Physiological, molecular, and environmental insights into plant nitrogen uptake, and metabolism under abiotic stresses

Abstract

Nitrogen (N) as an inorganic macronutrient is inevitable for plant growth, development, and biomass production. Many external factors and stresses, such as acidity, alkalinity, salinity, temperature, oxygen, and rainfall, affect N uptake and metabolism in plants. The uptake of ammonium (NH₄⁺) and nitrate (NO₃⁻) in plants mainly depends on soil properties. Under the sufficient availability of NO₃⁻ (>1 mM), low‐affinity transport system is activated by gene network NRT1, and under low NO₃⁻ availability (<1 mM), high‐affinity transport system starts functioning encoded by NRT2 family of genes. Further, under limited N supply due to edaphic and climatic factors, higher expression of the AtNRT2.4 and AtNRT2.5T genes of the NRT2 family occur and are considered as N remobilizing genes. The NH₄⁺ ion is the final form of N assimilated by cells mediated through the key enzymes glutamine synthetase and glutamate synthase. The WRKY1 is a major transcription factor of the N regulation network in plants. However, the transcriptome and metabolite profiles show variations in N assimilation metabolites, including glycine, glutamine, and aspartate, under abiotic stresses. The overexpression of NO₃⁻ transporters (OsNRT2.3a and OsNRT1.1b) can significantly improve the biomass and yield of various crops. Altering the expression levels of genes could be a valuable tool to improve N metabolism under the challenging conditions of soil and environment, such as unfavorable temperature, drought, salinity, heavy metals, and nutrient stress.

Abbreviations

• AMT
• ammonium transporter
• GDH
• glutamate dehydrogenase
• GOGAT
• glutamate synthase
• GS
• glutamine synthetase
• HATS
• high-affinity transport system
• LATS
• low-affinity transport system
• LHT1
• lysine-histidine-type transporter 1
• NR
• nitrate reductase
• NRT
• nitrate transporter
• NUE
• nitrogen-use efficiency

INTRODUCTION

Nitrogen (N) is a fundamental inorganic macronutrient and is essential in greater quantity for the growth, development, biomass production, and yield of plant by direct control on the metabolic functions in plants (X. Fan et al., 2017; Naz et al., 2019). It is an integral part of amino acids, proteins, nucleic acids, lipids, chlorophyll (most importantly), and many other N-containing metabolites (Kusano et al., 2011). The changes in physiological processes of plants in response to N fertilization determined the quality and quantity of crop yields (Y. Chen et al., 2013). However, plants used N in the predominant form of nitrate (NO₃⁻) and then assimilated it into amino acid using glutamine synthetase (GS), nitrate reductase (NR), glutamate dehydrogenase (GDH), and glutamate synthase (GOGAT) pathways (Tang et al., 2019). The primary substrates for protein synthesis in plant cells is amino acid, which become the main constituents of plant biomass through the classification, modification, transport, and storage processes (M. I. R. Khan et al., 2016).

Plants have naturally been evolved to survive in heterogeneous environmental conditions and are exposed to various biotic and abiotic stresses (Lin et al., 2016). Further, abiotic stress factors such as acidity, alkalinity, and salinity can adversely influence N fixation and symbiotic pathways (Lin et al., 2012; Wahid et al., 2007). A large number of external factors, including temperature, oxygen, and rainfall, are involved to impede the formation of N fixation and symbiosis (Lin et al., 2016; Wahid et al., 2007). The physiology and biochemistry of N in plants is multi-step process consisting of acquisition, assimilation, transport, and remobilization for different cellular metabolic processes. The uptake and assimilation of N are the key processes that are closely integrated with each other to cope up the N deficit of the entire plant. In plants, N metabolism is rapidly synchronized to respond in the internal and external stimuli, and multiple mechanisms regulate the metabolic pathway, such as the involvement of transporters and metabolic enzymes (Miflin & Habash, 2002). Considering this discussion, many studies have focused on the understandings of different abiotic stresses on plant N uptake, growth, morphology, and biomass production. However, our review aims to understand the effects of environmental and edaphic factors on N availability and uptake, and the use of physiological and molecular approaches to gain insights into plant N metabolism under various abiotic stress conditions.

NITROGEN SOURCES AND UPTAKE IN PLANTS

The major forms of inorganic N are NH₄⁺, NO₃⁻, and nitrite (NO₂⁻), whereas organic N occurs in many forms, such as amino acids, proteins, nucleic acids, nucleotides, and urea. Organic N must be converted to NH₄⁺, known as N mineralization. Further, soil microorganisms converted different form of N into NO₃⁻ or NH₄⁺. While organic N, such as crop residues and animal wastes are rich in N, is also added to the soil, legume crops are used as a source of N-fixing and as a green manure. However, N from these sources will be converted to NO₃⁻ before the use of crop under anerobic condition of flooded rice. The major fluxes in the N cycle include denitrification (using NO₃⁻ to oxidize organic C, yielding N₂), oxidation of NH₄⁺ to NO₃⁻ by chemosynthetic bacteria, assimilation of NH₄⁺, N fixation by bacteria, and the excretion of NH₄⁺ by heterotrophs (Dodds & Whiles, 2020).

Rhizosphere soil has more microbes, hence maintaining water uptake, improved nutrient availability, and growth and development of plants (McNear, 2013). Symbiotic microorganisms help to improve plant ability to cope with phytopathogens, induce systematic resistance, promote plant growth, and yield by colonizing plant roots. For example, plant growth-promoting rhizobacteria (PGPR) minimizes the detrimental effects of phytopathogens on plant biomass (Harman & Uphoff, 2019; Sathya et al., 2017). PGPR have been shown to increase plant growth by various mechanisms, including fixing atmospheric N, improving iron influx, biosynthesis of plant hormones (auxin and cytokinin), and solubilization of nutrients (Ryu et al., 2020). Pseudomonas, Bacillus, Rhizobium, and Azospirillum species have been found to significantly enhance crop growth and have become important components for the development of sustainable agriculture (Igiehon & Babalola, 2018). Several key genes are responsible for N uptake from rhizosphere soil and N assimilation into amino acids (Castro-Rodríguez et al., 2016, 2017). Genetic modification of host plant for root-related genes promotes the development and proliferation of symbiotic microorganisms, which are important tools to provide sustainable crop production (Clouse & Wagner, 2021).

Malate is a primary C source incorporates into the bacterium through carboxylate process, and, as a result, rhizobium improves nutrient acquisition and N availability for plant (Figure 1). Plants secrete several organic acids in soil to neutralize the effects of OH⁻ ions and maintain soil pH. Plants acquire inorganic N in the form of NH₄⁺ and NO₃⁻, and their availability depends on soil properties, including pH, texture, moisture content, and organic soil (Rennenberg et al., 2010). Nitrate acquisition is mediated by a transporters’ gene family comprising 68 PtNPF genes that encode peptide transporters and NO₃⁻ (Hachiya & Sakakibara, 2017). While NH₄⁺ acquisition is mediated by 22 genes, it is subdivided into two subfamilies: AMT1 and AMT2 (where AMT is ammonium transporter) (Calabrese et al., 2017; Wu et al., 2015). Overexpression of NO₃⁻ transporters (OsNRT2.3a and OsNRT1.1b) can significantly improve the biomass and yield of rice (X. Fan et al., 2017). Moreover, Frugier et al. (2000) conducted a study on Medicago sativa to show that the Mszpt2-1 gene (Kruppel-like zinc finger protein) is involved in vegetative organogenesis, development of N-fixing nodule, and controlling nodule differentiation. They also suggested that zinc finger transcription family may regulate various other plant developmental processes.

