Peculiarity of the early metabolomic response in tomato after urea, ammonium or nitrate supply

Nitrogen (N) is the nutrient most applied in agriculture as fertilizer (as nitrate, Nit; ammonium, A; and/or urea, U, forms) and its availability strongly constrains the crop growth and yield. To investigate the early response (24 h) of N-deficient tomato plants to these three N forms, a physiological and molecular study was performed. In comparison to N-deficient plants, significant changes in the transcriptional, metabolomic and ionomic profiles were observed. As a probable consequence of N mobility in plants, a wide metabolic modulation occurred in old leaves rather than in young leaves. The metabolic profile of U and A-treated plants was more similar than Nit-treated plant profile, which in turn presented the lowest metabolic modulation with respect to N-deficient condition. Urea and A forms induced some changes at the biosynthesis of secondary metabolites, amino acids and phytohormones. Interestingly, a specific up-regulation by U and down-regulation by A of carbon synthesis occurred in roots. Along with the gene expression, data suggest that the specific N form influences the activation of metabolic pathways for its assimilation (cytosolic GS/AS and/or plastidial GS/GOGAT cycle). Urea induced an up-concentration of Cu and Mn in leaves and Zn in whole plant. This study highlights a metabolic reprogramming depending on the N form applied, and it also provide evidence of a direct relationship between urea nutrition and Zn concentration. The understanding of the metabolic pathways activated by the different N forms represents a milestone in improving the efficiency of urea fertilization in crops.

Consequences of excess urea application on photosynthetic characteristics and nitrogen metabolism of Robinia pseudoacacia seedlings

Urea is the most frequently used nitrogen (N) fertilizer worldwide. However, the mechanisms in plants to cope with excess urea are largely unknown, especially for woody legumes that can meet their N demand by their own N2-fixation capacity. Here, we studied the immediate consequences of different amounts of urea application and exposure duration on photosynthesis, N metabolism, and the activity of antioxidative enzymes of Robinia pseudoacacia seedlings. For this purpose, seedlings were grown for 3 months under normal N availability with rhizobia inoculation and, subsequently, 50 mg N kg-1 was applied to the soil twice with urea as additional N source. Our results show that excess urea application significantly promoted photosynthesis, which increased by 80.3% and 84.7% compared with CK after the 1st and 2nd urea applications, respectively. The increase in photosynthesis translated into an increase in root and nodule biomass of 88.7% and 82.0%, respectively, while leaf biomass decreased by 4.8% after the first application of urea. The N content in leaves was 92.6% higher than in roots, but excess urea application increased the N content of protein and free amino acids in roots by 25.0%, and 43.3%, respectively. Apparently, enhanced root growth and N storage in the roots constitute mechanisms to prevent the negative consequences of excess N in the shoot upon urea application. Nitrate reductase (NR) activity of leaves and roots increased by 74.4% and 26.3%, respectively. Glutathione reductase (GR) activity in leaves and roots was enhanced by 337% and 34.0%, respectively, but then decreased rapidly to the initial level before fertilization. This result shows that not only N metabolism, but also antioxidative capacity was transiently promoted by excess urea application. Apparently, excess urea application initially poses oxidative stress to the plants that is immediately counteracted by enhanced scavenging of reactive oxygen species via enhanced GR activity.

Nitrogen nutrition and xylem sap composition in Zea mays: effect of urea, ammonium and nitrate on ionomic and metabolic profiles

In plants the communication between organs is mainly carried out via the xylem and phloem. The concentration and the molecular species of some phytohormones, assimilates and inorganic ions that are translocated in the xylem vessel play a key role in the systemic nutritional signaling in plants. In this work the composition of the xylem sap of maize was investigated at the metabolic and ionomic level depending on the N form available in the nutrient solution. Plants were grown up to 7 days in hydroponic system under N-free nutrient solution or nutrient solution containing N in form of nitrate, urea, ammonium or a combination of urea and ammonium. For the first time this work provides evidence that the ureic nutrition reduced the water translocation in maize plants more than mineral N forms. This result correlates with those obtained from the analyses of photosynthetic parameters (stomatal conductance and transpiration rate) suggesting a parsimonious use of water by maize plants under urea nutrition. A peculiar composition in amino acids and phytohormones (i.e. S, Gln, Pro, ABA) of the xylem sap under urea nutrition could explain differences in xylem sap exudation in comparison to plants treated with mineral N forms. The knowledge improvement of urea nutrition will allow to further perform good agronomic strategies to improve the resilience of maize crop to water stress.

A mechanistic mathematical model for describing and predicting the dynamics of high-affinity nitrate intake into roots of maize and other plant species

A fully mechanistic dynamical model for plant nitrate uptake is presented. Based on physiological and regulatory pathways and based on physical laws, we form a dynamic system mathematically described by seven differential equations. The model evidences the presence of a short-term positive feedback on the high-affinity nitrate uptake, triggered by the presence of nitrate around the roots, which induces its intaking. In the long run, this positive feedback is overridden by two long-term negative feedback loops which drastically reduces the nitrate uptake capacity. These two negative feedbacks are due to the generation of ammonium and amino acids, respectively, and inhibit the synthesis and the activity of high-affinity nitrate transporters. This model faithfully predicts the typical spiking behavior of the nitrate uptake, in which an initial strong increase of nitrate absorption capacity is followed by a drop, which regulates the absorption down to the initial value. The model outcome was compared with experimental data and they fit quite nicely. The model predicts that after the initial exposure of the roots with nitrate, the absorption of the anion strongly increases and that, on the contrary, the intensity of the absorption is limited in presence of ammonium around the roots.

