Excessive ammonium assimilation by plastidic glutamine synthetase causes ammonium toxicity in Arabidopsis thaliana

Differential Metabolic Responses of Lettuce Grown in Soil, Substrate and Hydroponic Cultivation Systems under NH4+/NO3− Application

Nitrogen (N) is an essential element for plant growth and development. The application of a balanced and optimal amount of N is required for sustainable plant yield. For this, different N sources and forms are used, that including ammonium (NH₄⁺) and nitrate (NO₃⁻). These are the main sources for N uptake by plants where NH₄⁺/NO₃⁻ ratios have a significant effect on the biomass, quality and metabolites composition of lettuce grown in soil, substrate and hydroponic cultivation systems. A limited supply of N resulted in the reduction in the biomass, quality and overall yield of lettuce. Additionally, different types of metabolites were produced with varying concentrations of N sources and can be used as metabolic markers to improve the N use efficiency. To investigate the differential metabolic activity, we planted lettuce with different NH₄⁺/NO₃⁻ ratios (100:0, 75:25, 50:50, 25:75 and 0:100%) and a control (no additional N applied) in soil, substrate and hydroponic cultivation systems. The results revealed that the 25% NH₄^+/75% NO₃⁻ ratio increased the relative chlorophyll contents as well as the biomass of lettuce in all cultivation systems. However, lettuce grown in the hydroponic cultivation system showed the best results. The concentration of essential amino acids including alanine, valine, leucine, lysine, proline and serine increased in soil and hydroponically grown lettuce treated with the 25% NH₄^+/75% NO₃⁻ ratio. The taste and quality-related compounds in lettuce showed maximum relative abundance with the 25% NH₄^+/75% NO₃⁻ ratio, except ascorbate (grown in soil) and lactupicrin (grown in substrate), which showed maximum relative abundance in the 50% NH₄^+/50% NO₃⁻ ratio and control treatments, respectively. Moreover, 1-O-caffeoylglucose, 1,3-dicaffeoylquinic acid, aesculetin and quercetin-3-galactoside were increased by the application of the 100% NH₄^+/0% NO₃⁻ ratio in soil-grown lettuce. The 25% NH₄^+/75% NO₃⁻ ratio was more suitable in the hydroponic cultivation system to obtain increased lettuce biomass. The metabolic profiling of lettuce showed different behaviors when applying different NH₄⁺/NO₃⁻ ratios. Therefore, the majority of the parameters were largely influenced by the 25% NH₄^+/75% NO₃⁻ ratio, which resulted in the hyper-accumulation of health-promoting compounds in lettuce. In conclusion, the optimal N applications improve the quality of lettuce grown in soil, substrate and hydroponic cultivation systems which ultimately boost the nutritional value of lettuce.

Investigation of the direct effect of a precision Ascophyllum nodosum biostimulant on nitrogen use efficiency in wheat seedlings

Reduction in the greenhouse gas (GHG) emissions and nitrogen (N) pollution of ground water by improving nitrogen use efficiency (NUE) in crops has become an intensively investigated research topic in pursuit of a more sustainable future. Although, distinct solutions have been proposed there are only a few reports documenting the detailed interplay between observed plant growth dynamics and changes in plant N related transcriptional and biochemical changes. It was previously demonstrated that the application of a formulated biostimulant (PSI-362) derived from Ascophyllum nodosum (ANE) improves N uptake in Arabidopsis thaliana and in barley. In this study, the effect of PSI-362 on the growth dynamics of wheat seedlings was evaluated at different biostimulant and N supplementation rates. Wheat grown on N deficient MS medium was also analysed from the first hour of the treatment until the depletion of the nutrients in the medium 9 days later. During this time the biomass increase measured for PSI-362 treated plants versus untreated controls was associated with increased nitrate uptake, with surplus N assimilated by the biomass in the form of glutamate, glutamine, free amino acids, soluble proteins, and chlorophyll. Phenotypical and biochemical analysis were supported by evaluation of differential expression of genetic markers involved in nitrate perception and transport (TaNRT1.1/NPF6.3), nitrate and nitrite reduction (TaNR1 and TaNiR1) and assimilation (TaGDH2, TaGoGAT, TaGS1). Finally, a comparative analysis of the precision biostimulant PSI-362 and two generic ANEs demonstrated that the NUE effect greatly differs depending on the ANE formulation used.

