Integrative response of plant mitochondrial electron transport chain to nitrogen source

Bicarbonate-Dependent Detoxification by Mitigating Ammonium-Induced Hypoxic Stress in Triticum aestivum Root

Ammonium (NH4+) toxicity is ubiquitous in plants. To investigate the underlying mechanisms of this toxicity and bicarbonate (HCO3-)-dependent alleviation, wheat plants were hydroponically cultivated in half-strength Hoagland nutrient solution containing 7.5 mM NO3- (CK), 7.5 mM NH4+ (SA), or 7.5 mM NH4+ + 3 mM HCO3- (AC). Transcriptomic analysis revealed that compared to CK, SA treatment at 48 h significantly upregulated the expression of genes encoding fermentation enzymes (pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), and lactate dehydrogenase (LDH)) and oxygen consumption enzymes (respiratory burst oxidase homologs, dioxygenases, and alternative oxidases), downregulated the expression of genes encoding oxygen transporters (PIP-type aquaporins, non-symbiotic hemoglobins), and those involved in energy metabolism, including tricarboxylic acid (TCA) cycle enzymes and ATP synthases, but upregulated the glycolytic enzymes in the roots and downregulated the expression of genes involved in the cell cycle and elongation. The physiological assay showed that SA treatment significantly increased PDC, ADH, and LDH activity by 36.69%, 43.66%, and 61.60%, respectively; root ethanol concentration by 62.95%; and lactate efflux by 23.20%, and significantly decreased the concentrations of pyruvate and most TCA cycle intermediates, the complex V activity, ATP content, and ATP/ADP ratio. As a consequence, SA significantly inhibited root growth. AC treatment reversed the changes caused by SA and alleviated the inhibition of root growth. In conclusion, NH4+ treatment alone may cause hypoxic stress in the roots, inhibit energy generation, suppress cell division and elongation, and ultimately inhibit root growth, and adding HCO3- remarkably alleviates the NH4+-induced inhibitory effects on root growth largely by attenuating the hypoxic stress.

Nitrogen sources differentially affect respiration, growth, and carbon allocation in Andean and Lowland ecotypes of Chenopodium quinoa Willd

Chenopodium quinoa Willd. is a native species that originated in the High Andes plateau (Altiplano) and its cultivation spread out to the south of Chile. Because of the different edaphoclimatic characteristics of both regions, soils from Altiplano accumulated higher levels of nitrate (NO3-) than in the south of Chile, where soils favor ammonium (NH₄ ⁺) accumulation. To elucidate whether C. quinoa ecotypes differ in several physiological and biochemical parameters related to their capacity to assimilate NO3- and NH₄ ⁺, juvenile plants of Socaire (from Altiplano) and Faro (from Lowland/South of Chile) were grown under different sources of N (NO3- or NH₄ ⁺). Measurements of photosynthesis and foliar oxygen-isotope fractionation were carried out, together with biochemical analyses, as proxies for the analysis of plant performance or sensitivity to NH₄ ⁺. Overall, while NH₄ ⁺ reduced the growth of Socaire, it induced higher biomass productivity and increased protein synthesis, oxygen consumption, and cytochrome oxidase activity in Faro. We discussed that ATP yield from respiration in Faro could promote protein production from assimilated NH₄ ⁺ to benefit its growth. The characterization of this differential sensitivity of both quinoa ecotypes for NH₄ ⁺ contributes to a better understanding of nutritional aspects driving plant primary productivity.

Light induced shoot-sourced transcription factor HY5 regulates the nitrate uptake of cotton by shoot-to-root signal transport

