthesize succinoglycan form empty nodules that are devoid of bacteria and bacteroids and are unable to fix nitrogen refs. 3 and 8). Genetic analyses led to the identification of a 25-kb cluster of exo genes (9-13) on the second symbiotic megaplasmid that are required for the production of succinoglycan (3,14,15), and biosynthetic roles have been assigned to most of these exo gene products (16). It has been reported that low molecular weight succinoglycan can partially rescue the symbiotic deficiencies of exo mutants (17,18). The active species was tentatively assigned as a tetramer of the octasaccharide subunit, but the exact structure is not known (17). Glazebrook and Walker (19) reported that R. meliloti RmlO21 derivatives carrying the expRi01 mutation synthesize a second EPS (EPS II) that is composed of disaccharide repeats of glucose and galactose carrying acetyl and pyruvyl modifications on the glucose and galactose, r...

Bacterial community assembly based on functional genes rather than species

“…Functions include homologs of the OmpA protein (COG2197), which is required for adhesion to both mammalian and fish epithelial cells in a range of Proteobacteria (23,24), Listeria internalin-like proteins, which enhance attachment and biofilm formation (25,26), and the widespread colonization island, which is essential for biofilm formation, colonization, and pathogenesis in a range of bacteria (27). Proteins related to the production and excretion of galactoglycan, or exopolysaccharide II, were more abundant in the U. australis community, and apart from forming part of the biofilm matrix, is also essential for the establishment and maintenance of symbiosis in several Rhizobium strains (28)(29)(30)(31)(32). GGDEF and EAL domain proteins, which are involved in the production and degradation, respectively, of bis-(3′-5′)-cyclic dimeric GMP (cyclic-di-GMP), were also detected at a higher abundance (33,34).…”

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

“…The forms of EPS II that are able to promote nodule formation are low-molecularweight polymers consisting of 15 to 20 disaccharide repeats. EPS II can act extracellularly, and the addition of as little as 7 pmol of the active forms of EPS II to an alfalfa seedling will allow S. meliloti mutants that are unable to make either succinoglycan or EPS II to form functioning nodules (66). This indicates that EPS II, like succinoglycan, probably acts as a signaling molecule and not a structural one.…”

“…Derivatives of strain Rm1021 with a nondisrupted expR gene produce large quantities of EPS II (7,122). Interestingly, EPS II production can substitute for succinoglycan and support infection thread formation and nodule invasion by S. meliloti exo mutants (66,121). The forms of EPS II that are able to promote nodule formation are low-molecularweight polymers consisting of 15 to 20 disaccharide repeats.…”

Specific binding of the regulatory protein ExpG to promoter regions of the galactoglucan biosynthesis gene cluster of Sinorhizobium meliloti - a combined molecular biology and force spectroscopy investigation

“…It has the ability to produce two acidic exopolysaccharides (EPSs), succinoglycan (EPS I) and galactoglucan (EPS II). EPS I is required for invasion of Medicago sativa root nodules by S. meliloti, but can be replaced by EPS II (Glazebrook and Walker, 1989;Gonzalez et al, 1996;Wang et al, 1999). EPS II consists of alternating glucose and galactose residues which are decorated by acetyl and pyruvyl groups (Her et al, 1990).…”