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Summary The exchange of chemical signals between soil bacteria (rhizobia) and legumes has been termed a molecular dialogue. As initially conceived in the early 1990s, it involved two main groups of molecules: nod gene‐inducing flavonoids from plants and the mitogenic lipochito‐oligosaccharide Nod factors of rhizobia. This review considers how subsequent research revealed the existence of a more complex set of interactions, featuring expanded roles for the original participants and contributions from additional plant and bacterial metabolites. Rhizobia respond to chemoattractants and growth‐enhancing compounds in root exudates, and several plant nonflavonoids possess nod gene‐inducing properties. Expression of non‐nod genes is induced by flavonoids; these include encoders of a type I secreted protein and the entire type III, and possibly also type IV, secretion systems. Many other genes and proteins in rhizobia are flavonoid‐inducible but their functions are largely unknown. Rhizobia produce far more Nod factor variants than was previously envisaged and their structures can be influenced by the pH of the environment. Other symbiotically active compounds or systems of rhizobia, some of them universally present, are: the surface polysaccharides, quorum‐sensing N‐acyl homoserine lactones, plant growth‐promoting lumichrome and two‐component regulatory systems.

Since the first volume of Bergey's Manual of Systematic Bacteriology was published, in 1984, two additional genera and several new species of stem-and root-nodulating bacteria have been proposed; further changes to the taxonomy of this group of organisms appear likely. This paper briefly reviews the current status of "Rhizobium" taxonomy and proposes minimal standards for the description of future genera and species belonging to this group of organisms. YThe taxonomy of the root-and stem-nodulating bacteria of legumes is in a state of transition. The classification of these organisms based on plant infection (3, 21) has been abandoned after extensive criticism (23, 78) and the demonstration that the genes coding for nodulation, host specificity, and nitrogen fixation in fast-growing rhizobia are located on transmissible symbiotic plasmids (6, 48, 53). Introduction of Adansonian and holistic approaches (10, 12, 23) led initially to a consolidation of species but more recently has resulted in the description of additional genera and species. In some cases, this has led to confusion as to the proper terminology for these organisms and how they should be distinguished.One of the functions of the subcommittees of the International Committee of Systematic Bacteriology is to recommend minimal standards for the valid publication of new taxa (38) and so to avoid the situation in which the literature includes many inadequately described bacteria for which no type strain is available. In this paper, the members of the International Subcommittee for the Taxonomy of Rhizobium and Agrobacterium briefly review recent developments in the taxonomy of root-and stem-nodulating bacteria and then propose minimal standards for their description.Current taxonomy of the root-nodule bacteria of legumes. After reviewing data on the numerical taxonomy, DNA mole percent G + C values, nucleic acid hybridizations, cistron similarities, serological relationships, extracellular polysaccharide composition, carbohydrate utilization patterns, and metabolic capacities, antibiotic sensitivities, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) protein-banding patterns, and speed of growth of rhizobia on laboratory media, Jordan (31, 32) separated the root-nodule bacteria into two genera, Rhizobium and Bradyrhizobium. Those organisms previously designated R . leguminosarum, R . trifolii, and R . phaseoli were combined into a single species, R. leguminosarum, with three biovar designations. Two other species, R. meliloti and R . loti, were listed in this genus, but slowly growing nodule bacteria were transferred to the genus Bradyrhizobium, with the designation of a single * Corresponding author. species, B . japonicum. This classification was clearly an interim one in that (i) it was based on rhizobia collected from only 15% of the 19,700 species of Leguminosae (1) and so was unlikely to be representative, (ii) it failed to consider the recently identified fast-growing rhizobia from soybean (34), and (iii) it grouped many slowly ...

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Gas chromatographic and mass spectrometric analyses of derivatized culture medium extracts were used to identify the products of flavonoid metabolism by rhizobia. A number ofRhizobium species and biovars degraded their nod gene-inducing flavonoids by mechanisms which originated in a cleavage of the C-ring of the molecule and which yielded conserved A-and B-ring products among the metabolites. In contrast, Pseudomonas putida degraded quercetin via an initial fission in its A-ring, and Agrobacterium tumefaciens displayed a nonspecific mode of flavonoid degradation which yielded no conserved A-or B-ring products. When incubated with rhizobia, flavonoids with OH substitutions at the 5 and 7 positions yielded phloroglucinol as the conserved A-ring product, and those with a single OH substitution at the 7 position yielded resorcinol. A wider range of structures was found among the B-ring derivatives, including p-coumaric, p-hydroxybenzoic, protocatechuic, phenylacetic, and caffeic acids. The isoflavonoids genistein and daidzein were also degraded via C-ring fission by Rhizobium fredii and Rhizobium sp. strain NGR234, respectively. Partially characterized aromatic metabolites with potential nod gene-inducing activity were detected among the products of naringenin degradation by Rhizobium leguminosarum bv. viciae. The initial structural modification of nod gene-inducing flavonoids by rhizobia can generate chalcones, whose open C-ring system may have implications for the binding of inducers to the nodD gene product.Flavonoids are polyphenolic secondary metabolites which are synthesized by plants via the expression of two multigeneencoded enzymes: phenylalanine ammonia lyase and chalcone synthase. Subgroups of compounds such as chalcones, flavanones, flavones, flavonols, and isoflavonoids occur in legume tissues, and they can be released (15) from roots into the rhizosphere, where some of them act as molecular signals to trigger the establishment of symbioses with bacteria in the family Rhizobiaceae (18,19). Their principal function is to interact with the nodD gene products of rhizobia and the subsequent transcriptional activation of other nod genes (17). Other effects of flavonoids on rhizobia include promotion of chemotactic responses (1) and stimulation of growth rate by unspecified mechanisms (8).Although rhizobia are known to utilize various aromatic compounds as carbon and/or energy sources by degrading them to catechol and protocatechuate and channelling these products after further enzymatic cleavage into the tricarboxylic acid cycle via the 0-ketoadipate pathway (3, 16), their capacity to degrade flavonoids has received little attention. Only two examples have been reported: the utilization of catechin by a Rhizobium sp. isolated from Leucaena leucocephala, with attendant formation of phloroglucinol carboxylic acid and protocatechuate (6), and a novel form of C-ring cleavage in a pentahydroxy flavone, quercetin, by Rhizobium loti (20). One report (4) has described an alteration in the types and amounts of formon...