Can Ammonium Stress Be Positive for Plant Performance?

Interplay between nitric oxide and inorganic nitrogen sources in root development and abiotic stress responses

Nitrogen (N) is an essential nutrient for plants, and the sources from which it is obtained can differently affect their entire development as well as stress responses. Distinct inorganic N sources (nitrate and ammonium) can lead to fluctuations in the nitric oxide (NO) levels and thus interfere with nitric oxide (NO)-mediated responses. These could lead to changes in reactive oxygen species (ROS) homeostasis, hormone synthesis and signaling, and post-translational modifications of key proteins. As the consensus suggests that NO is primarily synthesized in the reductive pathways involving nitrate and nitrite reduction, it is expected that plants grown in a nitrate-enriched environment will produce more NO than those exposed to ammonium. Although the interplay between NO and different N sources in plants has been investigated, there are still many unanswered questions that require further elucidation. By building on previous knowledge regarding NO and N nutrition, this review expands the field by examining in more detail how NO responses are influenced by different N sources, focusing mainly on root development and abiotic stress responses.

Seedlings of Poncirus trifoliata exhibit tissue-specific detoxification in response to NH4 + toxicity

Ammonium nitrogen (NH4 +-N) is essential for fruit tree growth, but the impact of excess NH4 +-N from fertilizer on evergreen citrus trees is unclear. In a climate chamber, 8-month-old citrus plants were exposed to five different hydroponic NH4 +-N concentrations (0, 5, 10, 15 and 20 mm) for 1 month to study effects of NH4 +-N on growth characteristics, N uptake, metabolism, antioxidant enzymes and osmotic regulatory substances. Application of 10 mm NH4 +-N adversely affected root plasma membrane integrity, root physiological functions, and plant biomass. MDA, CAT, POD, APX and SOD content were significantly correlated with leaf N metabolic enzyme activity (GOGAT, GDH, GS and NR). GDH was the primary enzyme involved in NH4 +-N assimilation in leaves, while the primary pathway involved in roots was GS-GOGAT. Under comparatively high NH4 + addition, roots were the main organs involved in NH4 + utilization in citrus seedlings. Our results demonstrated that variations in NH4 + concentration and enzyme activity in various organs are associated with more effective N metabolism in roots than in leaves to prevent NH4 + toxicity in evergreen woody citrus plants. These results provide insight into the N forms used by citrus plants that are important for N fertilizer management.

The activation of iron deficiency responses of grapevine rootstocks is dependent to the availability of the nitrogen forms

BACKGROUND

In viticulture, iron (Fe) chlorosis is a common abiotic stress that impairs plant development and leads to yield and quality losses. Under low availability of the metal, the applied N form (nitrate and ammonium) can play a role in promoting or mitigating Fe deficiency stresses. However, the processes involved are not clear in grapevine. Therefore, the aim of this study was to investigate the response of two grapevine rootstocks to the interaction between N forms and Fe uptake. This process was evaluated in a hydroponic experiment using two ungrafted grapevine rootstocks Fercal (Vitis berlandieri x V. vinifera) tolerant to deficiency induced Fe chlorosis and Couderc 3309 (V. riparia x V. rupestris) susceptible to deficiency induced Fe chlorosis.

RESULTS

The results could differentiate Fe deficiency effects, N-forms effects, and rootstock effects. Interveinal chlorosis of young leaves appeared earlier on 3309 C from the second week of treatment with NO3-/NH4+ (1:0)/-Fe, while Fercal leaves showed less severe symptoms after four weeks of treatment, corresponding to decreased chlorophyll concentrations lowered by 75% in 3309 C and 57% in Fercal. Ferric chelate reductase (FCR) activity was by trend enhanced under Fe deficiency in Fercal with both N combinations, whereas 3309 C showed an increase in FCR activity under Fe deficiency only with NO3-/NH4+ (1:1) treatment. With the transcriptome analysis, Gene Ontology (GO) revealed multiple biological processes and molecular functions that were significantly regulated in grapevine rootstocks under Fe-deficient conditions, with more genes regulated in Fercal responses, especially when both forms of N were supplied. Furthermore, the expression of genes involved in the auxin and abscisic acid metabolic pathways was markedly increased by the equal supply of both forms of N under Fe deficiency conditions. In addition, changes in the expression of genes related to Fe uptake, regulation, and transport reflected the different responses of the two grapevine rootstocks to different N forms.

CONCLUSIONS

Results show a clear contribution of N forms to the response of the two grapevine rootstocks under Fe deficiency, highlighting the importance of providing both N forms (nitrate and ammonium) in an appropriate ratio in order to ease the rootstock responses to Fe deficiency.

Moderate salinity and high ammonium/nitrate ratio enhance early growth in "summer wonder" lettuce cultivar

Because its impact in plant development and growth and its interaction with Na+ and Cl-, the supply of different N-forms to crops can be an easy-to-use tool with effective results on salinity tolerance. Here the effect of four N-NO3-/N-NH4+ ratios (mM; 2/0, 1.6/0.4, 0.4/1.6, 0/2) on adaptation to salt conditions (15 mM NaCl in a first experiment and 40 mM NaCl in a second experiment) was studied in young lettuce (cv "Summer wonder") plants. The experiments were carried out in greenhouse and under hydroponics conditions. The results show that this cultivar tolerates and adapts to moderate salinity by deploying several structural and physiological mechanisms; (i) increasing allocation of biomass to the root, (ii) increasing root Na+ uptake and storing it in the shoot and root tissues, (iii) increasing intrinsic water use efficiency and (iv) increasing root N and P uptake. The beneficial effect of salt exposure on growth was greater when the predominant N-form was N-NO3-. These plants with higher tissue N-NO3- concentration, decreased Cl- uptake and shoot and root Cl- concentration. Regardless of salt conditions, plants with a high proportion of N-NH4+ (1.6 mM) and a low proportion of N-NO3- (0.4 mM) had a greater growth and nitrogen use efficiency, that was associated with the improved uptake of nutrients, and the maintenance of water status.