Elongated hypocotyls 5 (HY5) is a transcription factor that can be induced by illumination and promotes nitrate uptake in Arabidopsis. However, whether GhHY5 regulates nitrate uptake in cotton is unknown. In this study, the cotton seedlings growing in light and dark conditions were treated with ¹⁵N-labeled nutrient solution to study whether the GhHY5 regulates nitrate uptake in cotton. The results showed that the ¹⁵N content and GhNRT1.1 expression in the light condition were higher than that in the dark condition, indicating that light induced the expression of GhNRT1.1 and subsequently promoted N uptake. Additionally, the expression of GhHY5 in the leaf and root of cotton was induced by light and the expression pattern of GhHY5 in the root was similar to that of GhNRT1.1. Furthermore, when the GhHY5 expression in the root was reduced, the ¹⁵N content and GhNRT1.1 expression were both decreased, indicating that the GhNRT1.1 expression was regulated by GhHY5. The root expression of GhHY5 was decreased in the grafted seedlings which the GhHY5 in the shoot was silenced by VIGS or the seedlings which the hypocotyl was girdled, but the expression of GhHY5 on one side root of the grafted cotton seedling was not changed if the GhHY5 was silenced on the other side root. Thus, we proposed that the light induced shoot-derived GhHY5 gene or GhHY5 protein may be transported from the xylem to the root, regulating the expression of GhHY5 and GhNRT1.1, and thus regulating N uptake at the root of cotton.

Implication of quantifying nitrate utilization and CO2 assimilation of Brassica napus plantlets in vitro under variable ammonium/nitrate ratios

BACKGROUND

Plantlets grown in vitro with a mixed nitrogen source utilize sucrose and CO2 as carbon sources for growth. However, it is very difficult to obtain the correct utilization proportions of nitrate, ammonium, sucrose and CO2 for plantlets. Consequently, the biological effect of ammonium/nitrate utilization, the biological effect of sucrose/CO2 utilization, and the ammonium/nitrate use efficiency for new C input derived from CO2 assimilation/sucrose utilization are still unclear for plantlets.

RESULTS

The bidirectional stable nitrogen isotope tracer technique quantified the proportions of assimilated nitrate and ammonium in Brassica napus plantlets grown at different ammonium/nitrate ratios. The utilization proportions of sucrose and CO2 could be quantified by a two end-member isotope mixing model for Bn plantlets grown at different ammonium/nitrate ratios. Under the condition that each treatment contained 20 mM ammonium, the proportion of assimilated nitrate did not show a linear increase with increasing nitrate concentration for Bn plantlets. Moreover, the proportion of assimilated CO2 did not show a linear relationship with the nitrate concentration for Bn plantlets. Increasing the nitrate concentration contributed to promoting the assimilation of ammonium and markedly enhanced the ammonium utilization coefficient for Bn plantlets. With increasing nitrate concentration, the amount of nitrogen in leaves derived from nitrate assimilation increased gradually, while the nitrate utilization coefficient underwent no distinct change for Bn plantlets.

CONCLUSIONS

Quantifying the utilization proportions of nitrate and ammonium can reveal the energy efficiency for N assimilation in plantlets grown in mixed N sources. Quantifying the utilization proportion of CO2 contributes to evaluating the photosynthetic capacity of plantlets grown with variable ammonium/nitrate ratios. Quantifying the utilization proportions of nitrate, ammonium, sucrose and CO2 can reveal the difference in the ammonium/nitrate use efficiency for new C input derived from CO2 assimilation/sucrose utilization for plantlets grown at variable ammonium/nitrate ratios.

The importance of utilizing nitrate (NO3-) over ammonium (NH4+) as nitrogen source during detoxification of exogenous thiocyanate (SCN-) in Oryza sativa

Thiocyanate (SCN^-) is a nitrogen-containing pollutant, which can be involved in the nitrogen (N) cycle and interferes with plant growth. The current study highlights a new insight into the N (nitrate [NO₃^-] and ammonium [NH₄⁺]) utilization ways in rice seedlings under SCN^- exposure to clarify the interactive effect on uptake and assimilation between these N-containing chemicals. Phenotypically, relative growth rates (RGR) of NO₃^--fed seedlings were significantly higher than NH₄⁺-fed rice seedlings at the same SCN^- concentration. Both N fertilizations have no significant influence on SCN^- content and its assimilation in rice seedlings. However, significant accumulation of NO₃^- and NH₄⁺ were detected in shoots prior to roots under SCN^- stress. Enzymatic assay and mRNA analysis showed that the carbonyl sulfide (COS) pathway of SCN^- degradation occurred in both roots and shoots of NO₃^--fed seedlings but only evident in roots of NH₄⁺-fed seedlings. Moreover, the effect of SCN^- on the activity of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT) was negligible in NO₃^--fed seedlings, while GOGAT activity was significantly inhibited in shoots of NH₄⁺-fed seedlings. Nitrogen use efficiency (NUE) estimation provided positive evidence in utilizing NO₃^- over NH₄⁺ as the main N source to support rice seedling growth during detoxification of exogenous SCN^-. Overall, SCN^- pollution has unexpectedly changed the rice preference for N source which shifted from NH₄⁺ to NO₃^-, suggesting that the interactions of SCN^- with different N sources in terms of uptake and assimilation in rice plants should not be overlooked, especially at the plant N nutritional level.

