Rhizobium lipopolysaccharide modulates infection thread development in white clover root hairs

Abstract

The interaction between Rhizobium lipopolysaccharide (LPS) and white clover roots was examined. The Limulus lysate assay indicated that Rhizobium leguminosarum bv. trifolii (hereafter called R. trifolii) released LPS into the external root environment of slide cultures. Immunofluorescence and immunoelectron microscopy showed that purified LPS from R. tnifolii 0403 bound rapidly to root hair tips and infiltrated across the root hair wall. Infection thread formation in root hairs was promoted by preinoculation treatment of roots with R. trifolii LPS at a low dose (up to 5 ,ug per plant) but inhibited at a higher dose. This biological activity of LPS was restricted to the region of the root present at the time of exposure to LPS, higher with LPS from cells in the early stationary phase than in the mid-exponential phase, incubation time dependent, incapable of reversing inhibition of infection by N03 or NH4', and conserved among serologically distinct LPSs from several wild-type R. trifolii strains (0403, 2S-2, and ANU843). In contrast, infections were not increased by preinoculation treatment of roots with LPSs from R. leguminosarum bv. viciae strain 300, R. meliloti 102F28, or members of the family Enterobacteriaceae. Most infection threads developed successfully in root hairs pretreated with R. trifolii LPS, whereas many infections aborted near their origins and accumulated brown deposits if pretreated with LPS from R. meliloti 102F28. LPS from R. leguminosarum 300 also caused most infection threads to abort. Other specific responses of root hairs to infection-stimulating LPS from R. trifolii included acceleration of cytoplasmic streaming and production of novel proteins. Combined gas chromatography-mass spectroscopy and proton nuclear magnetic resonance analyses indicated that biologically active LPS from R. trifolii 0403 in the early stationary phase had less fucose but more 2-0-methylfucose, quinovosamine, 3,6-dideoxy-3-(methylamino)galactose, and noncarbohydrate substituents (0-methyl, N-methyl, and acetyl groups) on glycosyl components than did inactive LPS in the mid-exponential phase. We conclude that LPS-root hair interactions trigger metabolic events that have a significant impact on successful development of infection threads in this Rhizobium-legume symbiosis.Establishment of an effective Rhizobium-legume symbiosis can be viewed as a process of cellular recognition and compatibility between bacterial and plant cells. The infection process involves bacterial attachment, root hair deformation, bacterial penetration of the root hair wall, formation and sustained development of the infection thread, bacterial release from infection threads within emerging root nodule cells, and bacterial differentiation into nitrogen-fixing bacteroids.The lipopolysaccharides (LPS) of rhizobia are likely to be involved in the infection process. They are major glycoconjugates on the surface of Rhizobium leguminosarum biovars * Corresponding author. t Present address: Laboratoire des Relationes Plantes-Microorganismes,

The Lipopolysaccharide of Sinorhizobium meliloti Suppresses Defense-Associated Gene Expression in Cell Cultures of the Host Plant Medicago truncatula

“…Another mutant study showed that not only the presence, but also the correct modification, of LPS is needed for optimal nodulation activity (Keating et al, 2002). Furthermore, purified LPS of the microsymbiont has been shown to promote infection in the case of the R. leguminosarum bv trifolii-white clover (Trifolium repens) symbiosis-and to cause the production of novel proteins (Dazzo et al, 1991). The principle of the ability of rhizobial LPS to suppress plant defense responses in the host was achieved by demonstrating the suppression effect on elicitor-induced oxidative burst (Albus et al, 2001).…”

Genetic Characterization of a Sinorhizobium meliloti Chromosomal Region Involved in Lipopolysaccharide Biosynthesis

“…One of the earliest events in the symbiotic dialog involves the secretion of flavonoid compounds by the host plant and the subsequent production of nodulation (Nod) factors by the rhizobia (38). Although the Nod factors are the best-characterized signal molecules of rhizobia, independent evidence supports the view that some of the bacterial surface polysaccharides are also active in signaling the plant (2,16,19,22,39,40,57,60). It has thus been shown that the pretreatment of alfalfa roots with specific fractions of Sinorhizobium meliloti exopolysaccharides (EPS) conferred on symbiosis-deficient EPS mutants the ability to develop nitrogen-fixing nodules at a significant rate (2,22,57,60).…”

Lipopolysaccharides as a communication signal for progression of legume endosymbiosis

“…Nodulation is still delayed, but the induction of aberrant structures is completely suppressed in the presence of LPS, and, upon release, the nifH-expressing bacteria can divide efficiently, completely fill the infected cells, and provide the plant with nitrogen. Although effects of purified LPSs on root hairs and the efficiency of wild-type nodulation have been reported (32), our data demonstrate a functional extracellular complementation of an LPS mutant. A signaling function in nodule development has been proposed for LPS, mainly as a suppressor of plant defense (6,32).…”

“…Although effects of purified LPSs on root hairs and the efficiency of wild-type nodulation have been reported (32), our data demonstrate a functional extracellular complementation of an LPS mutant. A signaling function in nodule development has been proposed for LPS, mainly as a suppressor of plant defense (6,32). Striking similarities exist between the ORS571-oac2 nodules and the indeterminate nodule phenotypes induced by LPS Ϫ mutants of S. meliloti (33,34) and Rhizobium leguminosarum bv.…”