Nitrate metabolism in tobacco leaves overexpressing Arabidopsis nitrite reductase

The AP2/ERF transcription factor MdDREB2A regulates nitrogen utilisation and sucrose transport under drought stress

Drought stress is one of the main environmental factors limiting plant growth and development. Plants adapt to changing soil moisture by modifying root architecture, inducing stomatal closure, and inhibiting shoot growth. The AP2/ERF transcription factor DREB2A plays a key role in maintaining plant growth in response to drought stress, but the molecular mechanism underlying this process remains to be elucidated. Here, it was found that overexpression of MdDREB2A positively regulated nitrogen utilisation by interacting with DRE cis-elements of the MdNIR1 promoter. Meanwhile, MdDREB2A could also directly bind to the promoter of MdSWEET12, which may enhance root development and nitrogen assimilation, ultimately promoting plant growth. Overall, this regulatory mechanism provides an idea for plants in coordinating with drought tolerance and nitrogen assimilation to maintain optimal plant growth and development under drought stress.

Plant physical defenses contribute to a latitudinal gradient in resistance to insect herbivory within a widespread perennial grass

PREMISE

Herbivore pressure can vary across the range of a species, resulting in different defensive strategies. If herbivory is greater at lower latitudes, plants may be better defended there, potentially driving a latitudinal gradient in defense. However, relationships that manifest across the entire range of a species may be confounded by differences within genetic subpopulations, which may obscure the drivers of these latitudinal gradients.

METHODS

We grew plants of the widespread perennial grass Panicum virgatum in a common garden that included genotypes from three genetic subpopulations spanning an 18.5° latitudinal gradient. We then assessed defensive strategies of these plants by measuring two physical resistance traits-leaf mass per area (LMA) and leaf ash, a proxy for silica-and multiple measures of herbivory by caterpillars of the generalist herbivore fall armyworm (Spodoptera frugiperda).

RESULTS

Across all genetic subpopulations, low-latitude plants experienced less herbivory than high-latitude plants. Within genetic subpopulations, however, this relationship was inconsistent-the most widely distributed and phenotypically variable subpopulation (Atlantic) exhibited more consistent latitudinal trends than either of the other two subpopulations. The two physical resistance traits, LMA and leaf ash, were both highly heritable and positively associated with resistance to different measures of herbivory across all subpopulations, indicating their importance in defense against herbivores. Again, however, these relationships were inconsistent within subpopulations.

CONCLUSIONS

Defensive gradients that occur across the entire species range may not arise within localized subpopulations. Thus, identifying the drivers of latitudinal gradients in herbivory defense may depend on adequately sampling the diversity within a species.

From cultivation to cancer: formation of N-nitrosamines and other carcinogens in smokeless tobacco and their mutagenic implications

Tobacco use is a major cause of preventable morbidity and mortality globally. Tobacco products, including smokeless tobacco (ST), generally contain tobacco-specific N-nitrosamines (TSNAs), such as N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-butanone (NNK), which are potent carcinogens that cause mutations in critical genes in human DNA. This review covers the series of biochemical and chemical transformations, related to TSNAs, leading from tobacco cultivation to cancer initiation. A key aim of this review is to provide a greater understanding of TSNAs: their precursors, the microbial and chemical mechanisms that contribute to their formation in ST, their mutagenicity leading to cancer due to ST use, and potential means of lowering TSNA levels in tobacco products. TSNAs are not present in harvested tobacco but can form due to nitrosating agents reacting with tobacco alkaloids present in tobacco during certain types of curing. TSNAs can also form during or following ST production when certain microorganisms perform nitrate metabolism, with dissimilatory nitrate reductases converting nitrate to nitrite that is then released into tobacco and reacts chemically with tobacco alkaloids. When ST usage occurs, TSNAs are absorbed and metabolized to reactive compounds that form DNA adducts leading to mutations in critical target genes, including the RAS oncogenes and the p53 tumor suppressor gene. DNA repair mechanisms remove most adducts induced by carcinogens, thus preventing many but not all mutations. Lastly, because TSNAs and other agents cause cancer, previously documented strategies for lowering their levels in ST products are discussed, including using tobacco with lower nornicotine levels, pasteurization and other means of eliminating microorganisms, omitting fermentation and fire-curing, refrigerating ST products, and including nitrite scavenging chemicals as ST ingredients.