Salt Stress-Related Mechanisms in Leaves of the Wild Barley Hordeum spontaneum Generated from RNA-Seq Datasets

This study aims to detect salt stress-related genes and mechanisms of the wild barley Hordeum spontaneum. Among the generated RNA-Seq datasets, several regulated transcripts are influenced by levels of cellular carbon, nitrogen and oxygen. Some of the regulated genes act on photorespiration and ubiquitination processes, as well as promoting plant growth and development under salt stress. One of the genes, encoding alanine:glyoxylate aminotransferase (AGT), participates in signaling transduction and proline biosynthesis, while the gene encoding asparagine synthetase (ASN) influences nitrogen storage and transport in plants under stress. Meanwhile, the gene encoding glutamate dehydrogenase (GDH) promotes shoot and root biomass production as well as nitrate assimilation. The upregulated genes encoding alpha-aminoadipic semialdehyde synthase (AASAS) and small auxin-up RNA 40 (SAUR40) participate in the production of proline and signaling compounds, respectively, while the gene encoding E3 ubiquitin-protein ligase regulates the carbon/nitrogen-nutrient response and pathogen resistance, in addition to some physiological processes under biotic and abiotic stresses via signal transduction. The gene encoding the tetratricopeptide repeat (TPR)-domain suppressor of STIMPY (TSS) negatively regulates the carbon level in the cell. In conclusion, this study sheds light on possible molecular mechanisms underlying salt stress tolerance in wild barley that can be utilized further in genomics-based breeding programs of cultivated species.

Chromosome groups 5, 6 and 7 harbor major quantitative trait loci controlling root traits in bread wheat (Triticum aestivum L.)

Identifying genomic regions for root traits in bread wheat can help breeders develop climate-resilient and high-yielding wheat varieties with desirable root traits. This study used the recombinant inbred line (RIL) population of Synthetic W7984 × Opata M85 to identify quantitative trait loci (QTL) for different root traits such as rooting depth (RD), root dry mass (RM), total root length (RL), root diameter (Rdia) and root surface areas (RSA1 for coarse roots and RSA2 for fine roots) under controlled conditions in a semi-hydroponic system. We detected 14 QTL for eight root traits on nine wheat chromosomes; we discovered three QTL each for RD and RSA1, two QTL each for RM and RSA2, and one QTL each for RL, Rdia, specific root length and nodal root number per plant. The detected QTL were concentrated on chromosome groups 5, 6 and 7. The QTL for shallow RD (Q.rd.uwa.7BL: Xbarc50) and high RM (Q.rm.uwa.6AS: Xgwm334) were validated in two independent F₂ populations of Synthetic W7984 × Chara and Opata M85 × Cascade, respectively. Genotypes containing negative alleles for Q.rd.uwa.7BL had 52% shallower RD than other Synthetic W7984 × Chara population lines. Genotypes with the positive alleles for Q.rm.uwa.6AS had 31.58% higher RM than other Opata M85 × Cascade population lines. Further, we identified 21 putative candidate genes for RD (Q.rd.uwa.7BL) and 13 for RM (Q.rm.uwa.6AS); TraesCS6A01G020400, TraesCS6A01G024400 and TraesCS6A01G021000 identified from Q.rm.uwa.6AS, and TraesCS7B01G404000, TraesCS7B01G254900 and TraesCS7B01G446200 identified from Q.rd.uwa.7BL encoded important proteins for root traits. We found germin-like protein encoding genes in both Q.rd.uwa.7BL and Q.rm.uwa.6AS regions. These genes may play an important role in RM and RD improvement. The identified QTL, especially the validated QTL and putative candidate genes are valuable genetic resources for future root trait improvement in wheat.

Copper toxicity compromises root acquisition of nitrate in the high affinity range

The application of copper (Cu)-based fungicides for crop protection plans has led to a high accumulation of Cu in soils, especially in vineyards. Copper is indeed an essential micronutrient for plants, but relatively high concentrations in soil or other growth substrates may cause toxicity phenomena, such as alteration of the plant's growth and disturbance in the acquisition of mineral nutrients. This last aspect might be particularly relevant in the case of nitrate ( NO 3 - ) , whose acquisition in plants is finely regulated through the transcriptional regulation of NO 3 - transporters and plasma membrane H⁺-ATPase in response to the available concentration of the nutrient. In this study, cucumber plants were grown hydroponically and exposed to increasing concentrations of Cu (i.e., 0.2, 5, 20, 30, and 50 µM) to investigate their ability to respond to and acquire NO 3 - . To this end, the kinetics of substrate uptake and the transcriptional modulation of the molecular entities involved in the process have been assessed. Results showed that the inducibility of the high-affinity transport system was significantly affected by increasing Cu concentrations; at Cu levels higher than 20 µM, plants demonstrated either strongly reduced or abolished NO 3 - uptake activity. Nevertheless, the transcriptional modulation of both the nitrate transporter CsNRT2.1 and the accessory protein CsNRT3.1 was not coherent with the hindered NO 3 - uptake activity. On the contrary, CsHA2 was downregulated, thus suggesting that a possible impairment in the generation of the proton gradient across the root PM could be the cause of the abolishment of NO 3 - uptake.