Superior glucose metabolism supports NH4+ assimilation in wheat to improve ammonium tolerance

The use of slow-release fertilizers and seed-fertilizers cause localized high-ammonium (NH4 +) environments in agricultural fields, adversely affecting wheat growth and development and delaying its yield. Thus, it is important to investigate the physiological responses of wheat and its tolerance to NH4 + stress to improve the adaptation of wheat to high NH4 + environments. In this study, the physiological mechanisms of ammonium tolerance in wheat (Triticum aestivum) were investigated in depth by comparative analysis of two cultivars: NH4 +-tolerant Xumai25 and NH4 +-sensitive Yangmai20. Cultivation under hydroponic conditions with high NH4 + (5 mM NH4 +, AN) and nitrate (5 mM NO3 -, NN), as control, provided insights into the nuanced responses of both cultivars. Compared to Yangmai20, Xumai25 displayed a comparatively lesser sensitivity to NH4 + stress, as evident by a less pronounced reduction in dry plant biomass and a milder adverse impact on root morphology. Despite similarities in NH4 + efflux and the expression levels of TaAMT1.1 and TaAMT1.2 between the two cultivars, Xumai25 exhibited higher NH4 + influx, while maintaining a lower free NH4 + concentration in the roots. Furthermore, Xumai25 showed a more pronounced increase in the levels of free amino acids, including asparagine, glutamine, and aspartate, suggesting a superior NH4 + assimilation capacity under NH4 + stress compared to Yangmai20. Additionally, the enhanced transcriptional regulation of vacuolar glucose transporter and glucose metabolism under NH4 + stress in Xumai25 contributed to an enhanced carbon skeleton supply, particularly of 2-oxoglutarate and pyruvate. Taken together, our results demonstrate that the NH4 + tolerance of Xumai25 is intricately linked to enhanced glucose metabolism and optimized glucose transport, which contributes to the robust NH4 + assimilation capacity.

Gasotransmitter ammonia accelerates seed germination, seedling growth, and thermotolerance acquirement in maize

Ammonia (NH₃), as an intermediate product of nitrogen metabolism, is recognized as a novel gasotransmitter (namely gaseous signaling molecule), its signaling role being revealed in plants. NH₃ exists in two different chemical forms, namely the weak base (free molecule: NH₃) and the weak acid (ammonium: NH₄⁺), which are generally in equilibrium with each other in plants. However, the effect of NH₃ on seed germination, seedling growth, and thermotolerance acquirement in maize remains unclear. Here, maize seeds were imbibed in the different concentrations of NH₃·H₂O (NH₃ donor), and then germinated and calculated seed germination rate at the various time points. Also, the 60-h-old seedlings were irrigated in the different concentrations of NH₃·H₂O, and then subjected to heat stress and counted survival rate. The data implied that the appropriate concentrations (6, 9, and 12 mM) of NH₃·H₂O accelerated seed germination as well as increased seedling height and root length compared with the control without NH₃ treatment. Also, the suitable concentrations (2 and 4 mM) of NH₃·H₂O improved tissue vitality, relieved an increase in malondialdehyde content, and enhanced survival rate of maize seedlings under heat stress compared with the control. These results firstly suggest that NH₃ could accelerate seed germination, seedling growth, and thermotolerance acquirement in maize.

Enhanced Stomatal Conductance Supports Photosynthesis in Wheat to Improved NH4+ Tolerance

The impact of ammonium (NH4+) stress on plant growth varies across species and cultivars, necessitating an in-depth exploration of the underlying response mechanisms. This study delves into elucidating the photosynthetic responses and differences in tolerance to NH4+ stress by investigating the effects on two wheat (Triticum aestivum L.) cultivars, Xumai25 (NH4+-less sensitive) and Yangmai20 (NH4+-sensitive). The cultivars were grown under hydroponic conditions with either sole ammonium nitrogen (NH4+, AN) or nitrate nitrogen (NO3-, NN) as the nitrogen source. NH4+ stress exerted a profound inhibitory effect on seedling growth and photosynthesis in wheat. However, these effects were less pronounced in Xumai25 than in Yangmai20. Dynamic photosynthetic analysis revealed that the suppression in photosynthesis was primarily attributed to stomatal limitation associated with a decrease in leaf water status and osmotic potential. Compared to Yangmai20, Xumai25 exhibited a significantly higher leaf K+ concentration and TaAKT1 upregulation, leading to a stronger stomatal opening and, consequently, a better photosynthetic performance under NH4+ stress. In conclusion, our study suggested stomatal limitation as the primary factor restricting photosynthesis under NH4+ stress. Furthermore, we demonstrated that improved regulation of osmotic substances contributed to higher stomatal conductance and enhanced photosynthetic performance in Xumai25.

The Influence of Poultry Manure-Derived Biochar and Compost on Soil Properties and Plant Biomass Growth

Promising methods for managing poultry manure (PM) include converting poultry manure through pyrolysis to biochar, which can be used for soil applications. The overall goal of this study was to determine the effects of poultry manure-derived biochar and compost on the soil and growth of cherry tomatoes. The biochar obtained at 475 °C was characterized by a relatively high organic matter content of 39.47% and nitrogen content of 3.73%, while it had the lowest C/N ratio of 8.18. According to the recommendations of the EBC, the biochar obtained at 475 °C demonstrated the most beneficial effects in terms of fertilizing potential. The composting of poultry manure with the straw was successful, and the limit of 60 °C was exceeded, which allowed for the hygienization of the compost. The produced compost and biochar are sanitary safe and do not exceed the limits of heavy metal content. The lowest plant biomass was obtained from growing medium A with 3.6 g wet weight (0.24 g dry weight). The measurements of the height of cherry tomatoes showed that growing media D, E, and F allowed the plants to obtain from 602 to 654 mm in height.