Core Ideas

• Uptake of ammonium (NH₄⁺) and nitrate (NO₃⁻) in plants mainly depends on soil properties.
• WRKY1 is a major transcription factor of the N regulation network in plants.
• The overexpression of NO₃⁻ transporters (OsNRT2.3a and OsNRT1.1b) significantly improves biomass and yield of crops.
• Higher expression of the AtNRT2.4 and AtNRT2.5T genes occurred and are considered as N remobilizing genes.
• Climatic conditions and agronomic practices significantly promoted nitrogen-use efficiency.

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MOLECULAR APPROACHES FOR UNDERSTANDING N REGULATION AND METABOLISM UNDER ABIOTIC STRESS

Nitrogen assimilation into various compounds

Different organic compound secreted by plant roots affects the quantity of different forms of nitrogen (S.-C. Fan et al., 2009). Ammonium and NO₃⁻ are absorbed by plants via their respective transporters. Uptake of N by plants is in the form of NO₃⁻, which needs to be reduced to NH₄⁺ before further assimilation for amino acid biosynthesis, followed by the biosynthesis of purines and pyrimidines (Schjoerring et al., 2002). Within plants, the reduction of NO₃⁻ is processed in two stages: (1) In the cytoplasm, a NR enzymes reduced nitrate into nitrite and (2) then reduced into ammonia in the chloroplasts. Further, glutamine was formed by the incorporation of ammonium into glutamate. However, other amino acids, such as asparagine, are called the GS-GOGAT pathway by transamination (Masclaux-Daubresse et al., 2006). The NH₄⁺ gets protonated at the physiological pH and is converted into NH₄⁺ ions. Moreover, the main amino acid is glutamic acid, while glutamine and asparagine are formed by the addition of other amino groups into glutamic acid and aspartic acid (Masclaux-Daubresse et al., 2006). These amides are transported through xylem vessels to other parts of the plants. While the enzyme GDH does not play a direct role in the assimilation, it protects the mitochondrial functions during the periods of high N metabolism and takes part in N remobilization (Masclaux-Daubresse et al., 2006).

Early studies on nucleic acids using N¹⁵-labeled urea and ammonium citrate on pigeons indicated that NH₄⁺ was enriched in the purines extracted from uric acid in the excreta. Results also showed that purines are being produced being a function of internal proteins synthesis and degradation, and N in the purine ring comes from a biologically labile pool of N that undergoes various conversions in the body (Barnes & Schoenheimer, 1943). Furthermore, Le Bozec and Moody (2009) reported that alkaloids are a group of organic compounds containing N in their molecular hetero-structure and play a crucial role in therapeutic effects, for example, vinca alkaloids, quinine, ephedrine, atropine, scopolamine, and caffeine.

Genes and transcription factors involved in N regulation and metabolism

At the molecular level, strategies to combat abiotic stresses can be categorized into pre- and post-genomic eras. The primary approach for identifying genes that confer tolerance to abiotic stresses, such as high UV radiation, extreme high and low temperatures, drought, salinity, and heavy metals, was forward genetics during the pre-genomic era. Marker-assisted selection was widely employed to confirm the transfer of tolerant genes or genomic regions into target plants. For example, the salinity tolerance of wheat was increased by adding HKT allele in durum wheat from Triticum monococcum (James et al., 2012). To transfer genes such as Kin1 in Arabidopsis, OsB28 in rice, and PKABA1 in wheat were utilized by molecular markers, which confer tolerance against salinity, cold, and drought stress, respectively (Winicov, 1998). Furthermore, alternative molecular approaches, including induced mutations, somaclonal variations, differential display, and differentiation screening, have been employed. However, these methods have shown limited success in elucidating the underlying mechanisms and identifying candidates for abiotic stress tolerance.

Nitrate is the inorganic form of N taken by plants most frequently and efficiently under various abiotic stresses (Houdusse et al., 2007) (Table 1). Nitrogen metabolism is controlled by changes in mRNA levels of several metabolites responsible for N uptake and assimilation. Nitrate is sensed in rhizosphere by the dual-affinity transporter and sensor (transceptor) and nitrate transporter (NRT), NRT1.1, which is also involved in transportation of auxin by NO₃⁻ suppression (Krouk et al., 2010). Under low NO₃⁻ availability conditions, it suppresses the lateral root formation by transporting auxin out of these roots. The NRT1.1 gene, formally named as CHL1, belongs to the NRT1 family of genes comprising 53 members in Arabidopsis (Tsay et al., 2007). This gene was first classified as low-affinity transport system (LATS) and expressed only after induced by NO₃⁻ concentrations suggesting its function as stimulate components in LATS. The absorption and utilization of NO₃⁻ in rice is a complex process, including absorption, translocation, and assimilation. In order to adapt to the changing environment, plants have evolved two NO₃⁻ transport systems, LATS, and high-affinity transport systems (HATS), and are responsible for NRT1/PTR (NPF) family members and NRT2 family members, respectively. The NRT2 family in rice includes OsNRT2.1, OsNRT2.2, OsNRT2.3a, OsNRT2.3b, and OsNRT2.4 (Liu et al., 2014; Song et al., 2020; Wei et al., 2018). Further, NO₃⁻ transport activity was related with the interaction of OsNAR2.1 with OsNRT2.1, OsNRT2.2, and OsNRT2.3a (Liu et al., 2014). Two members of NRT1 gene help in the uptake of NO₃⁻ from soil solution to plant roots, whereas three AMT genes are involved in NH₄⁺ uptake, and the members of NRT family are involved in N signaling, transportation, assimilation, and storage pathways. Cellular structure shows the magnified picture of root transport mechanisms via NO₃⁻ and NH₄⁺ transporters. There were about 400 deferentially expressed transcripts, among which NO₃⁻ transporter 1.5 (NRT1.5) and amino acid transporter (At4g38250) showed pronounced expression. Upregulation in HATS H NRT2.1 was also observed suggesting that NRT1.1 is essential for NO₃⁻ acquisition and regulation of subsequent genes (Muños et al., 2004). Under the N deficit conditions, such as drought and salinity, the expression of AtNRT2.1 and AtNRT2.2 is induced in the mature root instead of root tips. The AtNRT2.1 gene family comprises of seven genes in Arabidopsis (Orsel et al., 2002), and β-glucuronidase (GUS) expression directed by native promoter revealed the localization in epidermis, cortex, and endodermis of mature roots (Nazoa et al., 2003). The functions of NRT1.1 are influenced by the phosphorylation status of residue T101 and also regulates the expression of assimilating genes beside nitrate acquisition (B. Hu et al., 2015). The recognition and importance of NRT1.1 was observed by SAGE approach in Arabidopsis, and the study also focused on the comparisons between transcripts in the roots of mutant and wild plants grown in the presence of ammonium nitrate (Muños et al., 2004). In HATS, NRT2.1 is a major protein but it works in coordination with another NO₃⁻ transporter, namely, NAR2 (Okamoto et al., 2006; Orsel et al., 2006). It was discovered that NAR2, despite stabilizing NRT2.1, was also found in association with NRT2.1 in the form of tetramer comprising two subunits on plasma membrane (Yong et al., 2010). After the acquisition of N in the form of NO₃⁻, it has to be translocated into the aerial parts of plants because every cell needs N for regular functioning. As far as upward movement is concerned, petiole plays very important role in the distribution of NO₃⁻ toward leaves, and AtNRT1.4 is an important gene that acts as a transporter and is strongly expressed in petioles and midrib. In AtNRT1.8 mutants, xylem sap containing more NO₃⁻ was an indication that the protein localized in plasma membrane belongs to LATS functioning in NO₃⁻ transport across parenchyma cells to unload NO₃⁻ from xylem sap. These two genes, AtNRT1.5 and AtNRT1.8, encode the proteins that help NO₃⁻ loading in root stele and nitrate unloading from shoot or root, respectively (J.-Y. Li et al., 2010). Plants exhibit adaptive responses via modifying physiological and biochemical processes, consequently directing to the alterations in the expression of numerous genes during abiotic stresses. This gene expression analysis is typically performed using customized microarray chips, where DNA sequences, represented on microchips plant genes, are immobilized and used as substrates for hybridization. Through this process, the expression levels of different genes in a sample can be quantified, providing comprehensive valued content of the relative expression of genes in response to different abiotic stresses. However, it is important to note that microarray analysis of stress-responsive genes has certain technical limitations, such as cross-hybridization and background noise, that impact the accuracy and reliability of the results. Several extensive studies of microarray showed the effects of different stresses on gene expression in various plant species, including Arabidopsis, rice, wheat, corn, soybean, and tomato. The expression data resulting from these studies are publicly available through online databases such as Genevestigator.

