Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

Chloroplast gene sequence data suggest a single origin of the predisposition for symbiotic nitrogen fixation in angiosperms.

Abstract: Of the approximately 380 families of angiosperms, representatives of only 10 are known to form symbiotic associations with nitrogen-fixing bacteria in root nodules. The morphologically based classification schemes proposed by taxonomists suggest that many of these 10 families of plants are only distantly related, engendering the hypothesis that the capacity to fix nitrogen evolved independently several, if not many, times. This has in turn influenced attitudes toward the likelihood of transferring genes responsible for symbiotic nitrogen fixation to crop species lacking this ability. Phylogenetic analysis of DNA sequences for the chloroplast gene rbcL indicates, however, that representatives of all 10 families with nitrogen-fixing symbioses occur together, with several families lacking this association, in a single clade. This study therefore indicates that only one lineage of closely related taxa achieved the underlying genetic architecture necessary for symbiotic nitrogen fixation in root nodules.

Methods for evaluating biological nitrogen fixation

Abstract: Research into nitrogen fixation has expanded rapidly during the last two decades. The approach has been multi-disciplinary, involving chemistry, biochemistry, physiology, microbiology, genetics, agronomy and ecology. Success has attended several fields, particularly the biochemistry and genetics. It is increasingly likely that future advances will be made at disciplinary interfaces, such as chemistry and biochemistry or biochemistry and genetics: for example, two unresolved problems are the chemical nature of the iron-molybdenum cofactor of nitrogenase, and the biochemical role of the 17 gene products of the nifoperon. This book is therefore timely and should be useful for many years. Being a practical text, it should take longer to date than research reviews. It is divided into three parts: a short historical introduction, followed by two major sections on laboratory and field techniques. The first three chapters on laboratory techniques include the growth of free-living diazotrophs in batch and continuous cultures, direct (measurements of N uptake) and indirect (mainly measurements of acetylene reduction) methods of assessing rates of nitrogen fixation. The fourth chapter is on greenhouse studies with nitrogen-fixing legumes, and the fifth is a shorter account of nitrogen-fixing symbioses in nonlegumes. Chapter 6 deals with the purification and assay techniques for nitrogenase proteins, with an emphasis on the necessary anaerobic precautions to avoid oxygen damage. Chapter 7 discusses assays for enzymes associated with nitrogen fixation: glutamine synthetase and other amino acid enzymes, nitrate reductase, nitrite reductase and hydrogenase. The next chapter is devoted to purification techniques and methods of studying leghaemoglobin, and the final two chapters of Section I1 describe methods for identifying strains of diazotrophs and techniques for genetic studies. Field methods (Section 111) involve the study of legumes, bacterial associations with grasses and grain crops, the production and control of legume inoculants and the ecology of natural plant communities and soils. The final chapter reviews methods for studying nitrogen fixation by blue-green algae (Cyanobacteria). Most of these chapters are comprehensive, with admirable emphasis on detail and how to avoid error. In the main the authors have stuck to their brief, to describe practical methods and not review the field. The text is well supported by excellent diagrams, Figures and Tables. The techniques described range from the imaginative use of plastic sandwich bags, beer and baby bottles (not mentioned) to extended X-ray absorption edge and fine structure spectroscopy (‘EXAFS’), and the hazards to overcome extend from microbial contamination to destruction of field trials by elephants. The book contains useful tips of general application, e.g. on the storage of bacteria and the use of a sucrose cushion to protect and ease resuspension of centrifuged material. It is also well referenced. Biochemical interest should centre on chapters 6-8, describing nitrogenase, associated enzymes and leghaemoglobin respectively. Nitrogenase is the most complicated iron-sulphur protein; that in itself is a challenge. However, of more general interest are the details on anaerobic techniques and the critical assessment of assays involving enzymes associated with nitrogen fixation. What is wrong with the book? Not much. except the price, which should restrict its sale to serious nitrogen fixers. There is some overlapping, e.g. seed sterilization and preparative methods are described twice and anaerobic techniques are emphasized repeatedly. I feel that spectroscopic techniques such as e.p.r., Mossbauer and EXAFS deserved a separate chapter, and physiological experiments with free-living diazotrophs could have been given more attention. However, these are carping criticisms; if you can afford it, buy it. M. G. YATES

Hydrogen evolution: A major factor affecting the efficiency of nitrogen fixation in nodulated symbionts.

Abstract: Nitrogenase-dependent hydrogen evolution from detached legume nodules and from reaction mixtures containing cell-free nitrogenase has been well established, but the overall effect of hydrogen evolution on the efficiency of nitrogen fixation in vivo has not been critically assessed. This paper describes a survey which revealed that hydrogen evolution is a general phenomenon associated with nitrogen fixation by many nodulated nitrogen-fixing symbionts. An evaluation of the magnitude of energy loss in terms of the efficiency of electron transfer to nitrogen, via nitrogenase, in excised nodules suggested that hydrogen production may severely reduce nitrogen fixation in many legumes where photosynthate supply is a factor limiting fixation. With most symbionts, including soybeans, only 40-60% of the electron flow to nitrogenase was transferred to nitrogen. The remainder was lost through hydrogen evolution. In situ measurements of hydrogen evolution and acetylene reduction by nodulated soybeans confirmed the results obtained with excised nodules. In an atmosphere of air, a major portion of the total electron flux available for the reduction of atmospheric nitrogen by either excised nodules or intact nodulated plants was utilized in the production of hydrogen gas. Some non-leguminous symbionts, such as Alnus rubra, and a few legumes (i.e., Vigna sinensis) apparently have evolved mechanisms of minimizing net hydrogen production, thus increasing their efficiency of electron transfer to nitrogen. Our results indicate that the extent of hydrogen evolution during nitrogen reduction is a major factor affecting the efficiency of nitrogen fixation by many agronomically important legumes.

Endophytic Colonization and In Planta Nitrogen Fixation by a Herbaspirillum sp. Isolated from Wild Rice Species

Abstract: Nitrogen-fixing bacteria were isolated from the stems of wild and cultivated rice on a modified Rennie medium. Based on 16S ribosomal DNA (rDNA) sequences, the diazotrophic isolates were phylogenetically close to four genera: Herbaspirillum, Ideonella, Enterobacter, and Azospirillum. Phenotypic properties and signature sequences of 16S rDNA indicated that three isolates (B65, B501, and B512) belong to the Herbaspirillum genus. To examine whether Herbaspirillum sp. strain B501 isolated from wild rice, Oryza officinalis, endophytically colonizes rice plants, the gfp gene encoding green fluorescent protein (GFP) was introduced into the bacteria. Observations by fluorescence stereomicroscopy showed that the GFP-tagged bacteria colonized shoots and seeds of aseptically grown seedlings of the original wild rice after inoculation of the seeds. Conversely, for cultivated rice Oryza sativa, no GFP fluorescence was observed for shoots and only weak signals were observed for seeds. Observations by fluorescence and electron microscopy revealed that Herbaspirillum sp. strain B501 colonized mainly intercellular spaces in the leaves of wild rice. Colony counts of surface-sterilized rice seedlings inoculated with the GFP-tagged bacteria indicated significantly more bacterial populations inside the original wild rice than in cultivated rice varieties. Moreover, after bacterial inoculation, in planta nitrogen fixation in young seedlings of wild rice, O. officinalis, was detected by the acetylene reduction and 15N2 gas incorporation assays. Therefore, we conclude that Herbaspirillum sp. strain B501 is a diazotrophic endophyte compatible with wild rice, particularly O. officinalis.