Nitrogen Use Efficiency in Oil Palm Seedlings: Unraveling the Untapped Potential of Elevated External Ammonium Supply

Ammonium (NH4+) is an essential nitrogen source for plants, but excessive exposure can trigger stress responses that vary among and within different plant species. This study investigated the phenotypic variations in response to ammonium nutrition in five oil palm genotypes seedlings. Nitrate nutrition was used as a reference for a non-stressful condition, and three different nitrogen concentrations (5, 10, and 15 mM) were examined. Control groups without external nitrogen application were included for each genotype. Several parameters were analyzed, including plant growth, root length, gas exchange, fluorescence, chlorophyll, reducing sugars, amino acids, proteins, and nitrogen uptake. The results revealed a significant genotype effect, particularly between the interspecific OxG hybrid and the Elaeis guinensis genotypes. Ammonium nutrition increased shoot growth in all genotypes compared to nitrate nutrition. Additionally, there was a trend towards increased primary root length, amino acids, proteins, and nitrogen uptake under ammonium supply. These findings are promising, particularly considering the recommendation to use ammonium with inhibitors for environmental sustainability.

Benefits of biological nitrification inhibition of Leymus chinensis under alkaline stress: the regulatory function of ammonium-N exceeds its nutritional function

Introduction

The production of root exudates with biological nitrification inhibition (BNI) effects is a strategy adopted by ammonium-N (NH4+-N) tolerant plant species that occur in N-limited environments. Most knowledge on BNI comes from plant species that occur in acidic soils.

Methods

Here, combining field sampling and laboratory culture, we assessed the BNI-capacity of Leymus chinensis, a dominant grass species in alkaline grasslands in eastern Asia, and explored why L. chinensis has BNI ability.

Results and discussion

The results showed that L. chinensis has strong BNI-capacity. At a concentration of 1 mg mL^-1, L. chinensis' root exudates inhibited nitrification in soils influenced by Puccinellia tenuiflora by 72.44%, while DCD only inhibited it by 68.29%. The nitrification potential of the soil of L. chinensis community was only 53% of the P. tenuiflora or 41% of the Suaeda salsa community. We also showed that the supply of NH4+-N driven by L. chinensis' BNI can meet its requirements . In addition, NH4+-N can enhance plant adaptation to alkaline stress by regulating pH, and in turn, the uptake of nitrate-N (NO3--N). We further demonstrated that the regulatory function of NH4+-N is greater than its nutritional function in alkaline environment. The results offer novel insights into how L. chinensis adapts to high pH and nutrient deficiency stress by secreting BNIs, and reveal, for the first time, differences in the functional roles of NH4+-N and NO3--N in growth and adaptation under alkaline conditions in a grass species.

Critical Role of Regrowth Conditions in Post-Cryopreservation of In Vitro Plant Germplasm

Cryopreservation is an effective option for the long-term conservation of plant genetic resources, including vegetatively propagated crops and ornamental plants, elite tree genotypes, threatened plant species with non-orthodox seeds or limited seed availability, as well as cell and root cultures useful for biotechnology. With increasing success, an arsenal of cryopreservation methods has been developed and applied to many species and material types. However, severe damage to plant material accumulating during the multi-step cryopreservation procedure often causes reduced survival and low regrowth, even when the optimized protocol is applied. The conditions at the recovery stage play a vital role in supporting material regrowth after cryopreservation and, when optimized, may shift the life-and-death balance toward a positive outcome. In this contribution, we provide an overview of the five main strategies available at the recovery stage to improve post-cryopreservation survival of in vitro plant materials and their further proliferation and development. In particular, we discuss the modification of the recovery medium composition (iron- and ammonium-free), exogenous additives to cope with oxidative stress and absorb toxic chemicals, and the modulation of medium osmotic potential. Special attention is paid to plant growth regulators used at various steps of the recovery process to induce the desired morphological response in cryopreserved tissues. Given studies on electron transport and energy provision in rewarmed materials, we discuss the effects of light-and-dark conditions and light quality. We hope that this summary provides a helpful guideline and a set of references for choosing the recovery conditions for plant species that have not been cryopreserved. We also propose that step-wise recovery may be most effective for materials sensitive to cryopreservation-induced osmotic and chemical stresses.

Genetic variation underlying differential ammonium and nitrate responses in Arabidopsis thaliana

Nitrogen is an essential element required for plant growth and productivity. Understanding the mechanisms and natural genetic variation underlying nitrogen use in plants will facilitate the engineering of plant nitrogen use to maximize crop productivity while minimizing environmental costs. To understand the scope of natural variation that may influence nitrogen use, we grew 1,135 Arabidopsis thaliana natural genotypes on two nitrogen sources, nitrate and ammonium, and measured both developmental and defense metabolite traits. By using different environments and focusing on multiple traits, we identified a wide array of different nitrogen responses. These responses are associated with numerous genes, most of which were not previously associated with nitrogen responses. Only a small portion of these genes appear to be shared between environments or traits, while most are predominantly specific to a developmental or defense trait under a specific nitrogen source. Finally, by using a large population, we were able to identify unique nitrogen responses, such as preferring ammonium or nitrate, which appear to be generated by combinations of loci rather than a few large-effect loci. This suggests that it may be possible to obtain novel phenotypes in complex nitrogen responses by manipulating sets of genes with small effects rather than solely focusing on large-effect single gene manipulations.

