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Dissimilatory nitrate reduction to ammonium (DNRA), driven by nitrate-ammonifying bacteria, is an increasingly appreciated nitrogen-cycling pathway in terrestrial ecosystems. This process reportedly generates nitrous oxide (N 2 O), a strong greenhouse gas with ozone-depleting effects. However, it remains poorly understood how N 2 O is produced by environmental nitrate-ammonifiers and how to identify DNRA-derived N 2 O. In this study, we characterize two novel enzymatic pathways responsible for N 2 O production in Geobacteraceae strains, which are predominant nitrate-ammonifying bacteria in paddy soils. The first pathway involves a membrane-bound nitrate reductase (Nar) and a hybrid cluster protein complex (Hcp-Hcr) that catalyzes the conversion of NO 2 - to NO and subsequently to N 2 O. The second pathway is observed in Nar-deficient bacteria, where the nitrite reductase (NrfA) generates NO, which is then reduced to N 2 O by Hcp-Hcr. These enzyme combinations are prevalent across the domain Bacteria. Moreover, we observe distinctive isotopocule signatures of DNRA-derived N 2 O from other established N 2 O production pathways, especially through the highest 15 N-site preference (SP) values (43.0‰-49.9‰) reported so far, indicating a robust means for N 2 O source partitioning. Our findings demonstrate two novel N 2 O production pathways in DNRA that can be isotopically distinguished from other pathways.IMPORTANCEStimulation of DNRA is a promising strategy to improve fertilizer efficiency and reduce N 2 O emission in agriculture soils. This process converts water-leachable NO 3 - and NO 2 - into soil-adsorbable NH 4 + , thereby alleviating nitrogen loss and N 2 O emission resulting from denitrification. However, several studies have noted that DNRA can also be a source of N 2 O, contributing to global warming. This contribution is often masked by other N 2 O generation processes, leading to a limited understanding of DNRA as an N 2 O source. Our study reveals two widespread yet overlooked N 2 O production pathways in Geobacteraceae , the predominant DNRA bacteria in paddy soils, along with their distinctive isotopocule signatures. These findings offer novel insights into the role of the DNRA bacteria in N 2 O production and underscore the significance of N 2 O isotopocule signatures in microbial N 2 O source tracking., Competing Interests: The authors declare no conflict of interest.

Nitrogen (N) and phosphorus (P), as indispensable mineral elements, both play pivotal roles in plant growth and development. Despite the intimate association between nitrate signaling and inorganic phosphate (Pi) signaling, the regulatory function of Pi in N metabolism remains poorly understood. In this study, we observed that Pi deficiency leads to a reduction in the activity of nitrate reductase (NR), an essential enzyme involved in N metabolism. Furthermore, PHOSPHATE STARVATION RESPONSE 1 (PHR1), a key regulator of Pi signaling, exerts a negative impact on both NR activity and the expression of its coding gene NIA1. Importantly, our analysis utilizing yeast one-hybrid (Y1H) and electrophoretic mobility shift assay (EMSA) techniques reveals the direct binding of PHR1 to the NIA1 promoter via the P1BS motifs. Subsequent transient transcription expression assay (TTEA) demonstrates PHR1 as a transcriptional suppressor of NIA1. In addition, it was also observed that the SPX (SYG1/Pho81/XPR1) proteins SPX1 and SPX4 can attenuate the transcriptional inhibition of NIA1 by PHR1. Collectively, these findings reveal a mechanism through which PHR1-mediated Pi signal governs N metabolism, thus offering evidence for the precise modulation of plant growth and development via N-P interaction., Competing Interests: Declaration of competing interest All the authors declare that they have no conflict of interest., (Copyright © 2024 Elsevier GmbH. All rights reserved.)

Interactions between plants and microorganisms are pivotal for plant growth and productivity. Several plant molecular mechanisms that shape these microbial communities have been identified. However, the importance of nitric oxide (NO) produced by plants for the associated microbiota remains elusive. Using Arabidopsis thaliana isogenic mutants overproducing NO (nox1, NO overexpression) or down-producing NO (i.e. nia1nia2 impaired in the expression of both nitrate reductases NR1/NIA1 and NR2/NIA2; the 35s::GSNOR1 line overexpressing nitrosoglutathione reductase (GSNOR) and 35s::AHB1 line overexpressing haemoglobin 1 (AHB1)), we investigated how altered NO homeostasis affects microbial communities in the rhizosphere and in the roots, soil microbial activity and soil metabolites. We show that the rhizosphere microbiome was affected by the mutant genotypes, with the nox1 and nia1nia2 mutants causing opposite shifts in bacterial and fungal communities compared with the wild-type (WT) Col-0 in the rhizosphere and roots, respectively. These mutants also exhibited distinctive soil metabolite profiles than those from the other genotypes while soil microbial activity did not differ between the mutants and the WT Col-0. Our findings support our hypothesis that changes in NO production by plants can influence the plant microbiome composition with differential effects between fungal and bacterial communities., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Background: We have previously shown that the expression of a constitutively active nitrate reductase variant and the suppression of CLCNt2 gene function (belonging to the chloride channel (CLC) gene family) in field-grown tobacco reduces tobacco-specific nitrosamines (TSNA) accumulation in cured leaves and cigarette smoke. In both cases, TSNA reductions resulted from a strong diminution of free nitrate in the leaf, as nitrate is a precursor of the TSNA-producing nitrosating agents formed during tobacco curing and smoking. These nitrosating agents modify tobacco alkaloids to produce TSNAs, the most problematic of which are NNN (N-nitrosonornicotine) and NNK (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone). The expression of a deregulated nitrate reductase enzyme (DNR) that is no longer responsive to light regulation is believed to diminish free nitrate pools by immediately channeling incoming nitrate into the nitrate assimilation pathway. The reduction in nitrate observed when the two tobacco gene copies encoding the vacuolar nitrate transporter CLCNt2 were down-regulated by RNAi-mediated suppression or knocked out using the CRISPR-Cas technology was mechanistically distinct; likely attributable to the inability of the tobacco cell to efficiently sequester nitrate into the vacuole where this metabolite is protected from further assimilation. In this study, we used transcriptomic and metabolomic analyses to compare the nitrate assimilation response in tobacco plants either expressing DNR or lacking CLCNt2 function., Results: When grown in a controlled environment, both DNR and CLCNt2-KO (CLCKO) plants exhibited (1) reduced nitrate content in the leaf; (2) increased N-assimilation into the amino acids Gln and Asn; and (3) a similar pattern of differential regulation of several genes controlling stress responses, including water stress, and cell wall metabolism in comparison to wild-type plants. Differences in gene regulation were also observed between DNR and CLCKO plants, including genes encoding nitrite reductase and asparagine synthetase., Conclusions: Our data suggest that even though both DNR and CLCKO plants display common characteristics with respect to nitrate assimilation, cellular responses, water stress, and cell wall remodeling, notable differences in gene regulatory patterns between the two low nitrate plants are also observed. These findings open new avenues in using plants fixing more nitrogen into amino acids for plant improvement or nutrition perspectives., Competing Interests: Declarations. Ethics approval and consent to participate: Not applicable. Consent for publication: Not applicable. Competing interests: LB, JB, NS, and SG are employees of Philip Morris Products S.A. with NS and SG having equity or stocks. JL and RED are employees of North Carolina State University (NCSU). There are two patents relating to the content of this manuscript. Author LB has a patent (WO2014096283A2) and authors LB, JU, and RED have a joint patent (WO2016046288A1). Philip Morris Products S.A. is a tobacco manufacturer and was involved in the review and approval of the manuscript for publication., (© 2024. The Author(s).)

The proportionality of oxygen-to-nitrogen isotope effects ( 18 ε/ 15 ε) is used as a key isotopic signature of nitrogen cycling processes in the environment. Dissimilatory nitrate reduction is observed to have an 18 ε/ 15 ε proportionality of ~0.9 in marine and ~0.6 in freshwater/terrestrial ecosystems. The origins of this difference are uncertain, with both geochemical and biological factors conceivably at play. One potential factor is variation in the isotope effect of nitrate reduction among different forms of the nitrate reductase enzyme. NarG nitrate reductases are observed to typically have an 18 ε/ 15 ε of ~0.9. However, a recent study uncovered an exception, with Bacillus NarG enzymes having an 18 ε/ 15 ε proportionality of ~0.6. This provides an opportunity to investigate genetic controls on 18 ε/ 15 ε. Furthermore, this atypical NarG signature also raises the question of whether intrinsic isotope signatures can evolve as the enzymes that produce them accumulate mutations through time. Here, we present data from site-directed mutagenesis experiments of key NarG residues, which suggest that the distinct Bacillus 18 ε/ 15 ε cannot be caused by single mutations alone and is potentially uncommon in nature. Variation in the intrinsic isotope effects of an enzyme through time may thus require more extensive evolutionary changes., Competing Interests: Competing interests statement:The authors declare no competing interest.

