Tolerant mechanisms to O2 deficiency under submergence conditions in plants

Abstract

Wetland plants can tolerate long-term strict hypoxia and anoxic conditions and the subsequent re-oxidative stress compared to terrestrial plants. During O₂ deficiency, both wetland and terrestrial plants use NAD(P)⁺ and ATP that are produced during ethanol fermentation, sucrose degradation, and major amino acid metabolisms. The oxidation of NADH by non-phosphorylating pathways in the mitochondrial respiratory chain is common in both terrestrial and wetland plants. As the wetland plants enhance and combine these traits especially in their roots, they can survive under long-term hypoxic and anoxic stresses. Wetland plants show two contrasting strategies, low O₂ escape and low O₂ quiescence strategies (LOES and LOQS, respectively). Differences between two strategies are ascribed to the different signaling networks related to phytohormones. During O₂ deficiency, LOES-type plants show several unique traits such as shoot elongation, aerenchyma formation and leaf acclimation, whereas the LOQS-type plants cease their growth and save carbohydrate reserves. Many wetland plants utilize NH₄⁺ as the nitrogen (N) source without NH₄⁺-dependent respiratory increase, leading to efficient respiratory O₂ consumption in roots. In contrast, some wetland plants with high O₂ supply system efficiently use NO₃^- from the soil where nitrification occurs. The differences in the N utilization strategies relate to the different systems of anaerobic ATP production, the NO₂^--driven ATP production and fermentation. The different N utilization strategies are functionally related to the hypoxia or anoxia tolerance in the wetland plants.

Keywords: Anoxia; Hypoxia; Low O2 escape and low O2 quiescence strategies (LOES and LOQS); Nitrogen acquisition strategy; Re-oxidative stress; Respiration; Wetland plants.

Fig. 1

Regulations of sugar catabolism, fermentation, glycolysis, and major amino acid metabolism associated with NAD(P)⁺ regeneration and ATP production in terrestrial and wetland plants under O₂-deficient conditions. Blue arrows and letters indicate the reactions and enzymes in the up-regulated pathways when the mitochondrial electron transport and the TCA-cycle flux decrease under O₂-deficient conditions. Red letters indicate the regeneration of NAD(P)⁺ from NAD(P)H. In rice plants, the blue pathways contribute to their tolerance to long-term O₂ deficiency compared with the terrestrial plants. Some wetland plants such as rice also have a high ability to optimally regulate the pyruvate level by activation of pyrophosphate (PP_i)-dependent phosphofructokinase (PFK-PP_i) and pyruvate phosphate dikinase (PPDK) that consume PP_i instead of ATP for energy conservation. Besides glycolysis, PP_i is consumed to regulate the cytosolic pH by the tonoplast H⁺-pumping pyrophosphatase (H⁺-PP_iase) instead of H⁺-ATPase in wetland plants. Although two independent pathways for sucrose degradation contribute to the regulation of glycolytic flux in both terrestrial and wetland plants, the UDP-dependent sucrose synthase (SuSy) pathway is regarded as energetically more advantageous for survival under O₂-deficient conditions than the invertase (INV) pathway because here, PP_i is utilized instead of ATP. Sugar supply to glycolysis through starch mobilization is observed in species with developed storage organs such as tuber, rhizome, and endosperm. In NAD(P)H regeneration during the metabolisms of 2-oxoglutarate and glutamate associated with γ-aminobutyric acid (GABA) production, the glutamate dehydrogenase (GDH) pathway without ATP consumption is more efficient in energy consumption than the NAD(P)H-dependent glutamine: 2-oxoglutarate aminotransferase (GOGAT) pathway with ATP consumption. The accumulation of some amino acids such as GABA, alanine, and glutamate play an important role in avoiding carbohydrate loss not only during O₂-deficient conditions but also during the recovery phase of re-oxygenation after hypoxia/anoxia. Alanine accumulation by alanine aminotransferase (AlaAT) can operate non-circular TCA-cycle and gluconeogenesis under O₂ deficiency and re-oxygenation. Abbreviations are as follows: ADH, alcohol dehydrogenase; AlaAT, alanine aminotransferase; ALDH, acetaldehyde dehydrogenase; AspAT, aspartate aminotransferase; CoASH, coenzyme A; FK, fructokinase; GABA-T, GABA transaminase; GAD, glutamate decarboxylase; GHBDH, γ-aminobutyrate dehydrogenase; Glucose-1-P, glucose-1-phosphate; GS, glutamine synthetase; HXK, hexokinase; LDH lactate dehydrogenase; MDH, malate dehydrogenase; PCK, phosphoenolpyruvate carboxykinase; PDC, pyruvate decarboxylase; PDH; pyruvate dehydrogenase; PEPC, phosphoenolpyruvate carboxylase; PFK, ATP-dependent phosphofructokinase; PFK-PPi, PPi-dependent phosphofructokinase; PGI, phosphoglucoisomerase; PGM, phosphoglucomutase; Pi, phosphate; PK, pyruvate kinase; PPDK, pyruvate Pi dikinase; SSADH, succinate semialdehyde dehydrogenase; Starch Pase, starch phosphorylase; TCA, tricarboxylic acid; UDP, uridine diphosphate; UGPPase, UDP-glucose pyrophosphorylase; UTP, uridine triphosphate