OsEIL1 protects rice growth under NH4+ nutrition by regulating OsVTC1-3-dependent N-glycosylation and root NH4+ efflux

Rice is known for its superior adaptation to ammonium (NH₄⁺ ) as a nitrogen source. Compared to many other cereals, it displays lower NH₄⁺ efflux in roots and higher nitrogen-use efficiency on NH₄⁺ . A critical role for GDP-mannose pyrophosphorylase (VTC1) in controlling root NH₄⁺ fluxes was previously documented in Arabidopsis, but the molecular pathways involved in regulating VTC1-dependent NH₄⁺ efflux remain unclear. Here, we report that ETHYLENE-INSENSITIVE3-LIKE1 (OsEIL1) acts as a key transcription factor regulating OsVTC1-3-dependent NH₄⁺ efflux and protein N-glycosylation in rice grown under NH₄⁺ nutrition. We show that OsEIL1 in rice plays a contrasting role to Arabidopsis-homologous ETHYLENE-INSENSITIVE3 (AtEIN3) and maintains rice growth under NH₄⁺ by stabilizing protein N-glycosylation and reducing root NH₄⁺ efflux. OsEIL1 constrains NH₄⁺ efflux by activation of OsVTC1-3, but not OsVTC1-1 or OsVTC1-8. OsEIL1 binds directly to the promoter EIN3-binding site (EBS) of OsVTC1-3 in vitro and in vivo and acts to increase the transcription of OsVTC1-3. Our work demonstrates an important link between excessive root NH₄⁺ efflux and OsVTC1-3-mediated protein N-glycosylation in rice grown under NH₄⁺ nutrition and identifies OsEIL1 as a direct genetic regulator of OsVTC1-3 expression.

A revised view on the evolution of glutamine synthetase isoenzymes in plants

Glutamine synthetase (GS) is a key enzyme responsible for the incorporation of inorganic nitrogen in the form of ammonium into the amino acid glutamine. In plants, two groups of functional GS enzymes are found: eubacterial GSIIb (GLN2) and eukaryotic GSIIe (GLN1/GS). Only GLN1/GS genes are found in vascular plants, which suggests that they are involved in the final adaptation of plants to terrestrial life. The present phylogenetic study reclassifies the different GS genes of seed plants into three clusters: GS1a, GS1b and GS2. The presence of genes encoding GS2 has been expanded to Cycadopsida gymnosperms, which suggests the origin of this gene in a common ancestor of Cycadopsida, Ginkgoopsida and angiosperms. GS1a genes have been identified in all gymnosperms, basal angiosperms and some Magnoliidae species. Previous studies in conifers and the gene expression profiles obtained in ginkgo and magnolia in the present work could explain the absence of GS1a in more recent angiosperm species (e.g. monocots and eudicots) as a result of the redundant roles of GS1a and GS2 in photosynthetic cells. Altogether, the results provide a better understanding of the evolution of plant GS isoenzymes and their physiological roles, which is valuable for improving crop nitrogen use efficiency and productivity. This new view of GS evolution in plants, including a new cytosolic GS group (GS1a), has important functional implications in the context of plant metabolism adaptation to global changes.

CBL-Interacting Protein Kinase OsCIPK18 Regulates the Response of Ammonium Toxicity in Rice Roots