Oligochitosan fortifies antioxidative and photosynthetic metabolism and enhances secondary metabolite accumulation in arsenic-stressed peppermint

The current study was designed to investigate whether application of irradiated chitosan (ICn), a recently established plant growth promoter, can prove effective in alleviating arsenic (As) stress in peppermint, a medicinally important plant. This study investigated how foliar application of ICn alleviated As toxicity in peppermint (Mentha piperita L.). Peppermint plants were treated with ICn (80 mg L^-1) alone or in combination with As (10, 20, or 40 mg kg^-1 of soil, as Na₂HAsO₄·7H₂O) 40 days after transplantation (DAT), and effects on the growth, photosynthesis, and antioxidants were assessed at 150 DAT as stress severely decreases plant growth, affects photosynthesis, and alters enzymatic (ascorbate peroxidase, superoxide dismutase) and non-enzymatic (glutathione) antioxidants. When applied at 40 mg kg^-1, ICn significantly decreased the content of essential oil (EO) and total phenols in peppermint by 13.8 and 16.0%, respectively, and decreased phenylalanine ammonia lyase (PAL) and deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) activities by 12.8 and 14.6%, respectively. Application of ICn mitigated the disadvantageous effects caused by As toxicity in peppermint by enhancing activities of antioxidative enzymes and photosynthesis and increased accretion of secondary metabolism products (EOs and phenols). An enhancement of total phenols (increased by 17.3%) and EOs (36.4%) is endorsed to ICn-stimulated enhancement in the activities of PAL and DXR (65.9 and 28.9%, respectively) in comparison to the control. To conclude, this study demonstrated that foliar application of ICn (80 mgL^-1) effectively promoted the growth and physiology of peppermint and eliminated As-induced toxicity to achieve high production of EO-containing crops grown in metal-contaminated soils.

Physiological Characteristics and Transcriptomic Dissection in Two Root Segments with Contrasting Net Fluxes of Ammonium and Nitrate of Poplar Under Low Nitrogen Availability

To investigate physiological and transcriptomic regulation mechanisms underlying the distinct net fluxes of NH4+ and NO3- in different root segments of Populus species under low nitrogen (N) conditions, we used saplings of Populus × canescens supplied with either 500 (normal N) or 50 (low N) μM NH4NO3. The net fluxes of NH4+ and NO3-, the concentrations of NH4+, amino acids and organic acids and the enzymatic activities of nitrite reductase (NiR) and glutamine synthetase (GS) in root segment II (SII, 35-70 mm to the apex) were lower than those in root segment I (SI, 0-35 mm to the apex). The net NH4+ influxes and the concentrations of organic acids were elevated, whereas the concentrations of NH4+ and NO3- and the activities of NiR and GS were reduced in SI and SII in response to low N. A number of genes were significantly differentially expressed in SII vs SI and in both segments grown under low vs normal N conditions, and these genes were mainly involved in the transport of NH4+ and NO3-, N metabolism and adenosine triphosphate synthesis. Moreover, the hub gene coexpression networks were dissected and correlated with N physiological processes in SI and SII under normal and low N conditions. These results suggest that the hub gene coexpression networks play pivotal roles in regulating N uptake and assimilation, amino acid metabolism and the levels of organic acids from the tricarboxylic acid cycle in the two root segments of poplars in acclimation to low N availability.

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

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

Identification and Expression Profiling Analysis of the Cation/Ca2+ Exchanger (CCX) Gene Family: Overexpression of SlCCX1-LIKE Regulates the Leaf Senescence in Tomato Flowering Phase

Cation gradients in plant cellular compartments are maintained by the synergistic actions of various ion exchangers, pumps, and channels. Cation/Ca2+ exchanger (CCX) is one of the clades of the Ca2+/cation antiporter super family. Here, five SlCCX genes were identified in tomato. Sequence analysis indicated that SlCCXs have the conserved motifs as the CCX domain. Analysis of the expression level of each member of tomato CCX gene family under cation (Mg2+, Mn2+, Na+, and Ca2+) treatment was determined by qRT-PCR. Tomato CCX demonstrated different degrees of responding to cation treatment. Changes in SlCCX1-LIKE expression was induced by Mg2+ and Mn2+ treatment. Analysis of the expression of SlCCX genes in different tissues demonstrated that constitutive high expression of a few genes, including SlCCX1-LIKE and SlCCX5, indicated their role in tomato organ growth and development. Overexpression of SlCCX1-LIKE dramatically induced leaf senescence. Transcriptome analysis showed that genes related to ROS and several IAA signaling pathways were significantly downregulated, whereas ETH and ABA signaling pathway-related genes were upregulated in overexpression of SlCCX1-LIKE (OE-SlCCX1-LIKE) plants, compared with the wild type (WT). Moreover, overexpression of SlCCX1-LIKE plants accumulated more ROS content but less Mg2+ content. Collectively, the findings of this study provide insights into the base mechanism through which CCXs regulate leaf senescence in tomato.