Salmonella Typhimurium (STM), a zoonotic pathogen, can adjust its metabolic pathway according to the variations in the partial pressure of atmospheric oxygen and nitrate via fumarate nitrate reductase regulator (Fnr) and NarL, the response regulator for nitrate reductase. Both Fnr and NarL have been individually reported to be the contributors of virulent phenotypes of STM. Hypoxia along with nitrate-rich environment are prevalent in macrophages and the Salmonella-induced inflammatory lumen of the host's large intestine activates both fnr and narL genes. In this study, the double (fnr and narL) knockout STM showed a synergistic reduction in the swimming (62%), swarming (84%) and biofilm density (86%) phenotypes anaerobically in association with its significant aerobic attenuation. The intracellular replication of the double mutant was reduced by 2.3 logs in chicken monocyte-derived macrophages. Furthermore, the competitive index of the double mutant in liver and spleen was found to be 0.3 and 0.44 respectively at 120 h post-infection (PI) in mice. Surprisingly, no double mutant could be recovered from the infected mouse liver 3 days PI. Histopathological findings showed moderate infiltration of mononuclear cells in the large intestine of mice infected with double mutant, but severe infiltration was seen with the wild-type strain., (© 2024. The Author(s), under exclusive licence to Springer Nature B.V.)

Nitrate is a nutrient and signal that regulates gene expression. The nitrate response has been extensively characterized at the organism, organ, and cell-type-specific levels, but intracellular mRNA dynamics remain unexplored. To characterize nuclear and cytoplasmic transcriptome dynamics in response to nitrate, we performed a time-course expression analysis after nitrate treatment in isolated nuclei, cytoplasm, and whole roots. We identified 402 differentially localized transcripts (DLTs) in response to nitrate treatment. Induced DLT genes showed rapid and transient recruitment of the RNA polymerase II, together with an increase in the mRNA turnover rates. DLTs code for genes involved in metabolic processes, localization, and response to stimulus indicating DLTs include genes with relevant functions for the nitrate response that have not been previously identified. Using single-molecule RNA FISH, we observed early nuclear accumulation of the NITRATE REDUCTASE 1 (NIA1) transcripts in their transcription sites. We found that transcription of NIA1, a gene showing delayed cytoplasmic accumulation, is rapidly and transiently activated; however, its transcripts become unstable when they reach the cytoplasm. Our study reveals the dynamic localization of mRNAs between the nucleus and cytoplasm as an emerging feature in the temporal control of gene expression in response to nitrate treatment in Arabidopsis roots., (© 2024 John Wiley & Sons Ltd.)

Previous research on Magnetospirillum gryphiswaldense MSR-1 found that MSR-1 has a good denitrification nitrogen removal ability and specific application prospects in the sewage biological nitrogen removal field. Therefore, this study selected the essential denitrification gene NapA in MSR-1-wt for overexpression, and the overexpressed MSR-1-NapA was successfully constructed. Q-PCR amplification experiment and AGAR gel electrophoresis experiment proved that the relative transcription level of the NapA gene was increased by more than four times, and the denitrification ability of MSR-1-wt and MSR-1-NapA was further determined by enzyme activity experiment, denitrification experiment, and flow cytometry. The results showed that overexpression of the NapA gene increased nitrate reductase activity in MSR-1-NapA by more than four times. In the solution with a nitrate concentration of 118.33 ± 3.23 mgN/L, the denitrification efficiency of MSR-1-NapA was superior to that of MSR-1-wt, significantly enhancing both the denitrification and nitrogen removal capacities of MSR-1. This indicates its greater potential for biological nitrogen removal in wastewater treatment., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

High Pressure Hydrogenotrophic Denitrification (HPHD) provided a promising alternative for efficient and clean nitrate removal. In particular, the denitrification rates at low temperature could be compensated by elevated H 2 partial pressure. However, nitrite reduction was strongly inhibited while nitrate reduction was barely affected at low temperature. In this study, the nitrate reduction gradually recovered under long-term low temperature stress, while nitrite accumulation increased from 0.1 to 41.0 mg N/L. The activities of the electron transport system (ETS), nitrate reductase (NAR), and nitrite reductase (NIR) decreased by 45.8 %, 27.3 %, and 39.3 %, respectively, as the temperature dropped from 30 °C to 15 °C. Real time quantitative PCR analysis revealed that the denitrifying gene expression rather than gene abundance regulated nitrogen biotransformation. The substantial nitrite accumulation was attributed to the significant up-regulation by 54.7 % of narG gene expression and down-regulation by 73.7 % of nirS gene expression in hydrogenotrophic denitrifiers. In addition, the nirS-gene-bearing denitrifiers were more sensitive to low temperature compared to those bearing nirK gene. The dominant populations shifted from the genera Paracoccus to Hydrogenophaga under long-term low temperature stress. Overall, this study revealed the microbial mechanism of high nitrite accumulation in hydrogenotrophic denitrification at low temperature., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Transcription factors of the CYCLING DOF FACTOR (CDF) family activate in potato the SP6A FT tuberization signal in leaves. In modern cultivars, truncated StCDF1.2 alleles override strict SD control by stabilizing the StCDF1 protein, which leads to StCOL1 suppression and impaired activation of the antagonic SP5G paralog. By using DAP-seq and RNA-seq studies, we here show that StCDF1 not only acts as an upstream regulator of the day length pathway but also directly regulates several N assimilation and transport genes. StCDF1 directly represses expression of NITRATE REDUCTASE (NR/NIA), which catalyses the first reduction step in nitrate assimilation, and is encoded by a single potato locus. StCDF1 knock-down lines performed better in N-limiting conditions, and this phenotype correlated with derepressed StNR expression. Also, deletion of the StNR DAP-seq region abolished repression by StCDF1, while it did not affect NLP7-dependent activation of the StNR promoter. We identified multiple nucleotide polymorphisms in the DAP-seq region in potato cultivars with early StCDF1 alleles, suggesting that this genetic variation was selected as compensatory mechanism to the negative impact of StCDF1 stabilization. Thereby, directed modification of the StCDF1-recognition elements emerges as a promising strategy to enhance limiting StNR activity in potato., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

In sweet potato, rational nitrogen (N) assimilation and distribution are conducive to inhibiting vine overgrowth. Nitrate (NO 3 - ) is the main N form absorbed by roots, and cultivar is an important factor affecting N utilization. Herein, a hydroponic experiment was conducted that included four NO 3 - concentrations of 0 (N0), 4 (N1), 8 (N2) and 16 (N3) mmol L -1 with two cultivars of Jishu26 (J26, N-sensitive) and Xushu32 (X32, N-tolerant). For J26, with increasing NO 3 - concentrations, the root length and root surface area significantly decreased. However, no significant differences were observed in these parameters for X32. Higher NO 3 - concentrations upregulated the expression levels of the genes that encode nitrate reductase (NR2), nitrite reductase (NiR2) and nitrate transporter (NRT1.1) in roots for both cultivars. The trends in the activities of NR and NiR were subject to regulation of NR2 and NiR2 transcription, respectively. For both cultivars, N2 increased the N accumulated in leaves, growth points and roots. For J26, N3 further increased the N accumulation in these organs. Under higher NO 3 - nutrition, compared with X32, J26 exhibited higher expression levels of the NiR2, NR2 and NRT1.1 genes, a higher influx NO 3 - rate in roots, and higher activities of NR and NiR in leaves and roots. Conclusively, the regulated effects of NO 3 - supplies on root growth and NO 3 - utilization were more significant for J26. Under high NO 3 - conditions, J26 exhibited higher capacities of NO 3 - absorption and distributed more N in leaves and in growth points, which may contribute to higher growth potential in shoots and more easily cause vine overgrowth., (© 2024. The Author(s).)

The interplay between nitrogen and sulfur assimilation synergistically supports and sustains plant growth and development, operating in tandem to ensure coordinated and optimal outcomes. Previously, we characterized Arabidopsis CHLOROPHYLL A/B-BINDING (CAB) overexpression 2 (COE2) mutant, which has a mutation in the NITRIC OXIDE-ASSOCIATED (NOA1) gene and exhibits deficiency in root growth under low nitrogen (LN) stress. This study found that the growth suppression in roots and shoots in coe2 correlates with decreased sensitivity to low sulfur stress treatment compared to the wild-type. Therefore, we examined the regulatory role of COE2 in nitrogen and sulfur interaction by assessing the expression of nitrogen metabolism-related genes in coe2 seedlings under low sulfur stress. Despite the notable upregulation of nitrate reductase genes (NIA1 and NIA2), there was a considerable reduction in nitrogen uptake and utilization, resulting in a substantial growth penalty. Moreover, the elevated expression of miR396 perhaps complemented growth stunting by selectively targeting and curtailing the expression levels of GROWTH REGULATING FACTOR 2 (GRF2), GRF4, and GRF9. This study underscores the vital role of COE2-mediated nitrogen signaling in facilitating seedling growth under sulfur deficiency stress., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

L-lysine is an essential amino acid with broad applications in the animal feed, human food, and pharmaceutical industries. The fermentation production of L-lysine by Escherichia coli has limitations such as poor substrate utilization efficiency and low saccharide conversion rates. We deleted the global regulatory factor gene mlc and introduced heterologous genes, including the maltose phosphotransferase genes ( malAP ) from Bacillus subtilis , to enhance the use efficiency of disaccharides and trisaccharides. The engineered strain E . coli XC3 demonstrated improved L-lysine production, yield, and productivity, which reached 160.00 g/L, 63.78%, and 4.44 g/(L‧h), respectively. Furthermore, we overexpressed the glutamate dehydrogenase gene ( gdhA ) and assimilated nitrate reductase genes ( BsnasBC ) from B . subtilis , along with nitrite reductase genes ( EcnirBD ) from E . coli , in strain E . coli XC3. This allowed the construction of E . coli XC4 with a nitrate assimilation pathway. The L-lysine production, yield, and productivity of E . coli XC4 were elevated to 188.00 g/L, 69.44%, and 5.22 g/(L‧h), respectively. After optimization of the residual sugar concentration and carbon to nitrogen ratio, the L-lysine production, yield, and productivity were increased to 204.00 g/L, 72.32%, and 5.67 g/(L‧h), respectively, in a 5 L fermenter. These values represented the increases of 40.69%, 20.03%, and 40.69%, respectively, compared with those of the starting strain XC1. By engineering the substrate utilization pathway, we successfully constructed a high-yield L-lysine producing strain, laying a solid foundation for the industrial production of L-lysine.