Ammonium ( NH 4 + ) is one of the major nitrogen sources for plants. However, excessive ammonium can cause serious harm to the growth and development of plants, i.e., ammonium toxicity. The primary regulatory mechanisms behind ammonium toxicity are still poorly characterized. In this study, we showed that OsCIPK18, a CBL-interacting protein kinase, plays an important role in response to ammonium toxicity by comparative analysis of the physiological and whole transcriptome of the T-DNA insertion mutant (cipk18) and the wild-type (WT). Root biomass and length of cipk18 are less inhibited by excess NH 4 + compared with WT, indicating increased resistance to ammonium toxicity. Transcriptome analysis reveals that OsCIPK18 affects the NH 4 + uptake by regulating the expression of OsAMT1;2 and other NH 4 + transporters, but does not affect ammonium assimilation. Differentially expressed genes induced by excess NH 4 + in WT and cipk18 were associated with functions, such as ion transport, metabolism, cell wall formation, and phytohormones signaling, suggesting a fundamental role for OsCIPK18 in ammonium toxicity. We further identified a transcriptional regulatory network downstream of OsCIPK18 under NH 4 + stress that is centered on several core transcription factors. Moreover, OsCIPK18 might function as a transmitter in the auxin and abscisic acid (ABA) signaling pathways affected by excess ammonium. These data allowed us to define an OsCIPK18-regulated/dependent transcriptomic network for the response of ammonium toxicity and provide new insights into the mechanisms underlying ammonium toxicity.

Salicylate and glutamate mediate different Cd accumulation and tolerance between Brassica napus and B. juncea

Most hyperaccumulator plants have little economic values, and therefore have not been widely used in Cd-contaminated soils. Rape species are Cd hyperaccumulators with high economic values. Black mustard seed (Brassica juncea) has a higher accumulation ability and a higher tolerance for Cd than oilseed rape (Brassica napus), but its biomass is relatively low and its geographical distribution is limited. However, it is unknown why B. juncea (Bj) is more tolerant to and accumulates more Cd than B. napus (Bn). Here, we found that the differences in Cd accumulation and tolerance between the two species is mainly because Bj plants have higher levels of salicylic acid and glutamic acid than Bn plants. Exogenous salicylate and glutamate treatments enhanced Cd accumulation (salicylate + glutamate co-treatment doubled Cd accumulation level in Bn seedlings) but reduced oxidative stresses by increasing glutathione biosynthesis and activating phytochelatin-based sequestration of Cd into vacuoles. Our results provide a new idea to simultaneously improve Cd accumulation and Cd tolerance in B. napus.

Nitrate transporter NRT1.1 and anion channel SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity

Ammonium (NH₄ ⁺ ) and nitrate (NO₃ ^- ) are major inorganic nitrogen (N) sources for plants. When serving as the sole or dominant N supply, NH₄ ⁺ often causes root inhibition and shoot chlorosis in plants, known as ammonium toxicity. NO₃ ^- usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations, referred to as nitrate-dependent alleviation of ammonium toxicity. Our previous studies indicated a NO₃ ^- efflux channel SLAH3 is involved in this process. However, whether additional components contribute to NO₃ ^- -mediated NH₄ ⁺ detoxification is unknown. Previously, mutations in NO₃ ^- transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH₄ ⁺ . Whereas, in this study, we found when the high-NH₄ ⁺ medium was supplemented with low concentrations of NO₃ ^- , nrt1.1 mutant plants showed hyper-sensitive phenotype instead. Furthermore, mutation in NRT1.1 caused enhanced medium acidification under high-NH₄ ⁺ /low-NO₃ ^- condition, suggesting NRT1.1 regulates ammonium toxicity by facilitating H⁺ uptake. Moreover, NRT1.1 was shown to interact with SLAH3 to form a transporter-channel complex. Interestingly, SLAH3 appeared to affect NO₃ ^- influx while NRT1.1 influenced NO₃ ^- efflux, suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels. Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity through regulating NO₃ ^- transport and balancing rhizosphere acidification.

Decreased Solution pH and Increased K+ Uptake Are Related to Ammonium Tolerance in Hydroponically Cultured Plants