Integrated physiological and weighted gene co-expression network analysis reveals the hub genes engaged in nitrate-regulated alleviation of ammonium toxicity at the seedling stage in wheat (Triticum aestivum L.)

Wheat has a specific preference for NO₃ ^- and shows toxicity symptoms under high NH₄ ⁺ concentrations. Increasing the nitrate supply may alleviate ammonium stress. Nevertheless, the mechanisms underlying the nitrate regulation of wheat root growth to alleviate ammonium toxicity remain unclear. In this study, we integrated physiological and weighted gene co-expression network analysis (WGCNA) to identify the hub genes involved in nitrate alleviation of ammonium toxicity at the wheat seedling stage. Five NH₄ ⁺/NO₃ ^- ratio treatments, including 100/0 (N_a), 75/25 (N_r1), 50/50 (N_r2), 25/75 (N_r3), and 0/100 (N_n) were tested in this study. The results showed that sole ammonium treatment (N_a) increased the lateral root number but reduced root biomass. Increasing the nitrate supply significantly increased the root biomass. Increasing nitrate levels decreased abscisic acid (ABA) content and increased auxin (IAA) content. Furthermore, we identified two modules (blue and turquoise) using transcriptome data that were significantly related to root physiological growth indicators. TraesCS6A02G178000 and TraesCS2B02G056300 were identified as hub genes in the two modules which coded for plastidic ATP/ADP-transporter and WRKY62 transcription factors, respectively. Additionally, network analysis showed that in the blue module, TraesCS6A02G178000 interacts with downregulated genes that coded for indolin-2-one monooxygenase, SRG1, DETOXIFICATION, and wall-associated receptor kinase. In the turquoise module, TraesCS2B02G056300 was highly related to the genes that encoded ERD4, ERF109, CIGR2, and WD40 proteins, and transcription factors including WRKY24, WRKY22, MYB30, and JAMYB, which were all upregulated by increasing nitrate supply. These studies suggest that increasing the nitrate supply could improve root growth and alleviate ammonium toxicity through physiological and molecular regulation networks, including ROS, hormonal crosstalk, and transcription factors.

Redox post-translational modifications and their interplay in plant abiotic stress tolerance

The impact of climate change entails a progressive and inexorable modification of the Earth's climate and events such as salinity, drought, extreme temperatures, high luminous intensity and ultraviolet radiation tend to be more numerous and prolonged in time. Plants face their exposure to these abiotic stresses or their combination through multiple physiological, metabolic and molecular mechanisms, to achieve the long-awaited acclimatization to these extreme conditions, and to thereby increase their survival rate. In recent decades, the increase in the intensity and duration of these climatological events have intensified research into the mechanisms behind plant tolerance to them, with great advances in this field. Among these mechanisms, the overproduction of molecular reactive species stands out, mainly reactive oxygen, nitrogen and sulfur species. These molecules have a dual activity, as they participate in signaling processes under physiological conditions, but, under stress conditions, their production increases, interacting with each other and modifying and-or damaging the main cellular components: lipids, carbohydrates, nucleic acids and proteins. The latter have amino acids in their sequence that are susceptible to post-translational modifications, both reversible and irreversible, through the different reactive species generated by abiotic stresses (redox-based PTMs). Some research suggests that this process does not occur randomly, but that the modification of critical residues in enzymes modulates their biological activity, being able to enhance or inhibit complete metabolic pathways in the process of acclimatization and tolerance to the exposure to the different abiotic stresses. Given the importance of these PTMs-based regulation mechanisms in the acclimatization processes of plants, the present review gathers the knowledge generated in recent years on this subject, delving into the PTMs of the redox-regulated enzymes of plant metabolism, and those that participate in the main stress-related pathways, such as oxidative metabolism, primary metabolism, cell signaling events, and photosynthetic metabolism. The aim is to unify the existing information thus far obtained to shed light on possible fields of future research in the search for the resilience of plants to climate change.

Abscisic acid influences ammonium transport via regulation of kinase CIPK23 and ammonium transporters

Ammonium uptake at plant roots is regulated at the transcriptional, posttranscriptional, and posttranslational levels. Phosphorylation by the protein kinase calcineurin B-like protein (CBL)-interacting protein kinase 23 (CIPK23) transiently inactivates ammonium transporters (AMT1s), but the phosphatases activating AMT1s remain unknown. Here, we identified the PP2C phosphatase abscisic acid (ABA) insensitive 1 (ABI1) as an activator of AMT1s in Arabidopsis (Arabidopsis thaliana). We showed that high external ammonium concentrations elevate the level of the stress phytohormone ABA, possibly by de-glycosylation. Active ABA was sensed by ABI1-PYR1-like () complexes followed by the inactivation of ABI1, in turn activating CIPK23. Under favorable growth conditions, ABI1 reduced AMT1;1 and AMT1;2 phosphorylation, both by binding and inactivating CIPK23. ABI1 further directly interacted with AMT1;1 and AMT1;2, which would be a prerequisite for dephosphorylation of the transporter by ABI1. Thus, ABI1 is a positive regulator of ammonium uptake, coupling nutrient acquisition to abiotic stress signaling. Elevated ABA reduces ammonium uptake during stress situations, such as ammonium toxicity, whereas ABI1 reactivates AMT1s under favorable growth conditions.