Freezing stress can seriously affect plant growth and development, but the mechanisms of these effects and plant responses to freezing stress require further exploration. Here, we identified a NAM, ATAF1/2, and CUC2 (NAC)-family transcription factor (TF), NAC056, that can promote freezing tolerance in Arabidopsis. NAC056 mRNA levels are strongly induced by freezing stress in roots, and the nac056 mutant exhibits compromised freezing tolerance. NAC056 acts positively in response to freezing by directly promoting key C-repeat-binding factor (CBF) pathway genes. Interestingly, we found that CBF1 regulates nitrate assimilation by regulating the nitrate reductase gene NIA1 in plants; therefore, NAC056-CBF1-NIA1 form a regulatory module for the assimilation of nitrate and the growth of roots under freezing stress. In addition, 35S::NAC056 transgenic plants show enhanced freezing tolerance, which is partially reversed in the cbfs triple mutant. Thus, NAC056 confers freezing tolerance through the CBF pathway, mediating plant responses to balance growth and freezing stress tolerance., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Background: Glutamine synthetase (GS), glutamate synthase (GOGAT), and nitrate reductase (NR) are key enzymes involved in nitrogen assimilation and metabolism in plants. However, the systematic analysis of these gene families lacked reports in soybean ( Glycine max (L.) Merr.), one of the most important crops worldwide., Methods: In this study, we performed genome-wide identification and characterization of GS , GOGAT , and NR genes in soybean under abiotic and nitrogen stress conditions., Results: We identified a total of 10 GS genes, six GOGAT genes, and four NR genes in the soybean genome. Phylogenetic analysis revealed the presence of multiple isoforms for each gene family, indicating their functional diversification. The distribution of these genes on soybean chromosomes was uneven, with segmental duplication events contributing to their expansion. Within the nitrogen assimilation genes (NAGs) group, there was uniformity in the exon-intron structure and the presence of conserved motifs in NAGs. Furthermore, analysis of cis-elements in NAG promoters indicated complex regulation of their expression. RT-qPCR analysis of seven soybean NAGs under various abiotic stresses, including nitrogen deficiency, drought-nitrogen, and salinity, revealed distinct regulatory patterns. Most NAGs exhibited up-regulation under nitrogen stress, while diverse expression patterns were observed under salt and drought-nitrogen stress, indicating their crucial role in nitrogen assimilation and abiotic stress tolerance. These findings offer valuable insights into the genomic organization and expression profiles of GS , GOGAT , and NR genes in soybean under nitrogen and abiotic stress conditions. The results have potential applications in the development of stress-resistant soybean varieties through genetic engineering and breeding., Competing Interests: The authors declare that they have no competing interests., (© 2024 Elsanosi et al.)

Periplasmic nitrate reductase NapA from Campylobacter jejuni (C. jejuni) contains a molybdenum cofactor (Moco) and a 4Fe-4S cluster and catalyzes the reduction of nitrate to nitrite. The reducing equivalent required for the catalysis is transferred from NapC → NapB → NapA. The electron transfer from NapB to NapA occurs through the 4Fe-4S cluster in NapA. C. jejuni NapA has a conserved lysine (K79) between the Mo-cofactor and the 4Fe-4S cluster. K79 forms H-bonding interactions with the 4Fe-4S cluster and connects the latter with the Moco via an H-bonding network. Thus, it is conceivable that K79 could play an important role in the intramolecular electron transfer and the catalytic activity of NapA. In the present study, we show that the mutation of K79 to Ala leads to an almost complete loss of activity, suggesting its role in catalytic activity. The inhibition of C. jejuni NapA by cyanide, thiocyanate, and azide has also been investigated. The inhibition studies indicate that cyanide inhibits NapA in a non-competitive manner, while thiocyanate and azide inhibit NapA in an uncompetitive manner. Neither inhibition mechanism involves direct binding of the inhibitor to the Mo-center. These results have been discussed in the context of the loss of catalytic activity of NapA K79A variant and a possible anion binding site in NapA has been proposed., (© 2024. The Author(s), under exclusive licence to Society for Biological Inorganic Chemistry (SBIC).)

Mycobacterium abscessus ( Mab ) is an opportunistic pathogen afflicting individuals with underlying lung disease such as Cystic Fibrosis (CF) or immunodeficiencies. Current treatment strategies for Mab infections are limited by its inherent antibiotic resistance and limited drug access to Mab in its in vivo niches resulting in poor cure rates of 30-50%. Mab's ability to survive within macrophages, granulomas and the mucus laden airways of the CF lung requires adaptation via transcriptional remodeling to counteract stresses like hypoxia, increased levels of nitrate, nitrite, and reactive nitrogen intermediates. Mycobacterium tuberculosis ( Mtb ) is known to coordinate hypoxic adaptation via induction of respiratory nitrate assimilation through the nitrate reductase narGHJI . Mab , on the other hand, does not encode a respiratory nitrate reductase. In addition, our recent study of the transcriptional responses of Mab to hypoxia revealed marked down-regulation of a locus containing putative nitrate assimilation genes, including the orphan response regulator nnaR (nitrate/nitrite assimilation regulator). These putative nitrate assimilation genes, narK3 (nitrate/nitrite transporter), nirBD (nitrite reductase), nnaR , and sirB (ferrochelatase) are arranged contiguously while nasN (assimilatory nitrate reductase identified in this work) is encoded in a different locus. Absence of a respiratory nitrate reductase in Mab and down-regulation of nitrogen metabolism genes in hypoxia suggest interplay between hypoxia adaptation and nitrate assimilation are distinct from what was previously documented in Mtb . The mechanisms used by Mab to fine-tune the transcriptional regulation of nitrogen metabolism in the context of stresses e.g. hypoxia, particularly the role of NnaR, remain poorly understood. To evaluate the role of NnaR in nitrate metabolism we constructed a Mab nnaR knockout strain ( Mab ΔnnaR ) and complement ( Mab ΔnnaR+C ) to investigate transcriptional regulation and phenotypes. qRT-PCR revealed NnaR is necessary for regulating nitrate and nitrite reductases along with a putative nitrate transporter. Loss of NnaR compromised the ability of Mab to assimilate nitrate or nitrite as sole nitrogen sources highlighting its necessity. This work provides the first insights into the role of Mab NnaR setting a foundation for future work investigating NnaR's contribution to pathogenesis., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Simcox and Rohde.)

Objective: To explore the effect of intestinal nitrates on the growth of Klebsiella pneumoniae and its regulatory mechanisms., Methods: K. pneumoniae strains with nitrate reductase narG and narZ single or double gene knockout or with NarXL gene knockout were constructed and observed for both aerobic and anaerobic growth in the presence of KNO 3 using an automated bacterial growth analyzer and a spectrophotometer, respectively. The mRNA expressions of narG and narZ in K. pneumoniae in anaerobic cultures in the presence of KNO 3 and the effect of the binary regulatory system NarXL on their expresisons were detected using qRT-PCR. Electrophoretic mobility shift assays (EMSA) and MST analysis were performed to explore the specific regulatory mechanisms of NarXL in sensing and utilizing nitrates. Competitive experiments were conducted to examine anaerobic growth advantages of narG and narZ gene knockout strains of K. pneumoniae in the presence of KNO 3 ., Results: The presence of KNO 3 in anaerobic conditions, but not in aerobic conditions, promoted bacterial growth more effectively in the wild-type K. pneumoniae strain than in the narXL gene knockout strain. In anaerobic conditions, the narXL gene knockout strain showed significantly lowered mRNA expressions of narG and narZ ( P < 0.0001). EMSA and MST experiments demonstrated that the NarXL regulator could directly bind to narG and narZ promoter regions. The wild-type K. pneumoniae strain in anaerobic cultures showed significantly increased expressions of narG and narZ mRNAs in the presence of KNO 3 ( P < 0.01), and narG gene knockout resulted in significantly attenuated anaerobic growth and competitive growth abilities of K. pneumoniae in the presence of KNO 3 ( P < 0.01)., Conclusion: The binary regulatory system NarXL of K. pneumoniae can sense changes in intestinal nitrate concentration and directly regulate the expression of nitrate reductase genes narG and narZ to promote bacterial growth.

