Highway or byway: the metabolic role of the GABA shunt in plants

A novel NADP+-isocitrate dehydrogenase contributes to cadmium/lead detoxification and tolerance in plants

Excessive heavy metal pollutants in soil seriously damage ecological systems and the environment. Dianthus spiculifolius shows strong tolerance to Cd/Pb and readily accumulates both metals. Isocitrate dehydrogenase (IDH) is key enzyme in the tricarboxylic acid cycle, which is involved in the plant response to a variety of abiotic stresses. Previous transcriptomic analyses suggested that DsIDH in D. spiculifolius plays a role in Cd/Pb detoxification. In this study, we found that the transcript level of DsIDH was significantly increased under Cd/Pb stress. Transiently expressed DsIDH localized at the chloroplasts in tobacco leaves. Transgenic yeast lines overexpressing DsIDH showed increased tolerance to Cd and Pb and decreased accumulation of Cd and Pb. Compared with their respective wild types, transgenic Arabidopsis and D. spiculifolius overexpressing DsIDH showed increased IDH activity, increased tolerance to Cd/Pb, and decreased heavy metal contents. The increased activity of IDH significantly accelerated the decomposition of isocitrate and increased the production of α-ketoglutaric acid and NADPH, which reduced damage caused by the reactive oxygen species produced in response to Cd and Pb stresses. The DsIDH might be a novel tolerance-related candidate gene useful for decreasing the storage of toxic heavy metals in crops.

Amino Acid Metabolism and Transporters in Plant-Pathogen Interactions: Mechanisms and Implications

In the intricate landscape of plant-pathogen interactions, amino acids and their dedicated transporters emerge as pivotal players underpinning immune signalling and metabolic reprogramming. Amino acid metabolism serves as a linchpin in orchestrating systemic defence responses, with transporter-mediated amino acid homoeostasis intricately intertwined with immune pathways. This review synthesizes the dual roles of amino acids, including glutamate, proline, γ-aminobutyric acid, β-aminobutyric acid and pipecolic acid, as metabolic intermediates and signalling molecules that modulate defence responses. Complementing this metabolic framework, amino acid transporters, including LHT1 and members of the AAP and UMAMIT family, participate in plant defence against pathogens or provide nutrients to pathogens by regulating the transmembrane transport of amino acids. Their disease resistance or susceptibility functions are closely related to plant tissue-specificity and substrate-specificity. Additionally, this review explores the potential coordinated regulation between amino acid and sugar transporters in the context of plant-pathogen interactions. Looking ahead, future research should focus on resolving transporter mechanisms in resistance, dissecting regulatory hubs linking metabolism and transport, mapping nutrient fluxes at the host-pathogen interface and exploring the subcellular localization and transport direction of transporters to inform precision crop protection strategies.

Strategies to Improve γ-Aminobutyric Acid Biosynthesis in Rice via Optimal Conditions

The γ-aminobutyric acid (GABA), a ubiquitous non-protein amino acid in plants and animals, exhibits diverse biological activities and holds promise in human disease prevention and treatment. Prior studies have shown that germination could substantially elevate GABA levels in rice, but these investigations typically focused on limited germplasms, hindering the generalization of their findings. This study aims to identify optimal conditions for enriching GABA in a diverse set of 225 rice germplasms by examining the effects of various germination times, temperatures, and soaking solution pH levels, while elucidating the key factors influencing GABA enrichment in germinated brown rice. The optimal GABA enrichment in germinated brown rice was achieved under the following conditions: a germination temperature of 37 °C, a germination duration of 48 h, and a soaking solution pH of 5.5. Under these conditions, we found significant differences in GABA content among different germplasms. Subsequent correlation analyses demonstrated that GABA content showed significant positive correlations with embryo weight in brown rice, relative embryo weight, relative embryo weight in germinated brown rice, as well as glutamate (Glu) and proline (Pro) concentrations. Therefore, larger brown rice embryos, higher Glu and Pro content in germinated brown rice, and external Glu application contribute to increased GABA content. Our findings provide essential materials and theoretical insights for screening and developing GABA-rich functional rice germplasms, facilitating variety selection and breeding programs.

Effect of Hormonal Treatments on Cannabinoid Content Levels in Female Hemp (Cannabis sativa L.) Inflorescences

The diverse hormonal treatments applied to hemp (Cannabis sativa L.) carry significant implications for cultivation, and yield optimization across a range of applications, including fiber, seed, oil production, and the enhancement of medicinal compounds. However, there is no evidence concerning the long-term consequences of hormonal treatment. To determine the connection between the effects of hormonal treatment and cannabinoid accumulation, hemp plants were treated with γ-aminobutyric acid (GABA), abscisic acid (ABA), and salicylic acid (SA) to investigate their effects on gene expression and cannabinoid content levels in female inflorescences at 3 days and 4 weeks after treatment. The treatments influenced the transcript levels of five key cannabinoid biosynthesis genes, with 1.0 mM GABA significantly increasing OAC, THCAS, and CBCAS transcripts within 48 to 72 h. Additionally, 1.0 mM GABA led to a significant increase in tetrahydrocannabinol content by day three and significant increases in total cannabidiol and cannabinoid by week four. In addition, both ABA and SA induced transient, dose-dependent increases or decreases in gene expressions, but cannabinoid accumulation at 4 weeks showed no significant changes compared to the control. These results provide valuable insights for hormonal application in cultivation and the development of traits that enhance cannabinoid production in cannabis cultivation, which could significantly contribute to optimizing industrial applications.

Harnessing the dynamics of plant organic acids metabolism following abiotic stresses

Plants encounter various environmental stresses throughout development, including shade, high light, drought, hypoxia, extreme temperatures, and metal toxicity, all of which adversely affect growth and productivity. Organic acids (OAs), besides serving as intermediates in the tricarboxylic acid (TCA) cycle, play crucial roles in multiple metabolic pathways and cellular compartments, including mitochondrial metabolism, amino acid metabolism, the glyoxylate cycle, and the photosynthetic mechanisms of C4 and CAM plants. OAs contribute to stress tolerance by acting as root chelating agents, regulating ATP production, and providing reducing power for detoxifying reactive oxygen species (ROS). They also participate in the biosynthesis of solutes involved in stress signaling and osmoregulation, particularly during stomatal movements. This review explores how OAs regulate plant metabolism in response to specific abiotic stresses, emphasizing the increased production of malate, citrate, and succinate, which enhance resilience to water deficits, metal toxicity, and flooding. Since these mechanisms involve intricate metabolic networks, changes in OA metabolism present promising and underexplored potential for agriculture. Understanding these mechanisms could lead to innovative strategies for developing crops with greater resilience to climate change, whether through genetic manipulation or by selecting varieties with favorable metabolic responses to stress.

Reduced content of gamma-aminobutyric acid enhances resistance to bacterial wilt disease in tomato

Bacteria within the Ralstonia solanacearum species complex cause devastating diseases in numerous crops, causing important losses in food production and industrial supply. Despite extensive efforts to enhance plant tolerance to disease caused by Ralstonia, efficient and sustainable approaches are still missing. Before, we found that Ralstonia promotes the production of gamma-aminobutyric acid (GABA) in plant cells; GABA can be used as a nutrient by Ralstonia to sustain the massive bacterial replication during plant colonization. In this work, we used CRISPR-Cas9-mediated genome editing to mutate SlGAD2, which encodes the major glutamate decarboxylase responsible for GABA production in tomato, a major crop affected by Ralstonia. The resulting Slgad2 mutant plants show reduced GABA content, and enhanced tolerance to bacterial wilt disease upon Ralstonia inoculation. Slgad2 mutant plants did not show altered susceptibility to other tested biotic and abiotic stresses, including drought and heat. Interestingly, Slgad2 mutant plants showed altered microbiome composition in roots and soil. We reveal a strategy to enhance plant resistance to Ralstonia by the manipulation of plant metabolism leading to an impairment of bacterial fitness. This approach could be particularly efficient in combination with other strategies based on the manipulation of the plant immune system, paving the way to a sustainable solution to Ralstonia in agricultural systems.

