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.

Molecular Basis of Symbiotic Promiscuity

Plant growth promoting bacteria (PGPR) associations range in degree of bacterial proximity to the root and intimacy of association. In general, these can be separated into extracellular PGPR (ePGPR), existing in the rhizosphere, on the rhizoplane or in the spaces between cells of the root cortex, and intracellular PGPR (iPGPR), which exist inside root cells, generally in specialized nodular structures. The latter includes rhizobia and Frankia species, both of which fix nitrogen in symbiosis with higher plants. There has been considerable development in understanding signaling mechanisms of rhizobia (iPGPR) during the establishment of the rhizobia–legume symbiosis, and this may serve as a model of knowledge regarding cross-talk and plant growth promoting mechanisms. We provide a detailed review of this process, including plant-to-bacteria signal molecules, followed by bacterial perception and consequent production of bacteria-to-plant signals. A history of PGPR discovery is also provided, indicating progress in understanding each of the PGPR groups. Recent advances in understanding plant growth responses to microbial signals are reviewed, along with the research areas that require attention. Based on new understandings of signaling mechanisms in the iPGPR (rhizobia) and recent findings with ePGPR we are able to speculate regarding general patterns of signaling in the ePGPR.

Intracellular and extracellular PGPR: commonalities and distinctions in the plant–bacterium signaling processes

Rhizobium-legume nodulation: life together in the underground.

BIOCHEMISTRY OF POL YKETIDE SYNTHESIS: STRUCTURE OF THE SYNTHASES. . . . . . .. . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 40 THE GENES FOR FATTY ACID AND POLYKETIDE BIOSyNTHESIS. . . . . . . . . ...... 44 Fatty Acid Biosynthesis in Escherichia coli. . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . .. . . . . . 44 Polyketide Antibiotic Biosynthesis by Actinomycetes. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Putative Polyketides Involved in Nodulation by Rhizobium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Anthocyanin and Phytoalexin Biosynthesis in Flowering Plants. . . . . . . . . . . . . . . . . . . . . . . . . 49 MOLECULAR GENETICS OF FATTY ACID AND POLYKETIDE BIOSYNTHESIS. . . . . . . . . . . . . . . . ... . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . ... . . . . . . 50 Vertebrate Type I FAS Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 50 The Fungal Type I FAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 The Type I PKS of Penicillium patulum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 The Type II FAS of E. coli . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . 53 Type II Plant FAS Systems .... . . . . . . . . . . . . . . ....... . . . . . . . . .. . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . 54 Type 11 PKS Systems in Streptomyces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SS Macrolide Antibiotic Biosynthesis and the Module Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Plant Chalcone and Resveratrol Synthases . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 58 SPECIFICITY OF FAS AND PKS SYSTEMS: THE PROBLEM OF .. PROGRAMMING". . . . . . . . . . . . . . . . ... . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. 59

Molecular Genetics of Polyketides and Its Comparison to Fatty Acid Biosynthesis

SUMMARY
Many legumes respond to Rhizobium inoculation by developing unique structures known as nodules on their roots. The development of a legume nodule in which rhizobia convert atmospheric N2 into ammonia is a finely tuned process. Gene expression from both partners of the symbiosis must be temporally and spatially coordinated. Exactly how this coordination takes place is an area of intense study. Nodule morphogenesis appears to be elicited by at least two distinct signals: one from Rhizobium, a product of the nod genes (Nod factor), and a second signal, which is generated within plant tissues after treatment with Nod factor. The identity of the second signal is unknown but changes in the balance of endogenous plant hormones or the sensitivity of plant tissues to these hormones are likely to be involved. These hormonal changes may be triggered by endogenous flavonoids produced by the root in response to inoculation with Rhizobium. There is some controversy as to whether the legume nodule is an organ sui generis or a highly derived lateral root. A resolution of this question may become more critical as attempts to induce nodules on non-legume hosts, such as rice or maize, increase in number and scope.

/pdf/developmental-biology-of-legume-nodulation-3ol366huf6.pdf

Developmental biology of legume nodulation

https://www.cell.com/cell/pdf/0092-8674(86)90654-9.pdf

Organization, structure and symbiotic function of rhizobium meliloti nodulation genes determining host specificity for alfalfa

THE enzyme system known as nitrogenase is responsible for conversion of elementary nitrogen in microbial-plant symbiotic nitrogen fixing associations. A method has been developed for the isolation of nitrogenase mutants of Rhizobium meliloti for the study of genes regulating the synthesis of nitrogenase. This type of ineffective mutant cannot be selected directly since nitrogenase is only synthesised in bacteroids within the plant host.

Ilona Barabas

Papers

Organization, structure and symbiotic function of rhizobium meliloti nodulation genes determining host specificity for alfalfa

Evidence for common genetic determinants of nitrogenase and nitrate reductase in Rhizobium meliloti.