When the supply of inorganic carbon is limiting, photosynthetic cyanobacteria excrete nitrite, a toxic intermediate in the ammonia assimilation pathway from nitrate. It has been hypothesized that the excreted nitrite represents excess nitrogen that cannot be further assimilated due to the missing carbon, but the underlying molecular mechanisms are unclear. Here, we identified a protein that interacts with nitrite reductase, regulates nitrogen metabolism and promotes nitrite excretion. The protein, which we named NirP1, is encoded by an unannotated gene that is upregulated under low carbon conditions and controlled by transcription factor NtcA, a central regulator of nitrogen homeostasis. Ectopic overexpression of nirP1 in Synechocystis sp. PCC 6803 resulted in a chlorotic phenotype, delayed growth, severe changes in amino acid pools, and nitrite excretion. Coimmunoprecipitation experiments indicated that NirP1 interacts with nitrite reductase, a central enzyme in the assimilation of ammonia from nitrate/nitrite. Our results reveal that NirP1 is widely conserved in cyanobacteria and plays a crucial role in the coordination of C/N primary metabolism by targeting nitrite reductase., (© 2024. The Author(s).)

Understanding the physiological mechanism underlying nitrogen levels response to a low red/far-red ratio (R/FR) can provide new insights for optimizing wheat yield potential but has been not well documented. This study focused on the changes in nitrogen levels, nitrogen assimilation and nitrate uptake in wheat plants grown with and without additional far-red light. A low R/FR reduced wheat nitrogen accumulation and grain yield compared with the control. The levels of total nitrogen, free amino acid and ammonium were decreased in leaves but nitrate content was temporarily increased under a low R/FR. The nitrate reductase (NR) activity in leaves was more sensitive to a low R/FR than glutamine synthetase, glutamate synthase, glutamic oxalacetic transaminase and glutamic-pyruvic transaminase. Further analysis showed that a low R/FR had little effect on the NR activation state but reduced the level of NR protein and the expression of encoding gene TaNR1.2. Interestingly, a low R/FR rapidly induced TaPIL5 expression rather than TaHY5 and other members of TaPILs in wheat, suggesting that TaPIL5 was the key transcription factor response to a low R/FR in wheat and might be involved in the downregulation of TaNR1.2 expression. Besides, a low R/FR downregulated the expression of TaNR1.2 in leaves earlier than that of TaNRT1.1/1.2/1.5/1.8 in roots, which highlights the importance of NR and nitrogen assimilation in response to a low R/FR. Our results provide revelatory evidence that restricted nitrate reductase associated with downregulated TaNR1.2 and upregulated TaPIL5 mediate the suppression of nitrogen assimilation under a low R/FR in wheat., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Masson SAS.)

Although the sources of molecular hydrogen (H2) synthesis in plants remain to be fully elucidated, ample evidence shows that plant-based H2 can regulate development and stress responses. Here, we present genetic and molecular evidence indicating that nitrate reductase (NR) might be a target of H2 sensing that positively regulates nitrogen use efficiency (NUE) and seed size in Arabidopsis (Arabidopsis thaliana). The expression level of NR and changes of NUE under control and, in particular, low nitrogen supply were positively associated with H2 addition supplied exogenously or through genetic manipulation. The improvement in nitrate assimilation achieved by H2 was also mediated via NR dephosphorylation. H2 control of seed size was impaired by NR mutation. Further genetic evidence revealed that H2, NR, and nitric oxide can synergistically regulate nitrate assimilation in response to N starvation conditions. Collectively, our data indicate that NR might be a target for H2 sensing, ultimately positively regulating nitrate uptake and seed size. These results provide insights into H2 signaling and its functions in plant metabolism., Competing Interests: Conflict of interest statement. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Y.Z. and X.C. are employees and hold ownership interest (including patents) in Air Liquide (China) R&D Co., Ltd., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Main Conclusion: In P. aeruginosa, mutation of the gene encoding N-acyl-L-homoserine lactone synthase LasI drives defense and plant growth promotion, and this latter trait requires adequate nitrate nutrition. Cross-kingdom communication with bacteria is crucial for plant growth and productivity. Here, we show a strong induction of genes for nitrate uptake and assimilation in Arabidopsis seedlings co-cultivated with P. aeruginosa WT (PAO1) or ΔlasI mutants defective on the synthesis of the quorum-sensing signaling molecule N-(3-oxododecanoyl)-L-homoserine lactone. Along with differential induction of defense-related genes, the change from plant growth repression to growth promotion upon bacterial QS disruption, correlated with upregulation of the dual-affinity nitrate transceptor CHL1/AtNRT1/NPF6.3 and the nitrate reductases NIA1 and NIA2. CHL1-GUS was induced in Arabidopsis primary root tips after transfer onto P. aeruginosa ΔlasI streaks at low and high N availability, whereas this bacterium required high concentrations of nitrogen to potentiate root and shoot biomass production and to improve root branching. Arabidopsis chl1-5 and chl1-12 mutants and double mutants in NIA1 and NIA2 nitrate reductases showed compromised growth under low nitrogen availability and failed to mount an effective growth promotion and root branching response even at high NH 4 NO 3 . WT P. aeruginosa PAO1 and P. aeruginosa ΔlasI mutant promoted the accumulation of nitric oxide (NO) in roots of both the WT and nia1nia2 double mutants, whereas NO donors SNP or SNAP did not improve growth or root branching in nia1nia2 double mutants with or without bacterial cocultivation. Thus, inoculation of Arabidopsis roots with P. aeruginosa drives gene expression for improved nitrogen acquisition and this macronutrient is critical for the plant growth-promoting effects upon disruption of the LasI quorum-sensing system., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

While photoautotrophic organisms utilize inorganic nitrogen as the nitrogen source, heterotrophic organisms utilize organic nitrogen and thus do not generally have an inorganic nitrogen assimilation pathway. Here, we focused on the nitrogen metabolism of Rapaza viridis, a unicellular eukaryote exhibiting kleptoplasty. Although belonging to the lineage of essentially heterotrophic flagellates, R. viridis exploits the photosynthetic products of the kleptoplasts and was therefore suspected to potentially utilize inorganic nitrogen. From the transcriptome data of R. viridis, we identified gene RvNaRL, which had sequence similarity to genes encoding nitrate reductases in plants. Phylogenetic analysis revealed that RvNaRL was acquired by a horizontal gene transfer event. To verify the function of the protein product RvNaRL, we established RNAi-mediated knock-down and CRISPR-Cas9-mediated knock-out experiments for the first time in R. viridis and applied them to this gene. The RvNaRL knock-down and knock-out cells exhibited significant growth only when ammonium was supplied. However, in contrast to the wild-type cells, no substantial growth was observed when nitrate was supplied. Such arrested growth in the absence of ammonium was attributed to impaired amino acid synthesis due to the deficiency of nitrogen supply from the nitrate assimilation pathway; this in turn resulted in the accumulation of excess photosynthetic products in the form of cytosolic polysaccharide grains, as observed. These results indicate that RvNaRL is certainly involved in nitrate assimilation by R. viridis. Thus, we inferred that R. viridis achieved its advanced kleptoplasty for photoautotrophy, owing to the acquisition of nitrate assimilation via horizontal gene transfer., (© The Author(s) 2023. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Numerous environmental stresses have a significant impact on plant growth and development. By 2050, it is anticipated that high salinity will destroy more than fifty percent of the world's agricultural land. Understanding how plants react to the excessive use of nitrogen fertilizers and salt stress is crucial for enhancing crop yield. However, the effect of excessive nitrate treatment on plant development is disputed and poorly understood; so, we evaluated the effect of excessive nitrate supply and high salinity on abi5 plant growth performance. We demonstrated that abi5 plants are tolerant to the harmful environmental conditions of excessive nitrate and salt. abi5 plants have lower amounts of endogenous nitric oxide than Arabidopsis thaliana Columbia-0 plants due to their decreased nitrate reductase activity, caused by a decrease in the transcript level of NIA2, a gene encoding nitrate reductase. Nitric oxide appeared to have a critical role in reducing the salt stress tolerance of plants, which was diminished by an excess of nitrate. Discovering regulators such as ABI5 that can modulate nitrate reductase activity and comprehending the molecular activities of these regulators are crucial for the application of gene-editing techniques. This would result in the appropriate buildup of nitric oxide to increase the production of crops subjected to a variety of environmental stresses., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier GmbH. All rights reserved.)

Staphylococcus aureus is a pathogenic bacterium with a widespread distribution that can cause diverse severe diseases. The membrane-bound nitrate reductase NarGHJI serves respiratory function. However, little is known about its contribution to virulence. In this study, we demonstrated that narGHJI disruption results in the downregulation of virulence genes (e.g., RNAIII , agrBDCA , hla , psmα , and psmβ ) and reduces the hemolytic activity of the methicillin-resistant S. aureus (MRSA) strain USA300 LAC. Moreover, we provided evidence that NarGHJI participates in regulating host inflammatory response. A mouse model of subcutaneous abscess and Galleria mellonella survival assay demonstrated that the ΔnarG mutant was significantly less virulent than the wild type. Interestingly, NarGHJI contributes to virulence in an agr -dependent manner, and the role of NarGHJI differs between different S. aureus strains. Our study highlights the novel role of NarGHJI in regulating virulence, thereby providing a new theoretical reference for the prevention and control of S. aureus infection. IMPORTANCE Staphylococcus aureus is a notorious pathogen that poses a great threat to human health. The emergence of drug-resistant strains has significantly increased the difficulty of preventing and treating S. aureus infection and enhanced the pathogenic ability of the bacterium. This indicates the importance of identifying novel pathogenic factors and revealing the regulatory mechanisms through which they regulate virulence. The nitrate reductase NarGHJI is mainly involved in bacterial respiration and denitrification, which can enhance bacterial survival. We demonstrated that narGHJI disruption results in the downregulation of the agr system and agr -dependent virulence genes, suggesting that NarGHJI participates in the regulation of S. aureus virulence in an agr -dependent manner. Moreover, the regulatory approach is strain specific. This study provides a new theoretical reference for the prevention and control of S. aureus infection and reveals new targets for the development of therapeutic drugs., Competing Interests: The authors declare no conflict of interest.

