Exogenous GABA promotes adaptation and growth by altering the carbon and nitrogen metabolic flux in poplar seedlings under low nitrogen conditions

Exogenous GABA supplementation to facilitate Cr (III) tolerance and lipid biosynthesis in Chlorella sorokiniana

Microalgae possess the prospective to be efficiently involved in bioremediation and biodiesel generation. However, conditions of stress often restrict their growth and diminish different metabolic processes. The current study evaluates the potential of GABA to improve the growth of the microalga Chlorella sorokiniana under Cr (III) stress through the exogenous administration of GABA. The research also investigates the concurrent impact of GABA and Cr (III) stress on various metabolic and biochemical pathways of the microalgae. In addition to the control, cultures treated with Cr (III), GABA, and both Cr (III) and GABA treated were assessed for accurately analysing the influence of GABA. The outcomes illustrated that GABA significantly promoted growth of the microalgae, resulting in higher biomass productivity (19.14 mg/L/day), lipid productivity (3.445 mg/L/day) and lipid content (18%) when compared with the cultures under Cr (III) treatment only. GABA also enhanced Chl a content (5.992 μg/ml) and percentage of protein (23.75%). FAMEs analysis by GC-MS and total lipid profile revealed that GABA treatment can boost the production of SFA and lower the level of PUFA, a distribution ideal for improving biodiesel quality. ICP-MS analysis revealed that GABA supplementation could extend Cr (III) mitigation level up to 97.7%, suggesting a potential strategy for bioremediation. This novel study demonstrates the merits of incorporating GABA in C. sorokiniana cultures under Cr (III) stress, in terms of its potential in bioremediation and biodiesel production without disrupting the pathways of photosynthesis and protein production.

Dynamic nitrogen fixation in an aerobic endophyte of Populus

Biological nitrogen fixation by microbial diazotrophs can contribute significantly to nitrogen availability in non-nodulating plant species. In this study of molecular mechanisms and gene expression relating to biological nitrogen fixation, the aerobic nitrogen-fixing endophyte Burkholderia vietnamiensis, strain WPB, isolated from Populus trichocarpa served as a model for endophyte-poplar interactions. Nitrogen-fixing activity was observed to be dynamic on nitrogen-free medium with a subset of colonies growing to form robust, raised globular like structures. Secondary ion mass spectrometry (NanoSIMS) confirmed that N-fixation was uneven within the population. A fluorescent transcriptional reporter (GFP) revealed that the nitrogenase subunit nifH is not uniformly expressed across genetically identical colonies of WPB and that only ~11% of the population was actively expressing the nifH gene. Higher nifH gene expression was observed in clustered cells through monitoring individual bacterial cells using single-molecule fluorescence in situ hybridization. Through 15N2 enrichment, we identified key nitrogenous metabolites and proteins synthesized by WPB and employed targeted metabolomics in active and inactive populations. We cocultivated WPB Pnif-GFP with poplar within a RhizoChip, a synthetic soil habitat, which enabled direct imaging of microbial nifH expression within root epidermal cells. We observed that nifH expression is localized to the root elongation zone where the strain forms a unique physical interaction with the root cells. This work employed comprehensive experimentation to identify novel mechanisms regulating both biological nitrogen fixation and beneficial plant-endophyte interactions.

Gamma-Aminobutyric Acid Accumulation Contributes to Citrus sinensis Response against 'Candidatus Liberibacter Asiaticus' via Modulation of Multiple Metabolic Pathways and Redox Status