Modulation of plant nitrogen remobilization and postflowering nitrogen uptake under environmental stresses

Plants are sessile organisms that take up nitrogen (N) from the soil for growth and development. At the postflowering stage, N that plants require for seed growth and filling derives from either root uptake or shoot remobilization. The balance between N uptake and N remobilization determines the final carbon (C) and N composition of the seed. The N uptake and N remobilization mechanisms are regulated by endogenous signals, including hormones, developmental stage, and carbon/nitrogen ratio, and by environmental factors. The cellular responses to the environment are relatively well known. However, the effects of environmental stresses on the balance between N uptake and N remobilization are still poorly understood. Thus, this study aims to analyze the impact of environmental stresses (drought, heat, darkness, triggered defense, and low nitrate) on N fluxes within plants during seed filling. Using publicly available Arabidopsis transcriptome data, expression of several marker genes involved in N assimilation, transport, and recycling was analyzed in relation to stress. Results showed that the responses of genes encoding inorganic N transporters, N assimilation, and N recycling are mainly regulated by N limitation, the genes encoding housekeeping proteases are principally sensitive to C limitation, and the response of genes involved in the transport of organic N is controlled by both C and N limitations. In addition, ¹⁵N data were used to examine the effects of severe environmental stresses on N remobilization and N uptake, and a schematic representation of the major factors that regulate the balance between N remobilization and N uptake under the stress and control conditions was provided.

Transcriptomic and metabolomic profiles of Zea mays fed with urea and ammonium

The simultaneous presence of different N-forms in the rhizosphere leads to beneficial effects on nitrogen (N) nutrition in plants. Although widely used as fertilizers, the occurrence of cross connection between urea and ammonium nutrition has been scarcely studied in plants. Maize fed with a mixture of urea and ammonium displayed a better N-uptake efficiency than ammonium- or urea-fed plants (Buoso et al., Plant Physiol Biochem, 2021a; 162: 613-623). Through multiomic approaches, we provide the molecular characterization of maize response to urea and ammonium nutrition. Several transporters and enzymes involved in N-nutrition were upregulated by all three N-treatments (urea, ammonium, or urea and ammonium). Already after 1 day of treatment, the availability of different N-forms induced specific transcriptomic and metabolomic responses. The combination of urea and ammonium induced a prompt assimilation of N, characterized by high levels of some amino acids in shoots. Moreover, ZmAMT1.1a, ZmGLN1;2, ZmGLN1;5, ZmGOT1, and ZmGOT3, as well transcripts involved in glycolysis-TCA cycle were induced in roots by urea and ammonium mixture. Depending on N-form, even changes in the composition of phytohormones were observed in maize. This study paves the way to formulate guidelines for the optimization of N fertilization to improve N-use efficiency in maize and therefore limit N-losses in the environment.

Characterization of physiological and molecular responses of Zea mays seedlings to different urea-ammonium ratios

Despite the wide use of urea and ammonium as N-fertilizers, no information is available about the proper ratio useful to maximize the efficiency of their acquisition by crops. Ionomic analyses of maize seedlings fed with five different mixes of urea and ammonium indicated that after 7 days of treatment, the elemental composition of plant tissues was more influenced by ammonium in the nutrient solution than by urea. Within 24 h, similar high affinity influx rates of ammonium were measured in ammonium-treated seedlings, independently from the amount of the cation present in the nutrient solution (from 0.5 to 2.0 mM N), and it was confirmed by the similar accumulation of ¹⁵N derived from ammonium source. After 7 days, some changes in ammonium acquisition occurred among treatments, with the highest ammonium uptake efficiency when the urea-to-ammonium ratio was 3:1. Gene expression analyses of enzymes and transporters involved in N nutrition highlight a preferential induction of the cytosolic N-assimilatory pathway (via GS, ASNS) when both urea and ammonium were supplied in conjunction, this response might explain the higher N-acquisition efficiency when both sources are applied. In conclusion, this study provides new insights on plant responses to mixes of N sources that maximize the N-uptake efficiency by crops and thus could allow to adapt agronomic practices in order to limit the economic and environmental impact of N-fertilization.

