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 C16-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhibited a significantly reduced capacity to elicit infection thread (IT) formation in alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodF/nodL 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.

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…Aberrant, abortive infection threads are formed when Nod factor is supplied in a structurally incomplete form, or if Nod factor perception is disrupted 18,19,42 . An S. meliloti nodF nodL double mutant that produces Nod factor that lacks the acetyl substituent and has an altered acyl chain cannot effectively invade plant cells, and induces approximately 20% of colonized root hairs to form infection threads 19,42 . Those infection threads that are formed are aberrant and 'sac-like', and abort before reaching the base of the root hair cell 19 (FIG.…”

Nod factor‐induced host responses and mechanisms of Nod factor perception

“…Interestingly,, in the interaction with the noc/Fi? mutuant it was shown that, once the infection thread has passed the epidermis of the alfalfa root, infection proceeds normally (Ardourel et al, 1994). This indicates that in alfalfa also the control receptor is only active in the epidermis.…”

“…The activity in depolarization of NodRm-IV(C18: 1,8), carrying a saturated acyl chain, was severely reduced compared with that of NodRm-IV (C16:2,S), carrying the unsaturated acyl group specific for R. meliloti (Felle et aL, 1995), Although 0-acetylation of the nonreducing end sugar had no effect on the maximum of depolarization, the response to the O-acetylated factor was initially faster. Furthermore, markedly fewer infection threads were initiated by R. meliloti mutated in nodE or nodL which produces Nod factors that do not contain the appropriate unsaturated fatty acid or the O-acetyl group, respectively (Ardourel et aL, 1994), A strain mutated both in nodE and nodL, secreting Nod factors lacking the O-acetyl group and containing an inappropriate fatty acid had completely lost the abilitj' to induce infection threads (Adourel et aL, 1994), Substitutions at the reducing end of the Nod factors can have a dramatic effect on the activity to induce the epidermal responses. For example, desulphation of the NodRm factors reduces at least 1000-fold their activity to induce root hair deformation, ENOD12 expression and membrane potential depolarization on alfalfa (Journet et aL, 1994;Felle et aL, 1995), In addition, R. meliloti strains mutated in nodH, producing only non-sulphated NodRm factors, are unable to induce infection thread formation in alfalfa .…”

“…The bacterial nod genes are expressed during the whole infection process (Sharma & Signer, 1990; Schlaman et aL, 1991), These data suggest that after the second day Nod factor recognition still occurs and that a continuous activation of a receptor by Nod factor is required to allow the infection to progress. Therefore, it is very unlikely that Sym2 is the Nod factor receptor involved in inducing infection (uptake receptor, Ardourel et aL, 1994), It is plausible that another Nod factor receptor is involved and that this receptor, the signalling receptor, might induce both infection thread formation and deformation. When this hypothesis is correct and the signalling receptor is active in the epidermis, infection thread formation is induced in the epidermis of sym2 peas by R.L bv, viciae nodX^ strains.…”

“…This led Ardourel et al (1994) to propose the hypothesis that at least two different Nod factor receptors are present in the epidermis: a 'signalling' receptor and an 'uptake' receptor. The signalling receptor recognizes Nod factors even if the non-reducing end is modified and, upon binding, activates the induction of, for example, root hair deformation.…”