Towards identification of molecular mechanism in which the overexpression of wheat cytosolic and plastid glutamine synthetases in tobacco enhanced drought tolerance

Integrated physiological, transcriptomics and metabolomics analysis revealed the molecular mechanism of Bupleurum chinense seedlings to drought stress

Drought stress is a prominent abiotic factor that adversely influences the growth and development of Bupleurum chinense during its seedling stage, negatively impacting biomass and secondary metabolite production, thus affecting yield and quality. To investigate the molecular mechanism underlying the response of B. chinense seedlings under drought stress, this study employed comprehensive physiological, transcriptomic, and metabolomic analyses. The results revealed that under drought stress, the root soluble sugar and free proline content in B. chinense seedlings significantly increased, while the activities of SOD, POD, and CAT increased in the leaves. These findings indicate the presence of distinct response mechanisms in B. chinense to cope with drought stress. Integrated analysis further identified significant correlations between genes and metabolites related to amino acid biosynthesis in the leaves, as well as genes and metabolites associated with acetaldehyde and dicarboxylic acid metabolism. In the roots, genes and metabolites related to plant hormone signaling and the tricarboxylic acid (TCA) cycle showed significant correlations. These findings provide vital views into the molecular-level response mechanisms of B. chinense under drought stress. Moreover, this study establishes the groundwork for identifying drought-tolerant genes and breeding drought-resistant varieties, which could improve the drought tolerance of medicinal plants and have broader implications for agriculture and crop production in water-scarce areas.

Proteomic analysis of near-isogenic lines reveals key biomarkers on wheat chromosome 4B conferring drought tolerance

Drought is a major constraint for wheat production that is receiving increased attention due to global climate change. This study conducted isobaric tags for relative and absolute quantitation proteomic analysis on near-isogenic lines to shed light on the underlying mechanism of qDSI.4B.1 quantitative trait loci (QTL) on the short arm of chromosome 4B conferring drought tolerance in wheat. Comparing tolerant with susceptible isolines, 41 differentially expressed proteins were identified to be responsible for drought tolerance with a p-value of < 0.05 and fold change >1.3 or <0.7. These proteins were mainly enriched in hydrogen peroxide metabolic activity, reactive oxygen species metabolic activity, photosynthetic activity, intracellular protein transport, cellular macromolecule localization, and response to oxidative stress. Prediction of protein interactions and pathways analysis revealed the interaction between transcription, translation, protein export, photosynthesis, and carbohydrate metabolism as the most important pathways responsible for drought tolerance. The five proteins, including 30S ribosomal protein S15, SRP54 domain-containing protein, auxin-repressed protein, serine hydroxymethyltransferase, and an uncharacterized protein with encoding genes on 4BS, were suggested as candidate proteins responsible for drought tolerance in qDSI.4B.1 QTL. The gene coding SRP54 protein was also one of the differentially expressed genes in our previous transcriptomic study.

Effects of Biological Nitrogen Metabolism on Glufosinate-Susceptible and -Resistant Goosegrass (Eleusine indica L.)

Glufosinate is a broad-spectrum herbicide used to control most weeds in agriculture worldwide. Goosegrass (Eleusine indica L.) is one of the top ten malignant weeds across the world, showing high tolerance to glufosinate via different mechanisms that are not yet fully understood. This study revealed that nitrogen metabolism could be a target-resistant site, providing clues to finally clarify the mechanism of glufosinate resistance in resistant goosegrass populations. Compared to susceptible goosegrass (NX), the resistant goosegrass (AUS and CS) regarding the stress of glufosinate showed stronger resistance with lower ammonia contents, higher target enzyme GS (glutamine synthetase) activity, and lower GOGAT (glutamine 2-oxoglutarate aminotransferase) activity. The GDH (glutamate dehydrogenase) activity of another pathway increased, but its gene expression was downregulated in resistant goosegrass (AUS). Analyzing the transcriptome and proteome data of goosegrass under glufosinate stress at 36 h showed that the KEGG pathway of the nitrogen metabolism was enriched in glufosinate-susceptible goosegrass (NX), but not in glufosinate-resistant goosegrass (CS and AUS). Several putative target genes involved in glufosinate stress countermeasures were identified. This study provides specific insights into the nitrogen metabolism of resistant goosegrass, and gives a basis for future functional verification of glufosinate-tolerance genes in plants.