Induction of S-nitrosoglutathione reductase protects root growth from ammonium toxicity by regulating potassium homeostasis in Arabidopsis and rice

Ammonium (NH4+) is toxic to root growth in most plants already at moderate levels of supply, but mechanisms of root growth tolerance to NH4+ remain poorly understood. Here, we report that high levels of NH4+ induce nitric oxide (NO) accumulation, while inhibiting potassium (K+) acquisition via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4), leading to the arrest of primary root growth. High levels of NH4+ also stimulated the accumulation of GSNOR (S-nitrosoglutathione reductase) in roots. GSNOR overexpression improved root tolerance to NH4+. Loss of GSNOR further induced NO accumulation, increased SNO1/SOS4 activity, and reduced K+ levels in root tissue, enhancing root growth sensitivity to NH4+. Moreover, the GSNOR-like gene, OsGSNOR, is also required for NH4+ tolerance in rice. Immunoblotting showed that the NH4+-induced GSNOR protein accumulation was abolished in the VTC1- (vitamin C1) defective mutant vtc1-1, which is hypersensititive to NH4+ toxicity. GSNOR overexpression enhanced vtc1-1 root tolerance to NH4+. Our findings suggest that induction of GSNOR increases NH4+ tolerance in Arabidopsis roots by counteracting NO-mediated suppression of tissue K+, which depends on VTC1 function.

Physiological role of silicon in radish seedlings under ammonium toxicity

BACKGROUND

High concentrations of ammonium as the sole nitrogen source may result in physiological and nutritional disorders that can lead to reduced plant growth and toxicity. In this study, we hypothesized that ammonium toxicity in radish seedlings (Raphanus sativus L.) might be mitigated by the incorporation of silicon (Si) into applied nutrient solution. To examine this possibility, we conducted a hydroponic experiment to evaluate the effects of five concentrations of ammonium (1, 7.5, 15, 22.5, and 30 mmol L^-1 ) on the photosynthesis, green color index, stomatal conductance, transpiration, instantaneous water-use efficiency, and biomass production of radish in the absence and presence (2 mmol L^-1 ) of Si. The experimental design was a randomized block design based on a 2 × 5 factorial scheme with four replicates.

RESULTS

The highest concentration of applied ammonium (30 mmol L^-1 ) was found to reduce the photosynthesis, transpiration and total dry biomass of radish seedlings, independent of the presence of Si in the nutrient solution. However, at lower ammonium concentrations, the application of Si counteracted these detrimental effects, and facilitated the production of seedlings with increased photosynthesis, greater instantaneous water-use efficiency, and higher total dry biomass compared with the untreated plants (without Si). Transpiration and stomatal conductance were affected to lesser extents by the presence of Si.

CONCLUSION

These findings indicate that the addition of Si to nutrient solutions could provide an effective means of alleviating the unfavorable effects induced by ammonium toxicity at concentrations of less than 30 mmol L^-1 . © 2020 Society of Chemical Industry.

In Vivo Metabolic Regulation of Alternative Oxidase under Nutrient Deficiency-Interaction with Arbuscular Mycorrhizal Fungi and Rhizobium Bacteria

The interaction of the alternative oxidase (AOX) pathway with nutrient metabolism is important for understanding how respiration modulates ATP synthesis and carbon economy in plants under nutrient deficiency. Although AOX activity reduces the energy yield of respiration, this enzymatic activity is upregulated under stress conditions to maintain the functioning of primary metabolism. The in vivo metabolic regulation of AOX activity by phosphorus (P) and nitrogen (N) and during plant symbioses with Arbuscular mycorrhizal fungi (AMF) and Rhizobium bacteria is still not fully understood. We highlight several findings and open questions concerning the in vivo regulation of AOX activity and its impact on plant metabolism during P deficiency and symbiosis with AMF. We also highlight the need for the identification of which metabolic regulatory factors of AOX activity are related to N availability and nitrogen-fixing legume-rhizobia symbiosis in order to improve our understanding of N assimilation and biological nitrogen fixation.