Transcriptome analysis of tea (Camellia sinensis) leaves in response to ammonium starvation and recovery

The tea plant is a kind of ammonium-preferring crop, but the mechanism whereby ammonium (NH₄ ⁺) regulate its growth is not well understood. The current study focused on the effects of NH₄ ⁺ on tea plants. Transcriptomic analysis was performed to investigate the early- and late-stage NH₄ ⁺ deprivation and resupply in tea plants shoots. Through short- and long-term NH₄ ⁺ deficiency, the dynamic response to NH₄ ⁺ stress was investigated. The most significant effects of NH₄ ⁺ deficiency were found to be on photosynthesis and gene ontology (GO) enrichment varied with the length of NH₄ ⁺ deprivation. Enriched KEGG pathways were also different when NH₄ ⁺ was resupplied at different concentrations which may indicate reasons for tolerance of high NH₄ ⁺ concentration. Using weighted gene co-expression network analysis (WGCNA), modules related to significant tea components, tea polyphenols and free amino acids, were identified. Hence, NH₄ ⁺ could be regarded as a signaling molecule with the response of catechins shown to be higher than that of amino acids. The current work represents a comprehensive transcriptomic analysis of plant responses to NH₄ ⁺ and reveals many potential genes regulated by NH₄ ⁺ in tea plants. Such findings may lead to improvements in nitrogen efficiency of tea plants.

Is plastidic glutamine synthetase essential for C3 plants? A tale of photorespiratory mutants, ammonium tolerance and conifers

Agriculture faces the considerable challenge of having to adapt to a progressively changing climate (including the increase in CO₂ levels and temperatures); environmental impact must be reduced while at the same time crop yields need to be maintained or increased to ensure food security. Under this scenario, increasing plants' nitrogen (N) use efficiency and minimizing the energy losses associated with photorespiration are two goals of crop breeding that are long sought after. The plastidic glutamine synthetase (GS2) enzyme stands at the crossroads of N assimilation and photorespiration, and is therefore a key candidate for the improvement of crop performance. The GS2 enzyme has long been considered essential for angiosperm survival under photorespiratory conditions. Surprisingly, in Arabidopsis GS2 is not essential for plant survival, and its absence confers tolerance towards ammonium stress, which is in conflict with the idea that NH₄⁺ accumulation is one of the main causes of ammonium stress. Altogether, it appears that the 'textbook' view of this enzyme must be revisited, especially regarding the degree to which it is essential for plant growth under photorespiratory conditions, and the role of NH₄⁺ assimilation during ammonium stress. In this article we open the debate on whether more or less GS2 is a desirable trait for plant productivity.

Root endophyte differentially regulates plant response to NO3- and NH4+ nutrition by modulating N fluxes at the plant-fungal interface

In the soil, plant roots associated with fungi often encounter uneven distribution of nitrate (NO₃^- )/ammonium (NH₄⁺ ) patches, but the mechanism underlying N form-influenced plant-fungal interactions remains limited. We inoculated Arabidopsis with a root endophyte Phomopsis liquidambaris, and evaluated the effects of P. liquidambaris on plant performance under NO₃^- or NH₄⁺ nutrition. Under NO₃^- nutrition, P. liquidambaris inoculation promoted seedling growth, whereas under NH₄⁺ nutrition, P. liquidambaris suppressed seedling growth. Under high NH₄⁺ conditions, fungus-colonized roots displayed increased NH₄⁺ accumulation and NH₄⁺ efflux, similar to the effect of ammonium stress caused by elevated NH₄⁺ levels. Notably, this fungus excluded NH₄⁺ during interactions with host roots, thereby leading to increased NH₄⁺ levels at the plant-fungal interface under high NH₄⁺ conditions. A nitrite reductase-deficient strain that excludes NO₃^- but absorbs NH₄⁺ , decreased NH₄⁺ levels in Arabidopsis shoots and rescued plant growth and nitrogen metabolism under high NH₄⁺ levels. Transcriptomic analysis highlighted that P. liquidambaris had altered transcriptional responses associated with plant response to inorganic N forms. Our results demonstrate that fungus-regulated NO₃^- /NH₄⁺ dynamics at the plant-fungal interface alters plant response to NO₃^- /NH₄⁺ nutrition. This study highlights the essential functions of root endophytes in plant adaptation to soil nitrogen nutrients.

Nitrogen form differently modulates growth, metabolite profile, and antioxidant and nitrogen metabolism activities in roots of Spartina alterniflora in response to increasing salinity

Sodium tolerance and nitrogen-source preferences are two of the most fascinating and ecologically important areas in plant physiology. Spartina alterniflora is a highly salt-tolerant species and appears to prefer ammonium (NH₄⁺) over nitrate (NO₃^-) as an inorganic N source, presenting a suite of aboveground physiological and biochemical mechanisms that allows growth in saline environments. Here, we tested the interactive effects of salinity (0, 200, 500 mM NaCl) and nitrogen source (NO₃^-, NH₄⁺, NH₄NO₃) on some physiological and biochemical parameters of S. alterniflora at the root level. After three months of treatments, plants were harvested to determine root growth parameters and total amino acids, proline, total soluble sugars, sucrose, and root enzyme activity. The control (0 mM NaCl) had the highest root growth rate in the medium containing only ammonium and the lowest in the medium containing only nitrate. Except for NO₃^--fed plants, the 200 mM NaCl treatment generally had less root growth than the control. Under high salinity, NH₄⁺-fed plants had better root growth than NO₃^--fed plants. In the absence of salinity, NH₄⁺-fed plants had higher superoxide dismutase, ascorbate peroxidase, glutathione reductase, and guaiacol peroxidase activities than NO₃^--fed plants. Salinity generally promoted the activity of the principal antioxidant enzymes, more so in NH₄⁺-fed plants. Nitrogen metabolism was characterized by higher constitutive levels of glutamate dehydrogenase (GDH) activity under ammonia nutrition, accompanied by elevated total amino acids levels in roots. The advantage of ammonium nutrition for S. alterniflora under salinity was connected to high amino acid accumulation and antioxidant enzyme activities, together with low H₂O₂ concentration and increased GDH activity. Ammonium improved root performance of S. alterniflora, especially under saline conditions, and may improve root antioxidant capacity and N-assimilating enzyme activities, and adjust osmotically to salinity by accumulating amino acids.

