Fig. 3.
Different utilization strategies of inorganic nitrogen (N) source for the maintenance of ATP production caused by the difference in the O2 supply ability in wetland plants under O2-deficient condition. NAD(P)H produced mainly during glycolysis, lipid breakdown, and photosynthesis is oxidized to NAD(P)+ by the following two pathways competing for the oxidation, assimilation, and catabolic reduction of NO−3: NO−2-driven ATP production (A) or fermentation (B). The oxidation of NAD(P)H is shown by red letters and arrows. (A) As the species with high O2 supply ability can accelerate nitrification in their rhizosphere by high radial O2 loss (ROL) from the roots, they can utilize NO−2 produced from NO−3 by hypoxia-induced nitrate reductase (NR) as the electron acceptor in the mitochondrial electron transport chain (mETC) instead of O2. NO−2-driven ATP production enables NAD(P)H oxidation for regulating glycolysis, avoiding cytosolic anoxia, and anaerobic ATP synthesis, which is in the same order as that in the ATP through fermentation during hypoxia. Moreover, species with high potential for NR can oxidize NAD(P)H for N assimilation, and these species can acquire a large amount of N and productivity by the “synergistic effect of NH4+ and NO3−”. (B) The species with low O2 supply ability specializing in the assimilation of NH4+ that dominates the anaerobic soil may oxidize NAD(P)H through fermentation. NAD(P)H levels in the A and B pathway are regulated by glycolysis with pyrophosphate (PPi) utilization by PPi-dependent phosphofructokinase (PFK-PPi) and pyruvate phosphate dikinase (PPDK) instead of ATP and metabolisms of major amino acids such as the alanine, glutamate, 2-oxoglutarate, and γ-aminobutyric acid (GABA). Thus, in wetland plants, A and B pathways function as the N utilization strategy in maintaining the ATP production under anaerobic conditions. Abbreviations are as follows: ADH, alcohol dehydrogenase; AOX, alternative oxidase; bc1, cytochrome bc1; Class 1 Hb, class 1 hemoglobin; COX, cytochrome c oxidase; Cyt c, cytochrome c; GS, glutamine synthetase; GOGAT, glutamine oxoglutarate aminotransferase; GDH, glutamate dehydrogenase; IM, inner membrane; LDH, lactate dehydrogenase; NDs, mitochondrial NAD(P)H dehydrogenases; NiR, nitrite reductase; OM; outer membrane; PDC, pyruvate decarboxylase; TCA, tricarboxylic acid; UQH2, ubiquinol, I–V; mitochondrial complexes I–V