Post-Translational Modifications of Nitrate Reductases Autoregulates Nitric Oxide Biosynthesis in Arabidopsis

Nitric oxide (NO) is a regulator of growth, development, and stress responses in living organisms. Plant nitrate reductases (NR) catalyze the reduction of nitrate to nitrite or, alternatively, to NO. In plants, NO action and its targets remain incompletely understood, and the way NO regulates its own homeostasis remains to be elucidated. A significant transcriptome overlapping between NO-deficient mutant and NO-treated wild type plants suggests that NO could negatively regulate its biosynthesis. A significant increase in NO content was detected in transgenic plants overexpressing NR1 and NR2 proteins. In turn, NR protein and activity as well as NO content, decreased in wild-type plants exposed to a pulse of NO gas. Tag-aided immunopurification procedures followed by tandem mass spectrometry allowed identifying NO-triggered post-translational modifications (PTMs) and ubiquitylation sites in NRs. Nitration of tyrosine residues and S-nitrosation of cysteine residues affected key amino acids involved in binding the essential FAD and molybdenum cofactors. NO-related PTMs were accompanied by ubiquitylation of lysine residues flanking the nitration and S-nitrosation sites. NO-induced PTMs of NRs potentially inhibit their activities and promote their proteasome-mediated degradation. This auto-regulatory feedback loop may control nitrate assimilation to ammonium and nitrite-derived production of NO under complex environmental conditions.

Nitrite Reductase 1 Is a Target of Nitric Oxide-Mediated Post-Translational Modifications and Controls Nitrogen Flux and Growth in Arabidopsis

Plant growth is the result of the coordinated photosynthesis-mediated assimilation of oxidized forms of C, N and S. Nitrate is the predominant N source in soils and its reductive assimilation requires the successive activities of soluble cytosolic NADH-nitrate reductases (NR) and plastid stroma ferredoxin-nitrite reductases (NiR) allowing the conversion of nitrate to nitrite and then to ammonium. However, nitrite, instead of being reduced to ammonium in plastids, can be reduced to nitric oxide (NO) in mitochondria, through a process that is relevant under hypoxic conditions, or in the cytoplasm, through a side-reaction catalyzed by NRs. We use a loss-of-function approach, based on CRISPR/Cas9-mediated genetic edition, and gain-of-function, using transgenic overexpressing HA-tagged Arabidopsis NiR1 to characterize the role of this enzyme in controlling plant growth, and to propose that the NO-related post-translational modifications, by S-nitrosylation of key C residues, might inactivate NiR1 under stress conditions. NiR1 seems to be a key target in regulating nitrogen assimilation and NO homeostasis, being relevant to the control of both plant growth and performance under stress conditions. Because most higher plants including crops have a single NiR, the modulation of its function might represent a relevant target for agrobiotechnological purposes.

Competing Endogenous RNA Networks Underlying Anatomical and Physiological Characteristics of Poplar Wood in Acclimation to Low Nitrogen Availability

Although poplar plantations are often established on nitrogen (N)-poor soil, the physiological and molecular mechanisms underlying wood properties of poplars in acclimation to low N availability remain largely unknown. To investigate wood properties of poplars in acclimation to low N, Populus � canescens saplings were exposed to either 50 (low N) or 500 (normal N) �M NH4NO3 for 2 months. Low N resulted in decreased xylem width and cell layers of the xylem (the number of cells counted along the ray parenchyma on the stem cross section), narrower lumina of vessels and fibers, greater thickness of double fiber walls (the walls between two adjacent fiber cells), more hemicellulose and lignin deposition, and reduced cellulose accumulation in poplar wood. Consistently, concentrations of gibberellins involved in cell size determination and the abundance of various metabolites including amino acids, carbohydrates and precursors for cell wall biosynthesis were decreased in low N-supplied wood. In line with these anatomical and physiological changes, a number of mRNAs, long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) were significantly differentially expressed. Competing endogenous RNA regulatory networks were identified in the wood of low N-treated poplars. Overall, these results indicate that miRNAs-lncRNAs-mRNAs networks are involved in regulating wood properties and physiological processes of poplars in acclimation to low N availability.