Effect of Exogenous γ-Aminobutyric Acid (GABA) on the Growth, Photosynthetic Pigment, Antioxidant and GABA Metabolism of Festuca arundinacea (Tall Fescues) Under Cadmium Stress

γ-Aminobutyric acid (GABA), an endogenous amino acid widely found in living organisms, has important functions in plants such as regulating growth and development, maintaining carbon and nitrogen nutrient balance, and coping with adversity. In this study, we investigated the effects of exogenous 0.5 mmol/L GABA on the growth, antioxidant metabolism, and GABA shunt metabolism of tall fescue under 20 μmol/L Cd stress, using tall fescue (Festuca arundinacea) 'Ruby II' under hydroponics conditions. The results showed that (1) applying GABA for 3, 7, 11, and 15 d under Cd stress inhibited Cd transport from roots to leaves and promoted plant height, alleviating the effects of Cd stress on plant growth. (2) Exogenous 0.5 mmol/L GABA had an interesting regulatory effect on the activation of the antioxidant enzyme system induced by stress at different stages, which was accompanied by a decrease in malondialdehyde (MDA) contents and alleviated the degree of cell membrane lipid peroxidation under cadmium stress. Specifically, peroxidase (POD) enzyme activity reactions initially responded on the 3rd and 7th days of stress, and the changes in catalase (CAT) enzyme activities concentrated on the 11th and 15th days of the later stage. Ascorbate peroxidase (APX) enzyme was active throughout the whole stress period in the roots. Multiple factorial analyses further proved that the antioxidant pathway strongly influenced the survival and growth of tall fescue under stress in the presence of GABA. (3) Application of exogenous GABA activated the branching pathway for GABA synthesis from Glu decarboxylation (GABA shunt) with a higher contribution in the leaves, which induced changes in glutamate content, and plants maintained a higher endogenous GABA content and signal to regulate the plant antioxidant system and reduce cell membrane damage, thus improving the tolerance of plants to Cd stress.

Transforming tomatoes into GABA-rich functional foods through genome editing: A modern biotechnological approach

Gamma-aminobutyric acid (GABA) functions as an inhibitory neurotransmitter which blocks the impulses between nerve cells in the brain. Due to the increasing awareness about the health promoting benefits associated with GABA, it is also artificially synthesized and consumed as a nutritional supplement by people in some regions of the world. Though among the fresh vegetables, tomato fruits do contain a comparatively higher amount of GABA (0.07 to 2.01 mg g-1 FW), it needs to be further enhanced to fully impart its potential health benefits. Achieving this feat through classical breeding approaches is time and resource consuming, and is also associated with linkage drag. On the other hand, precise targeting of specific sites in the genome with less off- target effects is mediated by CRISPR/Cas9 genome editing tool and is widely used to overcome the barriers associated with traditional breeding approaches. Combining genome editing with speed breeding techniques can enable the rapid development of GABA-rich tomato cultivars, paving a way to unlock a new era of functional foods, where every bite contributes to cognitive well-being and holistic health. This review highlights the significance of GABA boosted functional foods and explores the potential of CRISPR/Cas9 technology for developing GABA enriched tomatoes.

Harnessing the power of genomics to develop climate-smart crop varieties: A comprehensive review

Abiotic stresses arising as consequences of climate change pose a serious threat to agricultural productivity on a global scale. Most cultivated crop varieties exhibit susceptibility to such environmental pressures as drought, salinity, and waterlogging. Addressing these abiotic stresses through agronomic means is not only financially burdensome but also often impractical, particularly in the case of abiotic stresses like heat stress. Cultivating resilient varieties that can withstand such pressures emerges as an economically feasible strategy to mitigate these challenges. Nevertheless, the development of stress-tolerant cultivars is hindered by the intricate nature of abiotic stress tolerance, often characterized by low heritability values. Compounding this complexity is the dynamic and multifaceted nature of these stresses, which impede conventional breeding efforts, rendering them painstakingly slow. The identification of molecular markers has emerged as a pivotal advancement in this arena. By pinpointing genomic regions associated with tolerance to abiotic stresses, these markers serve as effective tools for selection and trait introgression. In the post-genomic era, the proliferation of high-density SNP markers has revolutionized breeding strategies. Genomic selection, leveraging these markers, has become the method of choice for addressing polygenic traits with low heritability, such as abiotic stress tolerance. With the functional characterization of many genes being done, precise manipulation through genome editing techniques is gaining significant traction. This review delves into the application of molecular markers in breeding stress-tolerant crop varieties, alongside role of recent genomic techniques in enhancing abiotic stress tolerance. It also explores success stories and identifies potential targets for marker-assisted selection.

GABA does not regulate stomatal CO2 signalling in Arabidopsis

Optimal stomatal regulation is important for plant adaptation to changing environmental conditions and for maintaining crop yield. The guard cell signal γ-aminobutyric acid (GABA) is produced from glutamate by glutamate decarboxylase (GAD) during a reaction that generates CO2 as a by-product. Here, we investigated a putative connection between GABA signalling and the more clearly defined CO2 signalling pathway in guard cells. The GABA-deficient mutant Arabidopsis lines gad2-1, gad2-2, and gad1/2/4/5 were examined for stomatal sensitivity to various CO2 concentrations. Our findings show a phenotypical discrepancy between the allelic mutant lines gad2-1 and gad2-2-a weakened CO2 response in gad2-1 (GABI_474_E05) in contrast to a wild-type response in gad2-2 (SALK_028819) and gad1/2/4/5. Through transcriptomic and genomic investigation, we traced the response of gad2-1 to a deletion of full-length Mitogen-activated protein kinase 12 (MPK12) in the GABI-KAT line, thereafter renamed as gad2-1*. Guard cell-specific complementation of MPK12 in gad2-1* restored the wild-type CO2 phenotype, which confirms the proposed importance of MPK12 in CO2 sensitivity. Additionally, we found that stomatal opening under low atmospheric CO2 occurs independently of the GABA-modulated opening channel ALUMINIUM-ACTIVATED MALATE TRANSPORTER 9 (ALMT9). Our results demonstrate that GABA has a role in modulating the rate of stomatal opening and closing, but not in response to CO2per se.

Role of Neurotransmitters (Biomediators) in Plant Responses to Stress

Plants possess a complex signaling system that enables them to sense and adapt to various environmental stressors, including abiotic factors like extreme temperatures, drought, salinity, and toxic heavy metals. While the roles of hormones and signaling molecules in plant stress responses are well established, the involvement of neurotransmitters-traditionally linked to animal nervous systems-in plant stress physiology is a relatively underexplored area. Recent findings indicate that neurotransmitters such as gamma-aminobutyric acid, glutamate, serotonin, and dopamine play crucial roles in several physiological processes within plants. They regulate ion channels, adjust stomatal movements, modulate the production of reactive oxygen species, and influence gene expression. Evidence suggests that these neurotransmitters enhance antioxidant defense mechanisms and regulate stress-responsive pathways vital for plant stress tolerance. Additionally, under stressful conditions, neurotransmitters have been shown to impact plant growth, development, and reproductive activities. This review aims to illuminate the emerging understanding of neurotransmitters as key biomediators in plant responses to abiotic stress.

Stress-regulated Arabidopsis GAT2 is a low affinity γ-aminobutyric acid transporter

The four-carbon non-proteinogenic amino acid γ-aminobutyric acid (GABA) accumulates to high levels in plants in response to various abiotic and biotic stress stimuli, and plays a role in C:N balance, signaling, and as a transport regulator. Expression in Xenopus oocytes and voltage-clamping allowed the characterization of Arabidopsis GAT2 (At5g41800) as a low affinity GABA transporter with a K0.5GABA ~8 mM. l-Alanine and butylamine represented additional substrates. GABA-induced currents were strongly dependent on the membrane potential, reaching the highest affinity and highest transport rates at strongly negative membrane potentials. Mutation of Ser17, previously reported to be phosphorylated in planta, did not result in altered affinity. In a short-term stress experiment, AtGAT2 mRNA levels were up-regulated at low water potential and under osmotic stress (polyethylene glycol and mannitol). Furthermore, AtGAT2 promoter activity was detected in vascular tissues, maturating pollen, and the phloem unloading region of young seeds. Even though this suggested a role for AtGAT2 in long-distance transport and loading of sink organs, under the conditions tested neither AtGAT2-overexpressing plants, atgat2 or atgat1 T-DNA insertion lines, nor atgat1 atgat2 doubleknockout mutants differed from wild-type plants in growth on GABA, amino acid levels, or resistance to salt and osmotic stress.