TABLE 1 Genes responsible for N acquisition, accumulation, assimilation, and transport within the plant.

Gene	Plant	Functions	Effects on NO3−	Effect on NH4+	References
NPF6.1	Rice (Oryza sativa L.)	NO3− uptake; must be activated by NAC42 transcriptional factor	Increased uptake/concentration		(Bernal et al., 2012)
NPF7.4	Rice (Oryza sativa L.)	Determine axillary bud outgrowth, NO3− uptake	Increased in-flux/concentration		(Tang et al., 2019)
NPF7.7	Rice (Oryza sativa L.)	Two splicing variants transport distinct N forms; upregulate GS1.2, NPF6.5; Downregulate fine Culm1 (FC1), Dwarf3	Increased in-flux/concentration for variant 1 only	Increased in-flux/concentration for both variant, higher for variant 2	(W. Huang et al., 2019)
AMT1;1	Rice (Oryza sativa L.)	NH4+ uptake under low and high NH4+ conditions; N/K homeostasis		Increased uptake	(Konishi & Ma, 2021; C. Li et al., 2016)
AtNRT1.8	Arabidopsis (Arabidopsis thaliana L.)	Removal of NO3− from xylem vessels	Mediates low-affinity NO3− uptake		(J.-Y. Li et al., 2010)
AtNRT1.9	Arabidopsis (Arabidopsis thaliana L.)	Plasma membrane transporter expressed in the companion cells of root phloem	Increases NO3− in root phloem		(Y.-Y. Wang & Tsay, 2011)
AtNRT1.11 and AtNRT1.12	Arabidopsis (Arabidopsis thaliana L.)	Redistributing phloem N to the youngest tissues	Growth enhanced by increasing external NO3−		(Hsu & Tsay, 2013)
TaNRT2.3	Wheat (Triticum aestivum L.)	High-affinity NO3− uptake	Unknown		(Zhao et al., 2004)
OsNRT2.4	Rice (Oryza sativa L.)	Putative high-affinity NO3− transporter	Unknown		(Feng et al., 2011)
OsNRT2.3	Rice (Oryza sativa L.)	High-affinity NO3− transporter, sensing cytosolic pH changes	Improved grain yield and N-use efficiency		(Feng et al., 2011)
NAC2-5A	Wheat (Triticum aestivum L.)	Regulate expression of NO3− transporter and GS	Increased uptake		(M. Hu et al., 2018)
RDD1	Rice (Oryza sativa L.)	Upregulate AMT1;3, GS1;1; Uptake of N, P, K, Na, Mg, Cl, S, and Ca	Increased uptake	Increased uptake/accumulation	(Iwamoto & Tagiri, 2016)
N assimilation
AtNIA1 and AtNIA2	Promoted nitrate assimilation in Arabidopsis (Arabidopsis thaliana L.)	(Park et al., 2011)
Nii genes	NO3− is reduced to NH4+ by NiR enzyme	(Meyer & Stitt, 2001)
Glutamine synthetase	Assimilate NH4+ and from various internal nitrogen recycling pathways	(Bernard et al., 2009)
GS glutamine	Production of glutamate incorporated into other amino acids through the action of aminotransferases or transaminases	(Forde & Lea, 2007)
Transport of organic N in the phloem of rice and in the xylem of poplar	(Yamaya & Oaks, 2004)
Protein	Tobacco and ryegrass leaves, protein degradation during senescence coincides with increased levels of NH4+	(Masclaux et al., 2000; Mattsson & Schjoerring, 2003)
Amino acid	Protein breakdown converted to alternative amino acids and/or exported via the phloem to developing organs	(Hortensteiner & Feller, 2002)

The translocation of N toward aerial parts may be subjected to temporary storage in vacuole and can later be exported from vacuole toward growing organs or remobilized from older leaves into new leaves under N deficit conditions (Masclaux-Daubresse et al., 2010). Further, X. Fan et al. (2017) reported that AtNRT1.7 was characterized and found within the membrane of companion cells. Expression of AtNRT1.7 was induced during N starvation despite not being able to perform efficient N mobilization and resulted in stunted growth under abiotically stressed conditions (S.-C. Fan et al., 2009). Two other genes of NRT2 family (AtNRT2.4 and AtNRT2.5) were also considered as N-remobilizing genes, showing increased expression under N starvation and repressed under sufficient N (Orsel et al., 2002). Nevertheless, 12 genes are important in N acquisition and transport, and their expression varies with the changes in N concentrations in soil and plant (Figure 2; Table 1).