Phytosulfokine peptide optimizes plant growth and defense via glutamine synthetase GS2 phosphorylation in tomato

Phytosulfokine (PSK) is a plant pentapeptide hormone that fulfills a wide range of functions. Although PSK has frequently been reported to function in the inverse regulation of growth and defense in response to (hemi)biotrophic pathogens, the mechanisms involved remain largely unknown. Using the tomato (Solanum lycopersicum) and Pseudomonas syringae pv. tomato (Pst) DC3000 pathogen system, we present compelling evidence that the PSK receptor PSKR1 interacts with the calcium-dependent protein kinase CPK28, which in turn phosphorylates the key enzyme of nitrogen assimilation glutamine synthetase GS2 at two sites (Serine-334 and Serine-360). GS2 phosphorylation at S334 specifically regulates plant defense, whereas S360 regulates growth, uncoupling the PSK-induced effects on defense responses and growth regulation. The discovery of these sites will inform breeding strategies designed to optimize the growth-defense balance in a compatible manner.

Targeted and untargeted metabolomics reveals deep analysis of drought stress responses in needles and roots of Pinus taeda seedlings

Drought stress is one of major environmental stresses affecting plant growth and yield. Although Pinus taeda trees are planted in rainy southern China, local drought sometime occurs and can last several months, further affecting their growth and resin production. In this study, P. taeda seedlings were treated with long-term drought (42 d), and then targeted and untargeted metabolomics analysis were carried out to evaluate drought tolerance of P. taeda. Targeted metabolomics analysis showed that levels of some sugars, phytohormones, and amino acids significantly increased in the roots and needles of water-stressed (WS) P. taeda seedlings, compared with well-watered (WW) pine seedlings. These metabolites included sucrose in pine roots, the phytohormones abscisic acid and sacylic acid in pine needles, the phytohormone gibberellin (GA4) and the two amino acids, glycine and asparagine, in WS pine roots. Compared with WW pine seedlings, the neurotransmitter acetylcholine significantly increased in needles of WS pine seedlings, but significantly reduced in their roots. The neurotransmitters L-glutamine and hydroxytyramine significantly increased in roots and needles of WS pine seedlings, respectively, compared with WW pine seedlings, but the neurotransmitter noradrenaline significantly reduced in needles of WS pine seedlings. Levels of some unsaturated fatty acids significantly reduced in roots or needles of WS pine seedlings, compared with WW pine seedlings, such as linoleic acid, oleic acid, myristelaidic acid, myristoleic acid in WS pine roots, and palmitelaidic acid, erucic acid, and alpha-linolenic acid in WS pine needles. However, three saturated fatty acids significantly increased in WS pine seedlings, i.e., dodecanoic acid in WS pine needles, tricosanoic acid and heptadecanoic acid in WS pine roots. Untargeted metabolomics analysis showed that levels of some metabolites increased in WS pine seedlings, especially sugars, long-chain lipids, flavonoids, and terpenoids. A few of specific metabolites increased greatly, such as androsin, piceatanol, and panaxatriol in roots and needles of WS pine seedlings. Comparing with WW pine seedlings, it was found that the most enriched pathways in WS pine needles included flavone and flavonol biosynthesis, ABC transporters, diterpenoid biosynthesis, plant hormone signal transduction, and flavonoid biosynthesis; in WS pine roots, the most enriched pathways included tryptophan metabolism, caffeine metabolism, sesquiterpenoid and triterpenoid biosynthesis, plant hormone signal transduction, biosynthesis of phenylalanine, tyrosine, and tryptophan. Under long-term drought stress, P. taeda seedlings showed their own metabolomics characteristics, and some new metabolites and biosynthesis pathways were found, providing a guideline for breeding drought-tolerant cultivars of P. taeda.