New Insights on Arabidopsis thaliana Root Adaption to Ammonium Nutrition by the Use of a Quantitative Proteomic Approach

Nitrogen is an essential element for plant nutrition. Nitrate and ammonium are the two major inorganic nitrogen forms available for plant growth. Plant preference for one or the other form depends on the interplay between plant genetic background and environmental variables. Ammonium-based fertilization has been shown less environmentally harmful compared to nitrate fertilization, because of reducing, among others, nitrate leaching and nitrous oxide emissions. However, ammonium nutrition may become a stressful situation for a wide range of plant species when the ion is present at high concentrations. Although studied for long time, there is still an important lack of knowledge to explain plant tolerance or sensitivity towards ammonium nutrition. In this context, we performed a comparative proteomic study in roots of Arabidopsis thaliana plants grown under exclusive ammonium or nitrate supply. We identified and quantified 68 proteins with differential abundance between both conditions. These proteins revealed new potential important players on root response to ammonium nutrition, such as H⁺-consuming metabolic pathways to regulate pH homeostasis and specific secondary metabolic pathways like brassinosteroid and glucosinolate biosynthetic pathways.

Characteristics of Free Amino Acids (the Quality Chemical Components of Tea) under Spatial Heterogeneity of Different Nitrogen Forms in Tea (Camellia sinensis) Plants

Nitrogen (N) forms are closely related to tea quality, however, little is known about the characteristics of quality chemical components in tea under the spatial heterogeneity of different N forms. In this study, a split-root system, high performance liquid chromatography (HPLC), and root analysis system (WinRHIZO) were used to investigate free amino acids (FAAs) and root length of tea plants under the spatial heterogeneity of different N forms. Uniform. (U.) ammonium (NH₄⁺) (both compartments had NH₄⁺), U. nitrate (NO₃⁻) (both compartments had NO₃⁻), Split. (Sp.) NH₄⁺ (one of the compartments had NH₄⁺), and Sp. NO₃⁻ (the other compartment had NO₃⁻) were performed. The ranking of total FAAs in leaves were as follows: U. NH₄⁺ > Sp. NH₄⁺/Sp. NO₃⁻ > U. NO₃⁻. The FAA characteristics of Sp. NH₄⁺/Sp. NO₃⁻ were more similar to those of U. NO₃⁻. The contents of the important FAAs (aspartic acid, glutamic acid, and theanine) that determine the quality of tea, increased significantly in U. NH₄⁺. The total root length in U. NH₄⁺ was higher than that in the other treatments. More serious root browning was found in U. NO₃⁻. In conclusion, NH₄⁺ improved the accumulations of FAAs in tea leaves, which might be attributed to the root development.

Inorganic nitrogen form: a major player in wheat and Arabidopsis responses to elevated CO2

Critical for predicting the future of primary productivity is a better understanding of plant responses to rising atmospheric carbon dioxide (CO2) concentration. This review considers recent results on the role of the inorganic nitrogen (N) forms nitrate (NO3-) and ammonium (NH4+) in determining the responses of wheat and Arabidopsis to elevated atmospheric CO2 concentration. Here, we identify four key issues: (i) the possibility that different plant species respond similarly to elevated CO2 if one accounts for the N form that they are using; (ii) the major influence that plant-soil N interactions have on plant responses to elevated CO2; (iii) the observation that elevated CO2 may favor the uptake of one N form over others; and (iv) the finding that plants receiving NH4+ nutrition respond more positively to elevated CO2 than those receiving NO3- nutrition because elevated CO2 inhibits the assimilation of NO3- in shoots of C3 plants. We conclude that the form and amount of N available to plants from the rhizosphere and plant preferences for the different N forms are essential for predicting plant responses to elevated CO2.