Sustained carbon import supports sugar accumulation and anthocyanin biosynthesis during fruit development and ripening in blueberry (Vaccinium ashei)

Fruit ripening is a highly coordinated process involving molecular and biochemical changes that collectively determine fruit quality. The underlying metabolic programs and their transitions leading to fruit ripening remain largely under-characterized in blueberry (Vaccinium sp.), which exhibits atypical climacteric behavior. In this study, we focused on sugar, acid and anthocyanin metabolism in two rabbiteye blueberry cultivars, Premier and Powderblue, during fruit development and ripening. Concentrations of the three major sugars, sucrose (Suc), glucose (Glc), and fructose (Fru) increased steadily during fruit development leading up to ripening, and increased dramatically by around 2-fold in 'Premier' and 2- to 3-fold in 'Powderblue' during the final stage of fruit ripening. Starch concentration was very low throughout fruit development in both cultivars indicating that it does not serve the role of a major transitory carbon (C) storage form in blueberry fruit. Together, these patterns indicate continued import of C, likely in the form of Suc, throughout blueberry fruit development. Concentrations of the predominant acids, malate and quinate, decreased during ripening, and may contribute to increased shikimate biosynthesis which, in-turn, allows for downstream phenylpropanoid metabolism leading to anthocyanin synthesis. Consistently, anthocyanin concentrations were highest in fully ripened blue fruit. Weighted gene co-expression network analysis (WGCNA) was performed using a 'Powderblue' fruit ripening transcriptome and targeted fruit metabolite concentration data. A 'dark turquoise' module positively correlated with sugars and anthocyanins, and negatively correlated with acids (malate, quinate), was identified. Gene Ontology (GO) enrichment analysis of this module identified transcripts related to sugar, acid, and phenylpropanoid metabolism pathways. Among these, increased transcript abundance of a VACUOLAR INVERTASE during ripening was consistent with sugar storage in the vacuole. In general, transcript abundance of the glycolysis pathway genes was upregulated during ripening. The transcript abundance of PHOSPHOENOLPYRUVATE (PEP) CARBOXYKINASE increased during fruit ripening and was negatively correlated with malate concentration, suggesting increased malate conversion to PEP, which supports anthocyanin production during fruit ripening. This was further supported by the co-upregulation of several anthocyanin biosynthesis-related genes. Together, this study provides insights into important metabolic programs, and their underlying gene expression patterns during fruit development and ripening in blueberry.

Advances in Plant GABA Research: Biological Functions, Synthesis Mechanisms and Regulatory Pathways

The γ-aminobutyric acid (GABA) is a widely distributed neurotransmitter in living organisms, known for its inhibitory role in animals. GABA exerts calming effects on the mind, lowers blood pressure in animals, and enhances stress resistance during the growth and development of plants. Enhancing GABA content in plants has become a focal point of current research. In plants, GABA is synthesized through two metabolic pathways, the GABA shunt and the polyamine degradation pathway, with the GABA shunt being the primary route. Extensive studies have investigated the regulatory mechanisms governing GABA synthesis. At the genetic level, GABA production and degradation can be modulated by gene overexpression, signaling molecule-induced expression, transcription factor regulation, and RNA interference. Additionally, at the level of transporter proteins, increased activity of GABA transporters and proline transporters enhances the transport of glutamate and GABA. The activity of glutamate decarboxylase, a key enzyme in GABA synthesis, along with various external factors, also influences GABA synthesis. This paper summarizes the biological functions, metabolic pathways, and regulatory mechanisms of GABA, providing a theoretical foundation for further research on GABA in plants.

Effects of GABA on Oxidative Stress and Metabolism in High-Glucose Cultured Mongolian Sheep Kidney Cells

The Mongolian sheep, emblematic of the Inner Mongolian grasslands, is renowned for its exceptional stress resistance and adaptability to harsh environments, drawing considerable attention. Recent research has unveiled the novel role of γ-aminobutyric acid (GABA) in combating oxidative stress. This investigation examined how GABA impacts renal-cortex and medulla cells from Mongolian sheep exposed to high-glucose stress conditions, utilizing gene expression analysis and non-targeted metabolomics. Elevated glucose levels significantly reduced the viability of Mongolian sheep renal cells and increased reactive oxygen species (ROS) levels. Conversely, the introduction of GABA notably enhanced cell viability, reduced ROS production, and stimulated the expression of antioxidant genes (e.g., Gpx, SOD, CAT) in the renal cortex. In the renal medulla, CAT expression increased, while Gpx gene expression showed mixed responses. Metabolomics analysis indicated that high-glucose exposure altered various metabolites, whereas GABA alleviated the metabolic stress induced by high glucose through modulating glycolysis and the tricarboxylic acid cycle. In Mongolian sheep renal cells, GABA effectively mitigated oxidative damage triggered by high-glucose stress by upregulating antioxidant genes and regulating metabolic pathways, revealing insights into its potential mechanism for adapting to extreme environments. This finding offers a fresh perspective on understanding the stress resilience of Mongolian sheep and may provide valuable insights for research across diverse disciplines.

Impact of microplastic residues from polyurethane films on crop growth: Unraveling insights through transcriptomics and metabolomics analysis

The utilisation of coated controlled-release fertilizers (CRFs) leads to the persistence of residual plastic films in agricultural soils, posing a potential threat to crop health. This study investigates the impacts of four residual films (0.39 %, w/w) derived from CRFs in soil, including petrochemical polyether, bio-based polyether, castor oil polyester, and wheat straw polyester polyurethane on wheat growth. This study found that PecPEUR significantly reduced wheat plant height, stem diameter, leaf area, and aboveground fresh weight by 24.8 %, 20.2 %, and 25.7 %. Through an in-depth exploration of transcriptomics and metabolomics, it has been discovered that all residual films disrupted glycolysis-related metabolic pathways in wheat roots, affecting seedling growth. Among them, PecPEUR significantly reduced the fresh weight of aboveground parts by 20.5 %. In contrast, polyester polyurethane residue had no discernible impact on aboveground wheat growth. This was attributed to the enrichment of wheat root genes in jasmonic acid and γ-aminobutyric acid metabolic pathways, thus mitigating oxidative stress, enhancing stress resistance, and ensuring normal plant growth. This study, for the first time, provides comprehensive insights into the effects of polyurethane film residue on wheat seedling growth, underscoring its potential as a promising alternative to conventional plastics in soil.

Exploring the Organic Acid Secretion Pathway and Potassium Solubilization Ability of Pantoea vagans ZHS-1 for Enhanced Rice Growth

Soil potassium deficiency is a common issue limiting agricultural productivity. Potassium-solubilizing bacteria (KSB) show significant potential in mitigating soil potassium deficiency, improving soil quality, and enhancing plant growth. However, different KSB strains exhibit diverse solubilization mechanisms, environmental adaptability, and growth-promoting abilities. In this study, we isolated a multifunctional KSB strain ZHS-1, which also has phosphate-solubilizing and IAA-producing capabilities. 16S rDNA sequencing identified it as Pantoea vagans. Scanning electron microscopy (SEM) showed that strain ZHS-1 severely corroded the smooth, compact surface of potassium feldspar into a rough and loose state. The potassium solubilization reached 20.3 mg/L under conditions where maltose was the carbon source, sodium nitrate was the nitrogen source, and the pH was 7. Organic acid metabolism profiling revealed that strain ZHS-1 primarily utilized the EMP-TCA cycle, supplemented by pathways involving pantothenic acid, glyoxylic acid, and dicarboxylic acids, to produce large amounts of organic acids and energy. This solubilization was achieved through direct solubilization mechanisms. The strain also secreted IAA through a tryptophan-dependent metabolic pathway. When strain ZHS-1 was inoculated into the rhizosphere of rice, it demonstrated significant growth-promoting effects. The rice plants exhibited improved growth and root development, with increased accumulation of potassium and phosphorus. The levels of available phosphorus and potassium in the rhizosphere soil also increased significantly. Additionally, we observed a decrease in the relative abundance of Actinobacteria and Proteobacteria in the rice rhizosphere soil, while the relative abundance of genera associated with acid production and potassium solubilization, such as Gemmatimonadota, Acidobacteria, and Chloroflexi, as well as Cyanobacteria, which are beneficial to plant growth, increased. These findings contribute to a deeper understanding of the potassium solubilization mechanisms of strain ZHS-1 and highlight its potential as a plant growth-promoting rhizobacteria.

Improving postharvest quality and vase life of cut rose flowers by pre-harvest foliar co-applications of γ-aminobutyric acid and calcium chloride

Rose flowers (Rosa hybrida L.) are highly perishable and have a limited vase life. This study evaluated the effects of preharvest foliar applications of γ-aminobutyric acid (GABA) and calcium chloride (CaCl2), individually and combined, on antioxidant responses and vase life of cut Jumilia rose flowers. Treatments included foliar sprays of GABA at 0, 20, 40, and 60 mM and CaCl2 at 0, 0.75%, and 1.5%, applied in a factorial design within a completely randomized setup before harvest. Results showed GABA and CaCl2 interaction (especially, 60 mM GABA and 1.5% CaCl2) significantly increased enzymatic antioxidants including superoxide dismutase, catalase, and peroxidase, as well as non-enzymatic antioxidants such as flavonoids, carotenoids, phenolics, and antioxidant activity in petals compared to control. SOD activity in roses, treated with CaCl2 (1.5%) and GABA (60 mM), peaked at 7.86 units. mg-1 protein min-1, showing a nearly 2.93-fold increase over the control (2.68 units. mg-1 protein min-1). A parallel trend was observed for CAT activity. These treatments also reduced petal malondialdehyde content and polyphenol oxidase activity. Protein content and vase life duration increased in all treatments. Plants treated with a combination of GABA (20 mM) and CaCl2 (0.75%), GABA (60 mM) and CaCl2 (1.5%), or GABA (40 mM) individually exhibited the longest vase life duration. The co-application of GABA and CaCl2 improved the antioxidant activity and postharvest quality of cut roses by reducing PPO activity and MDA contents, increasing protein content and prolonging vase life. This treatment is a potential postharvest strategy to improve antioxidant capacity and delay senescence in cut roses.

