Effect of oxygen deficiency on nitrogen assimilation and amino acid metabolism of soybean root segments

Plants submitted to O-2 deficiency present a series of biochemical modifications, affecting overall root metabolism. Here, the effect of hypoxia on the metabolic fate of N-15 derived from (NO3)-N-15 (-), (NO2)-N-15 (-) and (NH4)-N-15 (+) in isolated soybean root segments was followed by gas chromatography-mass spectrometry, to provide a detailed analysis of nitrogen assimilation and amino acid biosynthesis under hypoxia. O-2 deficiency decreased the uptake of the nitrogen sources from the solution, as ratified by the lower (NO3)-N-15 (-) and (NH4)-N-15 (+) enrichment in the root segments. Moreover, analysis of endogenous NO2 (-) and (NH4)-N-15 (+) levels suggested a slower metabolism of these ions under hypoxia. Accordingly, regardless of the nitrogen source, hypoxia reduced total N-15 incorporation into amino acids. Analysis of N-15 enrichment patterns and amino acid levels suggest a redirecting of amino acid metabolism to alanine and gamma-aminobutyric acid synthesis under hypoxia and a differential sensitivity of individual amino acid pathways to this stress. Moreover, the role of glutamine synthetase in nitrogen assimilation both under normoxia and hypoxia was ratified. In comparison with (NH4)-N-15 (+), (NO2)-N-15 (-) assimilation into amino acids was more strongly affected by hypoxia and NO2 (-) accumulated in root segments during this stress, indicating that nitrite reductase may be an additional limiting step. NO2 (-) accumulation was associated with a higher nitric oxide emission. (NO3)-N-15 (-) led to much lower N-15 incorporation in both O-2 conditions, probably due to the limited nitrate reductase activity of the root segments. Overall, the present work shows that profound alterations of root nitrogen metabolism occur during hypoxic stress.