Short-term ammonium supply induces cellular defence to prevent oxidative stress in Arabidopsis leaves

Plants can assimilate nitrogen from soil pools of both ammonium and nitrate, and the relative levels of these two nitrogen sources are highly variable in soil. Long-term ammonium nutrition is known to cause damage to Arabidopsis that has been linked to mitochondrial oxidative stress. Using hydroponic cultures, we analysed the consequences of rapid shifts between nitrate and ammonium nutrition. This did not induce growth retardation, showing that Arabidopsis can compensate for the changes in redox metabolism associated with the variations in nitrogen redox status. During the first 3 h of ammonium treatment, we observed distinct transient shifts in reactive oxygen species (ROS), low-mass antioxidants, ROS-scavenging enzymes, and mitochondrial alternative electron transport pathways, indicating rapid but temporally separated changes in chloroplastic, mitochondrial and cytosolic ROS metabolism. The fast induction of antioxidant defences significantly lowered intracellular H₂ O₂ levels, and thus protected Arabidopsis leaves from oxidative stress. On the other hand elevated extracellular ROS production in response to ammonium supply may be involved in signalling. The response pattern displays an intricate plasticity of Arabidopsis redox metabolism to minimise stress in responses to nutrient changes.

Characteristics of NH4+ and NO3- fluxes in tea (Camellia sinensis) roots measured by scanning ion-selective electrode technique

As a vital beverage crop, tea has been extensively planted in tropical and subtropical regions. Nitrogen (N) levels and forms are closely related to tea quality. Based on different N levels and forms, we studied changes in NO3- and NH4+ fluxes in tea roots utilizing scanning ion-selective electrode technique. Our results showed that under both single and mixed N forms, influx rates of NO3- were much lower than those of NH4+, suggesting a preference for NH4+ in tea. With the increase in N concentration, the influx rate of NO3- increased more than that of NH4+. The NH4+ influx rates in a solution without NO3- were much higher than those in a solution with NO3-, while the NO3- influx rates in a solution without NH4+ were much lower than those in a solution with NH4+. We concluded that (1) tea roots showed a preference for NH4+, (2) presence of NO3- had a negative effect on NH4+ influx, and (3) NH4+ had a positive effect on NO3- influx. Our findings not only may help advance hydroponic tea experiments but also may be used to develop efficient fertilization protocols for soil-grown tea in the future.

Physiological role of alternative oxidase (from yeasts to plants)

Mitochondria of all so far studied organisms, with the exception of Archaea, mammals, some yeasts, and protists, contain, along with the classical phosphorylating cytochrome pathway, a so-called cyanide-insensitive alternative oxidase (AOX) localized on the matrix side of the mitochondrial inner membrane, and electron transport through which is not coupled with ATP synthesis and energy accumulation. Mechanisms underlying plentiful functions of AOX in organisms at various levels of organization ranging from yeasts to plants are considered. First and foremost, AOX provides a chance of cell survival after inhibiting the terminal components of the main respiratory chain or losing the ability to synthesize these components. The vitally important role of AOX is obvious in thermogenesis of thermogenic plant organs where it becomes the only terminal oxidase with a very high activity, and the energy of substrate oxidation by this respiratory pathway is converted into heat, thus promoting evaporation of volatile substances attracting pollinating insects. AOX plays a fundamentally significant role in alleviating or preventing oxidative stress, thus ensuring the defense against a wide range of stresses and adverse environmental conditions, such as changes in temperature and light intensities, osmotic stress, drought, and attack by incompatible strains of bacterial pathogens, phytopathogens, or their elicitors. Participation of AOX in pathogen survival during its existence inside the host, in antivirus defense, as well as in metabolic rearrangements in plants during embryogenesis and cell differentiation is described. Examples are given to demonstrate that AOX might be an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals.