Polyamine: A Potent Ameliorator for Plant Growth Response and Adaption to Abiotic Stresses Particularly the Ammonium Stress Antagonized by Urea

Polyamine(s) (PA, PAs), a sort of N-containing and polycationic compound synthesized in almost all organisms, has been recently paid considerable attention due to its multifarious actions in the potent modulation of plant growth, development, and response to abiotic/biotic stresses. PAs in cells/tissues occur mainly in free or (non- or) conjugated forms by binding to various molecules including DNA/RNA, proteins, and (membrane-)phospholipids, thus regulating diverse molecular and cellular processes as shown mostly in animals. Although many studies have reported that an increase in internal PA may be beneficial to plant growth under abiotic conditions, leading to a suggestion of improving plant stress adaption by the elevation of endogenous PA via supply or molecular engineering of its biosynthesis, such achievements focus mainly on PA homeostasis/metabolism rather than PA-mediated molecular/cellular signaling cascades. In this study, to advance our understanding of PA biological actions important for plant stress acclimation, we gathered some significant research data to succinctly describe and discuss, in general, PA synthesis/catabolism, as well as PA as an internal ameliorator to regulate stress adaptions. Particularly, for the recently uncovered phenomenon of urea-antagonized NH₄ ⁺-stress, from a molecular and physiological perspective, we rationally proposed the possibility of the existence of PA-facilitated signal transduction pathways in plant tolerance to NH₄ ⁺-stress. This may be a more interesting issue for in-depth understanding of PA-involved growth acclimation to miscellaneous stresses in future studies.

Optimization of a Cryopreservation Method for the Endangered Korean Species Pogostemon yatabeanus Using a Systematic Approach: The Key Role of Ammonium and Growth Regulators

Cryopreservation provides a secure long-term conservation option for rare and endangered plant species with non-orthodox or limitedly available seeds. Wide application of cryopreservation to biobank wild flora is hampered by the need to re-optimize nearly all protocol steps for every new species. We applied a systematic approach to simplify optimization of a multi-stage droplet-vitrification method for the endangered wetland Korean species, Pogostemon yatabeanus. This approach consisted of a standard procedure pre-selected based on material type and size, which was complemented with 11 additional treatments to reveal the most impactful conditions. Effect of ammonium nitrate at various protocol steps was also tested. The highest shoot tip survival (92%) and plant regeneration (90%) after cryopreservation were achieved using preculture with 10% sucrose followed by 40 min osmoprotection and 60 min treatment with vitrification solution A3-80% (33.3% glycerol + 13.3% dimethyl sulfoxide + 13.3% ethylene glycol + 20.1% sucrose) on ice. A three-step regrowth procedure starting with ammonium-free medium with 1 mg/L GA3 and 1 mg/L BA followed by ammonium-containing medium with and without growth regulators was essential for the development of healthy plants from cryopreserved shoot tips. This approach enables fast optimization of the cryopreservation procedure for new osmotic stress-sensitive plant species.

Physiological characteristics and miRNA sequencing of two root zones with contrasting ammonium assimilation patterns in Populus

BACKGROUND

The net ammonium fluxes differ among the different root zones of Populus, but the physiological and microRNA regulatory mechanisms are unclear.

OBJECTIVE

To elucidate the physiological and miRNA regulatory mechanisms, we investigated the two root zones displaying significant differences in net NH₄⁺ effluxes of P. × canescens.

METHODS

Populus plantlets were cultivated with 500 μM NH₄Cl for 10 days. Six plants were randomly selected to determine the net NH₄⁺ fluxes using a noninvasive microtest technique. High-throughput sequencing were used to determine the dynamic expression profile of miRNA among the different root zones of Populus.

RESULTS

Net NH₄⁺ efflux in zone I (from 0 to 40 mm from the root apex) was - 19.64 pmol cm^-2 s^-1 and in zone II (from 40 to 80 mm) it was - 43.96 pmol cm^-2 s^-1. The expression of eleven miRNAs was significantly upregulated, whereas fifteen miRNAs were downregulated. Moreover, eighty-eight target genes of the significantly differentially expressed miRNAs were identified in root zone II compared with zone I. Particularly, ptc-miR171a/b/e and their target, SCL6, were found to be important for the difference in net NH₄⁺ effluxes in the two root zones. Moreover, the expression of the target of ptc-miR169d, NFYA3 was upregulated in root zone II compared with root zone I, contributing to increased NH₄⁺ efflux and decreased NH₄⁺ assimilation in root zone II.

CONCLUSION

These results indicate that miRNAs regulate the expression levels of their target genes and thus play key roles in net NH₄⁺ fluxes and NH₄⁺ assimilation in different poplar root zones.

Drought and salinity: A comparison of their effects on the ammonium-preferring species Spartina alterniflora

Drought and salinity are the most serious environmental factors affecting crop productivity worldwide; hence, it is important to select and develop both salt- and drought-tolerant crops. The perennial smooth cordgrass Spartina alterniflora Loisel is unusual in that it is highly salt-tolerant and seems to prefer ammonium (NH₄ ⁺ ) over nitrate (NO₃ ^- ) as an inorganic N source. In this study, we determined whether Spartina's unique preference for NH₄ ⁺ enhances performance under salt and drought stress. Greenhouse experiments were conducted to compare the interactive effects of N source, salinity, and low water availability on plant performance (growth and antioxidant metabolism). Drought significantly reduced growth and photosynthetic activity in S. alterniflora, more so with NH₄ ⁺ than NO₃ ^- ; in contrast, NH₄ ⁺ enhanced growth under high salinity. The increased tolerance of S. alterniflora to salt stress in the presence of NH₄ ⁺ was linked to a high level of antioxidant enzyme activity, combined with low MDA content, EL, and H₂ O₂ production. In contrast, drought stress negated the growth advantages for S. alterniflora exposed to salt stress in the presence of NH₄ ⁺ . The susceptibility of S. alterniflora to drought was partly due to reduced antioxidant enzyme activities, thereby reducing the defense against the oxidative damages induced by osmotic stress. In conclusion, in contrast to salt stress, drought stress negates the beneficial effects of ammonium as an N source in the C₄ plant Spartina alterniflora.