Exogenous GABA Enhances Copper Stress Resilience in Rice Plants via Antioxidant Defense Mechanisms, Gene Regulation, Mineral Uptake, and Copper Homeostasis

The importance of gamma-aminobutyric acid (GABA) in plants has been highlighted due to its critical role in mitigating metal toxicity, specifically countering the inhibitory effects of copper stress on rice plants. This study involved pre-treating rice plants with 1 mM GABA for one week, followed by exposure to varying concentrations of copper at 50 μM, 100 μM, and 200 μM. Under copper stress, particularly at 100 μM and 200 μM, plant height, biomass, chlorophyll content, relative water content, mineral content, and antioxidant activity decreased significantly compared to control conditions. However, GABA treatment significantly alleviated the adverse effects of copper stress. It increased plant height by 13%, 18%, and 32%; plant biomass by 28%, 52%, and 60%; chlorophyll content by 12%, 30%, and 24%; and relative water content by 10%, 24%, and 26% in comparison to the C50, C100, and C200 treatments. Furthermore, GABA treatment effectively reduced electrolyte leakage by 11%, 34%, and 39%, and the concentration of reactive oxygen species, such as malondialdehyde (MDA), by 9%, 22%, and 27%, hydrogen peroxide (H2O2) by 12%, 38%, and 30%, and superoxide anion content by 8%, 33, and 39% in comparison to C50, C100, and C200 treatments. Additionally, GABA supplementation led to elevated levels of glutathione by 69% and 80%, superoxide dismutase by 22% and 125%, ascorbate peroxidase by 12% and 125%, and catalase by 75% and 100% in the C100+G and C200+G groups as compared to the C100 and C200 treatments. Similarly, GABA application upregulated the expression of GABA shunt pathway-related genes, including gamma-aminobutyric transaminase (OsGABA-T) by 38% and 80% and succinic semialdehyde dehydrogenase (OsSSADH) by 60% and 94% in the C100+G and C200+G groups, respectively, as compared to the C100 and C200 treatments. Conversely, the expression of gamma-aminobutyric acid dehydrogenase (OsGAD) was downregulated. GABA application reduced the absorption of Cu2+ by 54% and 47% in C100+G and C200+G groups as compared to C100, and C200 treatments. Moreover, GABA treatment enhanced the uptake of Ca2+ by 26% and 82%, Mg2+ by 12% and 67%, and K+ by 28% and 128% in the C100+G and C200+G groups as compared to C100, and C200 treatments. These findings underscore the pivotal role of GABA-induced enhancements in various physiological and molecular processes, such as plant growth, chlorophyll content, water content, antioxidant capacity, gene regulation, mineral uptake, and copper sequestration, in enhancing plant tolerance to copper stress. Such mechanistic insights offer promising implications for the advancement of safe and sustainable food production practices.

Emerging Trends in Non-Protein Amino Acids as Potential Priming Agents: Implications for Stress Management Strategies and Unveiling Their Regulatory Functions

Plants endure the repercussions of environmental stress. As the advancement of global climate change continues, it is increasingly crucial to protect against abiotic and biotic stress effects. Some naturally occurring plant compounds can be used effectively to protect the plants. By externally applying priming compounds, plants can be prompted to trigger their defensive mechanisms, resulting in improved immune system effectiveness. This review article examines the possibilities of utilizing exogenous alpha-, beta-, and gamma-aminobutyric acid (AABA, BABA, and GABA), which are non-protein amino acids (NPAAs) that are produced naturally in plants during instances of stress. The article additionally presents a concise overview of the studies' discoveries on this topic, assesses the particular fields in which they might be implemented, and proposes new avenues for future investigation.

Metabolite-mediated adaptation of crops to drought and the acquisition of tolerance

Drought is one of the major and growing threats to agriculture productivity and food security. Metabolites are involved in the regulation of plant responses to various environmental stresses, including drought stress. The complex drought tolerance can be ascribed to several simple metabolic traits. These traits could then be used for detecting the genetic architecture of drought tolerance. Plant metabolomes show dynamic differences when drought occurs during different developmental stages or upon different levels of drought stress. Here, we reviewed the major and most recent findings regarding the metabolite-mediated plant drought response. Recent progress in the development of drought-tolerant agents is also discussed. We provide an updated schematic overview of metabolome-driven solutions for increasing crop drought tolerance and thereby addressing an impending agricultural challenge.

Integrated Analysis of Transcriptome and Metabolome Reveals Differential Responses to Alternaria brassicicola Infection in Cabbage (Brassica oleracea var. capitata)

Black spot, caused by Alternaria brassicicola (Ab), poses a serious threat to crucifer production, and knowledge of how plants respond to Ab infection is essential for black spot management. In the current study, combined transcriptomic and metabolic analysis was employed to investigate the response to Ab infection in two cabbage (Brassica oleracea var. capitata) genotypes, Bo257 (resistant to Ab) and Bo190 (susceptible to Ab). A total of 1100 and 7490 differentially expressed genes were identified in Bo257 (R_mock vs. R_Ab) and Bo190 (S_mock vs. S_Ab), respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that "metabolic pathways", "biosynthesis of secondary metabolites", and "glucosinolate biosynthesis" were the top three enriched KEGG pathways in Bo257, while "metabolic pathways", "biosynthesis of secondary metabolites", and "carbon metabolism" were the top three enriched KEGG pathways in Bo190. Further analysis showed that genes involved in extracellular reactive oxygen species (ROS) production, jasmonic acid signaling pathway, and indolic glucosinolate biosynthesis pathway were differentially expressed in response to Ab infection. Notably, when infected with Ab, genes involved in extracellular ROS production were largely unchanged in Bo257, whereas most of these genes were upregulated in Bo190. Metabolic profiling revealed 24 and 56 differentially accumulated metabolites in Bo257 and Bo190, respectively, with the majority being primary metabolites. Further analysis revealed that dramatic accumulation of succinate was observed in Bo257 and Bo190, which may provide energy for resistance responses against Ab infection via the tricarboxylic acid cycle pathway. Collectively, this study provides comprehensive insights into the Ab-cabbage interactions and helps uncover targets for breeding Ab-resistant varieties in cabbage.

Recent advances in the biosynthesis and industrial biotechnology of Gamma-amino butyric acid

GABA (Gamma-aminobutyric acid), a crucial neurotransmitter in the central nervous system, has gained significant attention in recent years due to its extensive benefits for human health. The review focused on recent advances in the biosynthesis and production of GABA. To begin with, the investigation evaluates GABA-producing strains and metabolic pathways, focusing on microbial sources such as Lactic Acid Bacteria, Escherichia coli, and Corynebacterium glutamicum. The metabolic pathways of GABA are elaborated upon, including the GABA shunt and critical enzymes involved in its synthesis. Next, strategies to enhance microbial GABA production are discussed, including optimization of fermentation factors, different fermentation methods such as co-culture strategy and two-step fermentation, and modification of the GABA metabolic pathway. The review also explores methods for determining glutamate (Glu) and GABA levels, emphasizing the importance of accurate quantification. Furthermore, a comprehensive market analysis and prospects are provided, highlighting current trends, potential applications, and challenges in the GABA industry. Overall, this review serves as a valuable resource for researchers and industrialists working on GABA advancements, focusing on its efficient synthesis processes and various applications, and providing novel ideas and approaches to improve GABA yield and quality.

Exogenous phytosulfokine α alleviates chilling injury of loquat fruit via regulating sugar, proline, polyamine and γ-aminobutyric acid metabolisms

Postharvest loquat fruit is susceptible to chilling injury (CI) under cold stress. In this study, the effects of phytosulfokine α (PSKα) on sugar, proline, polyamine and γ-aminobutyric acid (GABA) metabolisms in loquat fruit during cold storage were investigated. The results showed that PSKα treatment significantly increased PSKα content along with up-regulating EjPSK3 and EjPSK6 expressions, and inhibited the increases of internal browning index, electrolyte leakage and malondialdehyde (MDA) content of loquat fruit. Besides, PSKα treatment maintained higher reducing sugar, proline, polyamines, and GABA contents in loquat fruit via activating biosynthesis pathway and suppressing catabolism pathway. More importantly, the results of correlation analysis indicated that PSKα content displayed positive correlations with reducing sugar, proline, polyamines and GABA contents. These findings suggested that the improved chilling tolerance in PSKα-treated loquat fruit was due to enhancing reducing sugar, proline, polyamines, and GABA contents, which might be modulated by endogenous PSKα signaling.

"Deciphering the enigmatic role of gamma-aminobutyric acid (GABA) in plants: Synthesis, transport, regulation, signaling, and biological roles in interaction with growth regulators and abiotic stresses."

