Rhizobium meliloti lipooligosaccharide nodulation factors: different structural requirements for bacterial entry into target root hair cells and induction of plant symbiotic developmental responses.

Abstract

Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factors are O-acetylated and N-acylated by specific Cl6-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhlbited a significantly reduced capacity to elicit infection thread (IT) formation ln alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodFlnodL mutants were unable to penetrate into legume hosts and to form ITs. Nevertheless, these mutants induced widespread cell wall tip growth in trichoblasts and other epidermal cells and were also able to elicit cortical cell activation at a distance. NodRm factor structural requirements are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses.

Isolation and Characterization of Nitrogen Fixing Rhizobia from Cultivated and Uncultivated Soils of Northern Tanzania

“…The rhizobium cell walls are composed of varying polysaccharide which provides another opportunity for plant partners choice using the ''lock and key'' cascade that determines the degree of plant and bacterial specificity [75]. Some researchers have reported that rhizobia of different genera can infect the same plant species but some plant species can strictly be infected by rhizobia from only one specific genera [78] [79].…”

Analysis of ENOD40 expression in alb1, a symbiotic mutant of Lotus japonicus that forms empty nodules with incompletely developed nodule vascular bundles

“…It has been demonstrated that Nod factors produced by rhizobial nod genes provoke initial plant responses leading to the formation of nodule primordia (Heidstra and Bisseling 1996). Formation of infection threads is known to be involved in the Nod factor-mediated signalling pathway (Ardourel et al 1994). In addition, bifurcated vascular bundles with abundant expression of ENOD40 are found to be induced by Nod factors alone (Stokkermans 1994;Minami et al 1996).…”

Lipo-chitooligosaccharidic nodulation factors and their perception by plant receptors

“…Nod factor signaling leading to root hair deformation, epidermal marker gene activation, and cortical cell divisions (CCD) in Medicago spp. depends on the presence of a sulfate group at the reducing glucosamine residue on the NodSm factor, whereas additional modifications at the nonreducing end of the backbone (O-acetylation and the nature of the acyl chain) are essential only for the infectionrelated events, including formation of shepherd's crooks, infection threads and repolarization of the cytoskeleton of root hairs [37]. This strict dependency on Nod factor structure is also seen for the formation of infection threads and nodule development from cortical cell division foci in pea (Pisum sativum) [38].…”

“…It was therefore hypothesized that, in Medicago spp. and pea, two complementary perception systems operate, one triggering a signaling cascade which leads to the formation of CCDs (Bsignaling^), and the second controlling the intracellular accommodation of rhizobia (Bentry^) [37].…”

“…Due to the specific role of HCL in rhizobial infection and the Nod factor structure-dependent phenotype of one of the hcl mutants, the corresponding gene could encode an Bentry receptor^while NFP could correspond to a less stringent Bsignaling receptor^ [36,37]. Synteny between the genetically well characterized SYM2 locus, which controls the formation of infection threads in pea, and chromosome 5 of M. truncatula led to the identification of an orthologous genomic region that contains, amongst others, 7 genes encoding LysM-RLKs, representing prime candidates for the entry receptor of M. truncatula [38,39].…”