N uptake, assimilation and isotopic fractioning control δ 15N dynamics in plant DNA: A heavy labelling experiment on Brassica napus L

In last decades, a large body of evidence clarified nitrogen isotope composition (δ¹⁵N) patterns in plant leaves, roots and metabolites, showing isotopic fractionation along N uptake and assimilation pathways, in relation to N source and use efficiency, also suggesting ¹⁵N depletion in plant DNA. Here we present a manipulative experiment on Brassica napus var. oleracea, where we monitored δ ¹⁵N of purified, lyophilized DNA and source leaf and root materials, over a 60-days growth period starting at d 60 after germination, in plants initially supplied with a heavy labelled (δ ¹⁵N_Air-N2 = 2100 mUr) ammonium nitrate solution covering nutrient requirements for the whole observation period (470 mg N per plant) and controlling for the labelled N species (ṄH₄, ṄO₃ and both). Dynamics of Isotopic Ratio Mass Spectrometry (IRMS) data for the three treatments showed that: (1) leaf and root δ ¹⁵N dynamics strictly depend on the labelled chemical species, with ṄH₄, ṄO₃ and ṄH₄ṄO₃ plants initially showing higher, lower and intermediate values, respectively, then converging due to the progressive NH₄⁺ depletion from the nutrient solution; (2) in ṄH₄ṄO₃, where δ¹⁵N was not affected by the labelled chemical species, we did not observe isotopic fractionation associated to inorganic N uptake; (3) δ¹⁵N values in roots compared to leaves did not fully support patterns predicted by differences in assimilation rates of NH₄⁺ and NO₃^-; (4) DNA is depleted in ¹⁵N compared to the total N pools of roots and leaves, likely due to enzymatic discrimination during purine biosynthesis. In conclusion, while our experimental setup did not allow to assess the fractionation coefficient (ε) associated to DNA bases biosynthesis, this is the first study specifically reporting on dynamics of specific plant molecular pools such as nucleic acids over a long observation period with a heavy labelling technique.

Transcriptomic and genome-wide association study reveal long noncoding RNAs responding to nitrogen deficiency in maize

BACKGROUND

Long noncoding RNAs (lncRNAs) play important roles in essential biological processes. However, our understanding of lncRNAs as competing endogenous RNAs (ceRNAs) and their responses to nitrogen stress is still limited.

RESULTS

Here, we surveyed the lncRNAs and miRNAs in maize inbred line P178 leaves and roots at the seedling stage under high-nitrogen (HN) and low-nitrogen (LN) conditions using lncRNA-Seq and small RNA-Seq. A total of 894 differentially expressed lncRNAs and 38 different miRNAs were identified. Co-expression analysis found that two lncRNAs and four lncRNA-targets could competitively combine with ZmmiR159 and ZmmiR164, respectively. To dissect the genetic regulatory by which lncRNAs might enable adaptation to limited nitrogen availability, an association mapping panel containing a high-density single-nucleotide polymorphism (SNP) array (56,110 SNPs) combined with variable LN tolerant-related phenotypes obtained from hydroponics was used for a genome-wide association study (GWAS). By combining GWAS and RNA-Seq, 170 differently expressed lncRNAs within the range of significant markers were screened. Moreover, 40 consistently LN-responsive genes including those involved in glutamine biosynthesis and nitrogen acquisition in root were identified. Transient expression assays in Nicotiana benthamiana demonstrated that LNC_002923 could inhabit ZmmiR159-guided cleavage of Zm00001d015521.

CONCLUSIONS

These lncRNAs containing trait-associated significant SNPs could consider to be related to root development and nutrient utilization. Taken together, the results of our study can provide new insights into the potential regulatory roles of lncRNAs in response to LN stress, and give valuable information for further screening of candidates as well as the improvement of maize resistance to LN stress.

Urea-functionalized amorphous calcium phosphate nanofertilizers: optimizing the synthetic strategy towards environmental sustainability and manufacturing costs

Nanosized fertilizers are the new frontier of nanotechnology towards a sustainable agriculture. Here, an efficient N-nanofertilizer is obtained by post-synthetic modification (PSM) of nitrate-doped amorphous calcium phosphate (ACP) nanoparticles (NPs) with urea. The unwasteful PSM protocol leads to N-payloads as large as 8.1 w/w%, is well replicated by using inexpensive technical-grade reagents for cost-effective up-scaling and moderately favours urea release slowdown. Using the PSM approach, the N amount is ca. 3 times larger than that obtained in an equivalent one-pot synthesis where urea and nitrate are jointly added during the NPs preparation. In vivo tests on cucumber plants in hydroponic conditions show that N-doped ACP NPs, with half absolute N-content than in conventional urea treatment, promote the formation of an equivalent amount of root and shoot biomass, without nitrogen depletion. The high nitrogen use efficiency (up to 69%) and a cost-effective preparation method support the sustainable real usage of N-doped ACP as a nanofertilizer.

Evidence that exogenous urea acts as a potent cue to alleviate ammonium-inhibition of root system growth of cotton plant (Gossypium hirsutum)

