The interaction between hydrogenase activity and carbon metabolism in nitrogen-fixing legume root nodules

Borthakur, Pritty B (1986). The interaction between hydrogenase activity and carbon metabolism in nitrogen-fixing legume root nodules. The Open University.



Rhizobium and Bradyrhizobium species fix atmos¬pheric nitrogen (N2) in symbiotic association with legumes. The endosymbiotic bacteroid-forms contain the nitrogenase enzyme complex which catalyzes the reduction of Ng to produce ammonia (NH3) using energy derived from the respiration of photosynthate supplied by the host plant. Hydrogen is also evolved as a by-product of the nitrogenase reaction and at least 25% of the energy is wasted through H2 evolution. However, some strains of rhizobia possess a hydrogen oxidising enzyme called ’uptake hydrogenase’ (Hup), that can recycle the H2 evolved as a by-product of N2 fixation and regenerate energy in the form of ATP.

This dissertation considers three topics relating to the physiology of symbiotic N2 fixation (i) what is the energy cost of symbiotic N2 fixation for the whole plant, and how significant are the losses associated with H2 evolution ? (ii) what factors limit the rate and efficiency of symbiotic N2 fixation ? (iii) Does the hydrogen recycling (uptake hydrogenase) system confer any advantages to the bioenergetics of symbiotic N2 fixation ? The scientific literature in each of these areas is reviewed, and some suggestions made concerning future experiments which might help to investigate the contribution of the uptake hydrogenase system to the energetics of symbiotic N2 fixation.

Rhizobia are obligate aerobes but the nitrogenase system is very sensitive to oxygen damage. Consequ¬ently, the root nodule is maintained as a microaerobic environment for the N2 fixing bacteroids. The O2 supply to the centre of nodules appears to be tightly regulated and in some cases the availability of O2 may limit the rate of respiration and hence the energy available for N2 fixation. In addition, the carbon supply to bacteroids is a major factor limiting the activity of root nodules. Environmental factors suchas fixed nitrogen in the soil, other mineral nutrients, temperature and moisture, etc, also affect nodulation and N2 fixation by rhizobia.

Experiments conducted with Hup+ and Hup- isogenic strains show that the Hup system is beneficial in the soybean-Bradyrhizobium symbiosis. In the pea-Rhizobium symbiosis the beneficial role of uptake hydrogenase is not yet established and more experiments are needed to determine the role of Hup in this symbiosis. Four future experiments are suggested.

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