Nano fertilizer synergist effects on nitrogen utilization and related gene expression in wheat

The application of nano materials is one of the current hot spots in agricultural production. The aim of this work was to evaluate the effects of different nano fertilizer synergists on nitrogen (N) utilization and related gene expression in wheat. The experiments were carried out in pot and field conditions at the West-Coast Economic New Area experimental base and Greenhouse of Qingdao Agricultural University. Seven treatments were set up: CK (compound fertilizer), T1 (compound fertilizer + 0.3% nano carbon synergist), T2 (compound fertilizer + 0.3% nano calcium carbonate synergist), T3 (compound fertilizer + 0.3% composite nano synergist), T4 (70% compound fertilizer + 0.3% nano carbon synergist), T5 (70% compound fertilizer + 0.3% nano calcium carbonate synergist), T6 (70% compound fertilizer + 0.3% composite nano synergist). The results showed that compared with CK, the N accumulation of T1, T2, T3, T4, T5 and T6 increased by 40-50%, 30-40%, 55-65%, 20-30%, 15-20% and 30-40%, respectively; and the N use efficiency increased by 12-19%, 9-18%, 16-22%, 5-17%, 4-16% and 10-20% respectively. And the gene expression levels of TaNRT2.2, TaNRT2.3, TaGS1 and TaGS2 in the treatments with synergistic phosphate fertilizer were significantly higher than those in the CK. The application of nano fertilizer synergist can significantly improve N accumulation, N use efficiency, and promote the expression of genes related to N transport and metabolism.

Identification of the glutamine synthetase (GS) gene family in four wheat species and functional analysis of Ta4D.GSe in Arabidopsis thaliana

Drought stress can negatively impact crop yield and quality. Improving wheat yields under drought stress is a major objective of agronomic research. Glutamine synthetase (GS) is a key enzyme of nitrogen metabolism that is critical to plant growth and development in abiotic stress response. However, to date, no systemic characterization of the GS genes has yet been conducted in wheat and its close relatives. We identified a total of 15 GS genes in Triticum aestivum (2n = 6x = 42; AABBDD), as well as 9 GS genes in Triticum dicoccoides (2n = 4x = 28; AABB), 6 in Aegilops tauschii (2n = 2x = 14; DD), and 5 in Triticum urartu (2n = 2x = 14; AA). The 35 GSs were further clustered into five lineages according to the phylogenetic tree. Synteny analysis revealed that the three subgenomes in bread wheat retained extensive synteny between bread wheat and its three relative species. We identified three up-regulated TaGSs (Ta4A.GSe, Ta4B.GSe, and Ta4D.GSe) from transcriptome data after drought and salt stress. Ta4D.GSe was subsequently used for further functional studies, and its subcellular localization were determined in Arabidopsis protoplasts. Its overexpression in Arabidopsis enhanced drought tolerance by increasing the ability of scavenging of reactive oxygen species (ROS) and osmotic adjustment. We identified GS gene family in four wheat species and performed comparative analyses of their relationships, chromosome locations, conserved motif, gene structure, and synteny. The subcellular localization of Ta4D.GSe was detected and its drought tolerance function was demonstrated. Taken together, these findings provide insight into the potential functional roles of the GS genes in abiotic stress tolerance. KEY MESSAGE: This report clearly shows detailed characterization of GS gene family in four wheat species and demonstrates that Ta4D.GSe plays an important role in enhancing drought tolerance by improving the scavenging of ROS and osmotic adjustment ability in Arabidopsis.

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

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

Genetic Control of Efficient Nitrogen Use for High Yield and Grain Protein Concentration in Wheat: A Review

The increasing global population and the negative effects of nitrogen (N) fertilizers on the environment challenge wheat breeding to maximize yield potential and grain protein concentration (GPC) in an economically and environmentally friendly manner. Understanding the molecular mechanisms for the response of yield components to N availability and assimilates allocation to grains provides the opportunity to increase wheat yield and GPC simultaneously. This review summarized quantitative trait loci/genes which can increase spikes and grain number by enhancing N uptake and assimilation at relative early growth stage, and 1000-grain weight and GPC by increasing post-anthesis N uptake and N allocation to grains.