Many plants grown with low-millimolar concentration of NH₄ ⁺ as a sole nitrogen source develop NH₄ ⁺ -toxicity symptoms. To date, crucial molecular identities and a practical approach involved in the improvement of plant NH₄ ⁺ -tolerance remain largely unknown. By phenotyping of upland cotton grown on varied nitrogen forms, we came across a phenomenon that caused sub-millimolar concentrations of urea (e.g., up 50 μM) to repress the growth inhibition of roots and whole plant cultivated in a NH₄ ⁺ -containing nutrient solution. A growth-recovery assay revealed that the relief in NH₄ ⁺ -inhibited growth required only a short-term exposure (≧12 h) of the roots to urea, implying that urea could elicit an internal signaling and be involved in antagonizing NH₄ ⁺ -sensitivity. Intriguingly, split-root experiments demonstrated that low urea occurrence in one root-half could efficaciously stimulate not only supplied root but also the root-half grown in NH₄ ⁺ -solution without urea, indicating the existence of urea-triggered local and systemic long-distance signaling. In the split-root experiment we also observed high arginase activity, strong arginine reduction and remarkable upregulation of polyamine biosynthesis-related genes (ADC1/2, SPDS and SPMS). Therefore, we suggest that external urea might serve as an effective cue (signal molecule) in an arginine-/polyamine-related process for ameliorating NH₄ ⁺ -suppressed root growth, providing a novel aspect for deeper exploring and understanding plant NH₄ ⁺ -tolerance.

Transcriptomic and network analyses reveal distinct nitrate responses in light and dark in rice leaves (Oryza sativa Indica var. Panvel1)

Nitrate (N) response is modulated by light, but not understood from a genome-wide perspective. Comparative transcriptomic analyses of nitrate response in light-grown and etiolated rice leaves revealed 303 and 249 differentially expressed genes (DEGs) respectively. A majority of them were exclusive to light (270) or dark (216) condition, whereas 33 DEGs were common. The latter may constitute response to N signaling regardless of light. Functional annotation and pathway enrichment analyses of the DEGs showed that nitrate primarily modulates conserved N signaling and metabolism in light, whereas oxidation–reduction processes, pentose-phosphate shunt, starch-, sucrose- and glycerolipid-metabolisms in the dark. Differential N-regulation of these pathways by light could be attributed to the involvement of distinctive sets of transporters, transcription factors, enriched cis-acting motifs in the promoters of DEGs as well as differential modulation of N-responsive transcriptional regulatory networks in light and dark. Sub-clustering of DEGs-associated protein–protein interaction network constructed using experimentally validated interactors revealed that nitrate regulates a molecular complex consisting of nitrite reductase, ferredoxin-NADP reductase and ferredoxin. This complex is associated with flowering time, revealing a meeting point for N-regulation of N-response and N-use efficiency. Together, our results provide novel insights into distinct pathways of N-signaling in light and dark conditions.

The NIN-like protein 5 (ZmNLP5) transcription factor is involved in modulating the nitrogen response in maize

Maize exhibits marked growth and yield response to supplemental nitrogen (N). Here, we report the functional characterization of a maize NIN-like protein ZmNLP5 as a central hub in a molecular network associated with N metabolism. Predominantly expressed and accumulated in roots and vascular tissues, ZmNLP5 was shown to rapidly respond to nitrate treatment. Under limited N supply, compared with that of wild-type (WT) seedlings, the zmnlp5 mutant seedlings accumulated less nitrate and nitrite in the root tissues and ammonium in the shoot tissues. The zmnlp5 mutant plants accumulated less nitrogen than the WT plants in the ear leaves and seed kernels. Furthermore, the mutants carrying the transgenic ZmNLP5 cDNA fragment significantly increased the nitrate content in the root tissues compared with that of the zmnlp5 mutants. In the zmnlp5 mutant plants, loss of the ZmNLP5 function led to changes in expression for a significant number of genes involved in N signalling and metabolism. We further show that ZmNLP5 directly regulates the expression of nitrite reductase 1.1 (ZmNIR1.1) by binding to the nitrate-responsive cis-element at the 5' UTR of the gene. Interestingly, a natural loss-of-function allele of ZmNLP5 in Mo17 conferred less N accumulation in the ear leaves and seed kernels resembling that of the zmnlp5 mutant plants. Our findings show that ZmNLP5 is involved in mediating the plant response to N in maize.

Transgenerational Response to Nitrogen Deprivation in Arabidopsis thaliana

Nitrogen (N) deficiency is one of the major stresses that crops are exposed to. It is plausible to suppose that a stress condition can induce a memory in plants that might prime the following generations. Here, an experimental setup that considered four successive generations of N-sufficient and N-limited Arabidopsis was used to evaluate the existence of a transgenerational memory. The results demonstrated that the ability to take up high amounts of nitrate is induced more quickly as a result of multigenerational stress exposure. This behavior was paralleled by changes in the expression of nitrate responsive genes. RNAseq analyses revealed the enduring modulation of genes in downstream generations, despite the lack of stress stimulus in these plants. The modulation of signaling and transcription factors, such as NIGTs, NFYA and CIPK23 might indicate that there is a complex network operating to maintain the expression of N-responsive genes, such as NRT2.1, NIA1 and NIR. This behavior indicates a rapid acclimation of plants to changes in N availability. Indeed, when fourth generation plants were exposed to N limitation, they showed a rapid induction of N-deficiency responses. This suggests the possible involvement of a transgenerational memory in Arabidopsis that allows plants to adapt efficiently to the environment and this gives an edge to the next generation that presumably will grow in similar stressful conditions.

