Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency

Plants mitigate ecosystem nitrous oxide emissions primarily through reductions in soil nitrate content: Evidence from a meta-analysis

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) and an ozone-depleting substance. The presence of plants in an ecosystem can either increase or decrease N2O emissions, or play a negligible role in driving N2O emissions. Here, we conducted a meta-analysis comparing ecosystem N2O emissions from planted and unplanted systems to evaluate how plant presence influences N2O emissions and examined the mechanisms driving observed responses. Our results indicate that plant presence reduces N2O emissions while it increases dinitrogen (N2) emissions from ecosystems through decreases in soil nitrate concentration as well as increases in complete denitrification and mineral N immobilization. The response of N2O emissions to plant presence was universal across major terrestrial ecosystems - including forests, grassland and cropland - and it did not vary with N fertilization. Further, in light of the potential mechanisms of N2O formation in plant cells, we discussed how plant presence could enhance the emission of N2O from plants themselves. Improving our understanding of the mechanisms driving N2O emissions in response to plant presence could be beneficial for enhancing the robustness for predictions of our GHG sinks and sources and for developing strategies to minimize emissions at the ecosystem scale.

Effects of the nitrogen form ratios on photosynthetic productivity of poplar under condition of phenolic acids

Phenolic acids can reduce nitrogen utilization rate of poplar, which seriously restrict the productivity of poplar plantation. In this study, three phenolic acid concentrations (T0, T1, and T2) and three ratios of nitrogen forms (NH₄⁺-N to NO₃^--were 1:3, 1:7, and 1:14) were chosen for orthogonal experiment on poplar (Populus × euramericana "Neva") seedlings to study the effects of the nitrogen form ratios on photosynthetic productivity of poplar under environment of phenolic acids. Results showed that photosynthetic physiology parameters were influenced by both phenolic acid concentration and nitrogen form ratio. The order of net photosynthetic rate (P_N) values obtained from 9 treatments were T1-1:3, T0-1:3, T2-1:3, T0-1:7, T1-1:7, T0-1:14, T2-1:7, T1-1:14, and T2-1:14 (from high to low). Under environment of phenolic acids, when poplar were treated with NH₄⁺-N to NO₃^--N ratio of 1:14, the major limitation factor of photosynthesis was non stomatal factor. When poplar were treated with NH₄⁺-N to NO₃-N ratio of 1:3, the major limitation factor of photosynthesis changed to stomatal factor. The leaf nitrogen content and total biomass were obviously positively related with P_N (p < 0.05). Phenolic acid inhibited photosynthetic productivity of poplar in a major way and this effect decreased with increase of the content of NH₄⁺-N.

Nitrate–Nitrite–Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants

Oxygen (O₂) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O₂ deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO₃⁻), nitrite (NO₂⁻), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N₂O) when supplied with NO₃⁻ and NO₂⁻, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N₂O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants’ survival during O₂-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N₂O budget.

Effect of uniconazole to soybean seed priming treatment under drought stress at VC stage

Studying the mechanism of drought stress in soybean root at vegetative cotyledon (VC) stage by soaking seeds with uniconazole revealed new insights into soybean stress physiology. Therefore, a completely random pot experiments with different time gradients for water cut-off (24, 48, 72, 96 and 120 h, respectively) were carried out with uniconazole (0.4 mgL-1) with respect to morphological, microscopic, ultramicroscopic, physiological, and molecular studies on varieties Hefeng55 (H50, drought tolerant variety) and Kenfeng16 (K16, drought susceptible variety). Results revealed that uniconazole effectively alleviated the inhibition on root growth caused by drought stress, increased the number of root tips, significantly reduced lignification of vessels, alleviated the damage of mitochondria and nucleus caused by drought stress, further strengthened osmotic adjustment system and antioxidant system, especially when the soil moisture content was less than 14%, broke expression restriction of IAA due to drought stress, and inhibited GA3 generation; finally, we found that high-intensity drought stress significantly increased the expression levels of GmNAC003, GmNAC004, GmNAC015, GmNAC020, GmHK07, GmRR01, GmRR02 and GmRR16 genes relating to drought tolerance, while uniconazole had a significant inhibitory effect on GmNAC003, GmNAC004, GmNAC015, GmNAC020, GmRR01, GmRR02 and GmRR16 genes. Our results provided a reference for the mechanism of drought resistance in legume and the effect of uniconazole on alleviating drought stress.