The periplasmic (NAP) and membrane-associated (Nar) nitrate reductases of Paracoccus denitrificans are responsible for nitrate reduction under aerobic and anaerobic conditions, respectively. Expression of NAP is elevated in cells grown on a relatively reduced carbon and energy source (such as butyrate); it is believed that NAP contributes to redox homeostasis by coupling nitrate reduction to the disposal of excess reducing equivalents. Here, we show that deletion of either dksA1 (one of two dksA homologs in the P. denitrificans genome) or relA / spoT (encoding a bifunctional ppGpp synthetase and hydrolase) eliminates the butyrate-dependent increase in nap promoter and NAP enzyme activity. We conclude that ppGpp likely signals growth on a reduced substrate and, together with DksA1, mediates increased expression of the genes encoding NAP. Support for this model comes from the observation that nap promoter activity is increased in cultures exposed to a protein synthesis inhibitor that is known to trigger ppGpp synthesis in other organisms. We also show that, under anaerobic growth conditions, the redox-sensing RegAB two-component pair acts as a negative regulator of NAP expression and as a positive regulator of expression of the membrane-associated nitrate reductase Nar. The dksA1 and relA / spoT genes are conditionally synthetically lethal; the double mutant has a null phenotype for growth on butyrate and other reduced substrates while growing normally on succinate and citrate. We also show that the second dksA homolog ( dksA2 ) and relA / spoT have roles in regulation of expression of the flavohemoglobin Hmp and in biofilm formation. IMPORTANCE Paracoccus denitrificans is a metabolically versatile Gram-negative bacterium that is used as a model for studies of respiratory metabolism. The organism can utilize nitrate as an electron acceptor for anaerobic respiration, reducing it to dinitrogen via nitrite, nitric oxide, and nitrous oxide. This pathway (known as denitrification) is important as a route for loss of fixed nitrogen from soil and as a source of the greenhouse gas nitrous oxide. Thus, it is important to understand those environmental and genetic factors that govern flux through the denitrification pathway. Here, we identify four proteins and a small molecule (ppGpp) which function as previously unknown regulators of expression of enzymes that reduce nitrate and oxidize nitric oxide.

Lelliottia amnigena PTJIIT1005 is a bacterium that utilizes nitrate as the sole nitrogen source and can remediate nitrate from media. The annotation was done related to nitrogen metabolic genes using the PATRIC, RAST tools, and PGAP from the genome sequence of this bacterium. Multiple sequence alignments and phylogenetic analysis of respiratory nitrate reductase, assimilatory nitrate reductase, nitrite reductase, glutamine synthetase, hydroxylamine reductase, nitric oxide reductase genes from PTJIIT1005 were done to find out sequence identities with the most similar species. The identification of operon arrangement in bacteria was also identified. The PATRIC KEGG feature mapped the N-metabolic pathway to identify the chemical process, and the 3D structure of representative enzymes was also elucidated. The putative protein 3D structure was analyzed using I-TASSER software. It gave good quality protein models of all nitrogen metabolism genes and showed good sequence identity with reference templates, approximately 81-99%, except for two genes; assimilatory nitrate reductase and nitrite reductase. This study suggested that PTJIIT1005 can remove N-nitrate from water because of having N-assimilation and denitrification genes., (© 2023. The Author(s).)

Nitrogen (N) is an essential element that plays an important role in crop biomass accumulation and quality formation. Increased crop yield is relied on excessive application of fertilizers, which usually leads to environmental pollution and unsustainable development. Thus, identification and characterization of genes involved in promoting nitrogen use efficiency is of high priority in crop breeding. The activity of nitrate reductase (NR) plays a critical role in nitrogen metabolism. In model plant Arabidopsis, NITRATE REDUCTASE 2 (NIA2), one of the two NRs, is responsible for about 90% of the NR activity. In this study, MdNIA2 gene in apple (Malus domestica) genome was screened out and identified by using AtNIA2 as bait. Phylogenetic analysis revealed that MdNIA2 had the closest evolutionary relationship with MbNIA from Malus baccata. Ectopic expression of MdNIA2 in Arabidopsis elevated the nitrogen use efficiency and increased root hair elongation and formation, resulting in promoted plant growth. Furthermore, the overexpression of MdNIA2 improved salt and drought tolerance in transgenic Arabidopsis and improved the salt tolerance of transgenic apple callus, and MdNIA2-reagualted NO metabolism might contribute to the abiotic stress tolerance. Overall, our data indicate the critical role of MdNIA2 in regulating nitrogen utilization efficiency and abiotic stress responses., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Masson SAS.)

Background: Brown macroalgae dominate temperate coastal ecosystems, and their productivity is typically limited by nitrate availability. As an economically important kelp, Saccharina japonica is the most productive farmed seaweed and needs to be supplemented with sufficient nitrate throughout the cultivation process. However, molecular characterization of genes involved in nitrogen assimilation has not been conducted in brown macroalgae., Results: Here, we described the identification of the nitrate reductase (NR) gene from S. japonica (SjNR). Using two different cloning methods for SjNR, i.e. rapid amplification of cDNA ends (RACE) and cDNA cloning alone, a single fragment was obtained respectively. According to results of sequence analysis between these two fragments, the tentative coding sequence in two clones, SjNR-L and SjNR-S, were suggested to represent two transcripts of the single copy SjNR, and the ATG of SjNR-S was located inside the third exon of SjNR-L. In the 5' upstream sequence of each transcript, promoter core elements, response elements, especially multiple N response elements which occurred in microalgal NR, were all predicted. Further sequence analysis revealed that both transcripts encoded all five domains conserved in eukaryotic plant NRs. RT-qPCR results showed that the transcription level of SjNR in juvenile sporophytes could be significantly induced by nitrate and inhibited by ammonium, which was in line with plant NRs. The recombinant SjNR-L and SjNR-S were all proved to have NR activity, suggesting that the single-copy gene SjNR might be regulated on transcription level based on alternative promoters and multiple transcriptional start sites. Moreover, both NADH and NADPH were found to be able to act as electron donors for SjNR alone, which is the first confirmation that brown algal NR has a NAD(P)H-bispecific form., Conclusion: These results will provide a scientific basis for understanding the N demand of kelp in various stages of cultivation and evaluating the environmental remediation potential of kelp in eutrophic sea areas., (© 2023. The Author(s).)

Background: Nitrogen (N) is an essential macronutrient for plant growth and development as it is an essential constituent of biomolecules. Its availability directly impacts crop yield. Increased N application in crop fields has caused environmental and health problems, and decreasing nitrogen inputs are in demand to maintain crop production sustainability. Understanding the molecular mechanism of N utilization could play a crucial role in improving the nitrogen use efficiency (NUE) of crop plants., Methods and Results: In the present study, the effect of low N supply on plant growth, physio-biochemical, chlorophyll fluorescence attributes, yield components, and gene expression analysis were measured at six developmental stages in rice cultivars. Two rice cultivars were grown with a supply of optimium (120 kg ha -1 ) and low N (60 kg ha -1 ). Cultivar Vikramarya excelled Aditya at low N supply, and exhibits enhanced plant growth, physiological efficiency, agronomic efficiency, and improved NUE due to higher N uptake and utilization at low N treatment. Moreover, plant biomass, leaf area, and photosynthetic rate were significantly higher in cv. Vikramarya than cv. Aditya at different growth stages, under low N treatment. In addition, enzymatic activities in cultivar Vikramarya were higher than cultivar Aditya under low nitrogen, indicating its greater potential for N metabolism. Gene expression analysis was carried out for the most important nitrogen assimilatory enzymes, such as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT). Expression levels of these genes at different growth stages were significantly higher in cv. Vikramarya compared to cv. Aditya at low N supply. Our findings suggest that improving NUE needs specific revision in N metabolism and physiological assimilation., Conclusion: Overall differences in plant growth, physiological efficiency, biochemical activities, and expression levels of N metabolism genes in N-efficient and N-inefficient rice cultivars need a specific adaptation to N metabolism. Regulatory genes may separately or in conjunction, enhance the NUE. These results provide a platform for selecting crop cultivars for nitrogen utilization efficiency at low N treatment., (© 2022. The Author(s), under exclusive licence to Springer Nature B.V.)

