The turn of the millennium from the twentieth to the twenty-first century provides an occasion to review our understanding of a biological process, biological nitrogen fixation, that is of prime importance for the continued survival of mankind. This process has provided a basis for maintaining soil fertility since the beginning of organised agriculture, yet its very existence was confirmed only just over a century ago.
In the intervening years, an enormous intellectual effort has dispersed much of the mystery surrounding biological nitrogen fixation. Biological fixation is widely exploited in agriculture, as are nitrogen fertilisers prepared for the last hundred years under extreme conditions of temperature and pressure. However, despite all our efforts, the fundamental nature of the reactions involved at the heart of the biological process remain unknown. This book aims to describe what we have learned in the last one hundred years or so about biological nitrogen fixation, about what its chemistry appears to be, and how it is applied in agriculture.
This ambitious objective has not been attempted recently. It is aimed at students and those who wish to enter these very challenging areas of research, and who need to learn the state of the art at the turn of the millennium. The authors are all acknowledged world experts in their fields. They have prepared concise, well referenced and authoritative accounts of their subjects.
This book provides a unique summary of the current state of knowledge that will be indispensable to all students and researchers, actual and potential, interested in biological nitrogen fixation. Newton Introduction 1 3 N2-fixing organisms 4 2 Symbiotic associations 4 1 Associative symbioses 5 1 Associations with animals 6 1 Free-living N2-fixing bacteria 6 1 Nitrogenases 6 11 Molybdenum nitrogenase Mo-nitrogenase 7 3 Alternative nitrogenases 10 1 Streptomyces thermoautotrophicus 11 1 nitrogenase Mechanism of nitrogenase action 12 3 Role of MgATP in nitrogenase catalysis 15 1 Electron transport to nitrogenase 16 1 Site-directed mutagenesis applied to 17 1 nitrogenase Nitrogen-fixation nif genes 17 5 Nitrogen-fixation gene cluster 18 1 Proposed functions for the nif-gene 18 3 products NifA-NifL regulatory system 21 1 Chemical systems 22 4 Reactivity models -- in non-aqueous 22 2 systems Reactivity models -- in aqueous solution 24 1 Structural models 25 1 Summary and outlook 26 1 References 27 8 Nitrogenase Structure 35 38 Paul M.
Benton Sanchayita Sen John W. Rubio Paul W. Mayer Patricia dos Santos Lance C. Seefeldt Dennis R. Dean Introduction 1 Background 5 Insight into the effects of amino acid 3 substitutions around the FeMo-cofactor on substrate reduction Rationale and strategy for isolation of an 3 acetylene-resistant MoFe protein Expanding the capacity of nitrogenase to 2 reduce longer-chain alkynes Acetylene interaction with the MoFe protein 1 in the resting state Summary and outlook 1 Acknowledgements 1 References 3 Regulation of Mo Nitrogenases 12 P.
Rudnick C. Under greenhouse conditions as well, no significant yield differences were observed when the plants were grown on dinitrogen versus nitrate as a nitrogen source Gibson, This indicates that legumes using BNF rather than nitrate nitrogen suffer no obvious yield penalties. Urea and ammonium sulfate are normally used to fertilize rice. An important point to note here is that in spite of the greater energy requirement for nitrate assimilation compared to ammonia assimilation , rice yields are better when grown on nitrate combined with ammonia as compared to ammonia alone Xiaoe and Xi, ; Ancheng et al.
The fact that no yield penalty exists for rice grown on nitrate and ammonia rather than ammonia alone suggests that energy may not be limiting.
The ability of plants to compensate for extra energy consumption cannot be ignored, as photosynthetic systems saturate at relatively low light intensity. Nevertheless, since source and sink metabolisms are tightly coupled, it is reasonable to assume that the extra energy consumption by roots would stimulate the production of biomass in shoots. It is well established that equilibrium between photosynthetic sugar synthesis in the chloroplast-containing leaf cells source tissues and sugar consumption by roots, fruits and grains sink tissues must be maintained for sustaining plant growth and survival.