Recognizing the hidden half in wheat: root system attributes associated with drought tolerance

Improving drought tolerance in wheat is crucial for maintaining productivity and food security. Roots are responsible for the uptake of water from soil, and a number of root traits are associated with drought tolerance. Studies have revealed many quantitative trait loci and genes controlling root development in plants. However, the genetic dissection of root traits in response to drought in wheat is still unclear. Here, we review crop root traits associated with drought, key genes governing root development in plants, and quantitative trait loci and genes regulating root system architecture under water-limited conditions in wheat. Deep roots, optimal root length density and xylem diameter, and increased root surface area are traits contributing to drought tolerance. In view of the diverse environments in which wheat is grown, the balance among root and shoot traits, as well as individual and population performance, are discussed. The known functions of key genes provide information for the genetic dissection of root development of wheat in a wide range of conditions, and will be beneficial for molecular marker development, marker-assisted selection, and genetic improvement in breeding for drought tolerance.

A 1232 bp upstream sequence of glutamine synthetase 1b from Eichhornia crassipes is a root-preferential promoter sequence

BACKGROUND

Glutamine synthetase (GS) acts as a key enzyme in plant nitrogen (N) metabolism. It is important to understand the regulation of GS expression in plant. Promoters can initiate the transcription of its downstream gene. Eichhornia crassipes is a most prominent aquatic invasive plant, which has negative effects on environment and economic development. It also can be used in the bioremediation of pollutants present in water and the production of feeding and energy fuel. So identification and characterization of GS promoter in E. crassipes can help to elucidate its regulation mechanism of GS expression and further to control its N metabolism.

RESULTS

A 1232 bp genomic fragment upstream of EcGS1b sequence from E. crassipes (EcGS1b-P) has been cloned, analyzed and functionally characterized. TSSP-TCM software and PlantCARE analysis showed a TATA-box core element, a CAAT-box, root specific expression element, light regulation elements including chs-CMA1a, Box I, and Sp1 and other cis-acting elements in the sequence. Three 5'-deletion fragments of EcGS1b upstream sequence with 400 bp, 600 bp and 900 bp length and the 1232 bp fragment were used to drive the expression of β-glucuronidase (GUS) in tobacco. The quantitative test revealed that GUS activity decreased with the decreasing of the promoter length, which indicated that there were no negative regulated elements in the EcGS1-P. The GUS expressions of EcGS1b-P in roots were significantly higher than those in leaves and stems, indicating EcGS1b-P to be a root-preferential promoter. Real-time Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) analysis of EcGS1b gene also showed higher expression in the roots of E.crassipes than in stems and leaves.

CONCLUSIONS

EcGS1b-P is a root-preferential promoter sequence. It can specifically drive the transcription of its downstream gene in root. This study will help to elucidate the regulatory mechanisms of EcGS1b tissue-specific expression and further study its other regulatory mechanisms in order to utilize E.crassipes in remediation of eutrophic water and control its overgrowth from the point of nutrient metabolism.

Manipulating Amino Acid Metabolism to Improve Crop Nitrogen Use Efficiency for a Sustainable Agriculture

In a context of a growing worldwide food demand coupled to the need to develop a sustainable agriculture, it is crucial to improve crop nitrogen use efficiency (NUE) while reducing field N inputs. Classical genetic approaches based on natural allelic variations existing within crops have led to the discovery of quantitative trait loci controlling NUE under low nitrogen conditions; however, the identification of candidate genes from mapping studies is still challenging. Amino acid metabolism is the cornerstone of plant N management, which involves N uptake, assimilation, and remobilization efficiencies, and it is finely regulated during acclimation to low N conditions and other abiotic stresses. Over the last two decades, biotechnological engineering of amino acid metabolism has led to promising results for the improvement of crop NUE, and more recently under low N conditions. This review summarizes current work carried out in crops and provides perspectives on the identification of new candidate genes and future strategies for crop improvement.