The ammonium (NH4+) tolerance of plants is an important issue in agriculture, associated with several plant characteristics. So far, plant tissue acidification has been shown as the primary cause of NH4+ toxicity. Suppressed plant growth caused by excess NH4+ can be counteracted by potassium (K+) application. However, the effects of NH4+ tolerances on the differences regarding pH changes together with K+ uptake remain to be determined. Here, we performed an 84 h hydroponic cultivation of five species with different NH4+ tolerances, subjected to three NH4+:NO3− solutions (0:100, 50:50, or 100:0), to investigate the pH changes and ion uptakes. Consequently, the solution pH was lowered over time to varying extents in the presence of NH4+. The NH4+-tolerant ageratum and lettuce, shown to be tolerant to NH4+ in this trial, rapidly lowered the solution pH, whereas extremely NH4+-sensitive salvia and cabbage only gave a minor decrease in the solution pH when grown with 100:0 NH4+:NO3−. Additionally, the increased external NH4+ level led to a substantial decline in the net cation influxes (K+, Ca2+, and Mg2+). As compared to solely NH4+-fed salvia and cabbage, solely NH4+-fed ageratum and lettuce ultimately showed a relatively greater net K+ influx. Taken together, this study discusses how the decreases in pH and K+ are related to NH4+ tolerance in five hydroponically cultured species.

Phloem iron remodels root development in response to ammonium as the major nitrogen source

Plants use nitrate and ammonium as major nitrogen (N) sources, each affecting root development through different mechanisms. However, the exact signaling pathways involved in root development are poorly understood. Here, we show that, in Arabidopsis thaliana, either disruption of the cell wall-localized ferroxidase LPR2 or a decrease in iron supplementation efficiently alleviates the growth inhibition of primary roots in response to NH₄⁺ as the N source. Further study revealed that, compared with nitrate, ammonium led to excess iron accumulation in the apoplast of phloem in an LPR2-dependent manner. Such an aberrant iron accumulation subsequently causes massive callose deposition in the phloem from a resulting burst of reactive oxygen species, which impairs the function of the phloem. Therefore, ammonium attenuates primary root development by insufficiently allocating sucrose to the growth zone. Our results link phloem iron to root morphology in response to environmental cues.

Root GS and NADH-GDH Play Important Roles in Enhancing the Ammonium Tolerance in Three Bedding Plants

Ammonium is a paradoxical nutrient because it is more metabolically efficient than nitrate, but also causes plant stresses in excess, i.e., ammonium toxicity. Current knowledge indicates that ammonium tolerance is species-specific and related to the ammonium assimilation enzyme activities. However, the mechanisms underlying the ammonium tolerance in bedding plants remain to be elucidated. The study described herein explores the primary traits contributing to the ammonium tolerance in three bedding plants. Three NH4+:NO3− ratios (0:100, 50:50, 100:0) were supplied to salvia, petunia, and ageratum. We determined that they possessed distinct ammonium tolerances: salvia and petunia were, respectively, extremely sensitive and moderately sensitive to high NH4+ concentrations, whereas ageratum was tolerant to NH4+, as characterized by the responses of the shoot and root growth, photosynthetic capacity, and nitrogen (amino acid and soluble protein)-carbohydrate (starch) distributions. An analysis of the major nitrogen assimilation enzymes showed that the root GS (glutamine synthetase) and NADH-GDH (glutamate dehydrogenase) activities in ageratum exhibited a dose-response relationship (reinforced by 25.24% and 6.64%, respectively) as the NH4+ level was raised from 50% to 100%; but both enzyme activities were significantly diminished in salvia. Besides, negligible changes of GS activities monitored in leaves revealed that only the root GS and NADH-GDH underpin the ammonium tolerances of the three bedding plants.

Nitrate alleviates ammonium toxicity in wheat (Triticum aestivum L.) by regulating tricarboxylic acid cycle and reducing rhizospheric acidification and oxidative damage

