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Content archived on 2024-04-16



To get a better understanding of the relationships between oxygen, nitrogen and carbon metabolism in the nodule and the expression and activity of the nitrogen fixing apparatus of bacteroids and evaluate the possibilities to manipulate regulatory circuits and metabolic fluxes in order to increase the symbiotic nitrogen fixation potential.
The project has established a consortium to study symbiotic nitrogen fixation which occurs in the plant nodule. Molecular mechanisms involved in gene regulation, signal perception, and transduction are to be studied. The knowledge gained will permit strategies to be developed to genetically manipulate the system in order to modify the regulatory circuits and metabolic fluxes and eventually increase the symbiotic nitrogen fixation potential.

Studies currently being undertaken are in the following areas:
oxygen regulation of nitrogen fixation gene expression in R meliloti and R leguminosarum;
carbon metabolism and efficiency of nitrogen fixation;
bacterial nitrogen metabolism;
plant nitrogen metabolism and nitrogen fixation.

A coherent view of the influence of major physical parameters such as oxygen concentration; carbon and nitrogen metabolism on the regulation of nitrogen fixation gene expression is emerging. The comparison of two symbiotic systems leads to the view that response to a given regulatory signal can result from different combinations of related but nonidentical regulatory proteins. The significant influence of nitrogen source on nif gene expression could provide a link between plant nitrogen metabolism and symbiotic efficiency.

The two symbiotic systems that the project will help improve are of agricultural importance. A major environmental problem exists due to the extensive use of fertilizers in the agricultural sector. Much of the synthetic nitrogen applied to crops is lost from the soil leading to problems such as eutrophy in lakes and rivers and water pollution. Therefore besides reduction in costs resulting from reduced use of fertilizers symbiotic nitrogen fixation provides a buffered mineral nitrogen source which is less susceptible to being washed out into the ground water thus less harmful to the environment.
Symbiotic nitrogen fixation results from the activity of the nitrogenase complex in the differentiated bacteroids inside the plant nodule. optimizing the efficiency of nitrogen fixation requires detailed knowledge of the various factors affecting the physiology of bacteroids and their interaction with the host plant cell. During recent years three physiological parameters have emerged as major determinants of the activity of nitrogen fixing bacteroids:1) reduced oxygen tension as a key signal for inducing the coordinate expression of the bacterial genes required for nitrogen fixation ; 2) carbon metabolism and especially dicarboxylic acid transport and utilisation which provides the energy required for nitrogen reduction ;3) nitrogen status of the host plant as part of a feed back control mechanism. We propose to identify the key molecules which respond to these physiological parameters and elucidate the regulatory pathways through which they operate as well as their possible interrelationships. We will use this knowledge to engineer strains and plant lines which will be tested as potential useful inoculants and improved symbiotic hosts. These studies Will be carried out on two main symbiotic systems of agricultural importance in Europe: Rhizobium meliloti alfalfa, Rhizobium leguminosarum bean-pea. Comparative studies will allow us to point out the general mechanisms and also differences which could be related to differences in physiological characteristics of each symbiotic system.


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Chemin de Borde-Rouge INRA

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Participants (5)