Urea Addition Promotes the Metabolism and Utilization of Nitrogen in Cucumber

Nitrogen (N) forms include ammonium [NH4+-N], nitrate [NO3−-N], and urea [CO(NH2)2]. Urea is the most common nitrogen fertilizer in agriculture due to its inexpensive price and high N content. Although the reciprocal influence between NO3−-N and NH4+-N is well known, CO(NH2)2 interactions with these inorganic N forms have been poorly studied. We studied the effects of different nitrogen forms with equal nitrogen on dry matter, yield, enzyme activity, and gene expression levels in cucumber. NO3−-N treatment with equal CO(NH2)2 promoted nitrate reduction, urea utilization, and the GS/GOGAT cycle but reduced the nitrate content. UR-2, NR-2, NR-3, NiR, GOGAT-1-1, and GS-4 were upregulated in response to these changes. NH4+-N treatment with equal CO(NH2)2 promoted nitrogen metabolism and relieved the ammonia toxicity of pure NH4+-N treatment. UR-2, GOGAT-2-2, and GS-4 were upregulated, and GDH-3 was downregulated in response to these changes. Treatment with both NO3−-N with added equal CO(NH2)2 and NH4+-N with added equal CO(NH2)2 enhanced the activities of GOGAT, GS, and UR and the amino acid pathway of urea metabolism; manifested higher glutamate, protein, chlorophyll, and nitrogen contents; and improved dry matter weight. A greater proportion of dry matter was distributed to the fruit, generating significantly higher yields. Therefore, the addition of urea to ammonium or nitrate promoted N metabolism and N utilization in cucumber plants, especially treatments with 50% NO3−-N + 50% CO(NH2)2, as the recommended nitrogen form in this study.

Integrated genomics, physiology and breeding approaches for improving nitrogen use efficiency in potato: translating knowledge from other crops

Potato plays a key role in global food and nutritional security. Potato is an N fertiliser-responsive crop, producing high tuber yields. However, excessive use of N can result in environmental damage and high production costs, hence improving nitrogen use efficiency (NUE) of potato plants is one of the sustainable options to address these issues and increase yield. Advanced efforts have been undertaken to improve NUE in other plants like Arabidopsis, rice, wheat and maize through molecular and physiological approaches. Conversely, in potato, NUE studies have predominantly focussed on agronomy or soil management, except for a few researchers who have measured gene expression and proteins relevant to N uptake or metabolism. The focus of this review is to adapt knowledge gained from other plants to inform investigation of N metabolism and associated traits in potato with the aim of improving potato NUE using integrated genomics, physiology and breeding methods.

Physiological and molecular responses in tomato under different forms of N nutrition

Urea is the most common nitrogen (N) fertilizer in agriculture, due to its cheaper price and high N content. Although the reciprocal influence between NO₃^- and NH₄⁺ nutrition are well known, urea (U) interactions with these N-inorganic forms are poorly studied. Here, the responses of two tomato genotypes to ammonium nitrate (AN), U alone or in combination were investigated. Significant differences in root and shoot biomass between genotypes were observed. Under AN+U supply, Linosa showed higher biomass compared to UC82, exhibiting also higher values for many root architectural traits. Linosa showed higher Nitrogen Uptake (NUpE) and Utilization Efficiency (NUtE) compared to UC82, under AN+U nutrition. Interestingly, Linosa exhibited also a significantly higher DUR3 transcript abundance. These results underline the beneficial effect of AN+U nutrition, highlighting new molecular and physiological strategies for selecting crops that can be used for more sustainable agriculture. The data suggest that translocation and utilization (NUtE) might be a more important component of NUE than uptake (NUpE) in tomato. Genetic variation could be a source for useful NUE traits in tomato; further experiments are needed to dissect the NUtE components that confer a higher ability to utilize N in Linosa.

Putative role of glutamine in the activation of CBL/CIPK signalling pathways during salt stress in sorghum

The salt overly sensitive (SOS) pathway is the only mechanism known for Na⁺ extrusion in plant cells. SOS pathway activation involves Ca²⁺-sensing proteins, such as calcineurin B-like (CBL) proteins, and CBL-interacting protein kinases (CIPKs). In this signalling mechanism, a transit increase in cytosolic Ca²⁺ concentration triggered by Na⁺ accumulation is perceived by CBL (also known as SOS3). Afterward, SOS3 physically interacts with a CIPK (also known as SOS2), forming the SOS2/SOS3 complex, which can regulate the number downstream targets, controlling ionic homeostasis. For instance, the SOS2/SOS3 complex phosphorylates and activates the SOS1 plasmalemma protein, which is a Na⁺/H⁺ antiporter that extrudes Na⁺ out of the cell. The CBL-CIPK networking system displays specificity, complexity and diversity, constituting a critical response against salt stress and other abiotic stresses. In a study reported in the journal Plant and Cell Physiology, we showed that NH₄⁺ induces the robust activation of transporters for Na⁺ homeostasis in root cells, especially the SOS1 antiporter and plasma membrane H⁺-ATPase, differently than does NO₃^-. Despite some studies having shown that external NH₄⁺ ameliorates salt-induced effects on ionic homeostasis, there is no evidence that NH₄⁺ per se or some product of its assimilation is responsible for these responses. Here, we speculate about the signalling role behind glutamine in CBL-CIPK modulation, which could effectively activate the SOS pathway in NH₄⁺-fed stressed plants.