In most algae, NO3- assimilation is tightly controlled and is often inhibited by the presence of NH4+. In the marine, non-colonial, non-diazotrophic cyanobacterium Synechococcus UTEX 2380, NO3- assimilation is sensitive to NH4+ only when N does not limit growth. We sequenced the genome of Synechococcus UTEX 2380, studied the genetic organization of the nitrate assimilation related (NAR) genes, and investigated expression and kinetics of the main NAR enzymes, under N or light limitation. We found that Synechococcus UTEX 2380 is a β-cyanobacterium with a full complement of N uptake and assimilation genes and NAR regulatory elements. The nitrate reductase of our strain showed biphasic kinetics, previously observed only in freshwater or soil diazotrophic Synechococcus strains. Nitrite reductase and glutamine synthetase showed little response to our growth treatments, and their activity was usually much higher than that of nitrate reductase. NH4+ insensitivity of NAR genes may be associated with the stimulation of the binding of the regulator NtcA to NAR gene promoters by the high 2-oxoglutarate concentrations produced under N limitation. NH4+ sensitivity in energy-limited cells fits with the fact that, under these conditions, the use of NH4+ rather than NO3- decreases N-assimilation cost, whereas it would exacerbate N shortage under N limitation., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Five molybdenum-dependent enzymes are known in eukaryotes. While four of them are under investigation since decades, the most recently discovered, (mitochondrial) amidoxime reducing component ((m)ARC), has only been characterized in mammals and the green algae Chlamydomonas reinhardtii. While mammalian mARCs have been shown to be involved in various signalling pathways, Chlamydomonas ARC was shown to be a nitric oxide (NO)-forming nitrite reductase. Similar to mammals, higher plants possess two ARC proteins. To test whether plant ARCs have a similar function in NO production to the function they have in C. reinhardtii, we analysed the enzymes from the model plant Arabidopsis thaliana. Both ARC1 and ARC2 from Arabidopsis could reduce N-hydroxylated compounds, while nitrite reduction to form NO could only be demonstrated for ARC2. Searching for physiological electron donors, we found that both ARC enzymes accept electrons from NADH via cytochrome b 5 reductase and cytochrome b 5 , but only ARC2 is able to accept electrons from nitrate reductase at all. Furthermore, arc-deficient mutant plants were similar to wildtype plants regarding growth and also nitrite-dependent NO-formation. Altogether, our results did not confirm the hypothesis that either ARC1 or ARC2 from Arabidopsis are involved in physiologically relevant nitrite-dependent NO-formation. In contrast, our data suggest that ARC1 and ARC2 have distinct, yet unknown physiological roles in higher plants., (© 2022 The Authors. The FEBS Journal published by John Wiley & Sons Ltd on behalf of Federation of European Biochemical Societies.)

Nitrous oxide (N 2 O) is a powerful greenhouse gas and an ozone-depleting compound whose synthesis and release have traditionally been ascribed to bacteria and fungi. Although plants and microalgae have been proposed as N 2 O producers in recent decades, the proteins involved in this process have been only recently unveiled. In the green microalga Chlamydomonas reinhardtii , flavodiiron proteins (FLVs) and cytochrome P450 (CYP55) are two nitric oxide (NO) reductases responsible for N 2 O synthesis in the chloroplast and mitochondria, respectively. However, the molecular mechanisms feeding these NO reductases are unknown. In this work, we use cavity ring-down spectroscopy to monitor N 2 O and CO 2 in cultures of nitrite reductase mutants, which cannot grow on nitrate or nitrite and exhibit enhanced N 2 O emissions. We show that these mutants constitute a very useful tool to study the rates and kinetics of N 2 O release under different conditions and the metabolism of this greenhouse gas. Our results indicate that N 2 O production, which was higher in the light than in the dark, requires nitrate reductase as the major provider of NO as substrate. Finally, we show that the presence of nitrate reductase impacts CO 2 emissions in both light and dark conditions, and we discuss the role of NO in the balance between CO 2 fixation and release.

Gaining insight into the behavior of bacteria at the single-cell level is important given that heterogeneous microenvironments strongly influence microbial physiology. The hybridization chain reaction (HCR) is a technique that provides in situ molecular signal amplification, enabling simultaneous mapping of multiple target RNAs at small spatial scales. To refine this method for biofilm applications, we designed and validated new probes to visualize the expression of key catabolic genes in Pseudomonas aeruginosa aggregates. In addition to using existing probes for the dissimilatory nitrate reductase ( narG ), we developed probes for a terminal oxidase ( ccoN1 ), nitrite reductase ( nirS ), nitrous oxide reductase ( nosZ ), and acetate kinase ( ackA ). These probes can be used to determine gene expression levels across heterogeneous populations such as biofilms. Using these probes, we quantified gene expression across oxygen gradients in aggregate populations grown using the a gar b lock b iofilm a ssay (ABBA). We observed distinct patterns of catabolic gene expression, with upregulation occurring in particular ABBA regions both within individual aggregates and over the aggregate population. Aerobic respiration ( ccoN1 ) showed peak expression under oxic conditions, whereas fermentation ( ackA ) showed peak expression in the anoxic cores of high metabolic activity aggregates near the air-agar interface. Denitrification genes narG, nirS, and nosZ showed peak expression in hypoxic and anoxic regions, although nirS expression remained at peak levels deeper into anoxic environments than other denitrification genes. These results reveal that the microenvironment correlates with catabolic gene expression in aggregates, and they demonstrate the utility of HCR in unveiling cellular activities at the microscale level in heterogeneous populations. IMPORTANCE To understand bacteria in diverse contexts, we must understand the variations in behaviors and metabolisms they express spatiotemporally. Populations of bacteria are known to be heterogeneous, but the ways this variation manifests can be challenging to characterize due to technical limitations. By focusing on energy conservation, we demonstrate that HCR v3.0 can visualize nuances in gene expression, allowing us to understand how metabolism in Pseudomonas aeruginosa biofilms responds to microenvironmental variation at high spatial resolution. We validated probes for four catabolic genes, including a constitutively expressed oxidase, acetate kinase, nitrite reductase, and nitrous oxide reductase. We showed that the genes for different modes of metabolism are expressed in overlapping but distinct subpopulations according to oxygen concentrations in a predictable fashion. The spatial transcriptomic technique described here has the potential to be used to map microbial activities across diverse environments.

In this study, we discussed the relationship between Entermorpha linza polysaccharide (EP) and Bacillus subtilis, which can transform nitrate. A sole carbon source experiment showed that Bacillus subtilis could utilize EP, and the bacterial density was maximally increased by 54.43% in the EP groups. The results of reducing sugar determination proved the secretion of polysaccharide-degrading enzymes. Scanning electron microscopy (SEM) showed that the EP groups had fewer spores and shrunken bacteria, indicating that EP could improve the growth environment and maintain bacterial integrity. Additionally, the ratios of periplasmic nitrate reductase (NAP), nitrite reductase (NIR), and dissimilatory nitrate reductase (D-NRase) in the EP groups were maximally increased by 107.22%, 84.70% and 36.10%, respectively. Transcriptome analysis further confirmed the above mentioned results. For example, the high expression of quorum sensing genes indicated that EP groups had higher bacterial density. Moreover, the high expression of antioxidant genes in the EP groups may be related to morphological integrity. Our study provides a basis for further discussion of the mechanism., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The extensive use of graphene oxide (GO) has resulted in its inevitable entry into the environment. It has been established that GO is detrimental to nitrogen accumulation in plants, as nitrogen is one of the most important nutrient for plant growth. However, its influence on nitrogen assimilation has not yet been investigated comprehensively. Based on the analysis of transcriptomics and nitrogen metabolites, this study showed that 400 mg L -1 GO exposure downregulated most of the genes encoding nitrogen-assimilating enzymes, including nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH). The activities of the above enzymes in wheat roots were also reduced with GO addition, and the activities of NR and GS, the rate-limiting enzymes of nitrate and ammonium assimilation, were approximately 75% and 76% lower with 400 mg L -1 GO supply, respectively, compared to those upon control treatment. Correspondingly, GO appears to exert a negative effect on multiple nitrogen assimilation products, including nitrous nitrogen, ammonium nitrogen, glutamine, glutamate, and soluble protein. In summary, this study showed that GO has adverse effects on the nitrogen assimilation of plants, and NR and GS are the most affected sites. Our findings would provide deeper insights into the physiological and molecular mechanisms underlying GO phytotoxicity., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Defects in RNA maturation and RNA decay factors may generate substrates for the RNA interference machinery. This phenomenon was observed in plants where mutations in some RNA-related factors lead to the production of RNA-quality control small interfering RNAs and several mutants show enhanced silencing of reporter transgenes. To assess the potential of RNAi activation on endogenous transcripts, we sequenced small RNAs from a set of Arabidopsis thaliana mutants with defects in various RNA metabolism pathways. We observed a global production of siRNAs caused by inefficient pre-mRNA cleavage and polyadenylation leading to read-through transcription into downstream antisense genes. In addition, in the lsm1a lsm1b double mutant, we identified NIA1, SMXL5, and several miRNA-targeted mRNAs as producing siRNAs, a group of transcripts suggested being especially sensitive to deficiencies in RNA metabolism. However, in most cases, RNA metabolism perturbations do not lead to the widespread production of siRNA derived from mRNA molecules. This observation is contrary to multiple studies based on reporter transgenes and suggests that only a very high accumulation of defective mRNA species caused by specific mutations or substantial RNA processing defects trigger RNAi pathways., (© 2022. The Author(s).)

The two homologous genes, NIA1 and NIA2 , encode nitrate reductases in Arabidopsis , which govern the reduction of nitrate to nitrite. This step is the rate-limiting step of the nitrate assimilation and utilization. Therefore, the regulation of NIA1 and NIA2 is important for plant development and growth. Although they are similar in sequence and structure, their regulations are different. Genetic analysis uncovers that NIA1, rather than NIA2, plays a predominant role in adopting to ABA stress. Although both long-term stress conditions can cause an improvement in NIA1 levels, a decrease in NIA1 levels under short-term treatments seems to be necessary for plants to switch from the growth status into the adopting status. Interestingly, the downregulation of the NR is distinct under different stress conditions. Under ABA treatment, the NR proteins are degraded via a 26S-proteasome dependent manner, while the transcriptional regulation is the main manner to rapidly reduce the NIA1 levels under nitrogen deficiency and NaCl stress conditions. These results indicate that under stress conditions, the regulation of NIA1 is complex, and it plays a key role in regulating the balance between growth and adaptation.