Fig. 2

Production and elimination of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in mitochondria and cytosol. H₂O₂, NO, O₂^·−, and ONOO⁻ produced under hypoxic stress conditions are detoxified by the mitochondrial electron transport chain (mETC) and ascorbate/glutathione cycle in the mitochondrial matrix to maintain a redox balance in the cells. The alternative oxidase (AOX) and type II NAD(P)H dehydrogenases (NDs), ND_ex, and ND_in (NDs located at the outer and inner surfaces of the mitochondrial inner membrane, respectively), can consume the accumulated reducing equivalents for maintaining the mitochondrial homeostasis. AOX has lower affinity to O₂ than cytochrome c oxidase (COX, complex IV); NDs, especially Ca²⁺ dependent ND_in, have lower affinity to NAD(P)H than complex I and nitrate reductase (NR). In NO scavenging under hypoxic stress condition, ascorbate can contribute to the reduction of NO to N₂O in the mitochondrial matrix. Ascorbate is converted to monodehydroascorbate by ascorbate peroxidase (APX), which also involves the scavenging of ONOO⁻ and converting it into NO, The NO generated is resupplied to mETC. Ascorbate can also participate in Class 1 hemoglobin (Class 1 Hb) regeneration from methemoglobin (metHb) in the cytosol. Abbreviations are as follows: Cyt c, cytochrome c; DHAR, dehydroascorbate reductase; GR, glutathione reductase; IM, inner membrane; IMS, inter-membrane space; MDHAR, monodehydroascorbate reductase; NO, nitric oxide; NR, nitrate reductase; OM, outer membrane; SOD, superoxide dismutase; TCA, tricarboxylic acid; UQH₂, ubiquinol

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 NO₃⁻: NO₂⁻-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 NO₂⁻ produced from NO₃⁻ by hypoxia-induced nitrate reductase (NR) as the electron acceptor in the mitochondrial electron transport chain (mETC) instead of O₂. NO₂⁻-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 (PP_i) utilization by PP_i-dependent phosphofructokinase (PFK-PP_i) 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