Huanglongbing (HLB; also known as citrus greening) is the most destructive bacterial disease of citrus worldwide with no known sustainable cure yet. Herein, we used non-targeted metabolomics and transcriptomics to prove that γ-aminobutyric acid (GABA) accumulation might influence the homeostasis of several metabolic pathways, as well as antioxidant defense machinery, and their metabolism-related genes. Overall, 41 metabolites were detected in 'Valencia' sweet orange (Citrus sinensis) leaf extract including 19 proteinogenic amino acids (PAA), 10 organic acids, 5 fatty acids, and 9 other amines (four phenolic amines and three non-PAA). Exogenous GABA application increased most PAA in healthy (except L-threonine, L-glutamine, L-glutamic acid, and L-methionine) and 'Candidatus L. asiaticus'-infected citrus plants (with no exception). Moreover, GABA accumulation significantly induced L-tryptophan, L-phenylalanine, and α-linolenic acid, the main precursors of auxins, salicylic acid (SA), and jasmonic acid (JA), respectively. Furthermore, GABA supplementation upregulated most, if not all, of amino acids, phenolic amines, phytohormone metabolism-related, and GABA shunt-associated genes in both healthy and 'Ca. L. asiaticus'-infected leaves. Moreover, although 'Ca. L. asiaticus' induced the accumulation of H2O2 and O2•- and generated strong oxidative stress in infected leaves, GABA possibly stimulates the activation of a multilayered antioxidative system to neutralize the deleterious effect of reactive oxygen species (ROS) and maintain redox status within infected leaves. This complex system comprises two major components: (i) the enzymatic antioxidant defense machinery (six POXs, four SODs, and CAT) that serves as the front line in antioxidant defenses, and (ii) the non-enzymatic antioxidant defense machinery (phenolic acids and phenolic amines) that works as a second defense line against 'Ca. L. asiaticus'-induced ROS in citrus infected leaves. Collectively, our findings suggest that GABA might be a promising alternative eco-friendly strategy that helps citrus trees battle HLB particularly, and other diseases in general.

Resilient and sustainable production of peanut (Arachis hypogaea) in phosphorus-limited environment by using exogenous gamma-aminobutyric acid to sustain photosynthesis

Globally, many low to medium yielding peanut fields have the potential for further yield improvement. Low phosphorus (P) limitation is one of the significant factors curtailing Arachis hypogaea productivity in many regions. In order to demonstrate the effects of gamma-aminobutyric acid (GABA) on peanuts growing under P deficiency, we used a pot-based experiment to examine the effects of exogenous GABA on alleviating P deficiency-induced physiological changes and growth inhibition in peanuts. The key physiological parameters examined were foliar gas exchange, photochemical efficiency, proton motive force, reactive oxygen species (ROS), and adenosine triphosphate (ATP) synthase activity of peanuts under cultivation with low P (LP, 0.5 mM P) and control conditions. During low P, the cyclic electron flow (CEF) maintained the high proton gradient (∆pH) induced by low ATP synthetic activity. Applying GABA during low P conditions stimulated CEF and reduced the concomitant ROS generation and thereby protecting the foliar photosystem II (PSII) from photoinhibition. Specifically, GABA enhanced the rate of electronic transmission of PSII (ETRII) by pausing the photoprotection mechanisms including non-photochemical quenching (NPQ) and ∆pH regulation. Thus, GABA was shown to be effective in restoring peanut growth when encountering P deficiency. Exogenous GABA alleviated two symptoms (increased root-shoot ratio and photoinhibition) of P-deficient peanuts. This is possibly the first report of using exogenous GABA to restore photosynthesis and growth under low P availability. Therefore, foliar applications of GABA could be a simple, safe and effective approach to overcome low yield imposed by limited P resources (low P in soils or P-fertilizers are unavailable) for sustainable peanut cultivation and especially in low to medium yielding fields.

Role of Gamma-Aminobutyric Acid in Plant Defense Response

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that acts as a defense substance and a signaling molecule in various physiological processes, and which helps plants respond to biotic and abiotic stresses. This review focuses on the role of GABA's synthetic and metabolic pathways in regulating primary plant metabolism, redistributing carbon and nitrogen resources, reducing the accumulation of reactive oxygen species, and improving plants' tolerance of oxidative stress. This review also highlights the way in which GABA maintains intracellular pH homeostasis by acting as a buffer and activating H⁺-ATPase. In addition, calcium signals participate in the accumulation process of GABA under stress. Moreover, GABA also transmits calcium signals through receptors to trigger downstream signaling cascades. In conclusion, understanding the role of GABA in this defense response provides a theoretical basis for applying GABA in agriculture and forestry and feasible coping strategies for plants in complex and changeable environments.