Nitric oxide (NO) modulates plant response to biotic and abiotic stresses by S-nitrosylation-mediated protein post-translational modification. Nitrate reductase (NR) and S-nitrosoglutathione reductase (GSNOR) enzymes are essential for NO synthesis and the maintenance of Nitric oxide/S-nitroso glutathione (NO/GSNO) homeostasis, respectively. S-nitrosoglutathione, formed by the S-nitrosylation reaction of NO with glutathione, plays a significant physiological role as the mobile reservoir of NO. The genome-wide analysis identified nine NR (NIA) and three GSNOR genes in the wheat genome. Phylogenic analysis revealed that the nine NIA genes +were clustered into four groups and the 3 GSNORs into two groups. qRT-PCR expression profiling of NIAs and GSNORs was done in Chinese spring (CS), a leaf rust susceptible wheat line showing compatible interaction, and Transfer (TR), leaf rust-resistant wheat line showing incompatible interaction, post-inoculation with leaf rust pathotype 77-5 (121-R-63). All the NIA genes showed upregulation during incompatible interaction in comparison with the compatible reaction. The GSNOR genes showed a variable pattern of expression: the TaGSNOR1 showed little change, whereas TaGSNOR2 showed higher expression during the incompatible response. TaGSNOR3 showed a rise of expression both in compatible and incompatible reactions. Before inoculation and after 72 h of pathogen inoculation, NO localization was studied in both compatible and incompatible reactions. The S-nitrosothiol accumulation, NR, and glutathione reductase activity showed a consistent increase in the incompatible interactions. The results demonstrate that both NR and GSNOR plays significant role in defence against the leaf rust pathogen in wheat by modulating NO homeostasis or signalling., (© 2022. The Author(s).)

Nitrogen is an essential nutrient for plant growth and development, and plays vital roles in crop yield. Assimilation of nitrogen is thus fine-tuned in response to heterogeneous environments. However, the regulatory mechanism underlying this essential process remains largely unknown. Here, we report that a zinc-finger transcription factor, drought and salt tolerance (DST), controls nitrate assimilation in rice by regulating the expression of OsNR1.2. We found that loss of function of DST results in a significant decrease of nitrogen use efficiency (NUE) in the presence of nitrate. Further study revealed that DST is required for full nitrate reductase activity in rice and directly regulates the expression of OsNR1.2, a gene showing sequence similarity to nitrate reductase. Reverse genetics and biochemistry studies revealed that OsNR1.2 encodes an NADH-dependent nitrate reductase that is required for high NUE of rice. Interestingly, the DST-OsNR1.2 regulatory module is involved in the suppression of nitrate assimilation under drought stress, which contributes to drought tolerance. Considering the negative role of DST in stomata closure, as revealed previously, the positive role of DST in nitrogen assimilation suggests a mechanism coupling nitrogen metabolism and stomata movement. The discovery of this coupling mechanism will aid the engineering of drought-tolerant crops with high NUE in the future., (Copyright © 2021 The Author. Published by Elsevier Inc. All rights reserved.)

Currently ectomycorrhizal research suffers from a lack of molecular tools specifically adapted to study gene expression in fungal symbionts. Considering that, we designed pReNuK, a cloning vector for transcriptional promoter studies in the ectomycorrhizal basidiomycete Laccaria bicolor. The pReNuK vector offers the use of a nuclear localizing and chromatin incorporating histone H2B-mCherry fluorescent reporter protein and it is specifically optimized for efficient transgene expression in Laccaria. Moreover, pReNuK is designed to work in concert with Agrobacterium-mediated transformation under hygromycin B resistance selection. The functionality of the pReNuK reporter system was tested with the constitutive Laccaria glyceraldehyde 3-phosphate dehydrogenase gene promoter and further validated with the nitrogen source regulated nitrate reductase gene promoter. The expression of the nucleus-directed H2B-mCherry reporter is highly stable in time. Moreover, the transformation of Laccaria with pReNuK and the expression of the reporter do not have negative effects on the growth of the fungus. The pReNuK offers a novel tool for studying in vivo gene expression regulation in Laccaria, the leading fungal model for ectomycorrhizal research., (Copyright © 2021. Published by Elsevier B.V.)

Mycoparasitic fungi can be utilized as biocontrol agents (BCAs) of many plant pathogens. Deciphering the molecular mechanisms of mycoparasitism may improve biocontrol efficiency. This work reports the first functional genetic studies in Ampelomyces, widespread mycoparasites and BCAs of powdery mildew fungi, and a molecular genetic toolbox for future works. The nitrate reductase (euknr) gene was targeted to reveal the biological function of nitrate assimilation in Ampelomyces. These mycoparasites live in an apparently nitrate-free environment, i.e. inside the hyphae of powdery mildew fungi that lack any nitrate uptake and assimilation system. Homologous recombination-based gene knock-out (KO) was applied to eliminate the euknr gene using Agrobacterium tumefaciens-mediated transformation. Efficient KO of euknr was confirmed by PCR, and visible phenotype caused by loss of euknr was detected on media with different nitrogen sources. Mycoparasitic ability was not affected by knocking out euknr as a tested transformant readily parasitized Blumeria graminis and Podosphaera xanthii colonies on barley and cucumber, respectively, and the rate of mycoparasitism did not differ from the wild type. These results indicate that euknr is not involved in mycoparasitism. Dissimilatory processes, involvement in nitric oxide metabolism, or other, yet undiscovered processes may explain why a functional euknr is maintained in Ampelomyces., Competing Interests: Declaration of competing interest The authors have no competing interests., (Copyright © 2021 British Mycological Society. Published by Elsevier Ltd. All rights reserved.)

Molybdenum (Mo) is an essential element for plant growth and is utilized by several key enzymes in biological redox processes. Rice assimilates molybdate ions via OsMOT1;1, a transporter with a high affinity for molybdate. However, other systems involved in the molecular transport of molybdate in rice remain unclear. Here, we characterized OsMOT1;2, which shares amino acid sequence similarity with AtMOT1;2 and functions in vacuolar molybdate export. We isolated a rice mutant harboring a complete deletion of OsMOT1;2. This mutant exhibited a significantly lower grain Mo concentration than the wild type (WT), but its growth was not inhibited. The Mo concentration in grains was restored by the introduction of WT OsMOT1;2. The OsMOT1;2-GFP protein was localized to the vacuolar membrane when transiently expressed in rice protoplasts. At the reproductive growth stage of the WT plant, OsMOT1;2 was highly expressed in the 2nd and lower leaf blades and nodes. The deletion of OsMOT1;2 impaired interorgan Mo allocation in aerial parts: relative to the WT, the mutant exhibited decreased Mo levels in the 1st and 2nd leaf blades and grains but increased Mo levels in the 2nd and lower leaf sheaths, nodes and internodes. When the seedlings were exposed to a solution with a high KNO3 concentration in the absence of Mo, the mutant exhibited significantly lower nitrate reductase activity in the shoots than the WT. Our results suggest that OsMOT1;2 plays an essential role in interorgan Mo distribution and molybdoenzyme activity in rice., (© The Author(s) 2021. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Understanding Nitrogen Use Efficiency (NUE) physiological and molecular mechanisms in high N demanding crops has become decisive for improving NUE in sustainable cropping systems. How the Nitrogen Utilization Efficiency (NUtE) component contributes to the NUE enhancement under nitrate limiting conditions in tomato remains to be elucidated. This study deals with the changes in several important nitrate metabolism related gene expressions (nitrate assimilation, transport, remobilization and storage/sequestration) engendered by short and long-term limiting nitrate exposure in two selected NUE-contrasting genotypes, Regina Ostuni (RO) and UC82, efficient and inefficient, respectively. At short-term, nitrate limiting supply triggered higher SlCLCa and SlNRT1.7 expressions in RO root and shoot, respectively, suggesting a higher nitrate storage and remobilization compared to UC82, explaining how RO withstood the nitrate deficiency better than UC82. At long-term, nitrate reductase (SlNR) and nitrite reductase (SlNIR) expression were not significantly different between nitrate treatments in RO, while significantly down-regulated under nitrate limiting treatment in UC82. In addition, SlCLCa and SlNRT1.8 transcript levels were significantly lower in RO, while those of SlNRT1.5 and SlNR appeared significantly higher. This suggested that the efficient genotype stored less nitrate compared to UC82, which was allocated and assimilated to the shoot. More interestingly, the expression of SlNRT2.7 was significantly higher in RO shoot compared to UC82 and strongly correlated to RO higher growth as well as to NUE and NUtE component. Our findings underlined the differential regulation of N-metabolism genes that may confer to NUtE component a pivotal role in NUE enhancement in tomato., (Copyright © 2021. Published by Elsevier Masson SAS.)