Alternative oxidase: distribution, induction, properties, structure, regulation, and functions

The respiratory chain in the majority of organisms with aerobic type metabolism features the concomitant existence of the phosphorylating cytochrome pathway and the cyanide- and antimycin A-insensitive oxidative route comprising a so-called alternative oxidase (AOX) as a terminal oxidase. In this review, the history of AOX discovery is described. Considerable evidence is presented that AOX occurs widely in organisms at various levels of organization and is not confined to the plant kingdom. This enzyme has not been found only in Archaea, mammals, some yeasts and protists. Bioinformatics research revealed the sequences characteristic of AOX in representatives of various taxonomic groups. Based on multiple alignments of these sequences, a phylogenetic tree was constructed to infer their possible evolution. The ways of AOX activation, as well as regulatory interactions between AOX and the main respiratory chain are described. Data are summarized concerning the properties of AOX and the AOX-encoding genes whose expression is either constitutive or induced by various factors. Information is presented on the structure of AOX, its active center, and the ubiquinone-binding site. The principal functions of AOX are analyzed, including the cases of cell survival, optimization of respiratory metabolism, protection against excess of reactive oxygen species, and adaptation to variable nutrition sources and to biotic and abiotic stress factors. It is emphasized that different AOX functions complement each other in many instances and are not mutually exclusive. Examples are given to demonstrate that AOX is an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals. This is the first comprehensive review on alternative oxidases of various organisms ranging from yeasts and protists to vascular plants.

HY5 regulates nitrite reductase 1 (NIR1) and ammonium transporter1;2 (AMT1;2) in Arabidopsis seedlings

HY5 (Long Hypocotyles 5) is a key transcription factor in Arabidopsis thaliana that has a pivotal role in seedling development. Soil nitrogen is an essential macronutrient, and its uptake, assimilation and metabolism are influenced by nutrient availability and by lights. To understand the role of HY5 in nitrogen assimilation pathways, we examined the phenotype as well as the expression of selected nitrogen assimilation-related genes in hy5 mutant grown under various nitrogen limiting and nitrogen sufficient conditions, or different light conditions. We report that HY5 positively regulates nitrite reductase gene NIR1 and negatively regulates the ammonium transporter gene AMT1;2 under all nitrogen and light conditions tested, while it affects several other genes in a nitrogen supply-dependent manner. HY5 is not required for light induction of NIR1, AMT1;2 and NIA genes, but it is necessary for high level expression of NIR1 and NIA under optimal nutrient and light conditions. In addition, nitrogen deficiency exacerbates the abnormal root system of hy5. Together, our results suggest that HY5 exhibits the growth-promoting activity only when sufficient nutrients, including lights, are provided, and that HY5 has a complex involvement in nitrogen acquisition and metabolism in Arabidopsis seedlings.

Disruption of the mitochondrial alternative oxidase (AOX) and uncoupling protein (UCP) alters rates of foliar nitrate and carbon assimilation in Arabidopsis thaliana

Under high light, the rates of photosynthetic CO2 assimilation can be influenced by reductant consumed by both foliar nitrate assimilation and mitochondrial alternative electron transport (mAET). Additionally, nitrate assimilation is dependent on reductant and carbon skeletons generated from both the chloroplast and mitochondria. However, it remains unclear how nitrate assimilation and mAET coordinate and contribute to photosynthesis. Here, hydroponically grown Arabidopsis thaliana T-DNA insertional mutants for alternative oxidase (AOX1A) and uncoupling protein (UCP1) fed either NO3 (-) or NH4 (+) were used to determine (i) the response of NO3 (-) uptake and assimilation to the disruption of mAET, and (ii) the interaction of N source (NO3 (-) versus NH4 (+)) and mAET on photosynthetic CO2 assimilation and electron transport. The results showed that foliar NO3 (-) assimilation was enhanced in both aox1a and ucp1 compared with the wild-type, suggesting that foliar NO3 (-) assimilation is probably driven by a decreased capacity of mAET and an increase in reductant within the cytosol. Wild-type plants had also higher rates of net CO2 assimilation (A net) and quantum yield of PSII (ϕPSII) under NO3 (-) feeding compared with NH4 (+) feeding. Additionally, under NO3 (-) feeding, A net and ϕPSII were decreased in aox1a and ucp1 compared with the wild type; however, under NH4 (+) they were not significantly different between genotypes. This indicates that NO3 (-) assimilation and mAET are both important to maintain optimal rates of photosynthesis, probably in regulating reductant accumulation and over-reduction of the chloroplastic electron transport chain. These results highlight the importance of mAET in partitioning energy between foliar nitrogen and carbon assimilation.