Gamma-aminobutyric acid (GABA) is an amino acid with a four-carbon structure, widely distributed in various organisms. It exists as a zwitterion, possessing both positive and negative charges, enabling it to interact with other molecules and participate in numerous physiological processes. GABA is widely distributed in various plant cell compartments such as cytoplasm mitochondria, vacuoles, peroxisomes, and plastids. GABA is primarily synthesized from glutamate using glutamate decarboxylase and participates in the GABA shunt within mitochondria, regulating carbon and nitrogen metabolism in plants The transport of GABA is regulated by several intracellular and intercellular transporters such as aluminium-activated malate transporters (ALMTs), GABA transporters (GATs), bidirectional amino acid transporters (BATs), and cationic amino acid transporters (CATs). GABA plays a vital role in cellular transformations, gene expression, cell wall modifications, and signal transduction in plants. Recent research has unveiled the role of GABA as a signaling molecule in plants, regulating stomatal movement and pollen tube growth. This review provides insights into multifaceted impact of GABA on physiological and biochemical traits in plants, including cellular communication, pH regulation, Krebs cycle circumvention, and carbon and nitrogen equilibrium. The review highlights involvement of GABA in improving the antioxidant defense system of plants, mitigating levels of reactive oxygen species under normal and stressed conditions. Moreover, the interplay of GABA with other plant growth regulators (PGRs) have also been explored.

Truncation of the calmodulin binding domain in rice glutamate decarboxylase 4 (OsGAD4) leads to accumulation of γ-aminobutyric acid and confers abiotic stress tolerance in rice seedlings

GABA (Gamma-aminobutyric acid) is a non-protein amino acid widely known as major inhibitory neurotransmitter. It is synthesized from glutamate via the enzyme glutamate decarboxylase (GAD). GAD is ubiquitous in all organisms, but only plant GAD has ability to bind Ca2+/calmodulin (CaM). This kind of binding suppresses the auto-inhibition of Ca2+/calmodulin binding domain (CaMBD) when the active site of GAD is unfolded resulting in stimulated GAD activity. OsGAD4 is one of the five GAD genes in rice genome. It was confirmed that OsGAD4 has ability to bind to Ca2+/CaM. Moreover, it exhibits strongest expression against several stress conditions among the five OsGAD genes. In this study, CRISPR/Cas9-mediated genome editing was performed to trim the coding region of CaMBD from the OsGAD4 gene, to remove its autoinhibitory function. DNA sequence analysis of the genome edited rice plants revealed the truncation of CaMBD (216 bp). Genome edited line (#14-1) produced 11.26 mg GABA/100 g grain, which is almost nine-fold in comparison to wild type. Short deletion in the coding region for CaMBD yielded in mutant (#14-6) with lower GABA content than wild type counterpart. Abiotic stresses like salinity, flooding and drought significantly enhanced GABA accumulation in #14-1 at various time points compared to wild-type and #14-6 under the same stress conditions. Moreover, upregulated mRNA expression in vegetative tissues seems correlated with the stress-responsiveness of OsGAD4 when exposed to the above-mentioned stresses. Stress tolerance of OsGAD4 genome edited lines was evidenced by the higher survival rate indicating the gene may induce tolerance against abiotic stresses in rice. This is the first report on abiotic stress tolerance in rice modulated by endogenous GABA.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01460-1.

Combined Proteomic and Metabolomic Analyses Reveal the Comprehensive Regulation of Stropharia rugosoannulata Mycelia Exposed to Cadmium Stress

The potential of Stropharia rugosoannulata as a microbial remediation material for cadmium (Cd)-contaminated soil lies in its capacity to absorb and accumulate Cd in its mycelia. This study utilized the TMT and LC-MS techniques to conduct integrated proteomic and metabolomic analyses with the aim of investigating the mycelial response mechanisms of S. rugosoannulata under low- and high-Cd stresses. The results revealed that mycelia employed a proactive defense mechanism to maintain their physiological functions, leading to reduced sensitivity to low-Cd stress. The ability of mycelia to withstand high levels of Cd stress was influenced primarily by the comprehensive regulation of six metabolic pathways, which led to a harmonious balance between nitrogen and carbohydrate metabolism and to reductions in oxidative stress and growth inhibition caused by Cd. The results provide valuable insights into the molecular mechanisms involved in the response of S. rugosoannulata mycelia to Cd stress.

γ-Aminobutyric acid (GABA) in N2-fixing-legume symbiosis: Metabolic flux and carbon/nitrogen homeostasis in responses to abiotic constraints

Nodule symbiosis is an energetic process that demands a tremendous carbon (C) cost, which massively increases in responses to environmental stresses. Notably, most common respiratory pathways (e.g., glycolysis and Krebs cycle) that sustain nitrogenase activity and subsequent nitrogen (N) assimilation (amino acid formation) display a noncyclic mode of C flux. In such circumstances, the nodule's energy charge could markedly decrease, leading to a lower symbiotic activity under stresses. The host plant then attempts to induce alternative robust metabolic pathways to minimize the C expenditure and compensate for the loss in respiratory substrates. GABA (γ-aminobutyric acid) shunt appears to be among the highly conserved metabolic bypass induced in responses to stresses. Thus, it can be suggested that GABA, via its primary biosynthetic pathway (GABA shunt), is simultaneously induced to circumvent stress-susceptible decarboxylating portion of the Krebs cycle and to replenish symbiosome with energy and C skeletons for enhancing nitrogenase activity and N assimilation besides the additional C costs expended in the metabolic stress acclimations (e.g., biosynthesis of secondary metabolites and excretion of anions). The GABA-mediated C/N balance is strongly associated with interrelated processes, including pH regulation, oxygen (O2) protection, osmoregulation, cellular redox control, and N storage. Furthermore, it has been anticipated that GABA could be implicated in other functions beyond its metabolic role (i.e., signaling and transport). GABA helps plants possess remarkable metabolic plasticity, which might thus assist nodules in attenuating stressful events.

Identification and function analysis of GABA branch three gene families in the cotton related to abiotic stresses

γ -aminobutyric acid (GABA) is closely related to the growth, development and stress resistance of plants. Combined with the previous study of GABA to promote the cotton against abiotic stresses, the characteristics and expression patterns of GABA branch gene family laid the foundation for further explaining its role in cotton stress mechanism. Members of GAD, GAB-T and SSADH (three gene families of GABA branch) were identified from the Gossypium hirsutum, Gossypium barbadense, Gossypium arboreum and Gossypium raimondii genome. The GABA branch genes were 10 GAD genes, 4 GABA-T genes and 2 SSADH genes. The promoter sequences of genes mainly contains response-related elements such as light, hormone and environment.Phylogenetic analysis shows that GAD indicating that even in the same species, the homologous sequences in the family. The GABA-T gene of each cotton genus was in sum the family had gene loss in the process of dicotyledon evolution. SSADH families Gossypium hirsutum, Gossypium barbadense, Gossypium arboreum and Gossypium raimondii were closely related to the dicot plants.GABA gene is involved in the regulation of salt stress and high temperature in Gossypium hirsutum.GABA attenuated part of the abiotic stress damage by increasing leaf protective enzyme activity and reducing reactive oxygen species production.This lays the foundation for a thorough analysis of the mechanism of GABA in cotton stress resistance.

Metabolomic Analysis of the Effect of Freezing on Leaves of Malus sieversii (Ledeb.) M.Roem. Histoculture Seedlings

Malus sieversii (Ledeb.) M.Roem. is the ancestor of cultivated apples, and is an excellent germplasm resource with high resistance to cold. Artificial refrigerators were used to simulate the low temperature of -3 °C to treat Malus sieversii (Ledeb.) M.Roem. histoculture seedlings. Observations were performed to find the effects of freezing stress on the status of open or closed stomata, photosystems, and detection of metabolomic products in leaves of Malus sieversii (Ledeb.) M.Roem. histoculture seedlings. The percentage of closed stomata in the Malus sieversii (Ledeb.) M.Roem. histoculture seedlings increased, the maximum fluorescence (Fm') excited by a strong light (saturating pulse) was weakened relative to the real-time fluorescence in its vicinity, and the quantum yield of unregulated energy dissipation was increased in PSII under freezing stress. The metabolites in the leaves of the Malus sieversii (Ledeb. M.Roem.) histoculture seedlings were analyzed by ultra-performance liquid chromatography-tandem mass spectrometry using CK, T12h, T36 h, and HF24h. Results demonstrated that cold stress in the Malus sieversii (Ledeb.) M.Roem. histoculture seedlings led to wilting, leaf stomatal closure, and photosystem damage. There were 1020 metabolites identified as lipids (10.2%), nucleotides and their derivatives (5.2%), phenolic acids (19.12%), flavonoids (24.51%), amino acids and their derivatives (7.75%), alkaloids (5.39%), terpenoids (8.24%), lignans (3.04%), organic acids (5.88%), and tannins (0.88%). There were 110 differential metabolites at CKvsT12h, 113 differential metabolites at CKvsT36h, 87 differential metabolites at T12hvsT36h, 128 differential metabolites at CKvsHF24h, 121 differential metabolites at T12hvsHF24h, and 152 differential metabolites at T36hvsHF24h. The differential metabolites in the leaves of the Malus sieversii (Ledeb.) M.Roem. seedlings grown under low-temperature stress mainly involved glycolysis, amino acid metabolism, lipid metabolism, pyrimidine metabolism, purine metabolism, and secondary metabolite metabolism. The Malus sieversii (Ledeb.) M.Roem. seedlings responded to the freezing stress by coordinating with each other through these metabolic pathways. The metabolic network of the leaves of the Malus sieversii (Ledeb.) M.Roem. histoculture seedlings under low temperature stress was also proposed based on the above pathways to deepen understanding of the response of metabolites of Malus sieversii (Ledeb.) M.Roem. to low-temperature stress and to lay a theoretical foundation for the development and utilization of Malus sieversii (Ledeb.) M.Roem. cultivation resources.

