Keys to Symbiotic Harmony

Abstract

At least three different sets of symbiotic signals (here, they are compared to locks and keys) are exchanged between legumes and rhizobia during nodule development. Flavonoids, the first of these, emanate from the plant and interact with rhizobial NodD proteins that serve as both environmental sensors and activators of transcription. A second set of signals is synthesized when NodD-flavonoid complexes activate transcription from nod boxes. Most of the genes immediately downstream of these promoters are involved in the synthesis of lipooligosaccharidic Nod factors that provoke deformation of root hairs and allow rhizobia to enter the root through infection threads. Fine-tuning of transcription of nodulation (nod) genes is probably related to sequence variations in individual nod boxes (there are 19 on the symbiotic plasmid of the broad-hostrange Rhizobium sp. strain NGR234). Other rhizobial products seem to be necessary for continued infection thread development, and these represent a third set of signals. Among them are extracellular polysaccharides (EPS) and related compounds, as well as proteins exported by the type three secretion system (TTSS). In the latter case, flavonoids also activate protein secretion, suggesting that the same keys can unlock different doors.Symbioses are nearly ubiquitous, and many are also persistent (56). Mutualistic, nitrogen-fixing associations between members of the plant family Leguminosae, and the soil bacteria Azorhizobium, Bradyrhizobium, Mesorhizobium, and Rhizobium (collectively called rhizobia) contribute substantially to plant productivity (111). Legume-Rhizobium symbioses are marriages between two vastly different genomes. In rhizobia, these include a chromosome plus zero to many plasmids, totalling 6 to 9 Mbp (71). In contrast, genomes of legumes are much larger, some comprising more than 20 chromosomes with total DNA contents that range from about 450 to 4,500 Mbp per haploid genome (1C) (1). As examples, the model legumes Lotus japonicus and Medicago truncatula have six or eight chromosomes totalling 450 and 500 Mbp/1C (14), respectively. Common beans (Phaseolus vulgaris) and related species (e.g., Vigna unguiculata) (both genera belong to the tribe Phaseoleae) have 11 chromosomes each (24) with DNA contents of 637 (1) and 540 Mbp/1C (50). Furthermore, several widely cultivated legumes such as Arachis hypogaea, Glycine max, and Medicago sativa are effectively polyploid, although in G. max most loci segregate as if they were diploid. Legume genomes are thus at least 50 times larger than those of their microsymbionts. Given this disparity, it is difficult to imagine that theirs is a marriage of equals. Nevertheless, their respective contributions are probably not vastly different (see reference 48). LEGUME BOWERS ARE DECORATED WITH FLAVONOIDSWhich partner initiates contact? Since plants are nonmotile, it is tempting to think that courtship begins when bacteria advance into the legume rhizosphere. Yet chemotaxis and motility among rhizobia are clearly not essential for no...

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

“…Recent papers have covered Nod factor structures, the synthesis of Nod factors, Nod factor signal transduction, and the relationship between host range and Nod factor structure. The reader is directed to previously published reviews (20,38,63,97,146,165,181,183) for a comprehensive discussion of these topics.…”

Phenolic acids act as signaling molecules in plant-microbe symbioses

“…71,72 Different aspects of the physiology of legume-Rhizobium symbiosis and mechanism of nitrogen fixation have been extensively investigated by various workers. 73,74 The establishment of the symbiosis requires signaling and recognition by both the partners; various signaling molecules are exchanged between the plant and the infecting bacteria to regulate nodule initiation, differentiation and functioning. Thus, many facets of plant-bacterium recognition, nodule formation and nitrogen fixation have been well studied, 75,76 but the mechanisms by which root nodules phenylpropanoids regulate the infection efficiency of rhizobia for nodule formation has not been studied in detail.…”

Role of Microsymbionts in Plant Microbe Symbiosis

“…Variable backbone length and substitutions at the reducing end also determine Nod factor host specificity [11]. Perception of Nod factor on plant root surface elicits a series of responses including root hair deformation, membrane potential depolarization, changes in ion fluxes, intracellular and extracellular alkalinization, induction of nodulin gene (ENOD) expression and nodule primordial development, which finally results in the mature nitrogen-fixing nodules formation [15].…”