Silicon fertigation and salicylic acid foliar spraying mitigate ammonium deficiency and toxicity in Eucalyptus spp. clonal seedlings

Nitrogen deficiency and toxicity, primarily in its ammonium form (NH₄⁺), can suppress plant growth and development. The use of silicon (Si) or salicylic acid (SA) may be an alternative to minimize the harmful effects of nutrient imbalances caused by NH₄⁺, thereby improving the photosynthetic efficiency of plants. The aim of the present study was to assess the action of fertigation-applied Si and SA foliar spraying in mitigating NH₄⁺ toxicity and deficiency in eucalyptus clonal seedlings. Two experiments were performed with eucalyptus clonal seedlings (Eucalyptus urophylla x Eucalyptus grandis), in a greenhouse. Both were carried out using a 4x2 factorial design and four concentrations of NH₄⁺ (5, 15, 30 and 60 mmol L^-1), in the absence and presence of Si (2 mmol L^-1), in experiment I; or with and without SA foliar application (10⁻² mmol L^-1), in experiment II, with six repetitions. Nitrogen content rose as a result of increasing N-NH₄⁺ concentration in the nutrient solution, and Si supplied via the nutrient solution was efficient in increasing the Si content in eucalyptus seedlings. The rise in N-NH₄⁺ concentration favored the maintenance of the photosynthetic apparatus, but high N-NH₄⁺ concentration increased energy loss through fluorescence and decreased the efficiency of photosystem II. The addition of Si to the nutrient solution proved to be beneficial to the photosynthetic apparatus by decreasing F₀ at 15 and 30 mmol L^-1 of NH₄⁺; and F_m at all NH₄⁺ concentrations studied. In addition, the beneficial element also increases F_v/F_m at all NH₄⁺ concentrations studied. SA foliar application was also efficient in reducing photosynthetic energy losses by decreasing F₀ and F_m at all NH₄⁺ concentrations studied. However, SA only increased the F_v/F_m at the high concentrations studied (30 and 60 mmol L^-1 of NH₄⁺). Nitrogen disorder by deficiency or N-NH₄⁺ toxicity reduced shoot dry mass production. The addition of Si to the nutrient solution and SA foliar application increased shoot dry mass production at all N-NH₄⁺ concentrations studied, and benefitted the photosynthetic apparatus by decreasing fluorescence and improving the quantum efficiency of photosystem II as well as dry mass production.

Arabidopsis MYB28 and MYB29 transcription factors are involved in ammonium-mediated alterations of root-system architecture

Ammonium (NH₄⁺) is known to produce alterations in root-system architecture, notably, by inhibiting primary root elongation and stimulating lateral root branching. This stimulation is associated with higher auxin transport promoted by apoplast acidification. Recently, we showed that MYB28 and MYB29 transcription factors play a role in ammonium tolerance, since its double mutant (myb28myb29) is highly hypersensitive toward ammonium nutrition in relation to altered Fe homeostasis. In the present work, we observed that primary root elongation was lower in the mutant with respect to wild-type plants under ammonium nutrition. Moreover, ammonium-induced lateral root branching was impaired in myb28myb29 in a Fe-supply dependent manner. Further research is required to decipher the link between MYB28 and MYB29 functions and the signaling pathway leading to root-system architecture modification by NH₄⁺ supply.

Arabidopsis thaliana transcription factors MYB28 and MYB29 shape ammonium stress responses by regulating Fe homeostasis

Although ammonium (NH₄⁺ ) is a key intermediate of plant nitrogen metabolism, high concentrations of NH₄⁺ in the soil provoke physiological disorders that lead to the development of stress symptoms. Ammonium nutrition was shown to induce the accumulation of glucosinolates (GSLs) in leaves of different Brassicaceae species. To further understand the link between ammonium nutrition and GSLs, we analysed the ammonium stress response of Arabidopsis mutants impaired in GSL metabolic pathway. We showed that the MYB28 and MYB29 double mutant (myb28myb29), which is almost deprived of aliphatic GSLs, is highly hypersensitive to ammonium nutrition. Moreover, we evidenced that the stress symptoms developed were not a consequence of the lack of aliphatic GSLs. Transcriptomic analysis highlighted the induction of an iron (Fe) deficiency response in myb28myb29 under ammonium nutrition. Consistently, ammonium-grown myb28myb29 plants showed altered Fe accumulation and homeostasis. Interestingly, we showed overall that growing Arabidopsis with increased Fe availability relieved ammonium stress symptoms and that this was associated with MYB28 and MYB29 expression. Taken together, our data indicated that the control of Fe homeostasis was crucial for the Arabidopsis response to ammonium nutrition and evidenced that MYB28 and MYB29 play a role in this control.