Fig. 4

Characteristics of low O₂ escape strategy (LOES) and low O₂ quiescence strategy (LOQS) to hypoxia/anoxia caused by flooding/submergence in wetland and terrestrial plants. Black solid and dashed lines are the networks of LOES (aerenchyma formation, shoot elongation, radial O₂ loss (ROL) barriers, and leaf acclimation) in wetland and terrestrial plants, respectively, while blue dashed lines indicate responses to suppress LOES in both plants; blue solid lines indicate the submerged regulatory network of LOQS in rice (wetland species). Four key factors, ROS accumulation, ethylene content, ATP depletion, and sucrose reserve decrease, involve the LOES and LOQS networks are shown in red letters. ROS production in hypoxic and anoxic stresses causes programmed cell death (PCD) in both plant types and involves the mechanisms of adventitious roots (ARs) emergence and aerenchyma formation. AR elongation in Arabidopsis (terrestrial plant) is promoted by the hypoxia signal and its formation is mediated by hypoxia-responsive HRE2, which is one of the group VII ethylene response transcription factors (ERFVIIs). High ethylene level inhibits the AR formation in Arabidopsis under hypoxic condition, although ARs are formed at low ethylene level. In contrast, in rice plants, ethylene has promotive effects on the AR formation and elongation. The contrasting regulation by ethylene on ARs may reflect different adaptive strategies in the flood-tolerant rice plants compared to the flooding-intolerant terrestrial species such as Arabidopsis. Leaf acclimation such as high specific leaf area (SLA), reoriented chloroplasts along with cell wall in leaf epidermis, thin cuticles and cell walls, development of dissected leaves underwater, and the maintenance of gas films can increase the net photosynthesis by decreasing the diffusion resistance for CO₂. The leaf plasticity could also result from the accumulation of ethylene and a decrease in CO₂ levels. Flooding/submergence causes ethylene accumulation, which triggers gibberellin (GA)-promoted cell elongation through the expansins (EXPs). In deep-water rice with LOES, ethylene promotes the induction of SNORKELs (SKs, SK1, and SK2) and GA elevation and the internodes of the shoots elongate rapidly to come out of the water surface. In the deep submergence lines of rice with LOQS, ethylene activates the submergence 1A-1 (SUB1A-1) promoting an increase in SLENDER RICE 1 (SLR1) and SLENDER RICE-Like 1 (SLRL1) transcription factors, which inhibit GA-mediated activation of gene expressions. This LOQS characteristic of rice can limit carbohydrate consumption by inhibiting shoot growth. Wetland plants develop shoot and root aerenchyma, ROL barriers, and elongated shoots elongation and these characteristics of LOES act synergistically with each other in enhancing the stability of O₂ and ATP availability in roots where nitrogen (N) uptake and active N assimilation take place. Abbreviations are as follows: ABA, abscisic acid; ADH, alcohol dehydrogenase; AlaAT, alanine aminotransferase; CIPK15, calcineurin B-like interacting protein kinase 15; HRE2, hypoxia-responsive ERF 2; PDC, pyruvate decarboxylase; QTL1 and 3, quantitative trait loci on chromosomes 1 and 3; SnRK1A, sucrose non-fermenting receptor kinase 1A; SuSy, sucrose synthase; SUB1A-1, submergence 1A-1

Fig. 5

Effects of NO₃⁻ (left) and NH₄⁺ (right) utilization as the sole nitrogen (N) source on root respiration and N acquisition in wetland plants. The downward (⇩) and upward (⇧) arrows indicate the decreasing and increasing responses, respectively. NO₃⁻ utilization results in a low root to shoot weight (S/R) ratio, which is unfavorable for O₂ supply. As the N uptake rate per root weight (NNUR) per root respiration rate decreases when the wetland plants utilize NO₃⁻, they develop the roots for N acquisition, consequently increasing the respiration of the whole roots. Therefore, NO₃⁻ utilization requires high O₂ supply to maintain productivity. In contrast, NH₄⁺ utilization results in a high S/R ratio, which is favorable for O₂ supply, and high NNUR per root respiration. Moreover, when NH₄⁺ concentrations increase, the wetland plants may assimilate NH₄⁺ in their shoots instead of their roots. These traits contribute to a decrease in the respiration of the whole root, and thus wetland plants can ensure NH₄⁺ utilization even under low O₂ supply. Photograph of Carex lyngbyei grown in 200 µM NO₃⁻ and NH₄⁺ treatments under hypoxic hydroponic culture for 1 month. Bar 5 cm