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A significant body of research has focused on the enzymes responsible for N assimilation and remobilization during plant growth and development (Miflin & Habash, 2002). Ammonium ion is incorporated in cells as amino acids via N enzymes such as GS and GOGAT and glutamine-oxoglutarate amino-transferase in plastids. Subsequently, NH₄⁺ assimilation occurs in the form of glutamine amide groups through GS (Coleman et al., 2012). The formation of glutamate, glutamine, and 2-oxoglutarate reacted in the presence of Gln synthetase and Glu synthase within chloroplast (McAllister et al., 2012). Glutamate synthesis occurs in the mitochondria in the presence of GDH by the reaction of NH₄⁺ and 2-oxoglutarate under specific conditions (McAllister et al., 2012). Transcription factors operate as major switches in plant N regulatory networks (Zuluaga & Sonnante, 2019). WRKY1 is a member of the WRKY family of TFs and has diverse regulatory functions in response to biotic and abiotic stresses. WRKY TFs have been shown to activate or repress transcription and, in some instances, have dual activator/repressor functions (Jia et al., 2015).

In a recent study on Arabidopsis, network analysis of WRKY1 predicted its role in N assimilation pathway, and WRKY1-1 null mutants showed genome-wide reprograming of transcripts in N and L signaling pathways but were redundant in functions compared with other WRKY family members. The transcriptome and metabolite profile showed variations in important metabolites of N assimilation, for example, glycine, glutamine, and aspartate (Heerah et al., 2019).

The first N-based transcription factor ANR1 identified for plant development was a root-specific MADS-box protein that regulates the production of lateral roots in response to nitrate (Zhang & Forde, 1998). Plants failed to produce normal lateral roots in response to NO₃⁻ supply in the rhizosphere, as ANR1 was induced by NO₃⁻ in roots by using reverse genetics approaches with reduced ANR1 levels (Zhang & Forde, 1998). However, later studies showed that ANR1 was influenced by N deficiency and repressed by NO₃⁻ resupply (Liao et al., 2008). The N feedback-dependent regulatory role of ANR1 in lateral root growth in plants. Moreover, GATA and DOF family transcription factors are also important in controlling N regulation in plants. In Arabidopsis, 30 transcription factors belonging to the GATA family are identified and are involved in C metabolism genes induced by NO₃⁻ concentrations in the shoots (Bi et al., 2005). While TF of DOF1 from maize was overexpressed in Arabidopsis and potato, resulting in the over-acceleration of N metabolites, alterations in organic acid metabolism, and overexpression in N and C metabolism genes (Yanagisawa et al., 2004). The promotor of NRT2.1 gene fused with GUS reporter gene suggested that the 150-bp region has some cis-regulatory elements for NRT2.1 by NO₃⁻ and sucrose (Girin et al., 2007).

Transporters’ role for N metabolism in cereals

The miR169 family of molecules plays a crucial role in the regulation of nitrogen transporters, particularly under conditions of N deprivation. When subjected to a scarcity of N, miR169 experiences a significant decrease in expression, while its target genes belonging to the NFYA family exhibit a substantial increase in expression in both the root and shoot tissues. Remarkably, when miR169a is artificially overexpressed, it acts as a suppressor of NFYA expression, resulting in heightened sensitivity to N starvation in plants. This sensitivity is accompanied by a downregulation of the NO₃⁻ transporter genes NRT2.1 and NRT1.1, indicating that miR169 plays a vital role in N uptake and redistribution within plants (Hu et al., 2018). In the context of maize, it has been observed that miR169 expression notably declines in N-deficient plants (Gao et al., 2019) (Table 1). Overexpression of OsNRT1.1A in rice greatly improved N utilization, grain yield, and maturation time, which was also significantly shortened. Similarly, bread wheat plants exposed to N-starved conditions exhibit an upregulation of NFYA genes (Lee et al., 2020) (Figure 1). The introduction of TaNFYA-B1, a specific NFYA gene, into soft wheat has been found to enhance lateral branching and promote the expression of NO₃⁻ transporters, resulting in improved N uptake and increased grain yields under N-deficient conditions (Lee et al., 2020). By considering these findings and drawing comparisons with existing literature, it is plausible to suggest that the CCAAT-TF WHAP6 could serve as an activator of N transport, making it a potential candidate gene for genetic enhancement programs aimed at increasing grain yield while reducing the reliance on fertilizers in durum wheat (Zuluaga & Sonnante, 2019).

Wheat is one of the main food crops in the world and is important for global food security (Zhu et al., 2008). The effects of climate, drought, and salinity on wheat are not only permeable but also affected by the antimetabolite of NO₃⁻ metabolism. Nagy et al. (2013) reported that GS is a good indicator of drought stress (abiotic), which can be used for the characterization of drought resistance of wheat varieties. Absolute growth and relative growth of different abiotic stress levels are better than differences in N metabolism. Improving the nitrogen-use efficiency (NUE) has been the main goal of agricultural research in recent years because N fertilizer has become the largest input cost. Driven by demand and production costs, the price of N fertilizer continues to rise (X. Fan et al., 2017; Naz et al., 2019). On the other hand, excessive use of N pollutes the environment and affects air, water, and soil quality. On the other hand, N runoff from farmlands also threatens the environment quality (Ben et al., 2019). Due to the interactions between environmental impact and genetic factors, the enhancement of NUE is complex (Figure 3).

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The NR restores NO₃⁻ in cytoplasm to nitrites, and then nitrites are converted into NH₄⁺ by NO₃⁻ reductase (NiR). In most soils, the NH₄⁺ transporter is received directly from the root nitrite transporters (NRTs) (Patterson et al., 2010). The sets of ion (Fe) transport proteins, NRTs, and AMTs are responsible for the receiving and transportation of NO₃⁻ and NH₄⁺, respectively. AMT protein is found in many plants, but is not analyzed and characterized comprehensively in wheat (Tian et al., 2017). Further, many different gene expressions are involved in the metabolism of amino acids such as glutamate, alanine, and aspartate after studies on NRTs, AMTs, NRs, and NiRs showed their role in the absorption and transportation of NO₃⁻ and NH₄⁺ or in the reduction of NO₃⁻ and nitrites (L. Li et al., 2019). The four NO₃⁻-transporter genes (TaNRT2.1, TaNRT2.2, TaNRT2.3, and TaNRT1.2) and two NH₄⁺-transporter genes (TaAMT1.1 and TaAMT1.2) in wheat genotypes have different N uptake efficiencies (T. Guo et al., 2014).