Exogenous γ-aminobutyric acid (GABA) alleviates nitrogen deficiency by mediating nitrate uptake and assimilation in Andrographis paniculata seedlings

γ-Aminobutyric acid (GABA) plays significant metabolic and signaling roles in plant stress responses. Recent studies have proposed that GABA alleviates plant nitrogen (N) deficient stress; however, the mechanism by which GABA mediates plant N deficiency adaptation remains not yet well understood. Herein we found in a medicinal plant Andrographis paniculata that 5 mmol L^-1 exogenous GABA promoted plant growth under N deficient (1 mmol L^-1 NO₃^-) condition, with remarkably increments in total N and NO₃^- concentrations in plants. GABA increased N assimilation and protein synthesis by up-regulating the activities and expression of N metabolic enzymes. GABA also increased the accumulation of α-ketoglutarate and malate, which could facilitate the assimilation of NO₃^-. Inhibition of NR by Na₂WO₄ counteracted the promoting effects of GABA on plant growth, and the effects of GABA were not affected by L-DABA and 3-MP, the inhibitors of GABA transaminase (GABA-T) and glutamate decarboxylase (GAD), respectively. These results suggested that the nutritional role of GABA was excluded in promoting plant growth under low N condition. The results of ¹⁵N isotopic tracing and NRTs transcription indicated that exogenous GABA could up-regulate NRT2.4 and NRT3.2 to increase plant NO₃^- uptake under N deficient condition. Interestingly, primidone, an inhibitor of GABA receptor, impeded the effects of GABA on plant growth and N accumulation. Thus, our results revealed that exogenous GABA acted as a signal to up-regulate NRTs via its receptor to increase NO₃^- uptake, and subsequently promoted NO₃^- assimilation to alleviate N deficiency in A. paniculata.

Exogenous γ-aminobutyric acid (GABA) improves salt-inhibited nitrogen metabolism and the anaplerotic reaction of the tricarboxylic acid cycle by regulating GABA-shunt metabolism in maize seedlings

Salinity stress hampers the growth of most crop plants and reduces yield considerably. In addition to its role in metabolism, γ-aminobutyric acid (GABA) plays a special role in the regulation of salinity stress tolerance in plants, though the underlying physiological mechanism remains poorly understood. In order to study the physiological mechanism of GABA pathway regulated carbon and nitrogen metabolism and tis relationship with salt resistance of maize seedlings, we supplemented seedlings with exogenous GABA under salt stress. In this study, we showed that supplementation with 0.5 mmol·L^-1 (0.052 mg·g^-1) GABA alleviated salt toxicity in maize seedling leaves, ameliorated salt-induced oxidative stress, and increased antioxidant enzyme activity. Applying exogenous GABA maintained chloroplast structure and relieved chlorophyll degradation, thus improving the photosynthetic performance of the leaves. Due to the improvement in photosynthesis, sugar accumulation also increased. Endogenous GABA content and GABA transaminase (GABA-T) and succinate semialdehyde dehydrogenase (SSADH) activity were increased, while glutamate decarboxylase (GAD) activity was decreased, via the exogenous application of GABA under salt stress. Meanwhile, nitrogen metabolism and the tricarboxylic acid (TCA) cycle were activated by the supply of GABA. In general, through the regulation of GABA-shunt metabolism, GABA activated enzymes related to nitrogen metabolism and replenished the key substrates of the TCA cycle, thereby improving the balance of carbon and nitrogen metabolism of maize and improving salt tolerance.

Combined Effect of Salt Stress and Nitrogen Level on the Primary Metabolism of Two Contrasting Hydroponically Grown Cichorium spinosum L. Ecotypes