Integrating transcriptome and metabolome to explore the growth-promoting mechanisms of GABA in blueberry plantlets

Tissue culture technology is the main method for the commercial propagation of blueberry plants, but blueberry plantlets grow slowly and have long growth cycles under in vitro propagation, resulting in low propagation efficiency. In addition, the long culturing time can also result in reduced nutrient content in the culture medium, and the accumulation of toxic and harmful substances that can lead to weak growth for the plantlets or browning and vitrification, which ultimately can seriously reduce the quality of the plantlets. Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that can improve plant resistance to various stresses and promote plant growth, but the effects of its application and mechanism in tissue culture are still unclear. In this study, the effects of GABA on the growth of in vitro blueberry plantlets were analyzed following the treatment of the plantlets with GABA. In addition, the GABA-treated plantlets were also subjected to a comparative transcriptomic and metabolomic analysis. The exogenous application of GABA significantly promoted growth and improved the quality of the blueberry plantlets. In total, 2,626 differentially expressed genes (DEGs) and 377 differentially accumulated metabolites (DAMs) were detected by comparison of the control and GABA-treated plantlets. Most of the DEGs and DAMs were involved in carbohydrate metabolism and biosynthesis of secondary metabolites. The comprehensive analysis results indicated that GABA may promote the growth of blueberry plantlets by promoting carbon metabolism and nitrogen assimilation, as well as increasing the accumulation of secondary metabolites such as flavonoids, steroids and terpenes.

P. Reddy G, Li Z, Chen F, Sun Y, Zhao Z. The nitrogen-dependent GABA pathway of tomato provides resistance to a globally invasive fruit fly

Introduction

The primary metabolism of plants, which is mediated by nitrogen, is closely related to the defense response to insect herbivores.

Methods

An experimental system was established to examine how nitrogen mediated tomato resistance to an insect herbivore, the oriental fruit fly (Bactrocera dorsalis). All tomatoes were randomly assigned to the suitable nitrogen (control, CK) treatment, nitrogen excess (NE) treatment and nitrogen deficiency (ND) treatment.

Results

We found that nitrogen excess significantly increased the aboveground biomass of tomato and increased the pupal biomass of B. dorsalis. Metabolome analysis showed that nitrogen excess promoted the biosynthesis of amino acids in healthy fruits, including γ-aminobutyric acid (GABA), arginine and asparagine. GABA was not a differential metabolite induced by injury by B. dorsalis under nitrogen excess, but it was significantly induced in infested fruits at appropriate nitrogen levels. GABA supplementation not only increased the aboveground biomass of plants but also improved the defensive response of tomato.

Discussion

The biosynthesis of GABA in tomato is a resistance response to feeding by B. dorsalis in appropriate nitrogen, whereas nitrogen excess facilitates the pupal weight of B. dorsalis by inhibiting synthesis of the GABA pathway. This study concluded that excess nitrogen inhibits tomato defenses in plant-insect interactions by inhibiting GABA synthesis, answering some unresolved questions about the nitrogen-dependent GABA resistance pathway to herbivores.

Improved lipid production and component of mycosporine-like amino acids by co-overexpression of amt1 and aroB genes in Synechocystis sp. PCC6803

Implementing homologous overexpression of the amt1 (A) and aroB (B) genes involved in ammonium transporter and the synthesis of mycosporine-like amino acids (MAAs) and aromatic amino acids, respectively, we created three engineered Synechocystis sp. PCC6803 strains, including Ox-A, Ox-B, and Ox-AB, to study the utilization of carbon and nitrogen in cyanobacteria for the production of valuable products. With respect to amt1 overexpression, the Ox-A and Ox-AB strains had a greater growth rate under (NH4)2SO4 supplemented condition. Both the higher level of intracellular accumulation of lipids in Ox-A and Ox-AB as well as the increased secretion of free fatty acids from the Ox-A strain were impacted by the late-log phase of cell growth. It is noteworthy that among all strains, the Ox-B strain undoubtedly spotted a substantial accumulation of glycogen as a consequence of aroB overexpression. Additionally, the ammonium condition drove the potent antioxidant activity in Ox strains with a late-log phase, particularly in the Ox-B and Ox-AB strains. This was probably related to the altered MAA component inside the cells. The higher proportion of P4-fraction was induced by the ammonium condition in both Ox-B and Ox-AB, while the noted increase of the P1 component was found in the Ox-A strain.

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.

Gamma-Aminobutyric Acid Supplementation Boosts the Phytohormonal Profile in 'Candidatus Liberibacter asiaticus'-Infected Citrus

The devastating citrus disease, Huanglongbing (HLB), is associated with 'Candidatus Liberibacter sp.' and transmitted by citrus psyllids. Unfortunately, HLB has no known sustainable cure yet. Herein, we proposed γ-aminobutyric acid (GABA) as a potential eco-friendly therapeutic solution to HLB. Herein, we used GC/MS-based targeted metabolomics combined with gene expression to investigate the role of GABA in citrus response against HLB and to better understand its relationship(s) with different phytohormones. GABA supplementation via root drench boosts the accumulation of endogenous GABA in the leaves of both healthy and 'Ca. L. asiaticus'-infected trees. GABA accumulation benefits the activation of a multi-layered defensive system via modulating the phytohormone levels and regulating the expression of their biosynthesis genes and some pathogenesis-related proteins (PRs) in both healthy and 'Ca. L. asiaticus'-infected plants. Moreover, our findings showed that GABA application stimulates auxin biosynthesis in 'Ca. L. asiaticus'-infected plants via the activation of the indole-3-pyruvate (I3PA) pathway, not via the tryptamine (TAM)-dependent pathway, to enhance the growth of HLB-affected trees. Likewise, GABA accumulation was associated with the upregulation of SA biosynthesis genes, particularly the PAL-dependent route, resulting in higher SA levels that activated CsPR1, CsPR2, CsPR5, and CsWRKY70, which are prominent to activation of the SA-mediated pathway. Additionally, higher GABA levels were correlated with an enhanced JA profile and linked with both CsPR3 and CsPR4, which activates the JA-mediated pathway. Collectively, our findings suggest that exogenous GABA application might be a promising alternative and eco-friendly strategy that helps citrus trees battle HLB.

Global proteome and lysine succinylation analyses provide insights into the secondary metabolism in Salvia miltiorrhiza

Danshen, belongs to the Lamiaceae family, and its scientific name is Salvia miltiorrhiza Bunge. It is a valuable medicinal plant to prevent and treat cardiovascular and cerebrovascular diseases. Lysine succinylation, a widespread modification found in various organisms, plays a critical role in regulating secondary metabolism in plants. The hairy roots of Salvia miltiorrhiza were subject to proteomic analysis to identify lysine succinylation sites using affinity purification and HPLC-MS/MS in this investigation. Our findings reveal 566 lysine succinylation sites in 348 protein sequences. We observed 110 succinylated proteins related to secondary metabolism, totaling 210 modification sites. Our analysis identified 53 types of enzymes among the succinylated proteins, including phenylalanine ammonia-lyase (PAL) and aldehyde dehydrogenase (ALDH). PAL, a crucial enzyme involved in the biosynthesis of rosmarinic acid and flavonoids, displayed succinylation at two sites. ALDH, which participates in the phenylpropane metabolic pathway, was succinylated at 8 eight sites. These observations suggest that lysine succinylation may play a vital role in regulating the production of secondary metabolites in Salvia miltiorrhiza. Our study may provide valuable insights for further investigation on plant succinylation, specifically as a reference point. SIGNIFICANCE: Salvia miltiorrhiza Bunge is a valuable medicinal plant that prevents and treats cardiovascular and cerebrovascular diseases. Lysine succinylation plays a critical role in regulating secondary metabolism in plants. The hairy roots of Salvia miltiorrhiza were subject to proteomic analysis to identify lysine succinylation sites using affinity purification and HPLC-MS/MS in this investigation. These observations suggest that lysine succinylation may act as a vital role in regulating the production of secondary metabolites in Salvia miltiorrhiza. Our study may provide valuable insights for further investigation on succinylation in plants, specifically as a reference point.