The NH₄⁺ transporters in plants belong to the multi-gene family. In Arabidopsis and rice, there are 6 and 10 AMTs, respectively, while there are at least 54 AMTs homologous in the wheat genome (Xuan et al., 2017). Depending on their sequence and morphology, these AMTs are divided into two subseries, AMT1 and AMT2, which are mainly expressed at the root and play a vital role in the intake of NH₄⁺ from soil (Mayer et al., 2006). Three wheat NH₄⁺ transport genes (TaAMT1; 1a, TaAMT1; 1b, and TaAMT1; 3a) can only be induced under compatible interaction of wheat with stem rust fungus (Zhong et al., 2014). The expression of the NO₃⁻ transport gene NRT2.6 was induced after plant pathogenic bacterium Erwinia amylovora (Dechorgnat et al., 2012). Wheat root transporter NRT1 and NRT2 families are activated under N-deficient conditions, and the adoption of NO₃⁻ is closely related to the expression of the gene TaNRT2.1, which is encoded as the root NO₃⁻ transmitter and appears to play an important role in the reception of NO₃⁻ after flowering. Sylvester-Bradley and Kindred (2009) identified NRT1 and NRT2 root family genes in bread wheat and explored their expression profiles under the roots of wheat seedlings during N deficiency. Therefore, N transporters could play an important role in N uptake from soil under low N availability and drought conditions. Moreover, Kasemsap and Bloom (2022) reported that grain and total biomass production per unit of N applied evaluated NUE. However, it is worth noting that higher yields are not always guranteed with an improved N acquisition. The transporter NPF7.4, overexpression of which led to increased uptake of NO₃⁻, reduced NO₃⁻ accumulation, and elevated concentration of the tissue amino acid, shows increased N assimilation. However, this increase in the acquisition of N effect results in the decreased biomass and grain production. Similarly, the knockout of the lysine-histidine-type transporter 1 (LHT1) was responsible for the transport of amino acid and contributed in the improvement of grain nutritional quality at maturity. However, this improvement resulted due to a decrease in the vegetative biomass, grain weight, and germination rate. Therefore, when defining and establishing NUE as breeding targets to reduce the inputs of nitrogen in agriculture, it is crucial to consider grain protein content. These considerations align with ongoing efforts aimed at enhancing the nutritional value of grains and represent promising strategies for achieving this goal (Swarbreck et al., 2019).

Intracellular enzymatic activities and N metabolism

Intracellular enzyme activity provides a detailed description of the behaviors of various enzymes, including GS, GOGAT, GDH, nitrate, and nitrite reductase, under abiotic stress conditions (Walker et al., 2018). Essential nutrients that play an important role in the physical chemistry and metabolism of an organism, such as in electron transfer proteins and as a cofactor as an enzyme (Figure 4). Glutamine synthase (GS) is the first and only enzyme that binds inorganic NH₄⁺ ions to organic compounds in the N absorption pathway (Miflin & Habash, 2002). The GS1 isoform is responsible for primary N assimilation and N remobilization at different stages of development or leaf senescence (Bernard et al., 2008). GS1 of leaves gains added significance during the sequential senescence of wheat caused by abiotic stress (Nagy et al., 2013). The activity of enzymes that convert organic N into inorganic N is essential for plant biomass accumulation, growth, and productivity (Yu et al., 2005). In plants, NO₃⁻ can be reduced to NH₄⁺ for the synthesis of proteins and other organic compounds (Garnett et al., 2009). Thus, the concentration of NH₄⁺ is an alternate pathway controlled by GDH, which is an isoenzyme known for partial removal of Cd toxicity under excess of NH₄⁺ under Cd toxicity. Precise gene expression codes, N assimilating proteins, and N transmissions using modern genomic methods in transcription proteomic groups, proteomics, or metabolic levels will allow the exploration of the molecular complexity of metal action, and this will also lead to a strategy to increase the resistance of plant to heavy metal toxicity under abiotic stress (Shruti & Dubey, 2010).

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NITROGEN UPTAKE AND METABOLISM UNDER ABIOTIC STRESSES

Temperature

Temperature higher than the physiological optimum under increasing global temperature is considered as heat stress, which is an important abiotic stress (Wahid et al., 2007) (Table 2). Temperature strongly influences N availability in soil by decreasing the photorespiration rate of C3 plants (Sardans & Peñuelas, 2012). Oxidative respiration processes are also temperature sensitive and may negatively affect cellular integrity, ion exchange, transportation material, and CO₂ evolution (Atkin & Tjoelker, 2003). Nitrogenase activities increase with temperature and generally higher N fixation is reported due to high enzymatic activity at higher temperature (Blais et al., 2012). It is assumed that increase in temperature leads to habitual expansion and can increase >27% N fixation ability (Hutchins et al., 2009). High temperature can lead to higher N emission into the atmosphere and O₃ accumulation, which eventually decrease crop productivity (Emberson et al., 2009), and lower production and higher mortality of plants (Nardone et al., 2010). High temperatures occurring on permanent or temporary basis can reduce seed germination, plant growth, and development and also affect nodule formation, thus changing yield by variation in the physiological and biochemical processes (Sathya et al., 2017; Wahid et al., 2007). Several studies have reported that symbiotic relationships between N fixing legumes and rhizobacteria lead to significant resilience toward temperature variations, such as Casuarina spp. being capable to grow at 35°C and Trifolium spp. at 30°C (Grover et al., 2011; Lindström & Mousavi, 2010). However, plants better adapted to extreme temperatures can develop a deeper root system and improve nodule formation. It has been proposed that nitrogenase activity depends on plant species and temperature region. Further, acute heat stress could have a modest negative impact on plant N status through increased dilution of N in tissue as a consequence of decreased specific uptake rate of N by roots (Mainali et al., 2014). Downregulation of N metabolism in common beans exposed to high temperatures was evidenced in the two routes of N acquisition: N fixation and subsequent NH₄⁺ assimilation as well as NO₃⁻ reduction (Hungria & Kaschuk, 2014). High temperature affects all components related to NUE, including morphology of root systems, soil mineral uptake, and symbiotic nitrogen fixation (Christophe et al., 2011). The deleterious effects of heat stress on the production of agriculture can be relieved by heat tolerance cultivars using different tools, such as conventional breeding and manipulation of regulatory genes (Jha et al., 2014).

TABLE 2 Effects of various abiotic stresses on N metabolism, growth, and biomass production of plants.