Combined Transcriptome and Proteome Analysis of Masson Pine (Pinus massoniana Lamb.) Seedling Root in Response to Nitrate and Ammonium Supplementations

Nitrogen (N) is an essential nutrient for plant growth and development. Plant species respond to N fluctuations and N sources, i.e., ammonium or nitrate, differently. Masson pine (Pinus massoniana Lamb.) is one of the pioneer plants in the southern forests of China. It shows better growth when grown in medium containing ammonium as compared to nitrate. In this study, we had grown masson pine seedlings in medium containing ammonium, nitrate, and a mixture of both, and performed comparative transcriptome and proteome analyses to observe the differential signatures. Our transcriptome and proteome resulted in the identification of 1593 and 71 differentially expressed genes and proteins, respectively. Overall, the masson pine roots had better performance when fed with a mixture of ammonium and nitrate. The transcriptomic and proteomics results combined with the root morphological responses suggest that when ammonium is supplied as a sole N-source to masson pine seedlings, the expression of ammonium transporters and other non-specific NH₄⁺-channels increased, resulting in higher NH₄⁺ concentrations. This stimulates lateral roots branching as evidenced from increased number of root tips. We discussed the root performance in association with ethylene responsive transcription factors, WRKYs, and MADS-box transcription factors. The differential analysis data suggest that the adaptability of roots to ammonium is possibly through the promotion of TCA cycle, owing to the higher expression of malate synthase and malate dehydrogenase. Masson pine seedlings managed the increased NH₄⁺ influx by rerouting N resources to asparagine production. Additionally, flavonoid biosynthesis and flavone and flavonol biosynthesis pathways were differentially regulated in response to increased ammonium influx. Finally, changes in the glutathione s-transferase genes suggested the role of glutathione cycle in scavenging the possible stress induced by excess NH₄⁺. These results demonstrate that masson pine shows increased growth when grown under ammonium by increased N assimilation. Furthermore, it can tolerate high NH₄⁺ content by involving asparagine biosynthesis and glutathione cycle.

Barley microRNAs as metabolic sensors for soil nitrogen availability

Barley (Hordeum vulgare) is one of the most important crops in the world, ranking 4th in the worldwide production. Crop breeders are facing increasing environmental obstacles in the field, such as drought, salinity but also toxic over fertilization which not only impacts quality of the grain but also an yield. One of the most prevalent mechanisms of gene expression regulation in plants is microRNA-mediated silencing of target genes. We identified 13 barley microRNAs and 2 microRNAs* that are nitrogen excess responsive. Four microRNAs respond only in root, eight microRNAs only in shoot and one displays broad response in roots and shoots. We demonstrate that 2 microRNAs* are induced in barley shoot by nitrogen excess. For all microRNAs we identified putative target genes and confirmed microRNA-guided cleavage sites for ten out of thirteen mRNAs. None of the identified microRNAs or their target genes is known as nitrogen excess responsive. Analysis of expression pattern of thirteen target mRNAs and their cognate microRNAs showed expected correlations of their levels. The plant microRNAs analyzed are also known to respond to nitrogen deprivation and exhibit the opposite expression pattern when nitrogen excess/deficiency conditions are compared. Thus, they can be regarded as metabolic sensors of the regulation of nitrogen homeostasis in plants.

Exogenous allantoin improves the salt tolerance of sugar beet by increasing putrescine metabolism and antioxidant activities

Allantoin as a nitrogen metabolite can improve the salt tolerance in plants, but its mechanism of action remain elusive. Herein, the effects of pretreatment with exogenous allantoin in salt tolerance were investigated in sugar beet. The seedlings were subjected to salt stress (300 mM Na⁺) without or with different allantoin concentrations (0.01, 0.1, and 1 mM). The effects of allantoin on plant growth, homeostasis, oxidative damage, osmoregulation, and polyamine metabolism were studied. The results showed that salt stress inhibited the net photosynthetic rate and plant growth, and caused oxidative damage. However, these adverse effects were mitigated by exogenous allantoin in a dose-dependent manner, especially at 0.1 mM. Allantoin reduced the accumulation of ROS by increasing the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and AsA content. Under salt stress, allantoin reduced the root concentrations of free putrescine (Put) but increased the free spermine (Spm) in leaves and roots. Furthermore, allantoin decreased the Na⁺/K⁺ ratio and promoted the accumulation of betaine and soluble sugars in leaves and roots. Under salinity conditions, allantoin may enhance the antioxidant system and improve ion homeostasis by enhancing putrescine and/or spermine accumulation. In addition, Pearson's correlation and principal component analysis (PCA) established correlations between physiological parameters, and significant differences between different concentrations of allantoin were observed. In total, exogenous allantoin effectively reduced the oxidative damage and ion toxicity in sugar beet, caused by salinity, this finding would be helpful in improving salt tolerance in plant.

Beneficial soil-borne bacteria and fungi: a promising way to improve plant nitrogen acquisition

Nitrogen (N) is an essential element for plant productivity, thus, it is abundantly applied to the soil in the form of organic or chemical fertilizers that have negative impacts on the environment. Exploiting the potential of beneficial microbes and identifying crop genotypes that can capitalize on symbiotic associations may be possible ways to significantly reduce the use of N fertilizers. The best-known example of symbiotic association that can reduce the use of N fertilizers is the N2-fixing rhizobial bacteria and legumes. Bacterial taxa other than rhizobial species can develop associative symbiotic interactions with plants and also fix N. These include bacteria of the genera Azospirillum, Azotobacter, and Bacillus, some of which are commercialized as bio-inoculants. Arbuscular mycorrhizal fungi are other microorganisms that can develop symbiotic associations with most terrestrial plants, favoring access to nutrients in a larger soil volume through their extraradical mycelium. Using combinations of different beneficial microbial species is a promising strategy to boost plant N acquisition and foster a synergistic beneficial effect between symbiotic microorganisms. Complex biological mechanisms including molecular, metabolic, and physiological processes dictate the establishment and efficiency of such multipartite symbiotic associations. In this review, we present an overview of the current knowledge and future prospects regarding plant N nutrition improvement through the use of beneficial bacteria and fungi associated with plants, individually or in combination.