Products of anaerobic metabolism in waterlogged roots of soybean are exported in the xylem

Waterlogging leads to hypoxia of the root system. Metabolic changes occur that enable the plant to tolerate the hypoxic stress. We investigated the export of organic acids, products of anaerobic metabolism, via xylem of waterlogged soybean (Glycine max) plants. Organic acids were quantified by GC-MS and their formation via aspartate metabolism investigated using [4-13C]aspartate. Elevated levels of malate were found together with variable amounts of other organic acids, notably lactate and succinate. Addition of [4-13C]aspartate to the medium led to isotopic enrichment of several organic acids in the xylem sap. Quantitatively, malate carried the highest amount of label among the organic acids. Labelling of succinate indicates its formation by reversal of the TCA-cycle from oxaloacetate. Since aspartate was a prominent amino acid of the phloem sap, it is suggested that this is an important source of malate exported in the xylem. The export of these organic acids will play the role of removing electrons from the hypoxic roots, representing an additional mechanism in the metabolic response to root hypoxia. Malate, normally considered an intermediate in succinate formation, is definitively a product of anaerobic metabolism.

Hypoxia induces stem and leaf nitric oxide (NO) emission from poplar seedlings

Hypoxia leads to NO formation in poplar roots. Additionally, either NO or a NO derivative is transported from the roots to the shoot causing NO emission from aboveground plant organs. Nitric oxide (NO) is involved in the response of plants to various forms of stress including hypoxia. It also seems to play an important role in stomatal closure during stress exposure. In this study, we investigated the formation of NO in roots of intact poplar (Populus × canescens) plants in response to hypoxia, as well as its dependence on nitrate availability. We further addressed the question if root hypoxia triggers NO emission from aboveground plant parts, i.e., stems and leaves of young poplar trees. Our results indicate that NO is formed in poplar roots in response to hypoxia and that this production depends on the availability of nitrate and its conversion product nitrite. As long as nitrate was available in the nutrient solution, NO emission of roots occurred; in the range of the nitrate concentrations (10-100 µM) tested, NO emission was widely independent on nitrate concentration. However, the time period in which NO was emitted and the total amount of NO emitted strongly depended on the nitrate concentration of the solution. Hypoxia also led to increased NO emissions from the leaves and stems of the trees. There was a tight correlation between leaf and stem NO emission of hypoxia-treated plants. We propose that NO is produced by nitrate reductase in the roots and either NO itself, a metabolic NO precursor, or a NO derivative is transported in the xylem sap of the trees from the roots to the shoot thereby mediating NO emission from aboveground parts of the plant.

Utilization of (15)NO3 (-) by nodulated soybean plants under conditions of root hypoxia

Waterlogging of soils is common in nature. The low availability of oxygen under these conditions leads to hypoxia of the root system impairing the development and productivity of the plant. The presence of nitrate under flooding conditions is regarded as being beneficial towards tolerance to this stress. However, it is not known how nodulated soybean plants, cultivated in the absence of nitrate and therefore not metabolically adapted to this compound, would respond to nitrate under root hypoxia in comparison with non-nodulated plants grown on nitrate. A study was conducted with (15)N labelled nitrate supplied on waterlogging for a period of 48 h using both nodulated and non-nodulated plants of different physiological ages. Enrichment of N was found in roots and leaves with incorporation of the isotope in amino acids, although to a much smaller degree under hypoxia than normoxia. This demonstrates that nitrate is taken up under hypoxic conditions and assimilated into amino acids, although to a much lesser extent than for normoxia. The similar response obtained with nodulated and non-nodulated plants indicates the rapid metabolic adaptation of nodulated plants to the presence of nitrate under hypoxia. Enrichment of N in nodules was very much weaker with a distinct enrichment pattern of amino acids (especially asparagine) suggesting that labelling arose from a tissue source external to the nodule rather than through assimilation in the nodule itself.

Recovery of nitrogen fixation after short-term flooding of the nodulated root system of soybean

Nitrogen fixation of terrestrial legumes is strongly and rapidly diminished under flooding. Although recovery is possible with the formation of aerenchyma, information is scarce regarding recovery after draining following short-term flooding, before the appearance of morphological adaptations. This study used soybean (Glycine max) plants nodulated with Bradyrhizobium elkanii to determine xylem sap glutamine as an indication of nitrogen fixation activity during recovery from different periods of flooding. Xylem glutamine levels showed rapid recovery (within 90 min) following periods of flooding up to 4 h. Recovery was progressively slower after longer periods of flooding. After 48 h flooding very little recovery could be observed within the first 120 min after draining but recovery was possible within 48 h. Consistent with the changes in xylem glutamine, direct measurements of apparent nitrogenase activity carried out immediately on draining revealed rapid recovery after flooding for 1 h and slow recovery following 48 h of flooding. In the latter case, nitrogenase activity largely recovered 24 h after draining. Experiments with (15)N(2) incorporation into amino acids exported in the xylem sap revealed that glutamine was by far the most highly labelled amino acid in sap collected over the first 30 min of exposure to the isotope. This is conclusive evidence that xylem sap glutamine is an immediate product of N(2) fixation and export. The changes in xylem sap glutamine seen on flooding (decline) and after draining (recovery) can therefore be attributed to changes in nitrogenase activity. The data show that xylem sap glutamine is a useful means for assessing changes in nitrogenase activity, especially when the root system is submersed in water and activity cannot be measured directly.

