Molecular basis of symbiotic promiscuity

Abstract

Eukaryotes often form symbioses with microorganisms. Among these, associations between plants and nitrogen-fixing bacteria are responsible for the nitrogen input into various ecological niches. Plants of many different families have evolved the capacity to develop root or stem nodules with diverse genera of soil bacteria. Of these, symbioses between legumes and rhizobia (Azorhizobium, Bradyrhizobium, Mesorhizobium, and Rhizobium) are the most important from an agricultural perspective. Nitrogen-fixing nodules arise when symbiotic rhizobia penetrate their hosts in a strictly controlled and coordinated manner. Molecular codes are exchanged between the symbionts in the rhizosphere to select compatible rhizobia from pathogens. Entry into the plant is restricted to bacteria that have the "keys" to a succession of legume "doors". Some symbionts intimately associate with many different partners (and are thus promiscuous), while others are more selective and have a narrow host range. For historical reasons, narrow host range has been more intensively investigated than promiscuity. In our view, this has given a false impression of specificity in legume-Rhizobium associations. Rather, we suggest that restricted host ranges are limited to specific niches and represent specialization of widespread and more ancestral promiscuous symbioses. Here we analyze the molecular mechanisms governing symbiotic promiscuity in rhizobia and show that it is controlled by a number of molecular keys.

FIG. 1

Invasion of legume root hairs by Rhizobium. (A) Rhizobia (rh) colonize the rhizosphere and attach to the root hairs (r). (B) Opening the “outer door.” Nod factors induce root hair curling and permit bacterial penetration at the center of infection (ci). The plant nucleus (n) precedes the growing infection thread(s) (it). (C) Crossing the inner doors. Still accompanied by the nucleus (n) an elongating infection thread (it) reaches the base of the root hair cell. (D) A developing infection thread ramifies (rit) near the nodule primordia formed by dividing cortical cells. (E) Bacteroids (b) are released from the infection thread (it) and form symbiosomes (s) in nodule cells. Granules of poly-β-hydroxybutarate (phb) accumulate in bacteroids surrounded by the peribacteroid membrane (pb). Other abbreviations: c, cortex; d, digestive vacuole; ep, epidermis; ed, endodermis.

FIG. 2

Early steps in nodulation of legumes showing that continued development of infection threads is under the control of the nodD1 gene in Rhizobium sp. NGR234. (A) Fluorescent image of a Vigna unguiculata root hair inoculated with a mutant incapable of producing NodNGR factors (NGRΔnodABC) and concomitantly treated with 10⁻⁷ M NodNGR(S) factors. Arrows point to rhizobia (rh). (B) Commencement of curling of a root hair stained with methylene blue. Extreme curling leads to the formation of a bright spot (bs), where rhizobia are often entrapped (same treatment as in panel A). (C) A bright-field image of a root hair inoculated with NGRΔnodABC::GUS3, treated with 10⁻⁷ M NodNGR(S) factors, and stained for β-glucuronidase activity. Arrows point to the entrapped rhizobia within the curl. (D) Fluorescent image of a root hair curled in the shape of a shepherd's crook, showing the center of infection (ci). (E) Experiment in which the nodABC mutant was replaced with NGRΔnodD1::GUS3, but incubated with 10⁻⁷ M NodNGR(S) factors, and stained for β-glucuronidase activity. Apparently, the nodD1 mutant lacks a factor(s) that is necessary for the continued development of infection threads (it). In its absence, infection threads abort (ait), forming a structure that resembles a cerebellum (cb). (F) Photomicrograph of root hairs inoculated with wild-type NGR234 marked with GUS3. Infection threads develop along the length of the root hair (dit). B. Relić and W. J. Broughton (unpublished results; see reference for further details).

FIG. 3

Examples of the five classes of rhizobial polysaccharides. (A) Structure of cyclic β-(1,6)-β-(1,3)-glucans common to B. japonicum. Redrawn from reference . (B) Acidic EPS of Rhizobium sp. strain NGR234 (60). EPS I (succinoglycan) of R. meliloti resembles the EPS of strain NGR234. (C) KPS of R. leguminosarum bv. trifolii (98). (D) The somatic K antigen of R. fredii USDA257 (83). (E) Core structure of the LPS of R. etli (84). Abbreviations: Gal, galactose; GalA, galacturonic acid; Glc, glucose; GlcA, glucoronic acid; Man, mannose; Kdo, 3-deoxy-d-manno-2-octulosonic acid; OAc, acetate group; Pyr, pyridine.