Stamnagathi (Cichorium spinosum L.) is an indigenous plant species well-known for its health-promoting properties. Salinity is a long-term issue with devastating consequences on land and farmers. Nitrogen (N) constitutes a crucial element for plant growth and development (chlorophyll, primary metabolites, etc.). Thus, it is of paramount importance to investigate the impact of salinity and N supply on plants’ metabolism. Within this context, a study was conducted aiming to assess the impact of salinity and N stress on the primary metabolism of two contrasting ecotypes of stamnagathi (montane and seaside). Both ecotypes were exposed to three different salinity levels (0.3 mM—non-saline treatment, 20 mM—medium, and 40 mM—high salinity level) combined with two different total-N supply levels: a low-N at 4 mM and a high-N at 16 mM, respectively. The differences between the two ecotypes revealed the variable responses of the plant under the applied treatments. Fluctuations were observed at the level of TCA cycle intermediates (fumarate, malate, and succinate) of the montane ecotype, while the seaside ecotype was not affected. In addition, the results showed that proline (Pro) levels increased in both ecotypes grown under a low N-supply and high salt stress, while other osmoprotectant metabolites such as γ-aminobutyric acid (GABA) exhibited variable responses under the different N supply levels. Fatty acids such as α-linolenate and linoleate also displayed variable fluctuations following plant treatments. The carbohydrate content of the plants, as indicated by the levels of glucose, fructose, α,α-trehalose, and myo-inositol, was significantly affected by the applied treatments. These findings suggest that the different adaptation mechanisms among the two contrasting ecotypes could be strongly correlated with the observed changes in their primary metabolism. This study also suggests that the seaside ecotype may have developed unique adaptation mechanisms to cope with high N supply and salinity stress, making it a promising candidate for future breeding programs aimed at developing stress tolerant varieties of C. spinosum L.

Exogenous γ-aminobutyric acid (GABA) mitigated salinity-induced impairments in mungbean plants by regulating their nitrogen metabolism and antioxidant potential

BACKGROUND

Increasing soil salinization has a detrimental effect on agricultural productivity.Therefore, strategies are needed to induce salinity-tolerance in crop species for sustainable foodproduction. γ-aminobutyric acid (GABA) plays a key role in regulating plant salinity stresstolerance. However, it remains largely unknown how mungbean plants (Vigna radiata L.) respondto exogenous GABA under salinity stress.

METHODS

Thus, we evaluated the effect of exogenous GABA (1.5 mM) on the growth and physiobiochemicalresponse mechanism of mungbean plants to saline stress (0-, 50-, and 100 mM [NaCland Na2SO4, at a 1:1 molar ratio]).

RESULTS

Increased saline stress adversely affected mungbean plants' growth and metabolism. Forinstance, leaf-stem-root biomass (34- and 56%, 31- and 53%, and 27- and 56% under 50- and 100mM, respectively]) and chlorophyll concentrations declined. The carotenoid level increased (10%)at 50 mM and remained unaffected at 100 mM. Hydrogen peroxide (H2O2), malondialdehyde(MDA), osmolytes (soluble sugars, soluble proteins, proline), total phenolic content, andenzymatic activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase(POD), glutathione reductase (GTR), and polyphenol oxidation (PPO) were significantlyincreased. In leaves, salinity caused a significant increase in Na+ concentration but a decrease inK+ concentration, resulting in a low K+/Na+ concentration (51- and 71% under 50- and 100- mMstress). Additionally, nitrogen concentration and the activities of nitrate reductase (NR) andglutamine synthetase (GS) decreased significantly. The reduction in glutamate synthase (GOGAT)activity was only significant (65%) at 100 mM stress. Exogenous GABA decreased Na+, H2O2,and MDA concentrations but enhanced photosynthetic pigments, K+ and K+/Na+ ratio, Nmetabolism, osmolytes, and enzymatic antioxidant activities, thus reducing salinity-associatedstress damages, resulting in improved growth and biomass.

CONCLUSION

Exogenous GABA may have improved the salinity tolerance of mungbean plants by maintaining their morpho-physiological responses and reducing the accumulation of harmfulsubstances under salinity. Future molecular studies can contribute to a better understanding of themolecular mechanisms by which GABA regulates mungbean salinity tolerance.

Recent advances of γ-aminobutyric acid: Physiological and immunity function, enrichment, and metabolic pathway

γ-aminobutyric acid (GABA) is a non-protein amino acid which naturally and widely occurs in animals, plants, and microorganisms. As the chief inhibitory neurotransmitter in the central nervous system of mammals, it has become a popular dietary supplement and has promising application in food industry. The current article reviews the most recent literature regarding the physiological functions, preparation methods, enrichment methods, metabolic pathways, and applications of GABA. This review sheds light on developing GABA-enriched plant varieties and food products, and provides insights for efficient production of GABA through synthetic biology approaches.