Abiotic stress	Response	References
Cadmium	N content response with increase in biomass of alfalfa	(Mobin, 2013)
	Inhabits N assimilation enzyme (NR, NiR, and GS) and respond plant growth	(J. Yang et al., 2019)
	Intercropping with Solanum nigrum L. reduced the Cd toxicity to wheat roots and decreased Cd uptake and accumulation in both the shoots and roots of the wheat	(L. Wang et al., 2023)
	Increase ROS and damage nodule to decrease N fixation	(Balestrasse et al., 2001)
Arsenic	Destroying cell membranes through oxidative damage	(Duan et al., 2015)
Lead	A 6% decline in the metal deposition within roots of inoculated maize was observed	(Rizvi & Khan, 2018)
	Produced a 75% decrease in the total number of nodules, due to 90% reduction in the number of rhizobial infections	(Pajuelo et al., 2008)
	Reduced nitrogenase activity in black gram and restrict N fixation	(Verma et al., 2015)
Heat	Negatively affected growth, yield, and N metabolism	(Wahid et al., 2007)
Copper	Reduction of plant biomass and inhibition of root growth	(Bernal et al., 2012)
Copper	Concentration of metals within plant organs increased with increasing concentrations of Cu	(Rizvi & Khan, 2018)
Copper and lead	Copper and lead had substantial distortive impact on root and leaf morphology	(Rizvi & Khan, 2018)
Temperature	High temperature can lead to higher N emission to atmosphere and O3 accumulation and decrease crop productivity	(Emberson et al., 2009)
	High temperature affects nodule formation at different growth stages and can lead to nodulation failure	(Sita et al., 2017)
pH	Extreme soil pH can negatively influence N metabolism in plant which reduces plant yield	(Ortiz-Ramirez et al., 2011)
	Acidic soil pH can facilitate NH4+ acquisition and assimilation into plant tissues	(Coskun et al., 2013)
	Soil alkalinity restricts the availability of plant nutrients, for example, Mg2+, Ca2+, H2PO4−, and Cl− through precipitation	(de Lacerda et al., 2003; Ghoulam et al., 2002)
Heat	Acute heat stress could, have a modest negative impact on plant N status through increased dilution of N in tissue as a consequence of decreased specific uptake rate of N by roots	(Mainali et al., 2014)
Drought	Initiate osmotic stress and reduced plant transpiration to decrease uptake and translocation of N	(Fahad et al., 2017)
Salt	The suppression of N assimilation enzyme activities under saline conditions may be attributed to salinity-induced osmotic changes as well as a decrease in NO3− uptake	(Debouba et al., 2007; de Souza Miranda et al., 2016)

Acidity and alkalinity

In agricultural soils, pH fluctuates greatly due to temperature, precipitation, ions imbalance, and soil properties that represent a major limiting factor for N availability and the ability of plants to uptake nutrients (Maathuis, 2009). Furthermore, N could be remobilized from old leaves to growing leaves during vegetative stage and from old leaves to seeds during reproductive stages (Diaz et al., 2008). Therefore, plant biomass is greatly influenced by pH variability and N availability in soil. For example, tea plants showed higher biomass production at pH 5, which gradually decreased at pH 6 as compared to pH 4, which suggests that tea plants were sensitive to acidic soil than alkaline (Ruan et al., 2007). Some other plants had showed better adaptability to acidic soil, for example, Lupinus angustifolius (Tang et al., 2019). Therefore, extreme soil pH can negatively influence N metabolism in plant, which reduces plant yield; however, plants have evolved to cope with abiotic soil stresses to improve plant N nutrition. Each plant species has different adaptability mechanisms to grow under severe alkaline and acidic soil conditions. Several studies have demonstrated that acidic soil pH can facilitate NH₄⁺ acquisition and assimilation into plant tissues (Coskun et al., 2013; Ortiz-Ramirez et al., 2011). Soil acidity also inhibit symbiotics relationships through restricting rhizobacterial growth, survival rate, suppress nodule formation, reduced photosynthetic activity, and N acquisition ability (C. W. Yang et al., 2009). The adverse effects of soil acidity on nodule formation have been reported in various legumes, for example, cowpea, pea, alfalfa, white clover, subclover, and bean even in the presence of rhizobia, which suggests that a pH of l < 5 results in nodulation failure (Ruan et al., 2007). Low soil pH affected legume growth and consequently inhibited symbiotic association between Bradyrhizobium and the host plant. Microbial community and modifications in plant root architecture play a critical role in improving plant growth, biomass, and regulating N metabolism. Based on these observations, it is yet to be addressed, for future avenue, that most plants able to survive under low pH condition are NH₄⁺ resistant.

Alkaline soil stress can disturb ion homeostasis and exhibits negative effects on N acquisition. Similarly, soil alkalinity restricts the availability of plant nutrients, for example, Mg²⁺, Ca²⁺, H₂PO₄⁻, and Cl⁻ through precipitation (de Lacerda et al., 2003). Plants respond to alkaline conditions by altering the metabolic mechanisms including ion transport, photosynthesis, phytohormone biosynthesis, and solute accumulation (R. Guo et al., 2017). Alkaline or high soil pH may disturb the root cell membrane and also alter its relative function (Martinière et al., 2018). For example, alkaline stress strongly decreased nutrient availability, inhibited photosynthetic activity, and growth of maize (Morales et al., 2020). Further, a significant interaction was found among Bradyrhizobium under drought conditions, which enhanced plant resistance to stress (A. Kumar & Verma, 2018).

Waterlogging

It is reported worldwide that 12% of cultivated areas are influenced by a major abiotic stress such as waterlogging (Shabala, 2011). In the waterlogged soil, the increase in the anaerobic activity enhanced many harmful substances and also the disturbance of the rhizosphere environment, leading to reduced mineral ions and important trace elements absorption, and hence showed the disturbance in the growth and development of roots and in the waterlogged soil, root growth is restrained, and discontinuing the N translocation in each organ of the plant is due to the limit of N fertilizer absorption, leading to decline efficiency of N usage (Ren et al., 2016). Moreover, the increase in the anaerobic activity inhibit crop growth, which affects crop and soil nutrients via suppressing the major metabolism of crops (Lawlor, 2002; Milroy et al., 2009; Ren et al., 2016). Nitrogen metabolism is important in the cellular adaptation to low oxygen stress in plants (Bailey-Serres et al., 2012). Further, key enzymes involved in the N metabolism are NR, glutamine (GS), GOGAT, and GDH. NR is a key enzyme and is sensitive to fluctuate in the environmental conditions and also adjust the process of N metabolism and assimilation (Kaiser et al., 2002). Limiting the reduction of NO₃⁻and oxygen under anaerobic conditions decreased the NR performance in the leaves of waterlogged plants (Hoff et al., 1992) and thus reduced the NH₄⁺ assimilation and the activities of GS and GOGAT (Robredo et al., 2011).