In plants under optimal light and at the normal carbon dioxide levels, sink limitation occurs when the rate of photosynthesis is limited by insufficient withdrawal of photosynthetic products generated in the green tissues through the Calvin—Benson cycle Sawada et al. It is intuitively envisaged that a nitrogen-fixing symbiosis in rice may be such a strong sink for photosynthate that yields would be impacted. Also, incidentally, in rice the actual grain yields are considerably lower than their maximum genetic potential.
Therefore, in planta nitrogen-fixing attribute may not significantly impact the present yield levels Ladha and Reddy, There has been a biotechnological interest to promote associative nitrogen fixation in non-legume crops that normally use large amounts of chemical fertilizers. Different nitrogen-fixing bacteria have been isolated from cereal roots by culture-dependent methods, and when used as plant inoculants they have varying degrees and strategies for plant growth promotion Kennedy et al. Some past efforts to increase nitrogen fixation in cereals by promoting pseudonodules with phytohormones failed.
Notably, recently obtained ammonium excreting mutants of some plant-associated diazotrophs were effective for promoting plant growth suggesting that they became capable of supplying nitrogen to their hosts.
Even though achieving genetically-modified nitrogen-fixing cereal crops is a complex process, the approaches that are being pursued at present are creating exciting possibilities for generating such plants in the foreseeable future. If so, the global environmental benefits of a reduced chemical fertilizer usage will be large, and we suppose that detrimental ecological consequences of nitrogen fixing cereals will be minimal.
Besides nitrogen, other agricultural inputs, such as phosphorus and water, may limit crop productivity. Mycorrhiza and plant cultivars with high phosphate use efficiency should be considered when developing nitrogen fixing cereals. However, we consider that not only the use of microbes and genetically modified plants will be required to achieve this goal, but a better crop management and efficient programs to control human population-growth are needed as well. JM-R contributed to the discussion, searched for references, and corrected the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. National Center for Biotechnology Information , U. Journal List Front Microbiol v. Front Microbiol. Published online Aug 9. Marco A. Julio C. Pallavolu M. Author information Article notes Copyright and License information Disclaimer.
This article was submitted to Microbial Symbioses, a section of the journal Frontiers in Microbiology. Received Mar 31; Accepted Jul The use, distribution or reproduction in other forums is permitted, provided the original author s and the copyright owner s are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
This article has been cited by other articles in PMC. Abstract Cereals such as maize, rice, wheat and sorghum are the most important crops for human nutrition. Keywords: rice, corn, wheat, sorghum, diazotrophic bacteria, root colonization, Rhizobium , Burkholderia. Introduction Cereals are grasses from the Poaceae family that were domesticated several thousand years ago in different geographical regions in order to take advantage of the edible components of their grain. Sources of Diazotrophic Bacteria Bacteria can get on to the plants either by root colonization from soil carryover, leaf litter Pfeiffer et al.
Open in a separate window. Culture-Independent Identification of Diazotrophs in Cereals By using a culture-independent approach, a better representation of existing diazotrophs may be obtained while the analysis of nifH transcripts has provided results on active diazotrophs. Diazotrophs May Be Human Pathogens It is not uncommon to isolate human or plant pathogens from plants and in many cases they are efficient growth promoting bacteria. Plants as Determinants of Bacterial Interactions Nitrogen fixation is highly variable depending on the associated diazotroph and the plant variety, but the host plant exerts a determinant effect by supplying the carbon and energy source for bacterial growth and nitrogen fixation.
Prospects for Engineering Cereal Plants Besides using associated bacteria to provide nitrogen to cereals other strategies involving the genetic modification of plants have been considered and are reviewed here.
Conclusion and Perspectives There has been a biotechnological interest to promote associative nitrogen fixation in non-legume crops that normally use large amounts of chemical fertilizers. Conflict of Interest Statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Purchase Nitrogen Fixation at the Millennium - 1st Edition. Print Book & E-Book. ISBN , The turn of the millennium from the twentieth to the twenty-first century Biological fixation is widely exploited in agriculture, as are nitrogen.
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