The Role of γ-Aminobutyric Acid (GABA) in the Occurrence of Adventitious Roots and Somatic Embryos in Woody Plants

The occurrence of adventitious roots and somatic embryos is a crucial step in micropropagation that frequently limits the application of this technique in woody plants. Recent studies demonstrated that they can be negatively or positively regulated with γ-aminobutyric acid (GABA), which is a four-carbon non-proteinous amino acid that not only acts as a main inhibitory neurotransmitter in mammals. It has been reported that GABA affects plant growth and their response to stress although its mode of action is still unclear. This review dealt with the effects of GABA on adventitious root formation and growth as well as on somatic embryogenesis. Furthermore, we focused on discussing the interaction of GABA with phytohormones, such as auxin, ethylene, abscisic acid, and gibberellin, as well as with the carbon and nitrogen metabolism during adventitious root development. We suggested that research on GABA will contribute to the application of micropropagation in the recalcitrant fruit and forest species.

Exogenous GABA improves the resistance of apple seedlings to long-term drought stress by enhancing GABA shunt and secondary cell wall biosynthesis

Drought stress is an important factor limiting apple production. γ-Aminobutyric acid (GABA) exists widely in plants and participates in the response to abiotic stress as a metabolite or signaling molecule. The role of exogenous GABA in apple plants, response to long-term drought stress remains unclear. Our study confirmed that exogenous GABA affects the drought resistance of apple plants under long-term drought stress. We found that 1 mM exogenous GABA improved the resistance of apple seedlings to long-term drought stress. The plants showed better growth, less reactive oxygen radical accumulation, less damage to cell membranes and greater active photosynthetic capacity. Under long-term drought stress, exogenous GABA facilitated GABA shunt, resulting in more accumulation of organic acids, namely citric acid, succinic acid and malic acid, in roots and stems of apple seedlings. In addition, exogenous GABA upregulated the expression of cellulose-related genes and lignin-related genes, and activated secondary cell wall-related transcription factors to synthesize more cellulose and lignin. A multiple factorial analysis confirmed that the GABA shunt and the biosynthesis of cellulose and lignin substantially contributed to the growth of apple seedlings with the application of exogenous GABA under long-term drought stress. Our results suggested that exogenous GABA improved the resistance of apple seedlings to long-term drought stress by enhancing GABA shunt and secondary cell wall biosynthesis.

Physiological and proteomic analyses of γ-aminobutyric acid (GABA)-treated tubers reveals that StPOD42 promotes sprouting in potato

Gamma-aminobutyric acid (GABA) is a nonproteinogenic amino acid that plays vital roles in plant growth and developmental processes. However, its role in regulating potato sprouting is unknown. Therefore, the physiological and molecular mechanisms underlying the sprouting process were assessed, and we found that GABA promoted sprouting after treatment for 50 d. In addition, the GABA and soluble sugar contents increased while the starch content decreased. To study the molecular mechanism by which exogenous GABA accelerates tuber sprouting, comparative proteomic analysis of tuber bud eyes was performed after GABA treatment for 48 h. Further analysis revealed 316 differentially abundant proteins (DAPs) that are mainly involved in fatty acid and sugar metabolism and cutin, suberin and wax biosyntheses. The qRT‒PCR results suggested that the GABA transaminase 2 (GABA-T2) and GABA-T3 expression levels showed the greatest decrease at 30 d of storage. Peroxidase 42 (StPOD42) expression showed the greatest increase at 30 d. Overexpression of StPOD42 in potato was found to promote tuber sprouting. Our results provide new insights into the role of GABA in regulating the sprouting process and indicate that StPOD42 is a target gene for molecular breeding to modulate potato sprouting.