Drought

Drought and heat stress have increased manyfold on a global scale and are adversely influencing crop growth and crop production, compromising food security (Miranda-Apodaca et al., 2020). Approximately, 1 billion ha of soil in world is severely affected from drought stress of various magnitude (H. A. Hussain, Hussain et al., 2018). The high frequency and more time period of drought stress affected crop production (Zhou, 2015). Simulation models play a crucial role in predicting the changing scenario of drought conditions due to climate change. The effort to enhance the resilience of strategic crops against the harmful effects of drought stress, inevitably resulting in reduced crop productivity, is essential for more sustainable agricultural practices (Seleiman et al., 2021). Drought stress is responsible for osmotic stress initiation and reduced plant transpiration to decrease uptake and translocation of N (Fahad et al., 2017). Further, root morphology, plant physiology, nutrients transport and assimilation, and yield productivity were significantly reduced under drought stress (Ahanger et al., 2021; Hussain et al., 2018). In the soil, water movement affected the nitrogen absorption process. Hence, first in the water, NH₄⁺ or NO₃⁻ is dissolved, then absorbed via roots and translocated to the aerial part of the plant (Alet et al., 2012). In the plants, the absorption of NH₄⁺ and NO₃⁻ is decreased due to drought stress. After roots absorptions, more concentration of NH₄⁺ is assimilated and very low amount of NO₃⁻ is assimilated in roots. Further, NR and NiR converted NO₃⁻ to NH₄⁺ (Rennenberg et al., 2006). The transcriptional abundance of genes and enzymes activities involved in the metabolism of N is affected under drought stress (L. Huang, Li, Shao et al., 2018; L. Huang, Li, Zhou et al., 2018). Nitrogen metabolism is much important; therefore, at higher N content in the leaves, the sensitivity of stomata increased to drought stress and retained higher photosynthetic capacity (Meng et al., 2016). Moreover, under drought stress, the photosynthetic rate decrease and thus reduced the NO₃⁻ reductase (NR) and GS activities directly or indirectly (Robredo et al., 2011). Effects of drought on N metabolism have largely been dependent on N concentration and crop cultivars (Bascuñán-Godoy, Sanhueza, Hernández et al., 2018; Bascuñán-Godoy, Sanhueza, Pinto et al., 2018). Various mineral elements (essential or beneficial) are being used either as soil or foliar fertilizers to reduce the negative effects of stresses to enhance abiotic stress tolerance in plants (Hasanuzzaman et al., 2017). During the rainfall season, several wetting and drying cycles of soils showed considerable modification in the microbial population, which regulates N metabolic pathways (Liu et al., 2022). During the drying cycle, nitrous oxide (N₂O) emission rate increased and ammonia-oxidizing bacterial abundance decreased. However, during heavy rainfall, ammonia oxidizing bacterial abundance and NH₄⁺ level increased and intermediate rainfall triggered N₂O fluxes (L. Wang et al., 2019). These findings illustrate that wet–dry cycles can impose considerable changes in the N cycle and N-fixing microbial populations. These effects are minimized through sporadic rainfall and rewetting of soils (Arce et al., 2018). Further, drought also restricts plant development and reduce N fixation through affecting plant physiological development and survival of soil microbial communities. Moreover, plants can be capable of tolerating ion toxicity, drought stress, ion hemostasis, and can neutralize the effect high or low pH on the roots of plant (Ruan et al., 2007).

Salinity

It is a major limiting factor in reducing agriculture production and farmland-use efficiency (Chen et al., 2022). The occurrence of soil salinity is not just caused by climatic factors such as drought, but also by indecent land cultivation practices (Cui et al., 2021). The status of soil N is depleting due to salinity stress and it is expected that soil fertility will decrease up to 50% in the middle of 21st century due to the continuous salinization of agricultural land (Aslam et al., 2017). Further, soil salinity imposes drastic effects on plant growth, survival, and soil fertility; therefore, soil salinity is a major limiting factor for sustainable agriculture, resulting in the reduction of 50%–80% crop yields (Panta et al., 2014). Further, salinity stress reduced microbial activity, suppressed the C and N cycling processes, and led to salt accumulation such as nitrates and nitrites (Qu et al., 2022). Nitrogen is the most important plant nutrient, which affected plant growth, because nitrogen is a part of several compound such as amino acids, amides and proteins, multiple ammonium compounds, and is involved through different mechanism of salt tolerance in plants (Arghavani et al., 2017; Mohamed, 2012; Rais et al., 2013; Zaki, 2016).The insufficiency of N in plants is caused by the imbalance of N transport via roots due to the decrease in the uptake of NO₃⁻ and NH₄⁺ in the presence of salt ions, which damage root membrane via creating osmotic instability and thus affected photosynthesis and crop growth (Acosta-Motos et al., 2017; A. Ahmad et al., 2013; Ashraf et al., 2017; N. A. Khan et al., 2014). In the saline soil, Cl⁻ ions compete with NO₃⁻ ions and decrease the uptake and transport of nitrogen in plants (Abdelgadir et al., 2005). Thus, under salinity stress, the uptake of N was decreased (Annunziata et al., 2017; Hütsch et al., 2016). Moreover, calcium (Ca) and nitrogen metabolism are closely related; the antagonistic interaction between chloride ions (Cl⁻) and nitrate ions (NO₃⁻) and the uptake of NO₃⁻ may also be impeded by various factors. First, excessive accumulation of salt ions can lead to the inactivation of NO₃⁻ transporters, hindering the uptake process (Balliu et al., 2015), and alterations in soil water potential can cause a reduction in water absorption, consequently affecting NO₃⁻ uptake (Ehlting et al., 2007). The presence of Cl⁻ ions can also interfere with specific membrane transport systems, leading to the decrease in assimilation of N and further subsequent analysis of amino acids (Queiroz et al., 2012). Further, under saline conditions, the suppression of N assimilation enzyme activities attributed to salinity induced osmotic changes and decrease the uptake of NO₃⁻ (de Souza Miranda et al., 2016). Moreover, the activity of NO₃⁻ reductase (NR) is influenced by various inner signals, with NO₃⁻ being the primary sign that triggers the transcription of NR genes (Kaiser et al., 2002). Salinity stress can impede the NO₃⁻ reduction process, which is responsible for supplying NH₃ for the analysis of new amino acids, primarily through the GS/GOGAT pathway (H. Wang et al., 2012). Further, due to rapid proteolysis, the salinity stress leads to an increase in the NH₄⁺ content in plants (R. Wang et al., 2007). Salinity negatively affects root hair growth, resulting in reduced nodule formation per plant and impaired N fixation capacity by nodules (Abiala et al., 2018). Rhizobacteria is capable of surviving under saline conditions such as Trichodesmium and can survive under 22–43 psu condition and shows higher growth rate and nitrogenase activity at 33–37 psu, implying that salinity shows no adverse impact on N fixation (Fu & Bell, 2003). Similarly, the yield of legumes is diminished under salinity stress due to the lack of symbiotic association between plant roots and soil microbial biota, indicating that salinity stress downregulates or inhibits this beneficial relationship (Abiala et al., 2018).

The information about the relationship between N metabolism and salinity in plants is less and often contradictory, underscoring the necessity to accumulate and analyze data. Moreover, the relationship between N and salinity is significantly different between field and greenhouse experiments, soil-based cultivation versus solution culture, and when using individual salts or mixtures of salts, as well as in short-term versus long-term investigations. Nonetheless, to comprehensively assess the impact of salinity on N metabolism, it is crucial to understand how salinity interferes with various steps of N uptake, assimilation, and the activities of N-assimilating enzymes. This review aims to address the effects of salinity on the metabolism of N, with a particular emphasis on how salinity disrupts N uptake, assimilation, and the activities of N-assimilating enzymes.