Elevated temperatures cause loss of seed set in common bean (Phaseolus vulgaris L.) potentially through the disruption of source-sink relationships

BACKGROUND

Climate change models predict more frequent incidents of heat stress worldwide. This trend will contribute to food insecurity, particularly for some of the most vulnerable regions, by limiting the productivity of crops. Despite its great importance, there is a limited understanding of the underlying mechanisms of variation in heat tolerance within plant species. Common bean, Phaseolus vulgaris, is relatively susceptible to heat stress, which is of concern given its critical role in global food security. Here, we evaluated three genotypes of P. vulgaris belonging to kidney market class under heat and control conditions. The Sacramento and NY-105 genotypes were previously reported to be heat tolerant, while Redhawk is heat susceptible.

RESULTS

We quantified several morpho-physiological traits for leaves and found that photosynthetic rate, stomatal conductance, and leaf area all increased under elevated temperatures. Leaf area expansion under heat stress was greatest for the most susceptible genotype, Redhawk. To understand gene regulatory responses among the genotypes, total RNA was extracted from the fourth trifoliate leaves for RNA-sequencing. Several genes involved in the protection of PSII (HSP21, ABA4, and LHCB4.3) exhibited increased expression under heat stress, indicating the importance of photoprotection of PSII. Furthermore, expression of the gene SUT2 was reduced in heat. SUT2 is involved in the phloem loading of sucrose and its distal translocation to sinks. We also detected an almost four-fold reduction in the concentration of free hexoses in heat-treated beans. This reduction was more drastic in the susceptible genotype.

CONCLUSIONS

Overall, our data suggests that while moderate heat stress does not negatively affect photosynthesis, it likely interrupts intricate source-sink relationships. These results collectively suggest a physiological mechanism for why pollen fertility and seed set are negatively impacted by elevated temperatures. Identifying the physiological and transcriptome dynamics of bean genotypes in response to heat stress will likely facilitate the development of varieties that can better tolerate a future of elevated temperatures.

Identification of transcription factors of nitrate reductase gene promoters and NRE2 cis-element through yeast one-hybrid screening in Nicotiana tabacum

BACKGROUND

This study aimed to identify the transcription factors of nitrate reductase genes (NIA1 and NIA2) promoters and hypothetical cis-element of NRE2. Based on the constructed cDNA library of Nicotiana tabacum K326, a yeast one-hybrid system was established using the Matchmaker® Gold Yeast One-Hybrid Library Screening System from Clontech. The transcription factors of NIA1 andNIA2 promoters and NRE2 cis-elements were screened.

RESULTS

After sequencing and bioinformatics analysis, 15 cDNA sequences were identified: 9 for NIA1 (including XP_016503563.1 and NP_001312236.1), 3 for NIA2 (including XP_016510250.1), and 3 for NRE2 (including XM_016576899.1). XP_016503563.1 was annotated in PREDICTED: CRM-domain containing factor CFM3, and NP_001312236.1chloroplastic/mitochondrial-like in Nicotiana tabacum. NP_001312236.1 was annotated in Sulfite oxidase-like of Nicotiana tabacum. XP_016510250.1 was annotated as PREDICTED: uncharacterized protein LOC107827596 in Nicotiana tabacum. XM_016576899.1 was annotated in PREDICTED: Nicotiana tabacum RING-H2 finger protein ATL16-like (LOC107759033).

CONCLUSION

A yeast one-hybrid library was successfully constructed. The identified transcription factors may provide a theoretical basis for the study of plant nitrate reductase.