Effect of Nickel Ions on the Physiological and Transcriptional Responses to Carbon and Nitrogen Metabolism in Tomato Roots under Low Nitrogen Levels

Nickel (Ni) is an essential trace element for plant growth and a component of the plant body that has many different functions in plants. Although it has been confirmed that nickel ions (Ni²⁺) havea certain regulatory effect on nitrogen (N) metabolism, there are not enough data to prove whether exogenous Ni²⁺ can increase the carbon (C) and N metabolism in the roots of tomato seedlingsunder low-nitrogen (LN) conditions. Therefore, through the present experiment, we revealed the key mechanism of Ni²⁺-mediated tomato root tolerance to LN levels. Tomato plants were cultured at two different N levels (7.66 and 0.383 mmol L⁻¹) and two different Ni²⁺ levels (0 and 0.1 mg L⁻¹ NiSO₄ 6H₂O) under hydroponic conditions. After nine days, we collected roots for physiological, biochemical, and transcriptome sequencing analyses and found that the activities of N assimilation-related enzymes decreased at LN levels. In contrast, Ni²⁺ significantly increased the activities of N assimilation-related enzymes and increased the contents of nitrate (NO₃⁻), ammonium (NH₄⁺), and total amino acids. Through root transcriptomic analysis, 3738 differentially expressed genes (DEGs) were identified. DEGs related to C and N metabolism were downregulated after LN application. However, after Ni²⁺ treatment, PK, PDHB, GAPDH, NR, NiR, GS, GOGAT, and other DEGs related to C and N metabolism were significantly upregulated. In conclusion, our results suggest that Ni²⁺ can regulate the C and N metabolism pathways in tomato roots to alleviate the impact of LN levels.

Acclimation Strategy of Masson Pine (Pinus massoniana) by Limiting Flavonoid and Terpenoid Production under Low Light and Drought

Low light and drought often limit the growth and performance of Masson pines (Pinus massoniana) in the subtropical forest ecosystem of China. We speculated that stress-induced defensive secondary metabolites, such as flavonoids and terpenoids, might influence the growth of Masson pines, considering the existence of tradeoffs between growth and defense. However, the mechanisms of Masson pines responsive to low light and drought at the levels of these two metabolites remain unclear. In the present work, the compositions of flavonoids and terpenoids, as well as their biosynthetic pathways, were revealed through metabolome and transcriptome analyses, respectively, coupled with a study on carbon allocation using a ¹³CO₂-pulse-labeling experiment in two-year-old seedlings under low light (LL), drought (DR), and their combined stress (DL) compared to a control (CK). A total of 35 flavonoids and derivatives (LL vs. CK: 18; DR vs. CK: 20; and DL vs. CK: 18), as well as 29 terpenoids and derivatives (LL vs. CK: 23; DR vs. CK: 13; and DL vs. CK: 7), were differentially identified in the leaves. Surprisingly, most of them were decreased under all three stress regimes. At the transcriptomic level, most or all of the detected DEGs (differentially expressed genes) involved in the biosynthetic pathways of flavonoids and terpenoids were downregulated in phloem and xylem under stress treatments. This indicated that stress treatments limited the production of flavonoids and terpenoids. The reduction in the ¹³C allocation to stems might suggest that it is necessary for maintaining the growth of Masson pine seedlings at the whole-plant level by attenuating energetic resources to the biosynthetic pathways of flavonoids and terpenoids when facing the occurrence of adverse environments. Our results provide new insight into understanding the acclimation strategy of Masson pines or other conifers in adverse environments.

Adaptive responses of carbon and nitrogen metabolisms to nitrogen-deficiency in Citrus sinensis seedlings

BACKGROUND

In China, nitrogen (N)-deficiency often occurs in Citrus orchards, which is one of the main causes of yield loss and fruit quality decline. Little information is known about the adaptive responses of Citrus carbon (C) and N metabolisms to N-deficiency. Seedlings of 'Xuegan' (Citrus sinensis (L.) Osbeck) were supplied with nutrient solution at an N concentration of 0 (N-deficiency), 5, 10, 15 or 20 mM for 10 weeks. Thereafter, we examined the effects of N supply on the levels of C and N in roots, stems and leaves, and the levels of organic acids, nonstructural carbohydrates, NH₄⁺-N, NO₃^--N, total soluble proteins, free amino acids (FAAs) and derivatives (FAADs), and the activities of key enzymes related to N assimilation and organic acid metabolism in roots and leaves.