Heavy metals

Heavy metal contamination has emerged as the most significant threat to plant growth and development, which impair plant physiological and nutrient uptake mechanisms. Cadmium (Cd) stress has been widely studied in terms of its effects on N metabolism (M. I. R. Khan et al., 2016) (Table 2). Cadmium (Cd) contamination enhances the accumulation of endogenous NH₄ salt in plants due to chemical process of free amino acids and other nitrogenous organic compounds, which increase protease activity (Chaffei, 2003). Activities of NR, GS, and GOGAT enzymes are also sensitive to Cd stress, such as the inhibition of NR, NiR, and GS assimilation enzyme activities under Cd stress, which reduced plant growth (Mobin, 2013). The process of N metabolism is critical for plants to mediate Cd toxicity in which self-adjustment of nitrogenous metabolites, for example, proline, glutathione (GSH; γ-glutamyl-cysteinyl-glycine), and phytochelatins, plays a very significant role (Sharma & Dietz, 2006). However, such response of plants in N metabolism to Cd toxicity is highly plant cultivar specific. For example, Cd stress induced changes in the activities of NR, GS, and GOGAT enzymes in rice plants and the response varied considerably between the five rice cultivars (W. Huang et al., 2015). In addition, Cd stress promoted free amino acids contents, which also differed significantly between the cultivars (Qian et al., 2015).

The toxicity of chromium (Cr) to plants will lead to changes in various enzymes related to N and carbohydrate metabolism, resulting in biochemical changes in plants (A. Ahmad et al., 2013). S. Kumar and Joshi (2008) mentioned that Cr (VI) stress caused a decrease in the activities of NR, NiR, GOGAT, GDH, and urease in the leaves, roots, shoots, and ear head of S. bicolor. Among them, Cr (VI) has a greater inhibitory effect on NiR (Gangwar & Singh, 2011). In addition, Cr toxicity is manifested as ammonia accumulation in the leaves of some crops, such as Kinnow mandarin plants (Shahid et al., 2018). The response of N metabolism to copper (Cu) stress is important to understand the behavior of N metabolism associated-enzymes in the presence toxic metals (Azmat & Khan, 2011). Reactive oxygen species produced under heavy metal stress cause decomposition of enzymes, resulting in reduced NR and NiR activities (Xiong et al., 2006). Nickel (Ni) toxicity to plants has emerged as a global problem, threatening agricultural production (Yusuf et al., 2011). The critical Ni concentration causing toxicity is more than 10 mg kg⁻¹ on dry mass basis but various in Ni-sensitive plant species (Kozlov, 2005). However, planting crops with Ni-tolerant bacteria such as Melastoma or Sarcotheca could increase biomass production and N uptake and reduce Ni accumulation in plant edible parts (Syam et al., 2016).

STRATEGIES TO IMPROVE N METABOLISM UNDER ABIOTIC STRESS

Response of plants to various stressed conditions such as extremes temperature, salinity, and drought is due to the intense molecular studies (Mittler, 2006). Further, Shimamura et al. (2006) reported that in wheat, the expression of cold responsive gene enhanced the freezing tolerance in transgenic plants. Moreover, the effects of environmental factors showed to control the expression of specific genes due to the transcription factor are important in single transduction pathways. In addition, the tolerance of freezing, drought, and salinity was increased in Arabidopsis plants due to the position expression of the dehydration-responsive element (DRE)-binding protein DREB1A (Kasuga et al., 1999). Further, drought tolerance was enhanced due to the transgene strategy in plants (Nelson et al., 2007). The heat stress severely affected soybean and the metabolomic views showed that several metabolites were downregulated and altered the homeostasis of general metabolism. Thus, the growth and development of soybean was influenced by changes in the metabolic pathways (Das et al., 2017).

Cultivation of cereals is a backbone in worldwide agriculture and food and industries, due to the concerns of human consumption, farmers, and agriculture-based industry. The growth and biological and grain yield of domesticated cereals is highly affected by extreme weather conditions caused by global climate change (Matamoros & Becana, 2021). Adequate N supply is essential for plant growth and development, whereas NO is regarded as a signaling molecule involved in abiotic stress tolerance in plants (Nabi et al., 2019). Further, N metabolism is closely related to sucrose metabolism, and enhancing sucrose metabolism is beneficial to nitrogen assimilation (Kobayashi et al., 2004). It can be interpreted that NO enhances N content in rice leaves as a defense by increasing osmotic regulation and enhancing the activities of GDH, sucrose synthase (SS), and sucrose phosphate synthase (SPS), thus increasing the adaptation of plants to salt stress (J. Huang et al., 2020). The change of the N metabolism ensures the proper grain filling process, which determines yield quality. The N uptake and assimilation are restrained during cold abiotic stress via the loss of activity of its two key enzymes such as GS and NR. Nitrogen metabolism is invariably affected by unfavorable environmental conditions, such as low temperature and drought abiotic stress. Further, the combination of two individual stresses can pose much higher threat to plants compared to the effects of a single stress factor (Mittler, 2006). In addition to the combined abiotic stresses from cold and other stresses, the effects of low temperature and drought on N assimilation have little investigated.

CONCLUSIONS AND PERSPECTIVES

The changes in physiological processes of plants in response to N fertilization determined the quality and quantity of crop yields. However, plants have naturally evolved to survive in very heterogeneous environments and are exposed to various kinds of biotic and abiotic stresses. This review emphasizes that understanding the physiological and molecular pathways of N metabolism under edaphic and climatic stresses provides opportunities to identify the targeting genes. The expression of these targeted genes using various biotechnological and genetic tools can help to develop crop cultivars with better abilities for N acquisition under many biotic and abiotic conditions. Using advanced genetic tools and knowledge, N uptake and NUE can be improved to increase crop yields while, subsequently, reducing environmental impacts of N fertilization. However, during the growing period, the availability patterns of macronutrients significantly differed from those in the conventional systems. However, NUE was significantly affected by the environmental conditions, such as the application of fertilizer, intensity of light, soil moisture, and temperature. Therefore, climatic conditions and agronomic practices significantly promoted NUE, and hence, it is essential to specify the interactions attribute of genotype × environment in contributing NUE and crop selecting in different agro-climatic environments. However, our review also stresses the need to understand the agronomic, environmental, and technological challenges in optimizing N uptake and mentalism in agroecosystems.

AUTHOR CONTRIBUTIONS

Kashif Akhtar: Data curation; project administration; writing—original draft; writing—review and editing. Noor ul Ain: Software; writing—review and editing. P. V. Vara Prasad: Funding acquisition; writing—review and editing. Misbah Naz: Data curation; methodology; writing—review and editing. Mehtab Muhammad Aslam: Formal analysis; methodology; writing—review and editing. Ivica Djalovic: Data curation; investigation; writing—review and editing. Muhammad Riaz: Methodology; writing—review and editing. Shakeel Ahmad: Data curation; writing—review and editing. Rajeev K. Varshney: Methodology; writing—review and editing. Bing He: Methodology; writing—review and editing. Ronghui Wen: Funding acquisition; supervision; writing—review and editing.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant No. 31560122), National Key R & D Program (2022YFD2301105-04), and the postdoctoral research fund by Guangxi University.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

This is a review article and no data are associated with the manuscript.

References

Abdelgadir, E. M., Oka, M., & Fujiyama, H. (2005). Characteristics of nitrate uptake by plants under salinity. Journal of Plant Nutrition, 28(1), 33–46. [DOI: https://dx.doi.org/10.1081/PLN-200042156]