Nitrogen assimilation system in maize is regulated by developmental and tissue-specific mechanisms

We found metabolites, enzyme activities and enzyme transcript abundances vary significantly across the maize lifecycle, but weak correlation exists between the three groups. We identified putative genes regulating nitrate assimilation. Progress in improving nitrogen (N) use efficiency (NUE) of crop plants has been hampered by the complexity of the N uptake and utilisation systems. To understand this complexity we measured the activities of seven enzymes and ten metabolites related to N metabolism in the leaf and root tissues of Gaspe Flint maize plants grown in 0.5 or 2.5 mM NO3 (-) throughout the lifecycle. The amino acids had remarkably similar profiles across the lifecycle except for transient responses, which only appeared in the leaves for aspartate or in the roots for asparagine, serine and glycine. The activities of the enzymes for N assimilation were also coordinated to a certain degree, most noticeably with a peak in root activity late in the lifecycle, but with wide variation in the activity levels over the course of development. We analysed the transcriptional data for gene sets encoding the measured enzymes and found that, unlike the enzyme activities, transcript levels of the corresponding genes did not exhibit the same coordination across the lifecycle and were only weakly correlated with the levels of various amino acids or individual enzyme activities. We identified gene sets which were correlated with the enzyme activity profiles, including seven genes located within previously known quantitative trait loci for enzyme activities and hypothesise that these genes are important for the regulation of enzyme activities. This work provides insights into the complexity of the N assimilation system throughout development and identifies candidate regulatory genes, which warrant further investigation in efforts to improve NUE in crop plants.

Both Free Indole-3-Acetic Acid and Photosynthetic Performance are Important Players in the Response of Medicago truncatula to Urea and Ammonium Nutrition Under Axenic Conditions

We aimed to identify the early stress response and plant performance of Medicago truncatula growing in axenic medium with ammonium or urea as the sole source of nitrogen, with respect to nitrate-based nutrition. Biomass measurements, auxin content analyses, root system architecture (RSA) response analyses, and physiological parameters were determined. Both ammonium and ureic nutrition severely affected the RSA, resulting in changes in the main elongation rate, lateral root development, and insert position from the root base. The auxin content decreased in both urea- and ammonium-treated roots; however, only the ammonium-treated plants were affected at the shoot level. The analysis of chlorophyll a fluorescence transients showed that ammonium affected photosystem II, but urea did not impair photosynthetic activity. Superoxide dismutase isoenzymes in the plastids were moderately affected by urea and ammonium in the roots. Overall, our results showed that low N doses from different sources had no remarkable effects on M. truncatula, with the exception of the differential phenotypic root response. High doses of both ammonium and urea caused great changes in plant length, auxin contents and physiological measurements. Interesting correlations were found between the shoot auxin pool and both plant length and the "performance index" parameter, which is obtained from measurements of the kinetics of chlorophyll a fluorescence. Taken together, these data demonstrate that both the indole-3-acetic acid pool and performance index are important components of the response of M. truncatula under ammonium or urea as the sole N source.

Molecular and physiological interactions of urea and nitrate uptake in plants

While nitrate acquisition has been extensively studied, less information is available on transport systems of urea. Furthermore, the reciprocal influence of the two sources has not been clarified, so far. In this review, we will discuss recent developments on plant response to urea and nitrate nutrition. Experimental evidence suggests that, when urea and nitrate are available in the external solution, the induction of the uptake systems of each nitrogen (N) source is limited, while plant growth and N utilization is promoted. This physiological behavior might reflect cooperation among acquisition processes, where the activation of different N assimilatory pathways (cytosolic and plastidic pathways), allow a better control on the nutrient uptake. Based on physiological and molecular evidence, plants might increase (N) metabolism promoting a more efficient assimilation of taken-up nitrogen. The beneficial effect of urea and nitrate nutrition might contribute to develop new agronomical approaches to increase the (N) use efficiency in crops.

Short-Term Treatment with the Urease Inhibitor N-(n-Butyl) Thiophosphoric Triamide (NBPT) Alters Urea Assimilation and Modulates Transcriptional Profiles of Genes Involved in Primary and Secondary Metabolism in Maize Seedlings

To limit nitrogen (N) losses from the soil, it has been suggested to provide urea to crops in conjunction with the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT). However, recent studies reported that NBPT affects urea uptake and urease activity in plants. To shed light on these latter aspects, the effects of NBPT were studied analysing transcriptomic and metabolic changes occurring in urea-fed maize seedlings after a short-term exposure to the inhibitor. We provide evidence that NBPT treatment led to a wide reprogramming of plant metabolism. NBPT inhibited the activity of endogenous urease limiting the release and assimilation of ureic-ammonium, with a simultaneous accumulation of urea in plant tissues. Furthermore, NBPT determined changes in the glutamine, glutamate, and asparagine contents. Microarray data indicate that NBPT affects ureic-N assimilation and primary metabolism, such as glycolysis, TCA cycle, and electron transport chain, while activates the phenylalanine/tyrosine-derivative pathway. Moreover, the expression of genes relating to the transport and complexation of divalent metals was strongly modulated by NBPT. Data here presented suggest that when NBPT is provided in conjunction with urea an imbalance between C and N compounds might occur in plant cells. Under this condition, root cells also seem to activate a response to maintain the homeostasis of some micronutrients.