RESULTS

N-deficiency elevated sucrose export from leaves to roots, C and N distributions in roots and C/N ratio in roots, stems and leaves, thus enhancing root dry weight/shoot dry weight ratio and N use efficiency. N-deficient leaves displayed decreased accumulation of starch and total nonstructural carbohydrates (TNC) and increased sucrose/starch ratio as well as a partitioning trend of assimilated C toward to sucrose, but N-deficient roots displayed elevated accumulation of starch and TNC and reduced sucrose/starch ratio as well as a partitioning trend of assimilated C toward to starch. N-deficiency reduced the concentrations of most FAADs and the ratios of total FAADs (TFAADs)/N in leaves and roots. N-deficiency reduced the demand for C skeleton precursors for amino acid biosynthesis, thus lowering TFAADs/C ratio in leaves and roots. N-deficiency increased (decreased) the relative amounts of C-rich (N-rich) FAADs, thus increasing the molar ratio of C/N in TFAADs in leaves and roots.

CONCLUSIONS

Our findings corroborated our hypothesis that C and N metabolisms displayed adaptive responses to N-deficiency in C. sinensis seedlings, and that some differences existed between roots and leaves in N-deficiency-induced alterations of and C and N metabolisms.

Effects of citric acid on antioxidant system and carbon-nitrogen metabolism of Elymus dahuricus under Cd stress

Exogenous citric acid (CA), which acts as an important intermediate product of the tricarboxylic acid (TCA) cycle, can enhance the TCA cycle activity and activate the branched operation of the TCA cycle, thus providing energy required for resistance to adverse conditions. However, the effects of CA application on TCA cycle-related metabolism under cadmium (Cd) were less reported. To investigate the effects of CA on the Cd tolerance of Dahurian wildrye grass (Elymus dahuricus), the growth, Cd accumulation, antioxidant systems and metabolic pathways of leaves and roots were investigated by a potted soil experiment with Cd (50 mg/kg) and CA (4 mmol/L) treatments. The results showed that Cd stress seriously affected growth and induced the production of reactive oxygen in clover leaves and roots, leading to membrane peroxidation and activation of the antioxidant defense system. Exogenous CA could not only effectively relieve the inhibition of Cd stress on growth and reduce the amount of reactive oxygen by increasing the antioxidant capacities but could also promote an increase in root Cd content. Metabolomic results showed that the application of CA increased the contents of sugars, sugar alcohols, and resistant substances, and promoted the metabolism of amino acids including γ-aminobutyric acid (GABA). These alterations contributed the significant enhancement of the Cd resistance, which may be related to the changes in the TCA cycle activity and the metabolism of the shikimic acid pathway in leaves and roots as well as GABA shunt in roots.

Genotypic Variation of Nitrogen Use Efficiency and Amino Acid Metabolism in Barley

Owing to the large genetic diversity of barley and its resilience under harsh environments, this crop is of great value for agroecological transition and the need for reduction of nitrogen (N) fertilizers inputs. In the present work, we investigated the diversity of a North African barley genotype collection in terms of growth under limiting N (LN) or ample N (HN) supply and in terms of physiological traits including amino acid content in young seedlings. We identified a Moroccan variety, Laanaceur, accumulating five times more lysine in its leaves than the others under both N nutritional regimes. Physiological characterization of the barley collection showed the genetic diversity of barley adaptation strategies to LN and highlighted a genotype x environment interaction. In all genotypes, N limitation resulted in global biomass reduction, an increase in C concentration, and a higher resource allocation to the roots, indicating that this organ undergoes important adaptive metabolic activity. The most important diversity concerned leaf nitrogen use efficiency (LNUE), root nitrogen use efficiency (RNUE), root nitrogen uptake efficiency (RNUpE), and leaf nitrogen uptake efficiency (LNUpE). Using LNUE as a target trait reflecting barley capacity to deal with N limitation, this trait was positively correlated with plant nitrogen uptake efficiency (PNUpE) and RNUpE. Based on the LNUE trait, we determined three classes showing high, moderate, or low tolerance to N limitation. The transcriptomic approach showed that signaling, ionic transport, immunity, and stress response were the major functions affected by N supply. A candidate gene encoding the HvNRT2.10 transporter was commonly up-regulated under LN in the three barley genotypes investigated. Genes encoding key enzymes required for lysine biosynthesis in plants, dihydrodipicolinate synthase (DHPS) and the catabolic enzyme, the bifunctional Lys-ketoglutarate reductase/saccharopine dehydrogenase are up-regulated in Laanaceur and likely account for a hyperaccumulation of lysine in this genotype. Our work provides key physiological markers of North African barley response to low N availability in the early developmental stages.