Ammonium (NH₄⁺) is one of the most important nutrients required by plants. However, a high concentration of NH₄⁺ as the sole nitrogen source suppresses plant growth. Although nitrate (NO₃^-) can alleviate NH₄⁺ toxicity, the mechanisms underlying this ability have not been fully elucidated. In this study, wheat plants were cultivated in hydroponic solution with 7.5 mM NO₃^- (control), 7.5 mM NH₄⁺ (sole ammonium, SA) or 7.5 mM NH₄⁺ plus 1.0 mM NO₃^- (ammonium and nitrate, AN). The results showed that compared with the control, the SA treatment significantly decreased root growth, protein content and the concentrations of most intermediates and the activity of enzymes from the tricarboxylic acid (TCA) cycle. Moreover, increased the activity of plasma membrane H⁺-ATPase and the rate of H⁺ efflux along roots, caused solution acidification, and increased the activity of mitochondrial respiratory chain complexes I-IV and the contents of protein-bound carbonyls and malondialdehyde in roots. SA treatment induced ultrastructure disruption and reduced the viability of root cells. Compared with the SA treatment, the AN treatment increased root growth, protein content, the concentrations of most intermediates and the activity of enzymes from the TCA cycle. Furthermore, AN treatment decreased the rate of H⁺ efflux, retarded medium acidification, decreased protein carbonylation and lipid peroxidation in roots and relieved ultrastructure disruption and increased the viability of root cells. Taken together, these results indicate that NO₃^--dependent alleviation of NH₄⁺ toxicity in wheat seedlings is closely associated with physiological processes that mediate TCA cycle, relieve rhizospheric acidification and decrease the production of ROS and oxidative damage.

Structural property, molecular regulation, and functional diversity of glutamine synthetase in higher plants: a data-mining bioinformatics approach

Glutamine synthetase (GS; E.C.6.3.1.2) is a key enzyme in higher plants with two isozymes, cytosolic GS1 and plastidic GS2, and involves in the assimilation and recycling of NH₄⁺ ions and maintenance of complex traits such as crop nitrogen-use efficiency and yield. Our present understanding of crop nitrogen-use efficiency and its correlation with the functional role of the GS family genes is inadequate, which delays harnessing the benefit of this key enzyme in crop improvement. In this report, we performed a comprehensive investigation on the phylogenetic relationship, structural properties, complex multilevel gene regulation, and expression patterns of the GS genes to enrich present understanding about the enzyme. Our Gene Ontology and protein-protein interactions analysis revealed the functional aspects of GS isozymes in stress mitigation, aging, nucleotide biosynthesis/transport, DNA repair and response to metals. The insight gained here contributes to the future research strategies in developing climate-smart crops for global sustainability.

The quality of compost was improved by low concentrations of fulvic acid owing to its optimization of the exceptional microbial structure

The influence of different concentrations of fulvic acid at 0, 100, 200, and 400 mg/kg was evaluated during the course of composting with straw and mushroom residues as substrates. The optimal concentration of fulvic acid is 100 mg/Kg based on microbial characteristics, chemical parameters, and germination index testing. Nearly 80% of the microbial taxa responded significantly to fulvic acid over the composting period, with a dynamic change of the co-occurrence network from complex to simple and then to complex. Fulvic acid accelerated the progress of composting and reduced the emission of gases at the thermophilic phase. The optimal concentration of fulvic acid enriched the beneficial microorganisms Aeribacillus, Oceanobacillus, and Rhodospirillaceae, and decreased the abundances of pathogenic microorganisms Corynebacterium, Elizabethkingia, and Sarcocystidae. This study indicates a new strategy to optimize the composting process using the biostimulant fulvic acid.

Genetic Engineering and Genome Editing for Improving Nitrogen Use Efficiency in Plants

Low nitrogen availability is one of the main limiting factors for plant growth and development, and high doses of N fertilizers are necessary to achieve high yields in agriculture. However, most N is not used by plants and pollutes the environment. This situation can be improved by enhancing the nitrogen use efficiency (NUE) in plants. NUE is a complex trait driven by multiple interactions between genetic and environmental factors, and its improvement requires a fundamental understanding of the key steps in plant N metabolism—uptake, assimilation, and remobilization. This review summarizes two decades of research into bioengineering modification of N metabolism to increase the biomass accumulation and yield in crops. The expression of structural and regulatory genes was most often altered using overexpression strategies, although RNAi and genome editing techniques were also used. Particular attention was paid to woody plants, which have great economic importance, play a crucial role in the ecosystems and have fundamental differences from herbaceous species. The review also considers the issue of unintended effects of transgenic plants with modified N metabolism, e.g., early flowering—a research topic which is currently receiving little attention. The future prospects of improving NUE in crops, essential for the development of sustainable agriculture, using various approaches and in the context of global climate change, are discussed.