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International audienceNitrogen fixation in legume nodules has been shown to be very sensitive to drought and other environmental constraints. It has been widely assumed that this decline in nitrogen fixation was a consequence of an increase in the so-called oxygen diffusion barrier and a subsequent impairment to bacteroid respiration. However, it has been recently shown that nitrogen fixation is highly correlated with nodule sucrose synthase (SS) activity under drought and other environmental stresses. Whether this correlation reflects a causative relationship or not has not been proven yet. The evidence presented here suggests that SS controls nitrogen fixation under mild drought conditions. However, nitrogen fixation cannot be enhanced only by increasing glycolytic flux, as under these conditions nodules become oxygen limited. Abscisic acid also induces a decline in nitrogen fixation that is independent of SS. The overall results suggest the occurrence of a complex regulation of nodule nitrogen fixation involving, at least, both carbohydrate and oxygen fluxes within the nodule.Vers la connaissance de la régulation de la fixation d'azote par les nodosités des légumineuses : enseignements tirés de la sécheresse, de l'acide abscissique et de l'accroissement de la disponibilité des photo-assimilats. La fixation d'azote par les nodosités des légumineuses est très sensible à la sécheresse ainsi qu'aux autres contraintes environnementales. Il était généralement supposé que la diminution de la fixation d'azote était due à un renforcement de la barrière à la diffusion de l'oxygène dans les nodosités qui entraîne une diminution de la respiration des bactéroïdes. Cependant, il a été montré récemment que la fixation de l'azote est fortement corrélée à l'activité de la saccharose synthase (SS) des nodosités de plants soumis à la sécheresse ou à d'autres contraintes environnementales. Que cette corrélation corresponde à une relation causale ou non, n'a pas encore été prouvé. Les résultats présentés ici suggèrent que la SS peut contrôler la fixation d'azote dans des conditions de sécheresse peu sévères. Cependant, la fixation de l'azote ne peut pas être accrue seulement par l'augmentation du flux de glucides vers les nodosités, car dans ces conditions l'oxygène devient un facteur limitant. Par ailleurs, l'acide abscissique induit également une limitation de la fixation d'azote indépendante de la SS. L'ensemble des résultats expérimentaux suggère ainsi l'existence d'un système complexe de régulation de la fixation d'azote qui dépend à la fois des flux de glucides et d'oxygène dans les nodosités

Imazamox and glyphosate represent two classes of herbicides that inhibit the activity of acetohydroxyacid synthase in the branched-chain amino acid biosynthesis pathway and the activity of 5-enolpyruvylshikimate-3-phosphate synthase in the aromatic amino acid biosynthesis pathway, respectively. However, it is still unclear how imazamox and glyphosate lead to plant death. Both herbicides inhibit amino-acid biosynthesis and were found to induce ethanol fermentation in plants, but an Arabidopsis mutant deficient in alcohol dehydrogenase 1 was neither more susceptible nor more resistant than the wild-type to the herbicides. In this study, we investigated the effects of the amino acid biosynthesis inhibitors, imazamox and glyphosate, on the pyruvate dehydrogenase bypass reaction and fatty acid metabolism in A. thaliana. We found that the pyruvate dehydrogenase bypass was upregulated following the treatment by the two herbicides. Our results suggest that the Arabidopsis aldehyde dehydrogenase 7B4 gene might be participating in the pyruvate dehydrogenase bypass reaction. We evaluated the potential role of the aldehyde dehydrogenase 7B4 upon herbicide treatment in the plant defence mechanism. Plants that overexpressed the ALDH7B4 gene accumulated less soluble sugars, starch, and fatty acids and grew better than the wild-type after herbicide treatment. We discuss how the upregulation of the ALDH7B4 alleviates the effects of the herbicides, potentially through the detoxification of the metabolites produced in the pyruvate dehydrogenase bypass.

tamate synthase and aspartate aminotransferase) were also affected by water-deficit stress. Two experiments were carried out to investigate the effects of water-deficit stress on carbon and nitrogen Key words: Pisum sativum, water stress, nitrogen metabolmetabolism of Pisum sativum nodules. In the first ism, nodule metabolism, pea, sucrose synthase. experiment, leaf y w was allowed to reach −1.0 MPa over a period of 14 d whilst in the second experiment −1.5 MPa was reached during the same time period.

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