Metabolic pathways engineering for drought or/and heat tolerance in cereals

Drought (D) and heat (H) are the two major abiotic stresses hindering cereal crop growth and productivity, either singly or in combination (D/+H), by imposing various negative impacts on plant physiological and biochemical processes. Consequently, this decreases overall cereal crop production and impacts global food availability and human nutrition. To achieve global food and nutrition security vis-a-vis global climate change, deployment of new strategies for enhancing crop D/+H stress tolerance and higher nutritive value in cereals is imperative. This depends on first gaining a mechanistic understanding of the mechanisms underlying D/+H stress response. Meanwhile, functional genomics has revealed several stress-related genes that have been successfully used in target-gene approach to generate stress-tolerant cultivars and sustain crop productivity over the past decades. However, the fast-changing climate, coupled with the complexity and multigenic nature of D/+H tolerance suggest that single-gene/trait targeting may not suffice in improving such traits. Hence, in this review-cum-perspective, we advance that targeted multiple-gene or metabolic pathway manipulation could represent the most effective approach for improving D/+H stress tolerance. First, we highlight the impact of D/+H stress on cereal crops, and the elaborate plant physiological and molecular responses. We then discuss how key primary metabolism- and secondary metabolism-related metabolic pathways, including carbon metabolism, starch metabolism, phenylpropanoid biosynthesis, γ-aminobutyric acid (GABA) biosynthesis, and phytohormone biosynthesis and signaling can be modified using modern molecular biotechnology approaches such as CRISPR-Cas9 system and synthetic biology (Synbio) to enhance D/+H tolerance in cereal crops. Understandably, several bottlenecks hinder metabolic pathway modification, including those related to feedback regulation, gene functional annotation, complex crosstalk between pathways, and metabolomics data and spatiotemporal gene expressions analyses. Nonetheless, recent advances in molecular biotechnology, genome-editing, single-cell metabolomics, and data annotation and analysis approaches, when integrated, offer unprecedented opportunities for pathway engineering for enhancing crop D/+H stress tolerance and improved yield. Especially, Synbio-based strategies will accelerate the development of climate resilient and nutrient-dense cereals, critical for achieving global food security and combating malnutrition.

Production of glycerolamines based conjugated γ-aminobutyric acids using microbial COX and LOX as successor enzymes to GAD

Gamma-aminobutyric acid (γ-ABA) can be produced by various microorganisms including bacteria, fungi and yeasts using enzymatic bioconversion, microbial fermentation or chemical hydrolysis. Regenerating conjugated glycerol-amines is valid by the intervention of microbial cyclooxygenase [COX] and lipooxygenase [LOX] enzymes produced via lactobacillus bacteria (LAB) as successor enzymes to glutamate decarboxylases (GAD). Therefore, the aim of this review is to provide an overview on γ-ABA production, and microbiological achievements used in producing this signal molecule based on those fermenting enzymes. The formation of aminoglycerides based conjugated γ-ABA is considered the key substances in controlling the host defense against pathogens and is aimed in increasing the neurotransmission effects and in suppressing further cardiovascular diseases.

Proposing Signaling Molecules as Key Optimization Targets for Intensifying the Phytochemical Biosynthesis Induced by Emerging Nonthermal Stress Pretreatments of Plant-Based Foods: A Focus on γ-Aminobutyric Acid

Emerging evidence has confirmed the role of emerging nonthermal stressors (e.g., electromagnetic fields, ultrasonication, plasma) in accumulating bioactive metabolites in plant-based food. However, the signal decoding mechanisms behind NonTt-driven phytochemical production remain unclear, hindering postharvest bioactive component intensification. This study aims to summarize the association between signaling molecules and bioactive secondary metabolite production under nonthermal conditions, demonstrating the feasibility of enhancing phytochemical accumulation through signaling molecule crosstalk manipulation. Nonthermal elicitors were found to be capable of inducing stress metabolisms and activating various signaling molecules, similar to conventional abiotic stress. A simplified pathway model for nonthermally induced γ-aminobutyric acid accumulation was proposed with reactive oxygen species and calcium signaling being versatile pathways responsive to nonthermal elicitors. Manipulating signal molecules/pathways under nonthermal conditions can intensify phytochemical biosynthesis. Further research is needed to integrate signaling molecule responses and metabolic network shifts in nonthermally stressed plant-based matrices, balancing quality modifications and intensification of food functionality potential.

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.

From Feasting to Fasting: The Arginine Pathway as a Metabolic Switch in Nitrogen-Deprived Chlamydomonas reinhardtii

The metabolism of the model microalgae Chlamydomonas reinhardtii under nitrogen deprivation is of special interest due to its resulting increment of triacylglycerols (TAGs), that can be applied in biotechnological applications. However, this same condition impairs cell growth, which may limit the microalgae's large applications. Several studies have identified significant physiological and molecular changes that occur during the transition from an abundant to a low or absent nitrogen supply, explaining in detail the differences in the proteome, metabolome and transcriptome of the cells that may be responsible for and responsive to this condition. However, there are still some intriguing questions that reside in the core of the regulation of these cellular responses that make this process even more interesting and complex. In this scenario, we reviewed the main metabolic pathways that are involved in the response, mining and exploring, through a reanalysis of omics data from previously published datasets, the commonalities among the responses and unraveling unexplained or non-explored mechanisms of the possible regulatory aspects of the response. Proteomics, metabolomics and transcriptomics data were reanalysed using a common strategy, and an in silico gene promoter motif analysis was performed. Together, these results identified and suggested a strong association between the metabolism of amino acids, especially arginine, glutamate and ornithine pathways to the production of TAGs, via the de novo synthesis of lipids. Furthermore, our analysis and data mining indicate that signalling cascades orchestrated with the indirect participation of phosphorylation, nitrosylation and peroxidation events may be essential to the process. The amino acid pathways and the amount of arginine and ornithine available in the cells, at least transiently during nitrogen deprivation, may be in the core of the post-transcriptional, metabolic regulation of this complex phenomenon. Their further exploration is important to the discovery of novel advances in the understanding of microalgae lipids' production.

Ultrasonic treatment to enhance seed germination and vigour of wheat (Triticum durum) in association with γ-aminobutyric acid (GABA) shunt pathway

Treatments of wheat (Triticum durum L.) seeds with sonication or hydropriming may enhance seed germination and vigour in association with γ-aminobutyric acid (GABA). Therefore, the objective of this study is to examine the effect of sonication and hydropriming treatments on seed germination of wheat through the characterisation of seed germination performance, GABA shunt metabolite level (GABA, glutamate, and alanine), and the level of glutamate decarboxylase (GAD) mRNA transcription. Wheat seeds were exposed to three treatments for 0, 5, 10, 15, and 20min: (1) sonication with water; (2) sonication without water; and (3) hydropriming without sonication. Treated seeds were evaluated for germination percentage, mean time to germinate, germination rate index in the warm germination test, and seedling emergence and shoot length in the cold test. GABA shunt metabolites level (GABA, glutamate, and alanine), and the level of GAD mRNA transcription were measured for the seeds after treatments and for seedlings during germination and cold tests. Seeds treated with sonication or hydropriming treatments had a higher germination rate index (faster germination) in the standard germination test, and higher seedling emergence and shoot length in the cold test. Seeds treated with sonication or hydropriming treatments showed an enhancement in GABA shunt and their metabolites (alanine and glutamate), and GAD mRNA transcription level compared to untreated-control seeds. In conclusion, the sonication or hydropriming treatments significantly improved the germination performance of wheat and enhanced GABA metabolism to maintain the C:N metabolic balance, especially under cold stress.

Metabolomic analysis reveals astaxanthin biosynthesis in heterotrophic microalga Chromochloris zofingiensis

The utilization of gibberellic acid-3, high carbon/nitrogen ratio and salinity concentration can effectively enhance astaxanthin biosynthesis in Chromochloris zofingiensis under the heterotrophic conditions, but the underlying mechanisms remained yet to be investigated. The metabolomics analysis revealed that enhancement of the glycolysis, pentose phosphate pathways (PPP), and tricarboxylic acid (TCA) cycle led to astaxanthin accumulation under the induction conditions. The increased fatty acids can significantly increase astaxanthin esterification. The addition of appropriate concentrations of glycine (Gly) and γ-aminobutyric acid (GABA) promoted astaxanthin biosynthesis in C. zofingiensis, as well as benefiting for biomass yield. With the addition of 0.5 mM GABA, the astaxanthin yield increased to 0.35 g·L^-1, which was 1.97-fold higher than that of the control. This study advanced understanding about astaxanthin biosynthesis in heterotrophic microalga, and provided novel strategies for enhanced astaxanthin production in C. zofingiensis.