The versatile GABA in plants

Gamma-aminobutyric acid (GABA) is a ubiquitous four-carbon, non-protein amino acid. GABA has been widely studied in animal central nervous systems, where it acts as an inhibitory neurotransmitter. In plants, it is metabolized through the GABA shunt pathway, a bypass of the tricarboxylic acid (TCA) cycle. Additionally, it can be synthesized through the polyamine metabolic pathway. GABA acts as a signal in Agrobacterium tumefaciens-mediated plant gene transformation and in plant development, especially in pollen tube elongation (to enter the ovule), root growth, fruit ripening, and seed germination. It is accumulated during plant responses to environmental stresses and pathogen and insect attacks. A high concentration of GABA elevates plant stress tolerance by improving photosynthesis, inhibiting reactive oxygen species (ROS) generation, activating antioxidant enzymes, and regulating stomatal opening in drought stress. The transporters of GABA in plants are reviewed in this work. We summarize the recent research on GABA function and transporters with the goal of providing a review of GABA in plants.

Differential Metabolomic Responses of Kentucky Bluegrass Cultivars to Low Nitrogen Stress

Kentucky bluegrass (Poa pratensis L.) is a cool-season turfgrass species that responds strongly to nitrogen (N), but the metabolomic responses of this grass species to N supply is unknown. The N-tolerant cultivar Bluemoon and N-sensitive cultivar Balin were exposed to normal N (15 mM) and low N (0.5 mM) for 21 days for identification of differentially expressed metabolites (DEMs) between normal N and low N treatments. Balin had more reductions of chlorophyll and total soluble protein concentrations and a higher accumulation of superoxide radicals under low N stress. A total of 99 known DEMs were identified in either cultivar or both including 22 amino acids and derivatives, 16 carbohydrates, 29 organic acids, and 32 other metabolites. In Bluemoon, β-alanine metabolism was most enriched, followed by alanine, aspartate, and glutamate metabolism, biosynthesis of valine, leucine, and isoleucine biosynthesis, and glycine, serine, and threonine metabolism. In Balin, alanine, aspartate, and glutamate metabolism were most enriched, followed by the tricarboxylic acid (TCA), glyoxylate and decarbohydrate metabolism, and carbon fixation. Bluemoon generally maintained higher TCA cycle capacity and had more downregulated amino acids, while changes in more organic acids occurred in Balin under low N stress. Some metabolite changes by low-N stress were cultivar-specific. The results suggested that regulation of metabolites related to energy production or energy saving could contribute to low N tolerance in Kentucky bluegrass.

GABA reverses salt-inhibited photosynthetic and growth responses through its influence on NO-mediated nitrogen-sulfur assimilation and antioxidant system in wheat

Gamma-aminobutyric acid (GABA) is a newly recognized signaling molecule participating in physiological processes, growth, and development of plants under optimal and stressful environments. In the present reported research, we investigated the role of GABA in imparting salt stress tolerance in wheat (Triticum aestivum L.). Exposure of wheat plants to 100 mM NaCl resulted in increased oxidative stress, glucose content, nitric oxide (NO) production together with reduced growth and photosynthetic traits of plants. Contrarily, GABA application improved nitrogen (N) metabolism, sulfur (S) assimilation, ion homeostasis, growth and photosynthesis under salt stress. Additionally, GABA mitigated oxidative stress induced by salt stress with the increased ascorbate-glutathione cycle and proline metabolism. The study with NO inhibitor, c-PTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy-3-oxide] in GABA experiment suggested that the impact of GABA on improvement of growth and photosynthesis under salt stress was mediated by NO and influenced N and S assimilation and antioxidant systems. The results suggested that the GABA has a significant potential in reversing the salt stress response in wheat plants, and GABA-mediated signals are manifested through NO.