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 (15)N derived from (15)NO(3)(-), (15)NO(2)(-) and (15)NH(4)(+) 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 (15)NO(3)(-) and (15)NH(4)(+) enrichment in the root segments. Moreover, analysis of endogenous NO(2)(-) and (15)NH(4)(+) levels suggested a slower metabolism of these ions under hypoxia. Accordingly, regardless of the nitrogen source, hypoxia reduced total (15)N incorporation into amino acids. Analysis of (15)N enrichment patterns and amino acid levels suggest a redirecting of amino acid metabolism to alanine and γ-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 (15)NH(4)(+), (15)NO(2)(-) assimilation into amino acids was more strongly affected by hypoxia and NO(2)(-) accumulated in root segments during this stress, indicating that nitrite reductase may be an additional limiting step. NO(2)(-) accumulation was associated with a higher nitric oxide emission. (15)NO(3)(-) led to much lower (15)N 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.

Involvement of nitrite in the nitrate-mediated modulation of fermentative metabolism and nitric oxide production of soybean roots during hypoxia

It is widely accepted that nitrate but not ammonium improves tolerance of plants to hypoxic stress, although the mechanisms related to this beneficial effect are not well understood. Recently, nitrite derived from nitrate reduction has emerged as the major substrate for the synthesis of nitric oxide (NO), an important signaling molecule in plants. Here, we analyzed the effect of different nitrogen sources (nitrate, nitrite and ammonium) on the metabolic response and NO production of soybean roots under hypoxia. Organic acid analysis showed that root segments isolated from nitrate-cultivated plants presented a lower accumulation of lactate and succinate in response to oxygen deficiency in relation to those from ammonium-cultivated plants. The more pronounced lactate accumulation by root segments of ammonium-grown plants was followed by a higher ethanol release in the medium, evidencing a more intense fermentation under oxygen deficiency than those from nitrate-grown plants. As expected, root segments from nitrate-cultivated plants produced higher amounts of nitrite and NO during hypoxia compared to ammonium cultivation. Exogenous nitrite supplied during hypoxia reduced both ethanol and lactate production and stimulated cyanide-sensitive NO emission by root segments from ammonium-cultivated plants, independent of nitrate. On the other hand, treatments with a NO donor or a NO scavenger did not affect the intensity of fermentation of soybean roots. Overall, these results indicate that nitrite participates in the nitrate-mediated modulation of the fermentative metabolism of soybean roots during oxygen deficiency. The involvement of mitochondrial reduction of nitrite to NO in this mechanism is discussed.

Analysis of alanine aminotransferase in various organs of soybean (Glycine max) and in dependence of different nitrogen fertilisers during hypoxic stress

Alanine aminotransferase (AlaAT) catalyses the reversible conversion of pyruvate and glutamate into alanine and oxoglutarate. In soybean, two subclasses were identified, each represented by two highly similar members. To investigate the role of AlaAT during hypoxic stress in soybean, changes in transcript level of both subclasses were analysed together with the enzyme activity and alanine content of the tissue. Moreover, the dependency of AlaAT activity and gene expression was investigated in relation to the source of nitrogen supplied to the plants. Using semi-quantitative PCR, GmAlaAT genes were determined to be highest expressed in roots and nodules. Under normal growth conditions, enzyme activity of AlaAT was detected in all organs tested, with lowest activity in the roots. Upon waterlogging-induced hypoxia, AlaAT activity increased strongly. Concomitantly, alanine accumulated. During re-oxygenation, AlaAT activity remained high, but the transcript level and the alanine content decreased. Our results show a role for AlaAT in the catabolism of alanine during the initial period of re-oxygenation following hypoxia. GmAlaAT also responded to nitrogen availability in the solution during waterlogging. Ammonium as nitrogen source induced both gene expression and enzyme activity of AlaAT more than when nitrate was supplied in the nutrient solution. The work presented here indicates that AlaAT might not only be important during hypoxia, but also during the recovery phase after waterlogging, when oxygen is available to the tissue again.