Background: Nitrogen (N) is an essential macronutrient to maintain plant growth and development. Plants absorb nitrate-N or ammonium-N in the environment and undergo reduction reactions catalyzed by nitrate reductase (NR), nitrite reductase (NIR), glutamine synthetase (GS), and glutamine oxoglutarate aminotransferase (GOGAT) within plants., Methods and Results: A total of 42 N assimilation-related genes (NAG) members were identified in rapeseed. Darwin's evolutionary pressure analysis showed that rapeseed NAGs underwent purification selection. Cis-element analysis revealed differences in the transcriptional regulation of NAGs between Arabidopsis and rapeseed. Expression analyses revealed that NRs were expressed mainly in old leaves, NIRs were expressed mainly in old leaves and lower stem peels, while the expression situation between different subfamilies of GSs and GOGATs was more complicated., Conclusions: Differential expression of NAGs suggested that they might be involved in abiotic stresses. The above results greatly enriched our understanding of NAGs' molecular characteristics and provided central gene resources for NAGs-mediated NUE improvement in rapeseed., (© 2021. The Author(s), under exclusive licence to Springer Nature B.V.)

The tomato accession LA1996, which carries a dominant allele of anthocyanin fruit (Aft) locus, accumulates anthocyanins in the epidermis of fruits when exposed to sunlight. The involvement of blue, UV-A, UV-B and a combination of these wavelengths on anthocyanin accumulation and the molecular mechanism of their regulation was investigated in LA1996. The most effective treatment for inducing anthocyanin biosynthesis in Aft fruits was co-irradiation with blue and UV-B (blue + UV-B) light. Finding the correlated genes is an important approach towards understanding their molecular mechanisms. In the present study, the nitrate reductase (NR) gene SlNIA was isolated using RNA-seq profiling of Aft fruits given different light treatments. The functions of NR-mediated anthocyanin induction by blue + UV-B were confirmed using a series of chemical treatments, followed by assessment of NR activity and nitric oxide (NO) detection. The expression of NR was highly induced by blue + UV-B, and this specificity was also confirmed with the enzyme activity of NR and the NO concentration. The NR inhibitors, which reduce NO generation, the expression levels of anthocyanin related genes and decreased anthocyanin accumulation in LA1996. Our results suggest that NR plays a key role in blue + UV-B-mediated anthocyanin accumulation in LA1996 fruits., (© 2020 German Society for Plant Sciences and The Royal Botanical Society of the Netherlands.)

Precise flowering timing is critical for the plant life cycle. Here, we examined the molecular mechanisms and regulatory network associated with flowering in Chinese cabbage ( Brassica rapa L.) by comparative transcriptome profiling of two Chinese cabbage inbred lines, "4004" (early bolting) and "50" (late bolting). RNA-Seq and quantitative reverse transcription PCR (qPCR) analyses showed that two positive nitric oxide (NO) signaling regulator genes, nitrite reductase ( BrNIR ) and nitrate reductase ( BrNIA ), were up-regulated in line "50" with or without vernalization. In agreement with the transcription analysis, the shoots in line "50" had substantially higher nitrogen levels than those in "4004". Upon vernalization, the flowering repressor gene Circadian 1 ( BrCIR1 ) was significantly up-regulated in line "50", whereas the flowering enhancer genes named SUPPRESSOR OF OVEREXPRESSION OF CONSTANCE 1 homologs ( BrSOC1s ) were substantially up-regulated in line "4004". CRISPR/Cas9-mediated mutagenesis in Chinese cabbage demonstrated that the BrSOC1-1/1-2/1-3 genes were involved in late flowering, and their expression was mutually exclusive with that of the nitrogen signaling genes. Thus, we identified two flowering mechanisms in Chinese cabbage: a reciprocal negative feedback loop between nitrogen signaling genes ( BrNIA1 and BrNIR1 ) and BrSOC1s to control flowering time and positive feedback control of the expression of BrSOC1s .

Wastewater treatment plants (WWTPs) are important for pollutant removal from wastewater, elimination of point discharges of nutrients into the environment and water resource protection. The anaerobic/anoxic/oxic (A2/O) process is widely used in WWTPs for nitrogen removal, but the requirement for additional organics to ensure a suitable nitrogen removal efficiency makes this process costly and energy consuming. In this study, we report mixotrophic denitrification at a low COD (chemical oxygen demand)/TN (total nitrogen) ratio in a full-scale A2/O WWTP with relatively high sulfate in the inlet. Nitrogen and sulfur species analysis in different units of this A2/O WWTP showed that the internal sulfur cycle of sulfate reduction and reoxidation occurred and that the reduced sulfur species might contribute to denitrification. Microbial community analysis revealed that Thiobacillus, an autotrophic sulfur-oxidizing denitrifier, dominated the activated sludge bacterial community. Metagenomics data also supported the potential of sulfur-based denitrification when high levels of denitrification occurred, and sulfur oxidation and sulfate reduction genes coexisted in the activated sludge. Although most of the denitrification genes were affiliated with heterotrophic denitrifiers with high abundance, the narG and napA genes were mainly associated with autotrophic sulfur-oxidizing denitrifiers. The functional genes related to nitrogen removal were actively expressed even in the unit containing relatively highly reduced sulfur species, indicating that the mixotrophic denitrification process in A2/O could overcome not only a shortage of carbon sources but also the inhibition by reduced sulfur of nitrification and denitrification. Our results indicate that a mixotrophic denitrification process could be developed in full-scale WWTPs and reduce the requirement for additional carbon sources, which could endow WWTPs with more flexible and adaptable nitrogen removal., Competing Interests: No authors have competing interests.

Although nitric oxide (NO) is a key regulatory molecule in plants, its function in plants under conditions of simulated acid rain (SAR) has not been fully established yet. In this study, exogenous sodium nitroprusside (SNP) at three different concentrations were applied to mung bean seedlings. Malondialdehyde (MDA), NO, hydrogen peroxide (H 2 O 2 ), antioxidant enzyme activities, and nitrate reductases (NR) were measured. Real time PCR was used to measure the NR expression. Compared to the control, the NR activity and NO content under the pH 2 SAR decreased by 79% and 85.6% respectively. Meanwhile, the SAR treatment reduced the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), while increased MDA content. Application of SNP could potentially reverse the adverse impact of SAR, depending on its concentration. For plants under the pH 2 SAR and 0.25 mM SNP condition, the activities of SOD, POD, APX increased by 123%, 291%, and 135.7% respectively, meanwhile, MDA concentration decreased by 43%, NR activities increased by 269%, and NO concentration increased by 123.6% compared with plants undergoing only pH 2 SAR. The relative expression of the NR1 gene was 2.69 times higher than that of pH 2 SAR alone. Overall, the application of 0.25 mM SNP eliminated reactive oxygen species (ROS) by stimulating antioxidant enzyme activities, reducing oxidative stress and mitigating the toxic effects of SAR on mung bean seedlings. This research provides a foundation for further research on the mechanism of NO on plants under SAR conditions.

Potassium chlorate (KClO 3 ) has been widely used to evaluate the divergence in nitrogen use efficiency (NUE) between indica and japonica rice subspecies. This study investigated the transcriptional regulation of major genes involved in the NUE in rice treated with KClO 3 , which acts as an inhibitor of the reducing activity of nitrate reductase (NR) in higher plants. A set of two KClO 3 sensitive nitrate reductase (NR) and two nitrate transporter (NRT) introgression rice lines (BC2F7), carrying the indica alleles of NR or NRT, derived from a cross between Saeilmi ( japonica , P1) and Milyang23 ( indica , P2), were exposed to KClO 3 at the seedling stage. The phenotypic responses were recorded 7 days after treatment, and samples for gene expression, physiological, and biochemical analyses were collected at 0 h (control) and 3 h after KClO 3 application. The results revealed that Saeilmi (P1, japonica ) and Milyang23 (P2, indica ) showed distinctive phenotypic responses. In addition, the expression of OsNR2 was differentially regulated between the roots, stem, and leaf tissues, and between introgression lines. When expressed in the roots, OsNR2 was downregulated in all introgression lines. However, in the stem and leaves, OsNR2 was upregulated in the NR introgression lines, but downregulation in the NRT introgression lines. In the same way, the expression patterns of OsNIA1 and OsNIA2 in the roots, stem, and leaves indicated a differential transcriptional regulation by KClO 3 , with OsNIA2 prevailing over OsNIA1 in the roots. Under the same conditions, the activity of NR was inhibited in the roots and differentially regulated in the stem and leaf tissues. Furthermore, the transcriptional divergence of OsAMT1.3 and OsAMT2.3 , OsGLU1 and OsGLU2, between NR and NRT, coupled with the NR activity pattern in the roots, would indicate the prevalence of nitrate (NO 3 ¯) transport over ammonium (NH 4 + ) transport. Moreover, the induction of catalase (CAT) and polyphenol oxidase (PPO) enzyme activities in Saeilmi (P1, KClO 3 resistant), and the decrease in Milyang23 (P2, KClO 3 sensitive), coupled with the malondialdehyde (MDA) content, indicated the extent of the oxidative stress, and the induction of the adaptive response mechanism, tending to maintain a balanced reduction-oxidation state in response to KClO 3 . The changes in the chloroplast pigments and proline content propose these compounds as emerging biomarkers for assessing the overall plant health status. These results suggest that the inhibitory potential of KClO 3 on the reduction activity of the nitrate reductase (NR), as well as that of the genes encoding the nitrate and ammonium transporters, and glutamate synthase are tissue-specific, which may differentially affect the transport and assimilation of nitrate or ammonium in rice.