Evaluation of GC/MS-Based 13C-Positional Approaches for TMS Derivatives of Organic and Amino Acids and Application to Plant 13C-Labeled Experiments

Low Nitrogen Input Mitigates Quantitative but Not Qualitative Reconfiguration of Leaf Primary Metabolism in Brassica napus L. Subjected to Drought and Rehydration

In the context of climate change and the reduction of mineral nitrogen (N) inputs applied to the field, winter oilseed rape (WOSR) will have to cope with low-N conditions combined with water limitation periods. Since these stresses can significantly reduce seed yield and seed quality, maintaining WOSR productivity under a wide range of growth conditions represents a major goal for crop improvement. N metabolism plays a pivotal role during the metabolic acclimation to drought in Brassica species by supporting the accumulation of osmoprotective compounds and the source-to-sink remobilization of nutrients. Thus, N deficiency could have detrimental effects on the acclimation of WOSR to drought. Here, we took advantage of a previously established experiment to evaluate the metabolic acclimation of WOSR during 14 days of drought, followed by 8 days of rehydration under high- or low-N fertilization regimes. For this purpose, we selected three leaf ranks exhibiting contrasted sink/source status to perform absolute quantification of plant central metabolites. Besides the well-described accumulation of proline, we observed contrasted accumulations of some "respiratory" amino acids (branched-chain amino acids, lysineand tyrosine) in response to drought under high- and low-N conditions. Drought also induced an increase in sucrose content in sink leaves combined with a decrease in source leaves. N deficiency strongly decreased the levels of major amino acids and subsequently the metabolic response to drought. The drought-rehydration sequence identified proline, phenylalanine, and tryptophan as valuable metabolic indicators of WOSR water status for sink leaves. The results were discussed with respect to the metabolic origin of sucrose and some amino acids in sink leaves and the impact of drought on source-to-sink remobilization processes depending on N nutrition status. Overall, this study identified major metabolic signatures reflecting a similar response of oilseed rape to drought under low- and high-N conditions.

SIMPEL: using stable isotopes to elucidate dynamics of context specific metabolism

The capacity to leverage high resolution mass spectrometry (HRMS) with transient isotope labeling experiments is an untapped opportunity to derive insights on context-specific metabolism, that is difficult to assess quantitatively. Tools are needed to comprehensively mine isotopologue information in an automated, high-throughput way without errors. We describe a tool, Stable Isotope-assisted Metabolomics for Pathway Elucidation (SIMPEL), to simplify analysis and interpretation of isotope-enriched HRMS datasets. The efficacy of SIMPEL is demonstrated through examples of central carbon and lipid metabolism. In the first description, a dual-isotope labeling experiment is paired with SIMPEL and isotopically nonstationary metabolic flux analysis (INST-MFA) to resolve fluxes in central metabolism that would be otherwise challenging to quantify. In the second example, SIMPEL was paired with HRMS-based lipidomics data to describe lipid metabolism based on a single labeling experiment. Available as an R package, SIMPEL extends metabolomics analyses to include isotopologue signatures necessary to quantify metabolic flux.

U-13C-glucose incorporation into source leaves of Brassica napus highlights light-dependent regulations of metabolic fluxes within central carbon metabolism

Plant central carbon metabolism comprises several important metabolic pathways acting together to support plant growth and yield establishment. Despite the emergence of 13C-based dynamic approaches, the regulation of metabolic fluxes between light and dark conditions has not yet received sufficient attention for agronomically relevant plants. Here, we investigated the impact of light/dark conditions on carbon allocation processes within central carbon metabolism of Brassica napus after U-13C-glucose incorporation into leaf discs. Leaf gas-exchanges and metabolite contents were weakly impacted by the leaf disc method and the incorporation of glucose. 13C-analysis by GC-MS showed that U-13C-glucose was converted to fructose for de novo biosynthesis of sucrose at similar rates in both light and dark conditions. However, light conditions led to a reduced commitment of glycolytic carbons towards respiratory substrates (pyruvate, alanine, malate) and TCA cycle intermediates compared to dark conditions. Analysis of 13C-enrichment at the isotopologue level and metabolic pathway isotopic tracing reconstructions identified the contribution of multiple pathways to serine biosynthesis in light and dark conditions. However, the direct contribution of the glucose-6-phosphate shunt to serine biosynthesis was not observed. Our results also provided isotopic evidences for an active metabolic connection between the TCA cycle, glycolysis and photorespiration in light conditions through a rapid reallocation of TCA cycle decarboxylations back to the TCA cycle through photorespiration and glycolysis. Altogether, these results suggest the active coordination of core metabolic pathways across multiple compartments to reorganize C-flux modes.