Resolving the Role of Plant NAD-Glutamate Dehydrogenase: III. Overexpressing Individually or Simultaneously the Two Enzyme Subunits Under Salt Stress Induces Changes in the Leaf Metabolic Profile and Increases Plant Biomass Production

NAD-dependent glutamate dehydrogenase (NAD-GDH) of higher plants has a central position at the interface between carbon and nitrogen metabolism due to its ability to carry out the deamination of glutamate. In order to obtain a better understanding of the physiological function of NAD-GDH under salt stress conditions, transgenic tobacco (Nicotiana tabacum L.) plants that overexpress two genes from Nicotiana plumbaginifolia individually (GDHA and GDHB) or simultaneously (GDHA/B) were grown in the presence of 50 mM NaCl. In the different GDH overexpressors, the NaCl treatment induced an additional increase in GDH enzyme activity, indicating that a post-transcriptional mechanism regulates the final enzyme activity under salt stress conditions. A greater shoot and root biomass production was observed in the three types of GDH overexpressors following growth in 50 mM NaCl, when compared with the untransformed plants subjected to the same salinity stress. Changes in metabolites representative of the plant carbon and nitrogen status were also observed. They were mainly characterized by an increased amount of starch present in the leaves of the GDH overexpressors as compared with the wild type when plants were grown in 50 mM NaCl. Metabolomic analysis revealed that overexpressing the two genes GDHA and GDHB, individually or simultaneously, induced a differential accumulation of several carbon- and nitrogen-containing molecules involved in a variety of metabolic, developmental and stress-responsive processes. An accumulation of digalactosylglycerol, erythronate and porphyrin was found in the GDHA, GDHB and GDHA/B overexpressors, suggesting that these molecules could contribute to the improved performance of the transgenic plants under salinity stress conditions.

High and Low Affinity Urea Root Uptake: Involvement of NIP5;1

Urea is the most widespread nitrogen (N) fertilizer worldwide and is rapidly degraded in soil to ammonium by urease. Ammonium is either taken up by plant roots or is further processed to nitrate by soil microorganisms. However, urea can be taken up by roots and is further degraded to ammonium by plant urease for assimilation. When urea is supplied under sterile conditions, it acts as a poor N source for seedlings or adult Arabidopsis thaliana plants. Here, the gene expression of young seedlings exposed to urea and ammonium nitrate nutrition was compared. Several primary metabolism and transport genes, including those for nitrate and urea, were differentially expressed in seedlings. However, urease and most major intrinsic proteins were not differentially expressed, with the exception of NIP6;1, a urea-permeable channel, which was repressed. Furthermore, little overlap with the gene expression with ammonium as the sole N source was observed, confirming that pure urea nutrition is not associated with the ammonium toxicity syndrome in seedlings. The direct root uptake of urea was increased under boron deficiency, in both the high and low affinity range. This activity was entirely mediated by the NIP5;1 channel, which was confirmed to transport urea when expressed in oocytes. The uptake of urea in the high and low affinity range was also determined for maize and wheat roots. The urea uptake by maize roots was only about half that of wheat, but was not stimulated by boron deficiency or N deficiency in either species. This analysis identifies novel components of the urea uptake systems in plants, which may become agronomically relevant to urea uptake and utilization, as stabilized urea fertilizers become increasingly popular.

The Urease Inhibitor NBPT Negatively Affects DUR3-mediated Uptake and Assimilation of Urea in Maize Roots

Despite the widespread use of urease inhibitors in agriculture, little information is available on their effect on nitrogen (N) uptake and assimilation. Aim of this work was to study, at physiological and transcriptional level, the effects of N-(n-butyl) thiophosphoric triamide (NBPT) on urea nutrition in hydroponically grown maize plants. Presence of NBPT in the nutrient solution limited the capacity of plants to utilize urea as a N-source; this was shown by a decrease in urea uptake rate and (15)N accumulation. Noteworthy, these negative effects were evident only when plants were fed with urea, as NBPT did not alter (15)N accumulation in nitrate-fed plants. NBPT also impaired the growth of Arabidopsis plants when urea was used as N-source, while having no effect on plants grown with nitrate or ammonium. This response was related, at least in part, to a direct effect of NBPT on the high affinity urea transport system. Impact of NBPT on urea uptake was further evaluated using lines of Arabidopsis overexpressing ZmDUR3 and dur3-knockout; results suggest that not only transport but also urea assimilation could be compromised by the inhibitor. This hypothesis was reinforced by an over-accumulation of urea and a decrease in ammonium concentration in NBPT-treated plants. Furthermore, transcriptional analyses showed that in maize roots NBPT treatment severely impaired the expression of genes involved in the cytosolic pathway of ureic-N assimilation and ammonium transport. NBPT also limited the expression of a gene coding for a transcription factor highly induced by urea and possibly playing a crucial role in the regulation of its acquisition. This work provides evidence that NBPT can heavily interfere with urea nutrition in maize plants, limiting influx as well as the following assimilation pathway.