Succinoglycan Is Required for Initiation and Elongation of Infection Threads during Nodulation of Alfalfa by Rhizobium meliloti

Abstract

Rhizobium meliloti Rm1021 must be able to synthesize succinoglycan in order to invade successfully the nodules which it elicits on alfalfa and to establish an effective nitrogen-fixing symbiosis. Using R. meliloti cells that express green fluorescent protein (GFP), we have examined the nature of the symbiotic deficiency of exo mutants that are defective or altered in succinoglycan production. Our observations indicate that anexoY mutant, which does not produce succinoglycan, is symbiotically defective because it cannot initiate the formation of infection threads. An exoZ mutant, which produces succinoglycan without the acetyl modification, forms nitrogen-fixing nodules on plants, but it exhibits a reduced efficiency in the initiation and elongation of infection threads. An exoHmutant, which produces symbiotically nonfunctional high-molecular-weight succinoglycan that lacks the succinyl modification, cannot form extended infection threads. Infection threads initiate at a reduced rate and then abort before they reach the base of the root hairs. Overproduction of succinoglycan by theexoS96::Tn5 mutant does not reduce the efficiency of infection thread initiation and elongation, but it does significantly reduce the ability of this mutant to colonize the curled root hairs, which is the first step of the invasion process. TheexoR95::Tn5 mutant, which overproduces succinoglycan to an even greater extent than theexoS96::Tn5 mutant, has completely lost its ability to colonize the curled root hairs. These new observations lead us to propose that succinoglycan is required for both the initiation and elongation of infection threads during nodule invasion and that excess production of succinoglycan interferes with the ability of the rhizobia to colonize curled root hairs.

LysM domains mediate lipochitin–oligosaccharide recognition and Nfr genes extend the symbiotic host range

“…Like the DZL strain on transgenic M. truncatula, several lipopolysaccharide-deficient (lps) mutants of S. melitoti induce infection threads penetrating the core nodule tissue and a Fix À phenotype (Campbell et al, 2003), but in contrast to the DZL strain, the lps mutants are usually endocytosed (Niehaus et al, 1998). The early arrest of infection thread growth observed in M. loti-inoculated transgenic M. truncatula resembles the phenotype reported for exopolysaccharide-deficient (exoÀ) S. meliloti mutants in wild-type alfalfa (Cheng and Walker, 1998). On the plant side, lectins were shown to be involved in host range presumably by recognizing bacterial exopolysaccharides (Diaz et al, 1989;van Rhijn et al, 2001).…”

The Pel and Psl polysaccharides provide Pseudomonas aeruginosa structural redundancy within the biofilm matrix

“…2). This is not surprising, as many polysaccharides are involved in the attachment of various bacterial species to abiotic surfaces (like plastic and glass), plant and mammalian surfaces (Cheng and Walker, 1998;Wang et al, 2004;Ledeboer and Jones, 2005). However, Pel did not appear to be an important attachment determinant.…”

Efficient rhizosphere colonization by Pseudomonas fluorescens f113 mutants unable to form biofilms on abiotic surfaces

“…Plasmids pDSK-GFPuv (Wang et al, 2007) or pHC60 (Cheng and Walker, 1998) were introduced in P. fluorescens strains by triparental mating and green fluorescence due to the presence of gfp was visualized by epifluorescence microscopy. A modification of the method of Sternberg and Tolker-Nielsen (Gjermansen et al, 2005) was used for biofilm formation under flow conditions, using LB diluted 1:10 as growth medium.…”