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.

Enhancement of growth in Ulva prolifera by diurnal temperature difference combined with nitrogen enrichment

Many studies have documented the responses of Ulva prolifera to environmental factors. However, the diurnal temperature differences and interactive effects of eutrophication are usually ignored. In this study, we selected U. prolifera as material to examine the effects of diurnal temperature on growth, photosynthesis and primary metabolites under two nitrogen levels. We cultured U. prolifera seedlings under two temperature conditions (22-22 °C: 22 °C during day and night; 22-18 °C: 22 °C during day and 18 °C at night) and two nitrogen levels (LN: 0.1235 mg L^-1; HN: and 0.6 mg L^-1). The results showed that 1) HN-grown thalli had higher growth rates, the chlorophyll a (Chl a) content, photosynthesis, superoxide dismutase (SOD) activity, soluble sugar, and protein contents under the two temperature conditions; 2) The growth of thalli was enhanced by 22-18 °C condition compared with 22-22 °C, but the increase was only significant under HN condition; 3) 22-18°C-grown thalli had a lower net photosynthetic rate, maximal quantum yield (F_v/F_m), and dark respiration rate (R_d) than those grown at 22-22 °C; 4) No significant effects of diurnal temperature difference were detected on the SOD activity and soluble sugar content under LN and HN conditions, while the soluble protein content was enhanced by 22-18 °C under LN condition; 5) The nitrogen affected metabolite variations in U. prolifera more significantly than the diurnal temperature difference. The metabolite levels in the tricarboxylic acid cycle, amino acid, phospholipids, pyrimidine, and purine metabolism pathways increased under HN condition. The levels of glutamine, γ-aminobutyrate (GABA), 1-aminocyclopropane-1-carboxylate (ACC), glutamic acid, citrulline, glucose, sucrose, stachyose, and maltotriose were enhanced by 22-18 °C, especially under HN condition. These results identify the potential role of the diurnal temperature difference and offer new insight into the molecular mechanisms for U. prolifera responses to eutrophication and temperature.

Integrating Enzyme Evolution and Metabolic Engineering to Improve the Productivity of Γ-Aminobutyric Acid by Whole-Cell Biosynthesis in Escherichia Coli

γ-Aminobutyric acid (GABA) is used widely in various fields, such as agriculture, food, pharmaceuticals, and biobased chemicals. Based on glutamate decarboxylase (GadBM4) derived from our previous work, three mutants, GadM4-2, GadM4-8, and GadM4-31, were obtained by integrating enzyme evolution and high-throughput screening methods. The GABA productivity obtained through whole-cell bioconversion using recombinant Escherichia coli cells harboring mutant GadBM4-2 was enhanced by 20.27% compared to that of the original GadBM4. Further introduction of the central regulator GadE of the acid resistance system and the enzymes from the deoxyxylulose-5-phosphate-independent pyridoxal 5'-phosphate biosynthesis pathway resulted in a 24.92% improvement in GABA productivity, reaching 76.70 g/L/h without any cofactor addition with a greater than 99% conversion ratio. Finally, when one-step bioconversion was applied for the whole-cell catalysis in a 5 L bioreactor, the titer of GABA reached 307.5 ± 5.94 g/L with a productivity of 61.49 g/L/h by using crude l-glutamic acid (l-Glu) as the substrate. Thus, the biocatalyst constructed above combined with the whole-cell bioconversion method represents an effective approach for industrial GABA production.

More Than a Diamine Oxidase Inhibitor: L-Aminoguanidine Modulates Polyamine-Related Abiotic Stress Responses of Plants

L-aminoguanidine (AG) is an inhibitor frequently used for investigating plant abiotic stress responses; however, its exact mode of action is not well understood. Many studies used this compound as a specific diamine oxidase inhibitor, whereas other studies used it for reducing nitric oxide (NO) production. Recent studies suggest its antiglycation effect; however, this remains elusive in plants. This review summarises our current knowledge about different targets of AG in plants. Our recommendation is to use AG as a modulator of polyamine-related mechanisms rather than a specific inhibitor. In the future overall investigation is needed to decipher the exact mechanisms of AG. More careful application of AG could give more insight into plant abiotic stress responses.

Phytohormones regulate the abiotic stress: An overview of physiological, biochemical, and molecular responses in horticultural crops

Recent changing patterns of global climate have turned out to be a severe hazard to the horticulture crops production. A wide range of biotic and abiotic stresses often affect plants due to their sessile nature. Horticultural crop losses are mainly caused by abiotic factors such as drought, salt, heat, cold, floods, and ultraviolet radiation. For coping up with these adversities, well-developed mechanisms have been evolved in plants, which play a role in perceiving stress signals and enabling optimal growth responses. Interestingly, the use of phytohormones for suppressing the impact of abiotic stress has gained much attention in recent decades. For circumvention of stress at various levels, including physiological, molecular, as well as biochemical, a sophisticated mechanism is reported to be provided by the phytohormones, thus labeling these phytohormones a significant role in plant growth and development. Phytohormones can improves tolerance against abiotic stresses by increasing seed germination, seedling growth, leaf photosynthesis, root growth, and antioxidant enzymes and reducing the accumulation of reactive oxygen species, malonaldehyde, and electrolyte leakage. Recent discoveries highlight the significant role of a variety of phytohormones including melatonin (MEL), Gamma-aminobutyric acid (GABA), jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BRs), and strigolactones (SLs) in abiotic stress tolerance enhancement of horticultural plants. Thus, current review is aimed to summarize the developmental concepts regarding role of phytohormones in abiotic-stress mitigation, mainly in horticultural crops, along with the description of recent studies which identified the role of different phytohormones in stressed environments. Hence, such a review will help in paving the path for sustainable agriculture growth via involvement of phytohormones in enhancement of abiotic stress tolerance of horticultural crops.

Integrated physiologic and proteomic analysis of Stropharia rugosoannulata mycelia in response to Cd stress

Soil Cd pollution seriously threatens environment and human health. Due to its ability to absorb and accumulate Cd in mycelia, Stropharia rugosoannulata could be a potential candidate for bioremediation of Cd-contaminated soils; however, the response mechanism of mycelia to Cd stress is still unclear. In this study, the physiologic and proteomic differences of S. rugosoannulata mycelia under 0.2 mg/L (low) and 2 mg/L (high) Cd stress were investigated. The results showed that Cd accumulation and mycelial growth inhibition exhibited a concentration-depended trend. Analysis of antioxidant system indicated that SOD, GR, GSH, GSSG and ASA played key roles in resisting the toxic effects of Cd. Via proteome analysis, 24 and 267 differentially expressed proteins (DEPs) were observed under low and high Cd stress, respectively. GO and KEGG analysis found that the mycelial growth inhibition might due to the down-regulation of some DEPs involved in "valine, leucine and isoleucine biosynthesis" and "tyrosine metabolism"; the certain tolerance to high Cd stress might attribute to the regulation of DEPs referred to energy metabolism and antioxidant system-related pathways, maintaining cellular energy homeostasis and removing ROS. These results provide a theoretical basis for further elucidation of response mechanisms in S. rugosoannulata to Cd stress.

Endocytosis of GABA receptor: Signaling in nervous system

GABA (ᵞ-aminobutyric acid), is the principal neurotransmitter known for its inhibitory role in chemical synapses. Being localized primarily in the central nervous system (CNS) it maintains a balance between excitatory (regulated by another neurotransmitter, glutamate) and inhibitory impulses. GABA acts by binding to their specific receptors GABA_A and GABA_B when released into the post-synaptic nerve terminal. Both of these receptors are responsible for fast and slow inhibition of neurotransmission, respectively. GABA_A is a ligand-gated ionopore receptor which opens the Cl^- ion channel and decreases the resting potential of the membrane resulting into inhibition of the synapse. On the other hand, GABA_B is a metabotropic receptor which increases the K⁺ ion levels preventing Ca⁺ ion release inhibiting the release of other neurotransmitters into the presynaptic membrane. The internalization and trafficking of these receptors is also conducted through distinct pathways and mechanism, discussed in detail in the chapter. Without the desired levels of GABA in the body, the psychological and neurological states of brain get hard to maintain. Various neurodegenerative diseases/disorders have been associated to low levels of GABA, such as anxiety, mood disorders, fear, schizophrenia, hungtington's chorea, seizures, epilepsy, etc. The allosteric sites present on GABA receptors have been proved to be potent drug targets to pacify the pathological states of these brain related disorders to an extent. Further in depth studies focussing on the subtypes of GABA receptors and their comprehensive mechanism are required to explore new drug targets and therapeutic avenues for effectual management of GABA related neurological diseases.