Isoform-Specific NO Synthesis by Arabidopsis thaliana Nitrate Reductase

Nitrate reductase (NR) is important for higher land plants, as it catalyzes the rate-limiting step in the nitrate assimilation pathway, the two-electron reduction of nitrate to nitrite. Furthermore, it is considered to be a major enzymatic source of the important signaling molecule nitric oxide (NO), that is produced in a one-electron reduction of nitrite. Like many other plants, the model plant Arabidopsis thaliana expresses two isoforms of NR (NIA1 and NIA2). Up to now, only NIA2 has been the focus of detailed biochemical studies, while NIA1 awaits biochemical characterization. In this study, we have expressed and purified functional fragments of NIA1 and subjected them to various biochemical assays for comparison with the corresponding NIA2-fragments. We analyzed the kinetic parameters in multiple steady-state assays using nitrate or nitrite as substrate and measured either substrate consumption (nitrate or nitrite) or product formation (NO). Our results show that NIA1 is the more efficient nitrite reductase while NIA2 exhibits higher nitrate reductase activity, which supports the hypothesis that the isoforms have special functions in the plant. Furthermore, we successfully restored the physiological electron transfer pathway of NR using reduced nicotinamide adenine dinucleotide (NADH) and nitrate or nitrite as substrates by mixing the N-and C-terminal fragments of NR, thus, opening up new possibilities to study NR activity, regulation and structure.

Response to early drought stress and identification of QTLs controlling biomass production under drought in pearl millet

Pearl millet plays a major role in food security in arid and semi-arid areas of Africa and India. However, it lags behind the other cereal crops in terms of genetic improvement. The recent sequencing of its genome opens the way to the use of modern genomic tools for breeding. Our study aimed at identifying genetic components involved in early drought stress tolerance as a first step toward the development of improved pearl millet varieties or hybrids. A panel of 188 inbred lines from West Africa was phenotyped under early drought stress and well-irrigated conditions. We found a strong impact of drought stress on yield components. This impact was variable between inbred lines. We then performed an association analysis with a total of 392,493 SNPs identified using Genotyping-by-Sequencing (GBS). Correcting for genetic relatedness, genome wide association study identified QTLs for biomass production in early drought stress conditions and for stay-green trait. In particular, genes involved in the sirohaem and wax biosynthesis pathways were found to co-locate with two of these QTLs. Our results might contribute to breed pearl millet lines with improved yield under drought stress.

Paenibacillus pabuli strain P7S promotes plant growth and induces anthocyanin accumulation in Arabidopsis thaliana

In this study, a novel plant growth-promoting rhizobacteria (PGPR), the bacterial strain Paenibacillus pabuli P7S (PP7S), showed promising plant growth-promoting effects. Furthermore, it induced anthocyanin accumulation in Arabidopsis. When co-cultivated with PP7S, there was a significant increase in anthocyanin content and biomass of Arabidopsis seedlings compared with those of the control. The quantitative reverse transcription-polymerase chain reaction analysis revealed higher expression of many key genes regulating anthocyanin and flavonoid biosynthesis pathways in PP7S-treated seedlings when compared with that of the control. Furthermore, higher expression of pathogen-related genes and microbe-associated molecular pattern genes was also observed in response to PP7S, indicating that the PGPR triggered the induced systemic response (ISR) in A. thaliana. These results suggest that PP7S promotes plant growth in A. thaliana and increases anthocyanin biosynthesis by triggering specific ISRs in plant.

Cloning and transcription analysis of the nitrate reductase gene from Haematococcus pluvialis

OBJECTIVES

To optimize the cultivation media for the growth rate of Haematococcus pluvialis and to study the transcription regulation of the algal nitrate reductase (NR), a key enzyme for nitrogen metabolism.

RESULTS

The NR gene from H. pluvialis hd7 consists of 5636 nucleotides, including 14 introns. The cDNA ORF is 2718 bp, encoding a 905 aa protein with three conserved domains. The NR amino acids of H. pluvialis hd7 are hydrophilic and have similarity of 72% compared to that of Dunaliella. NR transcription increased with an increase of nitrate concentration from 0.4 to 1 g/l. A deficiency of nitrogen increased NR transcription significantly. The transcription level of NR increased at phosphorus concentrations from 0.08 to 0.2 g/l, with a maximum at 0.08 g/l. The optimum parameters of medium component for transcription of NR and growth of H. pluvialis were 0.3 g NaNO₃/l, 0.045 g KH₂PO₄/l and 1.08 g sodium acetate/l.

CONCLUSIONS

This study provides a better understanding of nitrate regulation in H. pluvialis.