Anaerobic nitrate and ammonium metabolism in flood-tolerant rice coleoptiles

The tolerance of germinating rice seedlings to anaerobiosis cannot be fully accounted for by ethanolic fermentation alone. Nitrate metabolism (nitrate reduction to NH₄⁺ plus its subsequent assimilation) may provide an additional sink mechanism for excess protons and NADH produced during anaerobiosis. To follow the fate of nitrate, ¹⁵N-labelled nitrate and ammonium incorporation in aerobic and anaerobic rice coleoptiles was examined using ¹⁵N-edited ¹H NMR and gas chromatography-mass spectrometry methods. After 22 h of treatments, protein-free Ala, Glu, Gln, and γ-amino-butyrate were the main ¹⁵N-labelled products for both nitrate and ammonium-treated anaerobic rice coleoptiles, with Gln, Glu, and Ala being the most enriched. The total amount of ¹⁵N label incorporation into Ala and GAB increased significantly in response to anaerobiosis. The ¹⁵N-labelling pattern of Glu and Gln suggests that the GS/GOGAT system was primarily involved in ammonium assimilation whereas Glu dehydrogenase may play a role in nitrate assimilation. ¹⁵N incorporation into protein-derived amino acids was also significant and was more substantial in anaerobic than in aerobic rice coleoptiles, which indicate that protein biosynthesis remained active in anaerobic rice coleoptiles. Thus, anaerobic assimilation of inorganic N into amino acids, particularly Ala and Glu/GAB, may serve to supplement ethanolic fermentation in sustaining glycolysis and energy production in rice coleoptiles.