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Drought is one of the most limiting factors on crop productivity under Mediterranean conditions, where the leguminous species alfalfa (Medicago sativa L.) is extensively cultivated. Whereas the effect of drought on plant performance has been widely described at leaf and nodule levels, less attention has been given to plant-nodule interactions and their implication on metabolites exchange during a regrowth period, when water is limiting. For this purpose, physiological characterization and metabolite profiles in different plant organs and nodules were undertaken under water deficit, including regrowth after removal of aerial parts. In order to study in more detail how nitrogen (N) metabolism was affected by water stress, plants were labelled with N-enriched isotopic air (15N2) using especially designed chambers. Water stress affected negatively water status and photosynthetic machinery. Metabolite profile and isotopic composition analyses revealed that, water deficit induced major changes in the accumulation of amino acids (proline, asparagine, histidine, lysine and cysteine), carbohydrates (sucrose, xylose and pinitol) and organic acids (fumarate, succinate and maleic acid) in the nodules in comparison with other organs. The lower 15N-labeling observed in serine, compared with other amino acids, was related with its high turnover rate, which in turn, indicates its potential implication in photorespiration. Isotopic analysis of amino acids also revealed that proline synthesis in the nodule was a local response to water stress and not associated with a feedback inhibition from the leaves. Water deficit induced extensive reprogramming of whole-plant C and N metabolism, including when the aerial part was removedto trigger regrowth., This study was supported in part by the European research project PERMED (INCO- CT-2004-509140), the Spanish Ministry of Science and Technology (CGL2009-13079-C02) and i-COOP-suelos y legumbres (2016SU0016) funded by the National Research Council. GM was the recipient of a doctoral fellowship (Programa Nacional de Formación de Personal Universitario, AP2005–2916) sponsored by the Spanish Ministry of Education and Science.

International audience; Currently, the effects of global warming are one of the most important topics on the agendas of all governments and international economic and scientific organisations on the planet. Temperatures and rainfall will be especially subjected to increasing deregulation, and thus crop yields will be affected according to geographic location. Cellulosic materials, such as bast fibres, are considered one solution to decrease human environmental impact: they are a renewable resource, biodegradable and have a lower carbon emission than synthetic materials. However, their quality, yield and mechanical properties depend on environmental conditions during plant growth. In this paper, we explored the possible impact of seasonal drought linked to future climate change on flax plants and fibre quality. Two batches of the same textile flax plant cultivar were grown under two different field environmental conditions in the same year, one taken as a control under regular climatic conditions and the second one grown under drought stress. Carbon isotopic discrimination reveal an increase in water stress plant of the fibre δ13C, reflecting that plants are indeed suffering from drought stress from a physiological point of view. We characterized the mechanical properties, biochemistry and morphology parameters at both the stem and technical fibre scales. Our results showed that the plants of the two batches were morphologically different and that the drought-stressed plants were smaller, mainly in terms of the height of the stem (−28%) and diameter (−16%). Biochemical analyses highlighted a contrasting lignin content between the two batches. A difference in protein content was also measured, with an increased amount in stressed flax plants, with contrasting distributions revealed by tyrosine and tryptophan monitored by synchrotron UV fluorescence. In addition, polysaccharide composition was also quantified with an increase in mannose and an important decrease in glucose in the drought-stressed technical fibres. Surprisingly, despite the difference in biochemistry composition and morphological parameters, the mechanical properties of elementary flax fibres extracted from the two batches were not significantly different. This suggests that drought can affect the yield and biochemistry of the extracted technical flax fibres but does not necessarily impact the longitudinal mechanical performance of single fibres.

Glutamine synthetase (GS, EC 6.3.1.2) catalyzes the production of glutamine from,glutamate, ammonium and ATP. Although being essential in plants for N assimilation and recycling, kinetic commitments and transition states of the reaction have not been clearly established yet. Here, we examined C-12/C-13,N-14/N-15 and H2O/D2O isotope effects in Arabidopsis GS1 catalysis and compared to the prokaryotic (Escherichia coli) enzyme. A(14)N/N-15 isotope effect (V-15/K approximate to 1.015, with respect to substrate NH4+) was observed in the prokaryotic enzyme, indicating that ammonium utilization (deprotonation and/or amidation) was partially rate-limiting. In the plant enzyme, the isotope effect was inverse (V-15/K = 0.965), suggesting that the reaction intermediate is involved in an amidation-deamidation equilibrium favoring N-15. There was no C-12/C-13 kinetic isotope effect (V-13/K = 1.000), suggesting that the amidation step of the catalytic cycle involves a transition state with minimal alteration of overall force constants at the C-5 carbon. Surprisingly, the solvent isotope effect was found to be inverse, that is, with a higher turn-over rate in heavy water (V-D approximate to 0.5), showing that restructuration of the active site due to displacement of H2O by D2O facilitates the processing of intermediates. (C) 2016 Elsevier Masson SAS. All rights reserved.

International audience; In plants, glycolate oxidase is involved in the photorespiratory cycle, one of the major fluxes at the global scale. To clarify both the nature of the mechanism and possible differences in glycolate oxidase enzyme chemistry from C3 and C4 plant species, we analyzed kinetic parameters of purified recombinant C3 (Arabidopsis thaliana) and C4 (Zea mays) plant enzymes and compared isotope effects using natural and deuterated glycolate in either natural or deuterated solvent. The (12)C/(13)C isotope effect was also investigated for each plant glycolate oxidase protein by measuring the (13)C natural abundance in glycolate using natural or deuterated glycolate as a substrate. Our results suggest that several elemental steps were associated with an hydrogen/deuterium isotope effect and that glycolate α-deprotonation itself was only partially rate-limiting. Calculations of commitment factors from observed kinetic isotope effect values support a hydride transfer mechanism. No significant differences were seen between C3 and C4 enzymes.